| Summary: | 博士 === 國立成功大學 === 航空太空工程學系碩博士班 === 95 === The present study focuses on the noise problem associated with a vehicle’s exhaust pipe. A pulsated flow with weak pulsating blast waves from a vehicle engine is discharged out of the vehicle’s exhaust pipe. The discharged pulsated flow and the induced shock/vortex interactions that result in radiated noise are numerically investigated by the axi-symmetric Euler/Navier-Stokes solver with the parallel computational technology. The numerical method used is a fifth-order weighted essentially non-oscillation scheme of Jiang and Shu for spatial discretization and a fourth-order non-TVD Runge-Kutta method for time integration.
In order to effectively reduce the radiated noise due to shock/vortex interactions near the pipe exit, the pipe shape near the exit is redesigned by using a smoothed curved, expanding shape near the pipe exit, instead of the traditional straight shape. The detailed flow field downstream of the pipe and its sound pressure level are studied. When the flow field has become a quasi-steady state, the major propagation direction of sound wave for the straight-pipe case is found in the 60° direction, which is different from the 0° direction for the curved-pipe case. Moreover, noise produced by the straight- and curved-shape pipe with periodic pulsating blast waves are analyzed and compared. The periodic pulsating wave is obtained by fitting the measured pressure data from a motorcycle exhaust pipe at an engine speed of 5000rpm at an idle condition. It is found that the periodic pulsating waves can greatly influence the downstream flow and sound fields such as the vortex–ring size, vortex-center vorticity and sound pressure level (SPL). In consideration of the frequency response of the human ear, the A-scale SPL method is applied to analyze sound pressure level. Experimental and simulated sound pressure levels are computed and compared. The results of sound pressure levels for experiment and simulation show that their absolute error in the maximum A-weighting SPL is 0.8dBA for the curved-pipe case and 1.4dBA for the straight-pipe case. Moreover, their absolute error in the minimum A-weighting SPL is 1.7dBA for the curved-pipe case and 0.9dBA for the straight-pipe case. It is proved that the partially curved, expanding pipe can reduce the noise region downstream of the pipe.
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