This survey covers recent developments and applications of four skin-friction measurement techniques (oil-film interferometry, wall hot wire, surface fence and wall pulsed wire). Comparisons of the techniques with each other and with other methods are presented. Applications in attached and separated fully turbulent boundary layers and in highly accelerated laminar-like flows will be shown to demonstrate the application range and the limits of the various techniques.

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The effect of sweep on a transitional separation bubble behind a backward-facing step is investigated using direct numerical simulation (DNS). The Reynolds number based on step height and free stream component normal to the step, Reα=(HC∞ cos α)/ν, is kept constant at 3000 for sweep angles α between 0° and 60°. Results agree well with two experimental investigations. Up to α=40°, the mean flow follows the sweep-independence principle. For higher sweep angles, the size of the separated flow region is shortened considerably which is mainly due to an upstream shift of transition in the laminar shear layer emanating from the step and—to a lesser degree—to an increase in turbulent momentum flux uv¯. Fluctuations of wall-stress and wall pressure are largest near the reattachment location and their magnitude scales with (C∞ cos α)2 for all sweep angles. Transition is dominated by a Kelvin–Helmholtz-type instability of the free shear layer. The influence of skewing on the growth rate of instability waves is weak in accordance with linear stability theory predictions of Lu and Lele [J. Fluid Mech. 249, 441 (1993)]. The upstream shift of transition for α⩾50° is related to the way how disturbances spread sidewise in a wedge-shaped region inside of the shear layer.

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