Analysis of a free vortex wake model for the study of the rotor and near wake flow of a vertical axis wind turbine

The 3D dynamics of the near wake of a vertical axis wind turbine is simulated by a numerical model based on a free vortex wake/panel method. The model is first verified with respect to discretization sensitivity, stability and robustness. Results show little sensitivity to the discretization, requiring a medium spatial and temporal grid to reach convergence. Stability issues are addressed, identifying dependency of the simulation parameters and validity of the results. The in-rotor flow shows little or no effect of instabilities and the model proves to be an effective analysis tool for the flow at the blade level. Wake dynamics, especially the tip vortex evolution, is more subject to stability issues which have to be taken into consideration. The numerical results are then validated with experimental data to assess accuracy. The model is able to capture all the important dynamics in the near wake of a vertical axis wind turbine with good quantitative evaluation of the flowfield. Discrepancies in the comparison are discussed and limitations in the use of the model are presented.