Boundary condition on 2D square mesh

Legacy DOLFIN
1

Hello,I am beginner at FEniCS and i try to set up boundary conditions on my reaction diffusion problem, this pic is my schematic representation of 2D reaction diffusion system, i want the concentration at left to be constant value 1 and concentration gradient at bottom,top,right to be zero but i coundn’t find any example code like my problem. Could you guys give any idea or example to me.

conditionC
conditionC1169×1000 39.1 KB

this is my boundary condition code:

#boundary condition
c_left = 1.0  # Constant value on the left
c_rtb = 0.0  # Zero value on the right, top, and bottom

class BoundaryConditionExpresion(Expression):
    def eval(self,value,x):
        if near(x[0],0):
            value[0] = c_left
        else :
            value[0] = c_rtb
            
bc = DirichletBC(V, BoundaryConditionExpression(), 'near(x[1], 0) || near(x[1], 1) || near(x[0], 1)')

it gives error name ‘Expresstion’ is not defined

2

If you are a beginner of FEniCS, I would strongly recommend you to use DOLFINx, as it is recommended over legacy DOLFIN/FEniCS.
The tutorial at: The FEniCSx tutorial — FEniCSx tutorial
and in particular:
Setting multiple Dirichlet, Neumann, and Robin conditions — FEniCSx tutorial
should be of interest to you

3

Thank you for the tutorial, i will try it