In this paper, some schemes of reducing the aerodynamic noise are proposed for multi-blade centrifugal fans. Firstly, the vortex noise can be reduced mainly through the optimization of the geometric parameters of the impeller and volute and installing a rotation diffuser in the outlet of the impeller. Secondly, the discrete noise can be reduced mainly by changing the form of volute tongue. Finally, the experimental verification is carried out for the two improved fans, one is designed with optimization of inlet and outlet blade angles, and the other is installed a rotation diffuser in the outlet. The tests show that the noise of the improved fans is reduced obviously in comparison with the prototype.

Numerical simulation and analysis of fluctuating pressure on outside surface around model was conducted using large eddy simulation of soft ware Fluent. It is obtained that the scattergram of fluctuating pressure on outside surface of high-speed automobile. The result shows that the scattergram of fluctuating pressure by large eddy simulation coincides with the test results. It can be concluded that the calculations are available for project but the precision of simulation need be improved.

There are various active noise control algorithms for engine exhaust noise simulation. However, their control effects are different due to the different complexities. To find out a more effective control algorithm, a simulink model for simulation of noise control of an engine exhaust is established using MATLAB code, and the simulation is done based on the LMS algorithm. From the simulation, the limitation of the LMS algorithm is evaluated in the case that the error acoustic path exists, and the LMS algorithm is improved for better control effect.

The finite element method is used to analyze the acoustic characteristics of an expansionchamber muffler with its inlet and outlet pipes being coaxial. Its transmission loss (TL) is calculated. The result is compared with that of the onedimensional solutions based on planar wave theory. The limitation of onedimensional planar wave theory is discussed. The acoustic characteristics of eccentric outlet muffler, doubleoutlet muffler and doublecavity muffler are analyzed. The result indicates that the position and the number of outlet can affect the acoustic characteristic of a muffler for higher and medium frequencies; while the doublecavity muffler has high noiseelimination efficiency for lower and medium frequencies.