The Nanosecond Pulsed Dielectric Barrier Discharge Plasma Actuator for Boundary Layer Separation Control

Abstract

Since the mid-1990's, nonequilibrium plasmas have been brought into the limelight as an effective method of influencing flows of aeronautical interest, including shock position control, boundary layer momentum addition, and airfoil stall mitigation. Plasma actuation shows promise to address the time dependent process of dynamic stall as a nonintrusive, high bandwidth active control method. The highly dynamic phenomena of Retreating Blade Stall (RBS), as experienced by pitching blade rotorcraft, is a possible candidate for plasma flow control. This work investigates nanosecond pulsed Surface Dielectric Barrier Discharge (ns-SDBD) plasma actuators in their ability to mitigate stall by a two-pronged approach: Experimental demonstrations of separation mitigation and investigations into actuator plasma dynamics for improved design and understanding. Flow control is demonstrated on a NACA-0015 airfoil in constant angle of attack and pitching modes for Reynolds numbers $\text{Re}\leq6.8\times10^5$ and reduced frequencies $k\leq0.049$. In the steady stall condition, flow reattachment time and the number of pulses required for flow reattachment is explored, demonstrating effective reattachment of the flow. Dynamic stall mitigation is demonstrated for high peak angles of attack for an airfoil in the fully reversed flow condition, as seen in RBS. Observed drag changes are within 10\% for all tests. For the purposes of device design improvement, the plasma electric field dynamics are measured with the nonlinear laser diagnostic femtosecond Electric Field Induced Second Harmonic Generation (fs-EFISH), which allows for directionally sensitive and temporally resolved measurements. In-situ measurements of ns-SDBD's have traditionally been difficult to achieve due to surface effects, fast timescales, and the sub-millimeter plasma thickness at atmospheric pressure. Surface perpendicular electric field measurements have been collected in the plasma of ns-SDBD's in atmospheric and rarefied air. Averaged maximum ionization wave reduced electric fields are found to be $|E|/N=455.3\pm68.2~\text{Td}$. Qualitative field dynamics are supported by the literature.