Abstract:
The current invention provides significant performance improvements or significant energy savings for fans used in these applications: personal, industrial and automotive cooling, ventilation, vacuuming and dust removal, inflating, computer component cooling, propulsors for unmanned and manned air vehicles, propulsors for airboats, air-cushion vehicles, airships and model aircraft. Additionally, the invention provides higher performance such as higher lift and higher lift efficiency to small air vehicles. These advantages are achieved by using plasma actuators to provide active flow control effectors into thin fan blades and wing.

Description:
FIELD OF THE INVENTION 
     The present invention relates to enhancing aerodynamic performance by using plasma actuators to control air flow. The invention has specific application to fan and ducted-fan performance and flow control at low Reynolds and intermediate numbers. Dielectric barrier discharge plasma actuators are preferably used. 
     BACKGROUND OF THE INVENTION 
     Reynolds number (Re) is a dimensionless number that gives a measure of the ratio of inertial forces (V/ρ) to viscous forces (μ/L) and, consequently, it quantifies the relative importance of these two types of forces for given flow conditions. 
     Reynolds numbers frequently arise when performing dimensional analysis of fluid dynamics and heat transfer problems, and as such can be used to determine dynamic similitude between different experimental cases. They are also used to characterize different flow regimes, such as laminar or turbulent flow: laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion, while turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce random eddies, vortices and other flow fluctuations. 
     Reynolds number is generally defined as: Re=ρVL/μ 
     where: V is the mean fluid velocity in (SI units: m/s); L is the characteristic length of the structure (m); μ is the dynamic viscosity of the fluid (Pa·s or N·s/m 2 ); and ρ is the density of the fluid (kg/m 3 ) 
     For any shape, the parameter that is used as the characteristic length L is not given explicitly by physics, but rather is chosen by convention (and usually subscripted after the ‘Re’). 
     In boundary layer flow over a flat plate, experiments can confirm that, after a certain length of flow, a laminar boundary layer will become unstable and become turbulent. This instability occurs across different scales and with different fluids, usually when Re x ˜5 10 5 , where x is the distance from the leading edge of the flat plate, and the flow velocity is the ‘free stream’ velocity of the fluid outside the boundary layer. In conventional aerodynamics, the following approximate ranges apply for Reynolds number: 
     Extremely low: 1,000≦Re≦20,000 
     Very low: 20,000&lt;Re≦200,000 
     Conventional low: 200,000&lt;Re≦1,000,000 
     Intermediate: 1,000,000&lt;Re≦5,000,000 
     High: Re&gt;5,000,000 
     Fans are widely used for personal, industrial and automotive cooling, ventilation, vacuuming and dust removal, inflating, etc. Ducted, or shrouded, fans have for many years been used for propulsion of airboats, air-cushion vehicles, airships and model aircraft. They are also touted as the primarily propulsive system for so-called personal air vehicles, a number of which are under intense development (e.g. White, 2006; and Yoeli, 2002). Their main advantages are high static thrust and propulsion efficiency, while the duct acts to reduce blade noise and improve safety. The main factor limiting the performance of these blades is boundary layer separation, where the flow detaches from the blade surface. This leads to dramatic losses in performance and severe increases in noise and vibrations. The typical Reynolds number range is conventional low to intermediate. 
     In recent years, ducted fans have received renewed attention, particularly for the propulsion of small-scale (typically ˜500 mm) unmanned air vehicles (Fleming et al, 2003; Guerrero et al, 2003). In addition, there is a trend toward the design of even smaller air vehicles, known as “micro air vehicles” (MAVs; maximum dimension typically between 7.5 cm and 15 cm), for a variety of military and civil applications. One consequence of these smaller scales and relatively low tip speeds is a reduced fan blade Reynolds number, typically less than 50,000. At these Reynolds numbers, boundary layer transition does not occur and the boundary layer is susceptible to separation, which can result in a catastrophic loss of propulsion. The best performing blade profiles are thin, curved sections which do not produce a large pressure ΔP across the disk since leading-edge separation occurs at relatively low inflow angles. 
     A rapidly growing application of fans and ducted fans is their ubiquitous use for the cooling of modern high-speed computer systems at large scales and also at small scales such as on computer chips, motherboards and power supplies. Large scale server farms, which are collections or clusters of computer servers, are increasingly being used instead mainframe computers by large enterprises. The performance server farms (typically thousands or tens of thousands of processors) are typically limited by the performance of the cooling systems. At the small scales, typically, a fan blows air across a heat-sink that is attached to a particular component, such as a CPU. In modern designs, fan speed can be controlled based on a feedback principle and this is generally referred to as active cooling. However, modern high-speed processors require continuously greater cooling and this is generally accomplished using larger heat sinks and more powerful fans running at higher rpm. Apart from physical size limitations, these fans are increasing noisy and require greater input power. In fact, the noise generated by fans that are used to cool high-end processors, particularly within a small physical computer sizes, is often objectionable to the user. 
     The above mentioned problems were negotiated by designing more efficient blade sections, but this optimization process has now reached its limit. 
