Research Highlight: Brandon Dillon

Brandon is a PhD student working in the area of environmental hydraulics and sediment transport. He received his BS and MS degrees in Mechanical Engineering from Virginia Tech, and he is currently working on an experimental study examining the Mechanics Of Momentum Transport In An Alluvial System.

Current Research Summary: My PhD work focuses on the river morphology topic of incipient motion. The goals of this project are to investigate, quantify, and predict the threshold conditions of motion for alluvial material undergoing the action of turbulent river flows.

The so-called turbulent coherent motions of wall-bounded shear flows have been found to play an integral role in the process of sediment entrainment. These motions interact strongly with the alluvial bed – being both created by and consumed by momentum transfer processes active in the near-wall region. The type, quantity, strength, and scale of these coherent motions are dramatically affected by the nature of the river’s boundary roughness and form. However, as a fundamental consequence of the nature of turbulence, a more rigorous description of these motions has yet eluded scientists and mathematicians. The trouble impeding such a description (and perhaps the final but most serious difficulty in overcoming the Existence and Smoothness Problem) is the fact that these motions are both spatially and temporally non-local.

A consequence of this non-locality in the context of sediment transport becomes evident when analyzing the dislodging force experienced by a given sediment particle. This force is not merely a function of a characteristic (local) instantaneous velocity. But rather this force is a non-linear summation of a multitude of pressure, viscous, and advective processes being born, acting, and then dying out in the river body.

A river, aside from generating and destroying these motions as a function of boundary layer mechanics, also acts to transport these coherent motions. Dislodgment effects at one location are a chaotic but decidedly causal consequence of the time and spatial history of the various relevant quantifiable fields of the river system (e.g. velocity and pressure). On a longer timescale however, it can be seen that these histories are then a further consequence of the river system’s boundary conditions (e.g. bed bathymetry and mass sources), which are, over a smaller time scale, changed by the generation, transport, and destruction of momentum-dense turbulent fluid structures. In other words, to understand how rivers transport sediment, we must first understand how rivers transport momentum.

My research seeks to mathematically describe these coherent processes and to quantify their ability to impart motion to alluvial material.