INVESTIGATION OF WATERJET EFFLUX NOISE WITH NUMERICAL TECHNIQUES

Abstract

The URN resulting from a waterjet propulsion (WJP) is to be accurately estimated for stealth considerations. Such WJP systems are designed to operate either with a submerged jet (wet transom) or with a free surface jet (dry transom). The URN due to the operation of WJP system can be broadly categorized as pump noise and nozzle efflux noise. Pump noise includes mainly the cavitation noise and the tonal mechanical/hydrodynamic noise, whereas the nozzle efflux noise is due to jet impact and bubble entrainment. The present study attempts to implement advanced numerical techniques to estimate URN due to the nozzle efflux of a WJP in both wet and dry transom modes. Large Eddy simulations (LES) are carried out to simulate the hydrodynamics of turbulent multiphase flow behind both submerged and free surface water jets. The impact of water jet on free surface and subsequent bubble generation rate is captured using multiphase volume of fluids (VOF) technique. The impact noise is estimated by directly recording the fluctuating acoustic pressure at certain field points during the simulation. A blob detection algorithm is implemented to compute the bubble generation rate. A mathematical model to compute the bubble pulsation noise from bubble generation rate is validated and implemented. The estimated URN levels in wet and dry mode are presented and compared. Key inferences are presented which could be useful in further investigation of URN from water jet of stealth vessels, as well as to investigate bubble noise from propeller cavitation, air lubrication etc.