[petsc-users] ex42 question

[Jed Brown]

Matthew Knepley <knepley at gmail.com> writes:

> 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.

I'm not sure I believe these boundary conditions.  Try putting no-slip
all around the exterior and solve the algebraic problems accurately
(e.g., direct solver).  Since the constant over the whole domain is in
the space, you should be able to verify that the (global) integral of
divergence is 0.
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