Numerical simulations and low-order models of the two-way interaction between ocean current turbines and the background flow
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Abstract
Ocean Current Turbines (OCTs), which function similarly to wind and tidal turbines, represent a promising technology for harnessing the energy from oceanic currents such as the Gulf Stream. In planning the deployment of arrays of OCT devices, it is critical to consider the two-way interactions between the turbines and the ocean environment: temporally and spatially nonuniform flow fields are expected in the dynamic flow environments of western boundary currents, and include the presence of upstream shear and turbulence. These nonuniform flow conditions will affect power extraction, and the efficiency of the turbines when operating in isolation or as part of an array. Furthermore, models that are used in a predictive capability to compute the levelized cost of energy obtainable from such devices, or to optimize the layout of an array of turbines must be modified to account for the effects of such spatially and temporally inhomogeneous conditions. Similarly, the operation of OCT arrays can in turn influence the background flow in two significant ways, namely by contributing to the production of turbulence and through the generation of internal gravity waves that are radiated away from the point of origin. In this thesis, we have studied using detailed numerical simulations, the above two-way interaction between arrays of OCTs and the ocean environment. Insights developed from the simulations have guided the development of low-order wake interaction models capable of describing the effects of inhomogeneous flow conditions on array performance. A new, wake interaction modeling framework capable of capturing the detailed effects of turbulence and upstream shear on various performance parameters associated with OCTs arranged in any arbitrary configuration has been developed. The model accounts for the effects of turbulence and shear on the structure of the turbine wakes, specifically the extents of near- and far-wake regions. The analytical description for turbine wake is combined with an existing wake interaction model, the Unrestricted Wind Farm Layout Optimization model to predict the global power output from an array of OCTs. The resulting modelling framework accurately captures the effect of inlet turbulence and shear on the OCT farm power and efficiency, and can be applied to any array configuration. Results from the model were validated against both Large Eddy Simulations and Reynolds Averaged Navier-Stokes simulations, in which the OCTs were modeled using a Blade Element Momentum model. The dispersion of OCT wake turbulence through the background stratification of the ocean was investigated using Large Eddy Simulations for different levels of the density stratification. The effects of varying the strength of the stratification as well as the turbulent forcing were studied. Finally, the wake turbulence associated with OCT operation can drive the formation and radiation of internal gravity waves in the density-stratified background flow of ocean currents. Through detailed numerical simulations, the effect of the propagation of the internal waves on the background turbulent diffusivity was studied, and found to alter the transport properties of the ambient flow. The properties of the internal wave field, and its impact on background turbulent mixing was found to depend both on the Richardson number and the ambient, upstream turbulence.