[CIG-SHORT] Direction of traction-normal for Neumann BC

[Satoshi Okuyama]

Brad,

Thank you for quick answer.

I believe I made my pressure source cavity, but I will double-check that.
I am good at making this kind of simple mistakes. If I find my source IS
cavity, and still have the problem, I will make a minimum example that 
reproduce the problem.

I also thank you for the magma chamber example. I will give it a try later
but I have to update pylith first.

Regards,
---
Satoshi Okuyama


At Thu, 19 Jun 2014 07:56:17 -0700,
Brad Aagaard wrote:
> 
> [1  <text/plain; ISO-8859-1 (7bit)>]
> Satoshi,
> 
> The Neumann BC is intended to be applied to an external boundary. For a 
> spherical pressure source, the domain should not include the material 
> inside the sphere (it needs to be a cavity). This will result in a 
> consistent normal direction for the boundary.
> 
> If the spherical boundary is all one surface, then you can still run 
> into problems when PyLith initializes the boundary. It may find 
> horizontal normal directions. This means the default way it uniquely 
> defines the two tangential directions breaks down. The workaround is to 
> subdivide the surface into quadrants so that you can use a user-defined 
> up-direction to get consistent directions tangential and normal 
> directions for the Neumann BC. Attached is a small magma chamber example 
> that illustrates this.
> 
> We are working on a more detailed magma chamber and dike example for a 
> workshop next week and we will create an examples section under PyLith 
> User Resources (http://wiki.geodynamics.org/software:pylith:start) in 
> the next week and post it there.
> 
> Regards,
> Brad
> 
> 
> On 06/19/2014 02:09 AM, Satoshi Okuyama wrote:
> > Hello,
> >
> > Recently I started using pylith and I already love it. However, I have
> > an question about Neumann boundary condition;
> >
> > What determines the direction of positive traction-normal?
> >
> > or
> >
> > What determines the order of the vertices when pylith construct faces
> > from a group of vertices for boundary condition?
> >
> >
> > Here is my story,
> >
> > I am trying to simulate the deformation caused by a pressure source. I
> > created a mesh with spherical source and put all the vertices on source
> > surface into a group, then applied Neumann BC with just traction-normal.
> >
> > However, the deformation of the source was far from isotropic. I checked
> > the initial traction and found that deflation (traction toward source
> > center) is applied to some faces, while inflation is applied to the others.
> >
> > Following is an example of initial-traction output. I placed 5 vertices
> > on a plane of z=0 and formed 4 triangle face. Then I applied +1Pa of
> > traction-normal to this group.
> >
> > #######################################################################
> > # vtk DataFile Version 2.0
> > Simplicial Mesh Example
> > ASCII
> > DATASET UNSTRUCTURED_GRID
> > POINTS 5 double
> > -1.000000e+00 -1.000000e+00 0.000000e+00
> > 1.000000e+00 -1.000000e+00 0.000000e+00
> > 1.000000e+00 1.000000e+00 0.000000e+00
> > -1.000000e+00 1.000000e+00 0.000000e+00
> > 0.000000e+00 0.000000e+00 0.000000e+00
> > CELLS 4 16
> > 3  2 1 4
> > 3  3 0 4
> > 3  3 2 4
> > 3  4 1 0
> > CELL_TYPES 4
> > 5
> > 5
> > 5
> > 5
> > CELL_DATA 4
> > VECTORS initial_traction double
> > 0.000000e+00 0.000000e+00 -1.000000e+00
> > 0.000000e+00 0.000000e+00 1.000000e+00
> > 0.000000e+00 0.000000e+00 -1.000000e+00
> > 0.000000e+00 0.000000e+00 -1.000000e+00
> > #######################################################################
> >
> > As you see, 2nd cell (or face) receives traction of (0,0,1) while other
> > cells receives (0,0,-1). I noticed that if I consider 2 vectors - 1st
> > vertex to 2nd, and 1st to 3rd - the direction of the traction vector is
> > equal to the cross product of them.
> >
> > cell #1:
> >    v1: #2 -> #1 = ( 0,-2,0)
> >    v2: #2 -> #4 = (-1,-1,0)
> >    v1 x v2 = (0,0,-2)
> >
> > cell #2:
> >    v1: #3 -> #0 = ( 0,-2,0)
> >    v2: #3 -> #4 = ( 1,-1,0)
> >    v1 x v2 = (0,0,2)
> >
> > One step closer to the answer, I believe. But I have no idea how this
> > order is determined. The order of the vertices for 2nd cell is 3-0-4,
> > not 3-4-0. But why?
> >
> >
> > Regards,
> > ----
> > Satoshi Okuyama
> > _______________________________________________
> > CIG-SHORT mailing list
> > CIG-SHORT at geodynamics.org
> > http://lists.geodynamics.org/cgi-bin/mailman/listinfo/cig-short
> >
> 
> [2 magmachamber.tgz <application/x-compressed-tar (base64)>]
>