Abstract
The scope of the present study was to understand the wake characteristics of wind-turbines under various inflow shears. First, in order to verify the prediction accuracy of the in-house large-eddy simulation (LES) solver, called RIAM-COMPACT, based on a Cartesian staggered grid, we conducted a wind-tunnel experiment using a wind-turbine scale model and compared the numerical and experimental results. The total number of grid points in the computational domain was about 235 million. Parallel computation based on a hybrid LES/actuator line (AL) model approach was performed with a new SX-Aurora TSUBASA vector supercomputer. The comparison between wind-tunnel experiment and high-resolution LES results showed that the AL model implemented in the in-house LES solver in this study could accurately reproduce both performances of the wind-turbine scale model and flow characteristics in the wake region. Next, with the LES solver developed in-house, flow past the entire wind-turbine, including the nacelle and the tower, was simulated for a tip-speed ratio (TSR) of 4, the optimal TSR. Three types of inflow shear, N = 4, N = 10, and uniform flow, were set at the inflow boundary. In these calculations, the calculation domain in the streamwise direction was very long, 30.0 D (D being the wind-turbine rotor diameter) from the center of the wind-turbine hub. Long-term integration of t = 0 to 400 R/Uin was performed. Various turbulence statistics were calculated at t = 200 to 400 R/Uin. Here, R is the wind-turbine rotor radius, and Uin is the wind speed at the hub-center height. On the basis of the obtained results, we numerically investigated the effects of inflow shear on the wake characteristics of wind-turbines over a flat terrain. Focusing on the center of the wind-turbine hub, all results showed almost the same behavior regardless of the difference in the three types of inflow shear.
Highlights
Offshore-located wind-turbines are always clustered in wind farms to limit overall installation and maintenance expenses
Various turbulence statistics were calculated at t = 32.5–65 (R/Uin)
Parallel computation based on a hybrid large-eddy simulation (LES)/actuator line (AL) model approach was performed with a new SX-Aurora TSUBASA vector supercomputer
Summary
Offshore-located wind-turbines are always clustered in wind farms to limit overall installation and maintenance expenses. With the rotation of the blade of the wind-turbine, a region of wind-velocity deficit with temporal and spatial variations is formed downstream of the wind-turbine. The potential offshore market is the main driver for the development of large wind-turbines. This flow phenomenon is called wind-turbine wake. Wind-turbines in offshore wind farms operating in downwind wake flow are subjected to two main problems. The mutual interference of wind-turbine wake, which is a strongly nonlinear flow phenomenon, occurs especially in large offshore wind farms consisting of multiple wind-turbine groups. The mutual interference of wind-turbine wakes is an essential and inherent problem in offshore wind farms.
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