Optimum Thickness of Pressure-Sensitive Paint for Unsteady Measurements

Abstract

When pressure-sensitive paint (PSP) is used to measure unsteady pressures, the thickness of the paint layer critically affects both the PSP signal-to-noise ratio (SNR)and frequency response. Asthe thickness of a paint layer increases, the brightness of the signal, and thus the SNR, increases, whereas the frequency response decreases, resultinginattenuationoftheunsteadycomponentofthesignal.Inaddition,ifpressuree uctuationsandfrequencies are large and the paint layer is thick, the unsteady PSP signal may be distorted and the steady component may be offset from its true mean value. These observations suggest that, for each application, there is an optimum paint thickness where the “ unsteady SNR” (the ratio of unsteady signal amplitude to steady noise ) is a maximum and wherethe offset and distortion are small. Thishypothesisis explored by numerically evaluating published analytic solutions to a simplie ed model of the unsteady PSP problem. It was found that over a wide range of parameters the unsteady SNR is a maximum when the paint thickness corresponds to a 1.25 dB attenuation of the unsteady signal. At this optimum, the nondimensional time constant (h 2 !/D) is approximately 1.5.