由大粗糙元引起的高超声速边界层强制转捩在航天技术中有实际应用，因而近年来受到人们的广泛关注。虽然目前导致该转捩过程的内在机理尚不完全清楚，但有一点是明确的，即粗糙元的尾迹流场中存在强对流不稳定性。文章的出发点是研究这种对流不稳定模态是如何触发转捩的。首先通过CFD方法，计算出高超声速边界层中粗糙元的尾迹流场，并对其进行二维稳定性分析。结果发现，在传统不稳定Tollmien⁃Schlichting（ T⁃S）模态出现的临界Reynolds数之前，存在高增长率的无黏不稳定模态，表现为对称的余弦模态和反对称的正弦模态。然后对该不稳定模态在粗糙元尾迹流中的演化进行了模拟，验证了二维稳定性分析的结果，并考察了非平行性效应的影响。最后通过直接数值模拟，研究由这些不稳定模态触发转捩的全过程。结果表明，对流不稳定模态确实是导致边界层转捩的关键机制。该转捩过程的特点是，局部湍斑首先在不稳定模态特征函数的峰值附近出现，然后向全流场扩散。就文章研究的工况而言，余弦和正弦模态的相互作用对转捩的影响并不明显，且后者在转捩过程中起主导作用。%Roughness⁃induced transition in hypersonic boundary layers has been a topic of interest for decades, due to its relevance to practical problems. Although the mechanism of this transition process has not been well understood, it is now clear that strong convective instability exists in the wake of the roughness. The motivation of the current paper is to study how the transition is triggered by the convective instability modes. First, the wake of the roughness element in a hypersonic boundary layer was obtained by CFD approach, and 2D instability analysis on the obtained wake flow was performed. The results show that, high⁃growth⁃rate instability modes of inviscid nature are found at Reynolds numbers appreciably lower than the critical Reynolds number for the appearance of the unstable Tollmien⁃Schlichting ( T⁃S) modes, and these modes can be either varicose ( symmetric) or sinuous ( anti⁃symmetric) . Then, the evolution of the instability modes in the wake of the roughness element was numerically simulated, which confirmed the results of the 2D instability analysis and also showed the impact of the non⁃parallelism. Finally, through direct numerical simulation ( DNS) , the transition process trig⁃gered by these instability modes were investigated. It is confirmed that the convective instability modes play the leading role in boundary⁃layer transition. The scenario of transition is that, local turbulent spots firstly emerge in the vicinity of the peak of the eigen⁃function of the instability modes, and then spread into the whole flow field as convecting downstream. For the cases studied in this paper, the impact of the interaction between the varicose and sinuous modes on transition is not obvi⁃ous, and the latter plays the dominant role in the transition process.
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