Hydro-acoustical Study Based On Fine Analysis Of Flow, Hydro-acoustical Source And Sound Field

Hydro-acoustical noise is the pressure fluctuation propagating in thewater, which is induced by the unsteady and inhomogeneous water flowfield. Such noise is usually generated by the interaction among fluid orbetween fluid and solid boundary during the complex flow procedure.Hydro-acoustical noise is one of the main sources of the radiated and self-noise of water vehicles. The reduction of hydro-acoustical noise hassignificant importance for the acoustical environment protection in waters,particularly for the acoustical stealth and detection capabilities of the navalweapons. Hydro-acoustics is one fringe branch of hydrodynamics, which isto study the hydro-acoustical problems mainly dealing with thehydro-acoustical mechanism of the sound source and its control.Hydro-acoustical noise control with the aid of computationalhydro-acoustics (CHA) has become a hot issue thanking for the rapiddevelopment of CHA.Aiming at the generous noise problems induced by vortex flowsaround complex non-streamline-shaped structures, a systematic approachis established in this thesis for the location of sound sources, distinguishingof true and main sources, analysis of the noise generation mechanism anddesign of the control scheme. This approach is carried out based onmulti-step hybrid method of CHA, and through the fine numericalsimulation and analysis of the vortex flow, equivalent acoustical sourceand acoustical fields of three typical engineering cases (valve, cavity andsail). The distinguishing feature of this study is the fine analysis of thecorresponding flow, acoustical source and acoustical fields, namely: The 3-D instantaneous flow fields are simulated by LES, and the vortex flowstructures are analyzed through a topological study. The equivalentacoustical source fields in time and frequency domains are calculated fromthe flow fields based on the Lighthill’s acoustical analogy. The acousticalnear-fields and far-fields in frequency domain are completely calculated bydifferentially solving the Lighthill’s equation using finite and infiniteelement methods. With a systematic analysis of flow, acoustical source andacoustical fields, the acoustical source region is divided into severalregions with different hydro-acoustical mechanisms. Then the acousticalfields induced by the different source regions are calculated and compared,in order to check out the main and true sources. Finally, the noise reductionschemes are proposed according to the generation mechanisms of the mainsources.According to the comparisons between numerical and experimentalresults (their pressure losses and noise spectra) of valve cases, thesimulated flow, acoustical source and acoustical fields are basically agreedwith the experiment data. The numerical results indicate, under theoperating conditions of draining, the main low frequency (under500Hz)acoustical sources are induced by the high speed jet at the outlet of valve,and the main medium and high frequency (above500Hz) are induced bythe blocking effect of valve disc and its vortex wake. Under the operationconditions of drawing, the main acoustical sources are induced by the highspeed jet, the secondary vortex flow and their interaction around the innercorner of valve. The corresponding noise reduction schemes are proposedaccording to source locations and their generation mechanismsrespectively.The numerical results of cavity cases indicate, for the cavity (L/D=3),the main medium and high frequency noise sources are induced by a seriesof small vortexes generated by the instability of the shear layer at thecavity mouth, and the interaction between these small vortexes and thetrailing edge of cavity. The main low frequency noise source is induced by a large-sized vortex rotating in the cavity. For the cavity (L/D=1.25), themain low frequency source is induced by the interaction between a seriesof small vortexes generated by the instability of the shear layer and thelarge sized vortex in the cavity, the main high frequency sources arearound cavity. The noise reduction scheme is proposed to use theclapboard in the cavity. The effect of this noise reduction scheme is provedby the numerical simulation.The numerical results of sail cases indicate, the main noise sources arelocated at the conjunction trailing edge of the sail and main body, whereexist very complex vortex flow structures. It is proposed that these vortexstructures should be eliminated or reduced by the means of active flowfield control.The hydro-acoustical experiments are very difficult and expensive,because the low background noise and non-reflection acoustical boundaryconditions are very hard to be satisfied in the water. The CHA approachesassembled in this thesis provide an effective methodology for theengineering noise control.