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Pinching Out Surfaces
Surface 4
Surface 3
If surface grids do not have the same dimensions it may be necessary to pinch out one surface at the edge
of another. In most cases, this will occur when you are trying to add a small feature onto a larger one, say
a dam sitting in a valley. From the above figure it can be seen that the grid describing Surface 3 will be
much larger than the grid describing Surface 4. The overall problem domain will automatically default to
the largest surface grid, in this case Surface 3. The grid for Surface 4 fails to describe the surface over
the entire domain of the problem; therefore, Surface 4 must be pinched out where it contacts Surface 3.
This will cause Surface 4 to default to the shape of Surface 3, everywhere that Surface 4 is not described in
the domain of the problem. Note that a pinch out surface may be a surface above or below the surface
under consideration.
Pinching out of sufaces is a difficult feature to accommodate in a finite element solver and should be used
with caution. It is recommended that surfaces only pinch out at region boundaries. Surfaces which
randomly cross each other in 3D space will likely lead to tangled mesh errors. A workaround for this case
may be the use of a specified minimum layer thickness.
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Layers
A Layer is the area between two surfaces. Since there are 5 surfaces in the example illustration there are 4
layers in the problem.
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Assigning Soils
Each region-layer combination in the 3D model can be assigned a different soil. In the example there are
3 regions and 4 layers and thus 12 region-layer combinations. Each combination may either have a soil
assigned or be set as void. A void space is essentially air space. In the example Region 3 has been set as
void for all layers so there is a circular hole through the entire problem. Layer 1 and Layer 3 have each
been defined as a separate soil on region 1 and region 2. In Layer 2, Region 1 has been assigned Soil C
and Region 2 has been assigned Soil D. Finally, for Layer 4, Region 1 has been assigned the same Soil A
as Layer 1 and Region 2 is void.
5.1.2.6
Minimum Requirements
SVAIRFLOW requires certain information to be provided before a problem can be sent to the solver. This
chapter will summarize the information that is required by SVAIRFLOW to successfully solve a problem.