Unsteady

VestasFOAM 1.1.0 - LES/DES

Submitted by Yavor Hristov on May 5, 2015 - 12:00am
Main hypothesis

VestasFOAM 1.1.0 - DES is built upon the pimpleFoam solver packaged within the publically available OpenFOAM distribution [1]. The k-omega SST DES [2] turbulence model has been implemented in-house. If desired buoyancy can be selected through the Boussinesq approximation.

VestasFOAM - DES is used operationally to determine probability density functions of wind veer and wind shear and compliance with IEC standards for class A,B and C sites. This has been done with good success both forensically (i.e. once problems have been detected on old sites) and during the initial micro-siting activities when transient flow suspicions are raised on prospective sites.

VestasFOAM 1.1.0 - LES is built upon the SOWFA project led by NREL [3]. The SOWFA code has been modularized to fit within the VestasFOAM automated CFD workflows and linked to Vestas turbine libraries for efficient/automated case setup, execution and post-processing. Currently this is only valid on flat terrain/offshore.

For both LES/DES, grids are automatically generated in Pointwise [4] using a structured hyperbolic extrusion. Great care is taken to control grid quality, with small expansion ratios from terrain to rotor bottom, and uniform grid spacing through the turbine/wake areas. As with our steady process, when sufficiently vertically distanced from turbines, the horizontal mesh resolution is continually reduced in order to lower mesh size.

SOWFA - LES

Submitted by Matthew Churchfield on May 4, 2015 - 6:36pm
Main hypothesis

The large-eddy simulation (LES) solver within the Simulator for On/Offshore Wind Energy (SOWFA) is built upon the Open-source Field Operations And Manipulations (OpenFOAM) computational fluid dynamics (CFD) toolbox.  The solver is incompressible and uses the unstructured finite-volume formulation.  Buoyancy effects are included through a Boussinesq buoyancy forcing term.  Turbines are modeled with actuator lines.

Omnivor

Submitted by Emmanuel Branlard on May 4, 2015 - 6:11pm
Main hypothesis

Homogeneous Incompressible Newtonian fluid under conservative forces, viscous splitting assumption (separate convection/diffusion steps).

Omnivor is a vortex code that uses Lagrangian tracking of vorticity using low order singular and regularized vortex elements. Bodies may be represented using source elements.

Elements intensities may be a combination of prescribed intensities, intensities determined by solving of non-penetration condition or intensities determined using tabulated profile data and a Lifting line formulation.

EllipSys3D ABL

Tilman Koblitz's picture
Submitted by Tilman Koblitz on May 4, 2015 - 5:49pm
Main hypothesis

The EllipSys3D code is a multiblock finite volume discretization of the incompressible Reynolds Averaged Navier-Stokes (RANS) equations in general curvilinear coordinates.  The code uses a collocated variable arrangement, and Rhie/Chow interpolation [iv] is used to avoid odd/even pressure decoupling. As the code solves the incompressible flow equations, no equation of state exists for the pressure, and the SIMPLE algorithm of [v] is used to enforce the pressure/velocity coupling. The EllipSys3D code is parallelized with MPI for executions on distributed memory machines, using a non-overlapping domain decomposition technique.

The solution is advanced in time using a 2nd order iterative time-stepping (or dual time-stepping) method.  In each global time-step the equations are solved in an iterative manner, using under-relaxation. First, the momentum equations are used as a predictor to advance the solution in time.  At this point in the computation the flowfield will not fulfil the continuity equation. The rewritten continuity equation (the so called pressure correction equation) is used as a corrector making the predicted flowfield satisfy the continuity constraint.  This two step procedure corresponds to a single sub-iteration, and the process is repeated until a convergent solution is obtained for the timestep. When a convergent solution is obtained, the variables are updated, and we continue with the next timestep.

The three momentum equations are solved decoupled using a red/black Gauss-Seidel point solver. The solution of the Poisson system arising from the pressure correction equation is  accelerated using a multigrid method. In order to accelerate the overall algorithm, a three level grid sequence and local time stepping are used.

DWM-HAWC2

Submitted by Torben Juul Larsen on May 4, 2015 - 5:42pm
Main hypothesis

The Dynamic Wake Meandering method uses the wind speed deficit of the upstream turbine together with a meandering process in order to simulate the incoming flow field of the downstream turbine and thereby enabling detailed analysis of both production and loading through aeroelastic computations. The meandering process causes an intermittent appearance of the flow field with periods of full, half or no wake situation—varying from time to time driven by the low frequency large-scale natural turbulence.

DWM

Submitted by Gunner Chr. Larsen on May 4, 2015 - 5:39pm
Main hypothesis

The basic philosophy is to consider wakes as passive tracers continuously emitted from the wind farm turbines. The basic idea is a split of scales in the wake flow field, based on the conjecture that large turbulent eddies are responsible for stochastic wake meandering only, whereas small turbulent eddies are responsible for wake deficit attenuation and expansion in the meandering frame of reference as caused by turbulent mixing.