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Questions to Dynamic Mesh solver and diffusivity 

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January 10, 2008, 05:16 
Hi OFFolks,
working on a s

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Florian Krause
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Hi OFFolks,
working on a simulation of a sinking (rigid) particle in a water cylinder with a modified icoDyMFoam(OF1.4.1) I run in trouble with the mesh quality. I tried several diffusivity algorithms: directional: bad results after a relative short time quadratic: good results for mesh quality, even if the particle goes through half of the cylinder exponential: bad and confusing mesh moving after a short time It was more or less trial and error because I dont really know what the different diffusivity algorithms do and stand for. And I dont know what the values after the keyword stand for, like: exponential 2000 Does anyone know and has experience with the different diffusivity options and the influence of the values???Is there a documentation of this? For solvers I use: dynamicFvMesh dynamicMotionSolverFvMesh motionSolverLibs libfvMotionSolvers.so solver velocityComponentLaplacian I know from several "moving mesh" threads from the forum that there exits other solvers in OF, but does anyone know in which case it is better to use which solver? I know, alot of question to a often discussed topic. But I really appreciate any help and hints! Thanks & Kindest Regards, Flo 

January 10, 2008, 16:02 
Hi Florian,
Generally quad

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Frank Bos
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Hi Florian,
Generally quadratic inverseDistance diffusivity gives pretty good (if not the best) results for plunging bodies. For that kind of simulations I use the fvMotionSolver, velocityLaplacian. When you are encountering large rotations, the laplacian is not the right equation to use, a good alternative would be the displacementSBRStress motion solver. For really large rotation, subsets of meshes or user defined diffusivity could provide solutions......This is what I am dealing with now :S The following paper describes some theory about the laplacian motion solver: http://powerlab.fsb.hr/ped/kturbo/Op...Manuscript.pdf Regards, Frank
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January 10, 2008, 16:15 
How large is you body translat

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Frank Bos
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How large is you body translation / rotation ???
Frank
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January 10, 2008, 16:41 
Hi Frank,
by now I have no

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Florian Krause
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Hi Frank,
by now I have no (or very less) rotation of my particle, but I think large deformation. The paper from Jasak and Tukovic is definitly what I was searching for  its a good discription of the background of the solver and the diffusivity terms I will send you a two pictures  one of the deformed mesh and one of the undeformed mesh  to your eMail adress ok?! Regards, Florian 

January 10, 2008, 17:11 
ok.. I try to post them here

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Florian Krause
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ok.. I try to post them here
I use quadratic diffusivity in this case, and at this time I only have some nonorthogonal faces Regards, Florian 

January 10, 2008, 17:52 
In fact the screeshot of the d

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Florian Krause
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In fact the screeshot of the deformed mesh is not really representative and only gives an idea of the mesh deformation... My 3D Mesh only consists of hexa cells, but making a cut in paraView the visualization of the cut cells is very strange  something like RTrias...
But this is another, not soo important issue ;) Regards, Florian 

January 11, 2008, 03:04 
If the sphere is moving down a

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Frank Bos
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If the sphere is moving down all the way, you really need some cell layering.
Visualization of moving 3D meshes is really a pain, I use ensight to visualize complete cells instead of a cutting plane..... Frank
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January 11, 2008, 04:33 
Frank,
I agree with you reg

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Florian Krause
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Frank,
I agree with you regarding the use of additional/removal layer because my cell layers faces are parallel. But what happens when I have a unstructured Tet mesh? The structued Hex Mesh was only possible because I have only one sphere and buliding the more or less good hex mesh around the cylinder was not the easiest thing. For the add/rem cell layer I think I need something like: dynamicFvMeshLibs ("libtopoChangerFvMesh.so"); dynamicFvMesh topoChangerFvMesh; solver velocityLaplacian ; How can I define the max and min layer thickness in the dynamicMeshDict? Regards, Florian 

January 11, 2008, 04:44 
I haven't use cell layering be

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Frank Bos
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I haven't use cell layering before, so I don't know ....Try to look inside the code to find out how you define the dictionary....it's quit easy.
Frank
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January 11, 2008, 04:54 
I found a topo Change movingCo

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Florian Krause
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I found a topo Change movingCone example from the OF 1.4.1 dev source, I will check on it what is necassary for add/rem cell layers
Regards, Florian 

January 11, 2008, 06:10 
Florian, for the sphere are yo

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Srinath Madhavan (a.k.a pUl)
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Florian, for the sphere are you also solving for the conservation of angular momentum?


January 11, 2008, 20:35 
Hi pUI,
when I simulate the

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Florian Krause
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Hi pUI,
when I simulate the sinking process, I start with a (0 0 0) velcoity vector for the sphere. For the next iteration I calculate the new velocity for n+1 iteration with the resulting gravity force, the pressure/viscous force and the velocity from iteration n. Further, I calculate the moments on the sphere BUT I dont use the results for the moments to update the the velocity and dont take into account the possible influence of the moments on the rotation of the sphere. So, I dont think that I solve the conservation of angular momentum, but its a good hint for the future... ;) Is this what you mean? Regards, Florian 

January 11, 2008, 21:33 
yup, thanks

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Srinath Madhavan (a.k.a pUl)
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yup, thanks


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