| Summary: | 博士 === 國立成功大學 === 航空太空工程學系碩博士班 === 93 === The interaction of a planar blast wave with vortices is one of the fundamental and important topics in shock wave dynamics, since the problem is closely related to the aerodynamic noise generation in high-speed aircraft and the noise emitted from a vehicle’s exhaust system or an internal combustor. In this study, the problems of the planar blast-wave propagation over a wedge and of a planar blast wave discharged from a suddenly expanded duct are considered. A high-resolution Euler/Navier-Stokes solver developed by a 5th-order WENO scheme of Jiang and Shu is employed to investigate these two problems. The present Euler/Navier-Stokes solver is validated to be reasonably accurate.
For the problem of a blast wave propagating over a wedge surface, it is found that the boundary layer developed behind the first shock wave of the planar blast wave has influence on the type of shock reflection. Because the blast-wave intensity gradually becomes weaker during propagation, a transitional Mach reflection and a double Mach reflection which may occur in the case of shock-wave reflection are not found in the blast-wave reflection at an initial pressure ratio of 100. In this case, it is found that a single Mach reflection is simply transited to a regular reflection without intermediate types of shock-wave reflections.
For the problem of a blast wave passing through the duct with a sudden expansion, three cases of shock/vortex interactions are studied in detail. The induced vortices induced by the first shock wave are respectively arranged to interact with the second shock wave and a reflected shock wave at a different time sequence, and with both simultaneously. Basically, the first shock wave can induce two main vortices called as major and minor vortices. These two vortices form a pair and move together. In order to study the flow fields induced by the shock/vortex interactions, the computational shadowgraph and computational schlieren techniques are used. We found that the trajectory of the major vortex center may be different for these three cases, resulted from the effects of the expansion waves issuing from the sharp corner and of another small vortex. Moreover, the effects of different viscous models have influence on the induced vortex strengths. Thus the vortex strength obtained by the inviscid-flow model is stronger than those for the viscous-flow models, resulting in a lower pressure at the vortex center for the inviscid-flow case compared with the viscous case. Because of the boundary layer development along the duct wall after the sudden expansion, the first-shock reflection type is changed from a regular reflection to a single Mach reflection that can affect the arrival time of the reflected shock wave at the sharp corner. Finally, the mechanism of the vortex formation is mainly analyzed based on the inviscid-flow model. It is found that the compressibility term plays an important role in vorticity generation and that the baroclinic effect is more effective than the dilatation effect.
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