Incipient motion of sand and oil agglomerates

Incipient motion of sand and oil agglomerates: A numerical study on the mobility of particles under wave-induced forcing

Schippers, M.M.A.

Uijttewaal, W.S.J. (mentor)
Jacobsen, N.G. (mentor)
McCall, R.T. (mentor)
Zijlema, M. (mentor)
Weij, D. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2017-02-16

After the Deepwater Horizon oil spill in 2010, oil was released into the Gulf of Mexico and partly reached the shorelines. Here, it mixed with sand in the surf zone to form sand and oil agglomerates (SOAs). These cm-sized particles are heavier than water and are therefore found on the seabed. Years after the initial formation of these particles, they keep appearing on the beaches, despite earlier clean-up. Several studies to the formation and transport of these particles have been conducted, but traditional shear-stress based formula, which are generally used for sediment transport, do not predict the mobility of SOAs well. This study focuses on the incipient motion of SOAs and aims to propose an improved method of predicting SOA mobility under waves. Sliding and rotation are found to be the two governing mechanisms of incipient motion of SOAs. For both mechanisms and two particle shapes, spherical and disc-shaped, a mobility parameter is defined. A critical value of this parameter is derived from a force balance and moment of force balance and is a function of several parameters: the drag coefficient CD, the inertia coefficient CM, the lift coefficient CL, the location of the resulting forces relative to the point of rotation, the Keulegan-Carpenter number (KC), the friction coefficient and the relative distance of the point of rotation to the point of gravity. When the mobility number exceeds the critical value, initiation of motion of SOAs is expected. The forces on the object are simulated in 2DV with the CFD package OpenFOAM. The results of the numerical simulations indicate that the critical mobility parameter strongly correlates with the KC-number. For low KC-numbers, the inertia force has a significant contribution to the horizontal force and reduces the stability of the particles, so less hydrodynamic forcing is required for incipient motion. Sliding is the governing mechanism of incipient motion for disc-shaped particles, whereas rotation is expected for spherical particles. Burial reduces the mobility of the particle, as a smaller part of the particle disturbs the flow and soil pressures act as an additional stabilizing force. The results of this study can be applied to predict incipient motion of SOAs on a fixed bed. Due to the sensitivity of the mobility to SOA shape, density and diameter, a probabilistic method is recommended for predicting mobility of a collection of SOAs instead of a single particle.

OpenFOAM
Sand- and oil agglomerates
Incipient motion

http://resolver.tudelft.nl/uuid:3c81499e-17eb-4d77-9c86-a94df94a3eea

Student theses

master thesis

(c) 2017 M.M.A. Schippers