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June 13, 2008, 08:47 
Hello
I would like to know

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srinath
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I would like to know what the above bc sets? If i look in the directory for time 0, i find for example in p outlet { type waveTransmissive; field p; phi phi; rho rho; psi psi; gamma 1.4; fieldInf 18855.8; lInf 3; value uniform 18855.8; } What are lInf, fieldInf, psi. When is this bc applicable? 

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June 16, 2008, 13:17 
See:
http://www.openfoamwiki.

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David P. Schmidt
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February 23, 2009, 03:06 
Hi
If I am right, the waveT

#3 
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Hi
If I am right, the waveTransmissive BC mentioned above is only for compressible flow. There is one new bc called convective BC published in forum. But I am not sure we can simply obtained convection velocity needed and seems to setup the value fixed during calculation is not correct. Anyone has better idea? Regards XH 

July 14, 2010, 07:04 

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Tobias Holzinger
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Hello David,
I am actually optimizing the waveTransmissive BC by adding a plane wave filter. This filtering works quite fine, but as i am trying to extend the same BC for excitation i really need to understand the equation the waveTransmissive BC is based on... So far I arrived at some equation that looks somehow like: dp/dt=(1/dtLinf*DeltaCoeffs) p(t=n+1) + K/dt pInf Am I right so far?? Is there any location I can see the NOTES the code refers to?? Regards Tobias 

February 8, 2011, 12:58 

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Bertrand Delvaux
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Hello,
I would be interested as well... Did u find anything related to this equation in waveTransmissive? Regards, Bertrand 

February 9, 2011, 03:50 

#6 
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Tobias Holzinger
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Hi Bertrand,
I figured it out. Do you have the Paper Boundary conditions for direct simulations of compressible viscous flows T. J. Poinsot* and S. K. Lelef Center for Turbulence Research, Stanford University, Stanford, California 94305, USA NASA Ames Research Center, Moffett Field, California 94305, USA Received 23 February 1990; Revised 12 April 1991. Available online 24 September 2004. of Poinsot & Lele It is simply inserting the two LODI formulations (Eqs. 25+34) by eliminating L1. You receive a differential equation in p. L5 is replaced by K/Linf(ppinf). Then you have to reformulate it as a mixed BC. Hope this helps Tobias
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February 9, 2011, 07:16 

#7 
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Bertrand Delvaux
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Hello Tobias,
Thank you for the answer. I have the paper now, and tried to figure out the way you were suggesting. So, I would arrive at something like dp/dt + (u1c) dp/dx1 = K/Linf (ppinf) My questions are : 1. In the text, K is a constant : K=sig(1M^2)c/L, with sig a constant which gives "perfectly nonreflecting" when set to zero, meaning the mixedBoundary should be implemented such as dp/dt + (u1c) dp/dx1 = 0 ? 2. I bet u1 and x1 are related to the flow speed in one direction. Does that mean that this nonreflective Boundary Condition would be unidirectional or is there a possibility to impose this as the normalGradient mixed Boundary Condition? 3. What is the exact meaning of Linf? The length, counted from the border to specify when the "far field" condition is reached? I hope I don't bother too much with my many questions ... Thanks! Bertrand 

February 9, 2011, 07:51 

#8 
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Tobias Holzinger
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Hi Bertrand,
I'll try to answer your questions shortly... 1. setting sigma to zero theoretically causes nonreflectivity, but the relaxation to a wanted mean pressure won't take place, this leads to a drift of the mean values. I think Baum et. al computed an optimum sigma... 2. This BC only acts in normal direction 3. Yes, but some sources say you should use the domain length for optimum results. In my opinion you should try to chose a length that correlates with your first instable eigenfrequency. If you need more info, just ask...
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February 9, 2011, 08:03 

#9 
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Bertrand Delvaux
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Thank you for your insight!
I'll have a look for sigma and try different possibilities for Linf. How did you implement this Boundary Condition? Because Mixed Boundary Condition is supposed to be steadystate and doesn't allow for time derivative in its expression, if I am not wrong. I tried to look at it via GroovyBC, but again, the same problem : it looks like there is no other possibility than gradient expression, but no time derivative expression, or...? The only time derivative expression I could find is actually in convectiveOutlet. Do you have another Boundary Condition? Regards, Bertrand 

