SHORT PAPER Zifeng Yang • Partha Sarkar • Hui Hu Visualization of the tip vortices in a wind turbine wake Received: 30 December 2010 / Revised: 19 September 2011 / Accepted: 10 October 2011 / Published online: 12 November 2011 Ó The Visualization Society of Japan 2011 Abstract In the present study, an experimental study was conducted to characterize the formation and the evolution of the helical tip vortices and turbulent flow structures in the wake of a horizontal axis wind turbine model placed in an atmospheric boundary layer wind. A high-resolution particle image velocimetry system was used to make detailed flow field measurements to quantify the time evolution of the helical tip vortices in relation to the position of the rotating turbine blades in order to elucidate the underlying physics associated with turbine power generation and fatigue loads acting on the wind turbines. Keywords Tip vortex  Turbine wake aerodynamics  HAWT  PIV measurements 1 Introduction The wake of a wind turbine is typically divided into a near and a far wake. The near wake refers to the region from the turbine to approximately one rotor diameter downstream. In the near wake, the presence of the rotor is apparent by the number of blades, blade aerodynamics such as attached or stalled flows, 3-D effects and tip vortices. The far wake is the region beyond the near wake, where the actual rotor shape is less important. The main attentions for far wake flows are usually drawn in wake models, wake interference, turbulence models, and topographical effects (Vermeer et al. 2003). A significant feature in the near wake of a wind turbine is the helical tip vortices induced by the rotating blades. The evolution of the helical tip vortices has been found to affect the behavior of the turbulent wake flows downstream of wind turbines significantly. The tip vortices are also recognized as an important source of noise generation and blade vibration (Massouh and Dobrev 2007). A good physical understanding is essential in order to provide an accurate estimation of the dynamic wind loads required for the optimal mechanical design to improve the performance and fatigue lifetime of wind turbines. This requires a detailed knowledge about transient behavior of the turbulent wake flows downstream of wind turbines and the evolution of the helical tip vortices induced by the rotating turbine blades. While a number of studies have been conducted in the recent years to investigate wind turbine wake aerodynamics, very few experimental investigations can be found in literature to provide detailed field measurements to quantify the transient behavior of the helical tip vortices in the near wakes of wind turbines. Furthermore, most of the previous researches were conducted with the wind turbine models placed in flows with uniform incoming flow velocity and relatively low turbulence intensity. However, in reality, wind turbines always operate in atmospheric boundary-layer winds with significant wind shear and turbulence intensity along the elevation direction. Z. Yang  P. Sarkar  H. Hu (&) Department of Aerospace Engineering, Iowa State University, Ames, IA 50011, USA E-mail: huhui@iastate.edu J Vis (2012) 15:39–44 DOI 10.1007/s12650-011-0112-z