| Summary: | 博士 === 國立成功大學 === 航空太空工程學系碩博士班 === 91 === Environment protection issues have become important in recent years. Particularly, noise emitted from automobiles is a serious problem. Noise emitted from an internal combustion engine and an exhaust system can be classified as three types: turbulence noise, shell noise, and radiated noise. In the past, most papers were related to the radiated noise concentrated on shock/vortex interaction, which was one of the major sources of noise and was closely related to some aerodynamic problems. In this study, the problem of a planar blast wave discharged from a duct exit is considered. A numerical approach of a 5th-order WENO scheme of Jiang and Shu (1996) is used to investigate this problem. The fundamental study of planar blast wave/vortex interaction is a stepping-stone to understand the noise generation and the basic mechanism. The present results show that weak blast waves result in regular reflections associated with the blast-wave front while strong blast waves result in Mach reflections. The sound generated by the planar blast/vortex interaction is of quadrupolar nature, as in a shock/vortex interaction. The precursor generated by the blast/vortex interaction tends to decay like r-1 in the near field and r-1/2 in the far field as in the corresponding shock/vortex interaction. As for the second sound, it seems to decay like r-1/2 in the far field. The fourth sound generated by a planar blast/vortex interaction is found to exist in the present study, but not in the case of shock/vortex interaction.
It is also found that the number of sound is dependent on the strength of the blast wave. The secondary shock/vortex interaction plays an important role in the vorticity generation process. Moreover, it is found that the induced vortex pair rotates cyclically after the interaction, and the circulation (or strength) of the two vortices is constant when the incident blast wave has moved far downstream.
Generally, there are two kinds of methods for noise control, one is active control and the other is passive control. For active control, it needs additional devices to suppress noise and usually costs. For passive control, modifying the exhaust system geometry can be easily applied and is often used. In practical applications, modifying exhaust system geometry is a common and useful method to control noise, so a smoothed corner at the duct exit is adopted and is investigated with different values of two parameters – the smoothing angle (δ) and the radius of the smoothed surface (R), where R·δ is the circular arc length for the smoothed corner. The computed results show that weakening the vortex strength can successfully reduce the noises associated with the shock/vortex interaction due to the blast wave diffraction around the duct corner. Moreover, It is found that the maximum noise reduction occurs at θ = 0º (along the centerline of the duct) and the minimum noise reduction occurs at θ = 90º (along the vertical wall at the duct exit).
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