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Thanks for your quick reply. I was not saying UPrime2Mean is U^2. I calculate the UPrime and found it is the same as sqrt(UPrime2Mean). I got confused as eelcovv hinted " you can obtain the <u'^2> by subtracting the Umean squared from the Uprime2mean". I am not trying to offend anyone but find the right meaning of UPrime2Mean. Thanks jiejie |
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Averaging this would give <u'> (should be a value close to zero, right?). And then you compared to sqrt( UPrime2Mean ) (which is related to the rms value sqrt(<u'^2>) according to my understanding). If you are right then UPrime2Mean gives <u'>^2. Could you describe a little bit more in detail what you calculated? I really hope that UPrime2Mean_XX corresponds to <u_x'^2>, UPrime2Mean_YY to <u_y'^2> and so on. If not I have to recheck quite some of my results :D |
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I thought I might upset eelcovv as I was confused with his hint :D. Anyway, I actually output the flow field for every single time step and use the postprocessing utility sample to probe the velocity at a few locations. Then, I calculate the the veocity RMS by its definition, which should be the UPrime^2. I did this a while ago, I think the RMS value is very close to the sqrt of UPrime2Mean. That's why I am thinking UPrime2Mean is giving the correct result. jiejie |
Hi jiejie,
You are correct, my hint in an ealier post is misleading. The uprime2suare is already correct for U^2. So if you define U(t)=<U>+u'(t) then the uprime2square constains the in this order: <u'^2> <uv> <uw> <v'^2> <vw> <w'^2> no need to subtract anything. Forget my remark about that. If you want to plot for instance the turbulent kinetic energy k over a line sampled from you uprima2square you can just do plot 'sampledline_with_UPrime2Square' u 1:(0.5*($2+$5+$7)) cheers |
Hi eelcovv,
Thanks for the utility, it is really helpful! However I am not getting the results that I would expect. I have two time directories (0 and 90) and I want to get the average of the U field. Running the application I get the Umean file but the results are not correct, i.e. U in 0 : (0.1107 -0.0027 0.0006), U in 90: (0.0036 -0.1216 -0.0004) and the average I get is: (0.1017 -0.0126 0.0005). Could you give me any hint of what it can be wrong? Thanks! My controlDict: Code:
/*--------------------------------*- C++ -*----------------------------------*\ |
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I know this is an old thread, but I am trying to understand what's going on with the postAverage utility as well. Did you ever resolve the problem? I tried to replicate your results, but I instead get (0.075 -0.04233333333 0.0002666666667) in 90/UMean. Strange. I tried creating a single-cell mesh with a uniform velocity of (1 0 0) in timestep '0', and a uniform velocity of (2 0 0) in timestep '1'. Rather than a result of (1.5 0 0), I get (1.333333333 0 0)! I have attached the case for reference (see README file). Edit: This seems to be the same problem Yann was having: http://www.cfd-online.com/Forums/ope...tml#post384964 I think the key lies in the 'uniform/time' files saved in each timestep directory; the information in these files (e.g. deltaT) determines how the averaging is performed! Removing all 'uniform' directories via 'rm -r */uniform' (be careful) changes the results for me. However, it's still not quite right. For now, I am using ParaView's 'Temporal Statistics' filter and saving the resulting data to VTK (Save Data -> .vtm filetype). |
postAverage 'bug' fix
All,
EDIT: the original code for postAverage seems to work fine on OpenFOAM/2.2.0/051613. I was having problems with OpenFOAM/2.1.x/071612. If in doubt, check your setup with the case I posted above or something similar. I have found a solution, but so far it only works if your data is saved at consistent time intervals (1, 2, 3 etc., not 1, 3, 4, 4.5 5.1...). The utility was counting values from the first timestep twice when computing the average. So, for instance, the average of '1' and '2' was (1 + 1 + 2)/3 = 1.3333 instead of (1 + 2) / 2 = 1.5. This may or may not be a big deal; if you are averaging a lot of data with a small standard deviation, you won't even notice. However, if you are trying to average only a few time directories, it can make a big difference. I have fixed the problem by subtracting one from the startTime index 'timeI'. You should also clear or move your '(time)/uniform' directories, or else the '(time)/uniform/time' files will mess up the average. Perhaps changing the functionObject options in 'system/controlDict' would make this unnecessary and allow for averaging data saved at inconsistent time intervals, but I have not tried this yet. Here is my change to postAverage (see bolded line): Code:
\*---------------------------------------------------------------------------*/ Good luck, -Nuc |
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Hi everybody !
Thanks eelcovv for this very usefull tool ! However I am now trying to use it with a dynamic mesh case and I am facing some difficulties. In order to make it work with a mesh changing over the time I've added #include "dynamicFvMesh.H" in the top of the postAverage.C file, I've switched #include "createMesh.H" to #include "createDynamicFvMesh.H" and I've added the line mesh.readUpdate(); in the time loop. (see the file attached) When I run the tool it seems to do the calculations but at the end it gives me completely absurd results. Anybody knows how to make it work correctly with a dynamic mesh ? Thanks in advance |
Hello
Were you able to write the data only at last time step |
unable to run the code
sir,
plz tell me how to run the given code and how to get time averaged pressure or velocity for the whole domain in a dat file. |
The standard fieldAverage utility in OpenFOAM4.0 with the command "simpleFoam -postProcess" can also do this postAverage job.
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time average over the all time steps stored in the case file
Dear friends
I want to calculate the average of a variable like a tau over the time steps saved in the case file, and finally, get a Tau_mean file that is the time-average of all stored time steps in the last time directory. How can I do this with the postAverage tool? |
Extract Data from field average
Hi,
I have stored Tmean from 500 seconds to 1500 seconds using the field average function object. But, in the time directories, I have stored only last 50 time steps using the purge write option in the control dict. Can I use this utility to extract field (Tmean) from 1000 seconds to 1500 seconds using post processing. Please note that I have time directories stored from 1450 to 1500. Really appreaite if anyone can comment. |
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