Direct numerical simulation of transitional and turbulent round jets: Evolution of vortical structures and turbulence budget

Abstract

Direct numerical simulation of transitional and turbulent round jets is reported in a comparative framework. Such a comparison is central toward revealing the roles that molecular viscosity and vorticity intensification play in the evolution of jets. The initial and intermediate evolution is differentiated based on the assessment of the starting jet, roll-up frequency, dynamics of vortex rings, and emergence of the secondary instability. Long-term behavior is differentiated based on the assessment of preferred mode frequency, time averaged vortical structures, half jet-width, and volume flow rate obtained from the time-averaged velocity field. The present study demonstrates that viscous damping of cross-stream vorticity plays a key role in establishing helical instability as the dominant mode in long-term evolution of the transitional jet. On the contrary, varicose mode is dominant in the turbulent jet, despite preferred mode frequency being the same in both cases. Finally, a novel attempt is made toward comparing individual terms constituting turbulence budget between both regimes. Through such a comparison, relative dominance of various transport mechanisms governing the evolution of turbulence kinetic energy (K) is revealed. It is observed that terms accounting for a forward cascade of K from inertial to smallest scales are comparatively larger for the turbulent jet, while those accounting for the backscatter of K are comparatively larger for the transitional jet. It is also established that turbulence dissipation is evidently the same for both jets. Thus, the property of turbulence dissipation being independent of Reynolds number for turbulent jets can also be extrapolated to transitional jets.