Online

Hagan, Dan

Mechanical Engineering

2014

MS

Scouring is the process of soil or sediment erosion due to flowing water, which can lead to bed degradation and compromised transportation infrastructure. In the decade before 2000, over half of the 500 bridge failures in the United States were caused by flooding or scouring. To gain a better grasp of the effects of extreme weather events, such as Tropical Storm Irene, on the scouring process, this work is focused on a first principle understanding of the mechanism(s) of scour. The field of Computational Fluid Dynamics (CFD) is particu larly well suited to this task. Utilizing a Direct Numerical Simulation (DNS) code, the repeated impacts of a vortex dipole on a particle bed are simulated. The resulting scour characteristics and flow dynamics are investigated as a function of the Shields number. The vortex dipole propagates perpendicularly to the particle bed, resulting in the scouring of the bed and dissipation of the dipole. After completion of the scour event, the simulation is repeated four more times, where subsequent simulations use the scoured bed from the previous simulation as the initial bed form. This simulation series is conducted over a Shields number parameter space. The fluid phase is treated as a continuum and the discretized Navier-Stokes equations are solved down to the smallest scales of the flow on an Eulerian grid. The particles comprising the bed are represented by the Discrete Particle Model (DPM), whereby each individual particle is tracked in a Lagrangian framework. Particle-particle and particle-wall collisions are calculated using a soft-sphere model. The fluid phase and the solid phase are coupled through a forcing term in the fluid conservation of momentum equation, and a drag force in the particle equation of motion, governed by Newton's Second Law. Above the critical Shields number, the scour hole topography is not fundamentally altered with subsequent impacts until the scale of the scour hole reaches a critical value. At which point, the shape and scale of the scour hole significantly alters the behavior of the vortex dipole and results in strongly asymmetric scour topographies. This two-way coupling between the bed scour and the vortex dipole dynamics is the focus of this work.