Turbulent spots in hypersonic transitional planar and axisymmetric boundary layers

Experiments were conducted to investigate the characteristics of turbulent spots formed in transitional boundary layers developed over a flat plate and an axisymmetric cone placed in similar hypersonic free-stream environment of Mach number 5.85. The free-stream unit Reynolds number in the present work varied in the range of $(3.0{-}6.0)\times 10^6$ m −1 . Heat transfer measurement along the surface of both the test models was used to ascertain the state of boundary layer and to calculate the intermittency associated with the transitional boundary layer. Turbulent spots generated in the transitional boundary layer were characterised in terms of their leading–trailing-edge velocities, their streamwise length scales and their generation rates on both the test models. The leading edge of the turbulent spots developed over both the test models were found to be convecting at a speed equivalent to 90 % of the boundary layer edge speed. The trailing edge of the spots developed on a planar boundary layer traversed at a lower speed than its axisymmetric counterpart. Streamwise length scales of a turbulent spot developed in a planar boundary layer grew at a higher rate when compared with the axisymmetric boundary layer. Turbulent spot generation rates for both planar and axisymmetric boundary layers was found to be in the range of $10\,00\,000{-}30\,00\,000$ spots m −1 /s −1 .