For reference, Martin has registered an Issue on Bitbucket:
https://bitbucket.org/fenics-project/ffc/issue/7
Garth
On 2 September 2013 21:17, David Bernstein <[email protected]> wrote:
> Ok, yes that makes it work in both cases. Thanks for taking a look at it.
>
> Dave
>
>
> On Sep 2, 2013, at 1:54 AM, Martin Sandve Alnæs <[email protected]> wrote:
>
> It's a bug in tensor representation. As a temporary fix, try setting:
> parameters["form_compiler"]["representation"] = "quadrature"
> at the top to disable tensor representation.
>
> Martin
>
>
>
> On 2 September 2013 10:26, Cotter, Colin J <[email protected]>
> wrote:
>>
>> Hi Dave,
>> This term can't be integrated exactly because it is not polynomial, so
>> my first guess would be that different quadrature rules are used for those
>> two different expressions (if you don't suggest a quadrature rule then FFC
>> tries to guess one from the expression). Try fixing the quadrature rule in
>> that term and see if the results are still different.
>>
>> all the best
>> --Colin
>> ________________________________
>> From: [email protected] [[email protected]]
>> on behalf of David Bernstein [[email protected]]
>> Sent: 01 September 2013 20:35
>> To: [email protected]
>> Subject: [FEniCS] Question about nonlinear problem
>>
>> Hi Everyone, Recently I wrote a fenics python code for solving a nonlinear
>> advection diffusion equation which worked very well. However, I came across
>> one issue which I couldn't resolve and which is illustrated by the attached
>> code (simpler than original project but still seems to contain the problem).
>>
>> The example problem is identical to the one described here
>>
>> http://fenicsproject.org/documentation/tutorial/nonlinear.html
>>
>> except that the nonlinear term q(u) is (1-u)^(-2), i.e., the PDE is
>>
>> grad(grad(u)/(1-u)^2) = -f
>>
>> The attached code gives different results if q is written as "(1-u)**(-2)"
>> and "(1/(1-u))**2". Is there a reason for this? I would have thought they
>> should be equivalent.
>>
>> Thanks,
>> Dave
>>
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