# RIAM - COMPACT®

The RIAM-COMPACT natural terrain version model uses collocated grids in a general curvilinear coordinate system. The velocity components and pressure are deﬁned at the cell centers, and variables which result from the covariant velocity components multiplied by the Jacobian are deﬁned at the cell faces. As for the computational technique, the ﬁnite-diﬀerence method (FDM) is adopted, and an LES model is used for the turbulence model. In LES, a spatial ﬁlter is applied on the ﬂow ﬁeld to separate eddies of various scales into grid-scale (GS) components, which are larger than the computational grids, and subgrid-scale (SGS) components, which are smaller than the computational grids. Largescale eddies, that is, the GS components of turbulence eddies, are numerically simulated directly without relying on the use of a physically simpliﬁed model. The main eﬀect of small-scale eddies, that is, 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 and a spatially-ﬁltered Navier-Stokes equation are used.

The computational algorithm and the time-marching method are based on a fractional step (FS) method and the Euler explicit method, respectively. The Poisson’s equation for pressure is solved by the successive overrelaxation (SOR) method. For discretization of all the spatial terms except for the convective term, a second-order central diﬀerence scheme is applied. For the convective term, a third-order upwind diﬀerence scheme is applied. An interpolation technique based on 4-point diﬀerencing and 4-point interpolation by Kajishima is used for the fourth-order central diﬀerencing that appears in the discretized form of the convective term. In the weighting of the numerical dispersion term of the third-order upwind diﬀerencing, α = 3.0 is commonly applied in the Kawamura-Kuwahara Scheme. However, α is set to 0.5 in the present study to minimize the inﬂuence of numerical dispersion. For LES subgrid-scale modeling, the commonly used Smagorinsky model is adopted. A wall-damping function is used with a model coeﬃcient of 0.1. We have developed an LES-based model for analyzing neutral ﬂow over variable orography and applied it to the problem of proper site selection. Model performance was evaluated using data from wind tunnel tests over simple geometries and from a real site.

The topography in the computational domain is reconstructed using mainly the 50-melevation data of the Geography Survey Institute of Japan. Fine grid spacing is adopted near the wind turbines in order to reconstruct the topographical features in detail. The computational domain is set up in such a way that airﬂow characteristics at the turbine location are subject to topographical inﬂuences (upwind zone) and that eddies ﬂow out of the computational domain smoothly, and airﬂow at the turbine location is free from the inﬂuence of the outﬂow boundary (leeward zone). For analyses of airﬂow over steep topography, a buﬀer zone is established which surrounds the computational domain. The terrain in the buﬀer zone is ﬂat with an elevation close to zero meters and connects smoothly to the terrain in the computational domain.

## RIAM-COMPACT®

Submitted by Takanori UCHIDA on May 4, 2015 - 6:26pm
Software
Solver
LES
Regime
Turbulence
Turbulence closure
Turbulence model

Standard Smagorinsky Model

Atmospheric boundary layer
Coriolis
No
Atmospheric Stability
Atmospheric Stability
No
Canopy
Forest canopy
Yes
Wind farm
Wind turbine
Yes
Rotor model
Wake model
Wind farm range
References

「Proposal of Designed Wind Speed Evaluation Technique in WTG Installation Point
by Using the Meteorological Model and CFD Model」
Takanori UCHIDA et al., Reports of RIAM, Kyushu University, No.141, pp.1-12, 2011

「Investigation of the Causes of Wind Turbine Blade Damage at Shiratakiyama Wind Farm in Japan
―A Computer Simulation Based Approach―」
Takanori UCHIDA et al., Reports of RIAM, Kyushu University, No.141, pp.13-25, 2011

「Comparisons between the wake of a wind turbine generator operated at optimal tip speed ratio
and the wake of a stationary disk」
Takanori Uchida, Yuji Ohya and Kenichiro Sugitani, Modelling and Simulation in Engineering, Volume 2011

「New Evaluation Technique for WTG Design Wind Speed
using a CFD-model-based Unsteady Flow Simulation with Wind Direction Changes」
Takanori Uchida, Takashi Maruyama and Yuji Ohya, Modelling and Simulation in Engineering, Volume 2011

「Latest Developments in Numerical Wind Synopsis Prediction Using the RIAM-COMPACT® CFD Model
-Design Wind Speed Evaluation and Wind Risk (Terrain-Induced Turbulence) Diagnostics in Japan」
Takanori Uchida and Yuji Ohya, Energies, Vol.4, pp.458-474, 2011

「Verification of the Prediction Accuracy of Annual Energy Output at Noma Wind Park
by the Non-Stationary and Non-Linear Wind Synopsis Simulator, RIAM-COMPACT®」
T.Uchida and Y.Ohya, Journal of Fluid Science and Technology, Vol.3, No.3, pp.344-358, 2008

「Micro-siting technique for wind turbine generators by using large-eddy simulation」
Takanori Uchida and Yuji Ohya, Journal of Wind Engineering & Industrial Aerodynamics, Vol.96, pp.2121-2138, 2008

「Application of LES technique to diagnosis of wind farm by using high resolution elevation data」
Takanori Uchida and Yuji Ohya, JSME International Journal 「Environmental Flows」, Series B, Vol.49, No.3, pp.567-575, 2006

「Large-eddy simulation of turbulent airflow over complex terrain」
Takanori Uchida and Yuji Ohya, Journal of Wind Engineering & Industrial Aerodynamics, Vol.91, pp.219-229, 2003

「Numerical simulation of atmospheric flow over complex terrain」
Takanori Uchida and Yuji Ohya, Journal of Wind Engineering & Industrial Aerodynamics, Vol.81, pp.283-293, 1999

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