# CFDWind

**CFDWind** is a family of micro-scale models developed in CENER build on top the OpenFOAM CFD software (OpenFOAM, 2015). This family is currently composed of three branches which are very imaginatively labelled as version v1.x, v2.x and v3.x since the increase numbering reflects the added complexity in the atmospheric physic considered on each model.

Figure 1. Schematic diagram showing the naming convention, application and base publications describing the main physics of the CFDWind models

The three models are based on RANS/URANS equations for incompressible flows, in which turbulence closure is achieved using eddy-viscosity theory and two-equation closure schemes (k-ε, k-ω) modified for atmospheric flows.

As can be seen in the diagram above, the first two versions consider neutral stability and steady-state conditions while CFDWind3 is based on the Boussinesq approximation by including a buoyancy term together with the energy-transport equation in order to solve non-neutral atmospheric conditions.

More precisely, CFDWind1 deals with surface boundary layer (SBL) which complies with the Monin-Obukhov Similarity Theory (MOST) as proposed by Richards & Hoxey (1993) and Parente et al. (2011).

On the other hand, both v2 and v3 consider the whole Atmospheric Boundary Layer (ABL) structure by simulating the transition to geostrophic wind in the top boundary by adding the Coriolis force in the momentum equation as well as a length-scale limiter as proposed by Apsley and Castro (1997) for neutral conditions (CFDWind2) and further extended by Sogachev et al. (2012) and Koblitz et al. (2013) for non-neutral stratification (CFDWind3).

The coupling between pressure and velocity in the equations is solved by the well-known SIMPLE algorithm in case of the steady-state flows; whilst PISO algorithm is used to solve the coupling of the unsteady flows that emerge in the non-neutral cases.

When applied to any onshore non-flat site, all CFDWind models employ an automatic terrain mesh generator called **windMesh.**** **This software was jointly developed by CENER and the Barcelona Supercomputing Center (BSC) specifically to generate structured terrain-following meshes that maximise the quality of mesh parameters such as orthogonality and skewness by applying certain angle filters and elliptic smoothing techniques as can be observed in the following examples of Figure 2.

Figure 2. Example of meshes generated by windMesh, in which colors indicate cell's normals.

Different validation exercises of CFDWind1 and CFDWind2 can be found in Sanz-Rodrigo et al. (2011) and Chavez-Arroyo et al. (2014).

Figure 3. Example of the averaged wind speed obtained in complex terrain site.

**References**

- Apsley D., Castro I., 1997, A limited-length-scale k-ε model for the neutral and stably-stratified atmospheric boundary layer,
*Bound.-Lay. Meteorol*.**83**:75-78 - Chávez-Arroyo R, Sanz-Rodrigo J, Gancarski P. 2014 Modelling of atmospheric boundary-layer flow in complex terrain with different forest parameterizations.
*J. Physics: Conf. Series***524:**012119 - Koblitz T, Bechmann A, Sogachev A, Sørensen N and Réthoré P.E. 2013 Computational Fluid Dynamics model of stratified atmospheric boundary-layer flow.
*Wind Energy*. online. DOI: 10.1002/we.1684 - OpenFOAM 2015. OpenFOAM CFD Toolkit available at: http://www.openfoam.org/
- Parente A, Gorlé C, van Beeck J and Benocci C. 2011 Improved k–ε model and wall function formulation for the RANS simulation of ABL flows.
*J. Wind Eng. Ind. Aerodyn*.**99**267–278 ISSN 01676105 - Richards P . and Hoxey R. 1993 Appropriate boundary conditions for computational wind engineering models using k-Epsilon turbulence model.
*J. Wind Eng. Ind. Aerodyn*.**46**& 47 145–53 - Sanz-Rodrigo J, Cabezón D and García B. 2011 A systematic validation procedure for wind farm models in

neutral atmospheric conditions.*Proc. ICWE-13 Conf.* - Sanz-Rodrigo J, Cabezón D, Lozano S, and Martí I. 2009 Parameterization of the atmospheric boundary layer

for offshore wind resource assessment with a limited-length-scale k-ε model.*Proc. EWEA Conference.* - Sogachev A & Panferov O. 2006. Modification of two-equation models to account

for plant drag.*Bound.-Lay. Meteorol.***121**:229–266 - Sogachev A., 2009. A Note on Two-Equation Closure Modelling of Canopy Flow.
*Bound.-Lay. Meteorol*,**130**(3), 423–435 - Sogachev A, Kelly M and Leclerc M. 2012 Consistent Two-Equation Closure Modelling for Atmospheric Research: Buoyancy and Vegetation Implementations.
*Bound.-Lay. Meteorol.***145**:307–327