Synthetic jet actuators for mixing applications

The first preferred embodiment of an improved fluid mixing system comprises a synthetic jet actuator aligned perpendicular to a primary fluid flow. When the synthetic jet actuator is driven at a very high frequency, small scale mixing of the primary fluid flow can be effectively controlled. A second preferred embodiment for a mixing system comprises at least one synthetic jet actuator attached to the housing of a primary jet such that the direction of the synthetic jet flow will be parallel to the direction of the primary jet flow. If the two jets are allowed to operate at the same time, the synthetic jet actuator will have the effect of more effectively mixing the primary jet into the ambient fluid. Another embodiment of an improved mixing system comprises a synthetic jet actuator situated in a closed volume. The fluid flow created by the synthetic jet actuator in the closed volume will greatly aid mixing of the fluids in the chamber without injecting any new matter into the chamber.

FIELD OF THE INVENTION 
The present invention generally relates to fluid actuators for manipulation 
and control of fluid flow and, more particularly, to a fluid actuator in 
the form of a synthetic jet actuator for mixing fluids through the 
introduction of small scale perturbations into a flow field and direct 
control of the small scale mixing. 
BACKGROUND OF THE INVENTION 
The ability to manipulate and control the evolution of shear flows has 
tremendous potential for influencing system performance in diverse 
technological applications, including: mixing and combustion processes, 
lift and drag of aerodynamic surfaces, and thrust management. That these 
flows are dominated by the dynamics of a hierarchy of vortical structures, 
evolving as a result of inherent hydrodynamic instabilities (e.g., Ho & 
Huerre, 1984), suggests control strategies based on manipulation of these 
instabilities by the introduction of small disturbances at the flow 
boundary. A given shear flow is typically extremely receptive to 
disturbances within a limited frequency band and, as a result, these 
disturbances are rapidly amplified and can lead to substantial 
modification of the base flow and the performance of the system in which 
it is employed. 
There is no question, that suitable actuators having fast dynamic response 
and relatively low power consumption are the foundation of any scheme for 
the manipulation and control of shear flows. Most frequently, actuators 
have had mechanically moving parts which come in direct contact with the 
flow [e.g., vibrating ribbons (Schubauer & Skramstad J. Aero Sci. 14 
1947), movable flaps (Oster & Wygnanski, 1982), or electromagnetic 
elements (Betzig AIAA, 1981)]. This class of direct-contact actuators also 
includes piezoelectric actuators, the effectiveness of which has been 
demonstrated in flat plate boundary layers (Wehrmann 1967, and Jacobson & 
Reynolds Stan. U. TF-64 1995), wakes (Wehrmann Phys. Fl. 8 1965, 1967, and 
Berger Phys. Fl. S191 1967), and jets (Wiltse & Glezer 1993). Actuation 
can also be effected indirectly (and, in principle, remotely) either 
through pressure fluctuations [e.g., acoustic excitation (Crow & Champagne 
JFM 48 1971)] or body forces [e.g., heating (Liepmann et al. 1982, Corke & 
Mangano JFM 209 1989, Nygaard and Glezer 1991), or electromagnetically 
(Brown and Nosenchuck, AIAA 1995)]. 
Flow control strategies that are accomplished without direct contact 
between the actuator and the embedding flow are extremely attractive 
because the actuators can be conformally and nonintrusively mounted on or 
below the flow boundary (and thus can be better protected than 
conventional mechanical actuators). However, unless these actuators can be 
placed near points of receptivity within the flow, their effectiveness 
degrades substantially with decreasing power input. This shortcoming can 
be overcome by using fluidic actuators where control is effected 
intrusively using flow injection (jets) or suction at the boundary. 
Although these actuators are inherently intrusive, they share most of the 
attributes of indirect actuators in that they can be placed within the 
flow boundary and require only an orifice to communicate with the external 
flow. Fluidic actuators that perform a variety of "analog" (e.g., 
proportional fluidic amplifier) and "digital" (e.g., flip-flop) throttling 
and control functions without moving mechanical parts by using control 
jets to affect a primary jet within an enclosed cavity have been studied 
since the late 1950's (Joyce, HDL-SR 1983). Some of these concepts have 
also been used in open flow systems. Viets (AIAA J. 13 1975) induced 
spontaneous oscillations in a free rectangular jet by exploiting the 
concept of a flip-flop actuator and more recently, Raman and Cornelius 
(AIAA J. 33 1995) used two such jets to impose time harmonic oscillations 
in a larger jet by direct impingement. 
More recently, a number of workers have recognized the potential for MEMS 
(micro eclectro mechanical systems) actuators in flow control applications 
for large scale systems and have exploited these devices in a variety of 
configurations. One of a number of examples of work in this area is that 
of Ho and his co-investigators (e.g., Liu, Tsao, Tai, and Ho, 1994) who 
have used MEMS versions of `flaps` to effect flow control. These 
investigators have opted to modify the distribution of streamwise 
vorticity on a delta wing and thus the aerodynamic rolling moment about 
the longitudinal axis of the aircraft. 
Background Technology for Synthetic Jets 
It was discovered at least as early as 1950 that if one uses a chamber 
bounded on one end by an acoustic wave generating device and bounded on 
the other end by a rigid wall with a small orifice, that when acoustic 
waves are emitted at high enough frequency and amplitude from the 
generator, a jet of air that emanates from the orifice outward from the 
chamber can be produced. See, for example, Ingard and Labate, Acoustic 
Circulation Effects and the Nonlinear Impedance of Orifices, The Journal 
of the Acoustical Society of America, March, 1950. The jet is comprised of 
a train of vortical air puffs that are formed at the orifice at the 
generator's frequency. 
The concern of scientists at that time was only with the relationship 
between the impedance of the orifice and the "circulation" (i.e., the 
vortical puffs, or vortex rings) created at the orifice. There was no 
suggestion to combine or operate the apparatus with another fluid stream 
in order to modify the flow of that stream (e.g., its direction). 
Furthermore, there was no suggestion that following the ejection of each 
vortical puff, a momentary air stream of "make up" air of equal mass is 
drawn back into the chamber and that, as a result, the jet is effectively 
synthesized from the air outside of the chamber and the net mass flux out 
of the chamber is zero. There was also no suggestion that such an 
apparatus could be used in such a way as to create a fluid flow within a 
bounded (or sealed) volume. 
Such uses and combinations were not only not suggested at that time, but 
also have not been suggested by any of the ensuing work in the art. So, 
even though a crude synthetic jet was known to exist, applications to 
common problems associated with other fluid flows or with lack of fluid 
flow in bounded volumes were not even imagined, much less suggested. 
Evidence of this is the persistence of certain problems in various fields 
which have yet to be solved effectively. 
Vectoring of a Fluid Flow 
Until now, the direction of a fluid jet has chiefly been controlled by 
mechanical apparatus which protrude into a jet flow and deflect it in a 
desired direction. For example, aircraft engines often use mechanical 
protrusions disposed in jet exhaust in order to vector the fluid flow out 
of the exhaust nozzle. These mechanical protrusions used to vector flow 
usually require complex and powerful actuators to move them. Such 
machinery often exceeds space constraints and often has a prohibitively 
high weight. Furthermore, in cases like that of jet exhaust, the mechanism 
protruding into the flow must withstand very high temperatures. In 
addition, large power inputs are generally required in order to intrude 
into the flow and change its direction. For all these reasons, it would be 
more desirable to vector the flow with little or no direct intrusion into 
the flow. As a result, several less intrusive means have been developed. 
