Resumen de Experimental and CFD analysis of the flow in the wake of a vertical axis wind turbine
The main goal of this thesis is to analyse, experimentally and numerically, the flow around a vertical axis wind turbine. This thesis is divided in two blocks in which the wake of the different types of turbines Savonius (drag) and Darrieus (lift) are studied. The study of the wake of the turbine is an important topic to develop new turbines or increase their performance because it allows to know the interaction between the blades and the free flow.
The first block is focused on the experimental and numerical study of a scaled commercial vertical axis wind turbine with eight blades based in drag phenomenon. The analysis is focused in the determination of the flow studies in the wake of the turbine. The experiments are carried out in an open-jet wind tunnel and they mainly consist in two parts. In the first part, the wind velocity is set to 10 m/s to measure the power curve of the turbine and the result are compared with those of Savonius turbines in different wind tunnels. That configuration of the wind tunnel affects the performance of the turbine. The reduction of the number of blades of the turbine has been also analysed and it has been found that the performance increases. It has been found that the turbine with 4 blades has the highest performance. However, with four blades there is a position where the forces are in equilibrium and the turbine has problem of self-starting. In the second part, the wind velocity of wind tunnel is set to 4.5 m/s (Tip Speed Ratio (λ) of 0.25) to perform smoke visualization experiments. The smoke visualization technique has allowed the qualitative validation of the 2D simulations because the flow patterns visualized are similar to those numerically simulated. The numerical simulations have demonstrated that the four blades turbine generates higher torque than the turbine with eight blades.
The numerical simulations are an important tool to obtain information of the wake, but sometimes, the simulation has a high computational cost. For this reason, in the second block, a new model to predict the wake of vertical axis wind turbines is proposed and analysed. The model is based on the actuator disk and the Double Multiple Stream Tube methods. Specifically, the model, denoted as Multiple Actuator Block, is based on the definition, inside the computational domain, of multiple parallelepipedic blocks distributed along the path of the blades. Volumetric momentum sinks are imposed in these blocks to model the effect of the blades on the flow. In order to analyse the performance of the model a VAWT with three NACA0022 blades, whose numerical and experimental results are available in the literature, has been considered. Different types of simulations with the Multiple Actuator Block model have been carried out and have been compared with a complete finite volume simulation using the sliding mesh technique. This simulation requires about ten times more CPU time than the simulations using the Multiple Actuator Block model. It has been found that the large scale features of the far wake can be reproduced using the Multiple Actuator Block model applying in the block the forces, obtained from the complete finite volume simulation or obtained from a boundary-layer type code, when the blade is inside the block.
