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David March 13, 2004 14:30

Block Structures
I'm trying to model a 3 d pipe flow made of 5 blocks (a saquare shape in the middle surrounded by more blocks), as in the example in CFX4 Manual. I think i have followed it excatly but when volmsh reblocks pipe, it only joins 3 blocks together and leaves two blocks at opposite ends left inside the cylinder (i.e has internal geometry)!!! and i dont know why????

can any body give me any advice in creating block strucutres please,


David March 13, 2004 15:15

Re: Block Structures
I forgot to add that I am using CFX BUILD

Jeff March 13, 2004 16:38

Re: Block Structures

Check your global model tolerance. Build creates blocks with all faces meshed, thus adjoining blocks have matching sets of nodes. On the analysis form, these nodes get equivalenced (i.e. if two overlap, one gets thrown away). Build will only equivalence nodes that are closer than the global model tolerance. The tolerance should be set (under preferences) to roughly 1/10 of your smallest element size.

Set your model tolerance to an appropriate value and write the grid out again. You should see additional nodes being equivalenced (purple circles).

If the grid is very non-uniform, you can manually equivalence selected nodes with their own tolerance (see the help panel on how to do this).


David March 13, 2004 18:17

Re: Block Structures
Hi, I did try what you suggested but still have the same problem. The mesh seems to be the same all the way around and the structure of the blocks that make up the cylinder are all the same size. am doing something wrong with the method of creating the geometry?, i used the following method:

Created Point [1 0 0] & [2 0 0]

transformed point [1 0 0]

Create curve rotate 90 degree on the second point

so i ended up with a shape like this

* (1)


* * (1) curve created between (1)

I then used create curve PWL to draw straight lines to define a shape. Create Surface edge (selected all four edges), transformed this to make a base structure, defined a surface edge of a square in the middle and then simply extruded this to a length to create the model, applied the patches, mesh etc

does patches affect the reblocking when using volmsh!!

many thanks

Jeff March 19, 2004 18:21

Re: Block Structures
Try this....

1. Create your two points 2. Create|Curve|Rotate one point about the center one 90 degees 3. Repeat 2 more times so you have four curves with a point in the middle 4. Transform|Point|Scale and select the four outside points scaling by 0.8 0.8 0.8 5. Create|Surface|Vertex connecting the four new points to form the central square 6. Create|Surface|Curve to connect the edges of the square with the outside curves (4X) 7. Create|Solid|Extrude selecting the five surfaces and extruding in the Z direction by your desired length.

Patches ======= Some of the Build releases had an annoying default set which caused the cursor to select anything it touched instead of enclosing the entire entity. (See Preferences | Picking and choose "entire entity"). It's possible when you're setting your wall patches, you are also putting them inside the geometry.

Try this....

Turn the model sideways Set the picking method as explained above Set an inlet patch on one end (selecting 5 end faces only) Set an outlet patch on the other end (selecting 5 end faces only) You don't need walls, as the solver will automatically create a default WALL patch on the outside surfaces.

Orient the model so you're looking at the end view (in the inlet) before going to VOLMESH. When the model equivalences, you should see purple circles show up all around the central square and radially out from the square on ALL four O-grid intersections. If you don't, then the model is not equivalencing properly. If you do, and still have a problem, then patches are being placed between the blocks.

Hope this helps, Jeff

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