Jet vectoring can be achieved without active actuation using coanda effect, 
or the attachment of a jet to a curved (solid) surface which is an 
extension one of the nozzle walls (Newman, B. G. "The Deflexion of Plane 
Jets by Adjacent Boundaries-Coanda Effect," Boundary Layer and Flow 
Control v. 1, 1961 edited by Lachmann, G. V. pp. 232-265.). However, for a 
given jet momentum, the effect is apparently limited by the characteristic 
radius of the curved surface. The effectiveness of a coanda surface can be 
enhanced using a counter current flow between an external coanda surface 
and a primary jet. Such a system has been used to effect thrust vectoring 
in low-speed and high-speed jets by Strykowski et al. (Strykowski, P. J, 
Krothapalli, A., and Forliti D. J. "Counterflow Thrust Vectoring of 
Supersonic Jets," AIAA Paper No. 96-0115, AIAA 34th Aerospace Sciences 
Meeting, Reno, Nev., 1996.). 
The performance of coanda-like surfaces for deflection of jets can be 
further improved by exploiting inherent instabilities at the edges of the 
jet flow when it is separated from the surface. It has been known for a 
number of years that substantial modification of shear flows can result 
from the introduction of small perturbations at the boundaries of the 
shear flow. This modification occurs because the shear flow is typically 
hydrodynamically unstable to these perturbations. Such instability is what 
leads to the perturbations' rapid amplification and resultant relatively 
large effect on the flow. This approach has been used in attempts to 
control separating flows near solid surfaces. the flow typically separates 
in the form of a free shear layer and it has been shown that the 
application of relatively small disturbances near the point of separation 
can lead to enhanced entrainment of ambient fluid into the layer. Because 
a solid surface substantially restricts entrainment of ambient fluid, the 
separated flow moves closer to the surface and ultimately can reattach to 
the surface. This effect has been used as a means of vectoring jets near 
solid surfaces. See e.g., Koch (Koch, C. R. "Closed Loop Control of a 
Round Jet/Diffuser in Transitory Stall," PhD. Thesis, Stanford University, 
1990) (using wall jets along in a circular diffuser to effect partial 
attachment and thus vectoring of a primary round jet). 
Similar to mechanical deflectors, technologies that rely on coanda surfaces 
are limited because of the size and weight of the additional hardware. 
Clearly, a coanda collar in aerospace applications must be carried along 
at all times whether or not it is being used. 
Fluidic technology based on jet-jet interaction has also been used for flow 
vectoring or producing oscillations of free jets. Fluidic actuators 
employing control jets to affect a primary jet of the same fluid within an 
enclosure that allows for inflow and outflow have been studied since the 
late 1950's. These actuators perform a variety of "analog" (e.g., 
proportional fluidic amplifier) and "digital" (e.g., flip-flop) throttling 
and control functions in flow systems without moving mechanical parts 
(Joyce, 1983). In the "analog" actuator, the volume flow rate fraction of 
two opposite control jets leads to a proportional change in the volume 
flow rate of the primary stream out of two corresponding output ports. The 
"digital" actuator is a bistable flow device in which the control jets and 
Coanda effect are used to direct the primary stream into one of two output 
ports. 
These approaches have also been employed in free jets. Viets (1975) induced 
spontaneous oscillations in a free rectangular jet by exploiting the 
concept of a "flip-flop" actuator. More recently, Raman and Cornelius 
(1995) used two such jets to impose time harmonic oscillations in a larger 
jet by direct impingement. The control jets were placed on opposite sides 
of the primary jet and could be operated in phase or out of phase with 
each other. 
Use of a fluidic jet to vector another flow, while known for years, has 
been used with limited success. In particular, the only way known to 
vector a jet with another jet (dubbed a "control jet") of the same fluid 
was to align the control jet so that it impinges directly on the primary 
jet. Obviously, this involved injection of mass into the flow and has not 
been deemed very effective at vectoring the primary flow because it relies 
on direct momentum transfer between the jets for altering the direction of 
the primary jet. Direct momentum transfer is not economical in general and 
undesirable when the available power is limited (such as on board an 
aircraft). Furthermore, such control hardware is difficult and expensive 
to install because of the complex plumbing necessary to supply the control 
jet with fluid to operate. 
Modification of Fluid Flows About Aerodynamic Surfaces 
The capability to alter the aerodynamic performance of a given airframe by 
altering its shape (e.g., the "camber" of an airfoil) during various 
phases of the flight can lead to significant extension of the airframe's 
operating envelope. Geometric modification of lifting surfaces has so far 
been accomplished by using mechanical flaps and slats. However, because of 
the complex control system required, such devices are expensive to 
manufacture, install and maintain. Furthermore, flap systems not only 
increase the weight of the airframe, but also require considerable 
interior storage space that could be used for cargo, and additional 
ancillary hardware (e.g., hydraulic pumps, piping, etc.). In some 
applications, the weight penalty imposed by the flaps may more than offset 
their usefulness. 
In addition to the use of mechanical flaps, there has been considerable 
effort to enhance the aerodynamic performance of lifting surfaces by 
delaying flow separation and thus the loss of lift and increase in drag. 
Conventional methods for such flow control have primarily focused on delay 
of separation or inducement of reattachment by introducing small 
disturbances into the upstream wall boundary layer. Excitation methods 
have included external and internal acoustic excitation (Huang, Maestrello 
& Bryant, Expt. Fl. 15 1987), vibrating flaps (e.g., Neuberger & 
Wygnanski, USAF A TR-88 1987) and unsteady bleeding or blowing (e.g., 
Sigurdson & Roshko, AIAA 1985, and Seifert, Bachar, Koss, Shepshelovich & 
Wygnanski, AIAA J. 31 1993). These methods have been used with varying 
degrees of success. The effectiveness largely depends on the receptivity 
of the boundary layer to excitation within a relatively narrow bandwidth. 
Other efforts of designers to modify the flow about an aerodynamic surface 
have centered on injection of energy into the boundary layer of the flow 
in order to augment lift, reduce drag, delay turbulent onset, and/or delay 
flow separation. For example, the method disclosed by U.S. Pat. No. 
4,802,642 to Mangiarotty involves the retardation of a flow's transition 
to turbulence. However, this prior art does not and cannot change the 
effective aerodynamic shape of the airfoil. That is, the apparatus cannot 
change the direction of flow of the free stream fluid about the surface. 
Instead, the apparatus propagates acoustic excitation above the 
Tollmien-Schlichting frequency in an attempt to disrupt 
Tollmien-Schlichting waves as they begin to form and thereby delay the 
onset of turbulence. Although this method changes the drag characteristic 
of a lifting surface, the mean velocity field and thus apparent 
aerodynamic shape of the surface remain unchanged. 
Such devices as slots and fluid jets have also been extensively employed to 
inject energy into the boundary layer in order to prevent flow separation. 
Recently, efforts have turned to the use of piezoelectric or other 
actuators to energize the boundary layer along an aerodynamic surface. 
See, e.g., U.S. Pat. No. 4,363,991 to Edleman. These techniques, which 
employ acoustic excitation, change the surface aerodynamic performance by 
suppressing the naturally occurring boundary layer separation. This method 
requires the flow state to be vulnerable to specific disturbance 
frequencies. Although effective at delaying flow separation, none of these 
devices alter the apparent aerodynamic shape or mean velocity field of a 
given aerodynamic surface. Even though the changes in lift and drag that 
are caused by separation can be somewhat restored, there is no effect 
before separation occurs and the locus of the stagnation points remain 
largely unchanged. Therefore, before the present invention, there was no 
way to alter the effective shape of an aerodynamic surface without the 
complexity, high expense, and weight penalty of mechanical flaps or slats. 
Mixing of Fluids at the Small Scale Level 
In a somewhat different field of study, the ability to effectively control 
the evolution of the shear layer between two streams of similar fluids 
(gas or liquid) may have great impact on the mixing between the two 
streams (e.g., mixing a hot exhaust plume with cold ambient air). The 
boundary between the two streams forms the turbulent flow region known as 
a "shear layer." Hydrodynamic instabilities in this shear layer induce a 
hierarchy of vortical structures. Mixing between the two streams begins 
with the entrainment of irrational fluid from each stream by the 
large-scale vortical structures. These vortical structures scale with 
geometric features of the flow boundary (e.g., nozzle diameter of a jet, 
vortex generators, etc.) and they are critical to the mixing process 
between the two streams by bringing together in close contact volumes of 
fluid from each stream in a process that is referred to as entrainment. 
