Fluorescence Imaging Study of Impinging Underexpanded Jets

Abstract

An experiment was designed to create a simplified simulation of the flow through a hole in the surface of a hypersonic aerospace vehicle and the subsequent impingement of the flow on internal structures. In addition to planar laser-induced fluorescence (PLIF) flow visualization, pressure measurements were recorded on the surface of an impingement target. The PLIF images themselves provide quantitative spatial information about structure of the impinging jets. The images also help in the interpretation of impingement surface pressure profiles by highlighting the flow structures corresponding to distinctive features of these pressure profiles. The shape of the pressure distribution along the impingement surface was found to be double-peaked in cases with a sufficiently high jet-exitto-ambient pressure ratio so as to have a Mach disk, as well as in cases where a flow feature called a recirculation bubble formed at the impingement surface. The formation of a recirculation bubble was in turn found to depend very sensitively upon the jet-exit-toambient pressure ratio. The pressure measured at the surface was typically less than half the nozzle plenum pressure at low jet pressure ratios and decreased with increasing jet pressure ratios. Angled impingement cases showed that impingement at a 60° angle resulted in up to a factor of three increase in maximum pressure at the plate compared to normal incidence. Nomenclature

Steam-Jet Evaluation for Predicting Leakage Behavior and Interpretation of Experimental Verification

“…When the jet impact distance is short (L/D � 3) and the upstream pressure is 7 MPa, the pressure is maximal at the center of the impact surface, and a locally high pressure is formed at approximately 2.5 D in the radial direction. Tis increase in the local pressure is attributed to the efect of compression waves formed at the boundary of the jet [8].…”

“…Te supersonic jet fow is formed as an overexpanded fow when the pressure at the nozzle outlet (rupture) is lower than the back pressure, and the pressure at the nozzle outlet increases as the pressure upstream of the nozzle increases. If this pressure is higher than the ambient pressure, an underexpanded jet is formed [7,8]. When the nozzle pressure ratio (NPR) is small, the shape of the jet is formed as a continuous cell structure.…”

“…When the nozzle pressure ratio (NPR) is small, the shape of the jet is formed as a continuous cell structure. On the contrary, when NPR increases as the nozzle upstream pressure increases, a highly underexpanded jet with complex physics occurs as shown in Figure 1 [8,9]. Te fow discharged from the nozzle rapidly expands owing to the imbalance caused by the high pressure, and an expansion wave is generated.…”