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

#1 
New Member
Join Date: Feb 2013
Posts: 3
Rep Power: 5 
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  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,311
Rep Power: 28 
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 
Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
symmetry boundary conditions in cfx  lost.identity  CFX  41  May 22, 2013 07:21 
CFX13 Post Periodic interface  EtaEta  CFX  7  December 8, 2011 18:15 
hexagonal boundary conditions  s_h  Main CFD Forum  0  August 9, 2009 22:15 
Help with boundary conditions  Dan  CFX  0  April 3, 2006 11:32 
A problem about setting boundary conditions  lyang  Main CFD Forum  0  September 19, 1999 18:29 