On the measurement of local surface friction on a flat plate by means of preston tubes

Abstract

Measurements of local surface friction were made by Preston's method using a round pitot attached to the surface ('Preston .tube') and were compared with the values given by differentiation of overall measurements of surface friction by the wake-traverse method and with determinations by means of surface pitots of the type used by Stanton. The results establish that the calibration .curve for Preston tubes on a flat plate is similar to that obtained in pipe flow but that, for a given Preston-tube reading, the local surface friction is about 11 per cent higher on the flat plate than in a circular pipe. Corresponding differences were observed in the constants of the semi-logarithmic inner law for the boundary-layer velocity profile, of almost the right amount to account for the calibration differences.