Settings
The value of the Kinematic viscosity describes the resistance of the air to deformation. It is defined as the ratio of the viscosity to the density of the air.
The Density of mesh to be applied around the model is controlled by percentage reference. This specific refinement is utilised for the model simplification and the flow calculation. The default density (20%) normally results in a relatively low number of finite volumes and a relatively fast calculation. The minimum percentage is 10%. It entails a rather coarse mesh with the smallest number of volumes. The higher the density of the mesh, the smaller the size of the finite volume cells will be. The results are accordingly more precise, but the calculation will need more time due to the greater number of volumes. Setting the maximum mesh density (100%) leads to very fine meshes with millions of volumes. The calculation of 3D flow on such meshes is on the edge of the capabilities of current PCs, with a calculation time of several hours to several days.
Note
For further information, see the Chapter 'Computational Mesh and Model Simplification'.
The Boundary layers option controls whether the finite volume mesh next to the surfaces of the model is refined in a special way. This refinement gives better results near the boundaries of the model (see the Image 'Finite volume mesh with five boundary layers'). It is highly recommended to activate the boundary layers and define the number of layers NL when the surface roughness is to be taken into account.
The Level of detail controls the simplification of the model in RWIND Simulation where specific corrections of the RFEM/RSTAB model are required at, e.g., boundaries or corners (see the Image 'Model with levels of detail'). You can define levels of detail between 0 (very coarse modelling of details with a wide mesh) to 4 (many details with a very fine mesh). The default value 2 represents a good compromise between simplification of the model and calculation speed. Alternatively, you can define the Detail Size manually.
In this section of the dialog box, you can set the Number of the first load case to be created. By default, the first unused number is set.
The Keep RWIND Simulation results if mesh is deleted option enables you to preserve the loads created by RWIND Simulation when the RFEM/RSTAB model is modified afterwards. No new calculation by RWIND Simulation will be required if you change some material, adjust the support conditions, or apply other modifications which do not have an impact on the CFD analysis.
As the loads of the RWIND Simulation are connected to mesh points, the results of the CFD analysis which are needed for RFEM/RSTAB are stored within the model data. It means that the RWIND Simulation results are always part of the model when this option is enabled. If the mesh is deleted in RFEM due to some modification, the loads created by RWIND Simulation can be extrapolated to the new situation without running a new CFD analysis. There may be greater loads on fewer elements, for example. If a member or surface is deleted, its loads will be lost.
To apply the RWIND Simulation results that are still available in the background after having modified the RFEM/RSTAB model, use the [Calculate LC in Background] or [Calculate All in Background] buttons. Do not use [Open in RWIND Simulation]. A warning is shown before the results will be redistributed.
All results will nevertheless be deleted if some calculation parameter is changed in the “RWIND Simulation window”.
If there is any significant topological change of the model, the Keep RWIND Simulation results if mesh is deleted option must not be used.
Note
The loads cannot be adequately distributed across the geometry of the new model. For this reason, the feature is disabled by default. Reusing the internal RWIND results can be justified only if the old and new meshes do not differ significantly. The final decision must be taken by the user.
To clear the temporary data existing in the background, use the [Delete RWIND Simulation Results] button in the 'Load Cases' tab.
The Maximum number of iterations does not mean that all iterations actually have to be carried out: If the calculation converges with less iterations, it is stopped. This option can be useful to avoid infinite loops or apply short calculations for testing purposes.
The Convergence criterion represents the stop limit for the calculation. As soon as the residual pressure falls below this limit, the calculation is terminated. Note that no less than 300 iterations are carried out, however.
When you activate the Use Potential Flow to calculate initial conditions option, a linearised version of non-viscous Navier Stokes equations is used to generate the start conditions.
The Use second-order numerical scheme check-box controls which numerical scheme is used for divergence terms (fluxes). It is not activated by default so that the calculation is carried out according to first order. The first-order numerical discretization generally yields better convergence than the second-order scheme. The second-order discretization is usually more accurate, however.
The Close openings smaller than option enables you to control how openings in the model are to be treated. By default, RWIND Simulation takes account of all openings or gaps in its analysis. This means that openings that you have modelled for, e.g., windows or doors are assumed as open surfaces. To close these openings, you can enter their Absolute size as a limit. Alternatively, the value of that limit can be defined as a Percentage of the model size. The Image 'House with closed openings' shows the result of that feature applied to the simplified model of RWIND Simulation.
Note
Make sure to close the surfaces of imported beam models. Otherwise the wind load will be applied only to the members.
Note
As the automatic recognition of openings is a demanding topological task, it may be required to adjust openings manually.
This window section is accessible when the Use uniform turbulence at the inlet option has been activated in the Wind Load tab (see the Chapter 'Wind Load').
Note
By default, the Calculate turbulence parameters from the intensity of turbulence option is set.
This means that the parameters k and ε (or ω, depending on the selected model of turbulence) are determined automatically from the global turbulence intensity value defined in the Wind Load tab. When you clear the check box, you can define the parameters manually in the text boxes below. The Turbulence kinetic energy represents the mean kinetic energy characteristic for eddies which is produced by fluid shear or friction. The Rate of dissipation of turbulence energy describes the deduction of energy due to the airflow. When you have the selected the "k-omega" model of turbulence in the Wind Load tab (see the Chapter 'Wind Load'), you can specify the Specific rate of dissipation of turbulence energy instead.
For detailed information on turbulence kinetic energy, see CFD Online.
The settings of this window section have an effect on the load application of the generated member loads on the model.
- Concentrated: The loads result in concentrated loads at relative distances along every member. The intervals of the load application points are usually very small, depending on the density of the mesh.
- Uniform: For every member, constant loads are created along the member length. Only one uniform member load is applied for each global direction.
- Trapezoidal: Similar to uniform loads, the concentrated loads are levelled along the member. They are converted into a trapezoidal distribution, however, to approximate the actual gradients.
Note
In many cases, the Uniform distribution is accurate enough.
For the export of the RFEM/RSTAB model, the Export optimized member topology option is set by default.
It means that member elements are converted into simplified sections of the RWIND Simulation model. Every cross-section is replaced by a rectangular section which encompasses the outlines of the original one. By means of this simplification, the meshing of the member is less detailed so that the simulation can be performed much faster. For further details, see FAQ 4165.
If want to analyse the objects contained in the current Visibility (see Chapter 9.9.1.2 of the RFEM manual) instead of the entire model, select the Export active objects only option.
The Export Terrain model option enables you to use the terrain geometry defined in the Dlubal application for the simulation.
In RFEM and RSTAB, you can model terrain features by means of importing visual objects (see Chapter 11.3.9 of the RFEM manual).
Specify the number(s) of the relevant Visual objects representing the terrain model that are to be exported for the analysis.
The 
button clears the list of defined numbers, while the All option applies all visual objects.
In the RWIND Simulation program, the terrain model type will be allocated accordingly (see image to the left).
For more information on the Terrain Model, see the Chapter 'General'.
It is likewise possible to consider neighbouring buildings or other relevant objects in the simulation. Select the Export Surrounding model option and specify the number(s) of the Visual objects that are to be exported. Additionally, you can export CAD/BIM models which have been defined in the RFEM/RSTAB model (see Chapter 12.2.2 of the RFEM manual). The BIM workflow using IFC files is described in detail in a Knowledge Base article. In the RWIND Simulation program, both surrounding models and CAD/BIM models will be classified as secondary models (see the Chapter 'General').