     Achieving sustained flight of micro air vehicles (MAVs) brings significant challenges due to their small dimensions and low flight speeds. For so-called mini air vehicles, which operate in the 10,000&lt;Re&lt;300,000 range, efficient systems can be designed by managing boundary-layer transition via passive tripping at multiple locations. However, at Reynolds numbers routinely experienced by MAVs (Re&lt;100,000), conventional low-Reynolds-number airfoils perform poorly or generate no useful lift. Some of the best-performing airfoils in this Re range are cambered flat plates and airfoils with a thickness-to-chord ratio of approximately 5%. There are various definitions for MAY dimensions and weight, although one common definition refers to large (b˜15 cm and m˜90 g) and small (b˜8 cm and m˜30 g) MAVs. To maximize wing area, these vehicles typically have low-aspect-ratio (AR) wings (1&lt;AR&lt;2) for which typical Reynolds numbers during loiter are in the range of 20,000&lt;Re&lt;80,000, based on the aforementioned specifications. Innovative designs with larger-aspect-ratio wings can result in an even lower Reynolds number range. 
     The challenge of developing useful lift intensifies with yet smaller vehicles required to fly at even lower flight speeds. This includes the development of so-called nano Unmanned air Vehicles (UAVs) for which the missions include flying within confined areas. These are commonly termed Nano Air Vehicles (NAVs) and are defined as weighing less than 10 g, with dimensions smaller than 7:5 cm, and speeds between 0.5 and 7:5 m/s. 
     The significant difficulty associated with generating lift at Re&lt;20,000 has led many to pursue biologically inspired approaches in which the flight of small birds and insects is mimicked to a greater or lesser degree. 
     It is well known that a fan (or wind pump) can also be used as a turbine. The most common of these is the horizontal axis (axial flow) wind turbine, where wind turns the turbine blades that, in turn, drive a generator. 
     Patent application WO07106863A; titled “Methods and apparatus for reducing noise via a plasma fairing”; to Thomas Flint; discloses a plasma fairing for reducing noise generated by, for example, an aircraft landing gear is disclosed. The plasma fairing includes at least one plasma generating device, such as a single dielectric barrier discharge plasma actuator, coupled to a body, such as an aircraft landing gear, and a power supply electrically coupled to the plasma generating device. When energized, the plasma generating device generates plasma within a fluid flow and reduces body flow separation of the fluid flow over the surface of the body. 
     US application 20020195089A; titled “Self contained air flow and ionization method, apparatus and design for internal combustion engines” to Zetmeir Karl; disclosed method to enhance the performance of internal combustion engines by the creation of a swirling vortex, using principles of electrostatics in using tribology and coulomb forces, the utilization of dielectric properties of polymers in an air driven rotating electrophorus and the chemistry of enhanced combustion gases and combustion itself in a single self-contained apparatus and does so without the convention and application of external voltage 
     US application 20040195462; titled “Surface plasma discharge for controlling leading edge contamination and crossflow instabilities for laminar flow”; to Fedorov Alexander and Malmuth Norman; discloses a system and method for controlling leading edge contamination and crossflow instabilities for laminar flow on aircraft airfoils that is lightweight, low power, economical and reliable. Plasma surface discharges supply volumetric heating of the supersonic boundary layers to control the Poll Reynolds number and the cross flow Reynolds number and delay transition to turbulent flow associated with the leading edge contamination and crossflow instabilities. 
     US application 20040118973; titled “Surface plasma discharge for controlling forebody vortex asymmetry” to Federov Alexander et. al.; discloses a system and method for rapidly and precisely controlling vortex symmetry or asymmetry on aircraft forebodies to avoid yaw departure or provide supplemental lateral control beyond that available from the vertical tail surfaces with much less power, obtrusion, weight and mechanical complexity than current techniques. This is accomplished with a plasma discharge to manipulate the boundary layer and the angular locations of its separation points in cross flow planes to control the symmetry or asymmetry of the vortex pattern. 
     GENERAL BACKGROUND INFORMATION MAY BE FOUND IN THE FOLLOWING REFERENCES 
     
         
         1. Carr, L. W., “Progress in the analysis and prediction of dynamic stall” AIAA Journal of Aircraft, Vol. 25, No. 1, 1988, pp. 6-17. 
         2. Corke, T. C. He C. and Patel, M. P., “Plasma flaps and slats: An application of weakly ionized plasma actuators,” AAIA Paper 2004-2127, 2nd AIAA Flow Control Conference, Portland, Oreg., 2004. 
         3. Fleming, J., Jones, T., Ng, W., Gelhausen, P. and Enns, D., “Improving Control System Effectiveness for Ducted Fan VTOL UAVs Operating in Crosswinds,” AIAA Paper 2003-6514, 2 nd  AIAA “Unmanned Unlimited” Conference and Workshop and Exhibit, San Diego, Calif., Sep. 15-18, 2003. 
         4. Göksel, B. Greenblatt, D., Rechenberg I., Nayeri, C. N. and Paschereit, C. O., “Steady and Unsteady Plasma Wall Jets for Separation and Circulation Control,” AIAA Paper 2006-3686, 3rd AIAA Flow Control Conference, San Francisco, Calif., 5-8 Jun. 2006. 
         5. Göksel, B., Greenblatt, D., Rechenberg, I. Kastantin, Y., Nayeri, C. N. and Paschereit, C. O., “Pulsed plasma actuators for active flow control at MAY Reynolds Numbers,”  Notes on Numerical Fluid Mechanics and Multidisciplinary Design , Vol. 95, pp. 42-55, 2007. 
         6. Greenblatt D. and Wygnanski, I., “Use of Periodic Excitation to Enhance Airfoil Performance at Low Reynolds Numbers,”  AIAA Journal of Aircraft , Volume 38, Issue 1, 2001, pp. 190-192. 
