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July 13, 2014, 08:23 

#21  
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Quote:
thank you for your reply. I will try your methode. What can we tell through the plot of last initial pressure? Should it be decreasing? What I am doing is the DNS of turbulent pipe flow with Re5400, I would be glad to share the results with you, which Info do you need? Regards, Xu Chu 

July 13, 2014, 08:52 

#22  
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Quote:
is your approach DNS or you are using a LES model? I'm trying to simulate Moser's paper for Re_tau=395, but when I refined my grids, the secondary vortexes were dissipated and the U+ vs y+ plot didn't look like the Moser's. although for a coarse grid, the results are approximately the same as Moser's! 

July 13, 2014, 09:15 

#23  
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July 13, 2014, 11:07 

#24  
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August 30, 2016, 15:08 

#25 
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Mahdi Hosseinali
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Hi,
Has anyone else experienced this situation? I'm using the turbulentInlet BC and getting crazy pressure fluctuations all over my domain which is not physical. I need pressure data of my domain and they are getting buried in this unrealistic fluctuations. Will appreciate any help. 

November 3, 2016, 16:17 

#26 
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Declan
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I think what you are talking about is known as 'checkerboarding', a problem occuring on colocated meshes where velocity and pressure become decoupled and pressure field becomes physically unrealistic. There is another thread about it here:
http://www.cfdonline.com/Forums/mai...dproblem.html I had a problem like this and solved it by making my mesh much finer. 

November 16, 2016, 11:48 

#27 
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Mahdi Hosseinali
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Openfoam uses a RhieChow pressure velocity coupling:
http://www.cfdonline.com/Wiki/Veloc...ssure_coupling So checkerboard pressure is not the case. The problem is divergence of the velocity (when interpolated or randomly generated) is not zero. The pressure correction step is based on continuity residual and this throws off the pressure at inlet. Usually inlet P BC is zero gradient and outlet is fixedvalue, which makes a nice constant slope gradient of pressure all the way from inlet to outlet. There's a new BC on foam16 named turbulentDFSEMinlet, but I'm having trouble using it when I scale my problem. Also there's another method called flow rescaling (mentioned in Poletto's paper) that contains the unphysical pressure to inlet rather than the whole domain but I couldn't find much about it. 

June 7, 2017, 08:42 

#28  
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Jorden
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Quote:
The original pitzDaily with turbulent inlet fluctuations makes use of the inletOutlet for the outlet velocity. I can imagine that a zeroGradient outlet reflects the pressure fluctuations back into the domain, generating these unexpected fluctuations. 

March 14, 2018, 05:48 

#29 
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Mohamed Elghorab
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Dear Foamer,
I'm stuck in this problem now . Does anyone of you find a solution for this problem? the problem is this pressure fluctuations affect on the behavior of the velocity fluctuation so I can't get accurate results even for the velocity Thanks in advance Mohamed 

March 14, 2018, 06:23 

#30 
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Uwe Pilz
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I recommend st start with small fluctuations and look where the fluctuations arise form. Typical it is a boundary. You may think about the b.c. then.
__________________
Uwe Pilz  Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950) 

March 14, 2018, 09:58 

#31  
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Mohamed Elghorab
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Quote:
Dear Uwe, the problem is the inlet boundary condition is a multi equation B.C and its values differs from t1 to t2 according to the value of this difference the pressure fluctuations make this problem. but still I'm working on it now to apply a time filter to control the value of the velocity fluctuation in each time step. Thanks Mohamed 

March 15, 2018, 02:28 

#32 
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Santiago Lopez Castano
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Some commments:
1. If you are simulating turbulent flows, where fluctuations are resolved up to some sgs scale, why you find it strange to get fluctuations in one of the fields (p)? Have you made statistics on the pressure? Plotting the power spectrum of pressure should tell you whether the field is "correct". 2. Using schemes other than CD for "explicit" LES in OF is not correct. Youll be producing "aliasing errors" in the field. 3. Monitoring convergence when doing transient simulations is rather useless. You should consider monitoring mass conservation along with some volume average quantity (tke, helicity) to see "convergence. 4. Using Co = 0.5 is rather high for LES/DNS, specially on an algorithm like PISO which has an inconsistent treatment of the convective term. You want to keep Co ~ 0.1. My 0.02 cents 

March 16, 2018, 07:05 

#33  
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Mohamed Elghorab
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Quote:
Dear Santigo, Thanks for your comments. I'm using LES with CD scheme (Linear) with the algorithm pimple. In my trials I'm keeping the Co under 0.1 as you said but still the problem is that at some output time steps the pressure at the outlet is higher than that one at the inlet. and this affects the statistics of the velocity such as the decay of the homogeneous turbulence which I'm trying to get the output is not smooth and not identical even after the average in time and space. like the attached curves. I tried to use Advection outlet B.C for the velocity but still the same. 

March 16, 2018, 07:17 

#34  
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Santiago Lopez Castano
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Quote:
One last thing, don't use PIMPLE unless you have a very bad grid and/or complex geometry. Use it as last resort. Go to PISO instead. 

March 16, 2018, 09:24 

#35 
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Mohamed Elghorab
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Dear Santiago,
I'm using OFv1612+ and trying to get bypass transition in a rectangular channel. So I'm implementing an inlet B.C which gives isotropic homogeneous turbulence with control of many parameters. I'm using LES pimpleFoam as a solver and pimple algorithm with WALE as a SGS. the mesh is not complex as all of it are hex. The problem is that at some output time steps it gives pressure at the outlet higher than that of the inlet without any back flow and this affects the turbulence statics during the channel. I tried many outlet B.Cs fro the velocity and the pressure (it supposed that the normal is zeroGradient for the velocity and fixed value (zero) for the pressure) but the case still exist. For the schemes I'm using CD schemes. and the Co in most cases not exceed 0.1 Could you please tell me if you need any extra data about the case? Thanks in advance Mohamed 

June 19, 2018, 07:53 
Continuity in incompressible LES

#36 
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Aaron Endres
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Dear all,
I also observed the previously described pressure fluctuations in the domain of incompressible LES. After thinking about it and doing some testing, I came to the conclusion that this pressure fluctuation makes sense. Due to the time dependent inlet velocity, the mass inflow is also time dependent. When the mass flow decreases, the fluid has to be decelerated (continiuity in the incompressible case). This is achieved by a positive pressure gradient over the domain. This is why sometimes the outlet pressure is higher than the inlet pressure. The temporal average of the pressure gradient however is still negative. I hope these thoughts are of some help to future forum visitors. 

June 19, 2018, 08:27 

#37 
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Mohamed Elghorab
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Dear Aaron,
Thanks for your reply. Actually it's totally true what you mentioned. but in my case the problem was that the mean pressure gradient was also fluctuated between positive and negative. After many trials, I managed this problem by adding a correction factor for the velocity to keep constant Average mass flow rate (continuity) at the inlet of the channel. Thanks Mohamed 

August 14, 2018, 08:40 

#38  
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Quote:
Hi, what was the outcome of the ticket? There is no information visible when searching for the issue ID ... except that it seems to be no bug ... Greetings 

June 2, 2019, 23:27 

#39  
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Jianrui Zeng
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October 31, 2019, 06:53 

#40  
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Jianrui Zeng
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Quote:
The time step in my DNS case is 3e6 and I think it is relatively small. However, the max courant number is between 0.3 and 0.4. So should I decrease the time step to keep Co ~ 1? If the Co is not that low, cannot the case reach converged? Thank you. 

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fluctuation, les, pisofoam, pressure 
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