Is it possible to draw an ellipse in FEMPRO. I am trying to do an analysis on a horizontal atmospheric pressure vessel with semi-elliptical heads. It is quite long and I am concerned about the saddles, but I dont want to use ASME calculations, because I want to try to make the vessel as cheap as possible. I am running into issues however when try to model the heads. If anyone knows how to do this, if would be a great help.
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Drawing Ellipse
- Thread starter LOUDOG
- Start date
There are a few ways, but none of them are really that easy outside of drawing it in CAD and importing, but here are some suggestions:
1) Use a series of arcs to model one quadrant and then mirror to get the other three.
2) Draw it with a spline
3) Draw a rectangle and fillet the corners so that the ends are rounded. Divide the filleted corners into two or three segments. Use a spline and start at the junction between the two end fillets, then select one of the nodes that you created with the division of the fillet, then go to a point above the middle of the rectangle. Continue selecting symmetric points around your former rectangle. Make sure "interpolate" is unchecked.
As I mentioned, none of these are easy to get an accurate ellipse, but they will work. It is MUCH easier to develop it in a CAD package and then import. Just select the top surface for meshing and deselect everything else. Once you have meshed the elliptical surface, you can merge it with your other model. The trick to that is to make sure you are using an absolute mesh size instead of a percentage. If you go this route, you may want to mesh the head first. Then, you can mirror it to the other side and copy: join the edge to form the body. Be aware of where your saddles need to be so that you have sufficient mesh density in their area.
Hope this helps.
Garland E. Borowski, PE
Engineering Manager
Star Aviation
1) Use a series of arcs to model one quadrant and then mirror to get the other three.
2) Draw it with a spline
3) Draw a rectangle and fillet the corners so that the ends are rounded. Divide the filleted corners into two or three segments. Use a spline and start at the junction between the two end fillets, then select one of the nodes that you created with the division of the fillet, then go to a point above the middle of the rectangle. Continue selecting symmetric points around your former rectangle. Make sure "interpolate" is unchecked.
As I mentioned, none of these are easy to get an accurate ellipse, but they will work. It is MUCH easier to develop it in a CAD package and then import. Just select the top surface for meshing and deselect everything else. Once you have meshed the elliptical surface, you can merge it with your other model. The trick to that is to make sure you are using an absolute mesh size instead of a percentage. If you go this route, you may want to mesh the head first. Then, you can mirror it to the other side and copy: join the edge to form the body. Be aware of where your saddles need to be so that you have sufficient mesh density in their area.
Hope this helps.
Garland E. Borowski, PE
Engineering Manager
Star Aviation
- Thread starter
- #3
This helps very much. I have done it already by making concentric circles that continue to get smaller, and move outward to create the profile.
I have not done much importing besides solid models from ALIBRE. Can I select a surface imported from that programt to mesh like you suggested?
I have not done much importing besides solid models from ALIBRE. Can I select a surface imported from that programt to mesh like you suggested?
Yes. Build your solid model in Alibre and import it. Do a "Box Select: Surfaces" and select the entire model. Right click in the modeling screen and there is an option for "mesh parameters" or something like that. You can uncheck "Select Surface for Mesh". If you try to mesh at this point, nothing should mesh.
Then, "Point Select" the surfaces that make up the elliptical head. Right click again and re-select the "Select Surface for Meshing". Clearly, when you box select everything, you could hold the control key down, switch to a point select and deselect these so that they are not impacted by the original operation, but in the end, you just want the outer surfaces that make up the elliptical head to mesh.
Now you can mesh just the surfaces that are expected to do so. When you expand the surfaces in the tree, there is an indication as to which ones are selected for meshing, and I believe you can select and deselect them from the tree as well.
An alteranative is to build the elliptical head, but don't "hollow it out". This will give you probably 3 surfaces: the head in two surfaces and the interior face as one...the interior face my be two. Go ahead and mesh. Then, select the surface representing the interior face, right click and go to the bottom for "select subentities", choose lines and then hit delete. You may have to check to make sure that the remaining surfaces are complete and you may have to add some lines along the edge to close out the elements, but this way may be a little faster.
Then, "Point Select" the surfaces that make up the elliptical head. Right click again and re-select the "Select Surface for Meshing". Clearly, when you box select everything, you could hold the control key down, switch to a point select and deselect these so that they are not impacted by the original operation, but in the end, you just want the outer surfaces that make up the elliptical head to mesh.
Now you can mesh just the surfaces that are expected to do so. When you expand the surfaces in the tree, there is an indication as to which ones are selected for meshing, and I believe you can select and deselect them from the tree as well.
An alteranative is to build the elliptical head, but don't "hollow it out". This will give you probably 3 surfaces: the head in two surfaces and the interior face as one...the interior face my be two. Go ahead and mesh. Then, select the surface representing the interior face, right click and go to the bottom for "select subentities", choose lines and then hit delete. You may have to check to make sure that the remaining surfaces are complete and you may have to add some lines along the edge to close out the elements, but this way may be a little faster.
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