[Nek5000-users] convective boundary conditions

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

Because you impose a divergence in the last layer of elements, it emulates an acceleration (or suction) only in that last layer of elements without affecting the rest of your domain. 

The effect the two produce is similar in the sense that it ensure that turbulent vortices don't recirculate back into your domain through the outflow boundary.

Regards

Goran

________________________________________
From: nek5000-users-bounces at lists.mcs.anl.gov <nek5000-users-bounces at lists.mcs.anl.gov> on behalf of nek5000-users at lists.mcs.anl.gov <nek5000-users at lists.mcs.anl.gov>
Sent: Thursday, March 10, 2016 1:14 AM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] convective boundary conditions

Hi Goran,

thank you for your answer. However I still not understand very well what
happens in your simulation if you are using unphysical boundary
conditions... Activating turb_outflow subroutine, are you emulating a
suction (from the boundary to the inner domain)? In this case, is it not
affecting your physical results from your numerical simulation? Please,
correct me if I am wrong...

One more question, the effect that you are producing with the
turb_outflow subroutine, is it similar to applying convective boundary
condition?

Thanks again.
SL



El 10-03-2016 04:27, nek5000-users at lists.mcs.anl.gov escribió:
> SL,
>
> I had the same problem before. I've had to set rq = 200. If your flow
> is very turbulent, just turn up that value a lot.
>
> No, it's not physical because you're imposing a divergence on the last
> layer of elements. You can just exclude that part of your solution in
> your post processing.
>
> Regards,
>
> Goran
>
>
>> On Mar 9, 2016, at 21:58, "nek5000-users at lists.mcs.anl.gov"
>> <nek5000-users at lists.mcs.anl.gov> wrote:
>>
>> Hi Neks,
>>
>> regarding my question about convective boundary conditions:
>>
>> In the problem that I am solving, there are vortex traveling across
>> the boundary. Using O boundary conditions, the solution blows up (due
>> to the negative flux of the vortex). On the contrary, if I use ON, the
>> simulation continues running, but I get very strong reflections.
>>
>> In addition, although my flow is laminar, if I use turb_outflow
>> subroutine (with O boundary conditions) with rq=2, the simulation
>> works better, but at a certain moment, it also crashes. I am wondering
>> if I should increase rq to, let say 3, or maybe I should use a
>> different boundary condition, i.e. non reflective.
>>
>> I would like to ask, if I am using turb_outflow subroutine I am
>> imposing grad U>0 in the boundary (please correct me if I am wrong).
>> The question is, is it physical? What I am doing is
>> -pn+(1/Re)(n*grad(U))>0 ? What does it mean?
>>
>> Finally, I have seen that in 2015 you implemented some non-reflective
>> boundary conditions and they were working very well. Please, could you
>> help me with this issue? (i.e.: a piece of advise or some procedure to
>> follow...)
>>
>> Thank you very much in advance.
>> SL
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