Dilatational motions in the shape of travelling wave packets have been identified recently to be dynamically significant in hypersonic turbulent boundary layers. The present study investigates the mechanisms of their generation and their association with the solenoidal motions, especially the well-recognized near-wall self-sustaining process of the regeneration cycle between the velocity streaks and quasi-streamwise vortices. By exploiting the direct numerical simulation databases and orchestrating numerical experiments, we explore systematically the near-wall flow dynamics in the processes of the formation and transient growth of low-speed streaks. We conclude via theoretical ansatz that the nonlinearity related to the parallel density and pressure gradients close to the wall due to the restriction of the isothermal boundary condition is the primary cause of the generation of the dilatational structures at small scales. In fully developed turbulence, the formation and the existence of healthy dilatational travelling wave packets require the participation of the turbulence at scales larger than those of the near-wall regeneration cycles, especially the occurrence of the bursting events that generate vortex clusters. This is proven by the less intensified dilatational motions in the numerical experiments in which the Orr mechanism is alleviated and the vortical structures and turbulent bursts are weakened.

中文翻译：

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高超声速湍流边界层中近壁膨胀运动的产生

最近发现，行波包形状的膨胀运动在高超声速湍流边界层中具有动态显着性。本研究研究了它们的产生机制及其与螺线管运动的关联，特别是速度条纹和准流向涡流之间再生循环的公认的近壁自持过程。通过利用直接数值模拟数据库和组织数值实验，我们系统地探索了低速条纹形成和瞬态生长过程中的近壁流动动力学。我们通过理论分析得出结论，由于等温边界条件的限制，与靠近壁面的平行密度和压力梯度相关的非线性是小尺度膨胀结构产生的主要原因。在充分发展的湍流中，健康膨胀行波包的形成和存在需要比近壁再生循环更大尺度的湍流的参与，特别是产生涡团簇的爆发事件的发生。数值实验中膨胀运动强度较小，奥尔机制得到缓解，涡旋结构和湍流爆发减弱，证明了这一点。