Patent Publication Number: US-2021183739-A1

Title: Airflow control in active cooling systems

Description:
CROSS REFERENCE TO OTHER APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 62/949,383 entitled AIRFLOW CONTROL SYSTEM IN PIEZOELECTRIC COOLING FOR DEVICES filed Dec. 17, 2019 which is incorporated herein by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     As computing devices grow in speed and computing power, the heat generated by the computing devices also increases. Various mechanisms have been proposed to address the generation of heat. Active devices, such as fans, may be used to drive air through large computing devise, such as laptop computers or desktop computers. Passive cooling devices, such as heat spreaders, may be used in smaller, mobile computing devices, such as smartphones, virtual reality devices and tablet computers. However, such active and passive devices may be unable to adequately cool both mobile devices such as smartphones and larger devices such as laptops and desktop computers. Consequently, additional cooling solutions for computing devices are desired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the invention are disclosed in the following detailed description and the accompanying drawings. 
         FIGS. 1A-1E  depict an embodiment of an active cooling system including a centrally anchored cooling element. 
         FIGS. 2A-2B  depict embodiments of cooling elements usable in active cooling systems including centrally anchored cooling elements. 
         FIGS. 3A-3B  depict embodiments of cooling elements usable in active cooling systems including centrally anchored cooling elements. 
         FIGS. 4A-4C  are diagrams depicting an embodiment of an active cooling system. 
         FIGS. 5A-5E  depict an embodiment of an active cooling system formed in a tile. 
         FIGS. 6A-6C  depict embodiments of an active cooling system including an exhaust system used in a closed device. 
         FIGS. 7A-7C  depict embodiments of an active cooling system including an exhaust system used in an open device. 
         FIG. 8  is a plan view of an embodiment of an active cooling system usable with a heat-generating structure. 
         FIGS. 9A-9B  are diagrams of a portion of an embodiment of an active cooling system usable with a heat-generating structure. 
         FIG. 10  is a diagram depicting a side view of a portion of an embodiment of an active cooling system usable with a heat-generating structure. 
         FIG. 11  is a diagram depicting a side view of a portion of an embodiment of an active cooling system usable with a heat-generating structure. 
         FIGS. 12A-12B  are diagrams of a portion of an embodiment of an active cooling system usable with a heat-generating structure. 
         FIGS. 13A-13B  are diagrams of a portion of an embodiment of an active cooling system usable with a heat-generating structure. 
         FIG. 14  is a diagram depicting a side view of a portion of an embodiment of an active cooling system usable with a heat-generating structure. 
         FIG. 15  is a diagram depicting a side view of a portion of an embodiment of an active cooling system usable with a heat-generating structure. 
         FIG. 16  is a diagram depicting a side view of a portion of an embodiment of an active cooling system usable with a heat-generating structure. 
         FIG. 17  is a diagram depicting a side view of a portion of an embodiment of an active cooling system usable with a heat-generating structure. 
         FIG. 18  is a flow chart depicting an exemplary embodiment of a method for operating a cooling system. 
     
    
    
     DETAILED DESCRIPTION 
     The invention can be implemented in numerous ways, including as a process; an apparatus; a system; a composition of matter; a computer program product embodied on a computer readable storage medium; and/or a processor, such as a processor configured to execute instructions stored on and/or provided by a memory coupled to the processor. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. In general, the order of the steps of disclosed processes may be altered within the scope of the invention. Unless stated otherwise, a component such as a processor or a memory described as being configured to perform a task may be implemented as a general component that is temporarily configured to perform the task at a given time or a specific component that is manufactured to perform the task. As used herein, the term ‘processor’ refers to one or more devices, circuits, and/or processing cores configured to process data, such as computer program instructions. 
     A detailed description of one or more embodiments of the invention is provided below along with accompanying figures that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured. 
     As semiconductor devices become increasingly powerful, the heat generated during operations also grows. For example, processors for mobile devices such as smartphones, tablet computers, notebooks, and virtual reality devices can operate at high clock speeds, but produce a significant amount of heat. Because of the quantity of heat produced, processors may run at full speed only for a relatively short period of time. After this time expires, throttling (e.g. slowing of the processor&#39;s clock speed) occurs. Although throttling can reduce heat generation, it also adversely affects processor speed and, therefore, the performance of devices using the processors. As technology moves to 5G and beyond, this issue is expected to be exacerbated. 
     Larger devices, such as laptop or desktop computers include electric fans that have rotating blades. The fan that can be energized in response to an increase in temperature of internal components. The fans drive air through the larger devices to cool internal components. However, such fans are typically too large for mobile devices such as smartphones or for thinner devices such as tablet computers. Fans also may have limited efficacy because of the boundary layer of air existing at the surface of the components, provide a limited airspeed for air flow across the hot surface desired to be cooled and may generate an excessive amount of noise. Passive cooling solutions may include components such as a heat spreader and a heat pipe or vapor chamber to transfer heat to a heat exchanger. Although a heat spreader somewhat mitigates the temperature increase at hot spots, the amount of heat produced in current and future devices may not be adequately addressed. Similarly, a heat pipe or vapor chamber may provide an insufficient amount of heat transfer to remove excessive heat generated. 
     Varying configurations of computing devices further complicate heat management. For example, computing devices such as laptops are frequently open to the external environment while other computing devices, such as smartphones, are generally closed to the external environment. Thus, active heat management solutions for open devices, such as fans, may be inappropriate for closed devices. A fan driving heated fluid from the inside of the computing device to the outside environment may be too large for closed computing devices such as smartphones and may provide limited fluid flow. In addition, the closed computing device has no outlet for the heated fluid even if the fan can be incorporated into the closed computing device. Thus, the thermal management provided by such an open-device mechanism may have limited efficacy. Even for open computing devices, the location of the inlet and/or outlet may be configured differently for different devices. For example, an outlet for fan-driven fluid flow in a laptop may be desired to be located away from the user&#39;s hands or other structures that may lie within the outflow of heated fluid. Such a configuration not only prevents the user&#39;s discomfort but also allows the fan to provide the desired cooling. Another mobile device having a different configuration may require the inlets and/or outlets to be configured differently, may reduce the efficacy of such heat management systems and may prevent the use of such heat management systems. Thus, mechanisms for improving cooling in computing devices are desired. 
     A system including at least one heat-generating structure and a cooling system is described. The cooling system includes a cooling element and an exhaust system. The cooling element is in communication with a fluid and is configured to direct the fluid toward the heat generating structure(s) using vibrational motion. The exhaust system is configured to direct fluid away from the heat-generating structure to extract the heat and/or to draw the fluid toward the cooling element. In some embodiments, the exhaust system includes ducting configured to direct the fluid away from the heat-generating structure. The cooling system may have a total thickness of not more than three millimeters. In some embodiments, the system is incorporated into a mobile device. 
     In some embodiments, the system includes an inlet vent and an outlet vent. In such embodiment, the exhaust system is configured to direct the fluid from the heat-generating structure(s) toward the outlet vent. The cooling system is also configured to draw the fluid from the inlet vent toward the cooling element. The exhaust system may be configured to direct the fluid in a path past a plurality of components to the outlet vent. In some embodiments, the cooling system also includes an inlet subsystem and/or an outlet subsystem. The inlet subsystem includes an inlet cooling element in communication with the fluid. The inlet cooling element is configured to draw the fluid from the inlet vent using a first vibrational motion. The outlet subsystem includes an outlet cooling element in communication with the fluid. The outlet cooling element is configured to drive the fluid toward the outlet vent using a second vibrational motion. 
     In some embodiments, the cooling element has a first side distal from the heat-generating structure(s) and a second side proximal to the heat-generating structure(s). The exhaust system may also include chimneys and ducting. The chimneys are configured to direct the fluid from the heat-generating structure(s) toward the first side of the cooling element. The ducting is fluidically coupled with the chimneys and configured to direct the fluid from the chimneys to a location distal from the first side of the cooling element. In some such embodiments, the ducting is further configured to direct the fluid in a direction toward the heat-generating structure. For example, the heat-generating structure may include a heat spreader having an aperture therein. The ducting may be configured to direct the fluid through the aperture. 
     A system for cooling heat-generating structure(s) is also described. The cooling system includes cooling cells and an exhaust system. Each of the cooling cells includes a cooling element in communication with a fluid. The cooling element is configured to use vibrational motion to direct the fluid toward the heat-generating structure to extract heat from the heat-generating structure. The exhaust system is configured to direct fluid away from the heat-generating structure to extract the heat and/or to draw the fluid toward the cooling plurality of cooling cells. 
     The system may include an inlet vent and/or an outlet vent. The system is configured to draw the fluid from the inlet vent toward the cooling cells. The exhaust system is configured to direct the fluid from at least one heat-generating structure to the outlet vent. In some embodiments, the system includes an inlet subsystem and/or an outlet subsystem. The inlet subsystem includes an inlet cooling element in communication with the fluid. The inlet cooling element is configured to draw the fluid from the inlet vent using a first vibrational motion. The outlet subsystem includes an outlet cooling element in communication with the fluid. The outlet cooling element is configured to drive the fluid toward the outlet vent using a second vibrational motion. In some embodiments, the exhaust system includes ducting configured to direct the fluid away from the heat-generating structure(s). In some embodiments, the cooling cells have a total thickness of not more than three millimeters. 
     In some embodiments, the cooling element has a first side distal from the at least one heat-generating structure and a second side proximal to the heat-generating structure(s). The exhaust system may further include chimneys and ducting. The chimneys are configured to direct the fluid from the heat-generating structure(s) toward the first side of the cooling element. The ducting is fluidically coupled with the chimneys and configured to direct the fluid from the chimneys to a location distal from the first side of the cooling element. In some embodiments, the heat-generating structure includes a heat spreader having an aperture therein and ducting is configured to direct the fluid through the aperture. 
     A method is described. The method includes driving a cooling element to induce vibrational motion at a frequency. The cooling element is in communication with a fluid and configured to direct the fluid toward at least one heat-generating structure using the vibrational motion. The method also includes using an exhaust system to direct fluid away from the at least one heat-generating structure to extract heat and/or drawing the fluid toward the cooling element. In some embodiments, the exhaust system directs the fluid from the heat-generating structure(s) past components to an outlet vent. In some embodiments, the fluid is drawn from an inlet vent toward the cooling element. 
       FIGS. 1A-1E  are diagrams depicting an exemplary embodiment of active cooling system  100  usable with heat-generating structure  102  and including a centrally anchored cooling element  120 . For clarity, only certain components are shown.  FIGS. 1A-1E  are not to scale. Although shown as symmetric, cooling system  100  need not be. 
     Cooling system  100  includes top plate  110  having vent  112  therein, cooling element  120 , orifice plate  130  having orifices  132  therein, support structure (or “anchor”)  160  and chambers  140  and  150  (collectively chamber  140 / 150 ) formed therein. Cooling element  120  is supported at its central region by anchor  160 . Regions of cooling element  120  closer to and including portions of the cooling element&#39;s perimeter (e.g. tip  121 ) vibrate when actuated. In some embodiments, tip  121  of cooling element  120  includes a portion of the perimeter furthest from anchor  160  and undergoes the largest deflection during actuation of cooling element  120 . For clarity, only one tip  121  of cooling element  120  is labeled in  FIG. 1A . 
       FIG. 1A  depicts cooling system  100  in a neutral position. Thus, cooling element  120  is shown as substantially flat. For in-phase operation, cooling element  120  is driven to vibrate between positions shown in  FIGS. 1B and 1C . This vibrational motion draws fluid (e.g. air) into vent  112 , through chambers  140  and  150  and out orifices  132  at high speed and/or flow rates. For example, the speed at which the fluid impinges on heat-generating structure  102  may be at least thirty meters per second. In some embodiments, the fluid is driven by cooling element  120  toward heat-generating structure  102  at a speed of at least forty-five meters per second. In some embodiments, the fluid is driven toward heat-generating structure  102  by cooling element  120  at speeds of at least sixty meters per second. Other speeds may be possible in some embodiments. Cooling system  100  is also configured so that little or no fluid is drawn back into chamber  140 / 150  through orifices  132  by the vibrational motion of cooling element  120 . 
     Heat-generating structure  102  is desired to be cooled by cooling system  100 . In some embodiments, heat-generating structure  102  generates heat. For example, heat-generating structure may be an integrated circuit. In some embodiments, heat-generating structure  102  is desired to be cooled but does not generate heat itself. Heat-generating structure  102  may conduct heat (e.g. from a nearby object that generates heat). For example, heat-generating structure  102  might be a heat spreader or a vapor chamber. Thus, heat-generating structure  102  may include semiconductor components(s) including individual integrated circuit components such as processors, other integrated circuit(s) and/or chip package(s); sensor(s); optical device(s); one or more batteries; other component(s) of an electronic device such as a computing device; heat spreaders; heat pipes; other electronic component(s) and/or other device(s) desired to be cooled. 
     The devices in which cooling system  100  is desired to be used may also have limited space in which to place a cooling system. For example, cooling system  100  may be used in computing devices. Such computing devices may include but are not limited to smartphones, tablet computers, laptop computers, tablets, two-in-one laptops, hand held gaming systems, digital cameras, virtual reality headsets, augmented reality headsets, mixed reality headsets and other devices that are thin. Cooling system  100  may be a micro-electro-mechanical system (MEMS) cooling system capable of residing within mobile computing devices and/or other devices having limited space in at least one dimension. For example, the total height of cooling system  100  (from the top of heat-generating structure  102  to the top of top plate  110 ) may be less than 2 millimeters. In some embodiments, the total height of cooling system  100  is not more than 1.5 millimeters. In some embodiments, this total height is not more than 1.1 millimeters. In some embodiments, the total height does not exceed one millimeter. In some embodiments, the total height does not exceed two hundred and fifty micrometers. Similarly, the distance between the bottom of orifice plate  130  and the top of heat-generating structure  102 , y, may be small. In some embodiments, y is at least two hundred micrometers and not more than one millimeter. In some embodiments, y is at least two hundred micrometers and not more than three hundred micrometers. Thus, cooling system  100  is usable computing devices and/or other devices having limited space in at least one dimension. However, nothing prevents the use of cooling system  100  in devices having fewer limitations on space and/or for purposes other than cooling. Although one cooling system  100  is shown (e.g. one cooling cell), multiple cooling systems  100  might be used in connection with heat-generating structure  102 . For example, a one or two-dimensional array of cooling cells might be utilized. 
