On hairpin vortices induced by circular synthetic jets in laminar and turbulent boundary layers

Numerical simulations are carried out to investigate the hairpin vortices induced by single circular synthetic jets (SJ) in a laminar boundary layer (LBL) and a turbulent boundary layer (TBL) over a flat plate. Both boundary layers have the same freestream velocity 0.1. m/s. The momentum-thickness based Reynolds number is Reθ= 180 in the LBL and Reθ= 420 in the TBL. In both cases, the synthetic jet actuator (SJA) operates under the same conditions, i.e. the jet-to-freestream velocity ratio VR= 0.17 and the dimensionless stroke length L= 1.7. For the TBL simulation, large eddy simulation (LES) is used. The generation and evolution of the SJ-induced hairpin vortices and their accompanying vortex structures in both boundary layers are compared. It is found that in the TBL the hairpin vortex travels faster and is in a more bent shape than in the LBL. In addition, the hairpin vortex in the TBL is asymmetric, oscillates laterally, and dissipates fast, whereas it is symmetric and persistent in the LBL. Secondary vortices are observed in both cases. It is found the SJA ingestion also contributes to the generation of secondary vortices, and plays a major role in the TBL. The influence of hairpin vortices on both boundary layers is also examined in the context of flow separation control, through the investigation of the boundary layers' velocity profile, wall shear stress, and shape factor. The change of velocity and velocity gradient profiles due to the passage of hairpin vortices reveals that the influence of the hairpin vortex on the TBL is wider but much weaker than on the LBL. The current results suggest that a much stronger SJ is required for the current TBL in order to achieve similar flow separation control effect to that in the LBL.