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June 24, 2015, 11:42 |
unable to get log law in LES
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#101 |
Member
SM
Join Date: Dec 2010
Posts: 97
Rep Power: 15 |
Hi, I am doing a LES of channel flow using
cyclic boundary conditions and mentioning ubar in fvOptions stretched mesh with SR=1.15 first point y+ < 1 and smagorinsky without any wall function fvSchemes and fvSoultions are form channel395 tutorial dimensions,velocity and viscosity are different but give Re_{\tau}=450. When I plot u+ vs y+ I get a very bad profile as attached in log law region. Any suggestions on what might be going wrong? |
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June 25, 2015, 03:08 |
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#102 |
Senior Member
Cedric DUPRAT
Join Date: Mar 2009
Location: Nantes, France
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Dear Canopus,
Are you sure about your Reynolds number ? notice that Re is not Re_tau What are the size of you domain, velocity and viscosity ? Could you also post your fvScheme and fvSolution ? How have you calculated your y+ and u+ ? Have you plotted U = f(y) to see the global shape of your velocity profile, is it converged, flat at the center, or has it a parabolic shape ? how have you initialized your calculation ? These answers will help us to give you feed back on your calculation Regards, Cedric |
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June 25, 2015, 09:10 |
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#103 |
Member
SM
Join Date: Dec 2010
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Dear Cedric Thanks for taking your time out!
The details of the LES are as follows Domain 8h X 2h X 8h (x y z where x is streamwise, y normal and z spanwise) Reason for having large z is to serve as precursor of wide geometry Velocity (bulk) = 26m/s kinematic viscosity = 1.46e-05 perturbUChannel utility is used to assign initial field and then large number of time steps run to achieve turbulent flow Re(bulk) ~ 30000 -> Re(tau) = 450 (based on half channel width) U in x-y plane snapshot attached fvScheme and fvSolution are attached postprocessing: postChannel utiliy run as periodic in stremawise and spanwise direction file Uf.xy is used to get dU/dy | y=0 using first row of data uTau = sqrt(nu* |1dU/dy|y=0) ReTau = uTau*h/nu u+=u/uTau y+=y*uTau/nu U = f(y) is attached |
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June 26, 2015, 02:16 |
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#104 |
Senior Member
Cedric DUPRAT
Join Date: Mar 2009
Location: Nantes, France
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Dear Canopus,
that's strange ... Are you sure your statistics are converged. I mean how many cross-flow have you done before averaging ? you should not averaged from the begining of your calculation because the initial flow will be taken into account which is wrong. You're instantaneous snapshot looks good but if you are not converged, the average field will not be ok. And according to you first graph, your velocity looks more a laminar profile than a turbulent one. otherwise, are you sure about your dudy|y=0 calculation ? if utau is wrong, you might also have strange results. I hope this will help you, Cedric |
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June 26, 2015, 02:55 |
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#105 |
Member
SM
Join Date: Dec 2010
Posts: 97
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@Cedric Thanks again for your comments!
The flow through is 5 time based on u_Tau and is less I know. Do you think that its going to improve with more flow throughs? postChannel utility I believe computes based on collapsing streamwise and spanwise data of a particular instantaneous 3-D field. So I guess time evolution of flow is not taken account? Please Correct me if wrong. Is something wrong in calculation of u_Tau from Uf.xy obtained by running Code:
postChannel -latestTime |
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June 26, 2015, 04:20 |
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#106 |
Senior Member
Cedric DUPRAT
Join Date: Mar 2009
Location: Nantes, France
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Dear Canopus,
I'm sure that if you run a calculationo from scratch (using perturbU), starting your averaging procedure at t=0 and doing only 5 cross flow it's definitly not enough. I'm not sure to understand what is a cross flow based on utau ? You could try that: double you calculation time, and check if at the end, you have got the same results. if yes, that would mean that you had been already converged, if not, that you need to converge your calculation more. About the stretching, you need to check that you have at least 3, 4 points into your viscous layer (because you're not using wall function) to calculate the shear stress correctly. if you first points is at y+ = 0.3 and the second one a 10, that's not good. I hope this will help you Cedric |
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June 26, 2015, 04:52 |
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#107 |
Member
SM
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@Cedric thanks for your prompt response.
Dividing the channel length by friction velocity kind of gives flow through time 5. Of course one can also use bulk velocity and compute a flow through time. NB: 5 flow times is from the onset of turbulence and equals almost 50 flow times based on bulk velocity from onset of turbulence. Grid has 8 points in y+ < 10 with 1st point < 1 and stretching ratio =1.15 Any comments on postChannel & u_Tau calculation? |
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June 26, 2015, 05:32 |
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#108 |
Member
SM
Join Date: Dec 2010
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@ Cedric how did you compute the u+ vs y+ log plot in your first poset of thread?