         7. Greenblatt, D and Wygnanski, I., “The control of separation by periodic excitation,” Progress in Aerospace Sciences, Volume 37, Issue 7, 2000, pp. 487-545. 
         8. Greenblatt, D. and Wygnanski, I., “Effect of leading-edge curvature on airfoil separation control,”  AIAA Journal of Aircraft , Vol. 40, No. 3, 2002, pp. 473-481. 
         9. Greenblatt, D., Göksel, B. Schüle, C. Y. and Paschereit, C. O., “Dielectric Barrier Discharge Flow Control at Very Low Flight Reynolds Numbers,” 47 th  Israel Annual Conference on Aerospace Sciences, Dan Panorama Hotel, Tel Aviv and Technion Campus, Haifa, Feb. 21-22, 2007. 
         10. Greenblatt, D., “Dual Location Separation Control on a Semispan Wing,”  AIAA Journal , Vol. 45, No. 8, 2007, pp. 1848-1860. 
         11. Guerrero, I., Londenberg, W., Gelhausen, P. and Myklebust, A., “A Powered Lift Aerodynamic Analysis for the Design of Ducted Fan UAVs,” AIAA Paper 2003-6567, 2 nd  AIAA “Unmanned Unlimited” Conference and Workshop and Exhibit, San Diego, Calif., Sep. 15-18, 2003. 
         12. Poisson-Quinton, Ph. and Lepage, L., “Survey of French research on the control of boundary layer and circulation,” in Lachmann, G. V., “Boundary layer and Flow Control. Its Principles and Application”, Volume 1, Pergamon Press, New York, 1961, pp. 21-73. 
         13. Roth, J. R. and Dai, X. (2006) Optimization of the Aerodynamic Plasma Actuator as an Electrohydrodynamic (EHD) Electrical Device. AIAA Paper 2006-1203, 44th AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nev. 
         14. Seifert, A., Darabi, A. and Wygnanski, I., “Delay of airfoil stall by periodic excitation”, AIAA Journal of Aircraft, Vol. 33, No. 4, 1996, pp. 691-698. 
         15. Shyy, W., Berg, M. and Ljungqvist, D., “Flapping and Flexible Wings for Biological and Micro Air Vehicles”, Progress in Aerospace Sciences, Vol. 35, Issue 5, 1999, pp. 455-505. 
         16. Mueller T. J., “Aerodynamic Measurements at Low Reynolds Numbers for Fixed Wing Micro-Air Vehicles,” Presented at the RTO AVT/VKI Special Course on Development and Operation of UAVs for Military and Civil Applications, VKI, Belgium, Sep. 13-17, 1999. 
         17. Nagel, A., Levy, D. E. and Shepshelovich, M., “Conceptual Aerodynamic Evaluation of MINI/MICRO UAV,” AIAA Paper 2006-1261, 44th Aerospace Sciences Meeting and Exhibit, 9-12 Jan. 2006, Reno, Nev., 2006. 
         18. Roth, J. R., Sherman, D. and Wilkinson, S., “Boundary Layer Flow Control with One Atmosphere Uniform Glow Discharge Surface Plasma,” AIAA 1998-0328, 1998. 
         19. Weier, T. and Gerbeth, G., “Control of separated flows by time periodic Lorentz forces,” European Journal of Mechanics, B/Fluids, Vol. 23, 2004, pp. 835-849. 
         20. White. K. “The Skycar: Transportation of the Future,” CAMP InSight Magazine, December Issue, 2006, pp. 10-14. 
         21. Yoeli, R., “Ducted Fan Utility Vehicles and Other Flying Cars,” AIAA Paper 2002-5995, Biennial International Powered Lift Conference and Exhibit, Williamsburg, Va., Nov. 5-7, 2002. 
       
    
     SUMMARY OF THE INVENTION 
     The present invention relates to enhancing aerodynamic performance by using plasma actuators to control air flow. The invention has specific application to fan and ducted-fan performance and flow control at low Reynolds numbers. Dielectric barrier discharge plasma actuators are preferably used. 
     Recent experiments performed on a variety of airfoil shapes by the inventors, demonstrated a dramatic effect of lightweight Dielectric Barrier Discharge (DBD) plasma actuators on performance at very low Reynolds numbers (3,000&lt;Re&lt;75,000). The actuators were driven in a high frequency (kHz) “steady” mode and a pulsed mode where pulse frequencies and duty cycles were varied in a systematic fashion. Optimum reduced frequencies for generating maximum forces across the airfoil, in the presence of separated flow, were typically between 0.4 and 0.6 and significant performance improvements were achieved at low power (several milliwatts/cm). Moreover, profound differences in the response to reduced frequency and duty cycle were observed for the different airfoil shapes tested. 
     Given the fact that micro-scale fan blade performance suffers directly as a result of boundary layer separation, DBD plasma actuation emerges as a strong candidate to affect control. Indeed, the demonstration of control on open airfoils at Re&lt;75,000, combined with their light weight and low power operation renders them ideal for open or ducted fan applications. For a micro air vehicle propulsor, this would result in the generation of sufficient thrust for flight, or more efficient use of onboard fuel or battery power. For a computer fan, this would result in greater CFM at the same power or alternatively, the same CFM at significantly lower power. 