February 9, 2011, 10:04 

#10 
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Tobias Holzinger
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Hi Bertrand,
yes this BC is only for more or less steady state applications. It should also work for smooth time variant systems (a colleague of mine uses it for shock wave simulations). Yes I do have a better BC, which filters f&g waves, but unfortunately I can't give it to you before we published it. Maybe you should tell me what you want to simulate. Regards
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February 9, 2011, 15:22 

#11 
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Bertrand Delvaux
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Thank you for the reply,
Yes I understand about the still non published paper. Well, I don't use OF in any of the "classical" CFD purposes, but to investigate about the frequencies spectrum of a sound produced by the vocal tract (now approximated by a variable crosssection tube), so I use rhoPisoFoam, and definitely need some kind of nonreflective Boundary Condition at the far field, if that makes sense... Bertrand 

February 10, 2011, 03:37 

#12 
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Tobias Holzinger
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Hi Bertrand,
this BC should work fine. But I don't see your time variant system (speaking of mean values). Maybe you should try some grid expansion. This should also dampen a lot. The low frequencies then should be dampened by the BC.... Regards
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February 10, 2011, 03:56 

#13 
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Bertrand Delvaux
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Well,
Actually the acoustical results inside the tube are pretty fine (compared to experimental data) but outside the tube is another story. I need to have frequencies from 20 Hz until approx 6000 Hz, and therefore, the numerical schemes are too dispersive and dissipative outside my resonating cavity : damping subtle acoustic waves I'd like to study. Hence, I would need a nonreflective BC, of the type given in the paper from Stanford, mixing a normal gradient and a time derivative at the boundary of my domain. Regards 

February 10, 2011, 05:02 

#14 
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Tobias Holzinger
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But still, your simulation is not time variant in the sense of means...
Or am I missing something?
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February 10, 2011, 05:11 

#15 
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Bertrand Delvaux
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Well,
my mistake : I forgot to mention I am using a pulsating parabolic inlet, so timedepending (cf. infra) ... inlet { type groovyBC; variables "A=25;f=50;xp=pos().x;yp=pos().y;X=max(xp);Y=max(y p);"; valueExpression "( (sin(2*pi*f*time())>0) && (sqr(xp/X)+sqr(2*yp/(Y*sin(2*pi*f*time()))))<=1) ? vector(0, 0, ( A*(1(sqr(xp/X)+sqr(2*yp/(Y*sin(2*pi*f*time()))))) )) : vector(0, 0, 0)"; } 

February 10, 2011, 05:23 

#16 
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Tobias Holzinger
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Still, not in terms of means....
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Tobias Holzinger Chair of Thermodynamics, TU München 

February 10, 2011, 05:57 

#17 
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Bertrand Delvaux
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Well I could give it a try but I am not interested in mean values.


February 20, 2011, 17:20 

#18  
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Patty
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Quote:
I am looking for Wave Transmissive BC parameters to suppress instabilities under rhoPorousSimpleFoam. I have 2 questions: 1) considering your remark and according to your experience, do I have to understand that we have to try reduce Linf until oscillation occurs and than just increase it a bit to eliminate reflective waves or is there more subtle rule to apply to minimize the impact of Far Field position on modifying the actual value of P at the outlet? 2) Is it normal (as generally described in tutorials or reported examples) to apply in the dictionary the same pressure for outlet and Far Field position? What is the meaning? Best regards 

February 21, 2011, 03:08 

#19  
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Tobias Holzinger
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Hi Patty,
my experience might seem to be greater than it really is, but I'll try to report it to you. For the sake of sense I start answering your second question first: Quote:
Quote:
Hope this helps
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February 21, 2011, 17:53 

#20  
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Patty
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Quote:
Thanks for your explanation. Just one more precision: to have "buffer zones", I suppose you mean increasing the NUMBER of cells close to the outlet? It it so? Best regards 

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