Jets have been studied for near one century for their engineering applications. Similarity theory and closure model for turbulent plane jets have been developed. However, their experimental confirmations remain piecewise. Furthermore, the mechanism of plane jets in a surface-wave environment is still not well understood. For better understanding of the turbulent jet, especially in a wave environment, experiments were conducted in this study to investigate (1) the impingement of the plane jet onto a free surface; (2) the propagation of surface waves in current shear layers generated by the plane jet; (3) the interaction of turbulent plane jet with progressive waves; (4) the coherent structures of turbulent plane jet; and (5) the instability of turbulent plane jet....[ Read more ]

Jets have been studied for near one century for their engineering applications. Similarity theory and closure model for turbulent plane jets have been developed. However, their experimental confirmations remain piecewise. Furthermore, the mechanism of plane jets in a surface-wave environment is still not well understood. For better understanding of the turbulent jet, especially in a wave environment, experiments were conducted in this study to investigate (1) the impingement of the plane jet onto a free surface; (2) the propagation of surface waves in current shear layers generated by the plane jet; (3) the interaction of turbulent plane jet with progressive waves; (4) the coherent structures of turbulent plane jet; and (5) the instability of turbulent plane jet.

The experiments were carried out in a water tank which is 6m long, 0.4m wide and 0.4m high. Turbulent plane jets were measured with a two-component laser Doppler velocimetry (LDV) system and the jet flow fields were visualized with a laser-induced fluorescence (LIF) technique. A capacitance wave height gauge was utilized to measure the surface gravity waves.

Experiments on turbulent plane jets impinging onto a free surface in a stagnant ambient at a water depth of H/d=130 show the existence of four flow regimes in a plane jet: the zone of flow establishment (ZFE), the zone of established flow (ZEF), the zone of surface impingement (ZSI) and the zone of horizontal jet (ZHJ). In ZFE the flow is non-similar and characterized by the two free shear layers developing from the two edges of the jet orifice. In ZEF the flow properties are self-similar and agree very well with those based on the similarity theory. The jet spreads linear but with a larger spreading constant than free jet due to the re-entrainment. The vertical jet enters ZSI at about 77%H and then transforms into two horizontal surface jets in ZHJ.

In the experiment on wave-current interaction, the wave number was measured with a wave height gauge moving along the wave propagation direction. The waves were measured in an environment of currents, which were generated by the plane jets. In the upstream of the plane jets, the wave number first increases in the velocity- dominated current region and then decreases in the vorticity-dominated current region. The stronger the jet exit velocity, the larger the current effect on surface waves. Sirnilar effect of currents on wave amplitude is observed.

Plane jets in wave environments were observed to have the same flow regimes as in a stagnant ambient, i.e., ZFE, ZEF, ZSI and ZHJ. The self-similar feature in ZEF with a wider spreading of a plane jet was found in a wave environment. Wave-induced velocities ũ_i, wave-associated mean stresses ũ_i ũ_i]̄ and wave-induced turbulent Reynolds stresses r[tilde][tilde] _ij are obtained with a phase average scheme. The results show that the wave-induced flow field is dominated by the fundamental mode and the contribution from harmonic is negligible. The amplitude of the fundamental mode of w̃,r[tilde][tilde] _ww and r̃_uu, denoted by ŵ, r[circumflex][circumflex] _ww and r̂_uu and the wave-associated normal mean stress [w̃w̃]̄ have similar twin-peak distributions, indicating maximum energy transfer at about half-velocity location η=⁺_-0.16. There is an about 180° phase shift across the plane jets for the phase of the fundamental mode of w̃,r̃_ww and r̃_uu denoted by θ̃_w, θ̃_rww and [theta][theta] ̃_ruu.

Coherent vortex structures were observed in the plane jets when the progressive surface waves were imposed, even though no coherent vortex structures are shown in a stagnant ambient. The flow visualizations also show the plane jets are stable for H/d=130 even though surface waves are imposed.

Plane jets become unstable to exhibit a flapping motion when the jet exit velocity exceeds a critical velocity W_Ocr. The flapping frequency f is found to be inversely proportional to the square root of water depth H and the critical jet exit velocity to be linearly proportional to H. As a result, the Strouhal number defined by St=fd/W_Ocr is found to be proportional to the normalized water depth (H-zo)/d to a power of -3/2 with zo the virtual origin of the plane jet, which describes the critical condition for the self-excited flapping motion of plane jets.