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 freestream turbulence. Flows with an adiabatic wall and with uniform-flux heat transfer were explored. Conditional discrimination techniques were employed to examine the zones of flow within the transitional region. This analysis demonstrated that as much as one-half of the streamwise-component unsteadiness, and much of the apparent anisotropy, observed near the wall was produced, not by turbulence, but by the steps in velocity between the turbulent and inter-turbulent zones of flow. Within the turbulent zones u′/v′ ratios were about equal to those expected for equilibrium boundary-layer turbulence. Near transition onset, however, the turbulence kinetic energy within the turbulent zones exceeded fully turbulent boundary-layer levels. Turbulent-zone power-spectral-density measurements indicate that the ratio of dissipation to production increased through transition. This suggests that the generation of the full equilibrium turbulent boundary-layer energy cascade required some time (distance) and may explain the very high TKE levels near onset.
Boundary-Layer Transition in Accelerating Flows With Intense Freestream Turbulence: Part 2—The Zone of Intermittent Turbulence
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Blair, M. F. (September 1, 1992). "Boundary-Layer Transition in Accelerating Flows With Intense Freestream Turbulence: Part 2—The Zone of Intermittent Turbulence." ASME. J. Fluids Eng. September 1992; 114(3): 322–332. https://doi.org/10.1115/1.2910033
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