Effects of tip injection on the performance and near wake characteristics of a model wind turbine rotor Anas Abdulrahim, Ezgi Anık, Yashar Ostovan, O  guz Uzol * Department of Aerospace Engineering, METU Center for Wind Energy, Middle East Technical University (METU), 06800 Ankara, Turkey article info Article history: Received 9 December 2014 Received in revised form 25 July 2015 Accepted 7 November 2015 Available online xxx Keywords: Tip injection Wind turbine Tip vortex Tip vortex control Flow control abstract This paper presents an investigation of the effects of tip injection on the performance and near wake characteristics of a model wind turbine rotor. Experiments are conducted by placing a three-bladed horizontal axis wind turbine rotor at the exit of an open-jet wind tunnel. The rotor blades are non- linearly twisted and tapered with NREL S826 airfoil profile. The nacelle, hub and the blades are specif- ically designed to allow pressurized air to pass through and get injected from the blade tips while the rotor is rotating. Measurements of torque and thrust coefficient variations with the Tip Speed Ratio (TSR) as well as wake flow field surveys using Constant Temperature Anemometry are performed with and without tip injection. Results show that power and thrust coefficients both increase due to injection especially at TSR values higher than 3.5. Wake characteristics show a tip flow region that is radially pushed outwards with increased levels of turbulence occupying wider areas compared to the no- injection case. Up to two diameters downstream within the wake, the boundary between the wake and the freestream, which is normally dominated by the presence of the tip vortices, gets more diffused and turbulence levels are significantly increased due to tip injection. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction The pressure difference between the upper and lower surfaces of a wind turbine rotor blade results in a leakage of the flow from the lower to upper surface at the blade tip. When this leakage flow meets with the main stream, concentrated vortical structures get generated. These so-called tip vortices can cause a variety of per- formance losses for horizontal axis wind turbines. In addition, these vortices can cause structural and performance problems due to vortex-turbine interactions in successively arranged wind turbines in wind farms. In order to minimize such problems, controlling these vortices could be achieved by active or passive means. Passive Flow Control (PFC) methods do not adapt to changes in flow conditions. Never- theless, PFC methods have found application in real systems such as delta-wing vortex generators for separation control near the blade roots (e.g. Refs. [3,15]) or winglet like tip extensions for tip leakage control [2,8,13,25]. Active Flow Control (AFC), on the other hand, can be applied depending on the flow conditions and load requirements and is generally used to manipulate the local flow characteristics. AFC has been investigated in various applications mostly to control the boundary layer separation and transition characteristics (e.g. Refs. [26,14,6,9,10]). AFC for controlling the tip leakage characteristics and the size, vorticity and turbulence char- acteristics of the tip vortex using tip injection (also referred to as tip blowing) has also been proposed and investigated by previous re- searchers in various applications such as fixed wings (e.g. Refs. [5,11,18]), helicopter rotors (e.g. Refs. [7,12,28]) and turboma- chinery blades (e.g. Refs. [19,21,24] and [20]). Previous studies show that the strength, location, core structure, and number of vortices can be effectively manipulated by tip in- jection [16]. For instance [18], conducted an experimental study on the effects of chordwise varying tip injection on a rectangular wing with a NACA 0015 airfoil profile. Results show significant effects of the tip injection on the vortex characteristics such as the vortex size gets bigger with injection, the total pressure levels get reduced significantly near the vortex core, the wake size behind the wing gets affected showing a slightly thicker region as well as the wake entrainment characteristics gets modified resulting in vortex sep- aration from the wake zone. Moreover [5], investigated the effect of tip injection on the turbulence characteristics of the wake and the tip vortex. It was shown that tip injection causes significant * Corresponding author. E-mail address: uzol@metu.edu.tr (O. Uzol). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene http://dx.doi.org/10.1016/j.renene.2015.11.030 0960-1481/© 2015 Elsevier Ltd. All rights reserved. Renewable Energy 88 (2016) 73e82