_{1}

In the present study, wind conditions were numerically predicted for the site of the Bolund hill using the RIAM-COMPACT natural terrain version software, which is based on an LES turbulence model (CFD). In addition, airflow measurements were made using a split-fiber probe in the boundary layer wind tunnel. The characteristics of the airflow at and in the vicinity of the site of the Bolund Experiment were clarified. The study also examined the prediction accuracy of the LES turbulence simulations (CFD). The values of the streamwise (x) wind velocity predicted by the CFD model were generally in good agreement with those from the wind tunnel experiment at all points and heights examined, demonstrating the validity of CFD based on LES turbulence modeling.

Recently, the wind power industry has undergone rapid growth at an unprecedented rate across the world. This growth has been occurring because wind power generation has the best cost performance of all the renewable energies in terms of achieving a post-fossil fuel society and reducing CO_{2} emissions. There is no doubt that wind power is the leading renewable energy even in Japan. The authors are convinced that further dissemination of wind power generation will contribute on the global scale to “green innovation”, efforts to combat global warming.

In the field of wind power generation, as is the case in other fields, the use of CFD (Computational Fluid Dynamics) has increased rapidly and is being used for wind turbine deployment planning and estimation of the annual energy production of wind turbines. Given this background, validation testing of the prediction accuracy of CFD software used in the field of wind power generation has been advanced by various interested groups. One such validation testing was conducted as part of the Bolund Experiment [

The core technology of RIAM-COMPACT (Research Institute for Applied

Mechanics, Kyushu University, COMputational Prediction of Airflow over Complex Terrain) is under continuous development at the Research Institute for Applied Mechanics, Kyushu University [

Computation time had been an issue of concern for the RIAM-COMPACT software, which focuses on unsteady turbulence simulations. However, the present fluid simulation solver is compatible with multi-core CPUs (Central Processing Units) such as the Intel Core i7, which has drastically reduced the computation time, leaving no appreciable problems in terms of the practical use of the RIAM-COMPACT software. Furthermore, the RIAM-COMPACT software has successfully been made compatible with GPGPU (General Purpose computing on Graphics Processing Units). The concept of GPGPU is to widely apply the floating-point operation capacity of a GPU not only to graphics rendering but also to other numerical operations.

Subsequently, the numerical technique used for RIAM-COMPACT is described. Collocated grids in a general curvilinear coordinate system are used for the arrangement of variables in order to numerically predict local airflows over complex terrain with high accuracy while avoiding numerical instability. In these collocated grids, the velocity components and pressure are defined at the grid cell centers, and variables which result from the contravariant velocity components multiplied by the Jacobian are defined at the cell faces. As for the numerical method, the FDM (Finite-Difference Method) is adopted, and an LES model is used for the turbulence model. In LES, a spatial filter is applied to the flow field to separate eddies of various scales into GS (Grid Scale) components, which are larger than the computational grid cells, and SGS (Sub-Grid Scale) components, which are smaller than the computational grid cells. Large-scale eddies, i.e., the GS components of turbulence eddies, are directly numerically simulated without relying on the use of a physically simplified model. On the other hand, the main effect of small-scale eddies, i.e., the SGS components, is to dissipate energy, and this dissipation is modeled based on the physical considerations of the SGS stress.

For the governing equations of the flow, a spatially-filtered continuity equation for incompressible fluid (Equation (1)) and a spatially filtered Navier-Stokes equation (Equation (2)) are used:

∂ u ¯ i ∂ x i = 0 (1)

∂ u ¯ i ∂ t + u ¯ j ∂ u ¯ i ∂ x j = − ∂ p ¯ ∂ x i + 1 Re ∂ 2 u ¯ i ∂ x j ∂ x j − ∂ τ i j ∂ x j (2)

Supporting equations are given in Equations (3)-(8):

τ i j ≈ u ′ i u ′ j ¯ ≈ 1 3 u ′ k u ′ k ¯ δ i j − 2 ν S G S S ¯ i j (3)

ν S G S = ( C s f s Δ ) 2 | S ¯ | (4)

| S ¯ | = ( 2 S ¯ i j S ¯ i j ) 1 / 2 (5)

S ¯ i j = 1 2 ( ∂ u ¯ i ∂ x j + ∂ u ¯ j ∂ x i ) (6)

f s = 1 − exp ( − z + / 25 ) (7)

Δ = ( h x h y h z ) 1 / 3 (8)

Because mean wind speeds of approximately 10 m/s or higher are considered in the present study, the effect of vertical thermal stratification (density stratification), which is generally present in the atmosphere, is neglected. The computational algorithm and the time marching method are based on a FS (Fractional-Step) method [

In this section, the terrain elevation data and simulation set-up are described.

239˚, where the wind direction is defined such that 0˚ indicates northerly wind.

For the inflow conditions, as previously described in the discussion of _{ref} adopted in the present study is the value of the wind velocity at

the inflow boundary at the height of the maximum terrain elevation, H (=0.07 m, wind tunnel scale). The Reynolds number Re (=U_{ref}H/ν), which is based on U_{ref} (=6.08 m/s) and H (=0.07 m), is set to 2.8 × 10^{4} in order to match the value from the wind tunnel experiment.

Subsequently, an overview of the wind tunnel experiment performed in the present study will be given.

2.2 and 1.8 m in length, width, and height, respectively. (See http://venus.iis.u-tokyo.ac.jp/en/introduction/facility/fudo/index.html)

Measurements of the airflow field were made using a split-fiber probe, which is capable of detecting reversed flow (

_{ref}) at the measurement point as shown in the figure above. In _{ref})). An examination of

These small eddies then merge together. As a result, large vortices are formed, and are shed away to the downstream area of the model (arrow A in the figure).

Next, comparisons between the results from the numerical simulation (RIAM-COMPACT LES turbulence model) and those from the wind tunnel experiment are discussed.

ground surface to z* = 0.0286 H (2 mm in the wind tunnel), where H is the maximum terrain elevation (11 m; 0.07 m in the wind-tunnel experiment), the characteristic length scale in the present study. This range for applying the roughness model was selected based on the results of the wind tunnel experiment. Examinations of the case with a smooth surface (

In

wind velocity at points M1 to M4 indicated in

the examined points and heights, the results from the LES numerical simulations and wind tunnel experiment are in good agreement. Furthermore, at points M1 to M3, no significant differences due to the absence or presence of the surface roughness can be identified in the profiles of the mean wind velocity obtained from the LES numerical simulations. As described in the discussion of

In the present study, wind conditions were numerically predicted for the site of the Bolund Experiment using the RIAM-COMPACT natural terrain version software, which is based on an LES turbulence model (CFD). In addition, airflow measurements were made using a split-fiber probe in the boundary layer wind tunnel of the Institute of Industrial Science, University of Tokyo. The characteristics of the airflow at and in the vicinity of the site of the Bolund Experiment were clarified. The study also examined the prediction accuracy of the LES turbulence simulations (CFD). The values of the streamwise (x) wind velocity predicted by the CFD model were generally in good agreement with those from the wind tunnel experiment at all points and heights examined, demonstrating the validity of CFD based on LES turbulence modeling.

This work was supported by JSPS KAKENHI Grant Number 17H02053. Also, a wind tunnel experiment was supported by Dr. Makoto IIDA (Tokyo University). The author expresses appreciation to them.

Uchida, T. (2018) Large-Eddy Simulation and Wind Tunnel Experiment of Airflow over Bolund Hill. Open Journal of Fluid Dynamics, 8, 30-43. https://doi.org/10.4236/ojfd.2018.81003