[petsc-users] ex42 question

[Matthew Knepley]

On Wed, Apr 16, 2014 at 5:38 PM, Sang pham van <pvsang002 at gmail.com> wrote:

> Yes, It does converge toward 0-flow when I refine the mesh !
>
> In the code, coordinates are set by
>   ierr =
> DMDASetUniformCoordinates(da_prop,0.0+0.5*dx,1.0-0.5*dx,0.0+0.5*dy,1.0-0.5*dy,0.,0);CHKERRQ(ierr);
> Does it set coordinate for elements center here?
> When I want to try non-uniform grid, should I just modify coordinates
> attached with the DM, or I need more implementation?
>

Yes, I think that should be enough.

   Matt


> Many thanks.
> Sang
>
>
>
> On Wed, Apr 16, 2014 at 6:23 PM, Matthew Knepley <knepley at gmail.com>wrote:
>
>> On Wed, Apr 16, 2014 at 4:48 PM, Sang pham van <pvsang002 at gmail.com>wrote:
>>
>>> Hi Jed,
>>>
>>> I modified the ex43 code to enforce no-slip BCs on all boundaries.
>>> I run the code with volume force (0,-1) and isoviscosity. The expected
>>> result is Vx = Vy = 0 everywhere, and linearly decreasing pressure (from to
>>> to bottom).
>>> In the attached is plot of velocity field and pressure, so there is
>>> still a (light) flow in middle of the domain. Do you know why the solution
>>> is that, and what should I do to get the expected result?
>>>
>>
>> It sounds like it is due to discretization error. Your incompressibility
>> constraint is not verified element-wise
>> (I think ex43 is penalized Q1-Q1), so you can have some flow here. Refine
>> it and see if it converges toward
>> 0 flow.
>>
>>    Matt
>>
>>
>>> Thank you.
>>>
>>> Sang
>>>
>>>
>>>
>>>
>>> On Wed, Mar 26, 2014 at 2:38 PM, Jed Brown <jed at jedbrown.org> wrote:
>>>
>>>> Sang pham van <pvsang002 at gmail.com> writes:
>>>>
>>>> > Hi Dave,
>>>> > I guess you are the one contributed the ex42 in KSP's examples. I
>>>> want to
>>>> > modify the example to solve for stokes flow driven by volume force in
>>>> 3D
>>>> > duct. Please help me to understand the code by answering the following
>>>> > questions:
>>>> >
>>>> > 1. Firstly, just for confirmation, the equations you're solving are:
>>>> >      \nu * \nabla \cdot \nabla U - \nabla P = 0 and
>>>>
>>>> For variable viscosity, it must be formulated as in the example:
>>>>
>>>>   \nabla\cdot (\nu D U) - \nabla P = 0
>>>>
>>>>   where  D U = (\nabla U + (\nabla U)^T)/2
>>>>
>>>> >      \nabla \cdot U = 0
>>>> >
>>>> >       where U = (Ux,Uy,Uz), \nu is variable viscosity?
>>>> >
>>>> >  2. Are U and P defined at all nodes? (I googled the Q1Q1 element, it
>>>> looks
>>>> > like a box element with U and P defined at 8 corners).
>>>>
>>>> Yes.
>>>>
>>>> >  3. Are nodes' coordinate defined though the DA coordinates?
>>>>
>>>> Yes, though they are set to be uniform.
>>>>
>>>> >  4. How can I enforce noslip BC, and where should I plug in volume
>>>> force?
>>>>
>>>> Enforce the Dirichlet condition for the entire node.
>>>>
>>>
>>>
>>
>>
>> --
>> What most experimenters take for granted before they begin their
>> experiments is infinitely more interesting than any results to which their
>> experiments lead.
>> -- Norbert Wiener
>>
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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