Experimental investigation and reduced-order modeling of plasma jets in a turbulent boundary layer for skin-friction drag reduction

Abstract

Stereo particle imaging velocimetry measurements and reduced-order modeling are combined to provide a full picture of the interaction of plasma jets with a turbulent boundary layer (TBL). Three working modes of the plasma actuator are investigated, corresponding to a unidirectional jet (mode A), a steady crashing jet (mode B), and a spanwise oscillating jet (mode C). The results show that in mode C, a periodical alteration of two opposite wall jets can only be achieved at a low modulation frequency of 20 Hz. As the frequency increases to 100 Hz, the two unsteady wall jets collide in the middle, producing a meandering vertical jet column. In the cross-flow TBL, mode A induces a single streamwise vortex, which grows in size within the plasma actuation zone and decays rapidly in strength after propagating beyond. As a comparison, modes B and C produce a counter-rotating vortex pair during the interaction. The skin-friction drag variations within the plasma actuation zone are dominated by the cross-stream momentum transportation of streamwise vortices. In the vortex upwash zone where a strong shear is present, high levels of turbulent kinetic energy are produced. Physically, the spanwise shaking and vertical jumping of plasma jet heads contribute noticeably to turbulent fluctuation. Experimental evidence supports the simplification of a streamwise momentum equation into a nonlinear transportation-diffusion equation, resulting in a reduced-order streamwise vortex transportation model. Detailed comparison with the experimental data shows that this model is able to give a reasonable prediction of the cross-stream flow patterns and streamwise velocity variations within minutes.