Tech Briefs

A prior gauge concept is augmented with features to suppress spurious effects.

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.

Figure 1. Skin-Friction Gauges based on disks supported by cantilever beams are shown here greatly simplified to aid in understanding the underlying principles.

A generic gauge according to the present methodology is derived from a prior cantilever-beam skin-friction gauge (see Figure 1). Either gauge includes a housing designed to be mounted in a recess such that one of its end faces lies flush with the wall surface on which one seeks to measure skin friction. The gauge includes a cantilever beam that supports a disk, the outer face of which constitutes a sensing surface flush with the wall surface. The base of the cantilever is instrumented with strain gauges. The disk is separated from the wall-surface end face of the housing by a small gap, which allows for movement of the disk in response to forces exerted on the disk by the impinging flow. The readings of the strain gauges are taken as measures of flexing of the cantilever associated with movement of the disk, and, hence, of the wall shear exerted on the sensing surface by the flow.