Find the normal gradient of field variable at domain edges

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Dear all,

How can I find the gradient of a field variable on the edge of the domain. Example I am trying to find the gradient of gas saturation (s_g) to formulate an outflow boundary condition at the exit of a porous domain, and I cannot use the d-operator as it is not well defined, and I use the dtang operator as suggested. Here, the operator gives a finite value to x comp of gradient which is tangential to the edge but 0 to the y comp of gradient. I know s_g has a normal gradient to the boundary. Which operator will give me this value at the edge?

Best Regards Amit


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