idsession
code

Invited-6

B-2-1

Downwind Rotor Technologies forProf. Shigeo Yoshida
Large Scale Offshore Wind Turbines

Kyushu University

192

B-2-2

Investigation on effect of tower type
Dr. Bernhard Stoevesandt
on downwind turbines

Fraunhofer IWES

173

B-2-3

Blade Design Considerations due toTower Effect on Upwind andDr. Xabier Munduate
Downwind Turbines

CENER

Invited-7

B-2-4

Multi Rotor Systems for Large Scale
Dr. Peter Jamieson
Offshore Wind Power

Strathclyde University

171

B-2-5

Wake Deflection in Long Distancefrom a Yawed Wind Turbine

Tokyo University of Science33

B-2-6Fundamental Research on Wake
Structure of a Simple Vertical-Axis Mr. Tomohiro Tokiwa
Windmilltitle

Presenter name

Mr. Yuta Uemura

Affiliation

Doshisha University

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31-Oct. 16:15 - 18:15 [ Innovative onshore wind technology／Wind Turbine Technology (2) ]Investigation on effect of tower type on downwind turbines
Bernhard Stoevesandt1, Hamid Rahimi1 and Joachim Peinke2
1Fraunhofer IWES, 2University of Oldenburg

Abstract:
Due to new materials, control strategies and the fact, that noise is offshore not a driving factor, downwind turbines seem go
into a revival. So far Ming Yang, Hitachi and the Dutch developer 2-B Energy have been developing downwind concepts lately[1]. However, the effect of the tower shadow is a crucial question in the aerodynamics for such turbines. One question to beanswered is, how good the used blade element momentum (BEM) theory based load calculation models are for such a turbine
design. Another question is, how the tower effect could be minimized, since it is regarded as potentially problematic. Here,concerning the first questions, we compare measurement data from a wind tunnel experiment against computational fluid
dynamics (CFD) and BEM calculation. In the second part the tower effect of an assumed 5MW turbine following the NREL
5MW turbine [5] concept as a downwind configuration is compared between the standard tower and a lattice tower concept.
In BEM based calculations mostly either a wind deficit, from potential theory, or empirical models are used [1, 2]. Here, weinvestigate, how much this assumption holds using CFD simulations. We use measurement data from the NREL Phase VI
turbine for our research. The mesh generation was done using the bladeBlockMesher, developed by ForWind and Fraunhofer
IWES [3]. It creates a structured blockMesh around wind turbines blades. The rotor was meshed separately in a cylindricalmesh from the surrounding mesh. The latter has been meshed in a half-cylindrical way with the hub distance of 5D to the inlet
and crossflow cylinder walls and 10D distance to the outflow. Overall ca. 10 Million cells have been used. The k-omega SST
model has been used for unsteady RANS turbulence modeling [4]. The results from the BEM code FAST show anapproximately 20% higher value for the lift coefficient compared to CFD results. The CFD simulations show a drop of 50%
compared to the FAST simulation which has a drop of 70%. For the full scale wind turbine simulation with different tower
concepts the towers have been simulated using an actuator line concept for each situation. The blades were in this caseconsidered to be rigid an straight. The layout of the domain was chosen according to the before described simulation. Theoverall mesh consisted of about 30 million cells. We used a k-omega SST turbulence model for the unsteady RANS
simulations. The first results showed a weaker but less localized effect of the lattice tower wake against the monopile tower.
[1] Fairley P 2003 Two-bladed wind turbines make a comeback accessed 29 March 2016
URL https://www.technologyreview.com/s/528581/two-bladed-wind-turbines-make-a-comeback/[2] Hagen T R 2011[3] Rahimi H, Daniele E, Stoevesandt B and Peinke J 2016 Wind Engineering[4] Menter F and Esch T 2001 16th Brazilian Congress of Mechanical Engineering pp 26–30[5] Jonkman J, Butterfield S, Musial W and Scott G 2009 Definition of a 5-MW reference wind turbine for offshore systemdevelopment Tech. Rep. February

Aerodynamic Blade Design Considerations
due to Tower Effect on Upwind and Downwind Turbines
Xabier Munduate, Ainara Irisarri and Maria Aparicio
CENER

