A numerical study of dynamic flow patterns in supercritical jet flows for various swirl numbers

Three-dimensional large-eddy simulations (LES) of supercritical nitrogen injection are carried out over a range of swirl numbers ($0\le S<2$) to investigate the characteristics of the flow field. The swirl number is varied by varying the tangential velocity, keeping mass flow rate unchanged. Based on the observed flow dynamics, the swirl numbers are classified as (i) low, (ii) medium, or (iii) high. For low swirl numbers, recirculation zones are not observed and the flow pattern consists primarily of free shear layer instabilities arising out of forced convection that move helically around an intact jet potential core. For medium swirl numbers, a sudden drop in axial centerline velocity is observed due to large adverse pressure gradients. These gradients lead to recirculation regions in front of the injector forming a bubble-type aerodynamic bluff body outside the injector. As the swirl number is increased in this range, the bubble disappears forming a venturi-type flow with local acceleration of axial centerline velocity. For high swirl numbers, the recirculation region enters the injector, and a precessing vortex core (PVC) is observed in the flow field. Mean jet length and initial jet cone angle were found to have non-monotonic variations for medium and high swirl numbers. Finally, spectral analysis reveals the presence of a hydrodynamic frequency corresponding to shear layer instabilities and an acoustic mode corresponding to the injector for low and medium swirl numbers. For high swirl numbers, both these frequencies are suppressed noticeably and the dynamics is completely described by the PVC frequency.