Re: [Libmesh-users] Orientation of LibMesh Elements

[Kirk, B. (JSC-EG311) <ben...@na...>]

You can infer this from the 2d numbering.  For example, for the HEX8, the
base is oriented wrt (xi,eta) according to the QUAD4, and zeta is the
vertical direction.


On 2/16/10 3:04 PM, "Roy Stogner" <roy...@ic...> wrote:

> 
> 
> On Tue, 16 Feb 2010, Rahul Sampath wrote:
> 
>>  HEX8: 7        6
>>        o--------o
>>       /:       /|
>>      / :      / |
>>   4 /  :   5 /  |
>>    o--------o   |
>>    |   o....|...o 2
>>    |  .3    |  /
>>    | .      | /
>>    |.       |/
>>    o--------o
>>    0        1
>> 
>>     TET4:
>>        3
>>        o
>>       /|\
>>      / | \
>>     /  |  \
>>  0 o...|...o 2
>>     \  |  /
>>      \ | /
>>       \|/
>>        o
>>        1
>> 
>> However, you have not shown the coordinate axes. I need to know the
>> following:
>> 1) The relationship between the local coordinate system and the local
>> node numbering scheme. For  example, In Hex8 whether the edge 0-1 is
>> parallel to the local xi or eta axes.
>> 
>> 2) The relationship between the physical global coordinate system and
>> the local coordinate system.
>> 
>> I believe you must already be using some convention for both to be
>> able to compute the Jacobian of the transformation. It would be very
>> helpful if you could tell me where I can find this information.
> 
> To the best of my recollection:
> 
> For triangles and tets, the nodes are at (0,0,0), (1,0,0), (0,1,0),
> (0,0,1) in master coordinates
> 
> For tensor product elements, the geometry is [-1,1]^d and the edges
> are 0-1:xi, 0-3:eta, 0-4:zeta
> 
> For pyramids, the peak is at (0,0,1) and the base is the same as a
> quad.
> 
> Prisms are the tensor product of a triangle and an edge, with the
> triangle at zeta=-1 given the lower local node numbers.
> 
> I'm afraid that the only place where you can find this information
> solidly may be implicitly in the fe_lagrange*.C code, though; I'm not
> sure we've got it cleanly documented.  If you'd like to add such
> documentation, a patch (in ASCII so we can put it in the headers)
> would be appreciated.
> ---
> Roy
> 
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