# [Technical] Mesh movement during runtime

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May 23, 2013, 11:10
Mesh movement during runtime
#1
New Member

Matthias Stammen
Join Date: Oct 2010
Posts: 8
Rep Power: 15
Hello there,

although I already was searching here and there, I am still not sure how to use the diffusivity of the velocityLaplacian solver in the dynamicMeshDict.

I want to move my mesh during runtime depending on the pressure on the patch boundary "airfoil" (optimization of a 2D-airfoil).
But somehow the diffusivity (uniform, quadratic, motionDirectional etc.) does not change much the results and I always get some overlapping cells (see attachment). If I choose different values for uniform I also don't see any change in the result.

Here is the dynamicMeshDict I am using:
HTML Code:
```dynamicFvMesh           dynamicMotionSolverFvMesh;

motionSolverLibs       ("libDynamicFvMesh.so");

solver                  velocityLaplacian;

diffusivity             quadratic uniform; //uniform ; //(100);

distancePatches         (airfoil);```
Could anybody tell me what I am doing wrong?

Kind regards,
Matthias
Attached Images
 t925_diffusivity_uniform_1000.jpg (73.0 KB, 111 views)

 May 23, 2013, 12:00 #2 Senior Member     Marco A. Turcios Join Date: Mar 2009 Location: Vancouver, BC, Canada Posts: 740 Rep Power: 28 I'm not sure you can get the behaviour you want with the standard motion solvers. I'm not sure how you are coupling the pressure on the patch with the motionSolver. velocityLaplacian means that the displacement of the points is computed with velocity specified as initial and boundary conditions of the laplacian equation. The diffusivity for the points mainly affects how "stiff" the mesh is. The different options are there to modify how diffusivity is computed based on several criteria. I know there is one called inverseDistance that has the diffusivity be proportional to the inverse distance from certain patches (so its highest near the patches and then falls off as you move further). I think you will need some kind of distance dependant diffusivity to help things move along.

 May 23, 2013, 15:03 #3 New Member   Matthias Stammen Join Date: Oct 2010 Posts: 8 Rep Power: 15 Thanks, mturcios. The coupling of pressure and motionSolver I am doing via the pointMotionU. The diffusivity inverseDistance I also tried, but the results are just slightly different and I still get overlapping cells. But is it normal that different values, e.g. in motionDirectional, give no difference in the result? So, is my dynamicMeshDict correct in principle?

May 23, 2013, 15:52
#4
Senior Member

Marco A. Turcios
Join Date: Mar 2009
Posts: 740
Rep Power: 28
Quote:
 Originally Posted by MatzeS Thanks, mturcios. The coupling of pressure and motionSolver I am doing via the pointMotionU. The diffusivity inverseDistance I also tried, but the results are just slightly different and I still get overlapping cells. But is it normal that different values, e.g. in motionDirectional, give no difference in the result? So, is my dynamicMeshDict correct in principle?
The diffusivity is mainly affecting the stiffness of your system of points and how long/much the points are moved in the motionSolver step. You may want to try write the diffusivity to a pointField see what the absolute numbers are. There is nothing in the motionSolver about mesh quality, so you may want to check the mesh every few iterations and remesh when the quality is below a certain level. Good luck!

 August 16, 2020, 02:07 #5 Senior Member   Hojatollah Gholami Join Date: Jan 2019 Posts: 171 Rep Power: 7 Dear foamer I use dynamic mesh for "Flow-induced deformation of a flexible thin structure as manifestation of heat transfer enhancement" article. the geometry is same as turek-hron FSI benchmark. the quadratic inverseDistance uses 1/L^2 relation of laplacian equation of mesh motion, but when deflection amplitude increases the mesh deform near the moving boundary and make it distorted as below. I looking for a better diffusion parameter for this case. I think some mesh layers of the moving boundary should move with boundary (for example 3layers) and then diffusion spread in mesh domain to prevent distortion of mesh in first mesh layer of moving boundary. do you have any suggestion for any formulation to change the code base it? Thanks. salachnaj likes this.

 February 3, 2021, 04:23 #6 New Member   Conor Join Date: Oct 2016 Posts: 14 Rep Power: 9 Hi Hgholami, I am experiencing the same issue as you when using the displacementLaplacian solver with a cantilever foil. Have you been able to find a fix for this? Regards, Conor

February 3, 2021, 07:14
#7
Senior Member

Hojatollah Gholami
Join Date: Jan 2019
Posts: 171
Rep Power: 7
Hi Conor
I used velocity solver prefer than displacement solver. At now, the refVelocityLaplacian is prefer than other. Also "diffusivity quadratic inverseDistance" is good. But still it can't support large deflection. I also modified "quadratic" in motionDiffusivity for compatibility with geometry but still it is not sufficient. May you can use overset in foam-extend4.1. the solids4Foam for FSI problems use it.

Quote:
 Originally Posted by ConorMD Hi Hgholami, I am experiencing the same issue as you when using the displacementLaplacian solver with a cantilever foil. Have you been able to find a fix for this? Regards, Conor