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[Till B <question143963@xxxxxxxxxxxxxxxxxxxxx>]

Question #143963 on DOLFIN changed:
https://answers.launchpad.net/dolfin/+question/143963

    Status: Answered => Open

Till B is still having a problem:
Hi

Thank you two for your answers.

Anders:
With the working mesh, it solves a 37076 by 37076 matrix (which, however, is
singular). If I refine the mesh with the built-in refine function, I get a
279286 by 279286 matrix (and here I get the memory problems).
The mesh file itself (in xml format) is around 650 kb at the moment. I
already tried meshes with 30mb (and then reduced the mesh density until I
arrived at a working version). Do you mean that the mesh.xml file does not
need much hard disk storage? Or that processing the mesh information should
not cause much memory usage?

Andre:
The iterative stokes example does not work, it aborts with the message
"DOLFIN has not been configured with Trilinos or PETSc. Exiting.". I should
probably note that I use the precompiled Windows version of Fenics.

Thank you so far

Till


2011/2/3 Anders Logg <question143963@xxxxxxxxxxxxxxxxxxxxx>

> Your question #143963 on DOLFIN changed:
> https://answers.launchpad.net/dolfin/+question/143963
>
> Anders Logg proposed the following answer:
> On Thu, Feb 03, 2011 at 04:37:00PM -0000, Till B wrote:
> > New question #143963 on DOLFIN:
> > https://answers.launchpad.net/dolfin/+question/143963
> >
> > Hi,
> >
> > I recently installed the Fenics package and now I try to solve some
> problems with it. I want to simulate the flow of air around an obstacle.
> Eventually I want to include the temperature and see how a hot obstacle
> cools down in a colder air flow. However I encounter several problems,
> already with my baby example which does not even include the temperature. I
> am new in the CFD business. To begin, I adapted a few examples for
> Navier-Stokes solvers that I found in the Dolfin examples or on
> launchpad.net/nsbench (I tried both steady state and transient flow
> examples). I use a 3d mesh.
> >
> > My main problem occurs with mesh size. Unless I use a rather coarse mesh,
> I run out of memory, sometimes with a std::bad_alloc error, sometimes with
> the message that UMFPACK has run out of memory. It works with a mesh that
> has 7828 cells, but for example not, if I refine this mesh with the refine
> function of Dolfin.
> > Is there anything I can do about this? I could get more RAM, but I do not
> think that this will ultimately solve my problem. In the end I want to use
> much larger meshes.
> >
> > With this coarse mesh I do not get realistic results. Now matter how I
> set my inflow velocity, the speed of the fluid in the bulk is very low. When
> I use a 2d version of my example with a finer mesh, I get results that make
> sense.
> >
> > Is there a way to circumvent the memory issue?
> >
> > Thank you!
>
> What is the size of the linear systems you are solving? If you use P2
> elements in 3D you may very quickly get very large systems.
>
> I suspect the problem with memory allocation is a result of using a
> direct solver. UMFPACK allocates quite a lot of memory which makes it
> unsuitable for large problems.
>
> Try an iterative method and you should see that the memory usage is
> kept under control. The mesh itself should only require a minimal
> amount of storage.
>
> --
> Anders
>
> --
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