Effect of downstream control on stability and mixing of a vertical plane buoyant jet in confined depth

Abstract

The effect of downstream control on the mixing of a plane turbulent heated jet discharging vertically into confined depth is studied using the buoyancy extended k-ε model. The steady two-dimensional turbulent flow, temperature and turbulence fields are computed using the finite volume method on a high resolution grid. In the absence of a specific downstream control, the numerical predictions demonstrate three generic flow patterns for different jet discharges and environmental parameters: i) a flow with circulation cells of alternate rotation for non-buoyant discharge; ii) a stable buoyant discharge with the mixed fluid leaving the vertical jet region in a surface warm water layer; and iii) an unstable buoyant discharge with flow recirculation and re-entrainment of heated water. A stratified counterflow region always appears in the far-field for both stable and unstable buoyant discharges. The near field interaction and hence discharge stability is governed by only two dimensionless parameters - the discharge densimetric Froude number Fo and the depth to jet width ratio H/B. The computed velocity and temperature fields agree well with the laboratory flow-visualization and temperature measurements of Jirka & Harleman (1979). Numerical prediction of stability categories is in excellent agreement with experiments. For a given discharge and depth, it is found that the jet stability can be predicted regardless of downstream control, provided that the channel length exceeds about 6H. The effect of a strong downstream control close to the discharge primarily results in a flooded internal jump and the lowering of the interface level in the stratified counterflow region. Consistent with the detailed measurements of Andreopoulos, Praturi and Rodi (1986), the predictions show a clear reduction of the bulk dilution, although the effect of downstream control on the jet discharge stability is insignificant.