Abstract:
1. Introduction: This work presents the aerodynamic effect that the tower has on the rotor of upwind and downwind configurations andmay serve to obtain guidelines for the blade and airfoil design. The rotor configurations studied are:
•The baseline upwind three-bladed 10 MW INNWIND.EU rotor•The downwind three-bladed 10 MW INNWIND.EU rotor•
A downwind two-bladed design based on the 10 MW INNWIND.EU rotor
2. Analysis of the configurations: The results include the comparison of these three cases with and without the tower effect. The analysis
focus on the energy production AEP, the rotor power, the tip-speed ratio TSR and the angles of attack. The AEP obtained is higher for thebaseline rotor than for the rest of configurations. The reduction of AEP observed is close to 0.12% for the downwind three-bladedconfiguration, and it is approximately -2.08% for the downwind two-bladed design. The effect of the tower shadow reduces the AEP
calculated. This effect presents reductions close to -0.7% for the two-bladed downwind rotor, approximately -0.62% for the three-bladed
downwind rotor and even lower for the baseline case (-0.26%). Regarding the rotor power, the tower shadow effect leads to higher
frequencies and amplitudes. From 3 to 2 blades (both downwind), the decrease in rotor power is multiplied by a factor of 1.29. TSR has
been calculated for the different azimuth positions and it is clear that the standard deviation of the TSR is affected by the configuration.
Finally, the angle of attack at 50% span has been analyzed. The variation of angle of attack is low for the baseline case with respect to thedownwind configurations, and it is higher for the two-bladed design than for the downwind three-bladed configuration. The range of time
where the effect of the tower is evident (Δt) is wider for the upwind case than for downwind cases and it is significant with respect to theperiod (5.9%-12.3%). The effect in the rate ΔAoA/Δt related to the configuration from upwind to downwind configuration (3 bladed) isvery important (4.7 times). While the effect of the change is this variable due to the number of blades is lower (1.29 times).

3. Applications for design••The tower shadow effect can be only partially taken into account by the blade geometry design.
The way of considering the tower shadow effect in design could be focused on airfoil design: use airfoils designed to withstand unsteady
aerodynamics, airfoils where efficiency does not drop sharply with α variation.

4. Conclusions••

The conclusion is that the tower effect has an impact on both configurations; upwind and downwind.
As expected, considering the tower, less rotor power is measured with the three configurations studied. The AEP loss is higher for
downwind rotors, and higher for 2 bladed rotors.
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31-Oct. 16:15 - 18:15 [ Innovative onshore wind technology／Wind Turbine Technology (2) ][Invited Speaker]

The Multi Rotor Solution for Large Scale Offshore Wind PowerPeter Jamieson
University of Strathclyde, UKAbstract:
The multi rotor system (MRS) concept involves a multiplicity of small rotors on a single structure as compared todeveloping ever larger single rotor systems. The idea is not new. Designs of Honnef, for example, in the 1930’s show
MRS for onshore and offshore, but the motivation for MRS then lay in the difficulties in making very large single rotors
when steel was the main trusted engineering material. Heronemus in the 1970’s advocated MRS noting advantages instandardisation of rotor size. In work on advanced wind turbine concepts for the UK Department of Industry 1995
Jamieson noted an obvious but key advantage of the MRS in much increasing the ratio of system rotor swept area (valuein energy capture) to volume (cost in structure and power conversion) compared to an equivalent single rotor.
Present status is reviewed following an international workshop on MRS with presentations of Vestas on their 4 rotor, 900
kW prototype, laboratory and field testing of MRS at Kyushu University, the MOWIAN MRS tested at Kaiser WilhelmKoog and yaw system design work of HAW Hamburg. A major recent study of MRS for offshore deployment at 20 MW
scale was conducted in the Innwind.EU project within the European Wind Initiative (EWI) established under SET-Plan,
the Common European Policy for Energy Technologies. Results of this work and implications for future development
form the main part of this presentation.
The MRS benefits from standardisation of rotor size with higher production quantities and smaller unit size speeding
development, improving reliability and reducing cost. Project investment risk is greatly reduced in employing proventurbine technology at a manageable scale rather than developing ever larger units with longer development times forsystems and components and much greater exposure to cost impact of serial faults etc. With benefit from the scaling laws,
the weight and cost of the 45 rotors (each rated at 444 kW) and nacelles compromising a 20 MW multi rotor system thathas been developed within the Innwind.EU project is ~ 20% of that of a single equivalent rotor and nacelle system. Per
installed MW of offshore capacity, about 80% of the glass epoxy composite materials used in blades that are difficult torecycle will be saved.
The design developed within the Innwind project was subject to detailed analyses of aerodynamics, loads, support
structure, electrical integration and yawing systems. The total power of n rotors compared to n × power of a single rotor
operating in isolation is found to increase by 8% for the 45 rotor array. The Innwind project compared a number ofinnovations and results are presented. Levelised cost of energy was evaluated using a cost of energy model adapted fromthe NREL model and developed independently in another task of Inniwnd.EU. The 20 MW MRS results suggest morethan 20% reduction in LCOE based on technology aspects alone without consideration of commercial or industrial factors
such as reduced investment risk and standardisation.