Layers of entrained fluid are continuously stretched and folded at 
decreasing scales by vortical structures that evolve through the action of 
shear and localized instabilities induced by larger vortical structures. 
This process continues until the smallest vortical scales are attained and 
fluid viscosity balances the inertial forces. This smallest vortical scale 
is referred to as the Kolmogorov scale. Thus, a long-held notion in 
turbulence is that the smallest and largest turbulent motions are 
indirectly coupled through a cascade of energy from the largest to 
successively smaller scales until the Kolmogorov scale is reached and 
viscous diffusion can occur. Turbulent dissipation is the process by which 
(near the Kolmogorov scale) turbulent kinetic energy is converted into 
heat as small fluid particles are deformed. 
Scalar mixing (of heat or species, for example) is similar, but not 
identical to momentum mixing. Analogous to the Kolmogorov scale, the 
Batchelor scale is the smallest spatial scale at which an isoscalar 
particle can exist before scalar gradients are smoothed by the action of 
molecular diffusion. If scalar diffusion occurs on a faster scale than 
momentum diffusion, the Kolmogorov energy cascade breaks "packets" of 
scalars down into scales small enough that molecular scalar diffusion can 
occur (at the Batchelor scale). In this case, the Batchelor scale is 
larger than the Kolmogorov scale and turbulent motions persist at scales 
where scalar gradients have been smoothed out by diffusion. If, on the 
other hand, scalar diffusion occurs on a slower scale than momentum 
diffusion, turbulent motions stop (at the Kolmogorov scale) before the 
scalar gradients are smoothed out. Final mixing only occurs after laminar 
straining further reduces the size of the scalar layers. 
There is a substantial body of literature that demonstrates that mixing in 
shear flows can be influenced by manipulating the evolution of the large 
scale eddies (vortical structures) within the flow. Because the 
large-scale eddies result from inherent hydrodynamic instabilities of the 
flow, they can be manipulated using either passive or active devices. 
As noted above, although the entrainment process in turbulent shear flows 
is effected by the large-scale eddies, molecular mixing ultimately takes 
place at the smallest scales which is induced by a hierarchy of eddies of 
decreasing scales that continuously evolve from the largest scale eddies. 
Because the base flows are normally unstable at the large scales (and thus 
receptive to either passive or active control inputs), the traditional 
approach to controlling mixing at the small-scale has been indirect. 
Previous attempts to control small-scale mixing employing both passive and 
active control strategies have relied on manipulation of global two-and 
three-dimensional instability modes of the base flow with the objective of 
controlling mixing through the modification of the ensuing vortical 
structures. 
Passive control has primarily relied on (permanent) modification of the 
geometry of the flow boundary. For example, mixing of jet exhaust is often 
enhanced by corrugating the exhaust port of a jet. This corrugation 
creates the appearance of a number of lobes defined by raised and recessed 
curves which induce counter-rotating vortices, thus promoting mixing in 
the direction of the exhaust flow. Other passive devices for the promotion 
of mixing have included small tabs that act as vortex generators. The 
disadvantage of such mixing devices is that their geometry is fixed and 
thus their effectiveness cannot be adjusted for varying flow conditions. 
Conventional active control strategies overcome this deficiency because the 
control input can be adjusted. For example, one prior disclosure describes 
the manipulation of large scale eddies in a plane shear layer between two 
uniform streams using a small oscillating flap. However, because this 
approach depends on the classical cascading mechanism to transfer control 
influence to the scales at which molecular mixing occurs, mixing at the 
smallest scales in fully turbulent flows is only weakly coupled to the 
control input. More importantly, mixing control of this nature relies on a 
priori knowledge of the flow instabilities and associated eigenfrequencies 
of the particular flow. Specifically, this method also requires that the 
flow be unstable to a range of disturbances, a condition which is not 
always satisfied. 
Clearly, more efficient control of mixing in fully turbulent shear flows 
might be achieved by direct (rather than hierarchical) control of both the 
large-scale entrainment and the small-scale mixing processes. Such a 
control method has, before now, not been available but is enabled by 
synthetic jet actuators that are the subject of the present disclosure. 
Earlier work has demonstrated two important attributes of direct 
small-scale excitation: (1) that small-scale eddies can indeed be 
influenced by direct excitation of discrete wavenumbers within the 
dissipation range of a free shear layer, and (2) that this strategy also 
allows for efficient control of the large scale eddies. In the work of 
Wiltse and Glezer (1994), shear layer segments of a square air jet are 
forced near the jet exit plane at frequencies within the dissipation range 
of the base flow by planar, bimorph, piezoelectric actuators driven at 
resonance. These small-scale motions can have wavenumbers within one to 
two orders of magnitude of the Kolmogorov wavenumber of the base flow, 
thus enabling one to induce scalar mixing directly at the small scales, 
without relying on the conventional energy cascade. The dynamics of the 
large-scale eddies (and thus the entrainment process) is influenced by 
amplitude modulation of the excitation waveform. The work of Wiltse 
Nygaard and Glezer clearly demonstrate that the flow demodulates the 
excitation waveform, thus allowing simultaneous excitation at high and low 
frequencies, and thus of small- and large-scales, respectively. However, 
in all the prior art methodologies, intrusion or mass addition into the 
flow is required. Additionally, such mixing control is often ineffective 
in a bounded volume. 
Cooling of Heated Bodies 
Cooling of heat-producing bodies is a concern in many different 
technologies. Particularly, a major challenge in the design and packaging 
of state-of-the-art integrated circuits in single- and multi-chip modules 
(MCMs) is the ever increasing demand for high power density heat 
dissipation. While current technologies that rely on global forced air 
cooling can dissipate about 4 W/cm.sup.2, the projected industrial cooling 
requirements are 10 to 40 W/cm.sup.2 and higher within the next five to 
ten years. Furthermore, current cooling technologies for applications 
involving high heat flux densities are often complicated, bulky and 
costly. 
Traditionally, this need has been met by using forced convective cooling 
using fans which provide global overall cooling when what is often 
required in pinpoint cooling of a particular component or set of 
components. Furthermore, magnetic-motor-based fans have the problem of 
generating electromagnetic interference which can introduce noise into the 
system. 
In applications when there is a heat-producing body in a bounded volume, 
the problem of cooling the body is substantial. In fact, effective cooling 
of heated bodies in closed volumes has also been a long standing problem 
for many designers. Generally, cooling by natural convection is the only 
method available since forced convection would require some net mass 
injection into the system, and subsequent collection of this mass. The 
only means of assistance would be some mechanical fan wholly internal to 
the volume. However, often this requires large moving parts in order to 
have any success in cooling the heated body. These large moving parts 
naturally require high power inputs. But, simply allowing natural 
convective cooling to carry heat from the body producing it into the fluid 
of the volume and then depending on the housing walls to absorb the heat 
and emit it outside the volume is a poor means of cooling. 
SUMMARY OF THE INVENTION 
Briefly described, the present invention involves the use of improved 
synthetic jet actuators in novel mixing applications. Particularly, the 
present invention is concerned with a radically new approach to mixing 
control, based on concurrent manipulation of both the small- and 
large-scale vortical structures in a turbulent shear flow by means of a 
synthetic jet actuator. 
A first object of the present invention is to provide an improved device 
for asserting indirect, non-intrusive control over the mixing of a fluid 
flow. Most of the previous approaches to flow control can be classified as 
direct contact actuators. That is, prior art actuators generally have 
mechanically moving parts that come into direct contact with the flow in 
order to effect control authority. In contrast to these approaches, the 
fluidic technology based on synthetic jet actuators, which is the subject 
of the present invention, allows indirect assertion of control authority. 
