Application of LES CFD for flow simulation of an escarpment sitewith complex terrain
Graham Li1 and Takanori Uchida2
1Tsubasa Windfarm Design, Sunagawa 4-62-8, Tachikawa, Tokyo, Japan

Phone: +81 80 2379 2899, Email: graham@twd-wind.com

2

Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka, Japan

Phone: +81 92 583 7701 ,Email: takanori@riam.kyushu-u.ac.jp

Abstract: One of the wind turbines at Duogu wind farm, an existing operating wind farm locatedin China's Yunnan province experienced vibration problems. The turbine is located on top of avery steep cliff and the topography in the turbine vicinity is of complex nature. Analysis of turbine
operational data indicates the vibration problem occurred only from a particular wind direction.
With RIAM-COMPACT®, a LES CFD code was used to simulate the wind flow for the identified
wind direction. Simulation results predicted the presence of a small hill located upstream at thetop of the cliff is causing flow separation and turbulent wind conditions characterized by areverse flow region predicted to be present in the bottom part of the wind turbine rotor. This
simulation result agrees well with the deductions made from the vibration and turbine operational
data. The simulation was also carried out with RANS CFD commercial software METEODYN WT.
Significant turbulence and a reverse flow pattern were not predicted and distinct differences inthe shear profile at turbine location were identified between the flows simulated by the two CFD
software packages.

1. Site Description& BackgroundDougu windfarm is located in the city of Mengzi, Honghe prefecture, Yunnan province, China.
The windfarm started operation in 2012 and it consists of 33 Mingyang wind turbines of rated
capacity 1.5MW. The turbines have a hub height of 65m with rotor diameter of 82.6m. Theturbines are located on top of a cliff and aligned in a north-south direction with elevation ranges
from around 1850 to 2200 meters.The cliff has a height of around 600m with slopes exceeding
60 degrees in places. Aerial photos from Google Earth indicate vegetation is abundant at thebottom of the cliff but scarce along the cliff and in the vicinity of turbines.

Since the start of operations, one of the wind turbines, Turbine no.12 (T12) has experiencedvibration problems. T12 has the following coordinates: latitude 23.497622 degrees andlongitudes 103.442547 degrees. Wind farm operator Yunnan Huadian Dougu Wind Power
Corporation (YUDWPC) suspected the vibration issue is related to wind conditions. In mid-2015,YUDWPC approached Dr. Takanori Uchida, Associate Professor of the Research Institutefor Applied Mechanics, Kyushu University for simulating the wind flow around turbine T12.

2. Operation Problem
The vibration problem of turbine T12 was investigated by the operator YUDWPC and a report
was issued in April 2014 [1]. Stated in the report was that high vibration datawas recorded only
when the wind was blowing from the southwest. Wind direction on the ground level wasobserved to be in the reverse direction from that recorded by the nacelle anemometer. Analysisof the vibration dataindicates the vibration is in the Y-axis (vertical) direction. This suggests thevibration is associated with abnormal vertical wind shear across the wind turbine rotor. As shownin Fig.1, a figure extracted from the report, it was deduced that the presence of the small hill
located about 150m upstream from turbine T12 was causing the onset of turbulence and reverse
flow which led to the vibration recorded.

Fig.1 - Deduction made on the airflow upstream and in the vicinity of turbine T12

3. LES CFD Simulation by RIAM-COMPACT®
For LES CFD simulation, RIAM-COMPACT®, a software package developed by Kyushu
University was employed. The software uses a standard Smagorinsky turbulence model anddetails of the code can be found in a number of published papers[2]. For the simulation,SRTM-90m data was used for elevation data. Wind direction is set to true north at 247
degreesand the CFD model constructed is shown in Fig. 2 with the following details:

- Domain Size: 14km x 13.3km x 8.3km- Elevation: 1275m(min) - 2232m(max)- Calculation Grid Points: 300 x 400 x 60
- Total Number of Grid Points: 7.2 million- Grid Spacing: 8m-957m(x), 13m-72m(y), 1m-470m(z)Fig.2 - LES CFD domain and calculation mesh constructed by RIAM-COMPACT®

To increase calculation accuracy, the mesh is concentrated around the turbine positions in boththe x and y direction as shown in Fig.2. No roughness consideration is given in constructing theCFD calculation model. Atmospheric stability is set at neutral stability. After the calculation has
been stabilized, numerical results in the calculation domain are output for a real time of ten
minutes with an interval of one second.

