Control of Transitional and Turbulent Flows Using Plasma-Based Actuators

Abstract

Abstract : An exploratory numerical study of the control of transitional and turbulent separated flows by means of asymmetric dielectric-barrier-discharge (DBD) actuators is presented. The flow fields are simulated employing an extensively validated high-fidelity Navier-Stokes solver which is augmented with both phenomenological and first-principles models representing the plasma-induced body forces imparted by the actuator on the fluid. Several applications are considered, including suppression of wing stall, control of boundary layer transition on a plate, control of laminar separation over a ramp, and turbulent separation over a wall-mounted hump. Effective suppression of stall over a NACA 0015 airfoil at moderate Reynolds numbers is demonstrated using either co-flow or counter-flow pulsed actuators with sufficiently high frequency. By contrast, continuous actuation (simulated by a steady body force in the phenomenological model) is found to provide little control of separation. For continuous actuator operation, the first-principles approach is needed in order to reproduce the benefits of the inherently unsteady force induced by the plasma actuator. The pulsed-modulated unsteady plasma force is found to be more effective than a monochromatic radio-frequency forcing. These results highlight the greater importance of transition and turbulence enhancement mechanisms rather than pure wall-jet momentum injection for the effective use of DBD devices. As a consequence, meaningful computations require the use of three-dimensional large-eddy simulation approaches capable of capturing the effects of unsteady forcing on the transitional/turbulent flow structure. For a laminar boundary layer developing along a flat plate, a counter-flow DBD actuator is shown to provide an effective on-demand tripping device.