# Applying Shear Stress at a Boundary Condition

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 March 27, 2007, 12:53 Applying Shear Stress at a Boundary Condition #1 Stephen Guest   Posts: n/a Hello Everyone, I am attempting to apply a constant shear stress to an opening boundary condition and have been at a loss in how to specify this in CFX. Does anyone know how to implement this boundary condition? I am using CFX 11.0 Thanks in advance, Stephen

 March 27, 2007, 14:03 Re: Applying Shear Stress at a Boundary Condition #2 opaque Guest   Posts: n/a Dear Stephen, That boundary condition type is not implemented in ANSYS CFX.. Would you mind sharing what that BC models at an opening? I can see it for a wall (like wind on a free surface), but not for open boundaries.. Thanks, Opaque

 March 27, 2007, 14:44 Re: Applying Shear Stress at a Boundary Condition #3 Stephen Guest   Posts: n/a Thanks for the reply Ppaque, I am modeling the surface layer of the atmospheric boundary layer. Throughout the surface layer there is a constant shear stress equivalent to the density*friction velocity. This model will eventually be applied to modeling flow over bluff bodies within the surface layer. In order to take into account the shear stress from the flow above the domain, a constant shear stress must be applied to the top of the domain. Also, implementing an opening would not constrain the flow from entering and leaving the domain if required with the increase in the pressure due to the bluff body. I know it is possible to specify a velocity of the flow on an outlet and thus generate a shear stress on the flow but this seems like the backward method to solve the problem. This type of boundary condition is also sometimes implemented when modelling flow impinging on a plate, when taking the effects of the surroundings on the radiating fluid. I though there may be a way to implement this boundary condition through a user CEL function. Stephen

 March 27, 2007, 18:55 Re: Applying Shear Stress at a Boundary Condition #4 Glenn Horrocks Guest   Posts: n/a Hi, Can't you then work out the velocity distribution from the known shear stress? You can then apply the velocity distribution as an inlet boundary. CFX would work with this approach. Glenn Horrocks

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