[Nek5000-users] convective boundary conditions

Fri Mar 11 09:26:28 CST 2016

Hi SL,
I recommend as a first step the stabilized O condition, just to try. 
That is very easy to test. Please contact me directly via email so that 
we can discuss that (pschlatt at mech.kth.se).

Best regards,
Philipp

On 2016-03-10 13:35, nek5000-users at lists.mcs.anl.gov wrote:
> Dear Philipp,
>
> thank you very much. I am running a 3D numerical simulation of a jet
> with time dependent velocity conditions at the inlet surface
> (periodically ux=uy=uz=0). Since it is an open flow, I am having some
> troubles with the boundary conditions. Specially at the outlet surface,
> where I have a couple of vortex traveling across the boundary. The flow
> is laminar, however I have used ON boundary conditions and I have some
> reflections. I have also tried with turb_outflow subroutine, rq=2 (the
> simulation crash) and rq=100 (it is affecting the flow and it also has
> some reflections). I think that the solution to solve this problem could
> be using convective boundary conditions at the outlet...
>
> Regards,
> SL
>
> El 10-03-2016 23:16, nek5000-users at lists.mcs.anl.gov escribió:
>> 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
>>>> mailing list Nek5000-users at lists.mcs.anl.gov
>>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>>>
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