Mixing in the Intermediate Field of Dense Jets in Cross Currents

Abstract

This study presents an experimental and numerical study for an inclined (60$^o$ to horizontal) dense jet discharged into a coflowing current. The mixing and transport of the density current arising from the jet impingement on a horizontal bottom boundary is investigated. A light attenuation technique is employed to measure the layer-averaged concentration field over a region that extends 20$F_{r}D$ downstream and 9$F_{r}D$ laterally (where F = jet densimetric Froude number, and D = jet diameter). A comprehensive characterization of the resulting buoyant spread in both steady and unsteady cases is obtained. The concentration field is also computed using a three-dimensional (3D) shallow water equation model via the distributed entrainment sink approach that incorporates a near-field Lagrangian integral jet model (JETLAG) into the 3D model for dynamic simulation of the near-far field transition. The results show the occurrence of bifurcation in the gravitational spreading layer when the impinging dense jet is bent over, characterized by a crossflow Froude number $\mathbf{F} = U_{r}F_{r}$ of around 0.8 (where $U_{r}$ = ratio of ambient to jet velocity). The lateral concentration profiles are bimodal in shape; the concentration maximum is off-centered and 1.6–2 times the centerline value. The buoyant spreading is governed by buoyancy and inertia, and the spreading layer grows as x^\frac{2}{3} with downstream distance. For $\mathbf{F}\approx 0.4$, the upstream intrusion of the buoyant layer past the source is arrested; lateral gradients of concentrations are small and the dilution becomes constant downstream. In the intermediate range $\mathbf{F}\approx 0.4-0.8$, the profiles evolve gradually from top-hat to bimodal. The numerical prediction of the salient flow features as well as the intermediate field dilutions and spreading layer thickness are in good agreement with data.