     Therefore, there is clearly a need for, and it would be highly advantageous to have, a fan or ducted fan whose flow field is controlled by dielectric barrier discharge plasma actuators. 
     Fans are widely used for personal, industrial and automotive cooling, ventilation, vacuuming and dust removal, inflating, etc. Ducted, or shrouded, fans have for many years been the propulsors for airboats, air-cushion vehicles, airships and model aircraft. They are also touted as the primarily propulsive system for so-called personal air vehicles, a number of which are under intense development (e.g. White, 2006; and Yoeli, 2002—see attached disclosure). Their main advantages are high static thrust and propulsion efficiency, while the duct acts to reduce blade noise and improve safety. In recent years, ducted fans have received renewed attention, particularly for the propulsion of small-scale (typically ˜500 mm) unmanned air vehicles (Fleming et al, 2003; Guerrero et al, 2003—see attached disclosure). Nevertheless, there is at present a strong trend toward the design of even smaller air vehicles, known as “micro air vehicles” (MAVs; maximum dimension typically between 7.5 cm and 15 cm), for a variety of military and civil applications. One consequence of these smaller scales and relatively low tip speeds is a reduced fan blade Reynolds number, typically less than 50,000. At these Reynolds numbers, boundary layer transition does not occur and the boundary layer is susceptible to separation, which can result in a catastrophic loss of propulsion. The best performing blade profiles are thin, curved sections which do not produce a large pressure differential across the disk since leading-edge separation occurs at relatively low inflow angles. 
     Another application of fans and ducted fans at these length and velocity scales is their ubiquitous use for the cooling of modern high-speed computer chips, motherboards and power supplies. The efficacy of these fans is often quantified as the ratio of CFM to power input in Watts. Typically, a fan blows air across a heat-sink that is attached to a particular component, such as a CPU. In modern designs, fan speed can be controlled based on a feedback principle and this is generally referred to as active cooling. However, modern high-speed processors require continuously greater cooling and this is generally accomplished using larger heat sinks and more powerful fans running at higher rpm. Apart from physical size limitations, these fans are increasingly noisy and require greater input power. In fact, the noise generated by fans that are used to cool high-end processors, particularly within a small physical computer sizes, is often objectionable to the user. 
     The current invention offer an industrial fan implementing an innovative technology, which reduces power consumption up to 50% comparing to standard fans, and achieves a decrease of around 75% in noise level. Power consumption percentage of industrial fans is significant (especially in the fields of chemical manufacturing, paper manufacturing, petroleum and coal products manufacturing, as well as additional fields). 
     The current invention provides significant performance improvements or significant energy savings for these applications: personal, industrial and automotive cooling, ventilation, vacuuming and dust removal, inflating, etc. Computer component cooling, propulsors for unmanned and manned air vehicles, propulsors for airboats, air-cushion vehicles, airships and model aircraft. 
     Using DBD plasma actuators provides a way of introducing active flow control effectors into thin fan blades. 
     A paper entitled “Dielectric Barrier Discharge Flow Control at Very Low Flight Reynolds Numbers” to David Greenblatt, Berkant Göksel, Ingo Rechenberg; Chan Yong Schüle, Daniel Romann, and Christian O. Paschereit; published in AIAA JOURNAL Vol. 46, No. 6, June 2008 discloses results from experiments that were performed on a flat-plate airfoil and an Eppler E338 airfoil at low flight Reynolds numbers (Re&lt;140,000), in which dielectric barrier discharge plasma actuators were employed at the airfoil leading edges to effect flow control. The actuators were driven in a high-frequency (kilohertz) steady mode and a pulsed voltage mode in which pulse frequency and duty cycle were varied in a systematic fashion. Optimum reduced frequencies (F + ) for generating poststall lift were approximately between 0.2 and 2, and this was broadly consistent with zero-mass-flux slot-blowing data acquired at Reynolds numbers that were approximately 200 times higher. F +  is defined as the product of the pulsed frequency and distance from the actuator to the trailing edge of the wing (or blade) divided by the relative blade velocity. Nevertheless, profound differences in the response to reduced frequency and duty cycle were observed between the flat-plate and E338 airfoils. In general, actuation produced considerable performance improvements, including an increase in maximum lift coefficient of 0.4 to 0.8 and maintained elevated endurance at significantly higher lift coefficients. Actuation in the steady mode resulted in circulation control at Re=3000. Pulsed actuation also exerted a significant effect on the wake at prestall angles of attack, in which control of the upper-surface flat-plate bubble shedding produced significant differences in wake spreading and vortex shedding. The flat plate was also tested in a semispan-wing configuration (AR=6), and the effect of control was comparable with that observed on the airfoil. 
     The above discussion essentially demonstrated the concept at Reynolds numbers less than 140,000. However, increases in applied voltage to the actuators can render them applicable to much higher Reynolds numbers. In fact performance of the entire range of industrial fans, from the smallest to the largest can be significantly improved using DBD plasma actuators. The largest industrial fans have typical Reynolds numbers of several million, and it can be concluded that DBD plasma actuators can be applied from the extremely low to the intermediate Reynolds number range. 
     Accordingly, it is an aspect of the invention to provide a fan comprising: at least one fan blade; and at least one plasma actuator mounted on said fan blade. 
     In some embodiments the plasma actuator is dielectric barrier discharge plasma actuator. 
     In some embodiments the plasma actuator is driven in pulsed mode. 