     Cooling system  100  is in communication with a fluid used to cool heat-generating structure  102 . The fluid may be a gas or a liquid. For example, the fluid may be air. In some embodiments, the fluid includes fluid from outside of the device in which cooling system  100  resides (e.g. provided through external vents in the device). In some embodiments, the fluid circulates within the device in which cooling system resides (e.g. in an enclosed device). 
     Cooling element  120  can be considered to divide the interior of active cooling system  100  into top chamber  140  and bottom chamber  150 . Top chamber  140  is formed by cooling element  120 , the sides, and top plate  110 . Bottom chamber  150  is formed by orifice plate  130 , the sides, cooling element  120  and anchor  160 . Top chamber  140  and bottom chamber  150  are connected at the periphery of cooling element  120  and together form chamber  140 / 150  (e.g. an interior chamber of cooling system  100 ). 
     The size and configuration of top chamber  140  may be a function of the cell (cooling system  100 ) dimensions, cooling element  120  motion, and the frequency of operation. Top chamber  140  has a height, h 1 . The height of top chamber  140  may be selected to provide sufficient pressure to drive the fluid to bottom chamber  140  and through orifices  132  at the desired flow rate and/or speed. Top chamber  140  is also sufficiently tall that cooling element  120  does not contact top plate  140  when actuated. In some embodiments, the height of top chamber  140  is at least fifty micrometers and not more than five hundred micrometers. In some embodiments, top chamber  140  has a height of at least two hundred and not more than three hundred micrometers. 
     Bottom chamber  150  has a height, h 2 . In some embodiments, the height of bottom chamber  150  is sufficient to accommodate the motion of cooling element  120 . Thus, no portion of cooling element  120  contacts orifice plate  130  during normal operation. Bottom chamber  150  is generally smaller than top chamber  140  and may aid in reducing the backflow of fluid into orifices  132 . In some embodiments, the height of bottom chamber  150  is the maximum deflection of cooling element  120  plus at least five micrometers and not more than ten micrometers. In some embodiments, the deflection of cooling element  120  (e.g. the deflection of tip  121 ) has an amplitude of at least ten micrometers and not more than one hundred micrometers. In some such embodiments, the amplitude of deflection of cooling element  120  is at least ten micrometers and not more than sixty micrometers. However, the amplitude of deflection of cooling element  120  depends on factors such as the desired flow rate through cooling system  100  and the configuration of cooling system  100 . Thus, the height of bottom chamber  150  generally depends on the flow rate through and other components of cooling system  100 . 
     Top plate  110  includes vent  112  through which fluid may be drawn into cooling system  100 . Top vent  112  may have a size chosen based on the desired the acoustic pressure in chamber  140 . For example, in some embodiments, the width, w, of vent  112  is at least five hundred micrometers and not more than one thousand micrometers. In some embodiments, the width of vent  112  is at least two hundred fifty micrometers and not more than two thousand micrometers. In the embodiment shown, vent  112  is a centrally located aperture in top plate  110 . In other embodiments, vent  112  may be located elsewhere. For example, vent  112  may be closer to one of the edges of top plate  110 . Vent  112  may have a circular, rectangular or other shaped footprint. Although a single vent  112  is shown, multiple vents might be used. For example, vents may be offset toward the edges of top chamber  140  or be located on the side(s) of top chamber  140 . Although top plate  110  is shown as substantially flat, in some embodiments trenches and/or other structures may be provided in top plate  110  to modify the configuration of top chamber  140  and/or the region above top plate  110 . 
     Anchor (support structure)  160  supports cooling element  120  at the central portion of cooling element  120 . Thus, at least part of the perimeter of cooling element  120  is unpinned and free to vibrate. In some embodiments, anchor  160  extends along a central axis of cooling element  120  (e.g. perpendicular to the page in  FIGS. 1A-1E ). In such embodiments, portions of cooling element  120  that vibrate (e.g. including tip  121 ) move in a cantilevered fashion. Thus, portions of cooling element  120  may move in a manner analogous to the wings of a butterfly (i.e. in phase) and/or analogous to a seesaw (i.e. out of phase). Thus, the portions of cooling element  120  that vibrate in a cantilevered fashion do so in phase in some embodiments and out of phase in other embodiments. In some embodiments, anchor  160  does not extend along an axis of cooling element  120 . In such embodiments, all portions of the perimeter of cooling element  120  are free to vibrate (e.g. analogous to a jellyfish). In the embodiment shown, anchor  160  supports cooling element  120  from the bottom of cooling element  120 . In other embodiments, anchor  160  may support cooling element  120  in another manner. For example, anchor  160  may support cooling element  120  from the top (e.g. cooling element  120  hangs from anchor  160 ). In some embodiments, the width, a, of anchor  160  is at least 0.5 millimeters and not more than four millimeters. In some embodiments, the width of anchor  160  is at least two millimeters and not more than 2.5 millimeters. Anchor  160  may occupy at least ten percent and not more than fifty percent of cooling element  120 . 
     Cooling element  120  has a first side distal from heat-generating structure  102  and a second side proximate to heat-generating structure  102 . In the embodiment shown in  FIGS. 1A-1E , the first side of cooling element  120  is the top of cooling element  120  (closer to top plate  110 ) and the second side is the bottom of cooling element  120  (closer to orifice plate  130 ). Cooling element  120  is actuated to undergo vibrational motion as shown in  FIGS. 1A-1E . The vibrational motion of cooling element  120  drives fluid from the first side of cooling element  120  distal from heat-generating structure  102  (e.g. from top chamber  140 ) to a second side of cooling element  120  proximate to heat-generating structure  102  (e.g. to bottom chamber  150 ). The vibrational motion of cooling element  120  also draws fluid through vent  112  and into top chamber  140 ; forces fluid from top chamber  140  to bottom chamber  150 ; and drives fluid from bottom chamber  140  through orifices  132  of orifice plate  130 . 
     Cooling element  120  has a length, L, that depends upon the frequency at which cooling element  120  is desired to vibrate. In some embodiments, the length of cooling element  120  is at least four millimeters and not more than ten millimeters. In some such embodiments, cooling element  120  has a length of at least six millimeters and not more than eight millimeters. The depth of cooling element  120  (e.g. perpendicular to the plane shown in  FIGS. 1A-1E ) may vary from one fourth of L through twice L. For example, cooling element  120  may have the same depth as length. The thickness, t, of cooling element  120  may vary based upon the configuration of cooling element  120  and/or the frequency at which cooling element  120  is desired to be actuated. In some embodiments, the cooling element thickness is at least two hundred micrometers and not more than three hundred and fifty micrometers for cooling element  120  having a length of eight millimeters and driven at a frequency of at least twenty kilohertz and not more than twenty-five kilohertz. The length, C of chamber  140 / 150  is close to the length, L, of cooling element  120 . For example, in some embodiments, the distance, d, between the edge of cooling element  120  and the wall of chamber  140 / 50  is at least one hundred micrometers and not more than five hundred micrometers. In some embodiments, d is at least two hundred micrometers and not more than three hundred micrometers. 
     Cooling element  120  may be driven at a frequency that is at or near both the resonant frequency for an acoustic resonance of a pressure wave of the fluid in top chamber  140  and the resonant frequency for a structural resonance of cooling element  120 . The portion of cooling element  120  undergoing vibrational motion is driven at or near resonance (the “structural resonance”) of cooling element  120 . This portion of cooling element  120  undergoing vibration may be a cantilevered section in some embodiments. The frequency of vibration for structural resonance is termed the structural resonant frequency. Use of the structural resonant frequency in driving cooling element  112  reduces the power consumption of cooling system  100 . Cooling element  120  and top chamber  140  may also be configured such that this structural resonant frequency corresponds to a resonance in a pressure wave in the fluid being driven through top chamber  140  (the acoustic resonance of top chamber  140 ). The frequency of such a pressure wave is termed the acoustic resonant frequency. At acoustic resonance, a node in pressure occurs near vent  112  and an antinode in pressure occurs near the periphery of cooling system  100  (e.g. near tip  121  of cooling element  120  and near the connection between top chamber  140  and bottom chamber  150 ). The distance between these two regions is C/2. Thus, C/2=nλ/4, where λ is the acoustic wavelength for the fluid and n is odd (e.g. n=1, 3, 5, etc.). For the lowest order mode, C=λ/2. Because the length of chamber  140  (e.g. C) is close to the length of cooling element  120 , in some embodiments, it is also approximately true that L/2=nλ/4, where λ is the acoustic wavelength for the fluid and n is odd. Thus, the frequency at which cooling element  120  is driven, v, is at or near the structural resonant frequency for cooling element  120 . The frequency v is also at or near the acoustic resonant frequency for at least top chamber  140 . The acoustic resonant frequency of top chamber  140  generally varies less dramatically with parameters such as temperature and size than the structural resonant frequency of cooling element  120 . Consequently, in some embodiments, cooling element  120  may be driven at (or closer to) a structural resonant frequency than to the acoustic resonant frequency. 
     Orifice plate  130  has orifices  132  therein. Although a particular number and distribution of orifices  132  are shown, another number and/or another distribution may be used. A single orifice plate  130  is used for a single cooling system  100 . In other embodiments, multiple cooling systems  100  may share an orifice plate. For example, multiple cells  100  may be provided together in a desired configuration. In such embodiments, the cells  100  may be the same size and configuration or different size(s) and/or configuration(s). Orifices  132  are shown as having an axis oriented normal to a surface of heat-generating structure  102 . In other embodiments, the axis of one or more orifices  132  may be at another angle. For example, the angle of the axis may be selected from substantially zero degrees and a nonzero acute angle. Orifices  132  also have sidewalls that are substantially parallel to the normal to the surface of orifice plate  130 . In some, orifices may have sidewalls at a nonzero angle to the normal to the surface of orifice plate  130 . For example, orifices  132  may be cone-shaped. Further, although orifice place  130  is shown as substantially flat, in some embodiments, trenches and/or other structures may be provided in orifice plate  130  to modify the configuration of bottom chamber  150  and/or the region between orifice plate  130  and heat-generating structure  102 . 
     The size, distribution and locations of orifices  132  are chosen to control the flow rate of fluid driven to the surface of heat-generating structure  102 . The locations and configurations of orifices  132  may be configured to increase/maximize the fluid flow from bottom chamber  150  through orifices  132  to the jet channel (the region between the bottom of orifice plate  130  and the top of heat-generating structure  102 ). The locations and configurations of orifices  132  may also be selected to reduce/minimize the suction flow (e.g. back flow) from the jet channel through orifices  132 . For example, the locations of orifices are desired to be sufficiently far from tip  121  that suction in the upstroke of cooling element  120  (tip  121  moves away from orifice plate  13 ) that would pull fluid into bottom chamber  150  through orifices  132  is reduced. The locations of orifices are also desired to be sufficiently close to tip  121  that suction in the upstroke of cooling element  120  also allows a higher pressure from top chamber  140  to push fluid from top chamber  140  into bottom chamber  150 . In some embodiments, the ratio of the flow rate from top chamber  140  into bottom chamber  150  to the flow rate from the jet channel through orifices  132  in the upstroke (the “net flow ratio”) is greater than 2:1. In some embodiments, the net flow ratio is at least 85:15. In some embodiments, the net flow ratio is at least 90:10. In order to provide the desired pressure, flow rate, suction, and net flow ratio, orifices  132  are desired to be at least a distance, r 1 , from tip  121  and not more than a distance, r 2 , from tip  121  of cooling element  120 . In some embodiments r 1  is at least one hundred micrometers (e.g. r 1 ≥100 μm) and r 2  is not more than one millimeter (e.g. r 2 ≤1000 μm). In some embodiments, orifices  132  are at least two hundred micrometers from tip  121  of cooling element  120  (e.g. r 1 ≥200 μm). In some such embodiments, orifices  132  are at least three hundred micrometers from tip  121  of cooling element  120  (e.g. r 1 ≥300 μm). In some embodiments, orifices  132  have a width of at least one hundred micrometers and not more than five hundred micrometers. In some embodiments, orifices  132  have a width of at least two hundred micrometers and not more than three hundred micrometers. In some embodiments, the orifice separation, s, is at least one hundred micrometers and not more than one millimeter. In some such embodiments, the orifice separation is at least four hundred micrometers and not more than six hundred micrometers. In some embodiments, orifices  132  are also desired to occupy a particular fraction of the area of orifice plate  130 . For example, orifices  132  may cover at least five percent and not more than fifteen percent of the footprint of orifice plate  130  in order to achieve a desired flow rate of fluid through orifices  132 . In some embodiments, orifices  132  cover at least eight percent and not more than twelve percent of the footprint of orifice plate  130 . 
     In some embodiments, cooling element  120  is actuated using a piezoelectric. Thus, cooling element  120  may be a piezoelectric cooling element. Cooling element  120  may be driven by a piezoelectric that is mounted on or integrated into cooling element  120 . In some embodiments, cooling element  120  is driven in another manner including but not limited to providing a piezoelectric on another structure in cooling system  100 . Cooling element  120  and analogous cooling elements are referred to hereinafter as piezoelectric cooling element though it is possible that a mechanism other than a piezoelectric might be used to drive the cooling element. In some embodiments, cooling element  120  includes a piezoelectric layer on substrate. The substrate may be a stainless steel, Ni alloy and/or Hastelloy substrate. In some embodiments, piezoelectric layer includes multiple sublayers formed as thin films on the substrate. In other embodiments, the piezoelectric layer may be a bulk layer affixed to the substrate. Such a piezoelectric cooling element  120  also includes electrodes used to activate the piezoelectric. The substrate functions as an electrode in some embodiments. In other embodiments, a bottom electrode may be provided between the substrate and the piezoelectric layer. Other layers including but not limited to seed, capping, passivation or other layers might be included in piezoelectric cooling element. Thus, cooling element  120  may be actuated using a piezoelectric. 
