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May 22, 2020, 00:13 
skin friction coefficient and y+

#1 
Member
Join Date: Dec 2019
Posts: 30
Rep Power: 6 
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? 

May 22, 2020, 03:54 

#2 
Senior Member
Pedro Gomes
Join Date: Dec 2017
Posts: 466
Rep Power: 13 
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. 

May 22, 2020, 05:36 

#3 
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Akshay Koodly
Join Date: Aug 2017
Location: The Netherlands
Posts: 43
Rep Power: 8 
Just to clarify are you using the "cf_x" as output by SU2 (i.e. xcomponent of skin friction) or did you calculate the skin friction along the tangential direction to find the new y+ value?


May 22, 2020, 18:28 

#4 
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Join Date: Dec 2019
Posts: 30
Rep Power: 6 
I used the xcomponent of skin friction that SU2 output


May 25, 2020, 04:27 

#5 
Member
Akshay Koodly
Join Date: Aug 2017
Location: The Netherlands
Posts: 43
Rep Power: 8 
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 xcomponent 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 xcomponent.
The calculation in SU2 is in the Friction_Forces routine of this file https://github.com/su2code/SU2/blob/.../CNSSolver.cpp 

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skin friction coefficient, y plus 
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