AFRL is laying the groundwork for the development of revolutionary hypersonic aerospace vehicles (see Figure 1). Accordingly, AFRL engineers are examining the feasibility of replacing an air vehicle's traditional, mechanically or electrically actuated flight control surfaces (e.g., wing flaps) with plasma actuators that require no moving parts and are therefore potentially less expensive and more reliable. As part of the laboratory's Boundary Layers and Hypersonics program, the engineers conducted a wind tunnel test to evaluate the feasibility of using plasma actuators for airframe flight control.

Figure 1. Hypersonic vehicle concept
For this test, the engineers imposed a high electrical voltage between the test model's metal electrodes to ionize the surrounding air and create plasma—an electrically conductive state of matter characterized by the presence of a significant proportion of ionized molecules. In hypersonic flight, the thermal excitation associated with bow shock compression ionizes the air in a shock layer to create the required flow medium. The purpose of this test was to (1) investigate the effects of plasma discharges on hypersonic boundary layers, and (2) measure the local increase in static pressure on a flat plate subjected to streamwise plasma discharges.

AFRL's Mach 5 plasma channel is a low-density flow channel designed for the study of plasma dynamics and magnetoaerodynamic phenomena (see Figure 2). It consists of a two-dimensional conical nozzle operating at a stagnation temperature of about 280 K, and it relies upon a vacuum system to generate low-density airflows. The research team performed these plasma studies at a stagnation pressure of 370 torr. The engineers fabricated the entire plasma channel from acrylic plastic and assembled it with nylon screws in order to avoid unintentional grounding. The facility includes a test cabin to house a model support and instrumentation probes. The model for this test consisted of a 660 mm long ceramic plate with two copper electrodes embedded in the surface 317 mm apart and three pressure taps mounted on the centerline between the electrodes.1

Figure 2. Schematic of AFRL’s Mach 5 plasma channel
Figure 3. Model during a test, with a high-voltage
With the model at a 0° angle of attack, the team generated direct current electrical discharges between the model's two electrodes (see Figure 3). The discharge occurred in the streamwise direction, with the upstream electrode being the cathode. As a result of the discharge, plate surface pressure increased immediately downstream of the cathode. For a 50 mA (roughly 60 W) discharge, the pressure increased approximately 10%, and in the absence of a magnetic field, the pressure increment was nearly linear with applied power. Researchers consider the mechanism for the pressure increase to be boundary layer heating.

The Boundary Layers and Hypersonics program is focused on developing the knowledge of fluid physics needed to facilitate future revolutionary aerospace vehicle designs. The program strives to characterize, predict, and control high-speed fluid dynamics events, including boundary layer transition; shock/boundary layer and shock/shock interactions; and other airframe propulsion integration phenomena, such as real-gas effects, plasma aerodynamics, magnetohydrodynamics, and high-speed flow heat transfer. This test demonstrated that air diverted by plasma heating could successfully exert force on an aerodynamic surface in a hypersonic environment and, therefore, that the plasma actuator concept is a viable area for further study and development.

Ms. Melissa Withrow (Azimuth Corporation), formerly of the Air Force Research Laboratory's Air Vehicles Directorate, wrote this article. For more information, visit http://www.afrl.af.mil/techconn_index.asp. Reference document VA-H-06-05.

Reference

1 Kimmel, R. L., et al. "Effect of Surface Plasma Discharges on Boundary Layers at Mach 5." AIAA 2004-509, 42nd AIAA Aerospace Sciences Meeting and Exhibit, Jan 04.


Air Force Research Laboratory Technology Horizons Magazine

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

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