Transient flow analysis for horizontal axial upper-wind turbine

Research Inventy: International Journal of Engineering And Science
Vol.6, Issue 10 (November 2016), PP -31-36
Issn (e): 2278-4721, Issn (p):2319-6483, www.researchinventy.com
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Transient flow analysis for horizontal axial upper-wind turbine
Gwo-Chung Tsai , Wei-Shiue Li
Department of Mechanical and Electro-Mechanical Engineering, National ILan University,
I-Lan, Taiwan, ROC
Abstract : This study is to carry out a transient flow field analysis on the condition that the wind turbine is
working to generate turbine, the wind turbine operating conditions change over time, Purpose of this study is try
to find out the rule from the wind turbine changing over time . In transient analysis, the wind velocity on inlet
boundary and rotation speed in the rotor field will change over time, and an analytical process is provided that
can be used for future reference. At present, the wind turbine model is designed on the concept of upwind
horizontal axis type. The computer engineering software GH Bladed is used to obtain the relationship between
the rotor velocity and the wind turbine. Then the ANSYS engineering software is used to calculate the stress and
strain distribution in the blades over time. From the analytical result, the relationship between the stress
distribution in the blades and the rotor velocity is got to be used as a reference for future wind turbine structural
optimization.
keywords：transient flow field analysis, upwind wind turbine, horizontal axis, ANSYS.
I. Introduction
The earth is along with the global climate vicissitude and the petrochemical energy is dried up
gradually. The renewable and polluted lowly energy is subject to takes seriously, especially in February, 2005
Kyoto Protocol becomes effective, makes the explicit agreement to the greenhouse gas emissions decrement. In
each kind of emerging energy, the wind turbine generator most has one of prospects for development renewable
energy sources. In the past 10 years, installed global wind turbine capacity is 6,100 MW by 1996, around the
end of 2008 it grows to 120,791 MW. From 1997 to 2008, installed global wind turbine capacity had increased
by 35% annually, and growth of installed capacity from 1,280 MW to the end of 2008 of 27,056 MW, average
annual growth rate is about about 29% [1].
Wind turbine is a mechanism that change the wind's kinetic energy into electrical energy of the rotating
machinery, it is necessary to mention the theory proposed by the German aerodynamicists Albert Betz [2],
according to Betz assumptions Ideally, to calculate the horizontal axis wind turbine, the maximum turbine
coefficient (CP) is about 0.59, the energy available to wind generators is around 60%, but Betz flow model
discussed only consider the axial flow without considering the wake rotation. Then the theory proposed by
scholars Schmitz [3] considered the wind turbine swirl losses. Ji-Hang Luo [4] takes the rotor blades with three
different parts of NACA airfoil as a reference, and uses the main model of commercially available NORDEX
plant's N-50 800kW wind turbines as a simulation object. By using of STAR-CD CFD software for simulation,
he compares the wind power, flow and torque under different available wind speeds to find the better airfoil.
Cheng-Han Xie [5]carry out a simulation for the vertical-axis Giromail wind turbine, through experiments,
multi-pipe flow model and the software FLUENT to explore the start capacity and aerodynamic properties.
Since the operation of wind turbines, mainly by aerodynamic effects of wind rotor blades rotate,
thereby driving the generator inside the cabin to produce electricity. For the study of wind turbine structure,
wind load caused by the importance of structure is evident. This feature is that the use of the wind turbine
simulation model to establish the flow field domain model, simulate the operation of wind turbine rotor when
the wind load.
II. Shape design and flow field analysis
2.1 Wind turbine structure
This paper uses the horizontal axis upwind turbine as the design model. Wind turbine is commonly
classified as the horizontal axis type and vertical axis type. Among the horizontal axial wind turbine, there is a
more detailed classification which includes the upper-wind type and lower-wind type. According to the wind
direction, the rotor rotating against the wind before the tower bracket is called as upwind turbine, shape
information as shown in Fig.1. This model is made up of blades, generators, hub, nacelle and tower shown in
Fig.2. Upwind-type wind turbine model uses gearless direct-drive design, this type is usually taken an external
cooling generator with a fin shape, back the nacelle is laid in the back as a short axis pattern. The shape appears
to be more compact.

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The diameter of Wind turbine rotor is 112.18 meters, the length of leaves is 54.5 meters, the height of hub
is 84.05 meters, the tower owns a 84 meters height and conical shape, whose minimum and maximum diameter
are 3.4 meters, 6 meters respectively, the basic specifications shown in Table 1. The angle between blades and
rotor plane calls pitch angle or rotation angle, shown in Fig.3, the pitch angle used in this study is 0 °.
Tab.1 Basic specification
Rotor Position Upwind
Rated Turbine 5 (MW)
Rotor Diameter 112.18 (m)
Hub Height 84.05 (m)
Tower Diameter 3.4～6 (m)
Fig.3 Pitch angle
2.2 Flow field region and its size
The flow analysis of 5MW wind turbines includes two different fields which are outer flow field and
the rotor flow field, the related calculation area is shown in Fig. 4. The diameter of turbine blade is D=120m,
thickness is 6m. The total length of outer field is assigned to be 7D. The distance behind the blade to the rear
edge is 4D. The distance from the side edge of the blade the side edge of the outer field is 2.5D. The height of
the flow field is assigned to be 2D.
Fig.4 calculated flow area and its size