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31-Oct. 16:15 - 18:15 [ Innovative onshore wind technology／Wind Turbine Technology (2) ]Wake Deflection in Long Distance from a Yawed Wind Turbine
Yuta Uemura1, Hiroya Mamori1, Naoya Fukushima1, Makoto Yamamoto1 and Yasutada Tanabe2
1Tokyo University of Science, 2Japan Aerospace Exploration Agency

Abstract:
In wind farms where a large amount of wind turbines may lay on several rows, deficits of wind energy to the downstream
wind turbines occur, when the downstream turbines operate in the wakes generated by the upstream wind turbines. The
aerodynamic interaction between the wind turbines can cause output decrease of the downstream wind turbine. Furthermore, it
was confirmed that especially in the condition of wake half-overlapping, severe fluctuation of aerodynamic load occurs on thedownstream wind turbine blade. After all, to solve the problems such as the output decrease or increase of fatigue load,
avoidance of the wake interference to the downstream wind turbine by operation of wake deflection from the upstream windturbine is desired. In the present study, firstly, three-dimensional computations of single wind turbine are carried out withchanging yaw angles. For the result, focusing on velocity distribution in wake and aerodynamic loads on the yawed windturbine, we discuss the influence of yaw angle. Secondly, based on the results of the velocity distribution in wake for each yaw
angle, we move on to a simulation of avoidance of wake interference to downstream wind turbine, where two wind turbines areprepared. Special CFD solver “rFlow3D” which has been developed in JAXA (Japan Aerospace Exploration Agency) for
rotorcraft is used for the computations, because it has capability to handle moving grid systems. RANS (Reynolds-Averaged
Navier-Stokes) method with SA (Spalart-Allmaras) turbulence model is used considering its robust nature and shortcomputational time. In the solver, three-level overset grid system is adopted. The first level is the moving grids to resolve theflows around blades, the second is an inner background grid for covering the moving grid and capturing wake field with fineresolution, and the third is a much wider outer background grid which covers the inner background grid for preserving freestream conditions. Through the present study, the following characteristics are confirmed. The operation of wake deflection byadding the yaw angle causes enough wake skew angle even in far-wake. Furthermore, introduction of the yaw angle accelerates
progression of vortex dissipation and brings about early velocity recovery in the wake region. Meanwhile, the introduction
decreases amount of power generation of the yawed upstream wind turbine and increases fatigue load of flapwise moment
added to the blade root. In the final paper, the details of flow field, oscillation and performance characteristics of the yawed
wind turbine will be described.

Fundamental Research on Wake Structure of a Simple Vertical-Axis
Windmill
Tomohiro Tokiwa1, Yukinaga Kinoshita1, Hiroaki Mihara2, Tatsuya Inoue1 and Katsuya Hirata1
1Doshisha University, 2General Building Research Corporation of Japan

Abstract:
Our final purpose is to propose a new simple and durable windmill and to optimise it. This windmill is composed of arectangular flat plate rotating about a vertical-axis. The windmill is expected to attain rather low efficiency, however is suitable
for small, on-site and self-sufficient systems to generate power in disaster, isolated or remote regions/areas.
The windmill is the application of an autorotation; namely, a kind of self-sustained rotating motion known as ‘tumbling.’ The
tumbling is a rotating motion with the axis perpendicular to mainstream’s direction. The tumbling is not only purely-academic
concerns found in the falling motions of leaves and pieces of paper, but also a very important phenomenon in such actual
aspects as aeronautical and space engineering, ballistics and meteorology in addition to mechanical engineering whoseapplications are new renewable-energy converters like windmills and waterwheels, smart flow-control devices, mixers/diffusers
and so on. So, there have been some researches for the tumbling. However, we have not obtained enoughknowledge about the effect of aspect ratio AR upon the tumbling. This AR effect is important especially for renewableenergy converters from a practical point of view. Moreover, it is necessary to understand the wake structure past a rotating flat
plate in order to optimise windmill and waterwheels.
The present study is a fundamental approach on the wake structure of the windmill.More specifically, we conduct a series of
subsonic wind-tunnel experiments, in order to investigate the basic aerodynamic characteristics of a tumbling finite flat plate inuniform mainstream. Especially, we focus upon both the effects of an aspect ratio AR and a tip-speed ratio Ω* which is areduced form of a rotating rate n upon the tumbling of a finite flat plate, in addition to the Reynolds number Re. We should notethat the plate is forced to rotate with a constant speed, because we can easily control Ω* over a much wider range thanfreerotation or free-fall experiments. This is suitable as a fundamental approach in the first stage. We carry out
fluctuating-velocity measurements using a hot-wire anemometer together with the synchronized measurements of the plate’s
attack angleα which are used for phase-averaging analyses, in addition to flow visualisations. The tested range of a geometricalparameter AR is 0.67 – 5.0. The tested value of Ω* varies from 0.1 to 3.0. The tested value of Re varies from 3.0×103 –5.0×104. The tested range of another geometrical parameter λ concerning the plate’s cross section is 2.0×10-2 −5.0×10-2, which is small enough to be regarded as thin. As a result, we have revealed the effects of AR, Ω* and Re upon thetumbling of a flat plate. A frequency ratio fd/n as a reduced form of fd is more appropriate to consider flow modes than the flowmodes based on the Strouhal number St. The periodicity of fluctuating velocity of a rotating flat plate mainly depends AR andΩ*, but does not almost depend on Re.
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