Relatively high-pressure and temperature exhaust gases were discharged from automobile engine cylinders to exhaust pipes and coalesced, with their propagation, into weak shock waves, which eventually became major sources of exhaust gas noises. The suppression of these weak shock waves is one of the practical applications of shock wave research [1]. It was a goal of our research to establish a reliable concept of designing automobile silencers and mufflers. Traditionally the silencers and mufflers have been designed, in principle, based on the acoustic theory. However, the waves to suppress were not linear waves but weak shock waves. We intensively performed parametric study of surface roughness, perforation, distributed obstacles, and slightly complicated passages arranged in various shock tube geometries and at shock Mach number ranging from 1.05 to 3.00 in air by using double exposure holographic interferometry and shadowgraphs.
Then interferometric images so far collected were compared with numerical simulations, although, in those days, the levels of CFD were very fundamental, and we, presumably a little, deepened the understanding of physics of weak shock wave mitigation. Then the result of such a fundamental research was directed in part to the design of the silencer and muffler of Subaru Legacy 1993 version. In this presentation, we will summarize interferograms of the mitigation of weak shock waves so far collected over 20 years.
Figure 1 shows sequential interferograms of shock wave attenuation over a roughened surface. Experiments were performed in a 60mm ×150mm conventional shock tube at shock Mach number Ms = 1.41 in atmospheric air. Surface roughness on upper and bottom walls consists of grooves of 1mm width, 1.5mm interval, and 5mm in depth. The incident shock wave propagates from bottom to upward. The incident shock wave interacted with individual grooves creating expansion waves, which successively interact with the incident shock. With propagation the incident shock wave is attenuated.
(a) (b)
Fig.1 Sequential interferograms of shock wave mitigation along roughened walls in atmospheric air at 297.3K: (a) 90092808 delay time 1.0ms Ms = 1.413; (b) 90092810 delay time 0.74ms Ms = 1.412
[1] N. Sekine et al: Generation and propagation of shock waves in exhaust pipe of a 4 cycle automobile engine, Current Topics in Shock waves, Proc.17 ISSW&ST, Bethlehem (1989), pp.671-676.
