The accuracy of the measurement of turbulent skin friction by means of surface pitot-tubes and the distribution of skin friction on a flat plate

Abstract

An experimental study has been made of two-dimensional turbulent boundary-layer flow with zero pressure gradient. The investigation was made to determine the accuracy of a method proposed by Preston for measuring the local turbulent skin friction. The general momentum equation was used in conjunction with measurements of skill friction by Preston's method, to obtain increments in momentum thickness which could be compared with the measured values. The experiments were made on a smooth flat plate, 6 ft long, which spanned the working-section of a return-circuit wind tunnel. Transition was promoted by trip wires and glass-paper strips. A constant Reynolds number of 3.9 X 10 5 per foot was maintained throughout the experiments. The results, together with certain basic arguments, indicate that Preston's method will give the correct skin friction. For the range of Reynolds numbers covered in the experiments (1500 < Ro < 4000) the skin-friction coefficients obtained for different values of Ro (the Reynolds number based on momentum thickness), were approximately 10 per cent and 6 per cent less than the corresponding Prandtl-Schlichting and Schoenherr values respectively.