Measurement of turbulent spot convection rates in a transitional boundary layer

A new technique, based on the analysis of the signature of turbulent spots on the heat flux at the wall, is used to estimate the propagation rates of naturally occurring turbulent spots in a zero pressure gradient transitional boundary layer. Experiments were performed in a compression-heated transient aerodynamic facility using a flat plate with 32 thin-film heat transfer gauges along its centerline. Simultaneous wide-bandwidth heat transfer measurements at a number of streamwise locations were used to track the coherent transitional activity along the plate. The free-stream turbulence intensity was about 0.5%, and tests were performed at unit Reynolds numbers in the range 1.8 × 106−4.2 × 106 m−1. Special data reduction techniques were used to calculate the convection velocities of the leading and trailing edges of the turbulent spots, and these results compared well with those for artificially generated spots. The intermittency distribution in the transition zone showed good agreement with the “universal” distribution of Narasimha, confirming the reliability of this technique.