# MeteoDyn FAQ

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This article contains questions and answers on the use of MeteoDyn products (mainly Meteodyn WT and UrbaWind). Please feel free to add questions and answers here!

## General Information

### What is meteodyn WT?

Meteodyn WT is an automatic CFD software dedicated to the computation of the wind chracteristics in complex terrains. It is used in the wind energy industry for mico-sitting.

### What is UrbaWind?

UrbaWind is an automatic CFD software dedicated to the computation of the wind characteristics in urban environment. Three modules allow to calculate automatically the small wind energy potential, natural ventilation and pedestrian comfort.

### What do you mean by "automatic CFD software"?

The software are automatic in a sense that: 1. There is only one turbulence model, one equation model which calculates the tubulences mixing length. The model is adapted to the atmospheric boundary layer and is used to model the wind in large fields. 2. The generation of the mesh is integrated in the software, automatic with a refinement around the interest points (wind turbines, mapping) and the buildings. 3. The generation of the boundary conditions is automatic.

### How can I learn meteodyn WT and UrbaWind?

The user manual of the software explains all the functaionalities as well as physical theory. The user manual is provided with licensed version as well as in the demo (downloadable in te website of Meteodyn: http://www.meteodyn.com/en/downloads.html)

## meteodyn WT

### Turbulence

I wonder if meteodyn WT use a k-e model for turbulence?

• The turbulent fluxes in meteodyn WT are modeled by using the concept of "turbulent viscosity" considering a linear relationship between turbulent fluxes and the mean velocities gradients, as it is done in the k-e model. The turbulent viscosity is the product of a speed scale (square-root of the turbulent kinetic energy) and a length scale. The turbulent kinetic energy is computed as in the k-e model with a transport equation. The difference with the k-e model is that the length scale is not computed from the k-e dissipation equation but from empirical relationships deduced from numerous measurements in the atmospheric surface layer. The k-e model is not fitted for atmospheric flows because it cannot consider the large scales of the turbulence. In this way, meteodyn WT gets more realistic values of the turbulent viscosity.

Moreover, meteodyn WT takes into account the anisotropy of the turbulent viscosity in the atmosphere, particularly for the lateral turbulence (wind directions fluctuations). The stability effects and high roughness effects are also considered in the determination of the turbulent viscosity.

### Wind Energy

What kind of model is used for computing the wake effects?

• meteodyn WT uses a modified Park model. The wake decay constant is directly got from the turbulent intensity computed at the wind turbine generating the wake, instead (as in standard Park model) of evaluating it from the local roughness.

### Meteorology

What is done in meteodyn WT to deal with the thermal effects in the boundary layer? Is this done by using an energy equation to model buoyancy in the flow?

• Taking into account the thermal effects by introducing an energy equation would introduce some additional parameters and unknown variables which would make the work fastidious for the user (for example: how to know the temperature boundary conditions?).

Thus, meteodyn WT considers a more generalist approach. The thermal stability can be varied in the software thanks to a stability parameter. It varies from 0 (very unstable) to 9 (very stable). The parameter 2 refers to the near neutral case. This parameter not only modifies the wind profiles (velocity and turbulence) which enter the domain, but it affects also the way the mixing length used by the turbulence model is computed (this turbulence model was developed in order to handle atmospheric boundary layer problems).

### Mesh

1. What type of mesh is used in meteodyn WT?

• The mesh is mono-block structured boundary fitted with a refinment around the interest points (wind turbines and mapping)

2. What is the limit of cells number?

• A customer just found a limitation for the 64 bit version (around 21 million of cells). We are now working in order to break this limitation soon.

## UrbaWind

### Mesh

1. What type of mesh is used in UrbaWind?

• The mesh is Cartesian unstructured with a refinment around the buildings and the interest points.