Fluid mechanical simulations and development for vertical axis turbines

Abstract: The aerodynamics of vertical axis turbines is typically studied using streamtube, vortex or CFD models. This thesis focuses on the first two models, which are the computationally faster ones. The streamtube model is the fastest, allowing three-dimensional modeling of the turbine, but lacks a proper description of the flow through the turbine and does not include any time dependence in the solution. The vortex model used is two-dimensional, but gives a description of the flow around the turbine and can handle time dependence. Effects of a velocity profile and the inclusion of struts have been investigated using the streamtube model. Simulations with the velocity profile indicate that the vertical axis turbine should be quite insensitive to the profile (with respect to the power coefficient). If the applied profile is perpendicular to the rotational axis, the turbine generally performs better if the blade moves against the flow at the high velocity side of the profile. When including struts, the structural mechanics was included and the calculations shows that if turbines are designed for high flow velocities, additional struts are required, reducing the efficiency for lower flow velocities. Turbines in channels and turbine arrays were studied with the vortex model. The channel study included both the numerical parts of the simulations and the effects of channel width were investigated. On the numerical side, the most prominent result was that for wide channels, the number of revolutions until convergence is high. It was seen that smaller channels give higher power coefficients, as predicted by streamtube theory, but the increase in power coefficient with decreasing width was slower, than predicted by streamtube theory. Simulations on a turbine array were performed on five turbines in a row and in a zigzag pattern, where the mean power coefficients of the turbines in the array are higher than for a single turbine. The row configuration was also shown to obtain slightly higher power coefficients and being less sensitive to misalignments in flow direction than the zigzag pattern.