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

Thu Jun 19 12:00:53 PDT 2014

Francisco - one good way of debugging this stuff is to run it one line at a
time within cubit, and looking at the tree of volumes and surfaces to
figure out what it is referring to.

On Thu, Jun 19, 2014 at 11:54 AM, Brad Aagaard <baagaard at usgs.gov> wrote:

> 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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>>
>>
>>
>>
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