Measuring Skin Friction in Complex Flows That Include Shocks

A methodology for measuring skin friction (also known as wall shear) in complex flows typical of those inside supersonic-combustion ramjet (scramjet) engines has been developed. The flows inside scramjet engines are turbulent and include shocks. Because the dynamic pressures of such flows are high, even small skin-friction coefficients can significantly degrade engine performance. Hence, it is desirable to measure skin friction in order to gain better understanding of engine performance and to enable calibration of skin-friction submodels of computational fluid dynamics models used to design scramjet engines. The presence of shocks and the large heat fluxes associated with flows in scramjet engines add to the challenge of measuring skin friction, and the present methodology provides means to meet the challenge. Major elements of the methodology are a generic design for a gauge and techniques for processing the readings of the gauge such that the wall shear at the gauge location can be determined fairly accurately, even when a shock impinges directly on the gauge.