The effect of met mast on wind speed measurement and its correction by using computational fluid dynamics
Atsushi Yamaguchi, Takeshi Ishihara
The University of Tokyo, Tokyo, Japan
The effect of met mast on wind speed measurement was investigated by using the measured data at a met mast which satisfy the condition in IEC61400-12-1. By assuming that Doppler lidar can measure undisturbed wind speed, it was found that the wind speed decreases at the upstream of the mast and increases at the side of the mast. The numerical simulation by using computational fluid dynamics (CFD) was performed to quantitatively estimate the correction factor to consider the effect of the met mast. The numerical simulation gives the consistent results with the assumption that the lidar can measure undisturbed wind speed, if adequate methods are used. A correction method was proposed and it was shown that the proposed method is better compared with the conventional methods.
A met mast at Choshi offshore test site, on the east coast of Japan, was used. In this met mast, three cup anemometers were installed at same height so that the elongations between each anemometer are 120 degree. The configuration of the anemometers satisfies the conditions specified in IEC61400-12-1. A doppler lidar is also installed on the met mast. First, the wind speed ratio between each cup anemometers and the doppler lidar was calculated as a function of wind direction. Then, numerical simulation by using CFD was performed by changing the turbulence model and the modelling level of the met mast to investigate the applicability of each model. Finally, a correction method of met mast effect was proposed by using the result of CFD.
Wind speed measured at the anemometers upstream of the tower decreases up to 5% compared with the undisturbed wind speed while the wind speed at the side of the tower increases up to 5%. If the anemometer is in the wake of the tower, the wind speed measured at the anemometer is much smaller than the undisturbed wind. CFD can reasonably simulate this effect quantitatively if Large Eddy Simulation (LES) was used and all the detail of the met mast including the catwalks and the handrails was modelled. The proposed correction gives better estimation of the wind speed compared to the conventional method.
The effect of the met mast on the wind speed is not negligible even if the met mast satisfies the condition specified in IEC61400-12-1, not only in the wake of the mast but also in the upstream or the side of the mast. The CFD result is consistent with the assumption that the lidar can measure undisturbed wind speed. A correction method by using CFD was proposed and the estimated wind speed by using the proposed method shows good agreement with the lidar measurement.
The results obtained in this research imply that the use of the upstream anemometer as seen in some project may results in the underestimation or overestimation of the wind speed. The use of CFD to calculate the flow correction factor gives promising results. But it should be noted that the use of LES and the detailed tower model is required to obtain the correction factor quantitatively.