Another object of the present invention is for producing a synthetic jet of 
fluid synthesized from the working fluid of the medium where the synthetic 
jet actuator is deployed. Thus, linear momentum is transferred to the flow 
system without net mass injection into the system. 
Another object of the present invention is for direct control over both the 
large scale and small scale mixing processes within a fluid. As mentioned 
in the introduction, while entrainment of irrotational fluid in turbulent 
shear flows is effected by large-scale motions, molecular mixing 
ultimately takes place of the smallest scales. In most shear flows this 
mixing is normally induced by a hierarchy of vortical structures of 
decreasing scales, which ensue from hydrodynamic instabilities of the 
flow. The traditional approach to the control of mixing at the small 
scales has been indirect and has relied on the manipulation of global 
two-and-three-dimensional instability modes that lead to the appearance of 
large scale vortical structures. These vortices ultimately break down to 
smaller and smaller scale vortices in what is referred to as a cascading 
process and induces smaller and smaller scale motions that lead to 
molecular mixing. Because the traditional approach to mixing depends on 
this cascading mechanism to transfer control influence to the scales at 
which moleculars mixing occurs, the mixing at the smallest scales is only 
weakly coupled to the control input. 
Fluidic actuation using synthetic jets allows for exploitation of nonlinear 
mechanisms for amplification of disturbances in a very broad frequency 
band, and thus allows for a new approach to the control and enhancement of 
mixing in shear flows through small-scale vorticity. Small-scales that are 
within the dissipation range of the driven flow can be excited by 
adjusting the scale of the vortices that synthesize the synthetic jets, 
through the characteristic dimension of the orifice and the period of 
oscillations of the jet diaphragm. Thus mixing can be directly influenced 
at the scales in which it actually occurs within the driven flow. Large 
bandwidth is attainable using amplitude and frequency modulation of the 
resonant carrier frequency of these actuators. 
Another important attribute of the synthetic jet approach is that it 
enables the creation of a flow within a bounded volume. Particularly, 
effective mixing of fluids inside a bounded volume could be achieved 
without the addition of new species, need for a fluid source or drain, and 
without a mechanical stirring device, which may require a large power 
input and place additional geometric or contamination constraints on the 
designer. Some common applications of mixing in a bounded volume are 
mixing in chemical lasers and mixing for chemical or pharmaceutical 
products. In addition to these fields, the development of methods for 
enhancement of mixing through manipulation of the flow in which it occurs 
will have a direct impact on the performance of various other 
technologically important systems (e.g., propulsion, combustion, or in 
bio-engineering). 
I. Construction and Operation of Synthetic Jets 
The construction and operation of various synthetic jet actuators will 
first be described. These jets serve as the "hardware" for the present 
invention and are described in detail in patent application Ser. No. 
08/489,490. After discussing these devices, generally, several preferred 
embodiments of mixing apparatuses using synthetic jet actuators will be 
discussed. 
A. First Preferred Embodiment of a Synthetic Jet Actuator 
A first preferred embodiment of a synthetic jet actuator comprises a 
housing defining an internal chamber. An orifice is present in a wall of 
the housing. The actuator further includes a mechanism in or about the 
housing for periodically changing the volume within said internal chamber 
so that a series of fluid vortices are generated and projected in an 
external environment out from the orifice of the housing. The volume 
changing mechanism can be any suitable mechanism, for instance, a piston 
positioned in the jet housing to move so that fluid is moved in and out of 
the orifice during reciprocation of the piston. Preferably, the volume 
changing mechanism is implemented by using a flexible diaphragm as a wall 
of the housing. The flexible diaphragm may be actuated by a piezoelectric 
actuator or other appropriate means. 
Typically, a control system is utilized to create time-harmonic motion of 
the diaphragm. As the diaphragm moves into the chamber, decreasing the 
chamber volume, fluid is ejected from the chamber through the orifice. As 
the fluid passes through the orifice, the flow separates at the sharp 
edges of the orifice and creates vortex sheets which roll up into 
vortices. These vortices move away from the edges of the orifice under 
their own self-induced velocity. 
As the diaphragm moves outward with respect to the chamber, increasing the 
chamber volume, ambient fluid is drawn through the orifice into the 
chamber. Since the vortices are already removed from the edges of the 
orifice, they are not affected by the ambient fluid being entrained into 
the chamber. As the vortices travel away from the orifice, they synthesize 
a jet of fluid, a "synthetic jet," through entrainment of the ambient 
fluid. 
In addition to the basic design of a synthetic jet actuator, one may modify 
the design to enhance performance. This enhanced synthetic jet actuator 
comprises a housing defining an interior chamber and an orifice in one 
wall of the housing. This synthetic jet actuator has a device or mechanism 
for withdrawing fluid into the chamber and for forcing fluid out of the 
chamber through the orifice. At least one louver is attached to the 
housing and is aligned with an opening formed in the housing. The louver 
is a one-way valve and only permits fluid flow in one direction. Thus, the 
louver permits fluid flow either into the chamber through the opening or 
out of the chamber through the opening. 
The operation of the enhanced synthetic jet actuator can vary greatly 
depending upon whether the louver permits fluid to flow into the chamber 
or instead only permits fluid to flow out of the chamber. If the louver 
permits fluid flow into the chamber, then the synthetic jet actuator is 
able to withdraw fluid into the chamber through a greater surface area. 
The force of the jet formed by the synthetic jet actuator, however, is not 
decreased since all of the fluid exits the chamber through the orifice. 
The synthetic jet actuator with this configuration can operate at higher 
momentum during the outstroke. Alternatively, if the louver only permits 
fluid to flow out of the chamber, then the synthetic jet actuator will 
operate at higher momentum during the instroke. 
A synthetic jet actuator may have any suitable louver and any suitable 
mechanism or device for withdrawing fluid into the chamber and for forcing 
fluid out of the chamber. For instance, the louver may be a passive louver 
or an active louver, such as one whose position is at least partially 
controlled by a piezoelectric material. The device or mechanism may 
comprise a piston reciprocating within the chamber or may comprise a 
flexible diaphragm driven by piezoelectric actuation. 
B. Second Preferred Embodiment of a Synthetic Jet Actuator 
The synthetic jet actuator just described is not the only device capable of 
forming a synthetic jet stream. Indeed, there are several ways to build a 
synthetic jet actuator for use with the present invention. For example, in 
certain applications a constant suction synthetic jet actuator may be 
desirable. In this preferred embodiment, a synthetic jet actuator will 
typically be embedded in a body and operate through the outer surface of 
the body. There may be no room in the body for a piston or other volume 
changing means suggested by the first preferred embodiment. This second 
preferred embodiment provides a solution to such a potential problem. 
For the second preferred embodiment of a synthetic jet actuator, there are 
two concentric tubular sections, or pipes embedded in the solid body, 
normal to the outside surface. The outer of the two pipes is preferably 
connected to a source of fluid with a means for pulsing a fluid out of 
this pipe. The innermost of the two pipes is connected to an appropriate 
means for pulling fluid down this pipe from the ambient fluid above the 
planar surface, such as a vacuum or fluid pump. Situated such that its 
exit plane is slightly below the surface, in operation, the innermost pipe 
constantly pulls fluid down its length from the ambient fluid above the 
flat, planar surface. Meanwhile, the outer pipe is caused to pulse fluid 
into the ambient environment above the planar surface. Such an operation 
will cause a synthetic fluid jet to form above the constant suction 
synthetic jet actuator. 
Additionally, this embodiment allows a user to tailor the net mass flux 
into the system caused by the synthetic jet actuator. The source of fluid 
could be a compressor or other source separate from the depository of the 
fluid drawn into the innermost pipe. One could, therefore, tailor the 
system to yield a net mass increase, decrease, or no net mass flux in the 
system above the synthetic jet actuator. 
