Study of Normal Shock Behavior Near Second Throat
The location of normal shock, in most of the internal flows, plays an important role in deciding the start or unstart of a wind tunnel or intake model. In such cases, phenomena like shockwave boundarylayer interaction (SBLI), upstream and downstream pressure disturbances, change in geometry of the section, frictional effects, etc are the factors which can be associated with the shock oscillations. An attempt was made by Bogar et. al., to understand the normal shock oscillations inside the diverging section of a diffuser and the study concentrated more on the acoustical aspects of the supersonic flows. An analytical study, was presented by Culick and Rogers  arguing the stability and instability of normal shock in converging and diverging sections. This study also encompassed the acoustic field only. Later on, an experimental study on natural shock oscillations by Matsuo and Kim  proposed that upstream moving disturbances influence the normal shock location. Another experimental study on forced shock oscillations conducted by Ott et. al. claimed that shock displacement amplitude decreases with increasing excitation frequency. A more elaborate study was presented by Bur et. al. on forced oscillations proposing that shock oscillations are affected by wave fronts of second acoustic mode originating at second throat and run upstream. Recently, Bruce and Babinsky  studied this complex phenomenon of shock oscillations in a constant area duct and proposed that shock response to the back pressure variations, is by change in their relative strength, by moving so that their relative mach number matches the pressure jump. Current understanding has not yet reached the level where unsteady shock motion can be predicted reliably, especially in case of variable area duct. Since most of the intake designs are of varying cross section, and in real situations they encounter variations in stagnation pressure, the present study is to understand shock oscillations in the vicinity of the second throat (converging and diverging section) with the upstream stagnation pressure variations. Shock oscillations in diffuser have been observed with the increment and decrement in stagnation pressure.
KeywordsStagnation Pressure Normal Shock Schlieren Image Settling Chamber Shock Location
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