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2.3 Wind turbine and flow field meshing
Mesh method uses the Advancing Front method, which is non-structural mesh method suitable for
complex geometric shape, and built some denser mesh layers around the physical boundary.
The meshed model owns a total of 633,854 nodes and 3,674,692 elements, as shown in Tab.2. Element
pattern is four faces cone, outer flow field and wind turbine mesh results are shown in Fig.5 and Fig. 6.
Tab.2 nodes and element number
Total number of nodes 633854
Total number of elements 3674692
2.4 Boundary condition set
The boundary conditions are set by using the arrow symbol marking, shown in Fig.7. Black arrow
symbol is defined as Inlet, which means the entrance boundary prohibiting backflow occurrence, and set the
axial velocity as flow entrance velocity. Blue bi-directional arrow symbol is defined as Opening that sets the
relative pressure with the outside world is 0. The boundary conditions on blades, surface, wheel, cabin and
tower wall is set the wall boundary conditions which means non-sticky (Smooth Wall) and has a non-penetrating.
Fluid flows through the wall must be satisfied no slip condition, which means the fluid velocity on the wall is
equal to the speed of the wall.
The air flow conditions of 25℃, the air flow is single-phase flow regardless of sand, water and other
multiphase flow conditions, material parameters are given by the software and keep constant. Assume that all
processes in the solution process are the adiabatic process that means not considering heat conduction and solar
radiation and so on.
The simulation time is 90 seconds analyzing Inlet axial velocity on the boundary and the rotor speed
changing over time, the relationship between the inlet wind velocity and the time is shown in Fig.8, the
relationship between rotor speed and time is indicated in Fig.9, the purpose is to control the speed of wind
turbine changing over time. The maximum rotor speed of Upwind turbine is 14.093 RPM (at 34 seconds) and
the minimum value is 13.633 RPM (in 75 seconds), the difference between them is 3.26%.
Fig.7 boundary conditions set

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Fig.8 wind turbine inlet velocity vs. time graph
Fig.9 wind turbine rotating velocity vs. time graph
2.5 Coordinate system
Load data is output based on the generalized coordinate system, because the related results are
changing in different coordinate system, CFX software, data output, you need to pay special attention to the case
of coordinate system, the following are the output of the coordinate system load that Figure 10, according to this
coordinate system, the output force and torque. Post-processing output setting, where outputs were set by
calculating the load force and torque, the load position selector wheel (that is, the model of the three wind
turbine blades and hub), and direction of the axis of the output, respectively X, Y, Z three axis, Figure 11, Figure
12.
Fig.10 output coordinate system
Gust load data results
Upwind wind turbine data results are shown from Fig.13 to Fig.18. Data capture location is on the blades
of wind turbine rotor, respectively for the X axis, Y axis and Z axis, data capture target is force and torque. from

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the results of Upwind wind turbine model, it can be found maximum force is 630641 (N), maximum torque M is
5.53 × 107 (N ∙ m),the maximum value occurs at the 34th seconds of wind turbine operation, coinciding with the
maximum value of the rotor speed changing over time, through comparing the results of Fig.9 and Fig.13, Fig.9
and Fig.18, it can be observed the greater the rotor speed, the larger the load on the blades.
Fig.13 X-axis force vs. time graph
Fig.14 Y-axis force vs. time graph
Fig.15 Z-axis force vs. time graph
Fig.16 X-axis torque vs. time graph

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Fig.17 Y-axis torque vs. time graph
Fig.18 Z-axis torque vs. time graph
III. CONCLUSION
Through the results on load diagram at a few high points in time, it can be found that the greater the rotor
speed, the larger the load on the blades. In fact the wind machine in the load situation is very complicated, and
the actual wind conditions and wind machines are non-stable, so in the simulation of this paper the ideal wind
conditions are assumed. As for the analysis model, the pitch angle of the blade is not considered. In addition, the
impact of surface roughness to the wind change, heat conduction and multiphase flow problems are available for
further consideration.
ACKNOWLEDGEMENTS
The research is really thanked to the financial support of Ministry of Science and Technology, under Grants
MOST 105-3113-E-002 -003.
REFERENCE
[1]. Pure Turbine – Wind Energy Scenarios up to 2030, European Wind Energy Association, March 2008,www.ewea.org
[2]. Betz A. Wind-Energie und ihre Ausnutzung durch Windmuhlen[M], Gottingen:Vandenhoeck & Ruprecht, 1926.
[3]. Schmitz G. Theorie und Entwurf von Windradern optimaler Leistung[J], Wiss.Zeitschrift der Universitat Rostock, 1955.
[4]. Jie-hang Lo，Numerical Simulation on a Horizontal-Axis Wind Turbine with various Rotor blades，National Taiwan University
[5]. Technology Department of Mechanical Engineering, Master's thesis, 2006.
[6]. Cheng-Han Hsieh, experimental and numerical studies of torque and power generation in a vertical axis wind turbine, National
Cheng Kung University, Aerospace Engineering Department, Master's thesis, 2009.

Transient flow analysis for horizontal axial upper-wind turbine

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