Effect of lateral confinement on initial dilution of vertical round buoyant jet

Abstract

In relatively shallow coastal waters, a submarine outfall pipeline is often laid in a dredged trench and the wastewater jet discharges inside a concentric enclosure (a protective riser tube) before release at the seabed. The effect of such lateral confinement over a finite vertical length on the mixing of a buoyant jet is studied experimentally for a wide range of geometries. A confinement index β = Hr/(Dr - D) is proposed to categorize the confined buoyant jet, where Hr and Dr are the height and diameter of the confinement, respectively, and D is the jet diameter. Compared to the dilution of the corresponding free buoyant jet, it is found that mixing is impaired for β ≥ 3. For β » 1, the jet is highly confined and there is virtually no dilution within the riser. For β ≤ 1.5, the confinement has no effect. For 1.5 ≤ β ≤ 3, a wall-attached swirling plume with enhanced dilution is observed, and the mixing behavior is complex. Simple relations for predicting the initial dilution of a laterally confined plume are presented.