### fenics form, parameter update later

326
views
0
11 months ago by
Hi:

I am wondering if I write a form and then update them later, ex, in the time loop, update dt, will the form be updated also?

for example:
from dolfin import *

...
mesh=...
V=VectorFunctionSpace(mesh, str, order)

u=Function(V)
u_trial=TrialFunction(V)
u_test=TestFunction(V)

dt=0.1
a=Expression(..., x=1)

A=some_combination_of(u, split(u), dt, a)

F=derivative(A, u, u_test)
J=derivative(F, u, u_trial)
bc=...

for t<T:
update(dt)
update(a.x)
update(u)
solve(F==0, u, bc=bc, J=J)
​    t+=dt

Will the dt, a.x and u that is update be reflected in the final form before solve? As far as I know, the update u will do, as that provide a initial condition for newton solver if the problem is nonlinear.

Community: FEniCS Project

2
11 months ago by
Use Constant and Constant::assign. Functions can also be assigned with another Function

Edited to include an example of Expression
from dolfin import *

mesh = UnitSquareMesh(1, 1)
V = FunctionSpace(mesh, "CG", 1)
u, v = TrialFunction(V), TestFunction(V)

c = Constant(1.0)

a = u*v*dx
L = c*v*dx

u = Function(V)
print("u(0.5, 0.5):", u(0.5, 0.5))
solve(a == L, u)
print("u(0.5, 0.5):", u(0.5, 0.5))

c.assign(5.0)
w = Function(V)
solve(a == L, w)
print("w(0.5, 0.5):", w(0.5, 0.5))
u.assign(w)
print("u(0.5, 0.5):", u(0.5, 0.5))

e = Expression("val", val=1.0, degree=1)
j = e*v*dx
solve(a == j, u)
print("u(0.5, 0.5):", u(0.5, 0.5))

e.val = 5.0
solve(a == j, u)
print("u(0.5, 0.5):", u(0.5, 0.5))​
what if

F=derivative(A, u, u_test)
J=derivative(F, u, u_trial)

will the F and J be updated after update dt, u,and expression?
written 11 months ago by Yuxiang Lin
1
If you use Constant, Function and Expression, then yes. I will edit and add an example of Expression to my original answer.
written 11 months ago by Nate