To generate a finite volume mesh, the model must be topologically correct. In RWIND Simulation, model boundaries are defined by triangles, and the term "topologically correct" then means that these triangles must form a closed triangular mesh, i.e. each mesh edge has exactly two adjacent triangles, and that the triangles must not intersect or touch each other except common edges. In fact, the exact definition of a "topologically correct" model is more complicated, but we don't want to go into all the details here.
The problem is that CAD models are usually not topologically correct – triangles of a 3D object often intersect with triangles of another object, the model boundary is not closed, etc. The pre-processing of such models for CFD analysis can be very extensive and require 60-80% of an engineer’s time. Facilitating this work was therefore one of the main goals in the development of RWIND Simulation, which has been achieved by implementing the so-called simplified model. The simplified model actually represents a special mesh "shrink-wrapping" the original model. This mesh is topologically correct and can therefore be used as a model boundary for the generation of a 3D finite volume mesh. The simplified model will automatically correct most of the problems that would otherwise have to be corrected manually. Those include:
- Model simplification (see point 1 in Image 2.1): Details that are not relevant to the given simulation and could cause calculation instability due to insufficiently fine discretization are simplified.
- Intersection of triangles (see point 2 in Image 2.1)
- Open edges and surfaces with zero thickness (see point 3 in Image 2.1)
- Closing openings to prevent the air from flowing into the interior of the building (see point 4 in Image 2.1)
Image 2.2 shows some examples of automatically simplified models.
When simplifying the model, it is possible to specify the level of detail as well as the maximum size of openings to be closed. Using a simplified model to determine the wind load on a building is based on the following assumption: if the simplified model approximates the shape of the original model well, the load calculated for the simplified model will also be close to the correct values.
Though we expect the use of a simplified model to be the most common choice, the user can turn off this option and calculate with the model boundaries as they were imported. However, this option is only recommended for advanced users experienced in CFD analyses.