Mixing in the Intermediate Field of Dense Jets in Cross Currents

摘要

This study presents an experimental and numerical study for an inclined (60 degrees 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 20FrD downstream and 9FrD 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 F = UrFr 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(2/3) with downstream distance. For F approximate to 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 F approximate to 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. (C) 2015 American Society of Civil Engineers.