Controlling subsonic aerodynamic flow through the use of plasma actuators is an active area of research in both the Air Force (AF) and the general scientific community. A typical plasma actuator consists of two offset electrodes separated by a dielectric material (see Figure 1). Plasma forms as the voltage difference between the electrodes ionizes the surrounding gas. The electric field can then direct the charged particles in the plasma to transfer momentum to the surrounding, neutral (nonionized) air. Most of this momentum transfer occurs as a result of particle collisions. Experiments have demonstrated the ability of plasma actuators to reattach separated airflow at high angles of attack (see Figure 2), as well as to induce flow movement in an initially stationary air mass.1,2,3,4,5

The use of plasma actuators for flow control in AF weapon systems may offer several advantages compared to traditional flow control devices such as slats, flaps, and slots.

  1. Plasma actuators require no moving parts, which implies both lower costs and potential improvements in reliability and manufacturability.6,7,8
  2. Their low profile promises improvements related to weapon system size, weight, and aerodynamic drag.
  3. As electrical devices, plasma actuators are amenable to high-frequency control and therefore demonstrate effectiveness across a wide range of flow instabilities.
  4. Their ability to respond rapidly to varying conditions increases the weapon system’s aerodynamic agility.
  5. For some weapon systems, their use may eliminate control surfaces altogether, which would significantly impact the efficiency of loading, storing, and deploying the units.

Plasma-based flow control devices will also undoubtedly exhibit certain disadvantages with respect to conventional devices. For example, they may not perform well in adverse weather conditions. Furthermore, they require electrical power to operate (although researchers have, in fact, demonstrated the mitigation of these electrical power requirements through a “smart” actuator methodology).9

The ability of plasma actuators to achieve pronounced advantages over traditional flow control approaches in real-world weapon system scenarios remains to be seen. However, in order for scientists to fully explore the potential of such devices, they must first understand the physical phenomena on which they are based. While a number of organizations conducting experimental research in this area have produced significant insight into plasma actuator behavior, they lack similar progress in successfully modeling and simulating these devices. To address this deficiency, AFRL’s Computational Mechanics Branch has initiated efforts to develop high-fidelity computer models reflecting relevant plasma actuator physics.

In particular, it is the physics of the momentum transfer that AFRL scientists are intent on harnessing to control the flow of neutral gases such as air. Calculating the momentum transferred to the neutral gas surrounding a plasma actuator requires an understanding of the associated charged particle density and velocity distribution. Accordingly, the AFRL team is developing a computer code for modeling the density and velocity of the charged particles surrounding the plasma actuator. The momentum transfer caused by the plasma actuator operates on a time scale much shorter than the duration of subsonic particle flow over an airfoil. This inherent time difference allows scientists to integrate the plasma-induced momentum change over time and apply it as an averaged quantity. They can then use this time averaged momentum transfer in creating high-fidelity aerodynamic analysis codes that model the plasma actuator’s effect on the flow of the surrounding gas.

Experimental work has shown that altering the plasma actuator’s electrode geometry and dielectric permittivity, along with its applied voltage waveform, can significantly impact the strength of the resulting plasma actuation.10,11 Preliminary results also indicate that by placing one or more plasma actuators on an aerodynamic surface, designers can also considerably alter control effectiveness. AFRL’s ultimate goal is to use the new analysis codes to investigate plasma actuator design variables, ultimately aiding designers in optimizing plasma actuator flow control devices for use in future AF weapon systems.

Mr. Benjamin J. Case (SENTEL Corporation) and Mr. Steven Ellison, of the Air Force Research Laboratory’s Munitions Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp. Reference document MN-H-05-18.

References

  1. Roth, J. R. “Aerodynamic Flow Acceleration Using Paraelectric and Peristaltic Electrohydrodynamic Effects of a One-Atmosphere Uniform Glow- Discharge Plasma (OAUGDPTM).” Physics of Plasmas, vol 10, no 5 (2003).
  2. Ashpis, D. E. and Hultgren, L. S. “Demonstration of Separation Delay With Glow-Discharge Plasma Actuators.” Technical Paper 2003-1025, AIAA Press, Washington DC (2003).
  3. Corke, T. C. and Post, M. L. “Separation Control on High-Angle-of-Attack Airfoil Using Plasma Actuators.” Technical Paper 2003-1024, AIAA Press, Washington DC (2003).
  4. Enloe, C. L., et al. “Mechanisms and Responses of a Single Dielectric Barrier Discharge.” Technical Paper 2003-1021, AIAA Press, Washington DC (2003).
  5. Roth, J. R., Sherman, D. M., and Wilkinson, S. P. “Electrohydrodynamic Flow Control With a Glow-Discharge Surface Plasma.” AIAA Journal, vol 38, no 7 (2000).
  6. Roth, J. R., Sherman, D. M., and Wilkinson, S. P. “Boundary Layer Flow Control With a One-Atmosphere Uniform Glow-Discharge Surface Plasma.” Technical Paper 98-0328, AIAA Press, Washington DC (1998).
  7. Leger, L., Moreau, E., and Touchard, G. “Electrohydrodynamic Airflow Control Along a Flat Plate by a DC Surface Corona Discharge — Velocity Profile and Wall Pressure Measurements.” Technical Paper 2002-2833, AIAA Press, Washington DC (2002).
  8. Sierakowski, R., Chief Scientist, AFRL Munitions Directorate. Private communication (Sep 03).
  9. Corke, T. C. and Post, M. L. “Separation Control Using Plasma Actuators - Stationary and Oscillating Airfoils.” Technical Paper 2004-0841, AIAA Press, Washington DC (2004).
  10. Corke, T. C., et al. “Mechanisms and Responses of a Single Dielectric Barrier Plasma Actuator: Geometric Effects.” AIAA Journal, vol 42, no 3 (Mar 04).
  11. Corke, T. C., et al. “Mechanisms and Responses of a Single Dielectric Barrier Plasma Actuator: Plasma Morphology.” AIAA Journal, vol 42, no 3 (Mar 04).

Air Force Research Laboratory Technology Horizons Magazine

This article first appeared in the August, 2006 issue of Air Force Research Laboratory Technology Horizons Magazine.

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