Study On Aerodynamic Noise Characteristics And Numerical Predicting Method Of A Counter-Rotating Axial Fan

Problem about fan noise is one of the things that obsess the fan researchers and the users. This thesis took the study on a contra-rotating fan for example to discuss the problem about fan noise. In this thesis the LES technique was utilized to simulate the 3-D unsteady full flow field of the fan. The FFT technique was employed to analyze the numerical results from the CFD. In this thesis the reasons that raise and enhance the noise were studied and a new way to forecast fan noise was presented. The main parts of the thesis were presented below:Firstly, adopt the Realizableκ-εnumerical method conduct the steady simulation. The dates of the steady simulation would be used as the initial value to continue the unsteady simulation. Then use the LES method to finish the unsteady simulation.Secondly, several monitoring faces were arranged in the flow-field. The characters of pressure and vortex of every monitoring faces were studied. The monitoring points were arranged at every monitoring faces. Then the FW-H technique was used to simulate the aerodynamic noise of the flow-field. Broadband Sound showed at the inlet monitoring faces of the impellers and the tip clearance. Discrete spectrum noise showed at the blade passing frequencies and the harmonic frequencies of the two impellers.Thirdly, the pressure information on monitoring faces in a period time can be attained. Then the time domain information of pressure fluctuation on monitoring points can be computed. The FFT method is used to transfer it to frequency domain information on the monitoring points. Through analyzing the relevance between the pressure fluctuation and aerodynamic noise could judge the availability to identify the noise sources and noise characteristics by the method combining the CFD computing dates and FFT method.At last, a conclusion was get:The complexity of turbulence and the interaction of vortex and noise was the main reason that made it harder to predict the conter-rotating fan aerodynamic noise. Aerodynamic noise of fans is relative with the distribution of pressure. The characters of frequency and intensity of discrete spectrum noise were coincident with the pressure fluctuation characters. Discrete spectrum noise characters could be gotten from strength of vortex. Broadband noise sources could be identified from vortex distribution. The method of using the CFD result and FFT technique to study pressure fluctuation did well at the noise source and noise type prediction. The result of the aerodynamic noise numerical simulation was instructive for the development of low noise fans and the fans noise control.