Aerodynamic interaction of diffuser augmented wind turbines in multi-rotor systems Uli G € oltenbott a, * , Yuji Ohya b , Shigeo Yoshida b , Peter Jamieson c a Department of Aeronautics and Astronautics, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-koen, Kasuga 816-8580, Japan c Wind Energy DCT, University of Strathclyde, Royal Collage R336, Glasgow, G11XW, UK article info Article history: Received 11 August 2016 Received in revised form 18 January 2017 Accepted 2 May 2017 Available online 8 May 2017 Keywords: Multierotor systems Clustered wind turbines Diffuser augmented wind turbines Wind turbine wakes Upscaling of wind turbines abstract The most common wind turbine is the singleerotor, horizontal axis wind turbine. In order to reduce the cost of energy, upscaling of singleerotor wind turbines has been a major trend. Recent studies however show that for a given technology, the cost usually rises when upscaling, notably due to increased masses. To reach capacities beyond 10 MW, multierotor systems (MRS) have promising advantages over single erotor systems (SRS). On the other hand, diffuser augmented wind turbines (DAWTs) can significantly increase the performance of the turbine. In this research, brimmed DAWTs are introduced in a MRS. In wind tunnel experiments, the aerodynamics of two and three DAWTs, spaced in close vicinity in the same plane normal to a uniform flow, have been analyzed. Power increases of up to 5% and 9% for the two and three rotor configurations are respectively achieved in comparison to a singleerotor turbine. Hot ewire techniques used to measure the flow speed near the gap between the DAWTs in a MRS have shown an acceleration of the flow. Phenomena of bluff body flows are reviewed to analyze the physical dynamics of the flows in the MRS on the basis of the flow dynamics observed in a SRS. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction The most commonly used wind turbine for electricity genera- tion is the horizontal axis, singleerotor wind turbine. A major trend is to increase the size of the rotor in order to generate more energy and to reduce the cost of energy generated from the wind. This was notably possible due to technological advancements in fiber re- inforcements to manufacture longer blades. Currently, researchers focus on developing technologies to upscale singleerotor wind turbines to 10 MW and beyond. However, there are certain issues related to upscaling wind turbines which have been discussed by Jamieson and Branney [1] and Sieros et al. [2] who stated that upscaling usually results in an unfavorable weight increase for a given technology level. On the other hand, Hofmann and Sperstad [3] pointed out that low failure rates and short maintenance du- rations are crucial to achieve low cost of energy. A concept to overcome these issues for wind turbines is the multierotor system (MRS), defined as containing more than one rotor in a single structure. Upscaling by increasing the number of rotors instead of the diameter of a single rotor leads to significant savings in mass. A reduction in mass consequently reduces the overall cost of the system. Proposed as early as 1875 in Denmark [4] and later in the 1930s by Honnef in Germany, as reported by Jamieson [5], the multi-rotor concept has been further developed by Heronemus [6] in the 1970s, while Smulders et al. [7] later analyzed a two rotor configuration in a wind tunnel. In 2010, the South-West Research Institute con- ducted an analysis of a 7 rotor configuration in the NASA Langley wind tunnel to test the concept of Heronemus and found no negative interferences between the rotors [8]. Simulations of the aerodynamic performance for the same 7 rotor system were pre- sented by Chasapogiannis et al. [9]. In their CFD simulations using actuator discs, a power increase of 3% was predicted for the 7 rotor system. On a larger scale, structural considerations of a 20 MW multi-rotor system with 45 rotors have been presented by Jamieson and Branney [1]. They showed that mass and cost are reduced, in comparison to a single-rotor turbine with the same power, and that the structure is designed based on the aerodynamic forces rather than on the rotor loading. * Corresponding author. E-mail addresses: uli@riam.kyushu-u.ac.jp (U. G€ oltenbott), ohya@riam.kyushu- u.ac.jp (Y. Ohya), yoshidas@riam.kyushu-u.ac.jp (S. Yoshida), peter.jamieson@ strath.ac.uk (P. Jamieson). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene http://dx.doi.org/10.1016/j.renene.2017.05.014 0960-1481/© 2017 Elsevier Ltd. All rights reserved. Renewable Energy 112 (2017) 25e34