STUDIES OF SUPERSONIC NIXING AND ITS CONTROL

未来超音速燃烧冲压发动机的成功与否，在很大程度上取决于燃料-空气混合的效率。本项目旨在为超燃冲压发动机超音速混合增强技术的发展作出贡献。问题的关键在于，随着对流马赫数的增加，可压缩性抑制了混合层的增长。为了了解可压缩性如何阻碍湍流运动以及如何减轻其影响，我们分析了以前关于超音速混合层增长的重要结果。然后，我们建议使用流向涡来增强超声速混合，并在马赫数为2.4，2.5和4的超声速尾流中引入流向涡进行实验。实验结果表明，在非均匀流场中容易产生流向涡。虽然引入的流向涡在下游相当稳定，但即使在其形成期间也可以获得高的流体夹带率，这表明了增强超音速混合的能力。我们已经尝试并成功地通过破坏流向涡（强度、尺度、旋转和展向排配置的各种组合）来产生小尺度涡。它也成功地试图通过在它们的初始形成时嵌入其中的小尺度涡（和/或较小尺度的流向涡），通过它们在展向排配置中的相互作用，通过它们与入射弱激波的相互作用以及它们自身的不稳定性来引起它们的破裂。我们发现，流向涡可以发展暴力的不稳定性，无论是同向和反向旋转的展向行和崩溃到较小的尺度，导致有效的混合，特别是当每个涡的初始涡量分布满足瑞利准则的弯曲不稳定。这些结果表明，流向涡在增强超音速混合方面确实是很强大的。

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.