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All about subsonic plasma flow control
Surface dielectric barrier discharge was proved effective in subsonic plasma flow control. A great number of papers devoted to subsonic plasma flow control have appeared in the past ten years. The use of dielectric barrier discharge for flow control has been demonstrated in many applications. Examples include boundary layer acceleration, transition delay, lift augmentation on wings, separation control for low-pressure turbine blades, jet mixing enhancement, plasma flaps and slats, leading-edge separation control on wing sections, phased plasma arrays for unsteady flow control, and control of the dynamic stall vortex on oscillating airfoils.
1. Airfoil flow separation control
More than 70% lift force of aircraft is produced by wings. The lift-to-drag ratio and stall characteristic of the wing is of vital importance to the takeoff distance and climbing speed and the flight quality of the aircraft. In order to enhance the manoeuvrability and flexibility of the aircraft, a large angle of attack is used frequently. New technology should be employed in the development of aircraft of the next generation. Active flow control technologies are considered to be the most promising technology in the 21st century.
2. Flow separation control using microsecond and nanosecond discharge
Flow separation control by the microsecond and nanosecond discharge plasma aerodynamic actuation was presented. The control effects influenced by various actuation parameters were investigated.
The airfoil used was a NACA 0015. This shape was chosen because it exhibits well-known and documented steady characteristics as well as leading-edge separation at large angles of attack. The airfoil had a 12 cm chord and a 20 cm span. The airfoil was made of Plexiglas.
Twelve pressure ports were used to obtain the pressure distribution along the model surface. Fig. 1 shows the location of the pressure ports on the model’s surface. Three pairs of plasma aerodynamic actuators were mounted on the suction side of the airfoil.
The actuators were positioned 2% and 20% and 45% cord length of the airfoil. The plasma aerodynamic actuators were made from two 0.018mm thick copper electrodes separated by 1mm thick Kapton film layer. The electrodes were 4mm in width and 120mm in length.
They were arranged just in the asymmetric arrangement. A 1mm recess was molded into the model to secure the actuator flush to the surface. The pressure distribution along the airfoil surface was obtained by a Scanivalve with 96 channels having a range of ±11 KPa.
A pitot-static probe was mounted on the traversing mechanism. This was located at different positions downstream of the airfoil, on its spanwise centerline. Discrete points were sampled across the wake to determine the mean-velocity profile. The uncertainty of the measurement was calculated to be less than 1.5%.
The power supply used for microsecond discharge is 0-40 kV and 6-40 kHz, respectively. The output voltage and the frequency range of the power supply used for nanosecond discharge are 5-80 kV and 0.1-2 kHz, respectively. The rise time and full width half maximum (FWHM) are 190ns and 450ns, respectively.
The plasma aerodynamic actuation strength, which is related to the discharge voltage, is an important parameter in plasma flow control experiments. The flow control effects influenced by discharge voltage were investigated. Flow separates at the leading edge of the airfoil without discharge.
The pressure distribution has a plateau from leading edge to trailing edge which corresponds to global separation from the leading edge. When the microsecond discharge voltage is 13 kV and 14 kV, the flow separation can not be suppressed. As the microsecond discharge voltage increases to 15 kV, the actuation intensity increases and the flow separation is suppressed.
There is a 34.0% lift force increase and a 25.3% drag force decrease when the discharge voltage is 15 kV. When the millisecond discharge voltage increases to 16 kV, there is a 35.1% lift force increase and a 25.5% drag force decrease. The control effects for discharge voltage of 15 kV and 16 kV are approximately the same.
Thus, a threshold voltage exists for plasma aerodynamic actuation of different time scale. The flow separation can’t be suppressed if the discharge voltage is less than the threshold voltage. When the flow separation is suppressed, the lift and drag almost unchanged when the discharge voltage increases.
The initial actuation strength is of vital importance in plasma flow control. Once the flow separation is suppressed with an initial discharge voltage higher than the threshold voltage, the flow reattachment can be sustained even the discharge voltage was reduced to a value less than the threshold voltage, that is to say, the voltage to sustain the flow reattachment is lower than the voltage to suppress the flow separation in the same conditions.
We can make use of the results by managing the discharge voltage properly. A higher discharge voltage can be used to suppress the separation in the beginning, and then we can use a much lower discharge voltage to sustain the flow reattachment later. Not only the power consumption can be reduced obviously, but also the life-span and the reliability of the actuator can be increased greatly. I hope this information about Subsonic Plasma Flow Control will be helpful for you.