[Nek5000-users] convective boundary conditions

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

Dear all,
indeed we studied different way to have "better" outflow conditions for 
Nek. In particular we were interested in open (or semi-open) flows in 
which the nozzle condition does not properly work (at least to our 
experience, as the free-stream was disturbed). We have thus studied two 
different alternatives: the convective condition and the stabilized 
stress-free condition. The former follows essentially standard text 
books, whereas the latter was presented in a paper by Dong et al. in JCP 
(2014). If you are interested in any of these, please let me know a 
little bit better what case you are studying, such that we could prepare 
a code that suits your situation.

Best,
Philipp
KTH Mechanics

On 2016-03-10 07:21, nek5000-users at lists.mcs.anl.gov wrote:
> 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
>>> _______________________________________________ Nek5000-users
>>> mailing list Nek5000-users at lists.mcs.anl.gov
>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>> _______________________________________________ Nek5000-users
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>
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