Experimental study of near-wall underexpanded jet impingement on a flat plate using temperature-insensitive semi-transparent pressure-sensitive paint

Abstract

The aim of this study is to experimentally investigate the pressure field of underexpanded sonic jet impingement with a short nozzle–plate distance (L/Dn) and a large impingement angle (θ). The measurement of surface pressure is difficult using traditional non-transparent PSP due to optical obstruction. Here, a novel pressure-sensitive paint (PSP) with a high degree of transparency was used, which enabled back illumination and back imaging to resolve the issue of optical obstruction. The temperature sensitivity of the PSP was also greatly reduced using the self-assembled monolayer (SAM) technique, which reduced the effect of temperature variation on the impingement plate, as demonstrated via comparison with pressure tap data. With this technique, the effects of the nozzle–plate distance (L/Dn = 1, 2, and 3), impingement angle (θ = 90°, 60°, and 30°), and pressure ratio (PR = 1.2 and 1.5) on the pressure-field characteristics were evaluated in detail, with a focus on the fine flow structures near the impingement point. The surface pressure distribution in the wall jet region was found to be significantly influenced by the nozzle–plate distance, which could be related to variation in the flow field, based on a physical model of the sonic line. The obtained PSP results with a high spatial resolution revealed interesting flow physics that improve our understanding of high-speed near-wall jet impingement.