The Study On Numerical Analysis Of Fluid Noise And Acoustic Test With Towed Model

It is very important for modern military vessels to have a rather good acoustic performance today, so using the effective means to forecast and reduce ship noise has become a focus with difficulty for Navy's research. Hydrodynamic noise is one of the acoustic components for ship noise besides structural vibration noise and propeller one, it is closely related to the ship's self-noise, and also related to the ship radiated noise. In this paper some basal discussions about the hydrodynamic noise proposed with a focus on the part of vortex-induced noise, business software named Fluent is used to study acoustic characteristics of hydrodynamic noise about a certain surface ship. A new type of hydrodynamic noise model test technique using remote submersible model is also discussed in this paper.1. A typical model for the process of point vortex attenuation named Aoxin vortex is used to simulate the viscosity evolution of the two-dimensional point vortex in a free space, the matching expansion method is used for calculating radiation noise and the corresponding spectral analysis is also put out. The radiation noise of the point vortex in a free space is rather obvious only in the initial evolution process, and with the passage of time it attenuates rapidly; its frequency distribution is of the narrow band which is mainly located in low band, the peak of the spectrum distribution curve is very clear. The sound field of point vortex and its spectrum are relevant to the viscosity coefficient. With viscosity increasing, the peak sound pressure occurred with the time shift after, its magnitude will increase, and its main vibration frequency band will move to the direction of frequency increasing, vibration frequency range will slightly increase.2. Wing is a common attached body for engineering structures, its evolution of the tail vortex and radiation sound has certain characteristics. Elliptical wing and delta wing are studied respectivly in this paper, the matching expansion method is used to estimate its acoustic field, and the spectral analysis is also put out. It is shown that there is acoustic radiation accompanied by the tail vortex evolution. It is similar for the acoustic spectrum of elliptic wing and delta wing; there are two obvious peak bands for both of them which will drift to different frequency positions corresponding to different airfoils.3. The SST k -ωtwo equation turbulence model is used for the unsteady viscous three-dimensional flow simulation of the submarine. Then, based on Lighthill acoustic theory, the numerical calculation of sound field around the hull is put out and its near and far field characteristics are also numerically analyzed. The influence of the attack angle, flow velocity, model scale, Reynolds coefficient is discussed and the influence of the fairwater location to sound field is also considered. With the increasing of the flow vortex, the sound radiation is increase correspond.4. The VOF method and SST k -ωtwo equation turbulence model are used for three-dimensional surface warships to simulate their unsteady viscous flow. Then with the help of the Lighthill theory, the acoustic numerical calculation about the hull in near and far field is also carried out. The influence of free surface, current angle and flow velocity is discussed. It is shown that the acoustic radiation about the ship stern is greater than that about the ship bow, and the horizontal acoustic radiation of the mid-ship is significantly greater than that of the bow and stern. With the increasing of the flow vortex, the sound radiation is increase correspond, so the sound field could be improved by the fluid field improvement with better hull shape or properly using attach body. 5. A new acoustic test technque using remote model has been introduced to study the underwater noise of a ship. The remote model test project could be properly designed for the acoustic signal collecting. With the help of the appropriate data processing method, the acoustic sources could be analyzed successfully. Much valuable work about the fluid noise and propeller noise could be carried on with the help of remote model, and the low frequency noise which is especially interested for its long distance radiating with small attenuation could also be studied in this way. It is a rather good supplement to the traditional fluid model test.