The use of flush-mounted hot-film gauges to measure skin friction in unsteady boundary layers

Abstract

Flush-mounted hot-film gauges have proved very effective in measuring skin friction in steady laminar and turbulent boundary-layer flows. Their use is based on the analogy between momentum and heat transfer in the boundary layer. An extension of this technique for use with unsteady flows is presented, through the formulation of a more general relationship between the rates of heat and momentum transfer at the wall. The accuracy of the new formula and the range of its applicability are examined for the case of a periodic boundary layer, both in the laminar and turbulent regimes. This is accomplished by comparing the formula against exact numerical solutions of the differential equations. The present extension allows one to apply the hot-film technique to general unsteady-flow situations, including the measurement of the spectral density of wall-shear-stress fluctuations in steady turbulent flows.