Application of an optical pressure measurement technique with pressure-sensitive paint to blow-down supersonic wind tunnel testing

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Pressure-sensitive paint (PSP) attracts attention as a novel optical pressure measurement technique to obtain the full-field pressure distribution with very high spatial resolutions. However, conventional polymer-based PSPs cannot be used in short-duration supersonic wind tunnels of blow-down type due to the slow response to a rapid change in pressure. The aim of the present study is to establish PSP measurement technique applicable to aerodynamic tests in blow-down supersonic wind tunnels by improving the time response of PSP itself and the stability of excitation lights. The main results of the present work are as follows:1. The fluctuation of the output of a blue light-emitting diode (LED) array light source is extremely small (less than 0.2%), hence it is suitable for the illumination light for the PSP measurements in short-duration wind tunnels of blow-down type.2. The time constants of PSPs based on porous thin-layer chromatography plates are on the order of 10 microseconds. The fast-responding porous PSP has been applied to a surface pressure measurement in a Laval nozzle of the blow-down wind tunnel of Toyama Prefectural University, and pressure jumps due to a system of oblique shock waves generated in the nozzle wall and temporal fluctuations of the shock locations have been successfully visualized.3. The wall temperature change during a run seriously affects the pressure measurements. The use of a thermally-conductive adhesive tape to fix the porous PSP on a brass wall (as a heat sink) was effective to reduce the surface temperature change during the experiment.4. The signal-to-noise ratio (SNR) of a single image obtained is not enough due to the inevitable shot noise. It suggests that the image processing procedure for the noise reduction is important for PSP measurements in short-duration wind tunnels.