On the structure of a turbulent spot in a heated laminar boundary layer

An artificially generated turbulent spot was investigated experimentally in a heated boundary layer using a rake of mini-thermocouples. Simultaneous temperature traces were used to determine the spot's leading and trailing edge characteristics. The measurements on the centerline of the plate at a constant velocity and variable streamwise positions provided a Re_x range of 2.45–12.6 x 10⁵. At one axial station the free stream velocity was varied and off-axis measurements were obtained.The shape of the interface between the spot and the surrounding laminar flow was described by conventional and conditional histograms. The length of the spot's leading edge (i.e. the overhang) increased linearly with the downstream distance. Off-axis, the length of the overhang decreased slightly towards the wing tips but became larger with respect to the spot's streamwise extent. The trailing edge changed less significantly in the downstream direction.It was shown that when two spots are generated such that the leading edge of the upstream spot is in the calmed region of the downstream spot, the celerity of the upstream spot is decreased and the spots ultimately merge.