Influences of wall disturbances on coherent structures in supersonic turbulent boundary layers

Abstract

In the present study, the influences of wall disturbances on coherent structures and the corresponding turbulent transport in supersonic turbulent boundary layers are investigated. The free-stream Mach number is set as 2.0. The database is scrutinized to present the instantaneous distributions and spectral properties of momentum and heat transfer related flow quantities. In their important roles in enhancing the momentum and heat transfer, the wall disturbances lead to the energetic turbulent fluctuations with the length scales of the wall disturbance wavelength and its harmonics in the near-wall region. The large-scale motions are also excited with the spanwise length scales of the turbulent boundary layer thicknesses, which, by reasoning, is caused by nonlinear interactions between wall disturbances and turbulent motions at some certain scales. They also produce intense density and pressure fluctuations that penetrate the boundary layer by deforming the sonic surfaces and radiate towards the free stream, where the fluctuations remain isentropic processes in nature. During this process, the steady wall disturbances are distorted by the turbulence, therefore endued with the features of multi-scale and multi-frequency instead of remaining energetic at a single wavelength or frequency.