Simulations of a vertical axis turbine in a channel

The power coefficient of a turbine increases according to the predictions from streamtube theory for sites with a confined fluid flow.Here, a vertical axis turbine (optimized for free flow) has been simulatedby a two-dimensional vortex method, both in a channel and in freeflow. The first part of the study concerns the numerical parametersof channel simulations. It is found that for free flow andwide channels, a large number of revolutions are required for convergence(around 100 at the optimum tip speed ratio increasing with highertip speed ratio), while for smaller channels, the required number of revolutionsdecreases.The second part analyses changes in turbine performance bythe channel boundaries. The turbine performance increases whenthe channel width is decreased, although the results are below predictions from streamtube theory, and this difference increases with decreasingwidth. The differences are more prominent for smaller channels.It is also observed that the optimum tip speed ratio increases withdecreasing channel width. By increasing the chord, which decreases the optimumtip speed ratio, the power coefficient can be increased somewhat.