Experimental Study Of Noise Characteristics And Active Control Of Flow-Coupled Line Spectra In Ventilation Ducts

Fluid-induced acoustic resonance is an important noise source for ventilation duct systems with side-wall cavities.This problem is not only present in air ducts but is also prevalent in underwater vehicles and needs to be effectively suppressed.This dissertation investigates the characteristics of the narrowband resonance noise using a combination of numerical and experimental methods with a ventilation duct at low Mach numbers.On this basis,adaptive control algorithms are used to control the narrowband noise experimentally,and the effectiveness of active noise control techniques for the control of the fluid-induced resonance noise is verified.The specific innovative work is as follows:(1)An experimental system for the fluid-induced resonance noise was established,the installation structure and position of the microphone were designed and it was found that the resonance noise is a flow-cavity coupled phenomenon that arises from the flow excitation rather than the acoustic response inside the tube.The influence of the cavity depth,the incoming flow velocity and the length of the line at the back end of the cavity on the frequency of the narrowband noise is also analysed.The results show that the narrowband frequency decreases with increasing cavity depth and the incoming flow velocity;the narrowband frequency increases with increasing pipe length at the back end of the cavity and the corresponding incoming flow velocity also increases.(2)A combination of Large Eddy Simulation and the FW-H equation was used to investigate the mechanism of noise generation in the fluid-induced resonance.On the one hand,the flow field simulation results show that the turbulent vortices generated by the flow-induced cavity not only exist inside the cavity,but are also influenced by the flow in the main duct,creating vortices with higher flow velocities at the back end of the cavity.On the other hand,the sound field simulation results show that the simulation of the narrowband frequency agrees well with the experimental results,and the error between the simulation of the narrowband frequency and the experimental results at different cavity depths is around 3%,which verifies the effectiveness of the method.(3)Active control of narrowband noise using active noise control techniques.An active control system for narrowband noise is established,based on the Fx LMS algorithm to implement the control,and the effectiveness of the active control system is verified through experiments.The experimental results show that: the active control system can effectively suppress the narrowband noise energy upstream and downstream of the duct,and the noise at the fundamental and its first harmonic can achieve a noise reduction effect of 11 d B and 6d B respectively;the reference signal frequency is taken within a certain range of the system will have a control effect,and the optimal reference signal frequency and the frequency of the narrowband noise often have a large difference;the control system can effectively control the narrowband noise at the fundamental frequency while also noise reduction at its first harmonic.These characteristics are quite different from the narrowband noise under acoustic field excitation,indicating that the narrowband resonance noise has a strongly non-linear character.