[FLASH-USERS] Does FFT subroutine work with every domain discretization?‏‏

[Paul Ricker]

Marco,

Here are answers to your questions.

1a. The single-block FFT routines require mesh sizes that are powers of
     two. I don't know about PFFT. Given the oct-tree mesh, you would
     only encounter non-power-of-two global meshes when using multiple
     coarsest-level blocks.
1b. I have no intention of doing so myself. However, the interfaces to
     the FFT routines (fftsg.f etc.) are well-documented, so it shouldn't
     be difficult to modify FFT routines from a different package to
     plug into the FLASH single-block solver routines
     (gr_hgPoissonSolve*D.F90).
2.  Yes, you would just need a different Green's function. These are
     defined for different boundary conditions and dimensions in
     gr_hgPoissonSolve*D.F90 (for single blocks) and
     gr_pfftPoissonDirect.F90 (for PFFT).

Best,
Paul



On 10/01/2012 01:16 PM, Marco Mazzuoli wrote:
>      Dear all,
>
> in the application I developed, based on the Flash 4.0alpha architecture
> and the PARAMESH package, a (three-dimensional) Poisson problem is
> numerically solved by means of the multigrid method provided with the
> Flash package. Both pFFT and FFT are implemented into the Poisson
> solver. However, by making some preliminary tests, I have noticed that
> the fast Fourier transform is well performed only if the number of the
> discretization points in the three dimensions (e.g. NXB, NYB, NZB) is a
> multiple of 2.
> 1a) Does the FFT even work when blocks have a different size? Actually,
> it is quite common that FFT routines accept also array sizes which are
> also multiple of 3 and 5. Is it the case?
> 1b) Eventually, do you think it could be possible to replace the Flash's
> FFT with another FFT subroutine in order to support my request?
>
> 2) A secondary question deals with the Poisson solver. On the basis of
> Huang \& Greengard (2000) and Ricker (2008) algorithms, do you think it
> is feasible to modify the Poisson solver, distributed within the Flash
> package, in order to obtain a solver for a HelmHoltz problem?
>
> Thank you in advance for your helpfulness.
> Sincerely,
>
>      Marco Mazzuoli
>
>
>
> Ing. Marco Mazzuoli
> Dipartimento di Ingegneria
> delle Costruzioni, dell'Ambiente e
> del Territorio (DICAT)
> via Montallegro 1
> 16145 GENOVA-ITALY
> tel.  +39 010 353 2497
> cell. +39 338 7142904
> e-mail marco.mazzuoli at unige.it
>         marco.mazzuoli84 at gmail.com
>
>
>
>

-- 
Paul M. Ricker
Associate Professor of Astronomy
University of Illinois
http://sipapu.astro.illinois.edu/~ricker