Analysis of a Vertical-Axis Tidal Turbine Using the Variational Multiscale Formulation
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2022-09
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Abstract
This thesis first investigates the performance and near-wake characteristics of a full-scale vertical-axis tidal turbine under a uniform inflow and turbulent inflow with a 5% and 10% turbulence intensity. The governing equations of the flow field are the incompressible Navier-Stokes equations. As the turbine rotates throughout the simulation, these equations are expressed in a slightly different form referred to as the arbitrary Lagrangian-Eulerian (ALE) framework. The purpose of the ALE framework is to allow the mesh to move arbitrarily while the fluid moves independently of the mesh motion. From the available large eddy simulation (LES) formulations in the literature, the variational multiscale (VMS) formulation is used to discretize the system of equations. Unlike classical LES, the VMS formulation does not have any problems with specifying an appropriate filter for different flows. To study the effect of a turbulent inflow, a turbulence generation method referred to as Smirnov's random flow generation (RFG) is used. From the numerous turbulence generation methods available, Smirnov’s RFG was chosen as it can generate a turbulent velocity field that is divergence-free. A divergence-free velocity field ensures compatibility with the incompressible Navier-Stokes equations that govern the flow field and results in good numerical stability. While the performance of the turbine slightly reduced under a turbulent inflow compared to a uniform inflow, there was a negligible difference in its performance between the two turbulent inflow conditions. A turbulent inflow also resulted in large fluctuations of the instantaneous power coefficient. Lastly, the wake recovery was notably improved under a turbulent inflow. Next, the effect of a free surface on the performance and flow field of the turbine with different blade-strut configurations is studied. There was a negligible effect of the free surface on turbine performance and the flow field during deep immersion. Moreover, the tip-struts configuration was 15% more efficient than the quarter-struts configuration under deep immersion. Under shallow immersion, the performance of both blade-strut configurations reduced.
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Dhalwala, M. (2022). Analysis of a vertical-axis tidal turbine using the variational multiscale formulation (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.