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

[Francisco Delgado]

Hello, I've been following the magma chamber example and I get some errors
in Cubit when creating the geometry. In the geometry.jou file when I get to
lines 44 and 45, Cubit tells that those surfaces do not exist. What faces
are 35 and 36?? The two pieces of the upper surface??

Thanks


On Thu, Jun 19, 2014 at 10:56 AM, Brad Aagaard <baagaard at usgs.gov> wrote:

> 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
>>
>>
>
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-- 
--------------------------------------------------------
Francisco Delgado
PhD student in Geophysics
Cornell University
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