Unsteady heat transfer in subsonic boundary layers

Unsteady boundary layer transition experiments are performed in a modified Ludwieg tube setup. The transition is initiated by means of a moving bar me chanism which translates cylinders in front of a test plate. The intensity of the wakes shed by the cylinders is so large that transition starts at th e leading edge. In streamwise direction the wake-induced transition area gr ows and keeps its initial shape. Also experiments are performed in which th e moving cylinders are combined with a static grid. in the time between two wakes. individual turbulent spots develop which grow in streamwise directi on. These spots merge with the wake-induced transition until a completely t urbulent boundary layer is obtained. When the experimental results are tran sformed in intermittency distributions based on the: turbulent-to-laminar t ime fraction, the superposition principle proofs to be applicable. However, the intermittency distribution based on the mean heat flux cannot be deter mined unequivocally. This is due to the fact that there does not exist a un ique turbulent heat flux. This flux appears to be dependent on the origin o f the transition. (C) 2001 Elsevier Science Inc. All rights reserved.