# question about the channel395 tutorial

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 March 9, 2014, 01:22 question about the channel395 tutorial #1 Senior Member   Huang Xianbei Join Date: Sep 2013 Location: Yangzhou,China Posts: 283 Rep Power: 6 Hi, all I have just installed OF-2.1.1 which contains channelFoam solver. I'm doing a channel simulation. So firstly I want to clarify the accuracy of this solver. I ran the Allrun script in the channel395 tutorial and noticed something strange. 1. As for the case, Re_tau=395, nu=2e-5m^2/s, so u_tau=7.9e-3m/s and thus the pressure gradient can be calculated, gradP=u_tau^2/h=6.24e-5m/s^2, h is the half-width of the channel,h=1m. However, when monitoring the pressure gradient, the pressure gradient varies all the time and maintains a tendency to fall down and down. I'm confused that how can we identify the solution to reach a statical turbulent flow? 2. In the tutorial case, there are 2 folders, "0" and "0.org", take a look at into the "0" folder, it can be seen that all the initial fields are specified by nonuniform values. The only utility I know is the perturbU which can generate a parabolic velocity field for channel flow, but I don't know how to generate a proper "p" or "nusgs" field. Can someone figure it out?

 March 9, 2014, 03:01 #2 Senior Member   Huang Xianbei Join Date: Sep 2013 Location: Yangzhou,China Posts: 283 Rep Power: 6 After a period of searching , I continue my calculation for a much longer time, the velocity at the domain center oscillating around 0.15m/s(the Ubar=0.1335m/s), about 1.1times. So this is exactly what it should be. The first problem is solved. However, I'm still not quite understand the second problem. How can we initialize the p ,nusgs fields? Or my misunderstanding, just perturbU required for the velocity field?

 March 9, 2014, 09:13 #3 Senior Member   Huang Xianbei Join Date: Sep 2013 Location: Yangzhou,China Posts: 283 Rep Power: 6 In fact, if I use perturbU utility without change as this Code: ```// Expected turbulent Re(tau) Reynolds number with respect to utau Retau 395; // Streamwise component of flow. 0=x, 1=y, 2=z streamwise 0; // Spanwise component of flow. 0=x, 1=y, 2=z spanwise 2; // Halfheight of channel. This is the direction normal to both streamwise and // spanwise directions. h 1.0; // Set (overwrite) velocity to laminar profile setBulk false; // Perturb velocity with some cosine like perturbations perturb true; // Perturbation properties //wall normal circulation as a fraction of Ubar/utau duplus 0.25; //spanwise perturbation spacing in wall units betaPlus 200; //streamwise perturbation spacing in wall units alphaPlus 500; //transverse decay sigma 0.00055; //linear perturbation amplitude as a fraction of Ubar epsilon 0.005;``` The result U filed is quite different from the U in the "0" folder of the tutorial. I don't know how the U field comes from in the tutorial. If anyone knows , please tell me about that

 May 7, 2017, 08:48 #4 Member   Robert Ong Join Date: Aug 2010 Posts: 68 Rep Power: 8 Hi Huang, Have you got the answer on the second part of your question? I'm very interested to know the answer. Thanks, Robert

May 7, 2017, 23:20
#5
Senior Member

Huang Xianbei
Join Date: Sep 2013
Location: Yangzhou,China
Posts: 283
Rep Power: 6
Quote:
 Originally Posted by rob3rt 0ng Hi Huang, Have you got the answer on the second part of your question? I'm very interested to know the answer. Thanks, Robert
Hi, Robert
It has been a long time after I investigated the channel flow case. In my opinion, the fields are generated by a fully developed case so that the user can quickly has an idea of the right flow field.

Xianbei

 May 8, 2017, 00:34 #6 Member   Robert Ong Join Date: Aug 2010 Posts: 68 Rep Power: 8 Hi Huang, So, if the 0 folder has been defined a posteriori then what is the purpose for defining parameters in the 0.org folder? Thanks, Robert

May 8, 2017, 03:09
#7
Senior Member

Huang Xianbei
Join Date: Sep 2013
Location: Yangzhou,China
Posts: 283
Rep Power: 6
Quote:
 Originally Posted by rob3rt 0ng Hi Huang, So, if the 0 folder has been defined a posteriori then what is the purpose for defining parameters in the 0.org folder? Thanks, Robert
I would regard it as a guidence on setting up the boundary conditions for a case without posteriori.

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