# Wall Shear in Expression as boundary condition

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 May 10, 2016, 11:07 Wall Shear in Expression as boundary condition #1 New Member   Willian Andrećo Join Date: Dec 2015 Posts: 3 Rep Power: 10 Hi everyone, I have an expression that calculates an emission rate that I want to use as boundary condition for an wall as flux in, which is based on the shear stress. The expressions I`m using is: friction velocity = (Wall Shear/Density)^0.5 #unit [m s^-1] kl = 1e-6[m s^-1] + 6.3e-4[s^1.2 m^-1.2] * friction velocity^2.2 #unit[m s^-1] emission rate = 0.01[kg m^-3] * kl #unit [kg m^-2 s^-1] But, when I set this last expression in the additional variable flux, an error has occurred: Non-scalar variable 'Wall Shear' referenced by parameter 'Additional Variable Flux' in object '/FLOW:Flow Analysis 1/DOMAINefault Domain/BOUNDARY:interface/BOUNDARY CONDITIONS/ADDITIONAL VARIABLE:Concentration'. Non-scalars are only allowed as arguments to integrated quantity functions. I tried to simulate anyway, but in the calculation of my additional variable the program crashes. Do you know how can I solve this problem? I can input the values of shear stress from other simulation using initialize profile data. But I could not find a way to use these values in the expression.

 May 10, 2016, 21:02 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,771 Rep Power: 143 Two comments: * Does your equation work if you use the Wall Shear X components? Then you will have three equations for each XYZ direction. * I am not sure how well CFX can handle units with non-integer exponents. Most people do this in CFX by making the variables unitless so then you can use non integer exponents with no issues with units. Mazze[ITA] and jingxinrushui like this.

 May 11, 2016, 19:53 #3 New Member   Willian Andrećo Join Date: Dec 2015 Posts: 3 Rep Power: 10 Yes, CFX-Pre works with the wall shear components. I wrote the shear stress as (Wall Shear X^2 + Wall Shear Y^2 + Wall Shear Z^2)^0.5 Thank you!! Mazze[ITA] and jingxinrushui like this.

 Tags boundary condition, expressions, wall shear stress