Intermittent Flow and Thermal Structures of Accelerating Transitional Boundary Layers: Part 1—Mean Quantities

A conditional sampling technique was employed to separate the turbulent and nonturbulent parts of accelerated boundary layers undergoing laminar-turbulent transition on a uniformly heated flat plate. Tests were conducted with zero pressure gradient and two levels of streamwise acceleration parameter: K = 0.07 × 10⁻⁶ and 0.16 × 10⁻⁶. The conditionally sampled distribution of the skin friction coefficients revealed that the values for C_f in the nonturbulent and turbulent portions deviated significantly from the respective laminar and turbulent correlations. These deviations increased as acceleration increased. Reconstructing the local average C_f values using the laminar and fully turbulent correlations consistently overestimated the unconditioned C_f values. Using the conditionally sampled data for reconstructing C_f values provided better results, but does not necessarily result in the same unconditioned C_f values. The mean velocity profiles from the turbulent portions had the appearance of a low-Reynolds-number turbulent boundary layer with a large wake region. In the late transition region, as acceleration increased, the wake region in the turbulent portion was suppressed relative to the unconditioned result. The integral parameters, δ*, θ, and shape factor, H, were conditionally sampled and analyzed.