RESPONSE OF HOT-ELEMENT WALL SHEAR-STRESS GAUGES IN UNSTEADY TURBULENT FLOWS

Experimentally determined response characteristics of two types of hot-element wall shear stress gages are described for unsteady turbulent flows. One is the substrate-type thin-film gage and the other is a gage with a small cavity below a surface-level thin-film quartz-fiber element. Two substrate-type gages were tested: a quartz-substrate gage, and a glue-on gage with a thin flexible substrate. Steady-flow gage calibrations were used in reducing unsteady flow data. The cavity gage showed excellent response to a steplike flow, whereas the two substrate gages exhibited response times on the order of tens of seconds for the same flow. Steady-flow shear stress turbulence levels indicated by the cavity and substrate gages were 28 and 12%, respectively. In oscillating flows the substrate gages indicated lagging shear stress variations with significantly reduced amplitudes for oscillation frequencies ranging from 3 to 12 Hz, whereas the cavity gage exhibited a much more rapid response. Results indicate that the substrate gages register correct variations only for very low frequencies of oscillation.