Sedimentation from Buoyant Jets

An integral model is developed to describe sedimentation from a turbulent, buoyant jet injected at an angle into a stationary, uniform ambient fluid. Entrainment is modeled using the standard entrainment assumption and sediment is assumed to fall from the jet where the outward component of the fall velocity normal to the jet boundary exceeds the inward entrainment velocity. When appropriately scaled by source momentum and buoyancy fluxes, turbulent, buoyant jets may be described in terms of a single parameter: the angle ${\theta }_0$ between the flow and the horizontal at the virtual origin (which is close to the actual source for large initial densimetric Froude numbers). Including sedimentation in the model introduces a further parameter $w_s$ , which is the ratio of the fall speed of the sedimenting particles to a typical entrainment velocity (and so $w_S$ is a nondimensional fall speed). An important result is that this ratio is independent of the source speed, so that the proportion of the sediment load deposited near the source is independent of the flow rate. Sediment remaining in the plume beyond the near-source region is deposited when the plume spreads horizontally once it reaches the free surface. Results for plume shapes, deposition patterns, and the proportion of sediment load deposited in the near-source region (as functions of $\theta$ and $w_s$ ) are given. The results are supported by some preliminary laboratory experiments. The effects of flow in the ambient fluid are discussed briefly and a further parameter $u_F$ is introduced, which is the ratio of the ambient flow speed to a typical entrainment velocity (again this ratio is independent of the flow rate).