An effective surface flow controller for the manipulation of the properties of aerodynamic boundary layers, particularly the resulting body drag and airflow attachment, has been long sought after. Many ideas have been tested both in the laboratory and on aircraft, such as surface suction and blowing, riblets, magnetohydrodynamics (MHD), corona-induced ion wind, crossed microwave beam focusing and various micro-electronic mechanical structures (MEMS). The goal has always been a surface airflow controller that is able to manipulate the aerodynamic boundary layer while saving fuel (reducing power consumption) and improving the aircraft performance (reducing drag and delaying airflow separation).
The classical approach to aircraft aerodynamics is to create a fixed shape, or one nearly so, that can sustain flight over a wide range of flight conditions. The key limitation is that the aircraft aerodynamic properties are fixed before the aircraft leaves the ground and cannot be changed to adapt to varying conditions. With the addition of a surface flow controller, the aircraft could have both a fixed flight performance, due to its physical shape and a dynamic flight performance capability that is adaptive to the flight environment.
The ideal surface flow controller would modify the physical flight characteristics of the aircraft by adding a virtual flight characteristic that would arise from the skin of the aircraft only when required. An optimal surface flow controller would have no moving parts, and be low weight, highly power efficient, controlled electrically, and robust to environmental interactions, such as impacts and chemical stresses. Electrically generated and controlled plasma has the potential to be used as a flow controller embodying these characteristics and to lead to a practical means of electrohydrodynamic (EHD) shaping of the net aerodynamic flight performance of the aircraft. Figure 1, provided by our collaborators at the University of Tennessee, shows an effect that an OAUGDP® plasma actuator can have on a flow.
to the oncoming free-stream. Left - plasma OFF, Right - plasma ON.
Surface Plasma Actuators - AGT's Solution
AGT's atmospheric plasma produces charged reactive species that can be manipulated by the electric field used to generate them to induce a wide variety of aerodynamic boundary layer modifications. Previous academic efforts involving plasma aerodynamics have demonstrated the effectiveness of similar actuators for inducing vortices, increasing or reducing aerodynamic drag and thrust, and the reattachment of separated airflows.
AGT is currently funded to do research that will systematically optimize the effects of the OAUGDP® based airflow controller, specifically, the surface velocity and induced force, while minimizing the actuator's power consumption. The experimental data is being studied and will be used to develop a mathematical model suitable for engineering analysis.
The resulting flow control surfaces would be capable of enhancing the external flight characteristics of future munitions by increasing range via reductions in drag, increasing attack angles, preventing flow separation and stall, and providing electromagnetic steering. Similar surface airflow controllers could be used to modify the internal airflow dynamics of aircraft engines and inlets.
Plasma Aerodynamic Background
Atmospheric plasma is a partially ionized gas that has a tendency to remain overall electrically neutral over large volumes due to the atmospheric pressure induced collision frequency. There are two primary types of plasma: thermal and non-thermal. In thermal plasma such as an electrical arc, the kinetic energy of the charged particles and the background gas are roughly equivalent. A considerable amount of energy/heat must be added to maintain the plasma. In contrast, the electrons in non-thermal discharge plasmas such as in dielectric barrier discharges (filamentary discharge) or the OAUGDP® (uniform or diffuse glow discharge) have a higher kinetic energy than that of the ions and the background gas molecules.
For an airflow controller, the goal is to selectively transfer the input electrical energy to the electrons in order to generate charged ions through collisions, thus promoting the momentum exchange from the electric field accelerated ions to the airflow. This momentum exchange can be accomplished at a fraction of the energy required for a thermal plasma system.
When a non-thermal plasma is formed by electrically driven electrodes located on a surface, there are three theorized forms of EHD flow control that result in momentum exchange to the airflow. The first originates from an electrostatic body force that is created by the formation of the plasma, the second is due to an imposed DC field and subsequent DC mobility drift, and the last is due to electrical phasing of the generation of the surface plasma. The first and third have been the basis of most EHD plasma aerodynamics to date. Due to the exceptionally high applied voltage and real current required, the DC field acceleration technique has not been examined.
The electrostatic body force created during the plasma formation is unidirectional away from the electrode on the surface where the plasma is present. This is rather unusual given that the plasma is formed by a radio frequency, alternating voltage and thus one might assume that the net motion of the charged active species comprising the plasma discharge would average to zero. While theoretical explanations have been put forward, adequate explanation for origination of this electrostatic body force is still being debated. Experimental results thus far, have shown that the momentum exchange is highly dependant on the electric field, and subsequently, the applied voltage, the electrode design and the applied power.
Electrical phasing of the plasma generation, using not one, but several high voltage electrodes, is an attempt to couple the charge flow, not only away from the single generating electrode, local charge acceleration, but to the next electrode as well, global charge acceleration. Though research, by our colleagues at the University of Tennessee, have demonstrated this effect, the degree to which global charge coupling can be accomplished has not been fully realized. This approach brings significant advantages and is a part of ongoing research here at AGT.
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