Numerical Study of Laminar Unsteady Circular and Square Jets in Crossflow in the Low Velocity Ratio Regime

Abstract

The unsteady three-dimensional flow interactions in the near field of square and circular jets issued normally to a crossflow were predicted by direct numerical simulations, aiming to investigate the effect of the nozzle cross-section on the vortical structures formed in this region. The analysis focuses on jets in crossflow with moderate Reynolds numbers (Rej=200 and Rej=300) based on the jet velocity the characteristic length of the nozzle and low jet-to-cross-flow velocity ratios, 0.25≤R≤1.4, where the jets are absolutely unstable. In this regime, the flow becomes periodic and laminar, and three distinct wake flow configurations were identified: (1) symmetric shedding of hairpin vortices at Rej=200; (2) the formation of toroidal vortices as the legs of hairpin vortices merge and the vortices roll up at Rej=300 and R≤0.67; (3) asymmetric shedding of hairpin vortices in the square jet at Rej=300 and R≥0.9, where higher-frequency hairpin vortex shedding combines with a low-frequency spanwise oscillation in the counter-rotating vortex pair. The dynamics of each of these flow states were analyzed. Power spectral density plots show a measurable increase in the shedding frequencies in Rej=300 jets with R, and that these frequencies are consistently larger in circular jets.