Experimental investigation of two-dimensional Rayleigh–Taylor instability with controllable initial conditions

Experimental investigation of the Rayleigh–Taylor instability (RTI) and its dependence on initial conditions has been challenging, primarily due to the difficulty of creating a well-defined gaseous interface. To address this, a novel soap film technique was developed to create a discontinuous two-dimensional SF ₆ air interface with precisely controlled initial conditions. High-order modes were superimposed on a long-wavelength perturbation to study the influence of initial conditions on RTI evolution. Experiments conducted at Atwood numbers ranging from 0. 26 to 0. 66 revealed that bubble growth shows a weak dependence on both initial conditions and Atwood numbers, whereas spike growth is more influenced by these factors. Spike growth accelerates as the wavenumber of the imposed high-order modes decreases and/or the Atwood number increases. To quantify these effects, a variation on the previously developed potential flow model was applied, capturing the suppression of high-order modes and Atwood number dependence on RTI growth. In turbulent flow, the self-similar factors of bubbles and spikes exhibit minimal sensitivity to initial conditions. However, in relation to the Atwood number, the self-similar factors of bubbles (or spikes) demonstrate negligible (or significant) dependence. Comparisons with literature revealed that two-dimensional flows yield lower self-similar factors than three-dimensional flows. Furthermore, the discontinuity of the initial interface in this study, achieved through the soap film technique, results in faster spike growth compared with previous studies involving a diffusive initial interface. These findings provide critical insights into the nonlinear dynamics of RTI and underscore the importance of well-characterised initial conditions in experimental studies.