Law of the wall plot

majidsvn7 · December 1, 2019, 09:42

Hi all,
My case is a turbulent pipe flow which has been simulated in openFoam v7.
I wanna plot famous uplus-yplus graph (law of the wall) like the attached image and I don't know what shall I do?

Bazinga · December 2, 2019, 01:51

Well, you could use the sample utility to extract the velocity at the wall. Use some software (e.g. matlab, excel, python, whatever you prefer) to calculate your non-dimensional u+ and y+ and plot it. You would need to calculate the wall shear stress for this - I believe there might be a tool for this in OpenFOAM.

It's not as straightforward as I would like it to be, maybe someone has a better idea to get this more convenient.

majidsvn7 · December 2, 2019, 03:04

Quote:

Originally Posted by Bazinga

Well, you could use the sample utility to extract the velocity at the wall. Use some software (e.g. matlab, excel, python, whatever you prefer) to calculate your non-dimensional u+ and y+ and plot it. You would need to calculate the wall shear stress for this - I believe there might be a tool for this in OpenFOAM.

It's not as straightforward as I would like it to be, maybe someone has a better idea to get this more convenient.

I've read all the threads and post about law of the wall plot in cfd-online but I can't understand properly what shall I do for plotting this graph... Yes, there is a utility to calculate yPlus (min-max-avg) and wallShearStress in openFoam and I know how to using those for this purpose but I don't understand how can use these values ( single value) to plot the graph?! yPLus in each case have a range and I've force the case to have a specific range of yPlus ?!!!!

Bazinga · December 2, 2019, 03:19

These functions give you the value at the first nodal point at the wall. What you need is the velocity profile at the wall. For example, you use the sample utility to get u and y over a line. Then you need to calc u+ and y+ by yourself before plotting.

ari003 · February 17, 2021, 08:54

Quote:

Originally Posted by majidsvn7

I've read all the threads and post about law of the wall plot in cfd-online but I can't understand properly what shall I do for plotting this graph... Yes, there is a utility to calculate yPlus (min-max-avg) and wallShearStress in openFoam and I know how to using those for this purpose but I don't understand how can use these values ( single value) to plot the graph?! yPLus in each case have a range and I've force the case to have a specific range of yPlus ?!!!!

Hello Majid, hope that you are doing good. This thread has been quite off-dated I know but I'm also in the same situation of plotting the yPlus vs UPlus plot. Did you manage how to solve it?
If so could you please share the methodology? Will be really helpful.

Tobermory · February 21, 2021, 16:24

Generate a set of probes, normal to the wall, and output the mean velocity at each probe location. Also extract the wall shear stress, so that you can calculate u*. Now calculate U+ = U/u* and y+=y.u*/nu for each probe location and hey presto you can plot U+ vs y+.

December 2, 2019, 01:51		#2
Bazinga Senior Member Join Date: Jun 2012 Location: Germany, Bochum Posts: 230 Rep Power: 15	Well, you could use the sample utility to extract the velocity at the wall. Use some software (e.g. matlab, excel, python, whatever you prefer) to calculate your non-dimensional u+ and y+ and plot it. You would need to calculate the wall shear stress for this - I believe there might be a tool for this in OpenFOAM. It's not as straightforward as I would like it to be, maybe someone has a better idea to get this more convenient. majidsvn7 likes this.

February 21, 2021, 16:24		#6
Tobermory Senior Member Join Date: Apr 2020 Location: UK Posts: 666 Rep Power: 14	Generate a set of probes, normal to the wall, and output the mean velocity at each probe location. Also extract the wall shear stress, so that you can calculate u. Now calculate U+ = U/u and y+=y.u*/nu for each probe location and hey presto you can plot U+ vs y+. hulli likes this.

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December 2, 2019, 03:19		#4
Bazinga Senior Member Join Date: Jun 2012 Location: Germany, Bochum Posts: 230 Rep Power: 15	These functions give you the value at the first nodal point at the wall. What you need is the velocity profile at the wall. For example, you use the sample utility to get u and y over a line. Then you need to calc u+ and y+ by yourself before plotting.