Research on the mechanics of underwater supersonic gas jets

Abstract

An experimental research was carried out to study the fluid mechanics of underwater supersonic gas jets. High pressure air was injected into a water tank through converging-diverging nozzles (Laval nozzles). The jets were operated at different conditions of over-, full-and under-expansions. The jet sequences were visualized using a CCD camera. It was found that the injection of supersonic air jets into water is always accompanied by strong flow oscillation, which is related to the phenomenon of shock waves feedback in the gas phase. The shock wave feedback is different from the acoustic feedback when a supersonic gas jet discharges into open air, which causes screech tone. It is a process that the shock waves enclosed in the gas pocket induce a periodic pressure with large amplitude variation in the gas jet. Consequently, the periodic pressure causes the jet oscillation including the large amplitude expansion. Detailed pressure measurements were also conducted to verify the shock wave feedback phenomenon. Three kinds of measuring methods were used, i.e., pressure probe submerged in water, pressure measurements from the side and front walls of the nozzle devices respectively. The results measured by these methods are in a good agreement. They show that every oscillation of the jets causes a sudden increase of pressure and the average frequency of the shock wave feedback is about 5-10 Hz. underwater supersonic gas jet, pressure oscillation, flow visualization, shock wave feedback PACS: 47.55.Ca, 47.27.wg

Oscillation flow induced by underwater supersonic gas jets

“…For a supersonic gas jet in open air, the screech tone principle has been established [19,20], that is, wave feedback appears from the shear layer. Therefore, Shi et al [21] propose that the "back-attack" phenomenon may be better explained as shock wave feedback phenomenon. In this paper, we describe the "backattack" as a jet expansion feedback phenomenon to distinguish it from the jet oscillations which have smaller amplitude.…”

“…The operation conditions can be seen in Ref. [21] composed of (1) high amplitude and low frequency pressures caused by the "back-attack" or the jet expansion feedback; (2) medium amplitude and medium frequency pressures; (3) low amplitude and high frequency pressures caused by turbulence of the jet. It is highlighted in this paper that the jet bulge is responsible for the second part of the pressure oscillation.…”

Experimental and Theoretical Study of the Characteristics of Submerged Horizontal Gas Jets and Vertical Plunging Water Jets in Water Ambient

“…It should be pointed out; however, when the gas jets enter into the water initially, the pressure of the gas is not high enough to overcome the inertia effect of the water due to the large density ratio between the gas jet and the ambient water (Shi, 2010). The pressure inside the "gas bag" will accumulate and keep increasing, and once it is high enough to overcome the suppression of the water, the gas can expand freely to complete the expansion process.…”

“…This breaking phenomenon plays a very important role in causing the jet process to become more unsteady and turbulent. However, this mechanism cannot be observed in gas-gas jets due to their comparable density ratio (Shi, 2010). Christopher (2009) studied experimentally the phenomenon of the gas jets pinch-off produced by rectangular submerged nozzles in a water ambient using a photographic technique and he showed that the number of pinch-off events decreases in a logarithmic function when increasing the Mach number and when appropriately scaled by the total viewing length, and a geometric length scale (L Q ) that is relatively constant across all aspect ratio nozzles.…”

Estimation of the radioactive aerosols capture in accidental sequences of nuclear power plants

“…Observations of Dai et al [Dai, 2006] have shown that the injection process of high-speed gaseous jets in still water can induce large pressure pulsations upstream of the nozzle exit, and that the shock-cell structures in the over-and under-expanded jets can lead to strong changes in the hydrodynamic pressure. Shi et al [Shi, 2010] have shown that the injection of supersonic gaseous jets into water causes large flow oscillation, which can be related to shock waves reflecting in the gas phase and, complementarily, Tang et al [2011] has observed the back-attack phenomenon.…”