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March 20, 2013, 07:30 
airFoil2D  Calculating / Printing Reynolds number of flow

#1 
New Member
M. Abdul Akbar
Join Date: Feb 2013
Posts: 19
Rep Power: 5 
Dear Foamers,
I am running a slightly modified version of the airFoil2D tutorial in OpenFOAM. My question is; how do I calculate the Reynolds number of flow for any particular run ?. In other words is there any command or tool prebuilt in OpenFOAM wherein I can make the program print the Reynolds number ? Thanks in Advance 

March 20, 2013, 17:28 

#2 
New Member
Håkon Bartnes Line
Join Date: Mar 2013
Posts: 27
Rep Power: 5 
I don't think there is an automatic tool for this. You'll just have to choose a relevant velocity and length scale, and note down their values. For airfoils it's the convention to construct the Reynolds number with the chord length (distance from leading edge to trailing edge) and the inlet velocity. The kinematic viscosity can be found in "constant/transportProperties".


March 21, 2013, 09:30 

#3 
Senior Member
Dongyue Li
Join Date: Jun 2012
Location: Torino, Italy
Posts: 742
Rep Power: 9 
Hi,
Sorry for hajacking this thread. Does anyone check the yplus of this airFoil2D? Why yplus is so big in the boundary?Its about 17000.on the contrary in the main flow, yplus becomes zero. 

April 6, 2013, 20:39 

#4  
New Member
M. Abdul Akbar
Join Date: Feb 2013
Posts: 19
Rep Power: 5 
Quote:
Thanks for your response. Could you please check my details given below to see if I am calculating the Reynolds number correct or not. I have only included the relevant details from the relevant files to avoid clutter of data. Block Mesh File convertToMeters 1.00 Maximum ordinate used for the coordinates = 1.00 Transport properties file transportModel Newtonian; rho rho [ 1 3 0 0 0 0 0 ] 1; nu nu [ 0 2 1 0 0 0 0 ] 1e05; U internalField uniform (70 0 0); Based on the above inputs, I am calculating Reynolds number as, Re = 70 * 1 * 1 / 10^5 = 7,000,000. Please confirm if what I have calculated is correct or not. Or are there some other data too which determine the calculation of Reynolds number of flow. How about units ?. I have heard in an youtube tutorial video that the units for analysis and post processing are different in OpenFOAM or something like that. Could you please clarify on how to make the units consistent. Thanks in Advance. 

April 7, 2013, 14:09 

#5 
New Member
Håkon Bartnes Line
Join Date: Mar 2013
Posts: 27
Rep Power: 5 
Hi there,
Please keep in mind that since "nu (the kinematic viscosity) = mu (the dynamic viscosity) / rho (the density)", the Reynolds number is defined as "Re_L = U*L/nu = rho*U*L/mu". It doesn't matter in this case since rho is constant and equal to one, but the distinction would be significant if the flow were compressible, for instance. I don't know what the "maximum ordinate" means here. Is it the size of the bounding box? In that case, the Reynolds number on the wing would be significantly smaller than what you have calculated, since it's the length of the wing from tip to trailing edge which should be the characteristic length in this case. The units used in OpenFOAM are internally consistent, so as long as you get "convertToMeters" right, there are no nasty surprises that I can think of. I haven't heard of a mismatch between OpenFOAM and Paraview in this regard, but maybe some of the other foamers have? Good luck with your simulations, and don't hesitate to ask if there's anything else. Håkon 

April 7, 2013, 17:12 

#6 
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,209
Rep Power: 18 
hi Abdul akbar.
Hakon,your last sentence isn't correct here because there are some bodies that feels bad in themselves when some one wants to know more. It seems they enumerate the number of post and act jealously. I have received some messages that tells don't ask or "your post numbers is reaching mine" or "i don't want to answer for free" We can only be sorry for these people that don't know the value of knowledge. We only want to know more and more ant want to help each other more and more. 

Tags 
airfoil2d, cfd, nutilda, openfoam, reynolds number 
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