Flow Modulation and Mixing Enhancement of Highly Underexpanded Jet by Vortex Excitation

A highly underexpanded jet with a nozzle pressure ratio of 5.60 is excited by a vortex excitation. Modulations on flow characteristics by the vortex excitation are revealed by comparing the excited jet with the free jet through a large-eddy simulation (LES) technique. The modeling results have achieved good agreements with the available literature data in terms of time-averaged near-field variables, Mach disk dimensions, and dominant frequencies. A grid-convergence study is also performed to verify the fidelity of LES results. Ultimately, qualitative and quantitative comparisons of mean and instantaneous jet fields are conducted. It is demonstrated that the vortex excitation has enhanced the mixing of the injection nitrogen with the environment by a considerably increased mixing level and a faster mixing process in the middle jet. The vortex excitation modulates the buildup process of shocks and vortex structures, resulting in an intensified shock/shear-layer interaction and an accelerated turbulence transition process, which combine to improve mixing efficiency. The dominant instability mode is altered to a higher-order multiple helical mode from a single helical mode, and an intensified screech tone is achieved by the vortex excitation.