STUDIES OF SUPERSONIC NIXING AND ITS CONTROL

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The success of a future supersonic combustion ramjet engine highly depends on the efficiency of fuel-air mixing. The present project is aimed at contributing to the development of supersonic miximg enhancement techniques for Scramjet. The difficulty is in that the compressibility suppresses the growth of mixing layrs with increasing the convective Mach number. To see how the compressibility hampers the turbulent motions and how to alleviate its effects, we have analyzed important previous results for the growth of the supersonic mixing layrs. We then propose to use streamwise vortices to enhance the supersonic mixing and conduct experiments of introducing streamwise vortices into a supersonic wake at Mach numbers 2.4,2.5 and 4. The experiments verify that streamwise vortices can be easily generated. Although the streamwise vortices introduced are rather stable downstream a high rate of fluid entrainment into the vortices can be obtained even during their formation, indicating the capability of enhancing the supersonic mixing. We have tried and succeeded in generating small-scale eddies through the breakdown of the streamwise vortices (with various combinations of intensities, scales, rotations and in spanwise row configurations). It is also successfully tried to cause their breakdown by small scale eddies (and/or smaller scale streamwise vortices) embedded in them at their initial formation, by their mutual interaction in spanwise row configurations, by their interaction with incident weak shock waves, and by their own instabilities. We find that the streamwise vortices can develop violent instabilities for both co-and counter-rotating spanwise rows and breakdown to smaller scales to lead to efficient mixing, in particular when the initial vorticity distribution of each vortex satisfies Rayleigh criterion for the inflectional instability. With these results it is stressed that the streamwise vortices are really powerful in enhancing the supersonic mixing.