Discussion on mechanism of pressure-sensitive paint through application to high Knudsen number regime

The pressure-sensitive paint (PSP) is very useful and easy way to obtain the pressure distribution on surfaces. Usually the pressure distribution by the paint shows good agreement with that obtained by other methods, like a pressure tap. Here, in this study, we discuss on the property that the paint really shows, especially in high Knudsen number regime. In this regime, it is well known that a pressure tap shows good agreement with surface pressure, and not gas pressure. Therefore, a PSP experimental result is examined with a DSMC numerical result, and shows good agreement with surface pressure. To argue against the suggestion that "PSP measures pressure, not density" [Bell, J. H. et al., Annu. Rev. Fluid Mech., 33 (2001), 155206.], a temperature sensitivity is investigated in detail. From these arguments, it is suggested that the PSP measures a number flux toward a surface.