Paper 7: The Measurement of Skin Friction in Pressure Gradients Using a Static Hole Pair

Abstract

Two wall static holes of different sizes will give different readings of static pressure and the observed pressure difference is a function of the local skin friction. The static hole pair has, therefore, been proposed as a skin friction measurement device. This paper describes experiments which have been carried out to assess the accuracy of the static hole pair for the measurement of skin friction in favourable pressure gradients. The holes were formed in the wall of a pipe so that the device could easily be calibrated, and the favourable pressure gradient was then generated by inserting a central fairing. The skin friction values obtained from the device were compared with those measured by a Preston tube. Results showed that the static hole pair is capable of measuring skin friction within about 2 per cent, but a number of practical difficulties are involved, including the necessity to measure very small pressure differences. Brief consideration is given to the use of the static hole pair in adverse pressure gradients.