Direct Measurement of Skin Friction in Complex Fluid Flows

Abstract

This paper describes methods for the direct measurement of skin friction in fluid flows under a very wide variety of conditions. The challenges involved in different flow situations are highlighted, and representative instrument designs and measurement results are presented. The test conditions considered span the range from incompressible to hypersonic speeds; ambient to very high temperature including combusting flows; smooth, rough, and porous surfaces with and without injection and test times from 10-6 to essentially continuous runs. Both 2D and 3D flows are discussed. Accuracies of +/- 3-5% are presently attainable in the simplest cases, while +/- 10-15% is more typical for complex, 3D, high-speed flows at high temperatures. The final topic discussed is simultaneous measurements of skin friction and heat flux with the possible further additions of surface pressure and temperature.