# Effect of velocity ratio on the interaction between plasma synthetic jets and turbulent cross-flow

Research paper by Haohua Zong, Marios Kotsonis

Indexed on: 03 Mar '19Published on: 25 Apr '19Published in: Journal of fluid mechanics

#### Abstract

Plasma synthetic jet actuators (PSJAs) are particularly suited for high-Reynolds-number, high-speed flow control due to their unique capability of generating supersonic pulsed jets at high frequency ( ${>}5$  kHz). Different from conventional synthetic jets driven by oscillating piezoelectric diaphragms, the exit-velocity variation of plasma synthetic jets (PSJs) within one period is significantly asymmetric, with ingestion being relatively weaker (less than $20~\text{m}~\text{s}^{-1}$ ) and longer than ejection. In this study, high-speed phase-locked particle image velocimetry is employed to investigate the interaction between PSJAs (round exit orifice, diameter 2 mm) and a turbulent boundary layer at constant Strouhal number (0.02) and increasing mean velocity ratio ( $r$ , defined as the ratio of the time-mean velocity over the ejection phase to the free-stream velocity). Two distinct operational regimes are identified for all the tested cases, separated by a transition velocity ratio, lying between $r=0.7$ and $r=1.0$ . At large velocity and stroke ratios (first regime, representative case $r=1.6$ ), vortex rings are followed by a trailing jet column and tilt downstream initially. This downstream tilting is transformed into upstream tilting after the pinch-off of the trailing jet column. The moment of this transformation relative to the discharge advances with decreasing velocity ratio. Shear-layer vortices (SVs) and a hanging vortex pair (HVP) are identified in the windward and leeward sides of the jet body, respectively. The HVP is initially erect and evolves into an inclined primary counter-rotating vortex pair ( $p$ -CVP) which branches from the middle of the front vortex ring and extends to the near-wall region. The two legs of the $p$ -CVP are bridged by SVs, and a secondary counter-rotating vortex pair ( $s$ -CVP) is induced underneath these two legs. At low velocity and stroke ratios (second regime, representative case $r=0.7$ ), the trailing jet column and $p$ -CVP are absent. Vortex rings always tilt upstream, and the pitching angle increases monotonically with time. An $s$ -CVP in the near-wall region is induced directly by the two longitudinal edges of the ring. Inspection of spanwise planes ( $yz$ -plane) reveals that boundary-layer energization is realized by the downwash effect of either vortex rings or $p$ -CVP. In addition, in the streamwise symmetry plane, the increasing wall shear stress is attributed to the removal of low-energy flow by ingestion. The downwash effect of the $s$ -CVP does not benefit boundary-layer energization, as the flow swept to the wall is of low energy.