BOUNDARY-LAYER-TRANSITION IN ACCELERATING FLOWS WITH INTENSE FREESTREAM TURBULENCE .1. DISTURBANCES UPSTREAM OF TRANSITION ONSET

Hot-wire anemometry was employed to examine the laminar-to-turbulent transition of low-speed, two-dimensional boundary layers for two (moderate) levels of flow acceleration and various levels of grid-generated free stream turbulence. Flows with an adiabatic wall and with uniform-flux heat transfer were explored. All of the experimental test cases resulted in bypass-mode transitions, a conclusion based upon the observance of spots upstream of the theoretical minimum critical Reynolds number (three cases) or, for one case, upon the evidence that T-S mode amplification played no apparent role in the transition. Data obtained for the preonset stage indicate that the streamwise-component fluctuation-amplitude distributions, frequency distributions and outer-region waveforms of these bypass-mode transitions were similar to those reported in the literature for low-freestream-turbulence transitions. Within the zone upstream of the first appearance of turbulent spots: (1) The near-wall ( Y < delta/2) fluctuations were predominantly low-frequency (frequency approximately 115 of that of the most amplified T-S disturbances). (2) The maximum streamwise-component fluctuations occurred over the altitude band 0.3 < Y/delta < 0.4. (3) Very strong negative "spikes" in streamwise velocity were observed, just upstream of spot initiation, at boundary-layer altitudes near Y/delta = 0.6.