CFDWind3

Overview

CFDWind3 is a tool developed at CENER for the simulation of atmospheric flows whose latests implementations are carried out under the framework of the New European Wind Atlas project (NEWA). The current system is built under the open-source Computational Fluid Dynamics (CFD) tool-kit OpenFOAM version 2.4.0.

The model is designed to solve the unsteady Reynolds Navier-Stokes equations (URANS) for incompressible flows in which turbulence closure is achieved using eddy-viscosity theory and the modified (two-equation) k-ε closure scheme as proposed by Sogachev et al. (2012).

The solver is based on the Boussinesq approximation by including a buoyancy term in the momentum equations which, together with the solution of the energy-transport equation and additional source/sink terms in the turbulence closure, allows simulating the evolution of the diurnal cycle. On the other hand, only dry air is considered so neither humidity transport equation nor heat transfer by radiation or phase changes is included.

The Boussinesq approximation for incompressible flow is approached in OpenFOAM through the buoyantBoussinesqPimpleFoam solver which introduces the PISO-SIMPLE algorithm to solve the pressure-velocity-temperature coupling. This algorithm does not solve the continuity equation; instead, it solves a pressure Poisson equation that enforces continuity. Details about the solver algorithm for OpenFOAM can be found in OpenFOAM-PISO wiki description.

The original OpenFOAM solver is modified following the method proposed by Sanz-Rodrigo et al. 2017b. That is, Coriolis apparent force and real large scale forcing are used as model forcing. Forcing comes from the terms in WRF momentum& energy budget associated to the pressure gradient and the advection of momentum and temperature. These tendencies are obtained in the standard output of WRF following the method described in Lehner (2018a,b).

So far, the tendencies are only height and time-dependent which are valid for flat-terrain sites. Prior to be introduced in the microscale model, these terms are stored and averaged horizontally in a 45km area as described in Chavez-Arroyo et al. 2018. The implementations for incorporating the tendencies in OpenFOAM are built on top of the SOWFA project (Churchfield et al. 2014) developed at the U.S. National Renewable Energy Laboratory (NREL).

The surface conditions comply with the Monin-Obukhov Similarity Theory (MOST) for neutral-stability condition as proposed by Richards & Hoxey (1993) and Parente et al. (2011). These conditions are applied as wall functions to the fields of eddy-viscosity ηt, kinematic thermal conductivity αt,, turbulence dissipation ε, and Turbulent Kinetic Energy (TKE). So far, it has been found only small variations in the results obtained when including stability functions in the turbulence fields, namely αt and ηt. Therefore, up to this release, only mesoscale Dirichlet conditions are prescribed for the variations of temperature at the ground which are introduced through a wall temperature flux field that uses the algorithm proposed by Basu et al. (2008) to account for the atmospheric stability in the surface layer. To this end, following the code released as part of the SOWFA system, the dynamic values of velocity and temperature scales are computed based on the local flow, following MOST and Etling (1996) stability functions.