II. Applications and Advantages of Mixing with Synthetic Jet Actuators 
The devices capable of forming synthetic jets, and the improvement of using 
louvers, all have certain features common to the class of synthetic jets, 
which permit more effective mixing of fluids and greater control over the 
mixing process. The present invention involves these new and advantageous 
mixing applications. A brief description of the novel apparatus and 
process to which the present invention is directed as follows. 
A. Mixing with a Synthetic Jet Actuator in Free Fluid Flows 
Synthetic jet interactions with other fluid streams can be used in mixing 
applications. Previously, mixing of primary fluid flows with ambient fluid 
required either net mass injection into the flow or physical intrusion 
into the flow. These methods were not only marginally effective, but they 
were usually expensive to install or difficult to maintain as well. 
However, use of synthetic jet actuators in mixing of free flows avoids the 
need to physically intrude into the flow and gives the user better control 
of mixing through direct small scale vortex manipulation. 
In a first preferred embodiment of a fluid mixing system, a synthetic jet 
actuator is aligned perpendicular to a primary fluid flow. When the 
synthetic jet actuator is driven at a very high frequency, small scale 
mixing of the primary fluid flow can be effectively controlled. 
Furthermore, through amplitude modulation, simultaneous excitation of both 
small and large scales is possible. Small scale manipulation can be 
effected by synthetic jet actuators where the scale of perturbations 
induced by the synthetic jet actuator is adjusted to be within the 
dissipation range of the affected flow. 
In fact, use of a synthetic jet actuator is not the only available method 
of perturbing the flow, but many methods of perturbation in a primary flow 
shear layer would yield similar results. While it is clear that direct 
excitation at the molecular scale is usually impractical with current 
technology, it is nonetheless possible to operate at the Kolmogorov scale 
or even considerably smaller. This enables one using some means of high 
frequency perturbation to induce mixing directly at the small scales 
without relying on the conventional energy cascade. This produces a vast 
improvement over the prior art in both control and effectiveness of 
mixing. 
Additional advantages to this mixing enhancement technique are found in the 
fact that such high frequency excitation increases the dissipation of a 
primary flow's turbulent energy. This reduction in turbulent energy may 
lead to reduction in turbulent drag may be expected if this technique is 
applied in a boundary layer. In addition, this technique can be used to 
reduce noise caused by the large scale vortical structures in free shear 
flows. 
Excitation of the flow at frequencies corresponding to the Batchelor scale 
will increase scalar dissipation (molecular mixing of scalar quantities, 
such as concentration or temperature) in much the same manner as 
excitation at the Kolmogorov frequency increases turbulent kinetic energy 
dissipation. 
Aligning a synthetic jet actuator perpendicular to the primary fluid flow 
is not the only way for a synthetic jet actuator to enhance mixing of the 
primary flow with ambient fluid. In some potential applications, such as 
mixing jet engine exhaust with ambient fluid in order to better cool the 
exhaust, it may be desirable to vector a primary jet as well as enhance 
mixing of the flow. 
In the most simple system for a "parallel" mixing apparatus, a synthetic 
control jet is attached to the top housing of a primary jet such that the 
direction of its flow will be parallel to the direction of the primary jet 
flow. If the two jets are allowed to operate at the same time, the 
synthetic jet actuator will have the effect of vectoring the primary jet. 
If the synthetic control jet orifice is near the exhaust plane of the 
primary jet, then the primary jet flow will be vectored toward the 
synthetic control jet. On the other hand, if the synthetic control jet 
orifice is a enough distance behind the exhaust plane of the primary jet, 
then the primary jet will be vectored away from the control jet due to the 
synthetic jet flow negotiating the ninety-degree turn at the end of the 
primary jet housing and directly impinging into the flow of the primary 
jet. 
In this configuration, not only will the jet be vectored toward (or away 
from) the synthetic jet actuator but the primary jet fluid will also be 
more effectively mixed with the ambient fluid due to excitation of the 
large scale eddies in the flow. Vectoring with the much weaker synthetic 
jet actuator causes the primary jet to entrain much more ambient fluid 
into its stream. In fact, the primary jet flow rate can increase by 300% 
over the unvectored primary jet's flow rate. It is noteworthy that the 
difference in the flow rate between the forced and unforced primary jet 
flow is much larger than the flow rate of the synthetic control jet alone. 
In one application of this preferred embodiment, such entrainment of 
ambient fluid and the resultant mixing could easily help cool hot jet 
engine exhaust. 
B. Mixing Fluids in a Bounded Volume with Synthetic Jet Actuators 
In contrast to conventional jets, a unique feature of synthetic jet 
actuators is that they are normally synthesized from the working fluid of 
the flow system in which they are deployed. Therefore, synthetic jet 
actuators may be used to create fluid flows in bounded volumes, where 
conventional jets could never be used. In particular, synthetic jet 
actuators in bounded volumes are extremely effective in mixing the working 
fluid in the bounded volume. Although equally true in open flow systems, 
one should be reminded that synthetic jet actuators in closed systems do 
not need any complex piping to function and do not inject any fluid into 
the system. This is not to mention the low energy requirements and the 
fact that conventional jets are, by their very nature, unusable in bounded 
volume situations due to the necessity of fluid injection. 
By use of a synthetic jet actuator in a closed volume, control of mixing 
can be enhanced through small-scale vorticity manipulation and reduction 
of contamination will be effected through use of a jet which injects no 
foreign matter into the environment. The fluid flow created by the jet 
will greatly aid mixing of the fluids in the chamber without injecting any 
new matter into the chamber. Furthermore, use of a synthetic jet actuator 
will enable greater control of the mixing due to excitation at the 
Kolmogorov scale. 
Other features and advantages will become apparent to one with skill in the 
art upon examination of the following drawings and detailed description. 
All such additional features and advantages are intended to be included 
herein within the scope of the present invention, as is defined by the 
claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It will be obvious to those skilled in the art that many modifications and 
variations may be made to the preferred embodiments of the present 
invention as described hereafter without substantially departing from the 
spirit and scope of the present invention. All such modifications and 
variations are intended to be included herein within the scope of the 
present invention, as is set forth in the appended claims. 
As with the brief description above, the construction and operation of 
various synthetic jet actuators will first be described. These jets serve 
as the "hardware" for the present invention and are described in greater 
detail in patent application Ser. No. 08/489,940. After discussing these 
devices, generally, several preferred embodiments of mixing apparatuses 
using synthetic jet actuators will be discussed. 
I. Synthetic Jet Actuator Hardware 
A. First Preferred Embodiment 
FIG. 1A depicts a synthetic jet actuator 10 comprising a housing 11 
defining and enclosing an internal chamber 14. The housing 11 and chamber 
14 can take virtually any geometric configuration, but for purposes of 
discussion and understanding, the housing 11 is shown in cross-section in 
FIG. 1A to have a rigid side wall 12, a rigid front wall 13, and a rear 
diaphragm 18 that is flexible to an extent to permit movement of the 
diaphragm 18 inwardly and outwardly relative to the chamber 14. The front 
wall 13 has an orifice 16 of any geometric shape. The orifice 
diametrically opposes the rear diaphragm 18 and connects the internal 
chamber 14 to an external environment having ambient fluid 39. 
The flexible diaphragm 18 may be controlled to move by any suitable control 
system 24. For example, the diaphragm 18 may be equipped with a metal 
layer, and a metal electrode may be disposed adjacent to but spaced from 
the metal layer so that the diaphragm 18 can be moved via an electrical 
bias imposed between the electrode and the metal layer. Moreover, the 
generation of the electrical bias can be controlled by any suitable 
device, for example but not limited to, a computer, logic processor, or 
signal generator. The control system 24 can cause the diaphragm 18 to move 
periodically, or modulate in time-harmonic motion, and force fluid in and 
out of the orifice 16. 