     In some embodiments, cooling system  100  includes chimneys (not shown) or other ducting. Such ducting provides a path for heated fluid to flow away from heat-generating structure  102 . In some embodiments, ducting returns fluid to the side of top plate  110  distal from heat-generating structure  102 . In some embodiments, ducting may instead directed fluid away from heat-generating structure  102  in a direction parallel to heat-generating structure  102  or perpendicular to heat-generating structure  102  but in the opposite direction (e.g. toward the bottom of the page). For a device in which fluid external to the device is used in cooling system  100 , the ducting may channel the heated fluid to a vent. In such embodiments, additional fluid may be provided from an inlet vent. In embodiments, in which the device is enclosed, the ducting may provide a circuitous path back to the region near vent  112  and distal from heat-generating structure  102 . Such a path allows for the fluid to dissipate heat before being reused to cool heat-generating structure  102 . In other embodiments, ducting may be omitted or configured in another manner. Thus, the fluid is allowed to carry away heat from heat-generating structure  102 . 
     Operation of cooling system  100  is described in the context of  FIGS. 1A-1E . Although described in the context of particular pressures, gap sizes, and timing of flow, operation of cooling system  100  is not dependent upon the explanation herein.  FIGS. 1B-1C  depict in-phase operation of cooling system  100 . Referring to  FIG. 1B , cooling element  120  has been actuated so that its tip  121  moves away from top plate  110 .  FIG. 1B  can thus be considered to depict the end of a down stroke of cooling element  120 . Because of the vibrational motion of cooling element  120 , gap  152  for bottom chamber  150  has decreased in size and is shown as gap  152 B. Conversely, gap  142  for top chamber  140  has increased in size and is shown as gap  142 B. During the down stroke, a lower (e.g. minimum) pressure is developed at the periphery when cooling element  120  is at the neutral position. As the down stroke continues, bottom chamber  150  decreases in size and top chamber  140  increases in size as shown in  FIG. 1B . Thus, fluid is driven out of orifices  132  in a direction that is at or near perpendicular to the surface of orifice plate  130  and/or the top surface of heat-generating structure  102 . The fluid is driven from orifices  132  toward heat-generating structure  102  at a high speed, for example in excess of thirty-five meters per second. In some embodiments, the fluid then travels along the surface of heat-generating structure  102  and toward the periphery of heat-generating structure  102 , where the pressure is lower than near orifices  132 . Also in the down stroke, top chamber  140  increases in size and a lower pressure is present in top chamber  140 . As a result, fluid is drawn into top chamber  140  through vent  112 . The motion of the fluid into vent  112 , through orifices  132 , and along the surface of heat-generating structure  102  is shown by unlabeled arrows in  FIG. 1B . 
     Cooling element  120  is also actuated so that top  121  moves away from heat-generating structure  102  and toward top plate  110 .  FIG. 1C  can thus be considered to depict the end of an up stroke of cooling element  120 . Because of the motion of cooling element  120 , gap  142  has decreased in size and is shown as gap  142 C. Gap  152  has increased in size and is shown as gap  152 C. During the upstroke, a higher (e.g. maximum) pressure is developed at the periphery when cooling element  120  is at the neutral position. As the upstroke continues, bottom chamber  150  increases in size and top chamber  140  decreases in size as shown in  FIG. 1C . Thus, the fluid is driven from top chamber  140  (e.g. the periphery of chamber  140 / 150 ) to bottom chamber  150 . Thus, when tip  121  of cooling element  120  moves up, top chamber  140  serves as a nozzle for the entering fluid to speed up and be driven towards bottom chamber  150 . The motion of the fluid into bottom chamber  150  is shown by unlabeled arrows in  FIG. 1C . The location and configuration of cooling element  120  and orifices  132  are selected to reduce suction and, therefore, back flow of fluid from the jet channel (between heat-generating structure  102  and orifice plate  130 ) into orifices  132  during the upstroke. Thus, cooling system  100  is able to drive fluid from top chamber  140  to bottom chamber  150  without an undue amount of backflow of heated fluid from the jet channel entering bottom chamber  10 . 
     The motion between the positions shown in  FIGS. 1B and 1C  is repeated. Thus, cooling element  120  undergoes vibrational motion indicated in  FIGS. 1A-1C , drawing fluid through vent  112  from the distal side of top plate  110  into top chamber  140 ; transferring fluid from top chamber  140  to bottom chamber  150 ; and pushing the fluid through orifices  132  and toward heat-generating structure  102 . As discussed above, cooling element  120  is driven to vibrate at or near the structural resonant frequency of cooling element  120 . Further, the structural resonant frequency of cooling element  120  is configured to align with the acoustic resonance of the chamber  140 / 150 . The structural and acoustic resonant frequencies are generally chosen to be in the ultrasonic range. For example, the vibrational motion of cooling element  120  may be at frequencies from 15 kHz through 30 kHz. In some embodiments, cooling element  120  vibrates at a frequency/frequencies of at least 20 kHz and not more than 30 kHz. The structural resonant frequency of cooling element  120  is within ten percent of the acoustic resonant frequency of cooling system  100 . In some embodiments, the structural resonant frequency of cooling element  120  is within five percent of the acoustic resonant frequency of cooling system  100 . In some embodiments, the structural resonant frequency of cooling element  120  is within three percent of the acoustic resonant frequency of cooling system  100 . Consequently, efficiency and flow rate may be enhanced. However, other frequencies may be used. 
     Fluid driven toward heat-generating structure  102  may move substantially normal (perpendicular) to the top surface of heat-generating structure  102 . In some embodiments, the fluid motion may have a nonzero acute angle with respect to the normal to the top surface of heat-generating structure  102 . In either case, the fluid may thin and/or form apertures in the boundary layer of fluid at heat-generating structure  102 . As a result, transfer of heat from heat-generating structure  102  may be improved. The fluid deflects off of heat-generating structure  102 , traveling along the surface of heat-generating structure  102 . In some embodiments, the fluid moves in a direction substantially parallel to the top of heat-generating structure  102 . Thus, heat from heat-generating structure  102  may be extracted by the fluid. The fluid may exit the region between orifice plate  130  and heat-generating structure  102  at the edges of cooling system  100 . Chimneys or other ducting (not shown) at the edges of cooling system  100  allow fluid to be carried away from heat-generating structure  102 . In other embodiments, heated fluid may be transferred further from heat-generating structure  102  in another manner. The fluid may exchange the heat transferred from heat-generating structure  102  to another structure or to the ambient environment. Thus, fluid at the distal side of top plate  110  may remain relatively cool, allowing for the additional extraction of heat. In some embodiments, fluid is circulated, returning to distal side of top plate  110  after cooling. In other embodiments, heated fluid is carried away and replaced by new fluid at the distal side of cooling element  110 . As a result, heat-generating structure  102  may be cooled. 
       FIGS. 1D-1E  depict an embodiment of active cooling system  100  including centrally anchored cooling element  120  in which the cooling element is driven out-of-phase. More specifically, sections of cooling element  120  on opposite sides of anchor  160  (and thus on opposite sides of the central region of cooling element  120  that is supported by anchor  160 ) are driven to vibrate out-of-phase. In some embodiments, sections of cooling element  120  on opposite sides of anchor  160  are driven at or near one hundred and eighty degrees out-of-phase. Thus, one section of cooling element  120  vibrates toward top plate  110 , while the other section of cooling element  120  vibrates toward orifice plate  130 /heat-generating structure  102 . Movement of a section of cooling element  120  toward top plate  110  (an upstroke) drives fluid in top cavity  140  to bottom cavity  150  on that side of anchor  160 . Movement of a section of cooling element  120  toward orifice plate  130  drives fluid through orifices  132  and toward heat-generating structure  102 . Thus, fluid traveling at high speeds (e.g. speeds described with respect to in-phase operation) is alternately driven out of orifices  132  on opposing sides of anchor  160 . The movement of fluid is shown by unlabeled arrows in  FIGS. 1D and 1E . 
     The motion between the positions shown in  FIGS. 1D and 1E  is repeated. Thus, cooling element  120  undergoes vibrational motion indicated in  FIGS. 1A, 1D, and 1E , alternately drawing fluid through vent  112  from the distal side of top plate  110  into top chamber  140  for each side of cooling element  120 ; transferring fluid from each side of top chamber  140  to the corresponding side of bottom chamber  150 ; and pushing the fluid through orifices  132  on each side of anchor  160  and toward heat-generating structure  102 . As discussed above, cooling element  120  is driven to vibrate at or near the structural resonant frequency of cooling element  120 . Further, the structural resonant frequency of cooling element  120  is configured to align with the acoustic resonance of the chamber  140 / 150 . The structural and acoustic resonant frequencies are generally chosen to be in the ultrasonic range. For example, the vibrational motion of cooling element  120  may be at the frequencies described for in-phase vibration. The structural resonant frequency of cooling element  120  is within ten percent of the acoustic resonant frequency of cooling system  100 . In some embodiments, the structural resonant frequency of cooling element  120  is within five percent of the acoustic resonant frequency of cooling system  100 . In some embodiments, the structural resonant frequency of cooling element  120  is within three percent of the acoustic resonant frequency of cooling system  100 . Consequently, efficiency and flow rate may be enhanced. However, other frequencies may be used. 
     Fluid driven toward heat-generating structure  102  for out-of-phase vibration may move substantially normal (perpendicular) to the top surface of heat-generating structure  102 , in a manner analogous to that described above for in-phase operation. Similarly, chimneys or other ducting (not shown) at the edges of cooling system  100  allow fluid to be carried away from heat-generating structure  102 . In other embodiments, heated fluid may be transferred further from heat-generating structure  102  in another manner. The fluid may exchange the heat transferred from heat-generating structure  102  to another structure or to the ambient environment. Thus, fluid at the distal side of top plate  110  may remain relatively cool, allowing for the additional extraction of heat. In some embodiments, fluid is circulated, returning to distal side of top plate  110  after cooling. In other embodiments, heated fluid is carried away and replaced by new fluid at the distal side of cooling element  110 . As a result, heat-generating structure  102  may be cooled. 
     Using the cooling system  100  actuated for in-phase vibration or out-of-phase vibration, fluid drawn in through vent  112  and driven through orifices  132  may efficiently dissipate heat from heat-generating structure  102 . Because fluid impinges upon the heat-generating structure with sufficient speed (e.g. at least thirty meters per second) and in some embodiments substantially normal to the heat-generating structure, the boundary layer of fluid at the heat-generating structure may be thinned and/or partially removed. Consequently, heat transfer between heat-generating structure  102  and the moving fluid is improved. Because the heat-generating structure is more efficiently cooled, the corresponding integrated circuit may be run at higher speed and/or power for longer times. For example, if the heat-generating structure corresponds to a high-speed processor, such a processor may be run for longer times before throttling. Thus, performance of a device utilizing cooling system  100  may be improved. Further, cooling system  100  may be a MEMS device. Consequently, cooling systems  100  may be suitable for use in smaller and/or mobile devices, such as smart phones, other mobile phones, virtual reality headsets, tablets, two-in-one computers, wearables and handheld games, in which limited space is available. Performance of such devices may thus be improved. Because cooling element  120  may be vibrated at frequencies of 15 kHz or more, users may not hear any noise associated with actuation of cooling elements. If driven at or near structural and/or acoustic resonant frequencies, the power used in operating cooling systems may be significantly reduced. Cooling element  120  does not physically contact top plate  110  or orifice plate  130  during vibration. Thus, resonance of cooling element  120  may be more readily maintained. More specifically, physical contact between cooling element  120  and other structures disturbs the resonance conditions for cooling element  120 . Disturbing these conditions may drive cooling element  120  out of resonance. Thus, additional power would need to be used to maintain actuation of cooling element  120 . Further, the flow of fluid driven by cooling element  120  may decrease. These, issues are avoided through the use of pressure differentials and fluid flow as discussed above. The benefits of improved, quiet cooling may be achieved with limited additional power. Further, out-of-phase vibration of cooling element  120  allows the position of the center of mass of cooling element  100  to remain more stable. Although a torque is exerted on cooling element  120 , the force due to the motion of the center of mass is reduced or eliminated. As a result, vibrations due to the motion of cooling element  120  may be reduced. Moreover, efficiency of cooling system  100  may be improved through the use of out-of-phase vibrational motion for the two sides of cooling element  120 . Consequently, performance of devices incorporating the cooling system  100  may be improved. Further, cooling system  100  may be usable in other applications (e.g. with or without heat-generating structure  102 ) in which high fluid flows and/or velocities are desired. 
       FIGS. 2A-2B  depict plan views of embodiments of cooling systems  200 A and  200 B analogous to active cooling systems such as cooling system  100 .  FIGS. 2A and 2B  are not to scale. For simplicity, only portions of cooling elements  220 A and  220 B and anchors  260 A and  260 B, respectively, are shown. Cooling elements  220 A and  220 B are analogous to cooling element  120 . Thus, the sizes and/or materials used for cooling elements  220 A and/or  220 B may be analogous to those for cooling element  120 . Anchors (support structures)  260 A and  260 B are analogous to anchor  160  and are indicated by dashed lines. 
     For cooling elements  220 A and  220 B, anchors  260 A and  260 B are centrally located and extend along a central axis of cooling elements  220 A and  220 B, respectively. Thus, the cantilevered portions that are actuated to vibrate are to the right and left of anchors  260 A and  260 B. In some embodiments, cooling element(s)  220 A and/or  220 B are continuous structures, two portions of which are actuated (e.g. the cantilevered portions outside of anchors  260 A and  260 B). In some embodiments, cooling element(s)  220 A and/or  220 B include separate cantilevered portions each of which is attached to the anchors  260 A and  260 B, respectively, and actuated. Cantilevered portions of cooling elements  220 A and  220 B may thus be configured to vibrate in a manner analogous to the wings of a butterfly (in-phase) or to a seesaw (out-of-phase). In  FIGS. 2A and 2B , L is the length of the cooling element, analogous to that depicted in  FIGS. 1A-1E . Also in  FIGS. 2A and 2B , the depth, P, of cooling elements  220 A and  220 B is indicated. 
     Also shown by dotted lines in  FIGS. 2A-2B  are piezoelectric  223 . Piezoelectric  223  is used to actuate cooling elements  220 A and  220 B. In some embodiments, piezoelectric  223  may be located in another region and/or have a different configuration. Although described in the context of a piezoelectric, another mechanism for actuating cooling elements  260 A and  260 B can be utilized. Such other mechanisms may be at the locations of piezoelectric  223  or may be located elsewhere. In cooling element  260 A, piezoelectric  223  may be affixed to cantilevered portions or may be integrated into cooling element  220 A. Further, although piezoelectric  223  is shown as having particular shapes and sizes in  FIGS. 2A and 2B , other configurations may be used. 
