Transitional boundary layer spot in a fully turbulent environment

A spark was used to initiate and mark in time a turbulent spot in an initially laminar boundary layer. This marked spot of turbulence merged and interacted with the natural turbulent boundary layer generated by a row of spherical trips. By using a digital technique to align individual spot signatures, thus correcting for variations in the transit time to a given measurement station, a structure was tracked over a streamwise extent of 70 average turbulent boundary-layer thicknesses. The scale of the structure is of the order of 10δ in the streamwise direction becoming 2-3δ in the interface region of the boundary layer and is less than 4δ in the spanwise direction, in spite of the fact that no spanwise alignment was performed. The structure is characterized by a convection speed of 0.9 U_∞. It exhibits features in detailed agreement with those at the outer region of the turbulent boundary layer (interface region) and is consistent with existing two- and three-point space-time cortelations.