Morph mesh given displacement of boundary points

praveen · November 5, 2011, 02:12

Hello

Is there anything in openfoam which allows me to morph the interior mesh, given the displacement of the boundary mesh nodes ?

praveen

wyldckat · November 5, 2011, 04:04

Hi Praveen,

Does this tutorial suit your requirements: "tutorials/mesh/moveDynamicMesh/simpleHarmonicMotion"

Best regards,
Bruno

praveen · November 5, 2011, 07:05

Thanks. That seems relevant to me. I will check it out. Is it possible to specify the displacement of each individual boundary mesh point ?

ngj · November 5, 2011, 08:33

Hi Praveen,

Yes, it is possible to specify the displacement on the point level. This type of boundary condition can be put into two categories:

1. You know the motion by an algebraic equation, hence you loop over every boundary point and specify the displacement (Note: Some solvers use the boundary velocity, thus differentiate your algebraric equation with respect to time and evaluate it). Furthermore, if you are using tet-decomposition (available in 1.6-ext), you specify both the motion on the points and in the centers of the boundary faces. On the boundary, they are ordered as [<points> <face centers>].

2. You move the mesh based on results from the state of your simulation. Typically you can compute the motion in the face centers, and then you perform an interpolation to the points on the boundary. Again, be aware when you are using displacement or velocity solvers for the mesh motion.

With respect to the interpolation methods you can either look through the forum, or so-forth you have 1.6-ext installed, you can see an example of the implementation in the file "freeSurface.C" located somewhere in the "applications/solvers" directory.

A final comment: If the boundary you are moving also has a very fine boundary layer resolution, then my experience is that laplaceFaceDecomposition (1.6-ext) is the most robust combined with a very stiff mesh diffusivity next to that boundary. The diffusivity is the $\gamma$ term in the following Laplace equation for the velocity of the mesh motion:

$\nabla \gamma\boldsymbol\cdot\nabla\mathbf{u}_m=\boldsymbol{0}$

The diffusivity is specified in the dynamicMeshDict in <rootCase>/constant.

Good luck and kind regards,

Niels

Michael@UW · August 7, 2020, 13:04

Is there a type of boundary condition such that I can set the boundary positions directly? I know the boundary coordinates at each time step. I can do that by calculating the displacement or motion velocity but it will be simpler if I can set the coordinates of the boundary points directly.

November 5, 2011, 02:12	Morph mesh given displacement of boundary points	#1
praveen Super Moderator Praveen. C Join Date: Mar 2009 Location: Bangalore Posts: 343 Blog Entries: 6 Rep Power: 18	Hello Is there anything in openfoam which allows me to morph the interior mesh, given the displacement of the boundary mesh nodes ? praveen

November 5, 2011, 04:04		#2
wyldckat Retired Super Moderator Bruno Santos Join Date: Mar 2009 Location: Lisbon, Portugal Posts: 10,981 Blog Entries: 45 Rep Power: 128	Hi Praveen, Does this tutorial suit your requirements: "tutorials/mesh/moveDynamicMesh/simpleHarmonicMotion" Best regards, Bruno __________________ OpenFOAM: FAQ \| Getting started Forum: How to get help, to post code/output and forum guide Read this before sending me PM

November 5, 2011, 08:33		#4
ngj Senior Member Niels Gjoel Jacobsen Join Date: Mar 2009 Location: Copenhagen, Denmark Posts: 1,903 Rep Power: 37	Hi Praveen, Yes, it is possible to specify the displacement on the point level. This type of boundary condition can be put into two categories: 1. You know the motion by an algebraic equation, hence you loop over every boundary point and specify the displacement (Note: Some solvers use the boundary velocity, thus differentiate your algebraric equation with respect to time and evaluate it). Furthermore, if you are using tet-decomposition (available in 1.6-ext), you specify both the motion on the points and in the centers of the boundary faces. On the boundary, they are ordered as [<points> <face centers>]. 2. You move the mesh based on results from the state of your simulation. Typically you can compute the motion in the face centers, and then you perform an interpolation to the points on the boundary. Again, be aware when you are using displacement or velocity solvers for the mesh motion. With respect to the interpolation methods you can either look through the forum, or so-forth you have 1.6-ext installed, you can see an example of the implementation in the file "freeSurface.C" located somewhere in the "applications/solvers" directory. A final comment: If the boundary you are moving also has a very fine boundary layer resolution, then my experience is that laplaceFaceDecomposition (1.6-ext) is the most robust combined with a very stiff mesh diffusivity next to that boundary. The diffusivity is the $\gamma$ term in the following Laplace equation for the velocity of the mesh motion: $\nabla \gamma\boldsymbol\cdot\nabla\mathbf{u}_m=\boldsymbol{0}$ The diffusivity is specified in the dynamicMeshDict in <rootCase>/constant. Good luck and kind regards, Niels fumiya, npaliwal and Michael@UW like this.

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November 5, 2011, 07:05		#3
praveen Super Moderator Praveen. C Join Date: Mar 2009 Location: Bangalore Posts: 343 Blog Entries: 6 Rep Power: 18	Thanks. That seems relevant to me. I will check it out. Is it possible to specify the displacement of each individual boundary mesh point ?

August 7, 2020, 13:04		#5
Michael@UW Senior Member Join Date: Jan 2019 Posts: 125 Blog Entries: 1 Rep Power: 0	Is there a type of boundary condition such that I can set the boundary positions directly? I know the boundary coordinates at each time step. I can do that by calculating the displacement or motion velocity but it will be simpler if I can set the coordinates of the boundary points directly.