Amanullah Choudhry

Horizontal axis wind turbine dynamic stall predictions based on wind speed and direction variability

The onset of dynamic stall in horizontal axis wind turbines (HAWTs) is related to the rapid increase in the angle of attack caused by sudden changes in wind speed and direction. In order to relate the changes in wind speed and direction with the variations in the blade-section angle of attack, an analytical model is proposed to determine the regions of the blade affected by dynamic stall. The so-called threshold radius has been identified and defined as the percentage of the blade length from the horizontal axis wind turbines hub beyond which the probability of dynamic stall occurrence falls to zero. High quality wind data were acquired to determine the average wind conditions that serve as the model inputs. It is shown that the rate of change of wind speed, due to gusts or the average turbulence, can cause large regions of dynamic stall on the wind turbine blade. Other parameters, such as the yaw misalignment and the rate of change of yaw angle are shown to be the cause of asymmetrical distribution of threshold radius with azimuth and also serve to increase the affected regions. Finally it is shown that the type of airfoil used in the turbine blade also has a significant effect on the threshold radius due to the different limiting reduced frequencies.

Effects of Wake Interaction on Downstream Wind Turbines

The present article revisits the wake studies behind the NREL (National Renewable Energy Laboratory) Phase VI wind turbine inside a virtual wind tunnel that were recently performed at the University of Adelaide using Large Eddy Simulation (LES). A notable observation has been made in the current article, through comparisons of instantaneous contours of vorticity, velocity and turbulence intensity, that the regions of velocity deficits and high turbulence intensities in the wake are restricted to the regions of high vorticity. Therefore, for a downstream wind turbine, the smaller power production, the increased unsteady loads and the noise produced can directly be associated with the turbine blades passing through the streamwise vortices generated by the upstream wind turbine. In addition, a comparative analysis has been performed between the LES and semi-empirical models, used in the industry, to better understand the development of the wake inside the wind tunnel model. Finally, in order to illustrate the effects of wake on downstream wind turbines, a dynamic stall prediction model was used to determine the regions of the turbine blade affected by dynamic stall as a function of spacing between the turbines.