     In the embodiment shown in  FIG. 2A , anchor  260 A extends the entire depth of cooling element  220 A. Thus, a portion of the perimeter of cooling element  260 A is pinned. The unpinned portions of the perimeter of cooling element  260 A are part of the cantilevered sections that undergo vibrational motion. In other embodiments, anchor need not extend the entire length of the central axis. In such embodiments, the entire perimeter of the cooling element is unpinned. However, such a cooling element still has cantilevered sections configured to vibrate in a manner described herein. For example, in  FIG. 2B , anchor  260 B does not extend to the perimeter of cooling element  220 B. Thus, the perimeter of cooling element  220 B is unpinned. However, anchor  260 B still extends along the central axis of cooling element  220 B. Cooling element  220 B is still actuated such that cantilevered portions vibrate (e.g. analogous to the wings of a butterfly). 
     Although cooling element  220  A is depicted as rectangular, cooling elements may have another shape. In some embodiments, corners of cooling element  220 A may be rounded. Cooling element  220 B of  FIG. 2B  has rounded cantilevered sections. Other shapes are possible. In the embodiment shown in  FIG. 2B , anchor  260 B is hollow and includes apertures  263 . In some embodiments, cooling element  220 B has aperture(s) in the region of anchor  260 B. In some embodiments, cooling element  220 B includes multiple portions such that aperture(s) exist in the region of anchor  260 B. As a result, fluid may be drawn through cooling element  220 B and through anchor  260 B. Thus, cooling element  220 B may be used in place of a top plate, such as top plate  110 . In such embodiments, apertures in cooling element  220 B and apertures  263  may function in an analogous manner to vent  112 . Further, although cooling elements  200 A and  200 B are depicted as being supported in a central region, in some embodiments, one cantilevered section of the cooling element  220 A and/or  220 B might be omitted. In such embodiments, cooling element  220 A and/or  220 B may be considered to be supported, or anchored, at or near one edge, while at least part of at least the opposing edge is free to undergo vibrational motion. In some such embodiments, the cooling element  220 A and/or  220 B may include a single cantilevered section that undergoes vibrational motion. 
       FIGS. 3A-3B  depict plan views of embodiments of cooling systems  300 A and  300 B analogous to active cooling systems such as cooling system  100 .  FIGS. 3A and 3B  are not to scale. For simplicity, only cooling elements  320 A and  320 B and anchors  360 A and  360 B, respectively, are shown. Cooling elements  320 A and  320 B are analogous to cooling element  120 . Thus, the sizes and/or materials used for cooling elements  320 A and/or  320 B may be analogous to those for cooling element  120 . Anchors  360 A and  360 B are analogous to anchor  160  and are indicated by dashed lines. 
     For cooling elements  320 A and  320 B, anchors  360 A and  360 B, respectively, are limited to a central region of cooling elements  320 A and  320 B, respectively. Thus, the regions surrounding anchors  360 A and  360 B undergo vibrational motion. Cooling elements  320 A and  320 B may thus be configured to vibrate in a manner analogous to a jellyfish or similar to the opening/closing of an umbrella. In some embodiments, the entire perimeter of cooling elements  320 A and  320 B vibrate in phase (e.g. all move up or down together). In other embodiments, portions of the perimeter of cooling elements  320 A and  320 B vibrate out of phase. In  FIGS. 3A and 3B , L is the length (e.g. diameter) of the cooling element, analogous to that depicted in  FIGS. 1A-1E . Although cooling elements  320 A and  320 B are depicted as circular, cooling elements may have another shape. Further, a piezoelectric (not shown in  FIGS. 3A-3B ) and/or other mechanism may be used to drive the vibrational motion of cooling elements  320 A and  320 B. 
     In the embodiment shown in  FIG. 3B , the anchor  360 B is hollow and has apertures  363 . In some embodiments, cooling element  320 B has aperture(s) in the region of anchor  360 B. In some embodiments, cooling element  320 B includes multiple portions such that aperture(s) exist in the region of anchor  360 B. As a result, fluid may be drawn through cooling element  320 B and through anchor  360 B. The fluid may exit through apertures  363 . Thus, cooling element  320 B may be used in place of a top plate, such as top plate  110 . In such embodiments, apertures in cooling element  320 B and apertures  363  may function in an analogous manner to vent  112 . 
     Cooling systems such as cooling system  100  can utilize cooling element(s)  220 A,  220 B,  320 A,  320 B and/or analogous cooling elements. Such cooling systems may also share the benefits of cooling system  100 . Cooling systems using cooling element(s)  220 A,  220 B,  320 A,  320 B and/or analogous cooling elements may more efficiently drive fluid toward heat-generating structures at high speeds. Consequently, heat transfer between the heat-generating structure and the moving fluid is improved. Because the heat-generating structure is more efficiently cooled, the corresponding device may exhibit improved operation, such as running at higher speed and/or power for longer times. Cooling systems employing cooling element(s)  220 A,  220 B,  320 A,  320 B and/or analogous cooling elements may be suitable for use in smaller and/or mobile devices in which limited space is available. Performance of such devices may thus be improved. Because cooling element(s)  220 A,  220 B,  320 A,  320 B and/or analogous cooling elements may be vibrated at frequencies of 15 kHz or more, users may not hear any noise associated with actuation of cooling elements. If driven at or near the acoustic and/or structural resonance frequencies for the cooling element(s)  220 A,  220 B,  320 A,  320 B and/or analogous cooling elements, the power used in operating cooling systems may be significantly reduced. Cooling element(s)  220 A,  220 B,  320 A,  320 B and/or analogous cooling elements may not physically contact the plates during use, allowing resonance to be more readily maintained. The benefits of improved, quiet cooling may be achieved with limited additional power. Consequently, performance of devices incorporating the cooling element(s)  220 A,  220 B,  320 A,  320 B and/or analogous cooling elements may be improved. 
     In some embodiments, the cooling element may be anchored at one or more edges instead of at its center. For example,  FIGS. 4A-4C  depict an embodiment of cooling system  400  in which the edges of the cooling elements are anchored. 
       FIGS. 4A-4C  are diagrams depicting an exemplary embodiment of active cooling system  400  usable with a heat-generating structure  402 . For clarity, only certain components are shown and  FIGS. 4A-4C  are not to scale. Cooling system  400  is used in connection with a heat-generating structure  402 . Although shown as symmetric, cooling system  400  need not be symmetric. 
     Cooling system  400  includes cooling elements  410  and  420 . Cooling system  400  also includes orifice plate  430  having orifices  432  therein, top chamber  440  and bottom chamber  450  that may be analogous to orifice plate  130  having orifices  132  therein, top chamber  140  and bottom chamber  150 . Also shown are optional chimneys  460  used to direct fluid. 
     Cooling element  410  has a first side distal from heat-generating structure  402  and a second side proximate to heat-generating structure  402 . The first side of cooling element  410  is the top of cooling element  410  and the second side is the bottom of cooling element  410 . Cooling element  410  also has a passive vent  412  therein. In the embodiment shown, passive vent  412  is a centrally located aperture in cooling element  410 . In other embodiments, passive vent  412  may be located elsewhere. For example, passive vent  412  may be closer to one of the edges of cooling element  410 . Passive vent  412  may have a circular, rectangular or other shaped footprint. Although one passive vent  412  is shown, multiple passive vents might be used. 
     Cooling element  420  is between cooling element  410  and heat-generating structure  402 . In the embodiment shown, cooling element  420  is also between cooling element  410  and orifice plate  430 . Cooling elements  410  and  420  are separated by gap  442  and form a top chamber  440 . A bottom chamber  450  is formed between cooling element  420  and orifice plate  430 . Cooling element  420  also has active vents  422  therein. In the embodiment shown, active vents  422  are apertures located away from the central region of cooling element  420 . In other embodiments, active vents  422  may be located elsewhere. For example, an active vent may be centrally located in cooling element  420 . Although two active vents  422  are shown, another number (e.g. one, three, etc.) might be present. In some embodiments, active vents  422  are positioned such that the active vents  422  are not aligned with passive vent  412 . Active vents  422  may have circular, rectangular or other shaped footprints. In some embodiments, a single cooling element  410  or  420  which does not include a vent may be used in lieu of two cooling elements. 
     In some embodiments, cooling system  400  includes chimneys  460 . Chimneys  460  provide a return path for heated fluid to flow away from heat-generating structure  402 . In some embodiments, chimneys  460  return fluid to the side of cooling element  410  distal from heat-generating structure  402 . In the embodiment shown, chimneys  460  direct heated fluid substantially perpendicular to heat-generating structure  402  and toward the side of cooling element  410  distal from heat-generating structure  402 . In other embodiments, chimneys  460  may be omitted or configured in another manner. For example, chimneys may instead directed fluid away from heat-generating structure  402  in a direction parallel to heat-generating structure  402  or perpendicular to heat-generating structure  402  but opposite to the direction shown (e.g. toward the bottom of the page). If multiple cooling systems  400  are provided in an array, each cooling system  400  may include chimneys, only cooling systems  400  at the edges may include chimneys, other ducting may be provided at the edges or other locations in the array to provide a path for heated fluid to flow and/or other mechanisms may be used to allow heated fluid to be removed from the region proximate to heat-generating structure  402 . 
       FIG. 4A  depicts cooling system  400  in a neutral position. Thus, cooling elements  410  and  420  are shown as substantially flat. In operation, piezoelectric cooling elements  410  and  420  are actuated to vibrate between positions shown in  FIGS. 4B and 4C . Piezoelectric cooling elements  410  and  420  are, therefore, piezoelectric actuators. Operation of cooling system  400  is described in the context of  FIGS. 4B and 4C . Referring to  FIG. 4B , piezoelectric cooling element  410  has been actuated to move away from (deform to be convex) heat-generating structure  402 , while piezoelectric cooling element  420  has been actuated to move toward (deform to be concave) heat-generating structure  402 . This configuration is referred to as the suction arrangement. Because of the vibrational motion of piezoelectric cooling elements  410  and  420 , gap  442  has increased in size and is shown as gap  442 A. For example, in some embodiments, gap  442  has a height of at least ten and not more than twenty micrometers in the neutral position ( FIG. 4A ). Gap  442 A may have a height of at least twenty and not more than thirty micrometers in the suction arrangement ( FIG. 4B ). Thus, top chamber  440  has increased in volume, while bottom chamber  450  has decreased in volume. In the suction arrangement, the flow resistance of passive vent  412  (passive suction flow resistance) is low. Consequently, the pressure at passive vent  412  is low. In contrast, the flow resistance of active vent  422  (active suction flow resistance) is high. Consequently, the pressure at active vent  422  is high. Because of the low passive suction flow resistance, fluid is drawn into top chamber  440  through passive vent  412 . This is shown by arrows in  FIG. 4B . However, fluid does not flow out of (or flows out to a limited extent) active vent  422  because of the high passive suction flow resistance. However, active vent  422  is not physically closed in this configuration. For example, active vent  422  is not in contact with orifice plate  430  in the suction arrangement. 
       FIG. 4C  depicts an expulsion arrangement. Piezoelectric cooling element  410  has been actuated to move toward (deform to be concave) heat-generating structure  402 , while piezoelectric cooling element  420  has been actuated to move away from (deform to be convex) heat-generating structure  402 . Because of the vibrational motion of piezoelectric cooling elements  410  and  420 , gap  442  has decreased in size and is shown as gap  442 B. For example, in some embodiments, gap  442  has a height of at least ten and not more than twenty micrometers in the neutral position ( FIG. 4A ). Gap  442 B has a height of at least five and not more than ten micrometers in the expulsion arrangement ( FIG. 4C ). Thus, top chamber  440  has decreased in volume, while bottom chamber  450  has increased in volume. In the expulsion arrangement, the flow resistance of passive vent  412  (passive expulsion flow resistance) is high. Consequently, the pressure at passive vent  412  is high. In contrast, the flow resistance of active vent  422  (active expulsion flow resistance) is low. Consequently, the pressure at active vent  422  is low. Because of the low active expulsion flow resistance, fluid is expelled from top chamber  440  through active vent  422 , into bottom chamber  450  and through orifices  432 . This is shown by arrows in  FIG. 4C . However, fluid does not flow out of (or flows out to a limited extent) passive vent  412  because of the high passive expulsion flow resistance. Thus, passive vent  412  is considered closed and active vent  422  is considered open in the expulsion arrangement. However passive vent  412  is not physically closed in this configuration. For example, passive vent  412  is not in contact with cooling element  420  in the expulsion arrangement. Gap  442 B does not have a zero length. 
     Virtual valves may be considered to be formed at or near active vent  422  and passive vent  412 . A virtual valve has a high, but not infinite, flow resistance when closed. Thus, a virtual valve does not physically block flow but instead uses a high flow resistance or high pressure to throttle or prevent flow. A virtual valve has a significantly lower flow resistance or pressure when open, allowing flow. In some embodiments, the ratio of flow resistances or pressures between closed and open for a virtual valve is at least three and not more than ten. Thus, active vent  422  and its virtual valve (“active virtual valve”) are considered closed in the suction arrangement because the flow resistance is sufficiently high that little or no fluid flows through active vent  422  in the suction arrangement. Passive vent  412  and its virtual valve (“passive virtual valve”) are considered open in the suction arrangement because the pressure or flow resistance is sufficiently low to allow fluid to be drawn in to top chamber  440  through passive vent  412 . In contrast, active vent  422  and active virtual valve are considered open in the expulsion arrangement because the pressure or flow resistance is sufficiently low to allow fluid to flow through active vent  422  and be driven out of orifices  432 . Passive vent  412  and passive virtual valve are considered closed in the expulsion arrangement because the pressure or flow resistance is sufficiently high that little to no fluid is drawn through passive vent  412  in the expulsion arrangement. 
     Due to the vibrational motion of cooling elements  410  and  420  (and the attendant decrease in gap  442 A/ 442 B from  FIG. 4B  to  FIG. 4C ), the fluid is drawn in to top chamber  440  and through orifices  432 . The motion of the fluid is shown by arrows through orifices  432 . The fluid may spread as it travels away from orifice plate  420 , as shown by dashed lines and arrows for some orifices  432  in  FIG. 4C . The fluid deflects off of heat-generating structure  402  and travels along the channel between heat-generating structure  402  and orifice plate  430 . 
