Matias Avila

Finite element approximation of turbulent thermally coupled incompressible flows with numerical sub-grid scale modeling

Purpose – The purpose of this paper is to describe a variational multiscale finite element approximation for the incompressible Navier‐Stokes equations using the Boussinesq approximation to model thermal coupling.Design/methodology/approach – The main feature of the formulation, in contrast to other stabilized methods, is that the subscales are considered as transient and orthogonal to the finite element space. These subscales are solution of a differential equation in time that needs to be integrated. Likewise, the effect of the subscales is kept, both in the nonlinear convective terms of the momentum and temperature equations and, if required, in the thermal coupling term of the momentum equation.Findings – This strategy allows the approaching of the problem of dealing with thermal turbulence from a strictly numerical point of view and discussion important issues, such as the relationship between the turbulent mechanical dissipation and the turbulent thermal dissipation.Originality/value – The treatment...

A Parallel CFD Model for Wind Farms

We present a Computational Fluid Dynamics (CFD) modeling strategy for onshore wind farms aimed at predicting and opti- mizing the production of farms using a CFD model that includes meteorological data assimilation, complex terrain and wind turbine effects. The model involves the solution of the Reynolds-Averaged Navier-Stokes (RANS) equations together with a k-ɛ turbulence model specially designed for the Atmospheric Boundary Layer (ABL). The model involves automatic meshing and generation of boundary conditions with atmospheric boundary layer shape for the entering wind flow. As the integration of the model up to the ground surface is still not viable for complex terrains, a specific law of the wall including roughness effects is implemented. The wake effects and the aerodynamic behavior of the wind turbines are described using the actuator disk model, upon which a volumetric force is included in the momentum equations. The placement of the wind turbines and a mesh refinement for the near wakes is done by means of a Chimera method. The model is implemented in Alya, a High Performance Computing (HPC) multi physics parallel solver based on finite elements and developed at Barcelona Supercomputing Center.

A finite element dynamical nonlinear subscale approximation for the low Mach number flow equations

In this work we propose a variational multiscale finite element approximation of thermally coupled low speed flows. The physical model is described by the low Mach number equations, which are obtained as a limit of the compressible Navier-Stokes equations in the small Mach number regime. In contrast to the commonly used Boussinesq approximation, this model permits to take volumetric deformation into account. Although the former is more general than the latter, both systems have similar mathematical structure and their numerical approximation can suffer from the same type of instabilities. We propose a stabilized finite element approximation based on the variational multiscale method, in which a decomposition of the approximating space into a coarse scale resolvable part and a fine scale subgrid part is performed. Modeling the subscale and taking its effect on the coarse scale problem into account results in a stable formulation. The quality of the final approximation (accuracy, efficiency) depends on the particular model. The distinctive features of our approach are to consider the subscales as transient and to keep the scale splitting in all the nonlinear terms. The first ingredient permits to obtain an improved time discretization scheme (higher accuracy, better stability, no restrictions on the time step size). The second ingredient permits to prove global conservation properties. It also allows us to approach the problem of dealing with thermal turbulence from a strictly numerical point of view. Numerical tests show that nonlinear and dynamic subscales give more accurate solutions than classical stabilized methods.

A CFD framework for offshore and onshore wind farm simulation

We present a wind simulation framework for offshore and onshore wind farms. The simulation framework involves an automatic hybrid high-quality mesh generation process, a pre-processing to impose initial and boundary conditions, and a solver for the Reynolds Averaged Navier-Stokes (RANS) equations with two different turbulence models, a modified standard k-epsilon model and a realizable k-epsilon model in which we included the Coriolis effects. Wind turbines are modeled as actuator discs. The wind farm simulation framework has been implemented in Alya, an in-house High Performance Computing (HPC) multi-physics finite element parallel solver. An application example is shown for an onshore wind farm composed of 165 turbines.

Mesh generation, sizing and convergence for onshore and offshore wind farm Atmospheric Boundary Layer flow simulation with actuator discs

Abstract A new mesh generation process for wind farm modeling is presented together with a mesh convergence and sizing analysis for wind farm flow simulations. The generated meshes are tailored to simulate Atmospheric Boundary Layer (ABL) flows on complex terrains modeling the wind turbines as actuator discs. The wind farm mesher is fully automatic and, given the topography and the turbine characteristics (location, diameter and hub height), it generates a hybrid mesh conformal with the actuator discs and refined upwind and downstream. Moreover, it presents smooth element size transitions across scales and avoids extending high-resolution areas to all the domain. We take advantage of our automatic and robust mesher to study the mesh convergence of our RANS solver with linear elements, obtaining quadratic mesh convergence for a quantity of interest in all the tested cases. In addition, we quantify the mesh resolution at the terrain surface and at the actuator discs required to achieve a given numerical error in simulations in onshore and offshore frameworks. Finally, we present the generated meshes and the simulation results for an offshore and an onshore wind farm. We analyze in detail one particular wind direction for both cases, and for the onshore wind farm we use our automatic framework to estimate the yearly production of energy and measuring the error against the actual produced one.

Finite element dynamical subgrid-scale model for low Mach number flows with radiative heat transfer

Purpose – The purpose of this paper is to present a finite element approximation of the low Mach number equations coupled with radiative equations to account for radiative heat transfer. For high-temperature flows this coupling can have strong effects on the temperature and velocity fields. Design/methodology/approach – The basic numerical formulation has been proposed in previous works. It is based on the variational multiscale (VMS) concept in which the unknowns of the problem are divided into resolved and subgrid parts which are modeled to consider their effect into the former. The aim of the present paper is to extend this modeling to the case in which the low Mach number equations are coupled with radiation, also introducing the concept of subgrid scales for the radiation equations. Findings – As in the non-radiative case, an important improvement in the accuracy of the numerical scheme is observed when the nonlinear effects of the subgrid scales are taken into account. Besides it is possible to show...

Microscale model comparison (benchmark) at the moderate complex forested site Ryningsnäs

complex forested site Ryningsnäs Stefan Ivanell1, Johan Arnqvist1, Matias Avila2, Dalibor Cavar3, Roberto Aurelio Chavez-Arroyo4, Hugo Olivares-Espinosa1, Carlos Peralta5, Jamal Adib5, and Björn Witha6 1Uppsala University, Wind Energy Section, Campus Gotland, 621 67 Visby, Sweden 2Barcelona Supercomputing Center, BSC, Spain 3Wind Energy Department, Technical University of Denmark, Denmark 4National Renewable Energy Centre (CENER), Spain 5Wobben Research and Development MS GmbH, Germany 6ForWind Carl von Ossietzky Universität Oldenburg, Germany Correspondence to: Stefan Ivanell (stefan.ivanell@geo.uu.se)