Visualization study on boundary layer transition using surface arc plasma actuators

An experiment is conducted to investigate the effect of surface arc plasma actuators (SAPAs) on the transition of a laminar boundary layer on a flat plate in Ma = 3 flow. Schlieren snapshots and the root mean square (RMS) of the image intensity and pressure measurements are analyzed to determine the SAPAs' control performance. Four SAPAs in an array are placed in a spanwise direction to generate perturbations within the laminar boundary layer. The results indicate that a change from laminar to turbulent flow is achieved using SAPAs with a high repetition rate. The SAPAs introduce many thermal disturbance into the boundary layer which involves a lot turbulent structures. As a turbulent flow is formed downstream of the actuators, the RMS of the schlieren intensity (Irms) and the pressure fluctuations (Prms) increase. According to the RMS pressure measurements, the actuators with an actuation frequency of 10 kHz result in a 25% increase in the distance where the transition occurs. The formation of thermal gas bubbles causes trailing vortices associated with a strong shear effect downstream of the actuators. These streamwise vortices interact with the laminar boundary layer first, suggesting significant vortical activity in the boundary layer, whose thickness increases because the width of the Irms is increasing in spanwise direction. Amplification and breakdown of these disturbances eventually lead to the laminar-turbulent transition of the boundary layer. In addition, two higher actuation frequency modes are compared to determine the frequency effect on the laminar-turbulent transition. The Irms results indicate that the transition location moves upstream as the actuation frequency increases.