Depending on the specific application, the interior of the housing of the prior skin-friction gauge may contain a filler, which is typically an oil. As long as the flow speed is not too high, the oil is retained in the cavity by capillary action at the gap. The oil helps to suppress spurious components of gauge readings by damping vibrations and reducing intragauge thermal gradients associated with heat flux. Unlike the prior gauge, the present gauge includes a bellows, sealed to the disk and the housing in such a way as to define a cavity behind the disk. During assembly of the gauge, prior to sealing, the cavity is filled with an oil, with care taken to keep the oil free of air bubbles. In the present gauge, because of retention within the bellows, the relatively incompressible oil supports the cantilever beam against axial compression in the presence of uniform wall pressure on the sensing surface, thereby suppressing the spurious component of strain (and the associated spurious components of the strain-gauge readings) attributable to uniform wall pressure.

Figure 2. The Bending Moment (M0) attributable to the spatial variation of pressure (P3-P1) across the disk can be measured by use of the strain gauges at the top of the cantilever beam. These measurements can be used to correct for M0 in processing the readings of the other strain gauges to obtain the wall shear.

Another important spurious effect is that of apparent strains associated with temperature changes. This effect can be compensated by suitable processing of the outputs of additional strain gauges that are positioned to be especially responsive to the spurious effects. Yet another spurious effect is that of bending of the cantilever beam under the combined influences of acceleration and the inertia of the cantilever beam and disk. The readings of an accelerometer mounted next to the skinfriction gauge can be used to correct for this spurious effect by means of simple proportionalities among strain, wall shear, and acceleration.

An important element of the present methodology is an approach to designing skin-friction gauges for specific applications. This approach involves consideration of numerous phenomena within a multidisciplinary computational-simulation framework. The phenomena to be considered include the ones described above, plus such other relevant phenomena as instrumentation errors, settling times, filtering of measurement signals, and dynamic responses to multiple inputs.

This work was done by August J. Rolling of Virginia Polytechnic Institute and State University for the Air Force Research Laboratory.

AFRL-0055

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