Plasma actuator

Plasma actuators are a type of actuator currently being developed for active aerodynamic flow control. Plasma actuators impart force in a similar way to ionocraft. Plasma flow control has drawn considerable attention and been used in boundary layer acceleration, airfoil separation control, forebody separation control, turbine blade separation control, axial compressor stability extension, heat transfer and high-speed jet control. Dielectric barrier discharge (DBD) plasma actuators are widely utilized in airflow control applications. DBD is a type of electrical discharge commonly used in various electrohydrodynamic (EHD) applications. In DBDs, the emitter electrode is connected to a high-voltage source and exposed to the surrounding air, while the collector electrode is grounded and encapsulated within the dielectric material (see figure). When activated, they form a low-temperature plasma between the electrodes by application of a high-voltage AC signal across the electrodes. Consequently, air molecules from the air surrounding the emitter electrode are ionized, and are accelerated towards the counter electrode through the electric field.

Introduction Plasma actuators operating at the atmospheric conditions are promising for flow control, mainly for their physical properties, such as the induced body force by a strong electric field and the generation of heat during an electric arc, and the simplicity of their constructions and placements. In particular, the recent invention of glow discharge plasma actuators by Roth (2003) that can produce sufficient quantities of glow discharge plasma in the atmosphere pressure air helps to yield an increase in flow control performance.

Power supply and electrode layouts Either a direct current (DC), an alternating current (AC) power supply, or a microwave microdischarge can be used for different configurations of plasma actuators. One schematic of an AC power supply design for a dielectric barrier discharge plasma actuator is given here as an example. The performance of plasma actuators is determined by dielectric materials and power inputs, later is limited by the qualities of MOSFET or IGBT.

The driving waveforms can be optimized to achieve a better actuation (induced flow speed). However, a sinusoidal waveform may be preferable for the simplicity in power supply construction. The additional benefit is the relatively less electromagnetic interference. Pulse-width modulation can be adopted to instantaneously adjust the strength of actuation.

Manipulation of the encapsulated electrode and distributing the encapsulated electrode throughout the dielectric layer has been shown to alter the performance of the dielectric barrier discharge (DBD) plasma actuator. Locating the initial encapsulated electrode closer to the dielectric surface results in induced velocities higher than the baseline case for a given voltage. In addition, Actuators with a shallow initial electrode are able to more efficiently impart momentum and mechanical power into the flow. No matter how much funding has been invested and the number of various private claims of a high induced speed, the maximum, average speed induced by plasma actuators on an atmospheric pressure conviction, without any assistant of mechanical amplifier (chamber, cavity etc.), is still less than 10 m/s.

Influence of temperature The surface temperature plays an important role in limiting the usefulness of a dielectric barrier discharge plasma actuator. The thrust produced by an actuator in quiescent air increases with a power law of the applied voltage. For voltages greater than a threshold, the exponent of the power-law reduces limiting the thrust increase, and the actuator is said to have "saturated," limiting the actuator's performance. The onset of saturation can visually be correlated by the inception of filamentary discharge events. The saturation effect can be manipulated by changing the local surface temperature of the dielectric. Also, when dealing with real-life aircraft equipped with plasma actuators, it is important to consider the effect of temperature. The temperature variations encountered during a flight envelope may have adverse effects in actuator performance. It is found that for a constant peak-to-peak voltage the maximum velocity produced by the actuator depends directly on the dielectric surface temperature. The findings suggest that by changing the actuator temperature the performance can be maintained or even altered at different environmental conditions. Increasing dielectric surface temperature can increase the plasma actuator performance by increasing the momentum flux whilst consuming slightly higher energy.

Influence of rain Although plasma actuators have been extensively characterized for their performance as flow control devices, the notion that they might fail under adverse conditions such as dew, drizzle or dust makes them less popular in practical applications. Earlier publications have shown the effect of moisture, water adhesion, and even icing. A recent publication has simulated light rain by directly spraying water droplets on to a working plasma actuator and showed its effect on thrust recovery as the performance metric. It was shown that wet actuators quickly recover plasma glow, and gradually regain thrust comparable to the dry actuator.

Flow control applications Some recent applications of plasma actuation include high-speed flow control using localized arc filament plasma actuators, and low-speed flow control using dielectric barrier discharges for flow separation, replacing mechanical high-lift devices, 3D wake control, sound control, andsliding discharges. The present research of plasma actuators is mainly focused on three directions: (1) various designs of plasma actuators; (2) flow control applications; and (3) control-oriented modeling of flow applications under plasma actuation. In addition, new experimental and numerical methods are being developed to provide physical insights.

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