CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > SU2

skin friction coefficient and y+

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   May 22, 2020, 01:13
Default skin friction coefficient and y+
  #1
Member
 
Join Date: Dec 2019
Posts: 30
Rep Power: 6
jcownbey is on a distinguished road
I think there is a problem with the way SU2 calculates y+ or the skin friction coefficient. The values are not consistent with each other. I am using version 7.0.4

I am looking at the boundary layer of a Mach 6 nozzle. I used the following equation to calculate y+ for compressible flow as found in Viscous Fluid Flow (White):

y+ = y*sqrt(rho_inf*cfx/rho_w/2)*U_inf*rho_w/mu_w

where cfx is the skin friction coefficient in x as output by SU2. I checked this y+ value with the y+ value that is output by SU2. The values are several orders of magnitude off.

I first calculated velocity (U) by multiplying the Mach number by the speed of sound:

U = Mach*a
a = sqrt(gamma*R*T) where gamma and R are constant

I tried again with the velocity calculated by the momentum divided by the density. I got the same result.

My contour plots and values of other variables are reasonable and expected.

Does SU2 calculate the y+ value differently or is this a bug in the coding?
jcownbey is offline   Reply With Quote

Old   May 22, 2020, 04:54
Default
  #2
pcg
Senior Member
 
Pedro Gomes
Join Date: Dec 2017
Posts: 465
Rep Power: 13
pcg is on a distinguished road
This is what the code does:
FrictionVel = sqrt(fabs(WallShearStress)/Density);
YPlus[iMarker][iVertex] = WallDistMod*FrictionVel/(Viscosity/Density);

Which is equivalent to what you have provided you use rho_inf and U_inf consistently with how the friction coefficient was computed.
The reference values SU2 uses are printed in the beginning of the simulation on a large table.
pcg is offline   Reply With Quote

Old   May 22, 2020, 06:36
Default
  #3
Member
 
Akshay Koodly
Join Date: Aug 2017
Location: The Netherlands
Posts: 43
Rep Power: 8
akoodlyr is on a distinguished road
Just to clarify are you using the "cf_x" as output by SU2 (i.e. x-component of skin friction) or did you calculate the skin friction along the tangential direction to find the new y+ value?
akoodlyr is offline   Reply With Quote

Old   May 22, 2020, 19:28
Default
  #4
Member
 
Join Date: Dec 2019
Posts: 30
Rep Power: 6
jcownbey is on a distinguished road
I used the x-component of skin friction that SU2 output
jcownbey is offline   Reply With Quote

Old   May 25, 2020, 05:27
Default
  #5
Member
 
Akshay Koodly
Join Date: Aug 2017
Location: The Netherlands
Posts: 43
Rep Power: 8
akoodlyr is on a distinguished road
You should be using the skin friction coefficient tangential to the wall. Using the x component works when the flow is only along x like channels or flat plates. You can perhaps try to use the x-component only for a flat plate to check. In general, the wall shear stress in the y+ formula refers to shear stress along the wall and not only the x-component.



The calculation in SU2 is in the Friction_Forces routine of this file https://github.com/su2code/SU2/blob/.../CNSSolver.cpp
akoodlyr is offline   Reply With Quote

Reply

Tags
skin friction coefficient, y plus

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Finding Drag Force from Skin Friction Rob Wilk Main CFD Forum 0 May 8, 2020 07:04
how to calculate time averaged skin friction coefficient adilio FLUENT 2 January 28, 2017 06:00
Skin Friction Coefficient Chart Problem rosorio FLUENT 2 January 27, 2017 17:20
why in fluent report the skin friction coefficient is positive gomnam15010 FLUENT 14 March 19, 2013 02:46
the skin friction coefficient error from Fluent sutthinan Main CFD Forum 2 November 28, 2012 14:03


All times are GMT -4. The time now is 08:16.