Numerical Simulation Of Flow-induced Noise With Complex Moving Boundary Based On Immersed Boundary Method

The interaction between the fluid and the structure causes many problems,such as flow noise generation,acoustic propagation and noise control.It has received extensive attention in naval architecture and ocean engineering,aerospace engineering,vehicle engineering and other fields.In recent years,how to learn from nature for the design of bio-inspired robots with high thrust,high efficiency and low noise is a challenging and forward-looking topic.Bionic problems usually contain complex geometries and moving boundaries,traditional numerical methods face many difficulties.Meanwhile,the hybrid method for flow-induced noise prediction based on acoustic propagation equations is a hotspot in computational acoustics.This thesis focuses on the hybrid methods for solving flow-induced noise problems with complex moving boundaries.The sound generation and propagation processes of the typical bio-inspired locomotion(flapping flight and undulatory swimming)are explored.The main work and results are as follows:1.An immersed boundary method-acoustic perturbation equations(IBM-APE)approach based on ghost cell is proposed to simulate the acoustic propagation problems with complex geometries in the non-uniform flow fields.A high-accuracy parallel acoustic solver is developed.The present method can simulate the effect of non-uniform flow on the acoustic propagation.The high-order low-dissipation and low-dispersion schemes and the Open MP parallel technology are used in the method,which can simulate the flow-sound interaction problems efficiently and accurately.Besides,the ghost cell method is employed to deal with immersed boundary,which greatly enhances the simulation ability of the acoustic propagation problems with complex geometries.Several benchmark problems are simulated to verify the accuracy,convergence rate and parallel performance of the method.2.An IBM-based direct numerical simulation-acoustic perturbation equations(DNSAPE)hybrid method is proposed to deal with flow-induced noise problems with complex boundaries.The flow fields are first computed by solving incompressible Navier-Stokes(N-S)equations using a DNS solver.The acoustic fields are simulated by solving APE with source terms.The present method can study the sound generation and propagation processes.Besides,the method can effectively deal with flow-induced noise problems with complex geometries.The hybrid method is applied to simulate the sound generated by flow around the bluff body to verify the accuracy and reliability of the method.3.The DNS-APE hybrid method is improved for simulating flow-induced noise problems with moving boundaries.A boundary treatment method,which consists of motion boundary reconstruction,fresh cell interpolation and ghost cell calculation,is developed to solve the hydrodynamic and acoustic equations with moving boundaries.The method is applied to simulate the sound produced by the oscillating cylinder.Results indicate that the oscillating motion has an important impact on flow field and acoustic field.The vortex shedding can be changed by controlling the oscillation frequency of the cylinder to reduce the flow-induced noise.4.The sound generation and propagation processes of the typical bio-inspired locomotion(flapping flight and undulatory swimming)are investigated based on the hybrid method.Results indicate that the acoustic field generated by the bio-inspired locomotion is highly related to the period vortex shedding.The sound generation and acoustic characteristics depend on the lift and drag(thrust)acting on the motion body and the fluctuation of the force.For the foil with undulatory swimming,the effects of the oscillation frequency,phase difference and spacing on the flow performance and acoustic characteristics are studied.Results show that the acoustic field produced by the single foil presents a lift dipole pattern,the thrust and the radiated noise increase monotonically with increasing frequency.For the side-by-side foil system,a dipole-like pattern is discovered in the in-phase motion,whereas a monopole-like nature is presented in the anti-phase motion.The anti-phase system can achieve a lower radiated noise and a higher thrust compared with the in-phase one under the same condition.