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July 19, 2013, 08:49 
Velocity curve in channel flow by pisoFoam and LES

#1 
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Xiangyin Meng
Join Date: Jun 2012
Location: London
Posts: 69
Rep Power: 5 
Dear all,
I am struggling with a trouble in pisoFoam using LES for channel flow. Basically, the velocity curve from experiment has the maximum value of about 1.15 times of inlet uniform velocity, yeah? But I got much larger max value like 1.4 times of inlet uniform velocity, and it changes sharply from wall to centre. My question is, what parameters I may make wrong, or anything I can do to develop that? P.S. I am using oneEqEddy model. OF version 2.2.0. Cheers 

September 23, 2014, 09:14 

#2 
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Florian Ries
Join Date: Feb 2014
Location: Darmstadt, Germany
Posts: 88
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Hi,
that sounds like to coarse mesh. Which LES Model do you use??? Is your case really turbulent in the LES simulation. Is your case statistically stationary. Have you checked this?? kind regards Florian 

May 18, 2015, 08:19 

#3 
New Member
marzieh goodarzi
Join Date: Feb 2014
Location: TehranIran
Posts: 4
Rep Power: 3 
hi
I have the same problem but with pimple solver. I used oneEqEddy of LES model, and used mapped inlet Method, but the maximum velocity is 1.4 that exceed the experimental data. yplus of mesh is under 1 and about 162,000 node please help me ,I am confused Last edited by m.goudarzi; May 18, 2015 at 08:22. Reason: n 

May 19, 2015, 00:41 

#4 
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Florian Ries
Join Date: Feb 2014
Location: Darmstadt, Germany
Posts: 88
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hi m.goudarzi,
do you use Van Driest damping??? The OneEqModel behaves wrong at the wall. You have to use van Driest damping!! Or you can use dynOneEq (without Van Driest damping!), but there is a bug in this model Is your domain long enough??? Your domain has to be twice the longest turbulent structures (Nyquist criteria) i.e. in a pipe the largest turbulent structures is about 67 times the pipe diameter, this leads to a length of your domain of 16 times the diameter. Is your averaging time long enough?? How have you checked that?? Is your flow statistically statinoary before you start averaging?? How have you checked that? kind regards Florian 

May 23, 2015, 13:05 

#5  
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marzieh goodarzi
Join Date: Feb 2014
Location: TehranIran
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Hi dear Florian
Thank you for the quick reply Quote:
Quote:
Quote:
Thank you marzieh 

May 23, 2015, 13:21 

#6  
New Member
marzieh goodarzi
Join Date: Feb 2014
Location: TehranIran
Posts: 4
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This is my LES properties
Quote:


May 24, 2015, 05:13 

#7 
Member
Florian Ries
Join Date: Feb 2014
Location: Darmstadt, Germany
Posts: 88
Rep Power: 3 
Hi,
how do you simulate the channel??? I would do it like this: 1. Use pimpleFoam (no relaxation in final iteration, do residualControl with very low values!) 2. use fvOptions with pressureGradientExplicitSource (no mapping method!) 3. initialize your flow with isotropic turbulence (boxTurb) and superimpose a powerlaw profile 4. For LES use Gauss linear (no Upwind!!), corrected (or orthogonal for channel) and backward (ddt) 6. Lets say flowdirection is z and the heigth of the channel is h. As a result of the turbulence structures, the domain has to be: Lz=12h, Lx=4h, Ly=h 7. Run your simulation for 100 flow through domain and then start averaging. (How to average see tutorial channel95) 8. Include a model for your subgridstress tensor Tsgs in the oneEq. You need an averaged Tsgs for postprocessing. How to compare LES data with DNS data: With LES you simulate a filtered velocity field. The mean value of a filtered signal is equal with the mean value of the unfiltered signal. So you can compare a mean value from a LES with a mean value of a DNS. This is not true for the RMS value. The RMS value of a LES is smaller then the RMS value of the DNS. To compare this, you have to add the mean subgrid stress tensor TSgs to the covariance matrix of the LES. Averaging time: The random error Err of the average depends on the integral time scale Tint. In example, for the mean your random errror is proprtinal: Err ~ sqrt(2*Tint/tav) where tav is your averaging time. Tint is a function of space and is much higher near the wall. As a result of this, you have to average much longer at the wall. You should do a reference simulation with Smagorinsky model. For channel use Smagorinsky parameter Cs=0.05. kind regards Florian 

June 4, 2015, 14:45 
LES simulation of curved channel

#8 
New Member
marjan
Join Date: Aug 2013
Posts: 11
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Hi everyone, I model a curved channel with NS equation in curvilinear coordinates.
I imposed a fix pressure gradient in x direction which decomposed along the channel in curvilinear coordinate( stream wise and span wise direction) to drive the flow. So the pressure gradient alter in steam wise and span wise direction along the channel . But my solution is not stable. so i Think it s because of unbalance between the body forces. Does some one has any idea? Should I use a fixed pressure gradient in stream wise while imposing the centrifugal force as well? 

June 15, 2015, 05:12 
pisoFoam problems

#9 
New Member
Gizela
Join Date: May 2015
Posts: 6
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Hi everybody, i´ḿ new to openFoam and i´m trying to run pisoFoam for a channel with a pier inside. I received an error (see image) and from that i tried to change somethings in my boundary conditions but still giving the same error.
So, please I need your help to find out what is going wrong, where is my failure and how can I do it right. it is attached some files (blockMeshDict, U, p, epsilon). help please. Kind regards Gizela Last edited by Gizazu; June 24, 2015 at 11:02. 

June 21, 2015, 07:28 

#10 
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marzieh goodarzi
Join Date: Feb 2014
Location: TehranIran
Posts: 4
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Hi,
Thank you very much Florian for your answer I have done all these steps, but the result did not change I do not know where is the problem 

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