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The directionMixed Boundary Condition

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In this blog, quick reference notes about OpenFOAM are posted in a form of a summary to address a specific topic per post.
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The directionMixed Boundary Condition

Posted June 7, 2012 at 10:53 by Hisham
Updated June 7, 2012 at 13:45 by Hisham

A mixed boundary condition is a combination of a fixedValue (Drichilet) and a fixedGradient (Neumann) boundaries determined by a fraction value. When fraction = 1, the BC is fixedValue and when it is set to zero the BC is fixedGradient. Values in between calculate a value in between (see code).

The directionMixed BC allows changing the fraction value for different directions. Assuming a plane-strain stress analysis that needs fixation in X-direction (Dx = 0) and free Y-direction (zeroGradient) for the displacement. Note that the Z-direction is irrelevant. The BC would look like:
type directionMixed;
refValue uniform (0 0 0);
refGradient uniform (0 0 0);
valueFraction uniform (1 0 0 0 0 1);
The valueFraction has one rank higher than the variable (so as the gradient ). In this case it is a symmTensor [xx xy xz yy yz zz] because the variable is a vector. The refGradient is used for all components. The Left patch in this image was set to the aforementioned BC configurations and the top patch has a fixedValue disp.

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