Update ft04

[OsbertWang]

I use FEniCS installed in WSL of Windows 10.
The version is 2019.01.
Then I update demo files in the tutorial.
I change the plot code because WSL cannot use GUI.

"""
FEniCS tutorial demo program: Diffusion of a Gaussian hill.

  u'= Laplace(u) + f  in a square domain
  u = u_D             on the boundary
  u = u_0             at t = 0

  u_D = f = 0

The initial condition u_0 is chosen as a Gaussian hill.
"""

from __future__ import print_function
from dolfin import *
import time
import matplotlib.pyplot as plt

T = 2.0            # final time
num_steps = 50     # number of time steps
dt = T / num_steps # time step size

# Create mesh and define function space
nx = ny = 30
mesh = RectangleMesh(Point(-2, -2), Point(2, 2), nx, ny)
V = FunctionSpace(mesh, 'P', 1)

# Define boundary condition
def boundary(x, on_boundary):
    return on_boundary

bc = DirichletBC(V, Constant(0), boundary)

# Define initial value
u_0 = Expression('exp(-a*pow(x[0], 2) - a*pow(x[1], 2))',
                 degree=2, a=5)
u_n = interpolate(u_0, V)

# Define variational problem
u = TrialFunction(V)
v = TestFunction(V)
f = Constant(0)

F = u*v*dx + dt*dot(grad(u), grad(v))*dx - (u_n + dt*f)*v*dx
a, L = lhs(F), rhs(F)

# Create VTK file for saving solution
vtkfile = File('heat_gaussian/solution.pvd')

# Time-stepping
u = Function(V)
t = 0
for n in range(num_steps):

    # Update current time
    t += dt

    # Compute solution
    solve(a == L, u, bc)

    # Save to file and plot solution
    vtkfile << (u, t)
    plot(u)
    plt.savefig("u"+str(n)+".png")
    plt.cla

    # Update previous solution
    u_n.assign(u)

# Hold plot
# interactive()