Experimental investigation of water-air interface instability induced by a finite pulse

This study investigated the hydrodynamic instability on a liquid-gas interface and its dependence on initial conditions. A drop tower method was employed to generate a quasi-single-mode water-air interface and also finite pulse accelerations. The finite pulse was produced by releasing a water tank onto coil springs, achieving a peak acceleration of 193 times the gravity acceleration within 5 ms. The experiments highlighted the transition from Rayleigh-Taylor (RT) stabilization to near Richtmyer-Meshkov (near-RM) instability. The results demonstrated that bubble and spike development is dominated by RT stabilization during pulse acceleration and near-RM instability after pulse. The different behaviors of bubbles and spikes under high-Atwood-number conditions were observed, noting perturbation phase reversals and the formation of a high-speed water jet. Spectral analysis of the interface contour and time-varying Fourier mode amplitudes revealed that the bubble development is suppressed by nonlinear effect while the spike instability is markedly enhanced by flow focusing. A sink-flow model was developed to evaluate the water jet velocity induced by the depthwise flow focusing, validated through impact experiments on an initially unperturbed interface. Finally, a comprehensive nonlinear solution was established for quantifying the hydrodynamic instabilities on a water-air interface, incorporating variable acceleration, nonlinear effects, and flow focusing.