some turbulence CFD problems

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 December 29, 2016, 17:27 some turbulence CFD problems #1 Member   mohammad fazli Join Date: Sep 2015 Posts: 57 Rep Power: 4 Hello everybody I have solved a duct with my fortran code, so I wanna validate the results. thus my main questions are: exactly what is the correct way for post processing following terms? 1- I need shear stress of the surface so I calculate velocity gradient with upper cell. my problem is which upper cells. the cells that adopt their numbers with surface cell numbers? or closest one? 1- I wanna validate law of wall. so I need u_tau (friction velocity). I have friction velocities of surfaces. but what is the friction velocities of upper cell centers in domain. I know it is the amounts of cells on wall but for node in j=4 and i=30(4 node upper the wall) is friction velocity for j=1 and i=30 correct? or I should use the amounts for closest cell? I will be so happy to help me or introducing a tutorial.

 December 31, 2016, 02:05 #2 Member   mohammad fazli Join Date: Sep 2015 Posts: 57 Rep Power: 4 No answer???? really!!!

 December 31, 2016, 04:36 #3 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 3,562 Rep Power: 40 If you are solving DNS or LES with resolved BL, you can also adopt a simple first order accurate forward/backward normal derivative. Second order accuracy requires three nodes (one on the wall, two in the fluid region). The velocity u_tau is that defined by tau_wall = rho*u_tau^2 hiscense likes this.

 December 31, 2016, 08:29 #4 Member   mohammad fazli Join Date: Sep 2015 Posts: 57 Rep Power: 4 thank you Mr Denaro, my main question is about u_tau in domain, that is defined by the wall. imagine in two or three nodes over wall, what is the amount of u_tau???

December 31, 2016, 08:35
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Filippo Maria Denaro
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Quote:
 Originally Posted by mostanad thank you Mr Denaro, my main question is about u_tau in domain, that is defined by the wall. imagine in two or three nodes over wall, what is the amount of u_tau???
that is not clear to me

 December 31, 2016, 09:28 #6 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,548 Rep Power: 25 You calculate u_tau for the wall once. For the analysis you (probably) have in mind, this value of u_tau has to be the same for all distances from the wall. mostanad likes this. __________________ Please do not send me CFD-related questions via PM

December 31, 2016, 09:30
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Quote:
 Originally Posted by FMDenaro that is not clear to me
in the domain we have a wall and many grids above it. i can find the u_tau for the wall. this amount can be changed along the duct. so what is the amount of u_tau for nodes above the wall. because i wanna define y_plus and u_plus for all nodes in domain.

December 31, 2016, 10:10
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 Originally Posted by flotus1 You calculate u_tau for the wall once. For the analysis you (probably) have in mind, this value of u_tau has to be the same for all distances from the wall.
exactly flotus1,
but if we have various u_tau on wall and also displacement in nodes in normal direction so how you relate u_tau amounts on wall to upper grids? as you see in pictures we have four nodes. so we have u_tau for them. u_tau changes from 1 to 4. but for nodes in far from wall if we have displacement, what is the u_tau amount?
Attached Images
 grid.jpg (8.7 KB, 8 views)

 December 31, 2016, 14:39 #9 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,548 Rep Power: 25 Use the value from the nearest wall node. For higher accuracy you can interpolate the values of u_tau to the wall position with the closest wall-normal distance to the cell itself. utau_wall.jpg mostanad likes this. __________________ Please do not send me CFD-related questions via PM

 December 31, 2016, 15:31 #10 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 3,562 Rep Power: 40 There is a unique velocity scale in the inner region https://www.cfd-online.com/Wiki/Intr...urbulent_flows

December 31, 2016, 15:43
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Quote:
 Originally Posted by flotus1 Use the value from the nearest wall node. For higher accuracy you can interpolate the values of u_tau to the wall position with the closest wall-normal distance to the cell itself. Attachment 52934
It means that i should calculate the distance from walls(exactly from it's nodes) and find the nearest wall distance, also that node in wall?

 January 11, 2017, 23:07 #12 Member   Jingchang.Shi Join Date: Aug 2012 Location: Xi'an, China Posts: 69 Rep Power: 7 \U_tau is defined at the wall. If you've already calculated it , you can further calculate y^+ for any node. y^+ = y \nu / u_tau.

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