# How is a fixedValue BC extrapolated to first cell?

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 February 13, 2019, 05:21 How is a fixedValue BC extrapolated to first cell? #1 New Member   Join Date: Sep 2015 Posts: 13 Rep Power: 10 Hi all, I'm running a very simple case with simpleFoam (kEpsilon), where I have a rough wall patch. For this patch I choose to use the 'noSlip' BC for velocity which turns out to be a wrapper for fixedValue with value (0 0 0). 1) How is this fixed velocity at the wall extrapolated to the first cell? (Or where can I see how the extraploation is treated/defined?) 2) Other question - In my case I have roughness elements of length ~ 0.5 x dimension of the first cell. Is there a BC type where I can account for the effect on the velocity due to the roughness of the wall Thank you! Chris

 February 13, 2019, 05:30 #2 New Member   Join Date: Sep 2015 Posts: 13 Rep Power: 10 What I mean with 'extrapolate' is, how it affects the first cells value...

February 13, 2019, 08:24
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 Originally Posted by ChrisHa 2) Other question - In my case I have roughness elements of length ~ 0.5 x dimension of the first cell. Is there a BC type where I can account for the effect on the velocity due to the roughness of the wall Chris

Ok, if I'm not much mistaken, this is done through the nut*RoughWallFunction?

February 13, 2019, 16:43
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 Originally Posted by ChrisHa 1) How is this fixed velocity at the wall extrapolated to the first cell? (Or where can I see how the extraploation is treated/defined?) Chris
Is it correct that the velocity is 'extrapolated' via a modification of the viscosity at the wall in OpenFOAM, as described in Chapter 17.8.2 of 'The Finite Volume Method in Computational Fluid Dynamics: An Advanced Introduction with OpenFOAMŪ and Matlab'*?

Quote:
 If [...] the grid point is located in the inertial sublayer and the logarithmic wall functions are applied at the first interior point C. The implementation process involves computing the shear stress using the logarithmic wall function [...]. This shear stress is used in solving the momentum equation either by invoking its value directly as a source term (τwSb) or via a modified viscosity at the wall μw [...].
*Moukalled, F., Mangani, L., Darwish, M., 2016. The Finite Volume Method in Computational Fluid Dynamics: An Advanced Introduction with OpenFOAMŪ and Matlab, 1st ed. Springer International Publishing, Cham, s.l., 242 pp.

 Tags boundary condition, rough wall, velocity bc