Transitional boundary layers under the influence of strong free-stream turbulence, intensive wall cooling, and strong pressure gradients in hot gas flows

The effects of free-flow turbulence, wall cooling, and pressure gradients on the heat transfer and the boundary layer development on cooled surfaces were investigated. The measurements, performed in a plane boundary layer channel, show that even very intensive wall cooling does not change the transition behavior in boundary layers with free-stream turbulence, i.e., the outer turbulence dominates the boundary-layer-stabilizing effect of the wall cooling. The effects of free-stream turbulence in turbulent boundary layers are found to be independent of wall cooling. Wall cooling changes the boundary layer profile and enhances heat transfer. The combination of strong free-stream turbulence and strong free-stream acceleration results in stable laminar-turbulent intermediate states, accompanied by a drastic heat transfer enhancement compared to laminar flow. A numerical computation method, based on an extended integral method, was developed, and allowed a good confirmation of the experimental results, which can be directly applied to the design of cooled gas turbine blades.