Growth of turbulent spots in high-speed boundary layers on a flat plate

We investigate the development of isolated turbulent spots in supersonic flat plate boundary layers by direct numerical simulations. The spot structure is analysed and mean velocity profiles and Reynolds stresses are determined by averaging, for one flow parameter set, over an ensemble of simulations. Besides the Mach number also the Reynolds number affects the spot growth. Strong wall cooling leads to elongated turbulent spots. The asymptotic spreading of linear instabilities is investigated and its relation to the spot growth is discussed. The near-field reveals waves propagating away from the spots. Within the boundary layer they appear as distinct semi-circular patterns.