Measurements of skin friction in turbulent boundary layers at transonic speeds

Abstract

The first part of this report describes the design and construction of a floating element skin friction balance. This instrument, which is essentially an improved version of Dhawan's balance, was applied to measurements of local skin friction in the turbulent boundary layer of a smooth flat plate at high subsonic Mach numbers and supersonic Mach numbers up to M = 1.75. The measured skin friction coefficients are consistent with the results of other investigations at subsonic and also at high supersonic speeds. The principal difficulties which exist in comparing skin friction coefficients at various Mach numbers are discussed. The second part of this report describes the application of the Stanton tube technique to skin friction measurements near the base of a shock wave impinging upon a turbulent boundary layer. The floating element technique is inherently difficult to apply for skin friction measurements in non-uniform flow. Hence, a Stanton tube is calibrated by means of a floating element balance in a uniform flow field and then used to measure skin friction near the base of an impinging shock. Oblique shock waves were produced by two wedges of 2.5[degree] and 4.6[degree] semi-angles and a normal shock was generated by a choked channel. Skin friction and velocity profiles were obtained for these three cases at a free stream Mach number of 1.48.