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

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

Hi Paul,

     Thank you for your time on this problem. The geometry is a 2D box 
with no-slip velocity boundary conditions on all sides and the 
temperature boundary conditions are insulating on the side walls and the 
usual fixed temperatures on the top and bottom walls. I will try out 
your code immediately.

Regards,
Mani chandra
On 05/16/2010 09:14 AM, nek5000-users at lists.mcs.anl.gov wrote:
>
> 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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>>
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