[Nek5000-users] double mesh + solution at boundary + scalar blows up !

Mon Apr 24 10:01:57 CDT 2017

Dear Neks

thank you Paul for your help on my double mesh question !! 
it works perfectly, and I am now testing quantitatively the results against published data. 

I am having two issues, one is likely stupid, and the other is likely more complex.

stupid issue: When I try to plot the solution at the lower wall of the channel, it appears to be undefined. Is this normal / is there a way to access the solution at the lower wall of the channel ? (The velocity should vanish as I have no slip boundary conditions, but the scalar should not vanish as I have no-flux for the scalar.)

complicatd issue: 
the flow starts from 0, it is forced by an average pressure gradients, has laminar initial condition+30% random fluctuations and it converges nicely. 
after the turbulent flow becomes stationary, i introduce the scalar, which develops in a nice fluctuating plume, looking reasonable. however, after some times, for no apparent reason, the scalar blows up. it sometimes produces NaN, and sometimes it doesnt. Can anyone think a reason why this happens? Reynolds = Peclet, and the scalar is injected at the inlet through the boundary condition. It does not blow up upon reaching the outlet (where I have outflow BC). I tried moving the source and making it bigger and model it as a Gaussian instead of a step function to avoid Gibbs oscillations. I also tried changing the boundary conditions from no flux to absorption (following https://lists.mcs.anl.gov/mailman/htdig/nek5000-users/2014-July/002961.html). I also halved diffusivity, so Pe = 1/2 Re, but no luck.

is there any reason why the scalar should be more resolved than the flow ?
is there any known numerical issue that i’m ignoring ?

any hint would be fantastic.

thanks a bunch !
Agnese

> On Jan 21, 2017, at 7:00 PM, nek5000-users-request at lists.mcs.anl.gov wrote:
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>   1. Re: double mesh ? (nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>)
> 
> From: nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>
> Subject: Re: [Nek5000-users] double mesh ?
> Date: January 21, 2017 at 1:18:41 PM GMT+1
> To: "nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>" <nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>>
> Reply-To: nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>
> 
> 
> Dear Agnese,
> 
> Attached is an example that shows how to modify the connectivity for the thermal
> field to be inflow/outflow.
> 
> If your domain is generated with genbox and your flow is in the x direction then
> this should also work for 3D.
> 
> Note that no changes are required to the .rea file (you should have "P  " for both
> velocity and temperature in the .rea/.box file).
> 
> The changes occur in the .usr file.
> 
> Paul
> 
> From: nek5000-users-bounces at lists.mcs.anl.gov <mailto:nek5000-users-bounces at lists.mcs.anl.gov> [nek5000-users-bounces at lists.mcs.anl.gov <mailto:nek5000-users-bounces at lists.mcs.anl.gov>] on behalf of nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov> [nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>]
> Sent: Monday, January 16, 2017 9:31 AM
> To: nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>
> Subject: Re: [Nek5000-users] double mesh ?
> 
> Dear Paul 
> 
> thank you so much for your help !
> I am looking forward to see your example, it will be of great help !
> 
> could you tell me something more about the recycling BC as well ?
> 
> Thank you again for your help 
> Agnese
> 
>> 
>> From: nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>
>> Subject: Re: [Nek5000-users] double mesh ?
>> Date: January 16, 2017 at 4:05:21 PM GMT+1
>> To: "nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>" <nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>>
>> Reply-To: nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>
>> 
>> Dear Agnese,
>> 
>> Sorry for the long delay in answering this question.  The issue is that in fact it is possible
>> (and used to be a supported but rarely used feature).   The challenge, however, is how to
>> make it relatively easy in the context of the current setup for a variety of applications.
>> 
>> Nek supports a different gather-scatter handle for each field (velocity, temperature, passive
>> scalar, etc.)
>> 
>> We have a handle that allows the user access to the connectivity list, which would give
>> you an opportunity to "disconnect" the periodic faces for the temperature array.
>> 
>> I'll try to code up an example for the case of channel flow with a mesh generated by
>> genbox and post it shortly.
>> 
>> Incidentally, an alternative approach that we often use for more complex domains
>> would be to use a recycling boundary condition.   I think it would not be as efficient,
>> however, as periodicity for the case that you propose.
>> 
>> Once again, apologies for the delayed response.
>> 
>> Paul
>> 
>> From: nek5000-users-bounces at lists.mcs.anl.gov <mailto:nek5000-users-bounces at lists.mcs.anl.gov> [nek5000-users-bounces at lists.mcs.anl.gov <mailto:nek5000-users-bounces at lists.mcs.anl.gov>] on behalf of nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>[nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>]
>> Sent: Thursday, December 22, 2016 4:38 AM
>> To: nek5000-users at lists.mcs.anl.gov <mailto:nek5000-users at lists.mcs.anl.gov>
>> Subject: [Nek5000-users] double mesh ?
>> 
>> Hi
>> 
>> i am quite new, so i’m sorry if i’m getting things wrong.
>> 
>> I am trying to simulate a turbulent channel flow, with a passive scalar. 
>> in the streamwise direction, i would like to impose periodic boundary conditions for the flow, and non-periodic for the scalar.
>> in fact the scalar has a point source near the inlet, and i want to see how that develops downstream. 
>> 
>> as far as i understand this is not supported by nek5000, because the last mesh point is identified with the first mesh point when periodic BC are imposed.
>> correct ?
>> 
>> my question is : could i define two meshes, one for the scalar and one for the velocity ?
>> 
>> thanks a lot for your help !!!
>> agnese
>> 
>> 
>> Agnese Seminara
>> --------------------------------
>> CNRS
>> Laboratoire de physique de la matière condensée
>> Parc Valrose 
>> avenue J Vallot
>> 06108 Nice, France
>> +33 (0) 492 076 775
>> http://sites.unice.fr/site/aseminara/ <http://sites.unice.fr/site/aseminara/>
>> 
>> 
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