Can you please elaborate a bit? I suspect that there is a stupid mistake in my post processing. |
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June 26, 2015, 08:00 |
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#109 |
Senior Member
Cedric DUPRAT
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as you can see it was few years ago. But as you proposed, I used postChannel to get Uf and then I calculated tau_w (or utau) to get y+ and u+. It's true that your plot U=f(y) looks good (turbulent) and that your u+ = f (y+) is close to a laminar profile. Could it be something like a factor 2 in tau_w due to face value vs cell center value ? If you are confident with your calculation, maybe you could do a longer calculation .... just to check that your averaging procedure is converged. Cedric |
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June 26, 2015, 09:48 |
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#110 | |
Member
SM
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Dear Cedric seems like some light at end of tunnel..
can you please elaborate - Quote:
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June 29, 2015, 03:13 |
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#111 |
Senior Member
Cedric DUPRAT
Join Date: Mar 2009
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Dear Canopus,
well, to calculate dudy|y=0 I guess your first point is y=0 U=0. The question is then where is your second points; the cell center or the face value between the 1st and the 2nd point. Depending of that, you might have a factor 2 in the wall normal direction. To answer this question, you have to check the postChannel routine. Cedric |
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June 29, 2015, 14:51 |
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#112 |
Member
SM
Join Date: Dec 2010
Posts: 97
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Dear Cedric
I will check the postChannel. But just a thought - As long as the velocity values correspond to the wall normal values it should not matter whether it is face center or cell center as u_Tau depends on difference of u and y for first set of points? |
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July 1, 2015, 15:16 |
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#113 |
New Member
Hans Barósz
Join Date: May 2014
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Hi,
I have just sent you a message! |
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July 2, 2015, 08:41 |
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#114 | |
Senior Member
Timofey Mukha
Join Date: Mar 2012
Location: Stockholm, Sweden
Posts: 118
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Quote:
Also, don't forget to use the mean velocity, not the instantaneous one. In my experience even the patch-averaged instantaneous u_tau changes quite a lot in time. To avoid concerns with u_tau things, I suggest that you grab some DNS data and compare with what you get in global coordinates. That can give you a reasonable idea of whether you are doing fine or not. Also, don't underestimate the amount of averaging time that is needed, and as Cedric points out, don't start the averaging directly, allow some time to flush away the transients related to ICs etc. I used 1000 seconds for the Re_tau = 395 simulations. |
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July 2, 2015, 09:21 |
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#115 |
Member
SM
Join Date: Dec 2010
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Dear Timofey
Thanks for your suggestions. I already started with uTau for Channel 395 on running postChannel -latestTime outputs graphs/ Uf.xy Uf.xy contains U vs y as two columns where y is the cell center co-ordinate. I have checked the 1st y point in Uf.xy is half of 1st y point in constant/polymesh/points. As I am using these two values for computing u_Tau I think division by 2 is not needed(hopefully?) postChannel doesn't do any time averaging but spatial averaging I suppose. So what do you mean velocity? When you talk about time averaging do you mean averaging output of postChannel over several time periods? |
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July 2, 2015, 09:28 |
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#116 | |
Senior Member
Timofey Mukha
Join Date: Mar 2012
Location: Stockholm, Sweden
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Quote:
It will try to find the UMean automatically, which is the time-averged velocty. I guess you are producing that with the fieldAverage function object, right? |
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July 2, 2015, 10:58 |
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#117 |
Member
SM
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Yes I do use fieldAverage function object and have UMean file in the time directory for which I run postChannel.
I case I just have to wait for large number of flow times!!! I hope you agree to not using the factor 2? |
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July 2, 2015, 17:54 |
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#118 | |
Senior Member
Timofey Mukha
Join Date: Mar 2012
Location: Stockholm, Sweden
Posts: 118
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Quote:
You can check out my setup and the results for Re_tau = 395 at https://bitbucket.org/lesituu/channel_flow_data But I think using the averaged velocity profile should work fine, it is the same calculation. |
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July 6, 2015, 19:51 |
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#119 | |
Member
Yousef
Join Date: Feb 2015
Posts: 40
Rep Power: 11 |
Quote:
You can also use pressure gradient to calculate u_tau as indicated in the turbulent flows by Pope: dp/dx = u_tau^2 / (2*delta) which delta is the channel half width you can look for dp/dx in the timedirectory/uniform/momentumSourceProperties which is indicated as "gradient" Hope it helps. |
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July 17, 2016, 23:05 |
RAS better that LES
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#120 |
Senior Member
Elham
Join Date: Oct 2009
Posts: 184
Rep Power: 16 |
Hi every body,
I am simulating turbulent flow inside a channel. The channel size is 60*10*10(mm) and number of mesh are 300*80*80. I want to produce fully turbulent flow by means of mapping method. I produced a relatively good result with RAS. When I map data to LES the results get worse and not turbulent at all. And it takes so long time to proceed, eg. a full week just to proceed 0.2 sec. I will appreciate if anyone can give me a clue. Regards, Elham |
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