     The motion between the positions shown in  FIGS. 4B and 4C  may be repeated. Thus, piezoelectric cooling elements  410  and  420  vibrate, drawing fluid through passive vent  412  from the distal side of cooling element  410 , into top chamber  440 , out of chamber  440  through active vent  422  and pushing the fluid through orifices  432  and toward heat-generating structure  402 . In some embodiments, the frequency/frequencies of vibration of cooling elements  410  and/or  420  are analogous to those of cooling element  120 . Further, in some embodiments, piezoelectric cooling element(s)  410  and/or  420  may be driven at or near the resonant frequency. The resonant frequencies of piezoelectric cooling element(s)  410  and  420  may also be desired to be close. In some embodiments, the resonant frequencies of piezoelectric cooling element(s)  410  and  420  are desired to be within one hundred Hertz. In some embodiments, feedback is used to maintain piezoelectric cooling element(s)  410  and/or  420  at or near resonance. In some embodiments, the resonant frequencies of cooling elements  410  and/or  420  are closely matched to the acoustic resonant frequencies of chamber(s)  440  and/or  450 . In some embodiments, the speed at which the fluid impinges on heat-generating structure  402  is in the ranges described herein for cooling system  100 . 
     As indicated in  FIG. 4C , the fluid driven toward heat-generating structure  402  may move substantially normal (perpendicular) to the top surface of heat-generating structure  402 . In other embodiments, the fluid motion may have a nonzero acute angle with respect to the normal to the top surface of heat-generating structure  402 . In either case, the fluid may thin and/or form apertures in the boundary layer of fluid at heat-generating structure  402 . The boundary layer in one case is indicated by the curved dotted lines at the top surface of heat-generating structure  402  in  FIG. 4C . As a result, transfer of heat from heat-generating structure  402  may be improved. The fluid deflects off of heat-generating structure  402 , traveling along the surface of heat-generating structure  402 . In some embodiments, the fluid moves in a direction substantially parallel to the top of heat-generating structure  402 . Thus, heat from heat-generating structure  402  may be extracted by the fluid. The fluid may exit the region between orifice plate  430  and heat-generating structure  402  at the edges of cooling system  400 . In the embodiment shown, chimneys  460  at the edges of cooling system  400  allow fluid to be carried away from heat-generating structure  402 . In other embodiments, heated fluid may be transferred further from heat-generating structure  402  in another manner. The fluid may return to the distal side of cooling elements  410  where the fluid may exchange the heat transferred from heat-generating structure  402  to another structure or to the ambient environment. The fluid may then be circulated through cooling system  400  to extract additional heat. In other embodiments, heated fluid is carried away and replaced by new fluid at the distal side of cooling element  410 . As a result, heat-generating structure  402  may be cooled. 
     Opening and closing of passive vent  412  (passive virtual valve) and active vent  422  (active virtual valve) to draw fluid into chamber  450  and expel fluid through orifices  432  is based upon dynamic changes to flow resistance. In some embodiments, the ratio of active suction flow resistance to active expulsion flow resistance is at least three. In some such embodiments, the ratio of active suction flow resistance to active expulsion flow resistance is not more than ten. In some embodiments, the ratio of passive expulsion flow resistance to passive suction flow resistance is at least three. In some such embodiments, the ratio of passive expulsion flow resistance to passive suction flow resistance is not more than ten. Thus, virtual valves corresponding to vents  410  and/or  420  may be opened and closed. These ratios of pressures may be considered to be due to the change in size of gap  442 / 442 A/ 442 B (e.g. five through thirty micrometers in some embodiments). In some embodiments, the difference in pressure between being open and closed is 0.1 atmosphere through 0.2 atmosphere. For example, the pressure at passive vent  412  in the suction arrangement may be at least 0.1 atmosphere and not more than 0.2 atmosphere less than the pressure at passive vent  412  in the expulsion arrangement. Similarly, the pressure at active vent  422  in the expulsion arrangement may be at least 0.1 atmosphere and not more than 0.2 atmosphere less than the pressure at active vent  422  in the suction arrangement. 
     Using the cooling system  400 , fluid may be drawn in through passive vent  412  (in the suction arrangement) and driven through active vent  422  and orifices  432  (in the expulsion arrangement). Thus, the fluid may efficiently dissipate heat from heat-generating structure  402  in a manner analogous to the fluid driven by cooling system  100 . Thus, performance of a device utilizing cooling system  400  may be improved. Further, cooling system  400  may be a MEMS device. Thus, cooling system  400  may small-having a total height not exceeding five hundred micrometers. Consequently, cooling systems  400  may be suitable for use in mobile devices, such as smart phones, other mobile phones, virtual reality headsets, tablets, two-in-one computers, wearables and handheld games, in which limited space is available. Active cooling system  400  may also be used in other compute devices. Because piezoelectric cooling element(s)  410  and/or  420  may be vibrated at ultrasonic frequencies, users may not hear any noise associated with actuation of cooling elements. If driven at or near resonance frequency for the first and second piezoelectric cooling element(s), the power used in operating cooling systems may be significantly reduced. 
       FIGS. 5A-5E  depict an embodiment of active cooling system  500  including multiple cooling cells configured as a tile, or array.  FIG. 5A  depicts a top view, while  FIGS. 5B-5E  depict side views.  FIGS. 5A-5E  are not to scale. Cooling system  500  includes four cooling cells  501 A,  501 B,  501 C and  501 D (collectively or generically  501 ), which are analogous to one or more of cooling systems described herein. More specifically, cooling cells  501  are analogous to cooling system  100 . In some embodiments, cooling cell(s)  501  may be analogous to cooling system  400  and/or another cooling system. Although four cooling cells  501  in a 2×2 configuration are shown, in some embodiments another number and/or another configuration of cooling cells  501  might be employed. In the embodiment shown, cooling cells  501  include shared top plate  510  having apertures  512 , cooling elements  520 , shared orifice plate  530  including orifices  532 , top chambers  540 , bottom chambers  550  and anchors (support structures)  560  that are analogous to top plate  110  having apertures  112 , cooling element  120 , orifice plate  130  having orifices  132 , top chamber  140 , bottom chamber  150  and anchor  160 . In some embodiments, cooling cells  501  may be fabricated together and separated, for example by cutting through top plate  510  and orifice plate  530 . Cooling elements  520  are driven out-of-phase (i.e. in a manner analogous to a seesaw). Further, as can be seen in  FIGS. 5B-5C  and  FIGS. 5D-5E  cooling element  520  in one cell is driven out-of-phase with cooling element(s)  520  in adjacent cell(s). In  FIGS. 5B-5C , cooling elements  520  in a row are driven out-of-phase. Thus, cooling element  520  in cell  501 A is out-of-phase with cooling element  520  in cell  501 B. Similarly, cooling element  520  in cell  501 C is out-of-phase with cooling element  520  in cell  501 D. In  FIGS. 5D-5E , cooling elements  520  in a column are driven out-of-phase. Thus, cooling element  520  in cell  501 A is out-of-phase with cooling element  520  in cell  501 C. Similarly, cooling element  520  in cell  501 B is out-of-phase with cooling element  520  in cell  501 D. By driving cooling elements  520  out-of-phase, vibrations in cooling system  500  may be reduced. 
     Cooling cells  501  of cooling system  500  function in an analogous manner to cooling system(s)  100 ,  400 , and/or an analogous cooling system. Consequently, the benefits described herein may be shared by cooling system  500 . Because cooling elements in nearby cells are driven out-of-phase, vibrations in cooling system  500  may be reduced. Because multiple cooling cells  501  are used, cooling system  500  may enjoy enhanced cooling capabilities. Further, multiples of individual cooling cells  501  and/or cooling system  500  may be combined in various fashions to obtain the desired footprint of cooling cells. 
       FIGS. 6A-6C  depict active cooling systems  600 A,  600 B and  600 C used in closed device  610 . Thus, the fluid (e.g. air) remains within the device. Referring to  FIG. 6A , closed device  610  may be a mobile device such as a smartphones, tablet computer, notebook, virtual reality device, and/or other computing device. Cooling system  600 A includes one or more cooling cell(s)  601  and exhaust system  680 . Cooling cell(s)  601  are analogous to cooling cells  501 , cooling systems  100  and/or  400  and their components described herein. Thus, cooling cells  601  may include cooling element(s) driven to undergo vibrational motion for example at the structural and/or acoustic resonant frequency of the chamber(s) therein, an orifice plate and other structures. However, for clarity, some such structures are not shown. Cooling cell(s)  601  also drive fluid at the speeds described herein using the vibrational motion. Further, cooling cells  601  and, in some embodiments, active cooling systems  600 A,  600 B, and/or  600 C have a low profile. For example, the total thickness of cooling cells  601  and/or active cooling systems  600 A,  600 B, and/or  600 C may be not exceed three millimeters. In some embodiments, the total thickness of cooling cells  601  and/or active cooling systems  600 A,  600 B, and/or  600 C may not exceed than two millimeters. 
     Heat-generating structure  602  may be an integrated circuit or other structure residing on substrate  670 . Heat-generating structure  602  is a source of heat and may be analogous to heat-generating structure  102  and/or  402 . Although described as a single component, in some embodiments, multiple components may be present in heat-generating structure  602  and cooled by cooling system  600 A. For example, heat-generating structure  602  may also include a heat spreader, vapor chamber, and/or other mechanism for spreading and/or reducing heat. Substrate  670  may be a printed circuit board or other structure. Also shown are components  664 ,  666  and  667  which may be integrated circuits or other components. Mechanisms for mounting components  664 ,  666  and  667  are not shown. Although not shown, internal and/or external temperature sensors as well as other components might be employed. Cover  604  that encloses device  610  is also shown. Thus, little to no fluid flow between the interior of device  610  and the exterior of device  610  occurs. Cooling system  600 A is attached in proximity to heat-generating structure  602 . For example, cooling system  600 A may be attached to a frame in proximity to heat-generating structure  602 . A jet channel between an orifice plate and heat-generating structure  602 , corresponding heat spreader and/or other heat-generating structure may be maintained to allow fluid flow. The flow of fluid in  FIG. 6A  is depicted by unlabeled arrows. 
     Cooling cell(s)  601  of cooling system  600 A operate in a manner analogous to cooling systems described herein. As can be seen by arrows in  FIG. 6A , cooler fluid (e.g. air) near cooling system  600 A is drawn toward cooling cell(s)  601 . Cooling cell(s)  601  drive fluid from the side distal side from heat-generating structure  602  to the side proximal to heat-generating structure  602 . Thus, fluid is driven toward heat-generating structure  602 . Heat from heat-generating structure  602  is transferred to the fluid. The fluid exits the region near heat-generating structure  602  carrying away heat from heat-generating structure  602 . 
     Exhaust system  680  directs fluid carrying heat from heat-generating structure  602  away from cooling cell(s)  601 . In some embodiments, exhaust system  680  includes ducting through which the fluid travels. The ducting may be enclosed (e.g. analogous to a pipe) or may be open (e.g. forming channels). The fluid travels through device  610  via exhaust system  680  to a region of device  610  distal from cooling cell(s)  601 . In some embodiments, exhaust system  680  carries fluid to a region sufficiently distant that heat may be transferred to and dissipated by one or more structures along the path of fluid flow. Stated differently, the fluid may follow a path at least partially through exhaust system  680  such that the fluid exiting cooling system  600 A passes one or more components within system  610  and reaches a location distal from cooling system  600 A before returning to the cooling cell(s)  601 . In some embodiments, the fluid passes a sufficient number or configuration of component(s) and/or a sufficient length of exhaust system  680  that at least ninety percent of the heat transferred from heat-generating structure  602  is removed from the fluid. In some embodiments, the fluid passes a sufficient number or configuration of component(s) and/or a sufficient length of exhaust system  680  that at least eighty percent of the heat transferred from heat-generating structure  602  is removed from the fluid. In some embodiments, the fluid passes a sufficient number or configuration of component(s) and/or a sufficient length of exhaust system  680  that at least fifty percent of the heat transferred from heat-generating structure  602  is removed from the fluid at steady state operation of heat-generating structure  602 . Other fractions of heat may be removed in some embodiments. In some embodiments, device  610  includes grooves in substrate  670  and/or case  604  and/or other features used direct the fluid flow after exiting exhaust system  680 . In some embodiments, the fluid&#39;s path may include a heat sink or other mechanism for dissipating heat. For example, cover  604  may be used to dissipate heat. Consequently, heated fluid exiting heat-generating structure  602  does not immediately return to the distal side of cooling cell(s)  601 . Stated differently, cooling system  600 A does not simply receive heated fluid from heat-generating structure  602  and drive the heated fluid back toward heat-generating structure  602 . 
     Active cooling system  600 A may provide the benefits of cooling system(s)  100 ,  400 , and/or  00 . Thus, cooling system  600 A may more efficiently and quietly cool heat-generating structure  602  at lower power. Thus, performance of heat-generating structure  602  may be improved. Additional cooling systems (not shown) can be employed and/or cooling system  600 A can be increased in size, for example by adding more cells, to cool additional portions of the device, such as components  664  and/or  667 . Because active cooling system  600 A includes exhaust system  680 , heated fluid may be better circulated within closed device  610 . As a result, heat from heat-generating structure  602  may be spread among various structures that may be better able to dissipate heat. Thus, performance of closed device  610  may be improved. 
     In addition, active cooling system  600 A may also be used to mitigate issues related to hot spots on cover  604  of device  610 A. In conventional devices which do not use active cooling system  600 A, hotspots typically develop in the back portion of the cover, just above the integrated circuit or heat spreader (corresponding to the portion of cover  604  just above heat-generating structure  602 ) due to radiation and/or free convection. To reduce the temperatures at these hotspots, conventional devices typically increase the distance between the heat spreader and back cover (e.g. increase the distance between heat-generating structure  602  and cover  604 ), place holes above the heat spreader/integrated chip area (e.g. place holes in cover  604  above heat-generating structure  602 ) and try to use a fan system to pull air through these holes, or place conductive tape on the back cover near the heat spreader/integrated circuit (e.g. place conductive tape in the region of cover  604  near heat-generating structure  602 ) to conduct heat away from the hot spot to conduct the heat away from the hot spot. However, there may be limited ability to increase the distance between the back cover and the heat spreader/integrated circuit in a conventional system. Typically there is a high flow resistance in the region of the heat spreader/integrated circuit. Thus, a fan may have limited ability to generate a sufficient flow in this region to mitigate hot spots. Further, fans may be unable to be used in devices such as smartphones. Similarly, the use of conductive tape provides only a limited ability to spread heat across the cover. In contrast, when active cooling system  600 A is turned on, cooler fluid (e.g. air) flowing toward active cooling system  600 A can not only be used to cool heat-generating structure  602 , but also naturally cools the region of cover  604  near active cooling system  600 A. For example, in some embodiments, the region near the inlet of active cooling system  600 A may be at least twenty degrees Celsius cooler than in fluid exiting the region near heat-generating structure  602 . In some embodiments, the region near the inlet of active cooling system  600 A may be at least thirty degrees Celsius cooler than in fluid exiting the region near heat-generating structure  602 . In some embodiments, the region near the inlet of active cooling system  600 A may be up to thirty-five degrees Celsius cooler than in fluid exiting the region near heat-generating structure  602 . Thus, hot spots area on cover  604  may be reduced or eliminated. 
