# How to obtain the wall shear stress

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

 April 3, 2023, 08:52 How to obtain the wall shear stress #1 Member   zhangqiang Join Date: Nov 2022 Posts: 43 Rep Power: 3 How can I obtain the wall shear stress from SU2? I find there is an issue to discuss this: https://github.com/su2code/SU2/issues/399. Is wall shear stress available now? If SU2 cannot output wall shear stress, how can I obtain it? There is a defination about wall shear stress: https://www.cfd-online.com/Wiki/Wall_shear_stress, but I have no idea about how to calculate the partial differential. Any suggestion is appreciated~~~

 April 3, 2023, 11:07 #2 Senior Member   bigfoot Join Date: Dec 2011 Location: Netherlands Posts: 597 Rep Power: 17 We output the skin friction coefficient: https://www.cfd-online.com/Wiki/Skin...on_coefficient Code: `VOLUME_OUTPUT= PRIMITIVE` And you can also load the solution in paraview and compute the gradients in the Cartesian coordinate system.

 April 4, 2023, 09:33 #3 Member   zhangqiang Join Date: Nov 2022 Posts: 43 Rep Power: 3 For the skin friction coefficient: Cf = 2*tw/(rho * U*U), where Cf is the skin friction coefficient, tw is the wall shear stress, rho is the density, U is the velocity. Am I correct? Can I calculate the wall shear stress by: tw = 0.5*Cf*rho*U*U? In this formular, U is the SU2 calculated velocity. If calculate the wall shear stress by formular: https://www.cfd-online.com/Wiki/Wall_shear_stress, my understanding is: twx = mu * ux, where mu is the viscosity, ux means the gradients of velocity in the x direction. twy = mu * uy; twz = mu * uz Finally, tw = sqrt(twx^2 + twy^2 + twz^2). Am I correct?

 April 4, 2023, 10:36 #4 Senior Member   bigfoot Join Date: Dec 2011 Location: Netherlands Posts: 597 Rep Power: 17 SU2 saves the 3 components of skin friction Cf_x, Cf_y, Cf_z, so if you multiply by the dynamic pressure 0.5*rho_inf*U_inf^2 you get the shear stresses. Note that you need to multiply by the reference density and velocity, which are the freestream values provided in the config file. So the skin friction coefficients are the wall shear stresses divided by a constant value.

 April 5, 2023, 22:28 #5 Member   zhangqiang Join Date: Nov 2022 Posts: 43 Rep Power: 3 tw_x = Cf_x*0.5*rho_inf*U_inf^2. Do you mean that the rho_inf is the reference density, and U_inf is the reference velocity? If the configure file set INC_DENSITY_REF=1.0, INC_VELOCITY_REF=1.0, the tw_x=0.5*Cf_x? Am I correct?

 April 6, 2023, 09:29 #6 Senior Member   bigfoot Join Date: Dec 2011 Location: Netherlands Posts: 597 Rep Power: 17 yes, for incompressible these are the reference values. For compressible, when you choose REF_DIMENSIONALIZATION= DIMENSIONAL you need to set the FREESTREAM_DENSITY and FREESTREAM_VELOCITY