Combined effects of elevated free-stream turbulence and streamwise acceleration on flow and thermal structures in transitional boundary layers

The combined effects of elevated free-stream turbulence intensity (FSTI) and streamwise acceleration on flow and thermal structures in transitional boundary layers were investigated experimentally on a heated flat plate with elevated FSTI levels ranging from 3 to 7%. The acceleration strength, based on the parameter, K [≡ (ν/ U ∞ 2)d U ∞/dx] , rangedfrom 0.39 × 10 −6 to 4.1 × 10 −6. A three-wire probe was used to measure detailed momentum and thermal boundary layer structures, including the streamwise and cross-stream velocity fluctuations, temperature fluctuation, Reynolds stresses, Reynolds heat fluxes, eddy viscosity, turbulent thermal diffusivity, and turbulent Prandtl number. The results show that FSTI has a greater effect than streamwise acceleration on flow and thermal structures in transitional boundary layers. However, the effect of streamwise acceleration on increasing the transition length is significant. Furthermore, streamwise acceleration suppresses the magnitude of uv and slows down the uv transition process relative to u′ even when elevated FSTI seems to have a dominant effect. Acceleration is also shown to suppress ϵ H more than ϵ M in the transitional region.