       FIG. 6B  depicts cooling system  600 B used in closed device  610 . Cooling system  600 B includes one or more cooling cell(s)  601 , that are analogous to cooling cells  501 , cooling systems  100  and/or  400  and their components described herein. Thus, cooling cells  601  may include cooling element(s) driven to undergo vibrational motion for example at the structural and/or acoustic resonant frequency, an orifice plate and other structures. However, for clarity, some such structures are not shown. Cooling system  600 B also includes exhaust system  682 . 
     Active cooling system  600 B and device  610  are analogous to cooling system  600 A and device  610  depicted in  FIG. 6A . Thus, heat-generating structure  602 , cover  604 , substrate  670 , and components  664 ,  666  and  667  of device  610  in  FIG. 6B  are analogous to heat-generating structure  602 , cover  604 , substrate  670 , and components  664 ,  666  and  667  of device  610  in  FIG. 6A . However, cooling system  600 B includes cooling cell(s)  601  and exhaust system  682 . 
     Cooling cell(s)  601  of cooling system  600 B operate in an analogous manner to cooling cell(s)  601  of cooling system  600 A. However, cooling cell(s)  601  receive cooled fluid from exhaust system  682 . Cooler fluid (e.g. air) near cooling system  600 A and within exhaust system  682  is drawn toward cooling cell(s)  601 . Cooling cell(s)  601  drive fluid from its side distal side from heat-generating structure  602  to the side proximal to heat-generating structure  602 . Thus, fluid is driven toward heat-generating structure  602 . Heat is transferred to the fluid. The fluid exits the region near heat-generating structure  602  carrying away heat from heat-generating structure  602 . The fluid travels through device  610 . In some embodiments, the fluid to a region of device  610  sufficiently distant that heat may be transferred to and dissipated by one or more structures along the path of fluid flow. Thus, fluid entering exhaust system  682  is cooler than fluid exiting the region near heat-generating structure  602 . Exhaust system  682  provides a path for the cooled fluid to return to cooling cell(s)  601  as well as to dissipate additional heat. In some embodiments, the fluid passes a sufficient number or configuration of components and/or a sufficient length of exhaust system  682  that at least ninety percent of the heat transferred from heat-generating structure  602  is removed from the fluid. In some embodiments, the fluid passes a sufficient number or configuration of components and/or a sufficient length of exhaust system  682  that at least eighty percent of the heat transferred from heat-generating structure  602  is removed from the fluid. In some embodiments, the fluid passes a sufficient number or configuration of components and/or a sufficient length of exhaust system  682  that at least fifty percent of the heat transferred from heat-generating structure  602  is removed from the fluid at steady state operation of heat-generating structure  602 . Other amounts of heat may be removed in some embodiments. In some embodiments, device  610  includes grooves in substrate  670  and/or case  604  and/or other features used direct the fluid flow exiting the region near heat-generating structure and returning to exhaust system  682 . In some embodiments, the fluid&#39;s path may include a heat sink or other mechanism for dissipating heat. For example, cover  604  may be used to dissipate heat. Consequently, heated fluid exiting heat-generating structure  602  does not immediately return to the distal side of cooling cell(s)  601 . Stated differently, cooling system  600 B does not simply drive heated fluid from heat-generating structure  602  back toward heat-generating structure  602 . Instead, cooler fluid received at and directed by exhaust system  682  enters cooling cell(s) and is driven toward heat-generating structure  602 . 
     Active cooling system  600 B shares the benefits of active cooling system  600 A. Thus, cooling system  600 B may more efficiently and quietly cool heat-generating structure  602  at lower power. Thus, performance of heat-generating structure  602  may be improved. Additional cooling systems (not shown) can be employed and/or cooling system  600 B can be increased in size, for example by adding more cells, to cool additional portions of the device, such as components  664  and/or  667 . Because active cooling system  600 B includes exhaust system  682 , heated fluid may be better circulated within closed device  610 . As a result, heat from heat-generating structure  602  may be spread among various structures that may be better able to dissipate heat. Thus, performance of closed device  610  may be improved. 
       FIG. 6C  depicts cooling system  600 C used in closed device  610 . Cooling system  600 C includes one or more cooling cell(s)  601 , that are analogous to cooling cells  501 , cooling systems  100  and/or  400  and their components described herein. Thus, cooling cells  601  may include cooling element(s) driven to undergo vibrational motion for example at the structural and/or acoustic resonant frequency, an orifice plate and other structures. However, for clarity, some such structures are not shown. Cooling system  600 C also includes exhaust system  680  and exhaust system  682 . 
     Cooling system  600 C and device  610  are analogous to cooling systems  600 A,  600 B and device  610  depicted in  FIGS. 6A-6B . Thus, heat-generating structure  602 , cover  604 , substrate  670 , and components  664 ,  666  and  667  of device  610  in  FIG. 6C  are analogous to heat-generating structure  602 , cover  604 , substrate  670 , and components  664 ,  666  and  667  of device  610  in  FIGS. 6A-6B . 
     Cooling cell(s)  601  of cooling system  600 C operate in an analogous manner to cooling cell(s)  601  of cooling systems  600 A and  600 B because cooling system  600 C includes exhaust system  680  and  682 . Thus, exhaust system  680  directs fluid from heat-generating structure  602  to regions of device  610  distal from cooling cell(s)  601  in a manner analogous to cooling system  600 A. Exhaust system  682  returns fluid to cooling cell(s)  601  in a manner analogous to cooling system  600 B. In some embodiments, the amount of heat in fluid dissipated in traversing device  610  is analogous to that described above. 
     Thus, cooling system  600 C may more efficiently and quietly cool heat-generating structure  602  at lower power. Performance of heat-generating structure  602  may be improved. Additional cooling systems (not shown) can be employed and/or cooling system  600 C can be increased in size, for example by adding more cells, to cool additional portions of the device, such as components  664  and/or  667 . Because active cooling system  600 C includes exhaust systems  680  and  682 , heated fluid may be better circulated within closed device  610 . As a result, heat from heat-generating structure  602  may be spread among various structures that may be better able to dissipate heat. Thus, performance of closed device  610  may be improved. 
       FIGS. 7A-7C  depict cooling systems  700 A,  700 B and  700 C used in open device  710 . Thus, the fluid (e.g. air) enters, travels through and exits the device. Referring to  FIG. 7A , open device  710  may be a mobile device such as a smartphones, tablet computer, notebook, virtual reality device, and/or other computing device. Cooling system  700 A includes one or more cooling cell(s)  701 , that are analogous to cooling cells  501  and  601 , cooling systems  100  and/or  400  and their components described herein. Thus, cooling cells  701  may include cooling element(s) driven to undergo vibrational motion for example at the structural and/or acoustic resonant frequency, an orifice plate and other structures. However, for clarity, some such structures are not shown. Cooling system  700 A also includes exhaust system  780 . 
     Device  710  is analogous to device  610 . Consequently, analogous structures have similar labels. Device  710  includes heat-generating structure  702  on substrate  770 , cover  704 , and additional components  764 ,  766  and  767  that are analogous to heat-generating structure  602 , substrate  670 , cover  604 , and additional components  664 ,  666 , and  667 , respectively. Mechanisms for mounting components  764 ,  766  and  767  are not shown. Although not shown, internal and/or external temperature sensors as well as other components might be employed. Cooling system  700 A is attached in proximity to heat-generating structure  702 . For example, cooling system  700 A may be attached to a frame in proximity to heat-generating structure  702 . A channel between an orifice plate and heat-generating structure  702 , corresponding heat spreader and/or other heat-generating structure may be maintained to allow fluid flow. The flow of fluid in  FIG. 7A  is depicted by unlabeled arrows. Device  710  also includes vents  790  and  792  that allow for fluid communication between the interior of device  710  and the external environment. In the embodiment shown, vent  790  operates as inlet  790  and vent  792  operates as outlet  792 . 
     Cooling cell(s)  701  of cooling system  700 A operate in a manner analogous to cooling systems described herein. As can be seen by arrows in  FIG. 7A , cooler fluid (e.g. air) near cooling system  700 A is drawn toward cooling cell(s)  701 . In particular, fluid from inlet  790  travels toward cooling cell(s)  701 . Cooling cell(s)  701  drive fluid from the side distal side from heat-generating structure  702  to the side proximal to heat-generating structure  702 . Thus, fluid is driven toward heat-generating structure  702 . Heat is transferred to the fluid. The fluid exits the region near heat-generating structure  702  carrying away heat from heat-generating structure  702 . Fluid carrying heat from heat-generating structure  702  is directed by exhaust system  780  away from cooling cell(s)  701 . 
     Exhaust system  780  may be analogous to exhaust system  680 . In some embodiments, exhaust system  780  includes ducting through which the fluid travels. The ducting may be enclosed or may be open, forming channels in device  710 . The fluid travels through device  710  to outlet  792 . Thus, fluid carrying heat from heat-generating structure  702  can be expelled from device  710  and new fluid from the outside environment drawn in through inlet  790  to cool heat-generating structure. Further, because exhaust system  780  directs the fluid to outlet  792  that is distal from cooling cell(s)  701 , heat may be transferred to and dissipated by one or more structures along the path. For example, heat may be transferred to cover  704  and/or component(s)  764 ,  766  and  767 . Thus, the fluid may be cooled at least somewhat before exiting device  710 . 
     In some embodiments, the fluid passes a sufficient number or configuration of component(s) and/or a sufficient length of exhaust system  780  that at least ninety percent of the heat transferred from heat-generating structure  702  is removed from the fluid. In some embodiments, the fluid passes a sufficient number or configuration of component(s) and/or a sufficient length of exhaust system  780  that at least eighty percent of the heat transferred from heat-generating structure  702  is removed from the fluid. In some embodiments, the fluid passes a sufficient number or configuration of component(s) and/or a sufficient length of exhaust system  780  that at least fifty percent of the heat transferred from heat-generating structure  702  is removed from the fluid at steady state operation of heat-generating structure  702 . Other amounts of heat may be dissipated in other embodiments. In some embodiments, device  710  includes ducting, grooves in substrate  770  and/or case  704  or other features used direct the fluid flow after exiting exhaust system  780 . In some embodiments, the fluid&#39;s path may include a heat sink or other mechanism for dissipating heat. For example, cover  704  may be used to dissipate heat. Consequently, heat may be efficiently removed from device  710 . 
     Active cooling system may share benefits of active cooling systems  100 ,  400 ,  500 ,  600 A,  600 B, and/or  600 C. Because cool fluid form outside of device  710  can be used to cool heat-generating structure  702 , heat management may be improved. Thus, performance of heat-generating structure  702  may be further improved. Additional cooling systems (not shown) can be employed and/or cooling system  700 A can be increased in size, for example by adding more cells, to cool additional portions of the device, such as components  764  and/or  767 . Further, exhaust system  780  can be used direct fluid to a distant outlet  792 . Thus, outlets  792 , and other structures in device  710 , may be placed where desired. Consequently, configuration of device  710  may be more flexible. In addition, some heat may be transferred to other portions of device  710  before exiting via vent  792 . As a result, the fluid exiting device  710  may be cooler than if vent  792  were placed close to heat-generating structure  702 . Thus, discomfort or injury to the user due to fluid flowing through outlet  792  may be mitigated or avoided. 
       FIG. 7B  depicts cooling system  700 B used in open device  710 . Cooling system  700 B includes one or more cooling cell(s)  701 , that are analogous to cooling cells  501 ,  601 ,  701  cooling systems  100  and/or  400  and their components described herein. Thus, cooling cells  701  may include cooling element(s) driven to undergo vibrational motion for example at the structural and/or acoustic resonant frequency, an orifice plate and other structures. However, for clarity, some such structures are not shown. Cooling system  700  also includes exhaust system  782 . 
     Active cooling system  700 B and device  710  are analogous to cooling system  700 A and device  710  depicted in  FIG. 7A . Thus, heat-generating structure  702 , cover  704 , substrate  770 , inlet  790 , outlet  792 , and components  764 ,  766  and  767  of device  610  in  FIG. 7B  are analogous to heat-generating structure  602 , cover  604 , substrate  670 , inlet  790 , outlet  792 , and components  664 ,  666  and  667  of device  610  in  FIG. 6A . 
     Cooling cell(s)  701  of active cooling system  700 B operate in an analogous manner to cooling cell(s)  701  of active cooling system  700 A. However, cooling cell(s)  701  receive fluid from inlet  790  via exhaust system  782 . Cooler fluid (e.g. air) near cooling system  700 A and within exhaust system  782  is drawn toward cooling cell(s)  701 . Cooling cell(s)  701  drive fluid from its side distal side from heat-generating structure  702  to the side proximal to heat-generating structure  702 . Thus, fluid is driven toward heat-generating structure  702 . Heat is transferred to the fluid. The fluid exits the region near heat-generating structure  702  carrying away heat from heat-generating structure  702 . The fluid travels through device  710  to outlet  792 . In some embodiments, heat may be transferred to portions of device  710  between heat-generating structure  702  and outlet  792 . In some embodiments, device  710  includes grooves in substrate  770  and/or case  704  and/or other features used direct the fluid flow after exiting the region near heat-generating structure  702 . In some embodiments, the fluid&#39;s path may include a heat sink or other mechanism for dissipating heat. For example, cover  704  may be used to dissipate heat. Consequently, heated fluid exiting device  710  via outlet  792  may be cooler than if outlet  792  were located close to heat-generating structure  702 . 
