Numerical simulation and characterization of site specific tidal flow turbulence

Abstract

The main area of study of this thesis was to research the effect of the bathymetry (the topography of the ocean floor) on the turbulent nature of a tidal energy site by way of high-resolution computational fluid dynamics simulations. Wall Modelled Large Eddy Simulations, for both flood and ebb tides, were carried out of a sampled area of the Fall of Warness, Scotland, which is the European Marine Energy Centre tidal energy site. Results from the simulations were validated with velocity and turbulence data from site measurements at the European Marine Energy Centre site. Both the simulation and the test data correspond to flows at 2.1 m/s turbine hub height reference velocity for ebb and flood tides.

Results show the influence of the sea bottom as a modifier of flow velocities and a driver of turbulence production at local areas important for the purposes of tidal converter performance and loading. High order turbulence parameters such as: Reynolds stresses, length scales, frequency spectra and turbulence intensities are analysed at different depths and locations in the domain, to better understand their local dependence on the changing bathymetry.

Turbulence data presents highly energetic coherent structures of a sufficiently large spatial and temporal size, connected to bathymetry variation. Both flood and ebb tidal simulations show that site bathymetry not only influences the mean flow, but also turbulent structure, energy efficiency of turbulent kinetic energy extracted from wall shear stress and three-dimensional distribution of Reynolds Stresses. Turbulence was found to be more isotropic for larger portions of the water column than found in previous simulations of roughened surface flows, with an especially chaotic flow area covering the bottom fifth of the water column filled with energy rich turbulent events.

Quadrant analysis for multiple depth probe locations is used to give a three-dimensional statistical understanding of bursting events, as modified by the seabed. The results show highly energetic coherent structures of a sufficiently large spatial and temporal size, connected to bathymetry variation. Ebb and flood turbulent features were compared in an effort to further understand the role bathymetry plays on tidal asymmetry. The temporal, spatial and energetic character of coherent turbulent structures were analyzed, within a context of the direction of the flow and its link to the seabed as a driving force for turbulence generation. Ebb tide was found to have a stronger turbulent intensity and production than flood tide for the same reference mean flow velocities.

The thesis adds to the knowledge of tidal energy site flow characterization and the turbulence of offshore environments by showing the strong imprint of bathymetry on tidal flows. The numerical methodology employed shows a guideline for studying the local and unique seabed bathymetry interactions with the tides. The use of high-resolution large eddy simulations is shown to be a tool capable of reproducing energy rich resolved structures vital to continuous advancement of energy extraction devices.