I'm trying to simulate water flowing from a canal into a pipe. I've tried many different loading situations, time steps, mesh sizes, etc. and I just can't get the simulation to complete. In almost every case I get a message that the analysis stopped with 1 warning and 0 errors. The warning is almost always that the time step is too small. I understand why that might be, but I've had other analyses complete despite that warning. I don't see anything in the analysis results that would indicate the problem.
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I have two different scenarios in this one, with gravity pointing two different directions. Neither of them will complete. Any help with this would be greatly appreciated.
Ok, I think I'm missing something fundamental here. I did the manometer exercise from the official 2012 courseware. It looks like the attached image. That analysis ran successfully.
I try to do the exact same thing with a nearly identical model, and I get the problem I mentioned above - the analysis fails. I'm convinced now it has something to do with my mesh - the mesh in the courseware exercise is very clean, and looks to have been manually created. I'm using the mesher (and I've tried various settings all to no avail). I end up with elements in the bends of the manometer like the second attached image.
Is the solver really that sensitive to the mesh? If so, how can I reliably get a mesh that won't lead to failure when I'm using complex geometry?
Another update - I created a mesh manually of just a long cylinder with a small initial volume, and I was finally able to get an analysis to complete. I created the mesh by dividing a 2" circle using 8 divisions, then created a MRS mesh 50" long with 100 copies (leading to a .5" vertical element dimension). I ran the analysis over 1.5 seconds using 200 steps.
I created a cylinder with identical geometry in Inventor, with the same initial volume, analysis parameters, and boundary conditions. This analysis failed after 21 steps.
I really hope I'm missing something fundmental here, otherwise the open channel analysis type doesn't appear to be at all useful.
It sounds to me as if the mesh is too coarse with your CAD models. In your hand-built model (and the manometer example), you are quaranteed to have the same number of elements in the cross-section along the entire length. With a CAD model and an automatic mesh, you are somewhat "at the mercy" of the solid mesher; that is, you do not have control over the number of elements (really the number of nodes) in the cross section. If there are too few nodes at a particular cross section, the flow can become chocked and difficult to converge.
Did you use the boundary layer mesh with the CAD models? That will put N number of elements at the wall and can help ensure there are at least a given number of nodes in the cross section. Please see "Getting Started > Help > In-Product Help" "Help > Autodesk Simulation > Mesh Models > Mesh Overview > Meshing CAD Solid Models > Model Mesh Settings > Solid".
John Holtz, P.E.
Senior User Experience Designer, Simulation
Current version of Mechanical & Multiphysics: 2013 SP1 (2013.01.00.0012 28-Jun-2012)
Thanks for the reply - I really appreciate it. I don't think this is a problem with the number of nodes. I took the simple cylinder example and decreased the mesh size significantly and I end up with the same problem. I don't think the analysis log is indicating lack of convergence - when it stops at a given time step, it hasn't reached the maximum number of iterations specified in the analysis parameters.
I've attached a screen capture of results when the analysis stopped, with nodes selected to give you an idea of their density. I've also attached the analysis log.