# Rates of Convergence¶

Copyright (C) 2010-2020 Luke Olson
Copyright (C) 2020 Andreas Kloeckner

MIT License Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
In [1]:
from firedrake import *
import numpy as np
import matplotlib.pyplot as plt


## A Boundary Value Problem¶

Consider $$u_{*} = \sin(\omega \pi x) \sin(\omega \pi y)$$ on the unit square with $\omega = 2$ as a start.

In [2]:
omega = 2


## Refine the Mesh and Check the Error¶

In [12]:
errsH0 = []
errsH1 = []
hs = []

for nx in [4, 8, 16, 32, 64, 128]: # , 256, 512]:
print(f"Now solving {nx}x{nx}...")
mesh = UnitSquareMesh(nx, nx)

V = FunctionSpace(mesh, "Lagrange", 1)
Vexact = FunctionSpace(mesh, "Lagrange", 7)

x = SpatialCoordinate(V.mesh())
u_exact = interpolate(sin(omega*pi*x[0])*sin(omega*pi*x[1]), Vexact)
f = 2*pi**2*omega**2*u_exact

# all four sides of the square
bc = DirichletBC(V, 0.0, [1, 2, 3, 4])
u = TrialFunction(V)
v = TestFunction(V)

L = f*v*dx

u = Function(V)
solve(a == L, u, bc)

EH0 = errornorm(u_exact, u, norm_type='L2')
EH1 = errornorm(u_exact, u, norm_type='H1')
errsH0.append(EH0)
errsH1.append(EH1)
hs.append(1/nx)

Now solving 4x4...
Now solving 8x8...
Now solving 16x16...
Now solving 32x32...
Now solving 64x64...
Now solving 128x128...

In [13]:
errsH0 = np.array(errsH0)
errsH1 = np.array(errsH1)
hs = np.array(hs)
rH0 = np.log(errsH0[1:] / errsH0[0:-1]) / np.log(hs[1:] / hs[0:-1])
rH1 = np.log(errsH1[1:] / errsH1[0:-1]) / np.log(hs[1:] / hs[0:-1])
print(rH0)
print(rH1)

[1.63571727 1.8993806  1.97405822 1.9934556  1.99840339]
[0.8332833  0.95536396 0.98862547 0.99714187 0.99928454]

In [15]:
plt.loglog(hs, errsH0, "o-", label="$H^0$ error")
plt.loglog(hs, errsH1, "o-", label="$H^1$ error")
plt.loglog(hs, hs**1, "o-", label="$h^1$")
plt.loglog(hs, hs**2, "o-", label="$h^2$")
plt.legend()

Out[15]:
<matplotlib.legend.Legend at 0x7fcc02d964d0>
• Now play with $\omega$ and the element order.
In [ ]: