Selfdefined expression doesn't give right values

1

Hey everyone, I am relatively new to Fenics I don’t really know my way around it.

I have a question about my UserExpression:

class circle_source(UserExpression):
    def __init__(self, radius, center, coeff, **kwargs):
        super().__init__(**kwargs)
        self._radius = radius
        self._center = center
        self._coeff = coeff
    def eval(self, value, x):
        if (x[0] - self._center[0])**2 + (x[1] - self._center[1])**2 < self._radius**2:
            value = self._coeff
        else:
            value = 0.0
    def value_shape(self):
        return ()

Now if I project this on my finite element space and print the vertex values or plot it, they are all pretty close to 0 namely something like 10^(-320), and I don’t know why that is so.

domain = Circle(Point(0.,0.),1.0,60)
mesh = generate_mesh(domain, 10, "cgal")
V = FunctionSpace(mesh, 'CG', 1)

radius = 0.45
center = [0.2,0.5]
coeff = 3.0
coef = circle_source(radius, center, coeff)
coef_FEM = project(coef, V)
plot(coef_FEM)

index

Does anyone know, what I am doing wrong with my expression.

2

You are overwriting the value array here, and you should use:

value[0] = ...