Session D17: Flow Control: Plasma Actuation

Active Turbulent Boundary Layer Drag Reduction Using Pulsed-DC DBD Plasma Actuators

Viscous drag reduction was achieved in zero pressure gradient turbulent boundary layers over a flat plate across a range of Mach numbers. The primary means of flow control was a spanwise array of Pulsed-DC dielectric barrier discharge (DBD) plasma actuators which provided forcing in the sublayer of the boundary layer. Drag was measured directly using a floating force balance mounted on linear air bearings. For flow diagnostics between un-actuated and actuated cases, an electronically isolated, floating constant-temperature hotwire anemometer was developed. Hotwire measurements showed a decrease in both the turbulence intensity and energy in the spectra in the wall-normal profiles. The ``bursting" frequency for un-actuated and actuated cases was measured using the Variable-Interval-Time-Averaging technique (outlined by Blackwelder and Kaplan 1976). A nearly linear correlation between the decrease in viscous drag and the number of ``burst" events was observed.    

The effect of spanwise modulated DBD plasma forcing on flow development in a laminar separation bubble

The effect of spanwise modulated disturbances on flow development within a separation bubble formed over a flat plate is investigated experimentally using two-component Particle Image Velocimetry. Spanwise uniform and non-uniform forcing are considered using novel surface mounted dielectric barrier discharge (DBD) plasma actuators. Characterization of the disturbances is performed, showing that the actuators produce streamwise jets within the active regions and no significant momentum elsewhere, thus enabling spanwise forcing at the desired wavelengths. When subjecting the separation bubble to these disturbances applied at the frequency matching that of the most unstable mode in the baseline flow, the bubble height reduces and mean reattachment shifts upstream with decreasing spanwise wavelength, which is a likely consequence of higher momentum coefficients for these cases. However, significant changes in the flow topology are shown to arise for relatively large wavelengths, and hence low momentum coefficients. These modifications are linked to changes in the development of the dominant coherent structures induced by the spanwise modulated perturbations.   

Wake behind circular cylinder excited by spanwise periodic disturbances using DBD plasma actuator array

We experimentally investigated flow control of the wake behind a circular cylinder excited by temporal periodic disturbances with spanwise phase variations using DBD plasma actuator array, motivated by reducing drag forces by suppressing development of large scale vortices. DBD plasma actuator were segmented in the spanwise direction, and phase differences were given to adjacent electrodes. This experiment was conducted at Re≃8000 and the wake was visualized by PIV. Compared to without forcing, when the phase difference is 180° and non-dimensional forcing frequency is higher than approximately 1.0, small vortices induced by periodic disturbance emerged in the free shear layer. Then, wavy vortex structures along spanwise direction caused by phase differences were observed in the free shear layer by stereo PIV and resulting in reduction of drag forces.   

Control of Shock Wave Configuration at M=2 Compression Ramp by Array of Filamentary Plasma

This experimental study considers the effect of an electric discharge on the flow structure near a 12-20 degree compression ramp in M=2 airflow. The tests were conducted in the supersonic wind tunnel SBR-50 at the University of Notre Dame. Stagnation temperature and pressure were varied in a range of 294-600 K and 1-3 bar, respectively, to attain various Reynolds numbers ranging from 3e5 to 2e6. Surface pressure measurements, schlieren visualization, electric measurements of the discharge parameters, and plasma imaging with a high-speed camera were used to evaluate the plasma control authority on the ramp pressure distribution. The plasma being generated in front of the compression ramp shifts the shock position from the ramp corner to the electrode location, forming a flow separation zone ahead of the ramp. There were found that the pressure on the compression surface reduces linearly with the plasma power, and the ratio of pressure change to the flow pressure is an increasing function of the ratio of plasma power to the flow enthalpy flux. The last parameter is defined as the task-related plasma control efficiency, which ranges from 200 to 300%.