Numerical Investigation On Flow-induced Noise Around Circular Cylinders At High Reynolds Number

There are many cylindrical structures on underwater vehicle,such as antennas and lift platforms.Flow-induced noise of these protruding structures at high speed becomes one of the most important factors on the concealment performance.Accurate prediction of flow-induced noise around circular cylinders by numerical simulation is of great significance to accurately evaluate the underwater vehicle noise and optimize the structure.However,due to high Reynolds number(Re)and the complex flow characteristics,the calculation of hydrodynamic noise is difficult and lack of studies.The hybrid method based on large eddy simulation(LES)and Lighthill's acoustic analogy theory is conducted to solve the hydrodynamic noise for a three-dimensional circular cylinder.The influences of different simulation models on the flow results and the acoustic results are analyzed and the simulation accuracy of the hydrodynamic noise in underwater structures is evaluated,which provides the technical means for accurate noise prediction of underwater vehicles.According to the aerodynamic noise experiment of tandem cylinders at a low Mach number,LES is applied to simulate the unsteady flow field around tandem cylinders and Lighthill's acoustic analogy theory based on finite element/infinite element is adopted to simulate the flow-induced noise.The analysis approach of the flow stability and the suitable parameters of the acoustic model are given,and the hybrid method to simulate flow-induced noise of circular cylinders at a low Mach number is obtained.By comparing the numerical results with the experimental results,the noise calculation method adopted in this paper is introduced and verified.On this basis,the flow and acoustic characteristics of an underwater finite-height cylinder are simulated corresponding to an experiment performed in a water tunnel and the detailed simulation method is studied.The influences of Y~+value,the grid scale?Z/D,CFL number,the sub-grid(SGS)models of LES and the model parameter on the flow simulation results are analyzed by comparing with the experiment results.A relatively accurate flow field calculation model and reliable transient flow results are obtained.After achieving the unsteady flow field,the FFT transform and the mesh interpolation are calculated to obtain the sound source information in the frequency domain to conduct the flow-induced noise simulation.The acoustic mesh,the infinite element base surface,and the filter function are discussed with respect to their influents on the hydrodynamic noise prediction results.A suitable acoustic model is obtained.Based on the discussion of different models and parameters,the accurate hybrid method for simulating the hydrodynamic noise of a circular cylinder at high Reynolds number is established.Finally,the influence of Reynolds number,pitch ratio and arrangement method on the flow induced noise of underwater single/double cylinders is calculated and analyzed by the hybrid method.Analysis results show that Reynolds number has a significant positive correlation with the sound pressure level in the calculated case.The reduction of the Reynolds number can effectively reduce the hydrodynamic noise.The acoustic directivity of the hydrodynamic noise at Re=4.3×10~4,1.0×10~5 and 1.8×10~5 is similar and the acoustic distribution is the same.The acoustic directivity at Re=2.5×10~5 is similar to that of the dipole and the radiation noise is the largest in the direction perpendicular to the flow.With the increase of the pitch ratio,the noise of tandem cylinders firstly increases and then decreases,and the noise is the largest at the critical spacing ratio(L/D=3).The hydrodynamic noise of double cylinders has obvious acoustic directivity and the noise in the tandem arrangement is the largest.The study is able to provide a detailed guidance basis for the investigation of the hydrodynamic noise of underwater bluff body.