Graduation Year

2023

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Andrés Tejada-Martínez, Ph.D.

Committee Member

Rasim Guldiken, Ph.D.

Committee Member

Wenbin Mao, Ph.D.

Committee Member

Mauricio Arias, Ph.D.

Committee Member

Ping Wang, Ph.D.

Keywords

Finite volume, Numerical analysis, RANS/LES, Stokes drift velocity, Turbulence

Abstract

Wind and wave-driven Langmuir circulation (LC) in the upper ocean mixed layer and in shallow inner shelf regions of the coastal ocean has been extensively studied via field measurements and numerical simulations by the ocean turbulence-physical oceanography community. LC consists of counter rotating vortices or cells induced by the wind and surface gravity waves. As with all turbulence regimes, LC is characterized by a wide range of scales, with the largest and most energetic LC scales serving to promote vertical transport (and thereby mixing). However, numerical studies and field measrements of Langmuir turbulence under the influence of coastal boundaries and its interactions with other coastal ocean processes are lacking. Fine-scale numerical studies such as large-eddy simulations (LES) of LC in inner coastal shelves have been limited to periodicity over horizontal directions, characteristic of a water column unaffected by coastal boundaries. As such, these simulations have been performed with pseudo-spectral solvers employing a highly accurate spectral discretization in the horizontal directions. To uncover and understand potential interactions between LC and other coastal ocean processes, it is necessary to extend eddy-resolving simulations of LC to non-spectral discretizations capable of handling non-periodic boundary conditions in lateral directions. To that extent, in this work a second-order accurate finite volume discretization is employed. First, the discretization is tested in LES of laboratory tank experiments of counter-rotating vortices, mimicking LC, with non-periodic boundaries. Good agreement is found between the LES simulation and the laboratory experiments in terms of mean speeds and resolved turbulent kinetic energy (TKE) and TKE dissipation rate. Second, the finite volume discretization is tested in LES of LC of the inner coastal shelf via comparison against pseudo-sepctral LES and field measurements. It is found that careful consideration must be given to the LES subgrid-scale model and its near-wall treatment, as various combinations of these two components can lead to inaccurate represention of the cells, in particular the vertical transport induced by the cells. A subgrid-scale model characterized by a velocity scale obtained from a combination of the norm of the resolved strain rate tensor and the norm of resolved vorticity tensor (referred to as the S-Omega model) together with van Driest damping of the eddy viscosity near the wall provides best results in terms of (a) the interaction between LC with the bottom boundary layer, (b) the lateral length scales of LC, and ultimately (c) the upward transport induced by the LC. Finally, a simulation is posed to understand the influence of LC on an idealized tidal intrusion front occurring during flood in an estuary. Without wind and without wind-wave interaction (i.e. without LC), the front propagates uniformly across the spanwise length of the channel. A wind blowing against the direction of front propagation causes a counterflow flow along the lateral (spanwise) walls of the channel. This counterflow to the bulk tidal flow causes the front to propagate slower near the walls, while intruding further in the direction of the tide along the middle of the channel. Inclusion of the wind-current interaction leads to the generation of LC, which serves to homogenize the overall current in the vertical and the spanwise directions, resulting in a spanwise-uniform propagation of the front, similar to the propagation of the front in the flow without wind and waves.

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