An experimental investigation of transverse jets in supersonic crossflow

Abstract

This thesis presents an experimental investigation of air, argon and helium sonic under-expanded jets transversely injected through a turbulent boundary layer into a supersonic Mach 3 flow. The aims of the thesis are to demonstrate the steady and unsteady flow features of Transverse Jets In Supersonic Crossflow (TJISC), to present the shear layer vortex shedding frequency and the related controlling parameters, to extract the dominating coherent structures, and to present the mean and fluctuating pressure loads on the wall around the jet port beneath the TJISC.

The flow field was visualized using two types of schlieren methods, meanwhile, the mean and pressure fluctuation distributions beneath the TJISC were measured by Pressure-Sensitive Paint (PSP) and a high-speed pressure transducer array, respectively.

Besides the general flow features, instantaneous schlieren images reveal the unsteady nature of the TJISC. The quasi-periodically shedding shear layer vortices interact with the adjacent shock system and cause intense quasi-periodical deformation of the shock system. The Mach disk and the barrel shock presented in the air and argon cases are absent in the helium cases. In the convective frame for the helium cases, these shear layer vortices travel at supersonic speed and generate a series of moving shock waves that are propagating along the shear layer. The penetration depth of the helium TJISC is slightly higher than the air and argon cases due to these moving shocks.

Power Spectral Density (PSD) of schlieren image pixel light intensity shows that the peak frequency of vortex shedding is inversely proportional to the momentum flux ratio J and this may be due to the level of compressibility. At the same J, the peak vortex shedding frequencies of the air and argon TJISC are similar, while the frequency of the helium TJISC is approximately double. Spectral Proper Orthogonal Decomposition (SPOD) and Dynamic Mode Decomposition (DMD) were applied to the schlieren data and coherent structures were extracted. The SPOD results show that the modal energy peak frequencies are consistent with the shear layer vortex shedding frequency, and the first mode that represents the shear layer vortices contains most of the modal energy. The SPOD results indicate that the flow field is relatively low-rank, and the shed vortices in the shear layer are dominant.

Pressure fluctuations along the centre line beneath the jet illustrate that signals of the most upstream transducer (upstream of jet port) are dominated by the separated boundary layer. The signals of the second upstream transducer are dominated by the fluctuations of the shear layer vortices and shock structures in the air and argon cases, while the signals of the helium cases are relatively broadband. At the most downstream locations, the PSD of the pressure fluctuations presents peaks that are generated by the wake structures near the wall. Pressure-sensitive paint results identify the high-pressure regions upstream of the jet port that are caused by the separation shock and the bow shock. A symmetric low-pressure region, a collision shock, and the wake structures are observed downstream of the jet port.

In conclusion, the TJISC is an unsteady flow field with complex fluid mechanics that are closely linked to the injectant gas properties. The peak shear layer vortex shedding frequency is inversely proportional to the J. This conclusion is confirmed by the SPOD and DMD data and the extract modes are addressed. Pressure loads beneath the jets were presented and linked to the unsteady flow structures and injected gas properties. This thesis provides detailed information on the TJISC and can provide some insights on designing scramjet engines.