Particle-driven intrusions into an ambient stratification

FRANS DE ROOIJ, STUART B. DALZIEL AND PAUL F. LINDEN
Department of Applied Mathematics and Theoretical Physics
The University of Cambridge, Silver Street, Cambridge CB3 9EW, UK
Phone: 44.1223.339737, Fax: 44.1223.337918, E-mail: f.derooij@damtp.cam.ac.uk

Gravity currents driven by the increased density due to suspended particles are a well-known phenomenon in oceanography. When a particle-driven gravity current, or turbidity current, runs along the bottom of the ocean down to a level where it has the same bulk density as the surrounding fluid, it lifts off the ocean bottom and becomes a horizontal intrusion. Particle-driven intrusions are also thought to play an important role in the flow in sedimentation tanks of sewage treatment plants, in which vertical particle concentration gradients are set up, corresponding to a continuous density stratification. With varying inflow conditions, gravity intrusions can occur, possibly inducing an unwanted increase of the particle concentration of the outflow.

The present work is a laboratory investigation of two-dimensional particle-driven intrusions into two-layer and continuous stratifications. The intrusions are generated by releasing a fixed volume of fluid with suspended particles into a stratified quiescent fluid in a long rectangular tank. Using an advanced image processing system, quantitative measurements were taken of the propagation speed and the generation of internal waves. In addition, the deposition of particles was measured along the length of the tank.

Particle-driven intrusions along a density interface in a two-layered stratification were seen to behave identically to saline intrusions immediately after release (see figure 1 ). They propagate away from the lock at a constant speed which is well predicted by the mathematical model for saline intrusions. However, as particles start settling out, the intrusion gradually slows down. The deposition of sediment on the floor of the tank is well predicted by a model for particle-driven currents along a rigid boundary.

Fig.1: intrusion in a step stratification

In a continuous stratification the initial mid-level intrusion slowly rose as it propagated, as shown in figure 2 . Its initially blunt nose became more and more pointed and slowed down until it was overtaken by a light current along the top boundary, carrying very few particles, and by a dense current along the bottom boundary, heavily loaded with particles. Strong interactions with internal waves were observed, pinching off patches of fluid from both currents.

Fig.2: intrusion in a continuous stratification

Theoretical models of these flows including the effects of particle settling, detrainment of fluid from the intrusion and advection of particles by return flows are currently being developed. A comparison of these models with the experiments and an extrapolation to a range of flows will be made.