Research On The Hydrodynamic Noise Mechanism Of Underwater Open Airfoil Cavity

There are inevitably holes of various sizes and shapes on the surface of underwater vehicles,and noise is inevitably generated during navigation,so the analysis of the flow noise mechanism of underwater opening is of great significance in engineering applications.The opening on the surface of an underwater vehicle will destroy the continuity of the movement of the fluid on its surface,resulting in the phenomenon of flow separation during the movement of the fluid.Therefore,the fluid movement at the opening is very unstable,and the pressure fluctuation at the opening will cause vibration and acoustic radiation of the opening and cavity.In general,for openings with cavities,the shear layer that comes out of the cavity mouth forms this unstable shear layer flow that fluctuates between the outflow field of the cavity and the internal flow field of the cavity,which directly radiates noise on the one hand:and affects the trailing edge of the opening on the other.This effect increases the local pulsating pressure at the trailing edge of the opening,excitting the cavity to vibrate and producing secondary acoustic radiation.Based on the airfoil shape of the underwater vehicle enclosure,this paper studies the noise of the underwater open airfoil cavity under flow excitation,and analyzes and summarizes the flow noise mechanism of the underwater open airfoil cavity,which is of great significance for the vibration and noise reduction of the submarine enclosure.In this paper,the flow noise mechanism of underwater open airfoil cavity is studied from three aspects: theoretical analysis,simulation calculation and experimental measurement.Through theoretical explanation and acoustic analogy,the flow noise source when there is a solid interface in the turbulence of motion is determined,and the influence of wall conditions on the coupling resonance of underwater open mouth is analyzed by the self-sustaining oscillation mechanism and coupling conditions of the shear layer of the underwater open mouth,and the conditions of standing wave resonance and self-sustaining oscillation of the shear layer in the cylindrical cavity are explained.Through the numerical simulation of the flow field,the flow of the internal and external flow fields of the underwater closed airfoil cavity and the underwater open airfoil cavity was simulated.The complex flow of the internal flow field of the underwater open airfoil cavity was analyzed: the shear layer was separated into two parts with the impact of the cavity opening,one part flowed downward along the flow direction,and the other part entered the cavity to form disturbances in the cavity and finally rolled up a large-scale vortex,and the shear layer generated pulsating pressure when it hit the edge,and the pulsating pressure went upward and then affected the shear layer shedding upstream.The results of the numerical simulation calculation of the flow field are used as the sound source of acoustic numerical simulation calculation,and the self-noise and radiated noise of the open airfoil cavity under different flow speeds are predicted by the acoustic numerical simulation calculation of the open airfoil cavity: the self-noise of the open airfoil cavity increases with the increase of the flow rate,and the multi-order formant can be excited at high flow rate,and the frequency of the first-order formant is roughly 232 Hz.Unlike acoustic radiation from closed airfoil cavities,acoustic radiation from open airfoil cavities increases with increasing flow velocity along the direction of the airfoil wall.The relationship between flow velocity and radiated sound power of open airfoil cavity was verified by experimental research on underwater open airfoil cavities: the radiated sound power of closed airfoil cavities was proportional to the 6.2 power of flow velocity,and the radiated sound power of open airfoil cavities was proportional to the 6.3 power of flow velocity.The acoustic resonance oscillation of the open airfoil cavity under different flow velocities was studied,and the self-noise characteristics of the underwater airfoil cavity were analyzed,and the self-noise of the open airfoil cavity was greater than that of the closed airfoil cavity,and the resonance peak caused by the oscillation of the fluid in the cavity appeared around 200 Hz,and the mean square sound pressure level of the formant was about proportional to the 7th power of the flow rate,and the amplitude of the sound pressure of the higher-order formant increased with the flow velocity higher than that of the first-order formant.The self-sustaining oscillation characteristics of the shear layer of the open airfoil cavity were analyzed: the resonance peak generated by the self-sustaining oscillation of the shear layer appeared around 10 Hz,the resonance peak frequency increased with the increase of the flow rate,St/Lc=f/U was approximately equal to 1.6,and the self-sustaining oscillation of the shear layer coexisted with the fluid oscillation in the open airfoil cavity.