     In some embodiments pulsing of said plasma actuator is at a reduced frequency between 0.2 and 2. 
     In some embodiments pulsing of said plasma actuator is at duty cycle of 1 to 25 percent. 
     In some embodiments the fan is an axial fan. 
     In some embodiments the fan further comprising a shroud around said at least one fan blade. 
     In some embodiments the fan is a centrifugal fan. 
     In some embodiments the dielectric barrier in said dielectric barrier discharge plasma actuator comprises a dielectric selected from a group comprising: Mylar; polyimide; Teflon; kapton; and quartz. 
     In some embodiments at least two rows of plasma actuator are mounted on said fan blade. 
     In some embodiments the fan blade further comprises a flap. 
     In some embodiments the flap is an upper surface leading edge device. 
     In some embodiments the flap is an upper surface trailing edge device. 
     In some embodiments the flap is a lower surface trailing edge device. 
     In some embodiments the fan further comprises power transmitter providing electrical energy to said plasma actuator mounted on said fan blade. 
     In some embodiments the fan further comprising plasma controller controlling said transmitted electrical power. 
     It is another aspect of the invention to provide a method for enhancing performance of a rotating wing system comprising the steps of: providing electrical power to the rotating part of said rotating wing system; and actuating plasma actuators installed on said rotating wings. 
     In some embodiments the method further comprises the step of controlling said electrical power provided to said rotating wings system. 
     In some embodiments the step of controlling said electrical power provided to said rotating wings system comprises switching electrical power among plurality of plasma actuators mounted on plurality of rotating wings. 
     In some embodiments the method further comprises step of controlling said electrical power provided to said rotating wings system comprises converting said provided electrical power to high voltage RF signal. 
     Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
       In the drawings: 
         FIG. 1  schematically depicts fan system according to an exemplary embodiment of the current invention. 
         FIG. 2   a  schematically depicts a profile of straight blade according to an exemplary embodiment of the current invention. 
         FIG. 2   b  schematically depicts a profile of curved blade according to an exemplary embodiment of the current invention. 
         FIG. 2   c  schematically depicts a profile of flapped blade according to an exemplary embodiment of the current invention. 
         FIG. 2   d  schematically depicts a profile of thick blade according to an exemplary embodiment of the current invention. 
         FIG. 3   a  schematically depicts a profile of complex blade according to an exemplary embodiment of the current invention. 
         FIG. 3   b  schematically depicts a profile of complex blade with deployed devices according to an exemplary embodiment of the current invention. 
         FIG. 4   a  schematically depicts a profile of complex blade having permeable trailing edge devices, in deployed state according to an exemplary embodiment of the current invention. 
         FIG. 4   b  schematically depicts a profile of complex blade having permeable trailing edge devices, in deployed state according to an exemplary embodiment of the current invention, showing the air flow around said blade. 
         FIG. 5  schematically depicts construction of a Dielectric Barrier Discharge (DBD) plasma actuator according to an exemplary embodiment of the current invention. 
         FIG. 6  schematically depicts construction of a Dielectric Barrier Discharge (DBD) plasma actuator according to an exemplary embodiment of the current invention. 
         FIG. 7  schematically depicts top views of several radial blowers according to exemplary embodiments of the current invention. 
         FIG. 8  schematically depicts fan system according to an exemplary embodiment of the current invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to enhancing aerodynamic performance by using plasma actuators to control air flow. The invention has specific application to fan and ducted-fan performance and flow control at low Reynolds numbers. Dielectric barrier discharge plasma actuators are preferably used. 
     Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
     The drawings are generally not to scale. 
     For clarity, non-essential elements were omitted from some of the drawings. 
     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural elements or steps, unless such exclusion is explicitly recited. 
     Axial Fan Application 
     The present invention will be described with respect to axial fans, but it is equally applicable to radial (centrifugal) and cross-flow fans, and these latter two will be described below. 
       FIG. 1  schematically depicts fan system  100  according to an exemplary embodiment of the current invention. 
     Fan system  100  comprises fan  101  having fan blades  110  rotating in direction  111  about hub  112 . 
     For clarity only two of the blades are marked. In this figure four identical, symmetrically positioned blades are seen. However it should be noted that number of blades may vary, for example 2, 3, 5 or more blades may be used. Optionally, unequal shaped blades may be used, however preferably fan system  100  is balanced in respect to its center of rotation  113 . 
     Optional duct (shroud)  130  may be placed around fan  101 . 
     Plasma actuators  120  are installed on at least one of blades  110 . In the depicted exemplary embodiment, two plasma actuators  120  and  120 ′ are installed along each blade  110 . First plasma actuator  120  is installed near the leading edge  115  of blade  110 . A second plasma actuator  120 ′ is installed between leading edge  115  and trailing edge  116  of blade  110 . However, as will be shown later, the number and location of plasma actuators may be different. 
     Consider the open or shrouded axial fans shown in  FIG. 1 . For the purposes of this description, both open and ducted fan are identical apart from the duct  130  that is shown in  FIG. 1 . Therefore discussion of the open fan and its components described with respect to  FIG. 1 , apply equally to both. The basic fan shown in  FIG. 1  consists of a hub to which four blades are attached. Naturally, there can be anything from 1 to several tens or hundreds of blades. The fan is driven by a motor (not shown) that causes it to rotate in a clockwise direction. The blades are shown to have a constant chord length (c), but in general this could vary along the span of the blade. For the fan as shown in the figure, air or the particular working fluid will be caused to flow away from the reader (into the page). 