     Active cooling system  700 B may share the benefits of active cooling system  700 A. Thus, cooling system  700 B may more efficiently and quietly cool heat-generating structure  702  at lower power. Cooling efficacy of active cooling system  700 B may be improved through the use of cooler fluid entering device  710  through inlet  790 . Thus, performance of heat-generating structure  602  may be improved. Additional cooling systems (not shown) can be employed and/or cooling system  700 B can be increased in size, for example by adding more cells, to cool additional portions of the device, such as components  764  and/or  767 . 
     Further, exhaust system  782  can be used direct fluid from inlet  790 , which may be cooler than other fluid in the region of active cooling system  710 . Thus, cooling of heat-generating structure  702  may be improved. In addition, inlet  790 , and other structures in device  710 , may be placed where desired. Consequently, configuration of device  710  may be more flexible. 
       FIG. 7C  depicts cooling system  700 C used in open device  710 . Cooling system  700 C includes one or more cooling cell(s)  701 , that are analogous to cooling cells  501 ,  601  and/or  701 , cooling systems  100  and/or  400  and their components described herein. Thus, cooling cells  701  may include cooling element(s) driven to undergo vibrational motion for example at the structural and/or acoustic resonant frequency, an orifice plate and other structures. However, for clarity, some such structures are not shown. Cooling system  700 C also includes exhaust system  780  and exhaust system  782 . 
     Cooling system  700 C and device  710  are analogous to cooling systems  700 A,  700 B and device  710  depicted in  FIGS. 7A and 7B . Thus, heat-generating structure  702 , cover  704 , substrate  770 , and components  764 ,  766  and  767  of device  710  in  FIG. 7C  are analogous to heat-generating structure  602 , cover  604 , substrate  670 , and components  664 ,  666  and  667  of device  610  in  FIGS. 7A-7B . 
     Cooling cell(s)  701  of cooling system  700 C operate in an analogous manner to cooling cell(s)  701  of cooling systems  700 A and  700 B because cooling system  700 C includes exhaust systems  780  and  782 . Thus, exhaust system  780  directs fluid from heat-generating structure  702  to outlet  792  that may be distal from cooling cell(s)  701  in a manner analogous to cooling system  700 A. Exhaust system  782  directs fluid traveling through inlet  790  from outside device  710  to cooling cell(s)  701  in a manner analogous to cooling system  700 B. In some embodiments, the amount of heat in fluid dissipated in traversing device  710  is analogous to that described above. 
     Thus, cooling system  700 C may more efficiently and quietly cool heat-generating structure  702  at lower power. Performance of heat-generating structure  702  may be improved. Additional cooling systems (not shown) can be employed and/or cooling system  700 C can be increased in size, for example by adding more cells, to cool additional portions of the device, such as components  764  and/or  767 . Because active cooling system  700 C includes exhaust systems  780  and  782 , heated fluid may be better delivered to outlet  792  (as well as structure(s) in the path to outlet  792 ) and cooler fluid directed from inlet  790 . As a result, heat from heat-generating structure  702  may be better managed. Thus, performance of closed device  610  may be improved. 
     Exhaust systems used in active cooling systems and in conjunction with cooling cells may take on various configurations.  FIGS. 8-15  depict some such configurations. However, other configurations may be employed. 
       FIG. 8  is a plan view of an embodiment of a cooling system  800  usable with a heat-generating structure. For clarity, only certain components are shown and  FIG. 8  is not to scale. Cooling system  800  includes cell(s)  801 , chimneys  806  and ducts  880 . Cooling cell(s)  801  are analogous to the cooling cells  100 ,  400 ,  501 ,  601 , and/or  701  described herein and may be used in connection with a semiconductor structure, electronic device, and/or other heat-generating structure. In some embodiments, a single cooling cell might be included, while in other embodiments multiple cells might be included. For simplicity, any apertures or vents for cooling cell(s)  801  are not shown. 
     Chimneys  806  provide a return path for fluid from near the heat-generating structure being cooled (on the proximal side of cooling element and not shown in  FIG. 8 ) to the distal side of cooling element. In the embodiment shown, there are four chimneys  806  for each cooling cell(s)  801 . In another embodiment, a different number of chimneys  806  may be used. Thus, warmed fluid (e.g. air) that was used to carry heat from a heat-generating structure flows from a region proximate to the structure being cooled, through chimneys  806 , to the side of cell  801  shown. 
     Ducting  880  controls the movement of fluid from the chimneys  806 . More specifically, ducting  880  carries the heat-carrying fluid from chimneys  806  to another location. Consequently, the fluid carried by chimneys  806  may be prevented from mixing with fluid that enters cell(s)  801  through aperture(s) not shown in  FIG. 8 ). Thus, the fluid that carries heat away from the heat-generating structure may not mix with the fluid entering cell  801  to be used for cooling the heat generating structure. As a result, cell  801  may have improved cooling performance. In addition, ducting  880  may provide additional mechanical stiffness for cell  801 . As a result, vibration of the cooling element(s) for cell  801  may be better mechanically isolated from other cooling cells (not shown in  FIG. 8 ). Consequently, the cooling element(s) for cell  801  may be able to be more readily driven at the desired frequency of vibration. Again, performance of cooling cell system  800  may be improved. 
       FIGS. 9A-9B  are a diagram of a portion of cooling system  900  usable with a heat-generating structure. For clarity, only certain components are shown and  FIGS. 9A-9B  are not to scale. Cooling system  900  includes a cell  901 , apertures  904 , chimneys  906  and ducts  910 . Also shown are orifices  922 . Because orifices  922  are typically in an underlying orifice plate, orifices  922  are shown with dashed lines. Cooling cell  901  is analogous to the cooling cells described herein and may be used in connection with a semiconductor structure or device. In other embodiments, multiple cells might be included. In the embodiment shown, two apertures  904  are included. In another embodiment, another number of apertures, such as single aperture, might be used. Apertures  904  may be in a cooling element analogous to vent  412  in cooling element  410  or may be in a top plate analogous to vent  112  in top plate  110 . Chimneys  906  provide a return path for fluid from near the structure being cooled (on the proximal side of cooling element and not shown in  FIG. 9 ) to the distal side of cooling element. In the embodiment shown, there are four chimneys  906  for each cooling cell  901 . In another embodiment, a different number of chimneys may be used. 
       FIG. 9B  depicts cell  901  with the air flow indicated by lines and unlabeled arrows. Thus, heated fluid (e.g. air) that was used to carry heat away from a heat-generating structure flows through chimneys  906  at the corners of cell  901  (shown by smaller arrows at chimneys  906 ). Ducting  910  carries the heated fluid from chimneys  906  to another location such that the heated fluid carried by chimneys  906  may not mix with fluid that enters cell  901  through apertures  904  and is to be used for cooling (fluid flow shown by larger arrows). As a result, cell  901  may have improved cooling performance. In addition, ducting  910  may provide additional mechanical stiffness for cell  901 . As a result, vibration of the cooling element(s) for cell  901  may be better mechanically isolated from other cooling cells (not shown in  FIG. 9 ). Consequently, the cooling element(s) for cell  901  may be able to be more readily driven at the desired frequency of vibration. Again, performance of cooling cell system  900  may be improved. 
       FIG. 10  is a diagram depicting a side view of a portion of active cooling system  1000  usable with a heat-generating structure  1002 . Active cooling system  1000  includes cooling cell  1001 , chimneys  1070  and ducting  1080 . In the embodiment shown, cooling cell  1001  includes two cooling elements (top actuator  1010  and bottom actuator  1020  analogous to cooling elements  410  and  420 , respectively), orifice plate  1030  with orifices  1032  therein, chambers (top chamber and bottom chamber), passive valve (also termed aperture  1012  herein) for the top actuator and active valve between the bottom actuator and orifice plate. In the embodiment shown, heat-generating structure  1002  includes a component  1002 A such as an integrated circuit and heat spreader  1002 B. Also shown are standoff  1005  that may be used to set the height of the jet channel, ducting  1080  and chimneys  1070  that are part of the exhaust system. Other and/or different components may be included. 
     Cooling cell  1001  functions in an analogous manner to active cooling system  400 . In other embodiments, another cooling cell such as cooling system  100  may be used. Consequently, the sizes and operation of cooling cell  1001  are analogous to that of cooling systems  100  and/or  400 . Thus, the fluid driven by cooling cell  1001  can cool the heat spreader and, therefore, the underlying integrated circuit (or other heat generating device). In some embodiments, other cooling cells, such as those described herein, may be used in connection with the chimneys and ducting shown in  FIG. 10 . 
     Fluid moves along the surface of the heat spreader  1002 B to chimneys  1070 . The fluid transports heat from heat spreader  1002 B (and thus heat-generating component  1002 ), cooling heat spreader  1002 B and integrated circuit  1002 A. Chimneys  1070  carry the heated fluid away from heat spreader  1002 B. Ducting  1080  can transport the heated fluid away from vent  1012 . Thus, the fluid carrying heat from heat spreader  1002 B may not readily mix with fluid drawn through the vent  1012  to cool heat spreader  1002 B. Consequently, system  1000  may better cool the integrated circuitry. In addition, ducting  1080  may provide additional mechanical vibration isolation to cooling system  1000 . Thus, the top actuator  1010  and bottom actuator  1020  may be more easily driven at the desired frequency or frequencies. Consequently, performance of the system  1000  and the device system  1000  is incorporated into may be improved. 
       FIG. 11  is a diagram depicting a side view of a portion of an embodiment of active cooling system  1100  usable with heat-generating structure  1102 . Active cooling system  1100  includes cooling cell  1101 , chimneys  1170  and ducting  1180 . Cooling cell  1101  is analogous to cooling system  100 . In the embodiment shown, cooling cell  1101  includes cooling element  1120  and top plate  1110  having vent  1112  therein, orifice plate  1130  having orifices therein, that are analogous to cooling element  112 , top plate  110 , vent  112 , orifice plate  130  and orifices  132 , respectively. In the embodiment shown, heat-generating structure  1102  includes a component  1102 A such as an integrated circuit and heat spreader  1102 B. Also shown are standoff  1105  that may be used to set the height of the jet channel, ducting  1180  and chimneys  1170  that are part of the exhaust system. Other and/or different components may be included. Cooling cell  1101  thus functions in an analogous manner to active cooling system  100 . In other embodiments, another cooling cell such as cooling system  400  may be used. Consequently, the sizes and operation of cooling cell  1101  are analogous to that of cooling systems  100  and/or  400 . Fluid is driven through orifices  1132  of orifice plate  1130 . The fluid transports heat from heat spreader  1102 B, cooling heat spreader  1102 B and integrated circuit  1102 A. Thus, the fluid can cool heat spreader  1102 B and, therefore, the underlying integrated circuit  1102 A (or other heat-generating device). In some embodiments, other cooling cells, such as those described herein, may be used in connection with the chimneys and ducting shown in  FIG. 11 . 
     In active cooling system  1100 , fluid moves along the surface of heat spreader  1102 B to chimneys  1170 . The fluid transports heat from heat spreader  1102 B, cooling heat spreader  1102 B and integrated circuit  1102 A. Chimneys  1170  carry the heated fluid away from heat spreader  1102 B and to ducting  1180 . Ducting  1180  transports the heated fluid along the top surface of cooling system  1100  and to the edge of cooling system  1100 . Fluid flows down toward heat spreader  1102 B and through aperture  1103 . Aperture  1103  may be in heat spreader  1102 B or at the edge of heat spreader  1102 B. Although fluid is shown as being transported only to one aperture  1103  at one side of cooling cell  1101 , in some embodiments, multiple apertures that may be at multiple sides of cooling cell  1101  can be used. Fluid is then transported away, for example toward an air vent or other cooling mechanism. Thus, fluid may exit the region below heat spreader  1102 B. This flow of fluid is shown by arrows in  FIG. 11 . Consequently, system  1100  may better cool the integrated circuitry. In addition, ducting  1180  may provide additional mechanical vibration isolation to cooling system  1100 . Thus, the cooling element  1120  may be more easily driven at the desired frequency or frequencies. Consequently, performance of the system  1100  and the device system  1100  is incorporated into may be improved. 
       FIGS. 12A-12B  are diagrams of a portion of an embodiment of active cooling system  1200  usable with a heat generating structure such as the integrated circuit shown.  FIG. 12A  depicts a plan view of cooling system  1200 , while  FIG. 12B  depicts a side view of a portion of cooling system  1200  include a single cooling cell  1201 . Cooling cell  1201  is analogous to cooling system  100 . In the embodiment shown, cooling cell  1201  includes cooling element  1220  and top plate  1210  having vent  1212  therein, orifice plate  1230  having orifices  1232  therein, that are analogous to cooling element  112 , top plate  110 , vent  112 , orifice plate  130  and orifices  132 , respectively. In the embodiment shown, heat-generating structure  1202  includes a component  1202 A such as an integrated circuit and heat spreader  1202 B. Also shown are standoff  1205  that may be used to set the height of the jet channel and ducting  1280 . Other and/or different components may be included. Cooling cell  1201  thus functions in an analogous manner to active cooling system  100 . In other embodiments, another cooling cell such as cooling system  400  may be used. Consequently, the sizes and operation of cooling cell  1201  are analogous to that of cooling systems  100  and/or  400 . Thus, the fluid can cool heat spreader  1202 B and, therefore, the underlying integrated circuit  1202 A (or other heat generating device). In some embodiments, other cooling cells, such as those described herein, may be used in connection with the ducting shown in  FIGS. 12A and 12B . 
     In active cooling system  1200 , heated fluid is transported between cells  1201  via ducting  1280  that is between orifice plate  1230  and heat spreader  1202 B. In this embodiment, chimneys are omitted. Although shown as including sidewalls, ducting  1280  may simply be additional space to which fluid may flow. Thus, fluid moves along the surface of heat spreader  1202 B and between cells  1201 . Ducting  1280  transports the heated fluid to the edge of cooling system  1200 . Fluid is then transported away, for example toward an air vent or other cooling mechanism. Consequently, system  1200  may better cool the integrated circuitry. In addition, ducting  1280  may provide additional mechanical vibration isolation to cooling system  1200 . Thus, actuator  1220  may be more easily driven at the desired frequency or frequencies. Consequently, performance of the system  1200  and the device that active cooling system  1200  is incorporated into may be improved. 
