[petsc-users] ex42 question

[Sang pham van]

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?

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
>
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