4. LES CFD (RIAM-COMPACT) Simulation Results
Fig.3 shows an instantaneous vector plot across turbine T12. This picture clearly shows flow
separation occurred at the small hill located 140m upstream from the turbine,and the onset of theformation of the recirculating vortex behind the hill. The turbulent flow extends downstream
forming a reverse flow region characterized by negative values of wind speed covering the lower
part of the wind turbine rotor.

The simulation results also indicate that the wind flow is relatively undisturbed above hub heightlevel. The U component wind speed time series during the ten-minute simulation at rotor top, hub,rotor bottom and surface level (10m) positions are plotted in Fig.4.

Reverse FlowFlow Separation

Fig.3 - Instantaneous vector plot at turbine T12(Full animation URL link: http://www.twd-wind.com/RC/Dougu_Vector_T12.gif)

Referring to Fig.4, it is obvious that the wind speed at surface (10m) and rotor bottom issignificantly lower and showing more fluctuations than the wind speed at the hub and top part ofthe rotor.

Fig.4 - Time Series of U componentwind speed at rotor top, hub, rotor bottom and groundsurface level at turbine T12

Wind speed varies between 15 to 20m/s at hub height and rotor top whereas for rotor bottom
wind speed fluctuates between minus 6.2m/s to 2m/s. Minus wind speed indicates the wind isflowing in reverse direction. During the ten-minute simulation, minus values account for 62% of
the total data at rotor bottom.

When the rotor bottom wind speed is at its minimum of minus 6.2m/s the wind speed at rotor top
is at 18.1m/s, hence a very large absolute wind speed difference of 24.3m/s. Excluding theminus wind speed data, the wind speed difference across the rotor face (between rotor top androtor bottom) has a maximum value of 18.7m/s and an average of 17.4m/s. The maximum wind
shear exponent is calculated to be 5.8 with an average value of 2.5, far exceeding the IEC
standard average shear value of 0.2.

The average, minimum and maximum values of the U component wind speed at turbine T12 areshown in Fig.5. The average values represent the average shear profile seen at turbine T12.
Referring to that, the reverse flow region results in a negative or very low wind speed fromground surface level to the rotor bottom of around 25m. Wind speed increases gradually from25m and starts leveling off at around 50m.

Fig.5 - Vertical Shear Profile predicted by RIAM-COMAPCT®
Average, minimum and maximum of U component wind speed variation with height5. RANS CFD Simulation by METEODYN WT
A number of comparison studies have been published between the RIAM-COMPACT® and other
RANS and LES-based software [3,4,5,6,7]. In general, these comparison studies show that
LES-based software performs better than RANS based in terms of flow reproducibility[6,7]. Inthis study, commercial software METEODYN WT (turbulence model: k-L RANS) was employed
and its results were compared with the results calculated by the RIAM-COMAPCT®.

The calculation parameters are shown in Table 1. The calculation domain is a radius of 10km forthe x-y direction with turbine T12 as center; z direction has a maximum of 200m.

Wind Direction is set at 247 degrees with minimum vertical and horizontal resolution set to 5m
and 2m respectively. Atmospheric stability is set to neutral. The calculation was completedsmoothly with computation convergence recoded at 99.3%.

Direction

247 degrees

Thermal stability class2

Smoothing - Whole domain

1

Forest model

Robust model

Minimum horizontal resolution

5m

Minimum vertical resolution

2m

Horizontal expansion coefficient

1.1

Vertical expansion coefficient

1.2

Mesh

2,346,300 points(225 x 237 x 44)

Maximum iteration numberConvergence

25
99.3%

Table 1 - Calculation parameters for METEODYN's calculation

6. RANS CFD (METEODYN WT) Simulation Results
METEODYN's calculation output includes the speed-up factor from height 20m to 200m at aninterval of 20m at turbine T12. These values are shown in Table 2. The speed-up factor is thewind speed ratio at the given height referencing the wind speed at height 10m. The speed-up
factor therefore resembles the vertical shear profile. Assuming a wind speed of 9.5m/s at 10m
height, wind speed at different heights can be calculated based on the speed-up factor and theresults are shown in Table 2.

