The wake of a wind turbine generator (WTG) operated at the optimal tip speed ratio is compared to the wake of a WTG with its rotor replaced by a stationary disk. Numerical simulations are conducted with a large eddy simulation (LES) model using a nonuniform staggered Cartesian grid. The results from the numerical simulations are compared to those from wind-tunnel experiments. The characteristics of the wake of the stationary disk are significantly different from those of the WTG. The velocity deficit at a downstream distance of

As a countermeasure against global warming, a substantial reduction in CO_{2} emissions has become an urgent issue. Accordingly, the effective use of wind power energy is attracting attention as a clean and environmentally friendly solution. In Japan, the number of wind power generation facilities has been rapidly increasing to achieve the goal of 300 million kW of wind-generated energy in 2010. These wind power generation facilities range from those with a few wind turbine generators (WTG) to large wind farms (WF) with dozens of WTGs.

Given this background, we have developed RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, Computational Prediction of Airflow over Complex Terrain), a nonstationary, nonlinear wind synopsis simulator that is capable of predicting the optimum sites for wind turbine construction to the pin-point level within a target area of a few km or less [

To achieve improved accuracy of the simulation result, a continuous effort has been made in the research and development of RIAM-COMPACT. As a part of this effort, a wake model of high accuracy is currently under development to evaluate the influence of the mutual interference between WTGs. The wake model will be able to determine an appropriate separation distance between WTGs to avoid the reduction of energy generation at a wind farm as a whole due to the mutual interference between WTGs at the farm. Development of such a wake model is necessary for the effective planning of wind farms, especially in countries with limited flat areas, including Japan, in which large WTGs are constructed in high concentrations.

When multiple WTGs are installed at a site, the following empirical values are generally considered appropriate for the separation distance between two WTGs: approximately ten-times the WTG rotor diameter in the streamwise direction and three times the WTG rotor diameter in the spanwise direction. A few wind-tunnel and field experiments have been conducted to study the wake flows of WTGs [

The present study examines the following two characteristics of the wake flow which will enable the construction of an accurate wake model. First, the mean wind velocity deficit is evaluated at the above-mentioned downstream distance of ten-times the rotor diameter of a single WTG. Second, the characteristics of the wake flow of a single WTG are compared to those of a stationary disk, which has been used as the basis of existing wake models. In both cases, the wake flow of the WTG operated at the optimal tip speed ratio (tip speed ratio at the maximum power output) is considered. The investigations are conducted with an LES model that uses a nonuniform staggered Cartesian grid system. The results from the numerical simulations are compared to those from wind-tunnel experiments.

LES simulations are conducted using a staggered Cartesian grid with variable grid spacing. The finite difference method (FDM) is adopted for the computational technique. For the subgrid scale (SGS) model, the mixed-time scale model [

Figure

View of the small-scale WTG in the wind tunnel.

Relationship between the tip speed ratio and the power coefficient of the wind turbine generator (WTG) tested in the wind-tunnel experiments.

Numerical simulations are conducted for (1) airflow past the WTG operated at the optimal tip speed ratio and (2) airflow past a WTG for which the rotor has been replaced by a stationary disk with a diameter identical to that of the rotor. The WTG with a stationary disk will be referred to simply as the stationary disk hereafter. The computational domain and boundary conditions applied in the simulations are summarized in Figure

Computational domain and boundary conditions.

Enlarged view of WTG.

Figures

Contour plots of the streamwise (

WTG operated at optimal tip speed ratio

Stationary disk

Contour plots of the streamwise (

WTG operated at optimal tip speed ratio

Stationary disk

In contrast, the characteristics of the wake flow behind the stationary disk are complex. A large area of reverse flow is formed immediately behind the stationary disk. As a result of the flow curling around the edge of the disk, the magnitude of the spanwise component of the flow behind the stationary disk is significantly larger than that behind the WTG (not shown). From the area of reverse flow, large vortices are shed periodically. The Strouhal number of the vortex shedding and the structure of the shed vortices behind the stationary disk are topics of high interest in fluid dynamics and will be investigated in future research. Figures

Enlarged view of Figure

WTG operated at optimal tip speed ratio

Stationary disk

Enlarged view of Figure

WTG operated at optimal tip speed ratio

Stationary disk

Figure

Flow visualization of the vortex structure of the wake flow. Rear view showing streamlines for the instantaneous field.

WTG operated at optimal tip speed ratio

Stationary disk

A significant difference between the wake flow of the WTG and that of the stationary disk is also evident in the time-averaged fields of the streamwise (

Contour plots of the streamwise (

WTG operated at optimal tip speed ratio

Stationary disk

Same as Figure

WTG operated at optimal tip speed ratio

Stationary disk

To investigate this finding quantitatively, spanwise and vertical profiles of the time-averaged streamwise velocity are calculated (Figures

Spanwise profiles of the time-averaged streamwise (

WTG operated at optimal tip speed ratio

Stationary disk

Same as Figure

WTG operated at optimal tip speed ratio

Stationary disk

The present study investigated the characteristics of the wake flow behind a single wind turbine generator (WTG) operated at the optimal tip speed ratio. Even at a downstream distance of as large as ten-times the rotor diameter of a single WTG, the wind velocity behind the center of the rotor was approximately 30–40% of the inflow wind velocity. This large value of the wind velocity deficit was likely attributable to the tip vortices formed at the blade tips, which suppressed the momentum exchange between the wake flow of the WTG and the surrounding flow.

The wake of the WTG was also compared to that of a WTG with the rotor replaced by a stationary disk because existing wake models are based on the wake of a stationary disk. The diameter of the stationary disk was set equal to that of the rotor of the WTG. The wake flow of the WTG and that of the stationary disk were significantly different from each other. The wake flow of the stationary disk was characterized by a large area of reverse flow immediately behind the disk. However, at a downstream distance of

Our future research topics include investigations of the effects of the inflow turbulence on a WTG and turbulence distributions in the wake of a WTG. Numerical simulations of wake flows behind multiple WTGs are also planned as a future project.