CAD MODELING FOR ACUCONSOLE

AcuConsole supports different CAD formats: Parasolid, Acis, and Pro/Engineer (Granite) part and assembly files. Besides the CAD formats, AcuConsole also has a discrete module that can be used to import Nastran and STL files.

Introduction:

AcuConsole requires the fluid volume to be modeled in the CAD packages. For example, to model flow through a pipe, the fluid region represented by the volume within the inner diameter of the pipe (and bounded by the inlet and outlet) has to be modeled in the CAD software. In addition, depending on the CAD kernel (Parasolid, Acis or Granite), requirements for a valid geometry for AcuConsole are slightly different.

Parasolid:

Both the binary and ASCII formats of Parasolid are supported in AcuConsole. Parasolid schema files are included in AcuConsole distribution to support older versions of Parasolid.

ACIS:

AcuConsole supports both binary and ASCII formats of ACIS files. If the CAD model consists of surfaces and solids, the surfaces should be created as separate parts and assembled with the solid part(s). If both the solid and the surface are modeled in the same part, AcuConsole does not unionize the surface and solid properly.

Granite:

Pro/Engineer is a product from PTC and uses the Granite modeling kernel. AcuConsole uses the Granite API to access the parts and assemblies generated in Pro/Engineer or other packages using the Granite kernel. Currently, there are two limitations of the Granite kernel: 1. Layers are not supported – if an assembly component is in a blanked layer it will still be modeled in AcuConsole. 2. Quilts are not supported and are ignored.

Pro/Engineer or Granite Part:

To ensure that a valid AcuConsole model is created from a Pro/Engineer or Granite part, the following requirements are imposed: • The part must have an absolute tolerance • Avoid self-intersecting surfaces. (Use the surface analysis tool in Pro/Engineer.)

Pro/Engineer or Granite Assembly:

To ensure that a valid AcuConsole model is created from a Pro/Engineer or Granite assembly, the following requirements are imposed, • All components of the assembly must use the same absolute tolerance value. • Solid parts cannot overlap. • Avoid self-intersecting surfaces.

Pro/Engineer modeling guidelines

Most Granite parts and assemblies are created in Pro/Engineer. We recommend the following guidelines while creating parts or assemblies in Pro/Engineer for AcuConsole, • Make sure that you use absolute accuracy and set the same accuracy values in all the parts in case of an assembly. From the Pro/Engineer GUI: o Tools→Options and set the “enable_absolute_accuracy” to yes. o Edit→Setup, Accuracy→Absolute→Enter Value • Assemble with the default coordinate system. • Use global interference check to make sure that there are no solid interferences and fix if there are interferences. • Use surface analysis tools to check for self-intersecting surfaces. • Check for short edges in the model. Even though a short edge definition depends on the relative dimensions of the model, in most cases we found that short edges less than 0.5mm should be avoided.

DISCRETE:

AcuConsole has a discrete module that is used to read STL and Nastran files and also to convert the CAD formats to ACUSIM discrete geometry files (.agm). Conversion of CAD to discrete functionality is extremely useful to generate meshes on operating systems that do not support the CAD packages used to generate the geometry files.

CAD to Discrete Conversion Process:

To create a discrete geometry (agm) from CAD file, say “pipe.prt”, use the following procedure in AcuConsole:

• Create a new database “pipe.acs” in AcuConsole. • Import “pipe.prt” using File→Import and make sure to select “water tight tessellations” with higher resolution in the import dialog. Water tight tessellations result in creating a surface mesh of the geometry before importing into AcuConsole. Users could potentially use the surface mesh as the input for volume mesh generation. • Convert CAD to discrete geometry using Tools→CAD2Discrete menu item. At this stage, AcuConsole de-associates itself from the CAD geometry. • Create the appropriate Surface (inlet, outlet and wall) and Volume (air) groups. • Select Tools→GenerateMesh to open the mesh generation dialog. Check “export ams file” and uncheck “launch acuMeshSim” to generate “pipe.agm” and “pipe.ams” files. Pipe.agm is the discrete geometry file that is used in conjunction with the “pipe.ams” file for mesh generation. • Mesh generation can be done either on the same machine or a different machine using acuMeshSim with the “pipe.ams” and “pipe.agm” files. There is an option to either modify the surface mesh or use the surface mesh as an input to volume mesh generation.