Height [m]Speed-up factor

Wind Speed [m/s]

20

1.850

17.58

40

1.952

18.54

60

2.002

19.02

80

2.013

19.12

100

2.008

19.08

120

1.998

18.98

140

1.987

18.88

160

1.977

18.78

180

1.968

18.70

200

1.959

18.61

Table 2 - Speed-up factor at turbine T12 and calculated wind speed

Fig.6 - Comparison of vertical shear profile between the wind flows simulated by
RIAM-COMPACT® and METEODYN

The wind speed figures in Table 2 are plotted in Fig.6 and the resulting wind shear profile iscompared with the shear profile (average values) predicted by RIAM-COMPACT®. It can beseen from Fig.6 that the shapes of the two profiles are similar from 50m upwards but distinctively
different below 50m. METEODYN does not seem to predict any flow separation and reverse flow
region and therefore there is no significant wind speed reduction between 25m and 50m, andalso no negative wind speed values below 25m as predicted by RIAM-COMPACT®. Numericalcomparison results are shown in Table 3.

RIAM-COMPACTMETEODYNWind Speed near rotor bottom [m/s at height]

0.11 [23.7m]

17.58 [20m]Wind Speed near rotor top [m/s at height]

18.0 [102.6m]

19.08 [100m]

Wind Speed Difference [m/s]

17.9

1.5

Average Shear Exponent

3.50.025

Average Shear Exponent exceeding IEC Standard

YESNO

Table 3 -Numerical comparison of vertical shear profile between RIAM-COMPACT® andMETEODYN

Referring to Table 3, across the wind turbine rotor face, RIAM-COMPACT predicted a large windspeed difference with a shear exponent exceeding the IEC standard value of 0.2 by a large
margin. In sharp contrast, METEODYN predicted a small wind speed difference with a shear
exponent of 0.025 which is significantly below the IEC standard.

7. Conclusions
In the present paper, a turbine which has vibration problems was simulated using the commercial
software METEODYN WT as well as RIAM-COMPACT®. It was deduced from the vibration andturbine operation data that a possible reverse flow region near the rotor bottom was the directcause of the vibration. Simulation results from LES-based code RIAM-COMPACT® predicts aflow separation upstream and a reverse flow region at the rotor bottom. Simulation results fromRANS-based software METEODYN produced a very different shear profile which suggests thereverse flow and the associated flow separation were not predicted.

References[1] Yunnan Huadian Dougu Wind Power Corporation, Temporary measures for safe operation forWind Turbine no.7 and no.12 of Dougu windfarm, 2014

[2] T. Uchida, Y. Ohya, Large-eddy simulation of turbulent airflow over complex terrain, J. Wind
Eng.Ind. Aerodyn. 91, pp.219–229, 2003

[3] T. Uchida, Reproducibility of Complex Turbulent Flow Using Commercially-Available CFD
Software- Report 1: For the Case of a Three-Dimensional Isolated-Hill With Steep Slopes Reports of Research Institute for Applied Mechanics, Kyushu University No.150, pp.47-59, 2016[4] T. Uchida, Reproducibility of Complex Turbulent Flow Using Commercially-Available CFD
Software - Report 2: For the Case of a Two-Dimensional Ridge With Steep Slopes - Reports ofResearch Institute for Applied Mechanics, Kyushu University, No.150, pp.60-70, 2016[5] T. Uchida, Reproducibility of Complex Turbulent Flow Using Commercially-Available CFD
Software- Report 3: For the Case of a Three-dimensional Cube - Reports of Research Institutefor Applied Mechanics, Kyushu University No.150,pp.71-83,2016[6] T. Uchida, F.Watanabe, S.Mikami, Analysis of the Airflow Field around a Steep,Three-dimensional Isolated Hill with Commercially Available CFD Software(2nd report inJapanese), Reports of Research Institute for Applied Mechanics, Kyushu University No.149,
pp.72-77, 2015[7] T. Uchida, F.Watanabe, S.Mikami, Analysis of the Airflow Field around a Steep,Three-dimensional Isolated Hill with Commercially Available CFD Software, Reports of ResearchInstitute for Applied Mechanics, Kyushu University No.149, pp.91-98,2015