Alternatively, a piezoelectric actuator could be attached to the diaphragm 
18. The control system would, in that case, cause the piezoelectric 
actuator to vibrate and thereby move the diaphragm 18 in time-harmonic 
motion. The method of causing the diaphragm 18 to modulate is not limited 
by the present invention. 
The operation of the synthetic jet actuator 10 will now be described with 
reference to FIGS. 1B and 1C. FIG. 1B depicts the synthetic jet actuator 
10 as the diaphragm 18 is controlled to move inward into the chamber 14, 
as depicted by arrow 26. The chamber 14 has its volume decreased and fluid 
is ejected through the orifice 16. As the fluid exits the chamber 14 
through the orifice 16, the flow separates at sharp orifice edges 30 and 
creates vortex sheets 32 which roll into vortices 34 and begin to move 
away from the orifice edges 30 in the direction indicated by arrow 36. 
FIG. 1C depicts the synthetic jet actuator 10 as the diaphragm 18 is 
controlled to move outward with respect to the chamber 14, as depicted by 
arrow 38. The chamber 14 has its volume increased and ambient fluid 39 
rushes into the chamber 14 as depicted by the set of arrows 40. The 
diaphragm 18 is controlled by the control system 24 so that when the 
diaphragm 18 moves away from the chamber 14, the vortices 34 are already 
removed from the orifice edges 30 and thus are not affected by the ambient 
fluid 39 being drawn into the chamber 14. Meanwhile, a jet of ambient 
fluid 39 is synthesized by the vortices 34 creating strong entrainment of 
ambient fluid drawn from large distances away from the orifice 16. 
B. Modification of the First Preferred Embodiment 
Synthetic Jets with Louvers 
In the first preferred embodiment, the synthetic jet actuator 10 had a 
flexible diaphragm 18 for forcing fluid into and out of a chamber 14. The 
flexible diaphragm 18 is described as being controlled by a control system 
24 which may comprise, inter alia, a processor or logic circuitry. The 
synthetic jet actuator, however, is not limited to the use of just a 
flexible diaphragm. For instance, in some applications, a moveable piston 
head may be preferred. In these applications, the piston head would be 
positioned within the chamber 14 opposite the orifice 16 and would be 
moved by any suitable mechanism, such as a piston rod, so as to 
reciprocate within the chamber 14. 
As opposed to the flexible diaphragm 18, the piston head would be able to 
move a larger mass of fluid and thus be able to produce fluid flows having 
larger momentums. With these stronger fluid flows, the synthetic jet 
actuator 10 in turn may operate more effectively in vectoring primary 
fluid flows, in altering aerodynamic surfaces, in promoting mixing of 
fluids, and in aiding heat transfer to or from a fluid. The use of a 
piston rather than the flexible diaphragm 18 will have other advantages 
and benefits which will be apparent to those skilled in the art. 
A synthetic jet actuator, such as actuator 10 shown in FIGS. 1A to 1C, can 
be modified to operate more efficiently at very high speeds. At a very 
high speed, after the fluid is forced out of the chamber 14 through the 
orifice 16, the diaphragm 18 or piston then quickly begins to move away 
from the orifice 16 and attempts to draw fluid back into the chamber 14. A 
limitation on the withdrawal of fluid back into the chamber 14 can 
decrease the force of the jet 36 and the effectiveness of the jet actuator 
10. Furthermore, if the fluid is compressible, the quick retraction of the 
flexible diaphragm 18 creates a vacuum within the chamber 14. As a result, 
the fluid that is drawn into the chamber 14 has less mass than that 
previously forced out of the chamber 14 and the subsequent jet 36 will, 
consequently, have less momentum. The inability to force an adequate mass 
of fluid into the chamber 14 therefore decreases the effectiveness of the 
jet actuator 10. 
A synthetic jet actuator 200 which can effectively operate at high speeds 
is shown in FIGS. 4A and 4B and comprises a housing 202 defining an 
interior chamber 206. The housing 202 has an upper wall 204 with an 
orifice 209 and at least one louver 205. In the preferred embodiment, the 
jet actuator 200 preferably has a plurality of louvers 205. Only two 
louvers 205 have been shown in the figures in order to simplify the 
description. The synthetic jet actuator 200 also comprises a piston head 
208 for reciprocating toward and away from the orifice 209 at a prescribed 
rate and stroke distance. The invention is not limited to any particular 
stroke distance or rate whereby the rate and stroke distance may be 
adjusted according to the particular needs of an application. 
FIG. 4A illustrates the jet actuator 200 at a time when the piston 208 is 
moving toward the orifice 209. As shown in the figure, the louvers 205 are 
in a closed position whereby a fluid flow 217 is forced out only through 
the orifice 209. The jet 217 produced by the jet actuator 200 is similar 
to the jet 36 produced by the jet actuator 10 and produces vortex sheets 
which roll into vortices and move away from the orifice 209. 
With reference to FIG. 4B, the louvers 205 open during the time that the 
piston 208 moves away from the orifice 209. With the louvers 205 opened, 
fluid may enter the chamber 206 not only through the orifice 209 in flow 
211 but also through the openings adjacent to the louvers 205 in flows 
212. These additional fluid flows 212 substantially increase the surface 
area by which fluid may enter the jet actuator 200 and enable the jet 
actuator 200 to force a sufficient amount of fluid into the chamber 206 
while the piston 208 moves away from the orifice 209. Since the jet 
actuator 200 is able to intake sufficient amounts of fluid within the 
chamber 206, the jet actuator 200 is able to maintain the momentum of the 
fluid flow 217 in subsequent strokes of the piston 208. 
In some applications, a fluid flow with larger momentum 211 into the 
chamber 206 of the jet actuator 200 and a smaller flow out of the orifice 
209 may be desirable. FIGS. 5A and 5B illustrate a jet actuator 200' which 
has a plurality of louvers 205' which become opened while the piston 208 
moves toward the orifice 209 and become closed while the piston 208 moves 
away from the orifice 209. As a result, during the down stroke of the 
piston 208, as shown in FIG. 5A, a large fluid flow 211' is forced through 
the orifice 209. During the up stroke of the piston 208, on the other 
hand, the louvers 205' become opened and fluid is permitted to exit the 
chamber 206 not only through orifice 209 in flow 217' but also through the 
openings adjacent louvers 205' in flows 219. Since the fluid has a greater 
surface area in which to exit the chamber 206, the momentum of the flow 
217' is substantially decreased. 
As should be apparent from FIGS. 4A, 4B, 5A, and 5B, the amount of fluid 
that is drawn into the chamber 206 or which is forced out of the chamber 
206 may be altered by using one or more louvers. With the jet actuator 
200, the louvers 205 increase the amount of fluid that enters the chamber 
206 while the louvers 205' in jet actuator 200' decrease the momentum of 
the jet 217' exiting the orifice 209. By adjusting the size and number of 
the openings covered by the plurality of louvers, the flow rates in and 
out of the chamber 206 may be altered. 
1. Alternate Types of Louvers 
The louvers in a synthetic jet actuator are one-way valves that permit 
fluid flow in one direction but which block flow in the opposite 
direction. As shown above in synthetic jet actuators 200 and 200', the 
louvers can permit fluid flow either into the chamber 206 or out of the 
chamber 206. The invention can be implemented with any suitable type of 
louver, such as either an active louver or a passive louver. A passive 
louver is simply a flap or valve which is hinged so as to open with fluid 
flow in one direction and which closes tight against the housing 202 of 
the jet actuator with fluid flow in the opposite direction. 
An active louver, such as louver 230 shown in FIGS. 7A and 7B, becomes 
opened or closed with the assistance of a force other than just the force 
of a fluid flow. In the example shown in FIGS. 7A and 7B, this other force 
may be generated by a piezoelectric material 232. With reference to FIG. 
