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.

A modified concept for generating DBD-based virtual spanwise wall oscillations is introduced in continuation of earlier efforts by Hehner et al. (2019) ["Stokes-layer formation under absence of moving parts – A novel oscillatory plasma actuator design for turbulent drag reduc-tion", Phys. Fluids 31, 051701]. Four groups of a multi-electrode actuator array are operated at interfering high-voltage signals to achieve an oscillating discharge intensity at 50 Hz beat frequency. The resulting velocity fields have been recorded with time-resolved planar high-speed particle image velocimetry so as to analyse the induced flow topology and wall-normal velocity profiles. A direct comparison of the results to Hehner et al. (2019) has indicated favourable effects of the new concept in terms of flow topology (i.e improved spanwise flow homogeneity, velocity magnitude increase and reduced lift-off), which renders the new operation concept of the actuator array particularly promising for Stokes-layer-like flow formations and, consequently, for turbulent drag reduction over a range of Reynolds numbers.