### Prescribed displacement on one point

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0
12 months ago by
Hello,

I'am trying to prescribed a Dirichlet boundary condition on only one point located on the boundary of a body.

I use the following script (I report only the part of the script in which I try to prescribe the boundary condition, the rest of the script is classical and works well).

def P0(x, on_boundary):
tol = 1.0E-14
return on_boundary and near(x[0], 0, tol) and near(x[1], 0, tol) and near(x[2], 0, tol)

bc_P0 = DirichletBC(V, Constant((0, 0, 0)) , P0)

And I obtained the following error:

*** Warning: Found no facets matching domain for boundary condition.

It seems that the definition of a boundary does not work for only one point.
If someone can help me, it would be great.

Thank you in avance

Xavier

Community: FEniCS Project
Why do you supply three arguments to the near() function? I'm not very experienced with Fenics yet, but thus far I have only seen it used with two arguments, where the first one describes the coordinate and the second the point on the coordinate (so in your case just scrapping the tol argument). This way I have successfully implemented a prescribed displacement Dirichlet boundary condition for a single point.
written 12 months ago by Alexandra K. Diem

4
12 months ago by

Something like this worked for me:

P0    = 'near(x[0], 0.0) && near(x[1], 0.0) && near(x[2], 0.0)'
bc_P0 = DirichletBC(V, Constant(0.0, 0.0, 0.0), P0, 'pointwise')
0
12 months ago by
Try (... OR ... OR ...) instead of ANDs.
0
12 months ago by
pointwise can work.

like
class Pinpoint(SubDomain):
def __init__(self, coords):
self.coords = np.array(coords)
SubDomain.__init__(self)
def move(self, coords):
self.coords[:] = np.array(coords)
def inside(self, x, on_boundary):
return np.linalg.norm(x-self.coords) < DOLFIN_EPS
​
0
5 months ago by
The issue is that 'pointwise' is not compatible with 'on_boundary'