     The cross-sectional profile of the fan blade can have a variety of generic shapes depending on the application. A number of typical examples of blade profile shapes, indicated by A-A in  FIG. 1 , are shown in  FIGS. 2   a - 2   d , namely a straight blade, a curved blade, a flapped blade, complex blade profile with deployable devices, respectively. 
     It should be noted that profile of the blade may change along the length of the blade. Similarly, location of actuator(s) may change along the length of the blade. As the relative air speed and direction changes along the length of a blade, blade profile and plasma actuation may be adopted to the changing air flow conditions along the blade. 
     For fans operated at different rotational speeds, the blade attack angle may be changed to adapt to the different rotational speeds. Similarly, actuation mode of plasma actuators  120  may be changed to adapt to different rotational speeds. Additionally or alternatively, different plasma actuators may be operated (or not operated) depending on rotational speed. Specifically, different plasma actuators may optionally be operated (or not operated) depending on rotational direction when the fan direction is reversed to create reverse thrust. 
     Plasma actuators, for example DBD plasma actuators, are placed at various locations on the blades, for example between the leading-edge and the trailing-edge. The blade can have one actuator, as shown in  FIG. 2   a ; it can have a multitude of actuators as shown in  FIG. 2   b ; it can have two actuators—one at the leading-edge and the other at a bend as shown in  FIG. 2   c ; or it can have multiple actuators that are used in combination with leading- and trailing-edge devices as shown in  FIG. 3 . 
       FIG. 2   a  schematically depicts a profile of straight blade  210  according to an exemplary embodiment of the current invention. 
     Straight blade  210  comprises a thin straight blade structure  211  and a plasma actuator  215  installed at leading edge  212  of blade  210 . 
       FIG. 2   b  schematically depicts a profile of curved blade  230  according to an exemplary embodiment of the current invention. 
     Curved blade  230  comprises a thin curved blade structure  231  and a plasma actuator  235  installed at leading edge  232  of blade  230 . 
     Additionally, curved blade  230  comprises plasma actuators  235 ′,  235 ″ and  235 ′″ installed on upper surface of blade  230  between leading edge  232  and trailing edge  233 . 
       FIG. 2   c  schematically depicts a profile of flapped blade  250  according to an exemplary embodiment of the current invention. 
     Flapped blade  250  comprises a thin flapped blade structure comprising front section  251  and rear section  251 ′ constructed a higher attack angle. 
     Flapped blade  250  additionally comprises a plasma actuator  255  installed at leading edge  252  of blade  250 . 
     Additionally, curved blade  230  comprises plasma actuator  235 ′ installed on upper surface of blade  250  between leading edge  252  and trailing edge  253 , preferably near joint  254  of front section  251  and rear section  251 ′. 
       FIG. 2   d  schematically depicts a profile of thick blade  270  according to an exemplary embodiment of the current invention. 
     Thick blade  270  comprises an aerodynamically shaped blade structure  271 . 
     Thick blade  270  additionally comprises a plasma actuator  275  installed at leading edge  272  of blade  270 . 
     Additionally, thick blade  270  comprises plasma actuators  276 ′ and  276 ″ installed on upper surface of blade  270  between leading edge  272  and trailing edge  273  of blade  270 . 
     Additionally, thick blade  270  comprises plasma actuators  277 ′ and  277 ″ installed on lower surface of blade  270  between leading edge  272  and trailing edge  273  of blade  270 . 
       FIG. 3   a  schematically depicts a profile of complex blade  370  according to an exemplary embodiment of the current invention. 
     Complex blade  370  comprises an aerodynamically shaped blade structure  371 . 
     Complex blade  370  additionally comprises a plasma actuator  375  installed at leading edge  372  of blade  370 . 
     Additionally, complex blade  370  may comprise a plasma actuator  376 ′ installed on Upper Surface Leading Edge Device (USLED)  386 ′ attached to leading edge  372  of blade  270  using front hinge  382 . 
     Additionally, complex blade  370  may comprise a plasma actuator  376 ″ installed on Upper Surface Trailing Edge Device (USTED)  386 ″ attached to trailing edge  373  of blade  270  using back hinge  383 . 
     Additionally, complex blade  370  may comprise a plasma actuator  377 ″ installed on Lower Surface Trailing Edge Device (LSTED)  387 ″ attached to trailing edge  373  of blade  270  using back hinge  383 . 
     Additionally, complex blade  370  may comprise a plasma actuator  377 ′ installed to lower surface of blade  371  between leading edge  372  and LSTED  387 ″. 
     It should be noted that USLED  386 ′, USTED  386 ″ and LSTED  387 ″ are depicted in  FIG. 3   a  in un-deployed configuration. Optionally, more upper or lower surface devices may be used and may use same or separate hinges. The exact shape, location and size of devices may also vary. Specifically, additional plasma actuators may be added. 
       FIG. 3   b  schematically depicts a profile of complex blade  370  with deployed devices according to an exemplary embodiment of the current invention. In this figure all of USLED  386 ′, USTED  386 ″ and LSTED  387 ″ are depicted in deployed state. It should be noted that optionally only some of USLED  386 ′, USTED  386 ″ and LSTED  387 ″ are deployed. Preferably, plasma actuators such as  376 ′,  376 ″ and  377 ″ are in operation while the corresponding device is deployed. 
