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February 8, 2013, 18:36 
How to implement Robin Boundary Conditions?

#1 
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Join Date: Feb 2013
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Hi, I'm currently working with a code that requires to assign the Boundary conditions using the coefficients on the general form of Robin Bc (a*u + b * du/dn = g)
From my understanding I can assign Dirichlet conditions by setting a=1, b=0, and g the desired specified value. Similarly for Neuman bc I need to set a=0, b=1, and g the desired value. is this correct so far? Now lets considered a quadrilateral element 12     34 where there is Neuman boundary defined over the element and dirichlet on the nodes 1 and 3 Since I'm doing groundwater modeling, lets say I have specified hydraulic head for nodes 1 and 3 H=10 m and recharge flux equal to 1e4 m/day. Lets say that the area of the element is 1e6, then 100 m^3/day are coming into that element. Using FE discretization the flux will split into 25 m^2/day per node. I assume that for the nodes 2 and 4 the coefficients have to be a=0, b=1, g=25. But what are the values for the nodes 1 and 3? Thank you George 

February 9, 2013, 04:52 

#2  
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Filippo Maria Denaro
Join Date: Jul 2010
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Quote:
Assume the point 3 is at x=y=0 coordinates. Assume a bilinear approximation on 1234: u(x,y) = c0 + c1*x +c2*y +c3*xy that over the boundary x=0 leads to du/dx = c1+c3*y Hence, your Robin Bc along edge 31 becomes a*(c2*y) + b*(c1+c3*y)= g where a,b, g must be known from your problem. 

Tags 
boundary condition, finite element method, robin conditions 
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