The aerodynamic behavior of a vertical axis wind turbine (VAWT) is analyzed by means of 2D particle image velocimetry (PIV), focusing on the development of dynamic stall at different tip speed ratios. The VAWT has an unsteady aerodynamic behavior due to the variation with the azimuth angle h of the blade's sections' angle of attack, perceived velocity and Reynolds number. The phenomenon of dynamic stall is then an inherent effect of the operation of a VAWT at low tip speed ratios, impacting both loads and power. The present work is driven by the need to understand this phenomenon, by visualizing and quantifying it, and to create a database for model validation. The experimental method uses PIV to visualize the development of the flow over the suction side of the airfoil for two different reference Reynolds numbers and three tip speed ratios in the operational regime of a small urban wind turbine. The field-of-view of the experiment covers the entire rotation of the blade and almost the entire rotor area. The analysis describes the evolution of the flow around the airfoil and in the rotor area, with special focus on the leading edge separation vortex and trailing edge shed vorticity development. The method also allows the quantification of the flow, both the velocity field and the vorticity/circulation (only the results of the vorticity/ circulation distribution are presented), in terms of the phase locked average and the random component. List of symbols

Tip vortex locations have been measured in the wake of a model rotor in both axial flow and yaw using quantitative flow visualization. For each setting, the axial force coefficient has been derived, as well, from measurements. The results agree well with those previously published on the Delft University of Technology model rotor. The main interest is to determine the tip vortex pitch, wake skew angle, wake expansion, and to physically interpret the data. The results also help to validate and construct models. The tip vortex location data complement the existing skewed wake velocity data from hot-wire anemometry, making it a valuable experimental database.

The implementation of wind energy conversion systems in the built environment has renewed the interest and the research on Vertical Axis Wind Turbines (VAWTs).The VAWT has an inherent unsteady aerodynamic behavior due to the variation of angle of attack and perceived velocity with azimuth angle. The phenomenon of dynamic stall is then an intrinsic effect of the operation at low tip speed ratios, impacting both loads and power.The complexity of the problem and the need for new design approaches for VAWTs for the built environment have driven the authors to focus this research on the CFD modeling of VAWTs on:• Comparing the results between commonly used turbulence models: Unsteady Reynolds Averaged Navier-Stokes -URANS (Spalart-Allmaras and k-ε) and large eddy models (Large Eddy Simulation and Detached Eddy Simulation). • Verifying the sensitivity of the model to its grid refi nement (space and time). • Evaluating the suitability of using Particle Image Velocimetry (PIV) experimental data for model validation.The current work investigates the impact of accurately modeling the separated shed wake resulting from dynamic stall, and the importance of validation of the fl ow fi eld rather than validation with only load data.The structure and magnitude of the wake are validated with PIV results, and it demonstrated that the accuracy of the different models in simulating a correct wake structure has a large impact in loads.

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An experimental and numerical analysis of radial flows in the near wake of a horizontal axis wind turbine is presented for both axial and yawed turbine conditions. Blade performance and wake development are affected because of radial velocities. The phenomenon has not been previously detailed, and current knowledge is limited to the mid to far wake regions. The stereo particle image velocimetry dataset of the model experiments in controlled conditions (MEXICO) rotor is used along with a 3D unsteady potential-flow panel model. The latter is used, after validation, to give a complete description of the radial velocities and give insight into wake development and geometry. For both axial and yawed flows, the radial velocity is found to increase from root to tip with some complex behaviour in the root and tip regions. For axial flow, radial velocities were found to be appreciable. For yawed flow, because of the in-plane freestream component, radial velocities are of the scale of the unperturbed flow especially when the blade is in its leeward and windward positions. The implications of this study are particularly relevant for blade element momentum analysis tools that rely on a purely 2D momentum balance approach. Moreover, this study enables further understanding of the wake development process in the proximity of the rotor. NOMENCLATUREx horizontal axis coordinate [m] y coordinate perpendicular to the rotor plane [m] z vertical axis coordinate [m] blade azimuth position [°] rotor yaw angle [°] U 1 freestream velocity [m s 1 ] c chord length [m] Â blade twist [°] potential function [m 2 s 1 ] rotational speed [rad s 1 ] r radial position [m] n normal direction vector to each panel [m] C doublet influence coefficient B source influence coefficient doublet strength [m 3 s 1 ] source strength [m 2 s 1 ] Á ratio of the total number of panels with respect to a reference simulation