     It should be noted the deployment angle may vary according to the operational condition such as air speed and attack angle. 
       FIG. 4   a  schematically depicts a profile of complex blade  470  having permeable trailing edge devices, in deployed state according to an exemplary embodiment of the current invention. 
     Complex blade  470  has similar construction to blade  370 . For clarity, some parts were omitted or left un-marked in this figure. 
     Complex blade  470  may comprise a plasma actuator  475  installed on Upper Surface Leading Edge Device (USLED)  475  attached to leading edge  372  of blade  270  using front hinge. In this figure, perforated USLED  475  is shown deployed. 
     In this figure, perforated USTED  486  is shown deployed in acute angle to air velocity vector. 
     In contrast to devices  386 ″, perforated USTED  486  is preferably constructed with air openings  496 ,  496 ′. 
     Similarly, perforated LSTED  487  is preferably constructed with air openings  497 ,  497 ′. 
     It should be noted that number of the openings, their size and shape may vary. 
       FIG. 4   b  schematically depicts a profile of complex blade having permeable trailing edge devices, in deployed state according to an exemplary embodiment of the current invention, showing the air flow around said blade. 
     Perforated USTED  486  is shown with an air opening  496 . Similarly, perforated LSTED  487  is shown with air opening  497 . 
     Air flow direction is depicted by arrowed curves  411 . 
     Trapped vortices are depicted by arrowed loops  421  and  422 . 
     When the leading-edge (LE) and trailing-edge (TE) devices are not deployed, the blade profile resembles that shown in  FIG. 2   d . A variety of possible configurations are shown in  FIGS. 3 and 4 . These are by no means exhaustive, but serve to illustrate possible configurations:
         Deployment of the LED alone;   Deployment of the USTED alone;   Deployment of the LED and USSTED;   All devices deployed  6 .       

     As a general rule these devices need not be solid and can be permeable, i.e. they may also have slots within them to allow air to partially flow through them, as shown on the trailing-edge devices of  FIG. 4 . Once the devices are deployed, any number of plasma actuators can be activated in order to (a) maximize the lift force; (b) minimize the drag force; or (c) maximize the aerodynamic efficiency of the (lift force/drag force). 
     One example of how the invention works can be seen with respect to  FIG. 4   b , based on the example shown in  FIG. 4   a . In this configuration, a very large and powerful vortex  421  is “trapped” on the upper surface of the blade. This has two effects: (i) the low pressure produces very high lift; (ii) the reverse flow produced so-called skin-friction thrust, thereby reducing drag. A second vortex  422  is trapped aft of the blade further increasing lift and reducing drag. 
     The Dielectric Barrier Discharge Actuator 
       FIG. 5  schematically depicts construction of a Dielectric Barrier Discharge (DBD) plasma actuator  500  according to an exemplary embodiment of the current invention. 
     RF high power supply  510 , supplying alternating high voltage is connected to air exposed electrode  515  and insulated electrode  525  which are separated by thin dielectric layer  520 . Optional insulation layer  530  insulating insulated electrode  525  from its environment. However, when DBD  500  is attached to a structure such as a fan blade made of electrically insulating material, the blade may serve as insulation layer  530 . 
     RF plasma  550  is generated in the location where air exposed electrode  515  and insulated electrode  525  are in proximity to each other. Gas discharge is created when an electric field of sufficient amplitude is applied to a volume of gas to generate electron-ion pairs through electron impact ionization of the neutral gas [7-10].  545  is the air jet (or gas jet) produced by the plasma actuator.  540  is the induced flow of air resulting from the air jet. Generation of the air jet as a result of the plasma produces momentum as any jet would in the direction  545 . This results in the creation of a net thrust by Newton&#39;s third law of motion. 
       FIG. 6  schematically depicts construction of a Dielectric Barrier Discharge (DBD) plasma actuator  500 ′ according to an exemplary embodiment of the current invention. 
     RF high power supply  510 , supplying alternating high voltage is connected to air exposed electrode  515  and to the blade structure  525 ′ acting as insulated electrode. Air exposed electrode  515  and blade structure  525 ′ are separated by thin dielectric layer  520  at the location where RF plasma  550  is to be created, but separated by thick insulating layer  520 ′ elsewhere. 
     Kempton film may be used as dielectric layer. Alternatively, other insulators such as Mylar or polyimide may be used. 
     The DBD actuators used in the demonstration of the invention had an asymmetric arrangement, consisting of two thin metal electrodes separated by a thin dielectric layer (see  FIG. 5 ). In the experimental system, the structure&#39;s thickness was approximately 200*10 −6  m, however other thickness may be used. Sufficiently high voltages, at “carrier” frequencies f c  between approximately 3 kHz and 10 kHz, are supplied to the actuator and cause the air to weakly ionize at the edges of the upper electrode. It should be noted that higher or lower frequency may be used according to embodiments of the invention. 
     These are regions of high electric field potential and in an asymmetric configuration, such as that shown in the figure; plasma is only generated at one edge. The plasma moves to regions of increasing electric field gradients and induces a 2-D wall jet in the flow direction  540  along the surface, thereby adding momentum to the boundary layer. 
     In some embodiments, the high voltage is supplied continuously, in other embodiments; the high voltage is switched on and off or is otherwise modulated. Specifically, in some preferred embodiments, the high voltage was modulated as a train of high frequency square pulses at a characteristic frequency and with a specific duty cycle. 
