Large Eddy Simulation of Multi-Scale Fractal Nozzle Jets: Effects on Mixing and Flow Development

This study investigates the influence of nozzle exit shape on the flow and mixing characteristics of subsonic jets using large-eddy simulations (LES). The focus is on comparing regular polygonal nozzle exits---pentagon, hexagon, heptagon, and octagon---with their first- and second-order multi-scale fractal counterparts of identical exit area and a fractal dimension of D_f = 1.5. A circular nozzle exit serves as the baseline for comparison. All cases are simulated at a Mach number of 0.6 and a Reynolds number of 20,000, representative of typical mixing applications. The fractal geometries are generated and implemented in the flow solver as inflow boundary conditions, enabling accurate representation of complex multi-scale boundaries. Building on prior findings that fractal plates enhance turbulent mixing by breaking up large-scale coherent structures and promoting rapid decorrelation in wakes [1], this study extends the concept to jet flows to examine its effect on jet penetration, spreading, and mixing efficiency. The results show that the introduction of multi-scale geometric features alters near-field vortex formation and accelerates the transition to turbulence, leading to increased entrainment and modified far-field spreading relative to both circular and regular polygonal nozzles. These findings highlight the potential of multi-scale fractal nozzle designs to tailor mixing performance in engineering applications such as combustion and propulsion.

Reference

[1] O. Es-Sahli, A. Sescu, M. Z. Afsar, and O. R. H. Buxton, “Investigation of wakes generated by fractal plates in the compressible flow regime using large-eddy simulations,” Physics of Fluids, vol. 32, no. 10, p. 105113, 2020, doi: 10.1063/5.0018712.