Experimenal study of the aerodynamics of a horizontal axis wind turbine

Abstract

One of the challenges of the wind energy community today is to improve the existing background on the aerodynamic phenomena of a Horizontal Axis Wind Turbine (HAWT), the prediction of the wind speed distribution on the rotor plane, and the estimation of the design loads. This dissertation aims at contributing to the fulfillment of these objectives.

In this way, this study assessed the feasibility of measuring the loads exerted on a HAWT blade by means of Stereoscopic Particle Image Velocimetry (SPIV), which is a non intrusive technique that provides with the whole 3D velocity field in a plane. Thus, with this PIV-Loads method, the velocity and pressure fields, as well as the resultant aerodynamic forces around a section of the blade, would be available simultaneously, without the need of modifying the model or disturbing the flow.

In order to achieve this goal progressively, the PIV-Loads method, based in a Momentum Equation contour-based approach, was firstly validated using DNS data, both for a laminar unsteady flow case, as for a velocity averaged turbulent flow. Secondly, the method was tested in the wind tunnel with a bidimensional problem, measuring forces in a stationary flat plate, for different angles of attack (with laminar and turbulent flow conditions). The force estimation results were compared with those provided by a high sensitive balance. Finally, the PIV-Loads method was applied to a HAWT model working both in axial and yaw flow conditions, measuring forces on a rotating blade for steady and unsteady cases. Final load calculations were compared with those resulting from a numerical simulation based in the Panel method approach. Bringing the project to completion, the near vortex wake of a HAWT was characterized by means of Time Resolved PIV.

Regarding the PIV-Loads methodology, load predictions are more reliable if the integration path does not cross a shear layer or a boundary layer. In addition, it is neither recommended to neglect the third velocity component when measuring forces on a rotating HAWT blade, nor to eliminate the velocity fluctuation terms when dealing with turbulent flows.

All implemented codes and experimental results were validated or compared with numerical or experimental alternative data showing good consistency. The conclusion is that the PIV-Loads method provides with precise results if the available velocity data is sufficiently accurate. However, any PIV errors such as lack of resolution, velocity gradients inside the interrogation window or laser reflections, may lead to uncertainties in the load measurements. Any future improvement in this sense will certainly lead to better results.