Oscillation Flow Induced by Underwater Supersonic Gas Jets from a Rectangular Laval Nozzle

Abstract

In present work, direct measurements of the interfacial behavior of water-submerged gas jets, with nozzle operated in over-expansion to highly under-expansion conditions, were performed using high-speed digital photography. The experimental results show that the two low-frequency oscillations, which are so called jet necking/bulging and necking/back-attack phenomenon, are also found in the region near present rectangular nozzle exit.As the nozzle pressure ratio increased, two distinct flow regimes can be observed: One that showed unstable gas/water interfacial characteristics for nozzle pressure ratio, NPR or Po/Pa≤ 10.17 when the jet pattern showed similar shape as the one issuing from axisymmetric nozzles, and the other with stable jet pattern for NPR > 18.48 when a 3D cross-like cross section jet gas/water boundary started to form and grow from the nozzle exit. Finally, numerical analysis on shockwave structures of the experimental nozzle models was carried out, and the submerged jet gas/water interface characteristics show good agreement with the jet boundaries predicted by numerical simulations. The numerical results indicate that the over-expanded flow in the four corner regions as well as the recompression shock wave may play a dominant role in the interface instability to result in necking/bulging and necking/back-attack phenomenon during over-expanded to slightly under-expanded conditions.