7A, when the louver 230 is in a closed state, a semi-rigid member 234 is 
in intimate contact with wall 204 of the synthetic jet actuator. The 
semi-rigid member 234 preferably overlaps a portion of the wall 204 so 
that the louver 230 remains in a closed state even when a fluid flow 236 
contacts the louver 230. As is known to those skilled in the art, the 
piezoelectric material 232 will deflect upon the application of an 
electrical signal. Thus, an electrical signal can be applied to the 
piezoelectric material 232 from a signal generator 239 to cause the 
piezoelectric material to deflect down to an open state shown in FIG. 7B. 
In the open state, a fluid flow 238 is permitted to travel through an 
opening 235 and exit the chamber or, as depicted in this example, enter 
the chamber. The exact manner in which an electrical signal is applied to 
the piezoelectric material 232 is known to those skilled in the art and, 
accordingly, has been omitted from the drawings in order to simplify the 
description of the invention. 
2. Louvered Jet Actuator as a Pump 
With reference to FIGS. 8A and 8B, a synthetic jet actuator 240 according 
to the invention may also operate as a pump transferring fluid from one 
side of a barrier 252 to the opposite side of the barrier 252. The jet 
actuator 240 comprises a housing 242 defining an interior chamber 249 and 
has a piston 248 reciprocating within the chamber 249. While the piston 
248 is moving in direction 251, as shown in FIG. 8A, louvers 243a and 243b 
are open and permit fluid flows 246a and 246b to enter through openings 
245a and 245b into the chamber 249. As the piston 248 moves in direction 
253, the louvers 243a and 243b become closed and louver 243c opens, 
thereby permitting a fluid flow 246c to exit through opening 245c in wall 
244. The reciprocation of the piston 248 within the actuator 240 therefore 
pumps fluid from one side of the barrier 252 to the opposite side of 
barrier 252. 
C. Second Preferred Embodiment 
Constant Suction Synthetic Jet Actuator 
The preferred embodiment for a constant suction synthetic jet actuator 511, 
a further improvement on the class of synthetic jet actuators, is depicted 
in FIG. 10. A constant suction synthetic jet 511 is particularly useful 
for the application of embedding a synthetic jet actuator into a solid 
body 512. 
The preferred embodiment 511 is comprised of an outer cylindrical section 
514 made similar to a pipe, and an inner cylindrical section 516. Although 
not limited to such an embodiment, the outer cylindrical section 514 and 
the inner cylindrical section 516 as depicted in FIG. 10 are concentric 
and approximately perpendicular to the outer surface 513 of the solid body 
512. Additionally, the outer cylindrical section 514 is embedded into the 
solid body 512 such that an upper rim 515 of the outer section is 
contiguous with the outer surface 513. By contrast, the inner cylindrical 
section 516 has an upper rim 531 which is some small distance below the 
outer surface 513 of the solid body 512. The particular diameters given to 
the outer cylindrical section 514 and the inner cylindrical section 516 
are not critical to the present invention. 
The outer cylindrical section 514 should preferably be connected by fluidic 
piping 521 to a fluid source 522. Along the path of the fluidic piping 521 
is a valve 523 which permits control of the fluid flow through the fluidic 
piping 521. The present invention, however, is not intended to be limited 
to the use of a valve 523 only. Any equivalent mechanism for stopping and 
restarting the flow of fluid would also function adequately and is 
included in the present invention. 
In operation, the valve 523 should preferably alternately stop and then 
release fluid through the fluidic piping 521 and into the outer cylinder 
514. This "on-off" operation is controlled by a suitable control system 
524, such as a microcomputer or other logic device. The frequency at which 
the control system 524 causes the gate valve 523 to operate should 
preferably be adjustable in order to control effectively the operation of 
the synthetic jet actuator. A computer control system would easily provide 
this level of control. 
The inner cylindrical section 516 is preferably connected by fluidic piping 
517 to a suction mechanism 518. Such a suction mechanism 518 may comprise 
a vacuum, a pump, or any other appropriate mechanism for providing a 
constant suction. As indicated by the name of this preferred embodiment, 
the suction mechanism 518 operates constantly during operation of the 
synthetic jet actuator 511 and the removed fluid can be pumped back into 
the blowing section. 
Therefore, in operation, the suction mechanism 518 creates a constant 
suction on an ambient fluid 524 above the outer surface 513 of the solid 
body 512. This action creates a constant flow of the ambient fluid 525 
into the inner cylindrical section 516 and through the fluidic piping 517. 
The operation of the constant suction synthetic jet 511 when ambient fluid 
525 is being pulled into the inner cylindrical section 516 is depicted in 
FIG. 11A. In FIG. 11A, the gate valve 523 is closed such that no fluid is 
ejected through the outer cylindrical section 514. This particular mode of 
operation is very much like the synthetic jet actuator 10 of FIGS. 1A-1C 
when the diaphragm or piston withdraws from the housing 11, thereby 
increasing the volume of the chamber 14. 
FIG. 11B depicts a mode of operation of the constant suction synthetic jet 
511 when the gate valve 523 is opened and fluid flows out through the 
outer cylindrical section 514. As the fluid goes by the upper rim 515 of 
the outer cylindrical section 514, vortices 526, 527 are formed, roll up, 
and move away. Vortices 532, 533, as depicted in FIG. 32B, have already 
moved a small distance away from the outer surface 513 of the body 512. 
The vortices 526, 527, 532, 533 entrain ambient fluid 525, as depicted by 
arrows 528a-528d. Thus, a synthetic jet actuator of fluid 529 is formed 
approximately normal to the outer surface 513 and moves away from the 
solid body 512. 
Since the upper rim 531 of the inner cylindrical section 516 is slightly 
below the outer surface 513 of the solid body 512, as fluid is ejected 
from the outer cylindrical section 514, some of the fluid will be pulled 
around the upper rim 531 of the inner cylinder 516 and into the fluidic 
piping 517, as depicted in FIG. 11B. However, because this occurs below 
the outer surface 513 of the solid body 512, the formation of the vortices 
526, 527 and the resulting fluid jet 529 is not affected by the constant 
suction. 
The constant suction synthetic jet actuator 511 alternates between the mode 
of operation depicted in FIG. 11A and the mode of operation depicted in 
FIG. 11B. However, as descried above with regard to the synthetic jet 
actuator 10 depicted in FIGS. 1A-1C, a constant jet of fluid 529 is formed 
above the opening in the outer surface 513 of the solid body 512. 
If it is desired, the fluid source 522 for the outer cylindrical section 
514 can be a storage container into which fluid from the ambient air 525 
is deposited after being drawn through the inner cylinder 516 by the 
suction mechanism 518. In this way, zero net mass is injected into the 
system. This feature may be desirable in some applications. However, the 
present invention is not limited to such a configuration. Where it does 
not matter whether any mass is ejected into the system, the source of 
fluid 522 for the outer cylinder 514 can be any fluidic chamber or 
environment. In this way, the net mass flow into or out of the synthetic 
jet actuator of this preferred embodiment can be tailored for the specific 
application. 
II. Mixing Applications of Synthetic Jet Actuators 
The devices capable of forming synthetic jets, as described above, have 
certain novel applications as improved mixing devices. The present 
invention involves these mixing devices, which are described in details as 
follows: 
A. Mixing a Free Flow with an Ambient Fluid 
The preferred embodiment for using a synthetic jet actuator 10 to mix a 
primary jet of fluid 113 with ambient fluid 114 through direct small scale 
vorticity manipulation is pictured in FIG. 2, denoted by reference numeral 
115. Although any means of high frequency excitation would yield similar 
results, use of a synthetic jet actuator 10 is depicted here. The 
synthetic jet actuator exhibits several inherent advantages over other 
mechanisms causing high frequency excitation. For example, a synthetic jet 
actuator is non-intrusive and causes no net mass change in the system. 
