# Mesh resolution

(Rebecca) Xuefei YUAN xy2102 at columbia.edu
Tue Dec 15 21:37:31 CST 2009

```Now PETSc treats the multigrid based on the half spacing strategy,
thus they always do odd number of grids.

R

Quoting Ryan Yan <vyan2000 at gmail.com>:

> Sorry, I forgot to cc to the group.
>
> BTW, if this is designed to have only odd number of points on the finer
> level grid, can anyone provide a hint on the reasoning behind this?
>
> Thanks a lot,
>
> Yan
>
> On Tue, Dec 15, 2009 at 10:09 PM, Ryan Yan <vyan2000 at gmail.com> wrote:
>
>> Hi Rebecca,
>> Thanks for the info. I noticed that rule; yet, it seems very difficult to
>> bring up a finest grid with *even* number of pts.
>>
>> Yan
>>
>>
>> On Tue, Dec 15, 2009 at 9:55 PM, (Rebecca) Xuefei YUAN <
>> xy2102 at columbia.edu> wrote:
>>
>>> The relation between the coarser grid and finer grid is always half
>>> spacing in either x-, y-, or z-direction.
>>>
>>> Thus, for your finest grid being 257x257x257, you need a coarser grid of
>>> 129x129x129 on the coarse grid if your level is 2.
>>>
>>>
>>> R
>>>
>>>
>>>
>>> Quoting Ryan Yan <vyan2000 at gmail.com>:
>>>
>>>  Hi All,
>>>> I was ignoring the importance of fixing the mesh size.
>>>> For instance, if I want to compare the effect of mesh sequencing on the
>>>> solver, I will use one level with 256 pt on each direction in 2d, in the
>>>> first run.
>>>>
>>>> My question is,  in the second run, how to set up the number of points on
>>>> the coarsest grid and pre-fix the level of the grid so that DMMG
>>>> can bring me the exact resolution of  256 pt on each direction on the
>>>> finest
>>>> grid. Only this way, the two different solver are solving the problem of
>>>> the
>>>> same
>>>> size.
>>>>
>>>> Thanks a lot,
>>>>
>>>> Yan
>>>>
>>>>
>>>
>>>
>>> --
>>> (Rebecca) Xuefei YUAN
>>> Department of Applied Physics and Applied Mathematics
>>> Columbia University
>>> Tel:917-399-8032
>>> www.columbia.edu/~xy2102 <http://www.columbia.edu/%7Exy2102>
>>>
>>>
>>
>

--
(Rebecca) Xuefei YUAN
Department of Applied Physics and Applied Mathematics
Columbia University
Tel:917-399-8032
www.columbia.edu/~xy2102

```