     It should be noted that using pulsed RF power at low duty cycle may save energy while maintaining reasonable performance. Power saving is useful, but it may be critical in applications were limited power is available such as in UAVs. In fan applications, power switching among plurality of actuators on the plurality of blades may be done by a controller at the fan&#39;s hub. Such power switching may reduce the instantaneous power transmission into the rotor to the average power used by the totality of the actuator without the need for power storage at the rotor. 
     It is pointed out that the DBD actuators are merely one type of plasma-based actuator and it is referenced for purposes of illustration only. 
     Application to Centrifugal and Crossflow Fans 
       FIG. 7  schematically depicts top views of several radial blowers according to exemplary embodiments of the current invention. 
     The description in the above section is relevant to axial fans specifically. However, the same concept may be applied to centrifugal fans (sometimes called radial blowers) and crossflow fans. Consider the impellers of two typical centrifugal fans, i.e. with backward inclined ( FIG. 7(   i )) and airfoil type vanes ( FIG. 7(   ii )), and the impeller of a crossflow fan, with forward curved vanes ( FIG. 7(   iii )). Clearly, the backward inclined blade profiles correspond to that shown in  FIG. 2   a . In a conventional centrifugal fan no DBD active flow control is applied. In this invention, DBD plasma actuation is applied at either or both of the edges of the impeller profile. A similar correspondence exists between the centrifugal fan airfoil type profile and that shown in  FIG. 2   d , and the forward curved profile and that shown in  FIG. 2   b . It is thus clear that the DBD plasma control of axial flow fan flows described herein may apply equally well to the centrifugal and crossflow fan vanes described here. 
       FIG. 8  schematically depicts fan system  800  according to an exemplary embodiment of the current invention. 
     Fan system  800  comprises an electrical motor  840  connected to plurality of fan wings  850  by axis  820 . Power cord  880  supplies main electrical power to motor  840  and to power transmitter  830 . Power transmitter  830  transmits electrical power to the rotating parts of fan system  800  using one of the abovementioned methods. Optional plasma controller  870  converts electrical power received from transmitter  830  to RF electrical signals and activate the plurality of plasma actuators  860  installed on fan wings  850 . 
     In all applications, electrical power needs to be supplied to the plasma actuators. In embodiments such as airplane wings or control surfaces, electrical power supply may be places within the fuselage or the wing and connected to the actuators. 
     In applications such as fans and other rotating structure, electrical power may be transferred from the stationary structure to the rotating parts by conduction, for example by using slip-ring. 
     Alternatively, electrical power may be transferred from the stationary structure to the rotating parts by induction or by microwave radiation. For example, an electrical transformer may be used for transferring electrical energy from the fan&#39;s stationary part to the rotating parts having: a stationary primary coil, attached to the fan stator and wound around the rotation axis receiving AC electrical power from main electrical grid of from a local power supply; and a secondary coil, electromagnetically coupled to said primary coil mounted on and rotating with the rotating part of the fan. 
     Alternatively, brushes used for transferring electrical energy to the rotor of the motor used for rotating the fan may be used also for supplying electrical energy to plasma actuators. Optionally, dedicated brushes may be used. Alternatively, in a brushless motor, induced electrical currents created in the rotor of the motor may be used for supplying electrical energy to plasma actuators. 
     In some embodiments, high frequency RF power is inductively or capacitively transferred from the stationary structure to the rotating parts at the carrier frequency used by the plasma actuator(s). 
     Power may be generated in the rotating part by placing magnets on the stator and having a coil or coils in the rotor. 
     Optionally, electrical power is conditioned in the rotor, for example is frequency is changed or its voltage changed or regulated or modulated before it is supplied to the actuator(s). Optionally a plasma controller mounted on the rotor regulates the distribution of electrical power to the plasma actuators. 
     Other Applications 
     It is well known that a fan (or wind pump) can also be used as a turbine. The most common of these is the horizontal axis (axial flow) wind turbine, where wind turns the turbine blades that, in turn, drive a generator. Thus, it is conceivable that DBD plasma actuators could be used on wind turbines in order to improve performance and control the flow separation phenomenon known as dynamic stall (see Carr, 1988). 
     Another application of the invention is for Micro Air Vehicles (MAVs) and Nano Air Vehicles (NAVs) and other Unmanned small Air Vehicles (UAV). Providing lift pose significant challenges due to their small dimensions and low flight speeds. For so-called mini air vehicles, which operate in the 10,000&lt;Re&lt;300,000 range, efficient systems can be designed by using plasma actuators. The challenge of developing useful lift intensifies with yet smaller vehicles required to fly at even lower flight speeds. This includes the development of so-called nano UAVs for which the missions include flying within confined areas. These are commonly termed nano air vehicles (NAVs) and are defined as weighing less than 10 g, with dimensions smaller than 7:5 cm, and speeds between 0.5 and 7:5 m/s. Propulsion systems used in these UAVs are usually (optionally electrically powered) propellers. In these applications, high efficiency lift and propulsion systems directly translates to one or few of: longer flight duration, higher payload, higher speed and/or higher maneuverability. 
     Finally, all of the above examples referred to airflows. However, similar applications can be found in liquid applications, particularly when the liquids are conductive or even weakly conductive such as in seawater. In these instances, plasma actuation would not be used, but control could be affected using Lorentz force type actuators of the design described by Weier et al (2004). 
     It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination. 
     Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.