Particle-driven gravity currents

Particle-driven gravity currents

Stovers, M.J.P.

Goeree, J.C. (mentor)

Mechanical, Maritime and Materials Engineering

Maritime & Transport Technology

2016-12-12

Gravity currents are common around the world, consider for instance avalanches and pyroclastic flows. The kind of gravity currents that this research focuses on occur below the water surface. During dredging operations soil is cut and transported, during many of these processes sediment is introduced into the environment. Consider for instance over-flow losses or the offloading of a dredging vessel through its bottom doors. The introduction of the sediment of greater density into the ambient water creates a gravity current. The small fractions of these gravity currents can travel a great distance and can cover parts of the seabed burying sea-life and damaging the environment. Furthermore dredging companies want to deposit the sand there where it is needed. The objective is to increase the knowledge of the behaviour of these currents and to obtain valuable experimental results that can be used for model validation. In this report the influence of initial concentration on the run-out length and current velocity is investigated and the behaviour of particles of different size with varying concentration has been mapped. Experiments involving a full-depth lock release of a fixed volume suspension of sand particles into a homogeneous fluid of lesser density are carried out over a range of initial concentration. The behaviour is modelled using the two-layer shallow water equations with manning friction and a particle conservation equation containing the settling velocity of the particles. The resulting gravity current passes through four phases as observed in the experiments and in previous work. There is first a slumping phase, during which the suspension collapses and the current is accelerated. This may be followed by a purely inertial phase, where the buoyancy force is balanced by the inertial force. This phase displays almost constant velocity and increases in length with increasing concentration. Then there follows a similarity phases that is initiated when the bore that is formed during slumping overtakes the front of the current. This phase shows a steep decrease in velocity and at the end, the viscous phase sets in when the velocity is sufficiently low that viscous forces start to dominate the flow. The numerical model displays all phases except the last viscous phase since viscous forces are neglected within the model. The experiments showed a maximum velocity during the inertial phase independent of concentration that is not captured by the model, with increasing concentration the model predicts increasing velocity due to increased potential energy. At low concentration the experiments showed that particles segregate over the run-out length of the current. Smaller particles travelled a greater distance than the bigger particles with a higher settling velocity. At high concentration the particle segregation does not occur and the PSD at the start of the run-out length of the current is also found at the end. The multiple fractions model with hindered settling effect shows the same trend of particle segregation reducing with increasing concentration, but was not able to match the PSD of the experiment at high concentration. The model predicts particle 2 segregation as the experiment showed none. Beyond a critical point of initial concentration of particles, the resulting dense current came to an abrupt halt at some point faster than currents with a lower initial concentration. This is a behaviour that is not captured by the model and needs more research.

shallow water equations
dredging
gravity
current, turbidity

http://resolver.tudelft.nl/uuid:d8f05a23-c444-4734-85e6-45afb5afe6fc

2019-12-12

Student theses

master thesis

(c) 2016 Stovers, M.J.P.