Abstract
Two different plasma actuation strategies for producing near-wall flow oscillations, namely the burst-modulation and beat-frequency mode, are characterized with planar particle image velocimetry in quiescent air. Both concepts are anticipated to work as non-mechanical surrogates of oscillating walls aimed at turbulent flow drag reduction, with the added benefit of no moving parts, as the fluid is purely manipulated by plasma-generated body forces. The current work builds upon established flow-control and proof-of-concept demonstrators, as such, delivering an in-depth characterization of cause and impact of the plasma-induced flow oscillations. Various operational parameter combinations (oscillation frequency, duty cycle and input body force) are investigated. A universal performance diagram that is valid for plasma-based oscillations, independent of the actuation concept is derived. Results show that selected combinations of body force application methods suffice to reproduce oscillating wall dynamics from experimental data. Accordingly, the outcomes of this work can be exploited to create enhanced actuation models for numerical simulations of plasma-induced flow oscillations, by considering the body force as a function of the oscillation phase. Furthermore, as an advantage over physically displaced walls, the exerted body force appears not to be hampered by resonances and therefore remains constant independent of the oscillation frequency. Hence, the effects of individual parameter changes on the plasma actuator performance and fluid response as well as strategies to avoid undesired effects can be determined.
Highlights
The development of plasma-based flow-control actuators has experienced a brief time span [1] leading to a considerable variety of numerous electrical and mechanical characterisation studies
The parameter space of streamwise traveling waves (StTWs) spans a spectrum of temporal oscillation frequencies f and streamwise wavenumbers κx [13], where the oscillation waveform has an additional effect on both drag control performance and efficiency [15]
The coordinate system (x, y, z) is such that the x axis is parallel to the electrodes, y is the wall-normal coordinate and z refers to the forcing direction of the plasma actuator (PA), which is across the electrodes
Summary
The development of plasma-based flow-control actuators has experienced a brief time span [1] leading to a considerable variety of numerous electrical and mechanical characterisation studies. These have led to rapid advances in plasma actuator (PA) technology, as summarized in reviews in the past two decades [2–6]. In the branch of turbulent flow control different concepts and mechanisms, aimed at viscous drag reduction, have been developed so far [7]. A promising concept is streamwise traveling waves (StTWs) of spanwise wall velocity [8], which impart a periodic fluid oscillation transverse to the mean-flow direction and modulated along the streamwise direction.
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