On coherent vortices in turbulent plane jets generated by surface water waves

Coherent vortices in turbulent plane jets as generated by surface water gravity waves were investigated experimentally. Experiments were carried out in a water tank of 6m long, 0.4m wide and 0.4m high with a water depth of 34cm. Plane jets were generated by ejecting water vertically from a slot orifice of a jet generator mounted on the bottom of the tank. The ejected water has the same temperature as the ambient water to avoid buoyancy effect. Regular surface gravity waves were generated by a flap-type wave-maker on one end of the tank and eliminated by a wave-absorber on the other end. Turbulent plane jets were visualized with a laser-induced fluorescence (LIF) technique and measured with a laser Doppler Velocimetry (LDV) system. It is found that strong alternative coherent vortices like the vortex-street structure were generated and developed in the self-similar region of the plane jets when the surface waves were imposed. Two vortices of opposite sign are generated for each period of the surface gravity waves. The mean properties of the plane jets in a wave environment remain self-similar as occurs in turbulent plane jets in a stagnant ambient. However, the jet spreading in wave environment is much larger than those without waves. Hence, the generation of the coherent vortices enhances the jet mixing. The magnitude of the wave-induced velocities, wave-associated mean stresses and wave-induced turbulent Reynolds stresses were obtained using a phase average scheme. The results show that the wave-induced flow field is dominated by the fundamental mode and the contribution from high order harmonics is negligible.