The Mechanism Of Laminar-turbulent Transition And The Characteristics Of Turbulence In A Supersonic Boundary Layer On A Flat Plate

In this paper, direct numerical simulation (DNS) has been done for the laminar-turbulent transition and the fully developed turbulence of a supersonic boundary layer on a flat plate with Mach number 4.5. Then studies of the essential mechanism of breakdown in laminar-turbulent transition and important characteristics of the fully developed turbulence have been carried on. Investigation of the method of generating inflow boundary conditions for the spatial mode direct numerical simulation (SDNS) of a fully developed turbulent boundary layer, using the results of temporal mode direct numerical simulation (TDNS) has also been done. Results obtained include:1. During the breakdown process in laminar-turbulent transition, the mechanism leading to the swift change of mean flow profile from laminar to turbulent and the rapid increase of disturbance energy was found to be the modification of mean flow profile by the disturbances, which greatly changed its stability characteristics. The unstable zones of the first mode and its maximum amplification rate were drastically enlarged, while the unstable zones of the second mode decreased gradually and even disappeared. Though the most unstable T-S wave was of second mode for laminar flow, during the breakdown process in laminar-turbulent transition, the first mode unstable waves would quickly be excited and enhanced, resulting in the rapid increase of disturbance energy, leading to the swift change of mean flow profile from laminar to turbulent. Namely the first mode unstable wave played the key role in the breakdown process. Though the mode and the amplitude of the initial disturbance waves would have impact on the time of beginning and the lasting time of the breakdown process in the transition, but the mechanism of breakdown is always the same. It was also found that only the sharp increase of skin-friction coefficient reflected the beginning of breakdown process in the transition.2. The normal-wise distribution of several statistical mean values of disturbances of a fully developed turbulence, such as the mean flow profile,...