An experimental investigation of the structure and dynamic behavior of two dimensional over-expanded air jets in water was conducted. To characterize the structure and time-dependant behavior of these jets, high speed digital video recording and static pressure distribution were obtained. Test conditions varied by setting different reservoir pressure.
Shadowgraph of the over-expanded jet into ambient air confirmed approximate Mach number at the jet exit of 2.0 that was slightly less than the value predicted by the ideal design calculation.
From the captured images, jet spreading angle was measured as a function of mass flow rate. At a mass flow rate less than 0.025kg/s, the spreading angle remained roughly constant. As mass flow rate increases beyond 0.025kg/s, the spreading angle increased with mass flow rate.
Time dependant instability was estimated by examination of the sequence of the video images. Periodic nature of the jet shape was observed and the frequency of the repetition was approximately 5-6 for all cases tested. Three length scales that characterize the geometry of the instability of submerged jet were defined. These Three characteristic length scales were defined as following. $L_1$ is the maximum width of the plume when the periodic instability occurs, $L_2$ is the width of the jet where secondary reverse flow entrained jet flow and $L_3$ is the distance from the jet exit to the location where entrainment of the secondary reverse flow occurs. The ratio of $L_1$ and $L_2$ decreased with increasing stagnation pressure, i.e. mass flow rate. $L_3$ increased with increasing stagnation pressure.
The temporal behavior of static pressure measurements also showed peak around frequency of 5, which corresponds the frequency obtained by visual measurements.
