Heat transfer measurements in transitional boundary layers

High velocity boundary layer transition experiments are performed in a Ludw ieg tube set-up. At a Mach number of 0.36, the transition is studied by usi ng several turbulence generating grids. These grids cause turbulence levels varying from 0.25% to 3.5%. It is found that, depending on the turbulence level, different intermittency distributions should be used to describe the transition zone well. For low turbulence levels, the Narasimha and the Joh nson models, which are based on turbulent spots, show good agreement with t he measurements. For intermediate levels, the front part of the transition zone follows a distribution which is described by turbulent spots which dec rease in size. In these cases, the latter part of transition also shows agr eement with the Narasimha and Johnson models. A major difference with the ' classical' intermittency distributions is obtained for high turbulence leve ls. Assuming that for these levels non-growing turbulent spots are initiate d in the whole transition zone, an exponential intermittency is derived. In the measurements these distributions indeed are found. (C) 2001 Elsevier S cience Ltd. All rights reserved.