Pass variables to the expression for the initial condition

I/O
1

I’m new to Fenics and I’m trying to solve the 1D Burgers equation using this script

I’m having problems passing variables to the expression for the initial condition. For instance, if I try to set the initial condition u(x,0) = np.pi * x as follows,

#  Define the initial condition.
#
  # u_init = Expression ( "x[0]", degree = 1 ) # <- the original IC, which works
  PI = np.pi  
  u_init = Expression ( "PI * x[0])", degree = 1 )  # <- my IC, which doesn't work

then the code doesn’t work and I get this error message:

------------------- Start compiler output ------------------------
/tmp/tmpi4_ubx8m/dolfin_expression_e1b233f340eedca27ef2c4df85830884.cpp: In member function ‘virtual void dolfin::dolfin_expression_e1b233f340eedca27ef2c4df85830884::eval(Eigen::Ref<Eigen::Matrix<double, -1, 1> >, Eigen::Ref<const Eigen::Matrix<double, -1, 1> >) const’:
/tmp/tmpi4_ubx8m/dolfin_expression_e1b233f340eedca27ef2c4df85830884.cpp:61:23: error: ‘PI’ was not declared in this scope; did you mean ‘pi’?
   61 |           values[0] = PI * x[0]);
      |                       ^~
      |                       pi

And I have the same problem if I try to give the viscosity, nu, as an argument.

Why does this happen? How could it be fixed?

2

Should be:

u_init = Expression ( "PI * x[0])", PI=PI, degree = 1 )
2 Likes
3

Ok, thank you very much, this way it works perfectly!