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

[Brad Aagaard]

Francisco,

Which version of CUBIT or Trelis are you using? I am using CUBIT 14.1. I 
did have to update the surface ids from when I originally used the 
script 3 years ago (I don't recall which version of CUBIT I was using at 
the time).

Brad


On 06/19/2014 11:49 AM, Francisco Delgado wrote:
> 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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