[Nek5000-users] [*] Re: RB convection with high Rayleigh numbers
nek5000-users at lists.mcs.anl.gov
nek5000-users at lists.mcs.anl.gov
Sat May 15 22:44:55 CDT 2010
Hi Mani,
I've added an example of a case that should run out to Ra=10^8
in the /benard directory of the repo (high_ray.blah is the case).
I've run this with lx1=5, lxd=9 and it will get to Ra=10^8.
(In reality, I ran tests with 4 separate runs, 10^k, k=5,6,7,8.
I then a created "high_ray.usr" file that would march through
this sequence in a single run...)
Once you establish the flow at the desired Ra, you can then
bump lx1 to say 7 or 8 --- that should be sufficient for Ra=10^8
I think. Even lx1=5 is looking pretty good with the given mesh.
Also, you can reduce the CFL in the .rea file and increase Torder
to 3, so that you will increase your temporal accuracy.
I initially set the parameters so that it would march quickly
to Ra=10^8.
The high-Ra test case uses periodic bcs and a uniform mesh in X.
If you want vertical walls then you should likely use a nonuniform
mesh in X. The "high_ray.box" file shows how the nonuniform
mesh in Y is created. A similar set up could be used for nonuniformity
in both X and Y.
Paul
On Sun, 16 May 2010, nek5000-users at lists.mcs.anl.gov wrote:
> Hi Paul,
>
> Thanks for the reply. I have a doubt whether one should increase the
> polynomial order or the number of elements with a fixed order for this case.
> As I understand from your illustration of the rotating cone case, increasing
> the polynomial order leads to a much better solution than with the same
> number of points generated using more elements with a lower order polynomial.
>
> Also how do I use the fast tensor-product solver? Is there some flag that I
> need to turn on?
>
> Regards,
> Mani chandra
> On 05/15/2010 09:05 PM, nek5000-users at lists.mcs.anl.gov wrote:
>>
>> Hi Mani,
>>
>> I've not tried to go this high yet, but there are several std
>> techniques to pushing the envelope.
>>
>> First, you should try to get an estimate of your expected resolution
>> requirements at the target Ra. Then, guess that you might want to
>> run that simulation at say, lx1=10 or 12; and build the corresponding
>> mesh (number of elements in x and y); but start the compuation at
>> much lower Ra, with lx1=4, say.
>>
>> Then work your way up in Ra, say by a factor of 10 in each successive
>> run, while also increasing lx1 as needed.
>>
>> Note that for the flow in a box, you might find it faster to use
>> the fast tensor-product solver for the pressure, if using the Pn-Pn-2
>> method --- this is not always the case, but for some very difficult
>> problems (e.g., highly refined meshes), it does work better.
>>
>> Paul
>>
>>
>> On Sat, 15 May 2010, nek5000-users at lists.mcs.anl.gov wrote:
>>
>>> Hi,
>>>
>>> I'm trying to simulate RB convection in a 2D box of aspect ratio 2:1.
>>> I'd like to simulate cases with Rayleigh numbers of the order of 10^7 -
>>> 10^8. But I'm barely able to do 10^5 and that too with a time step of
>>> 10^-6. For the mesh I took 8 spectral elements in the x direction and 4
>>> spectral elements in the y direction with 48 points within each of them.
>>> Is there any way one can reach higher Ra numbers?
>>>
>>> Regards,
>>> Mani chandra
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>>> Nek5000-users at lists.mcs.anl.gov
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