A comparative three-dimensional study of impulsive flow emanating from a shock tube for shock Mach number 1.6

A three-dimensional transient impulsive flow emanating from an open end of a 165-mm driver section shock tube is investigated numerically in the present study for a shock Mach number of 1.6. Here, the main objective was to find out an appropriate model to simulate the early evolution of the high Mach number transient flows by comparing the flow field obtained from two numerical models with both qualitative and quantitative experimental observations. The 3-D numerical simulations were performed by solving RANS equations using the shear stress transport K-ω model and large eddy simulation of the flow with the help of ANSYS CFX software. The experiments were performed with the simultaneous 2-D and 3-D particle image velocimetry systems and high-resolution smoke flow visualizations. The 2-D PIV results were used for comparing the numerical results obtained on a plane, and the 3-D PIV results were used to compare the azimuthal variations across the vortex ring. It is observed that the velocity field of the transient jet, Kelvin–Helmholtz (K–H) vortices at the trailing jet and the interaction of K–H vortices with the primary vortex ring were resolved well in LES quite similar to the experiments. However, the SST K-ω model resolved only the velocity field of the transient jet in the axial region similar to the experiments and dissipated the K–H vortices at the jet boundary. Though both models resolved the shear layer originated from the triple point, they did not predict the rollup and formation of vortices and their interactions observed in a 24 MP camera. This demands the use of a higher spatial resolution. It is also noticed that a substantial adverse pressure gradient experienced by the vortex ring and trailing jet during the early stage of evolution due to the presence of precursor/incident shock was responsible for the pairing up and merging of the shear layer vortices to form a stronger counter-rotating vortex ring. This adverse gradient also plays a dominant role in the azimuthal expansion (rapid increase in diameter) of the vortex ring