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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'How does the wind blow behind wind turbines and in wind farms?' taking place on Tuesday, 11 March 2014 at 16:30-18:00. The meet-the-authors will take place in the poster area.

Yuto Hiromori Mie University, Japan
Co-authors:
Yuto HIROMORI (1) F P Yasunari KAMADA (1) Takao MAEDA (1) Junsuke MURATA (1) Naoya MORI (1)
(1) Mie University, Tsu, Japan

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Abstract

A field experimental investigation on the wake characteristics of a HAWT in high turbulence intensity

Introduction

Increasing demand for wind energy power can be met by implementing large scale wind farms even in areas where terrain is more complex, and where the sites with high wind velocities are often located. Hence, it becomes important to study the flow behavior and characteristics of the wake developed behind wind turbine in a complex terrain.
In the present study, the flow behind a wind turbine installed in a quasi-complex terrain was experimentally investigated and the effect of turbulence intensity on the wake was verified.


Approach

In this study, a field experiment was carried out in Mie university accessory test site on a test wind turbine. General view of the test site is shown in Fig.1. Along with prevailing wind direction of the test site, a reference measurement mast was set up upstream the test wind turbine and wake measurement masts were set up in the downstream side. The reference coordinate system used in the data reduction is defined as: x-axis is inflow direction, y-axis is lateral direction, z-axis is vertical direction, when the origin is assumed to the center of the wind turbine rotor at the hub-height.



Test wind turbine
General view of the test wind turbine used in this field experiment is shown in Fig. 2. The test wind turbine was 3-bladed and upwind type HAWT. It had a maximum capacity of 30kW, a rotor diameter of D=2R=10.0 [m] and a hub-height of 13.4m. In addition, the pitch angle and the nacelle were set optionally.



Wind measurement system
In a field experiment, for better understanding of wind velocity distribution in the wake of a wind turbine, it is important to perform measurements in many points. In this study, the wake was measured in many points by using movable wake measurement mast A and B. Five and three (a total of 8) sonic anemometers were respectively installed on the wake measurement masts A and B. The installation interval is y/R=0.25 and the height is the hub-height. The measurement was carried out at the downstream position of x/D=2.0. The reference wind velocity used in the data reduction was observed by a reference wind measurement anemometer. The reference wind velocity was measured in the upper classes for prevailing wind at 10m and 13.3m in height.


Main body of abstract

Data reduction
All the data extracted from the database were 1-min average. The extracted data which were considered were the data which satisfied the following conditions; the averaged reference wind velocity URef was 5.5±0.25m/s and the wind direction was 0±5° at the prevailing wind direction.
In this test site, due to the topographical features of the terrain where the wind turbine is set up, there was a difference between the wind velocity measured at reference measurement point and the wake measurement points while the test wind turbine was parked. In order to exclude this wind velocity difference and properly evaluate the effect of the test wind turbine on the wake velocity variations, it is considered the non-dimensional wind velocity ratio UNR, which is defined as the ratio of non-dimensional wind velocity UN_O measured when the test wind turbine was operated and non-dimensional wind velocity UN_P measured when the test wind turbine was parked.



The value of UN_O and UN_P is averaged among all collected extracted data. Furthermore, the value of UN_O is defined as the ratio of the wake wind velocity UWake_O measured by sonic anemometers installed at wake measurement mast and the reference wind velocity URef_O when the test wind turbine was operated. Also the value of UN_P is the ratio of the wake wind velocity UWake_P and the reference wind velocity URef_P when the test wind turbine was parked.



Experimental Results
Axial velocity distribution in wake
In Fig.3, the axial velocity distribution in wake at the hub-height z/R=0 and downstream distance x/D=2.0 is represented. The vertical axis corresponds to non-dimensional wind velocity ratio UNR and the horizontal axis corresponds to non-dimensional horizontal position y/R. In order to analyze the wake distribution, the extracted data were averaged. The average value of extracted reference turbulence intensity TIRef is 0.28. This value corresponds to the wind turbine classes IIIA which is defined in terms of wind velocity and turbulence parameters by IEC(2).
In Fig.3, the wind velocity distribution shows the maximum value is close to the reference wind velocity (UNR=1.0) with increasing the distance from the center of rotor. Therefore, the position represented the minimum value of non-dimensional wind velocity ratio UNR is moved from prevailing wind direction downstream of the test wind turbine axis to the direction of +y/R, and the position is y/R=0.25. In this test site, a low hill existed on the +y/R side of the test wind turbine. The air flowing into the wind turbines has a tendency to slow down as it approaches the hill. The wake was deflected for the reason set forth above.



Influence of reference turbulence intensity
The wake distribution was classified by reference turbulence intensity TIRef to research the effect of reference turbulence intensity on the flow field within the wake. In Fig.4, the axial velocity distributions in wake at the hub-height z/R=0 and downstream distance x/D=2.0 with different turbulence reference intensities are represented. The vertical axis to non-dimensional wind velocity ratio UNR and the horizontal axis correspond to non-dimensional horizontal position y/R. The wake velocity distribution is shown in black circular plot for 0.15≦TIRef<0.25 and white circular plot for 0.15≦TIRef<0.25 respectively. It is found that the minimum value of the wind velocity in the wake increases with x/D=2.0 when reference turbulence intensity increases. That was caused by wind turbulence. The wind velocity in wake is recovered because wind turbulence has a stimulating effect on the mixture of the main flow and the wake.




Conclusion

In this study, a field experiment of measuring the wake behind the test wind turbine was carried out in a quasi-complex terrain. The wake was measured by using many sonic anemometers installed on wake measurement masts which set downstream the test wind turbine. The effects of inflow conditions and reference turbulence intensity on the flow field on the wake at the hub-height were considered. The main conclusions of this study are as follows.
-The axial velocity distribution in wake at the hub-height z/R=0 and downstream distance x/D=2.0 approaches the reference wind velocity with increasing the downstream distance from the center of the rotor. Furthermore, the position represented the minimum value of the wake wind velocity is moved from prevailing wind direction downstream of the test wind turbine axis to the direction of +y/R because there is a unique feature in the landscape of the test site. (In this test site, there is a low hill existed on the +y/R side of the test wind turbine and the air flowing into the wind turbines has a tendency to slow down as it approaches the hill.)
- The axial velocity distribution in wake at the hub-height z/R=0 and downstream distance x/D=2.0 varies with the level of the reference turbulence intensity. The wake deficit recovered well enough for larger reference turbulence intensity. That causes the wind turbulence has a stimulating effect on the mixture of the wake and non-wake wind flow. As a result, it is found that the wind velocity in wake is recovered.



Learning objectives
Further work must experimentally investigate the flow behind a wind turbine installed in a quasi-complex terrain and the effect of inflow conditions and reference turbulence intensity on the flow field on the wake field except hub-height of the wind turbine.


References
(1) Benjamin Martinez. “Wind resource in complex terrain with OpenFOAM”, Wind Energy MSc thesis (2011).
(2) IEC (International Electrotechnical Commission). “IEC 61400-1 INTERNATIONAL STANDARD”, Third edition (2005-08).