The origin of turbulent spots

It has been suggested that a turbulent spot is formed when a transient sepa ration occurs ill the laminar boundary layer and this criterion has been su ccessfully used by Johnson and Ercan (1996, 1997) to predict bypass transit ion for boundary layers subjected to a wide range of free-stream turbulence levels and streamwise pressure gradients. In the current paper Experimenta l results are presented that support the premise that the formation of turb ulent spots is associated with transient separation. Near-wall hot-wire sig nals in laminar and transitional boundary layers are analyzed statistically to produce probability distributions for signal level and trough frequency . In the laminar period the signal level is normally distributed, but durin g the inter-turbulent periods in the transitional boundary layer, the distr ibution is truncated at the lower end, i.e,, the lowest velocity periods in the signal disappear; suggesting that these are replaced during transition by the turbulent periods. The number of these events (troughs) also correl ates with the number of turbulent spots during early transition. A linear p erturbation theory is also used ill the paper to compute the streamlines th rough a turbulent spot and its associated calmed region. The results indica te that a hairpin vortex dominates the flow and entrains a low-momentum flu id stream from upstream with a high-momentum stream from downstream and the n ejects the combined stream into the turbulent spot. The hairpin can only exist if a local separation occurs beneath its nose and the current results suggest that this separation is induced when the instantaneous velocity in the near-wall signal drops below 50 percent of the mean.