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做出贡献。难点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声速混合层生长的重要结果。然后，我们提出利用流向涡增强超声速混合，并在马赫数为2.4、2.5和4的超声速尾迹中引入了流向涡的实验。实验证明，该方法可以很容易地产生流向涡。虽然引入的顺流涡旋在下游相当稳定，但即使在涡旋形成过程中，也能获得较高的流体夹带率，这表明涡旋具有增强超声速混合的能力。我们已经尝试并成功地通过分解流向涡旋（具有各种强度、尺度、旋转和展向行构型的组合）来产生小规模的涡旋。人们还成功地尝试通过在它们最初形成时嵌入的小规模涡流（和/或较小规模的流向涡）、它们在展向行构型中的相互作用、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导致它们的破坏。我们发现，顺流涡旋在共旋和反旋的展向行中都可以发展出剧烈的不稳定性，并分解成更小的尺度以导致有效的混合，特别是当每个涡旋的初始涡度分布满足瑞利不稳定性准则时。这些结果强调了流向涡对超声速混合的增强作用。

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.