It should be noted, however, that high frequency excitation can be affected 
by a number of actuators including fluidic actuators, synthetic jets, or 
piezoelectric actuators where the scale of the perturbations induced by 
the actuator is adjusted to be within the dissipation range of the flow. 
Conventional pulsed jets, moving flaps, and electromechanical actuators 
(such as speakers) can also be used to introduce the high frequency, high 
amplitude disturbances required locally. Ultrasonic devices can introduce 
even higher frequencies. Magnethohydrodynamic actuation can be used to 
nonintrusively introduce dissipative motions globally throughout the flow. 
It should be noted that with any of these techniques, it is possible to 
operate at the Kolmogorov scale, the Batchelor scale, or even smaller. 
In the preferred embodiment shown in FIG. 2, a square primary jet 116 is 
shown with a synthetic jet actuator 10 situated such that the flow 
direction of the jet actuator 10 (depicted by arrow 36) is perpendicular 
to the flow direction (depicted by arrow 113) of the primary jet 116. As 
the primary jet flow 113 passes out of the primary jet housing 117, a 
shear layer 118 is formed between the high speed jet fluid 113 and the 
ambient fluid 114. The hierarchy of vortical structures is formed within 
the shear layer between the jet and the ambient fluid. The synthetic jet 
actuator 10 is then activated and operated by vibration of the synthetic 
jet actuator diaphragm 18 at a very high frequency. Such high frequency 
excitation increases the dissipation of the primary jet turbulent energy 
and scalar mixing at a faster rate than if no synthetic jet actuator 10 
were used. 
It is also possible to effectively mix a fluid flow with an ambient fluid 
by a "parallel arrangement." That is, the synthetic jet actuator may be 
positioned such that the synthetic jet stream and the primary jet stream 
are parallel. The synthetic jet actuator, in such a configuration, is 
attached to a housing wall of a primary jet actuator. An orifice of the 
synthetic jet actuator is placed in the exhaust plane of the primary jet 
actuator. When both are operated at the same time, effective, controllable 
mixing will occur. 
In addition, mixing may be accomplished with a synthetic jet actuator of 
the first preferred embodiment having louvers or a synthetic jet actuator 
of the second preferred embodiment, the concentric cylinder configuration. 
The present invention is not limited by the particular configuration of 
the synthetic jet actuator. 
B. Mixing Fluids in a Bounded Volume 
The preferred embodiment of a closed volume mixing apparatus 124 is 
pictured in FIG. 3. The mixing apparatus 124 comprises a rectangularly 
cubic housing 125 defining a sealed chamber 126. The chamber 126 is 
defined in the two-dimensional depiction of FIG. 3 by a lower housing wall 
128, an upper housing wall 127, and a right housing wall 129. The final 
end of the mixing apparatus chamber 126 is comprised of the front wall 13 
of a synthetic jet actuator 10 (as depicted in FIG. 1A) with the orifice 
16 of the actuator 10 facing the inside the mixing apparatus chamber 126. 
One wall of the synthetic jet actuator housing 12 comprises a flexible 
material comprising a diaphragm 18. The diaphragm 18 is caused to vibrate 
at a high frequency by a control system 24. When the diaphragm 18 is 
excited by the control system 24, a synthetic jet stream 36 is produced in 
the chamber 126 originating in the region around the orifice 16 in the 
synthetic jet actuator housing 12 and propagating in a direction away from 
the housing wall constituting the diaphragm 18. The vortices within this 
turbulent flow 36, aid the mixing of fluid in the chamber 126. 
As with mixing of free fluid flows, described above, the preferred 
embodiment of a bounded volume mixing apparatus may incorporate other 
embodiments of synthetic jet actuators without departing from the 
principles of the present invention. Particularly, bounded volume mixing 
may be accomplished with a synthetic jet actuator of the first preferred 
embodiment having louvers or a synthetic jet actuator of the second 
preferred embodiment, the concentric cylinder configuration. The present 
invention is not limited by the particular configuration of the synthetic 
jet actuator. 
C. Louvered Mixing Apparatus 
In addition to the potential use of a louvered synthetic jet actuator in 
the mixing embodiments described above, FIGS. 6A-6B illustrate the 
preferred embodiment of a synthetic jet actuator 220 operating as a mixing 
apparatus. The synthetic jet 220, as with jets 200 and 200', comprise a 
piston 208 that reciprocates in directions 213 and 215 in order to 
alternately force fluid into and out of chamber 206. While the piston 208 
moves in direction 213, louvers 225 open and permit fluid from region B to 
travel through passage 222a and enter into the chamber 206 as a flow 223a 
and for fluid from region C to travel through passage 223b and enter into 
the chamber 206 as a flow 223b. Also at this time, a fluid flow 224 from 
region A is drawn in through the orifice 209 into the chamber 206 and 
becomes mixed with the fluid flows 223a and 223b from regions B and C, 
respectively. While the piston 208 moves in direction 215, the louvers 225 
are closed and fluid in chamber 206 is forced out of the orifice 209 in a 
flow 227. The flow 227 is a fairly strong flow and is defined by vortices 
which further promote mixing of the fluids from regions A, B, and C. 
With the synthetic jet actuator 220, the fluids from regions B and C may be 
the same fluid and, furthermore, may be different than the fluid within 
region A. Thus, during operation, the jet actuator 220 will receive fluid 
flows 224, 223a, 223b from regions A, B, and C, respectively, and mix 
these fluids within the chamber 206. The invention, however, is not 
limited to having only two different types of fluids and may have a 
greater number of different fluids flowing through individual passages 
into the chamber 206. Alternatively, the fluids from regions B and C may 
be the same type of fluid and will be mixed together with the fluid in 
region A. 
In addition to varying the number of different types of fluids, the jet 
actuator 220 may also vary the flow rates from the various regions. For 
instance, in the example shown in FIGS. 6A and 6B, the surface areas of 
openings 228a and 228b can be adjusted relative to each other and relative 
to the surface area of the orifice 209 in order to selectively control the 
rates of fluid flows 223a, 223b, and 224. Because the flow rates can be 
adjusted, the jet actuator 220 can produce mixtures of fluids having a 
range of concentrations. For example, by increasing the surface area of 
opening 228 relative to opening 228b, the mixture within the chamber 206 
will have a larger amount of fluid, and thus a higher concentration, of 
fluid from region B than from region C. The synthetic jet actuator 220 
would be ideal for applications, such as an automotive fuel injector, 
where two or more fluids need to be accurately mixed. 
The louvers according to the invention are not limited to use solely within 
a synthetic jet actuator but may be embodied in other types of jets or 
apparatuses. For instance, as shown in FIG. 9, a jet apparatus 260 
comprises a housing 262 defining an interior chamber 261. The chamber 261 
initially contains an oxidizer, such as ambient air, which is subsequently 
mixed with a combustible fuel, such as gasoline. The fuel is added to the 
chamber 261 by opening louvers 265a and 265b to permit the fuel to flow 
through passages 263 and 264 and through openings 266a and 266b. Once the 
fuel is added to the chamber 261, the louvers 265a and 265b are closed and 
a spark plug 268 introduces a spark into the chamber 261 between its pair 
of contacts 270a and 270b. The spark combusts the fuel and, due to the 
increase in volume of fluid within the chamber 261, causes a jet flow 272 
to exit the chamber 262. Rather than having two passages 263 and 264 for 
the introduction of fuel into the chamber 261, the jet actuator 260 may 
only have one passage or, alternatively, may have one of these passages 
263 or 264 introduce the oxidizing agent. Other variations in the design 
of the jet actuator should be apparent to those skilled in the art. 
It would be apparent to one skilled in the art that many variations and 
modifications may be made to the preferred embodiment as described above 
without substantially departing from the principles of the present 
invention. All such variations and modifications are intended to be 
included herein and are within the scope of the present invention, as set 
forth in the following claims.