       FIGS. 13A-13B  are diagrams of a portion of an embodiment of active cooling system  1300  usable with a heat generating structure such as the integrated circuit shown.  FIG. 13A  depicts a particular cell  1301 , while  FIG. 13B  depicts multiple cells  1301 . Cooling cell  1301  is analogous to cooling system  100 . In the embodiment shown, cooling cell  1301  includes cooling element  1320  and top plate  1310  having vent  1312  therein, orifice plate  1330  having orifices  1332  therein, that are analogous to cooling element  112 , top plate  110 , vent  112 , orifice plate  130  and orifices  132 , respectively. In the embodiment shown, heat-generating structure  1302  includes a component  1302 A such as an integrated circuit and heat spreader  1302 B. Also shown are standoff  1305  that may be used to set the height of the jet channel and ducting  1380 . Other and/or different components may be included. 
     Cooling cell  1301  functions in an analogous manner to active cooling system  100 . In other embodiments, another cooling cell such as cooling system  400  may be used. Consequently, the sizes and operation of cooling cell  1301  are analogous to that of cooling systems  100  and/or  400 . Fluid is then driven through orifices  1332  of orifice plate  1330 . The fluid transports heat from heat spreader  1302 B, cooling heat spreader  1302 B and integrated circuit  1302 A. Thus, the fluid can cool heat spreader  1302 B and, therefore, the underlying integrated circuit  1302 A (or other heat generating device). In some embodiments, other cooling cells, such as those described herein, may be used in connection with the features shown in  FIGS. 13A and 13B . 
     In active cooling system  1300 , fluid moves along the surface of heat spreader  1302 B toward the exit path. The fluid transports heat from heat spreader  1302 B, cooling heat spreader  1302 B and integrated circuit  1302 A. The exit path guides the fluid away from cells  1301 . Fluid may be transported away, for example toward an air vent or other cooling mechanism. This flow of fluid is shown by arrows in  FIGS. 13A-13B . Consequently, system  1300  may better cool the integrated circuitry. Consequently, performance of the system  1300  and the device active cooling system  1300  is incorporated into may be improved. 
       FIG. 14  is a diagram depicting a side view of a portion of an embodiment of active cooling system  1400  usable with heat-generating structure  1402 . Active cooling system  1400  includes cooling cell  1401  and ducting  1480 . Cooling cell  1401  is analogous to cooling system  100 . In the embodiment shown, cooling cell  1401  includes cooling element  1420  and top plate  1410  having vent  1412  therein, orifice plate  1430  having orifices therein, that are analogous to cooling element  112 , top plate  110 , vent  112 , orifice plate  130  and orifices  132 , respectively. In the embodiment shown, heat-generating structure  1402  includes a component  1402 A such as an integrated circuit and heat spreader  1402 B. Ducting  1480  is part of the exhaust system. Other and/or different components may be included. Cooling cell  1401  thus functions in an analogous manner to active cooling system  100 . In other embodiments, another cooling cell such as cooling system  400  may be used. Consequently, the sizes and operation of cooling cell  1401  are analogous to that of cooling systems  100  and/or  400 . Fluid is driven through orifices  1432  of orifice plate  1430 . The fluid transports heat from heat spreader  1402 B, cooling heat spreader  1402 B and integrated circuit  1402 A. Thus, the fluid can cool heat spreader  1402 B and, therefore, the underlying integrated circuit  1402 A (or other heat-generating device). In some embodiments, other cooling cells, such as those described herein, may be used in connection with the chimneys and ducting shown in  FIG. 14 . 
     In active cooling system  1400 , fluid moves along the surface of heat spreader  1402 B to ducting  1480 . The fluid transports heat from heat spreader  1402 B, cooling heat spreader  1402 B and integrated circuit  1402 A. Ducting  1480  transports the heated fluid along the top surface of heat spreader  1402 B and past the edge of cooling system  1400 . Fluid flows down toward heat spreader  1402 B and through aperture  1403 . Aperture  1403  may be in heat spreader  1402 B or at the edge of heat spreader  1402 B. Although fluid is shown as being transported to apertures  1103  at both sides of cooling cell  1401 , in some embodiments, multiple apertures and/or apertures only at a single side of cooling cell  1401  may be used. Fluid is then transported away, for example toward an air vent or other cooling mechanism. Thus, fluid may exit the region below heat spreader  1402 B. Further, if multiple cooling cells are used, apertures  1403  may only be located at edge(s) of the array of cooling cells or not between every cell. This flow of fluid is shown by arrows in  FIG. 14 . Consequently, system  1400  may better cool the integrated circuitry. In addition, ducting  1480  may provide additional mechanical vibration isolation to cooling system  1400 . Thus, the cooling element  1420  may be more easily driven at the desired frequency or frequencies. Consequently, performance of the system  1400  and the device system  1400  is incorporated into may be improved. 
       FIG. 15  is a diagram depicting a side view of a portion of an embodiment of active cooling system  1500  usable with heat-generating structure  1502 . Active cooling system  1500  includes cooling cell  1501  and ducting  1580 . Cooling cell  1501  is analogous to cooling system  100 . In the embodiment shown, cooling cell  1501  includes cooling element  1520  and top plate  1510  having vent  1512  therein, orifice plate  1530  having orifices therein, that are analogous to cooling element  112 , top plate  110 , vent  112 , orifice plate  130  and orifices  132 , respectively. In the embodiment shown, heat-generating structure  1502  includes a component  1502 A such as an integrated circuit and heat spreader  1502 B. Ducting  1580  is part of the exhaust system. Other and/or different components may be included. Cooling cell  1501  thus functions in an analogous manner to active cooling system  100 . In other embodiments, another cooling cell such as cooling system  400  may be used. Consequently, the sizes and operation of cooling cell  1501  are analogous to that of cooling systems  100  and/or  400 . Fluid is driven through orifices  1532  of orifice plate  1530 . The fluid transports heat from heat spreader  1502 B, cooling heat spreader  1502 B and integrated circuit  1502 A. Thus, the fluid can cool heat spreader  1502 B and, therefore, the underlying integrated circuit  1502 A (or other heat-generating device). In some embodiments, other cooling cells, such as those described herein, may be used in connection with the chimneys and ducting shown in  FIG. 15 . 
     In active cooling system  1500 , fluid moves along the surface of heat spreader  1502 B to ducting  1580 . The fluid transports heat from heat spreader  1502 B, cooling heat spreader  1502 B and integrated circuit  1502 A. Ducting  1580  transports the heated fluid along the top surface of heat spreader  1502 B, at the edges of cooling system  1500 . Fluid is then transported away, for example toward an air vent or other cooling mechanism. This flow of fluid is shown by arrows and indicator that fluid flows out of the plane of the page in ducting  1580 . Consequently, system  1500  may better cool the integrated circuitry. In addition, ducting  1580  may provide additional mechanical vibration isolation to cooling system  1500 . Thus, the cooling element  1520  may be more easily driven at the desired frequency or frequencies. Consequently, performance of the system  1500  and the device it is used in may be improved. 
       FIG. 16  is a diagram depicting a perspective view of a portion of an embodiment of active cooling system  1600  usable with heat-generating structure  1602 . Portions of structures that are covered are indicated by a dotted line. Active cooling system  1600  includes cooling cells  1601  (of which only one is labeled) and ducting  1680 . In the embodiment shown, cooling cells  1601  are arranged in an array  1603 . Cooling cells  1601  may be analogous to cooling system(s)  100  and/or  400 . Consequently, the sizes and operation of cooling cell  1601  are analogous to that of cooling systems  100  and/or  400 . Heat-generating structure  1602  may be a heat spreader. Also shown as part of active cooling system are connector  1640  through which electrical connection may be made to cooling cells  1601  and electronics  1650  used in controlling (e.g. driving cooling elements for) cooling cells  1601 . 
     In active cooling system  1600 , heat is transferred from heat-generating structure  1602  to the fluid. The fluid is directed to ducting  1680  at the edges of array  1603 . Ducting  1680  carries the heated fluid along the surface of heat-generating structure  1602 . The fluid is directed by ducting  1680  to past the edge of heat-generating structure  1602 . Fluid is then transported away, for example toward an air vent or other cooling mechanism. Some or all of the fluid may then return to the top of active cooling system  1600  and be reintroduced to cooling cells  1601 . Consequently, system  1600  may better cool the heat-generating structure  1602 . In addition, ducting  1680  may provide additional mechanical vibration isolation to cooling system  1600 . Thus, the cooling elements in cooling cells  1601  may be more easily driven at the desired frequency or frequencies. Consequently, performance of the system  1600  and the device system  1600  is used in may be improved. 
       FIG. 17  is a diagram depicting a perspective view of a portion of an embodiment of active cooling system  1700  usable with heat-generating structure  1702 . Portions of structures that are covered are indicated by a dotted line. Active cooling system  1700  includes cooling cells  1701  (of which only one is labeled) and ducting  1780 . In the embodiment shown, cooling cells  1701  are arranged in an array  1703 . Cooling cells  1701  may be analogous to cooling system(s)  100  and/or  400 . Consequently, the sizes and operation of cooling cell  1701  are analogous to that of cooling systems  100  and/or  400 . Heat-generating structure  1702  may be a heat spreader. Also shown as part of active cooling system are connector  1740  through which electrical connection may be made to cooling cells  1701  and electronics  1750  used in controlling (e.g. driving cooling elements for) cooling cells  1701 . 
     In active cooling system  1700 , heat is transferred from heat-generating structure  1702  to the fluid. The fluid is directed to ducting  1780  at the edges of array  1703  and/or between the rows and columns of cells. For example, additional ducting (not shown in  FIG. 17  for clarity) may carry fluid from between the rows and columns of array  1703  to the edges where ducting  1780  resides. Ducting  1780  carries the heated fluid along the surface of heat-generating structure  1702 . Fluid may be transported away, for example toward an air vent or other cooling mechanism. Some or all of the fluid may return to the top of active cooling system  1700  and be reintroduced to cooling cells  1701 . Consequently, system  1700  may better cool the heat-generating structure  1702 . In addition, ducting  1780  may provide additional mechanical vibration isolation to cooling system  1700 . Thus, the cooling elements in cooling cells  1701  may be more easily driven at the desired frequency or frequencies. Consequently, performance of the system  1700  and the device system  1700  is used in may be improved. 
       FIG. 18  is a flow chart depicting an exemplary embodiment of method  1800  for operating a cooling system. Method  1800  may include steps that are not depicted for simplicity. Method  1800  is described in the context of active cooling systems  100 ,  600 C and  700 C. However, method  1800  may be used with other cooling systems including but not limited to systems and cells described herein. 
     One or more of the cooling element(s) in a cooling system is actuated to vibrate, at  1802 . At  1802 , an electrical signal having the desired frequency is used to drive the cooling element(s). In some embodiments, the cooling elements are driven at or near structural and/or acoustic resonant frequencies at  1802 . The driving frequency may be 15 kHz or higher. If multiple cooling elements are driven at  1802 , the cooling elements may be driven out-of-phase. In some embodiments, the cooling elements are driven substantially at one hundred and eighty degrees out of phase. Further, in some embodiments, individual cooling elements are driven out-of-phase. For example, different portions of a cooling element may be driven to vibrate in opposite directions (i.e. analogous to a seesaw). In some embodiments, individual cooling elements may be driven in-phase (i.e. analogous to a butterfly). In addition, the drive signal may be provided to the anchor(s), the cooling element(s), or both the anchor(s) and the cooling element(s). Further, the anchor may be driven to bend and/or translate. Also at  1802 , feedback from the cooling element(s) is used to adjust the driving current. In some embodiments, the adjustment is used to maintain the frequency at or near the acoustic and/or structural resonant frequency/frequencies of the cooling element(s) and/or cooling system. Resonant frequency of a particular cooling element may drift, for example due to changes in temperature. Adjustments made at  1102  allow the drift in resonant frequency to be accounted for. 
     Exhaust system(s) are used to direct fluid from and/or to the cooling system, at  1804 . The exhaust system may carry heated fluid from the heat-generating structure to an outlet and/or location distal from the cooling cell(s). Similarly, the exhaust system may carry cooler fluid from an inlet or location with the device to the cooling cells. 
     For example, a cooling element, such as cooling element  120 , in one or more of cooling cells  601  and/or  701  may be driven at its structural resonant frequency/frequencies, at  1802 . This resonant frequency may also be at or near the acoustic resonant frequency for the top chamber (e.g. top chamber  140 ). This may be achieved by driving piezoelectric layer(s) in anchor  160  (not shown in  FIGS. 1A-1F ) and/or piezoelectric layer(s) in cooling element  120 . Further,  1802  may be performed for all of cooling cells  601  and/or  701  or some of cooling cells  601  and/or  701 . Multiple cooling cells  601  and/or  701  may be driven in-phase or out-of-phase at  1802 . In addition,  1802  may be performed in response to various conditions being fulfilled. For example, a surface temperature of a structure reaching or exceeding a threshold, an input from a component device  610  or  710 , an input from a user, and/or another condition being fulfilled may result in cooling element(s) of cooling cell(s)  601  and/or  701  being energized. 
     Also at  1802 , feedback is used to maintain the cooling element of cooling cell(s)  601  and/or  701  at resonance. In some embodiments in which multiple cooling elements are driven, the cooling elements phase is also maintained at  1802 . For example, cooling elements may be driven and maintained at one hundred and eighty degrees out-of-phase. Thus, the efficiency of cooling elements in driving fluid flow through cooling systems  600 C and/or  700 C and onto the surface of heat-generating structure  602  and/or  702  may be maintained. 
     At  1804 , exhaust systems  680  and/or  780  are used to direct heated fluid away from heat-generating structure  602  and/or  702 . Exhaust systems  680  and/or  780  are also used to direct heated fluid away from cooling cells  601  and/or  70 . Similarly, exhaust systems  682  and/or  7802  are used to direct cooler fluid toward the entrance to cooling cells  601  and/or  701 . Thus, active cooling systems may more efficiently and quietly cool heat-generating devices at lower power. 
     Thus, various embodiments of cooling systems, their components, exhaust systems, ducting and method of operations have been disclosed. Various features may be omitted and/or combined in ways not explicitly disclosed herein. As a result, cooling of heat-generating structures may be improved. 
     Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed embodiments are illustrative and not restrictive.