[Nek5000-users] [*] Re: RB convection with high Rayleigh numbers

[nek5000-users at lists.mcs.anl.gov]

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
>>> _______________________________________________
>>> Nek5000-users mailing list
>>> Nek5000-users at lists.mcs.anl.gov
>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
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