Variable-density mixing in turbulent jets with coflow

Abstract

Two sets of experiments are performed to study variable-density effects in turbulent round jets with coflow at density ratios, $s=4.2$ and $s=1.2$. Ten thousand instantaneous realisations of simultaneous two-dimensional particle image velocimetry and planar laser-induced fluorescence at three axial locations in the momentum-dominated region of the jet allow us to calculate the full turbulent kinetic energy (t.k.e.) budgets, providing insights into the mechanisms of density fluctuation correlations both axially and radially in a non-Boussinesq flow. The strongest variable-density effects are observed within the velocity half-width of the jet, $r_{\tilde{u} _{1/2}}$. Variable-density effects decrease the Reynolds stresses via increased turbulent mass flux in the heavy jet, as shown by previous jet centreline measurements. Radial profiles of turbulent flux show that in the lighter jet t.k.e. is moving away from the centreline, while in the heavy jet it is being transported both inwards towards the centreline and radially outwards. Negative t.k.e. production is observed in the heavy jet, and we demonstrate that this is caused by both reduced gradient stretching in the axial direction and increased turbulent mass fluxes. Large differences in advection are also observed between the two jets. The air jet has higher total advection caused by strong axial components, while density fluctuations in the heavy jet reduce the axial advection significantly. The budget mechanisms in the non-Boussinesq regime are best understood using effective density and velocity half-width, $\unicode[STIX]{x1D70C}_{eff}\bar{u}_{1,CL}^{3}/r_{\tilde{u} _{1/2,eff}}$, a modification of previous scaling.