Study On Acoustic Radiation Mechanisms Of Flow-induced Underwater Open Cavity

The vibration and noise of an underwater vehicle in the low speed test show that acoustic radiation at some spectrums of low frequency band is the main characteristic of radiated noise. In order to satisfy requirement of quiet underwater vehicle, the first task is to study on mechanisms of the low frequency noise spectrum and seek effective ways to control the sound radiation. One of main noise sources of underwater vehicle is the hydrodynamic noise. Holes arranged on the surface of the cavity structure are the most likely to produce low-frequency peak hydrodynamic noise. The current research and published results on vibration and sound radiation of flow-induced open cavity are quite a lot, but mostly limited to simple open cavity structure in the air. While the research and published results study on the underwater complex open cavity are seldom.The water hole and cavity structures of underwater vehicle are selected as the major research objects in present study. The mechanisms of flow-induced self-sustained oscillation, the direct acoustic radiation, the second acoustic radiation, hydrodynamic resonance and water elastic resonance are studied in detail by theoretical analysis, numerical simulation and experiment. The major research contents are presented as follows,1、A 3D simplified open cavity model of underwater vehicle is established. The excitation force distribution of flow-induced open cavity is obtained based on a hybrid CFD numerical method. The mechanism of open cavity flow around the cavity is investigated by the vorticity contours.2、Considering the rigid structure boundary, the effects of parameters(such as the cavity geometry parameters, flow field parameters and the porous arrangement, etc.) on the oscillation mechanism and the direct acoustic radiation mechanism of flow-induced open cavity are discussed. Research reveals that the direct acoustic radiation energy of flow-induce open cavity enhances, and fundamental frequency becomes bigger with the increasing of stream velocity. Flow-induced direct acoustic radiation of open cavity can be effectively reduced by using the circle or square shape of the cavity mouth or the 30-45 degree neck angles of hole designs.3、Based on the first-order shear deformable plate theory and Timoshemko beam theory, the semi-analytical dynamic model of rectangular cavity(the stiffened multi-plate combination structure) is formulated by the modified variational method. The advantages of the mathematical model and semi-analytical method are very convenient to select the sub domain displacement function for vibration problems with different boundary conditions and different loads, and have fast convergence, high accuracy and computational efficiency.4、Considering the elastic structure boundary,the second acoustic radiation of flow-induce random vibration is obtained by the using the simulation software. The second acoustic radiation results show that the peak values of frequency spectrum correspond to characteristic frequencies of the wet modes and excitation force without the primary and secondary.5、The Helmholtz resonator model and acoustic-fluid structure interaction(A-FSI) model are established. The contributions of aft peak tank resonance acoustic radiation for the noise spectrum measured are discussed by these models. The study shows that Helmholtz resonator resonance of aft peak tank is not the main acoustic radiation mechanism of underwater vehicle measured in low frequency band, while the water- elastic resonance may be the main acoustic radiation mechanism when the hydrodynamic oscillation frequencies are close to the 32.9Hz and 34.4Hz of A-FSI modal.6、Considering the actual parameters of the new gravity water tunnel of Shanghai Jiao Tong University, the plant open cavity flow experiment is designed and preformed. Some parts of vibration and acoustic radiation mechanisms of flow-induced open cavity have been verified in the experiment.