Chapter 7: Geometry Branch and Tools

• The default units are meters unless the default was changed in the User Preferences dialog or when launching from CAD.
• For Pro/Engineer geometries, changing the units system only changes the analysis length unit; it does not change any dimensions in the model.
• For geometries originating in other CAD systems, changing the units system will open a dialog that contains a single check-box to Change Units.
• If this box is checked, the dimensions of the geometry are converted to the new units, and the physical sizes are unchanged. For example, if the object was 1 meter long, and units are changed to mm, it will be 1000 mm long. (Same size, different value).
• If this box is unchecked, the geometry will be resized. For example, if the object was 1 meter long, and units are changed to mm, it will become 1 mm long. (Different size, same value.)
• If the geometry is resized (box unchecked) after the units are changed (box checked), the dimensional values are reset to their original values in the selected units system. For example, a 1 m cube is opened into Autodesk Simulation CFD:
  • The units are changed from m to mm, and the Change Units box is checked. All sides of the cube are now 1000 mm.
  • The units are changed again from mm to in, and the Change Units box is unchecked. All sides of the cube are now 1 in.
  • This prevents the geometry from being resized "infinitely," and provides a way to "reset" the geometry back to the original dimensional values.
  • If it is necessary to resize the geometry beyond the scope of the Change Units function in Autodesk Simulation CFD, we recommend modifying the geometry in the CAD tool.

Note: All designs in a design study must have the same length units.

• The length units of each new design added to a design study are automatically set to be the same as the other design (or designs) within the study.
• Changing the length units of an individual design causes the length units of every design to be changed as well.
• Length units cannot be changed if any design in the study has a scenario that has been run (results have been computed).

Edge Merging unifies edges that meet at a common vertex at an inflection angle less than a specified tolerance. This tool provides the benefit of reducing the number of edges, especially small ones, which leads to reduced overall mesh density, and faster analysis times.

The inflection point is determined by the included tangency angle which has an upper limit in the Edge Merging dialog of 15 degrees.

Edge Merging will have no effect if more than two edges meet at a vertex (as shown on the left). Likewise, it will not merge edges that meet at an included angle larger than the specified angle (shown on the right):

Small Object Removal is a geometry repair tool, not a part suppression tool. It is better to suppress parts from the model in the CAD tool. This tool is designed to remove very small surfaces and edges that are typically too small to be seen.

Sliver surfaces often either prevent successful mesh generation or result in an excessive mesh density.

The box on the left has a very tiny sliver loop in the middle of the top surface. With this loop, the mesh density was very high in this region. The box on the right shows the effect after it was removed by Small Object Removal. The mesh density was considerably less.

The typical CAD model consists of the solid parts, but not fluid parts. The fluid region is usually contained within and around the solids, but in most cases is not explicitly constructed as part of the geometric model.

Geometry that consists only of solid parts will typically have openings where the working fluid (air, water, etc.) will enter and leave:

In this state, such a model is not suitable for a flow analysis (because of the lack of a flow part).

The Void Fill tool provides a facility to create capping surfaces that bound a water-tight internal void. The surfaces and volume that are created are actual geometry that can have boundary conditions, materials, etc., and are meshed as part of the simulation model.

Many devices are immersed in a fluid (air or water, for example). A key factor in the design of such parts is how the fluid flows around the device. Examples include:

• Motorcycles and bicycles
• Aircraft
• Automobiles
• Electronic devices subject to external natural convection

In all of these cases, the volume that surrounds the device is rarely included as part of the production CAD model. To analyze the flow, it is necessary to add a surrounding volume to the model.

With the External Volume tool, the surrounding air (or fluid) can be constructed directly on the simulation model within Autodesk Simulation CFD, without having to add it to the CAD geometry.