Localized thermal perturbations for control of turbulent shear flows

Thermal perturbations initiated by pulsed plasmas and lasers have demonstrated impressive control authority in various high-speed turbulent shear flows, yet questions on fundamental physics and amplitude scaling remain unanswered. The control mechanism is believed to stem from thermal energy deposition (Joule heating), and there is a vital difference in the flow receptivity to these perturbations in comparison to more traditional momentum-based devices employed for active flow control. This paper provides a chronological description of the author's research on energy deposition-based flow control actuators with specific focus on surface plasma discharges. Early demonstrations of both success and failure for boundary-layer separation control are reviewed and assessed with the benefit of hindsight. This sets the stage for more recent investigations of flow physics and scaling in both free and reattaching incompressible turbulent shear layers. The majority of the presented results are taken from experiments by the author employing nanosecond-pulse-driven dielectric barrier discharge (NS-DBD) plasma actuators, although laser energy deposition is included for context. A general review of NS-DBD plasma actuators for active flow control is also provided. The paper is presented in a manner that conveys the thinking of the author and the wider community at the time of investigation.