Modification of fluid flow about bodies and surfaces through...

Details Modification of fluid flow about bodies and surfaces through... Modification of fluid flow about bodies and surfaces through...

2002-03-08

2003-11-11

Poon, Peter M (Department: 3643)

Aeronautics and astronautics

Aircraft sustentation

Sustaining airfoils

C244S199100, C244S200000

Reexamination Certificate

active

06644598

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to the control of fluid flow about solid surfaces and, more particularly, to a synthetic fluid actuator embedded in a solid surface downstream from an obstruction on the solid surface such as to emit a synthetic jet stream out of the surface and modify the characteristics of fluid flowing over and about the surface.
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: lift and drag of aerodynamic surfaces, flow reattachment to wings, and aircraft stall 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. S
191 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 primarily with the relationship between the impedance of the orifice and the eddies (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.
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.
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.
Much of the recent work on flow control techniques with the objective of extending the post stall flight envelope of various airfoil configurations has focused on the manipulation of flow separation at moderate and large angles of attack either at the leading edge or over flaps (e.g., Seifert et al.,
Oscillatory Blowing: A Tool to Delay Boundary
-
Layer Separation,
AIAA, 1993). This has been typically accomplished by exploiting the instability of the separating shear layer and its receptivity to time-periodic actuation (e.g., pulsed blowing) on the time scale of the flow about the airfoil, which results in a Coanda-like unsteady reattachment. Active control techniques that have achieved varying degrees of separation control by manipulation of the unstable separated free shear layer have included external and internal acoustic excitation (e.g., Ahuja and Burrin,
Control of Flow Separation by Sound,
AIAA, 1984, and Hsiao et al.,
Control of Wall
-
Separated Flow by Internal Acoust

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