Nastran:

The discrete model creation process explained in section 2.1 is the basis for Nastran file support. Surface meshes created in packages like ANSA can be imported into AcuConsole as Nastran files. If the Nastran files have surface grouping information, it will be preserved in AcuConsole.

STL:

Both ASCII and binary STL file formats are supported in AcuConsole. AcuConsole assumes that the STL files do not have any slivers or gaps. AcuConsole does not have the functionality to clean up bad geometry. As such, healing gaps etc. has to be done in the software writing the STL. The user provides a feature angle to generate/segment the faces from the STL data. Modeling Examples:

SolidWorks Modeling Example 1 – CAD model for simple pipe flow simulation

The following example illustrates the CAD modeling process for creating a fluid volume for a simple pipe flow. A pipe of diameter 1mm and length 50 mm is created.

Step 1: Launch SolidWorks and select File→New menu item to create a new part. Select “Part” in the “New SolidWorks Document” dialog as shown in Figure 1.

Figure 1: create a new part in SolidWorks

Step 2: To create a pipe model, a sketch of the cross section of the pipe has to be created first. To sketch the circle, click on the Sketch icon in the toolbar as shown in Figure 2.

Figure 2: Switch to sketch mode

Step 3: To draw a circle, select the “Circle” icon from the toolbar as shown in Figure 3. You can modify the radius of the circle in the “Properties” block. For this example, set the radius value to 0.5mm to give a “1mm” diameter pipe [The dimensions used in this session are mm].

Figure 3: Sketch the pipe cross section

Step 4: Once the circular cross section has been sketched, it has to be extruded to create the pipe geometry. Select the icon for “Extruded Boss” under “Features” in the tool bar. Select the circle sketched in Step 3. Select “Blind” under “Direction 1” block and provide the “D1” value of 50mm as shown in Figure 4.

Figure 4: Extruding 1mm dia. circle by 50mm

Step 5: Save the part as a Parsolid file, pipe.x_t, as shown in Figure 5. The model can now be imported into AcuConsole. The model has three faces and one region. Each of these three faces can be used to create Surface groups (inlet, outlet and wall) in AcuConsole for setting boundary conditions. There will be one region that can be used to create a Volume group (air).

Figure 5: Save As a parasolid text file

SolidWorks Modeling Example 2 - CAD model for simulating flow over a cylinder.

Step 1: Create a new part in SolidWorks as in the first example.

Step 2: In sketch mode, draw a square of 50mm x 50mm representing the flow cross- section as shown in Figure 6.

Figure 6: Cross section representing the flow inlet

Step 3: Extrude the rectangle by 250mm to create a box as in Figure 7. (Typically, the flow domain downstream of the cylinder should be extending about 200 diameters from the cylinder).

Figure 7: Extrude 50mm x 50 mm square by 250mm

Step 4: To create the fluid volume, the cylinder region has to be cut out from the box created in Step 3. The Extruded Cut feature shown in Figure 8 is used to create the cut. Create an extruded cut feature and sketch a circle of diameter 1mm, representing the cross-section of the cylinder on the side face of the box as shown in Figure 9. For “Direction 1”, select “Through All” to cut through all faces in the extrusion direction.

Figure 8: Create an extruded cut feature

Figure 9: Extrude the circular cross section sketched on side face

Step 5: Figure 10 shows the fluid volume created in SolidWorks. Save the model as a Parasolid file and import it into AcuConsole. There are seven faces in the model (6 for the box and 1 for the cylinder). These faces can be classified into inflow, outflow, wall and freestream “Surface” groups in AcuConsole for boundary conditions.

Figure 10: Fluid volume for simulating flow over a cylinder

Additional Notes: Figure 10 shows the origin of the model at the center of the inlet face. Setting up advanced mesh controls and using probes to monitor the flow downstream of the cylinder could be easier with the origin at the center of the cylinder. In Step 3 of this example, instead of extruding the square in one direction by 250mm, extrude the square by 200mm in the downstream direction and 50mm in the upstream direction. Then follow the rest of the steps as before to locate the cylinder center at the origin.