Estimating turbulent spot initiation rates from transition lengths in hypersonic boundary layers

Experimental evidence indicates that boundary layers in hypersonic flows transition from being laminar to turbulent via the mechanism of formation and propagation of turbulent spots. Turbulent spots have been detected using either surface heat transfer gauge signals or pressure traces, or they have been visualized using high-speed schlieren techniques. The extent of the transition zone is strongly dependent on the rate of initiation of turbulent spots and spot propagation parameters. However, it is not possible at present to predict the turbulent spot initiation rates in hypersonic transitional flows. As a first step toward developing an effective predictive model for the purpose, a methodology is developed to enable estimation of the spot initiation rate and intermittency distribution from experimental heat transfer data. The methodology is validated by comparison with experimental data where both the heat transfer and intermittency were measured. The methodology is then used in a predictive mode on three experimental datasets, and it is found to give good agreement. This enables prediction of other important variables as well, including skin friction and a wide variety of other boundary-layer properties in the transition zone at hypersonic Mach numbers