#!/usr/bin/env python3
"""Test the general MG solver with Dirichlet boundary conditions.
Here we solve::
alpha phi + div{beta grad phi} + gamma . grad phi = f
with::
alpha = 1.0
beta = cos(2*pi*x)*cos(2*pi*y) + 2.0
gamma_x = sin(2*pi*x)
gamma_y = sin(2*pi*y)
f = (-16.0*pi**2*cos(2*pi*x)*cos(2*pi*y) + 2.0*pi*cos(2*pi*x) +
2.0*pi*cos(2*pi*y) - 16.0*pi**2 + 1.0)*sin(2*pi*x)*sin(2*pi*y)
This has the exact solution::
phi = sin(2.0*pi*x)*sin(2.0*pi*y)
on [0,1] x [0,1]
We use Dirichlet BCs on phi.
For the coefficients we do not have to impose the same BCs, since that
may represent a different physical quantity. beta is the one that
really matters since it must be brought to the edges. Here we take
beta to have Neumann BCs. (Dirichlet BCs for beta will force it to 0
on the boundary, which is not correct here)
"""
import matplotlib.pyplot as plt
import numpy as np
import pyro.mesh.boundary as bnd
import pyro.multigrid.general_MG as MG
import pyro.util.io_pyro as io
from pyro.mesh import patch
from pyro.util import compare, msg
# the analytic solution
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def true(x, y):
return np.sin(2.0*np.pi*x)*np.sin(2.0*np.pi*y)
# the coefficients
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def alpha(x, _y):
return np.ones_like(x)
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def beta(x, y):
return 2.0 + np.cos(2.0*np.pi*x)*np.cos(2.0*np.pi*y)
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def gamma_x(x, _y):
return np.sin(2*np.pi*x)
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def gamma_y(_x, y):
return np.sin(2*np.pi*y)
# the righthand side
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def f(x, y):
return (-16.0*np.pi**2*np.cos(2*np.pi*x)*np.cos(2*np.pi*y) +
2.0*np.pi*np.cos(2*np.pi*x) + 2.0*np.pi*np.cos(2*np.pi*y) -
16.0*np.pi**2 + 1.0)*np.sin(2*np.pi*x)*np.sin(2*np.pi*y)
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def test_general_poisson_dirichlet(N, store_bench=False, comp_bench=False, bench_dir="tests/",
make_plot=False, verbose=1, rtol=1.e-12):
"""
test the general MG solver. The return value
here is the error compared to the exact solution, UNLESS
comp_bench=True, in which case the return value is the
error compared to the stored benchmark
"""
# test the multigrid solver
nx = N
ny = nx
# create the coefficient variable
g = patch.Grid2d(nx, ny, ng=1)
d = patch.CellCenterData2d(g)
bc_c = bnd.BC(xlb="neumann", xrb="neumann",
ylb="neumann", yrb="neumann")
d.register_var("alpha", bc_c)
d.register_var("beta", bc_c)
d.register_var("gamma_x", bc_c)
d.register_var("gamma_y", bc_c)
d.create()
a = d.get_var("alpha")
a[:, :] = alpha(g.x2d, g.y2d)
b = d.get_var("beta")
b[:, :] = beta(g.x2d, g.y2d)
gx = d.get_var("gamma_x")
gx[:, :] = gamma_x(g.x2d, g.y2d)
gy = d.get_var("gamma_y")
gy[:, :] = gamma_y(g.x2d, g.y2d)
# create the multigrid object
a = MG.GeneralMG2d(nx, ny,
xl_BC_type="dirichlet", yl_BC_type="dirichlet",
xr_BC_type="dirichlet", yr_BC_type="dirichlet",
coeffs=d,
verbose=verbose, vis=0, true_function=true)
# initialize the solution to 0
a.init_zeros()
# initialize the RHS using the function f
rhs = f(a.x2d, a.y2d)
a.init_RHS(rhs)
# solve to a relative tolerance of 1.e-11
a.solve(rtol=1.e-11)
# alternately, we can just use smoothing by uncommenting the following
# a.smooth(a.nlevels-1,50000)
# get the solution
v = a.get_solution()
# compute the error from the analytic solution
b = true(a.x2d, a.y2d)
e = v - b
enorm = e.norm()
print(" L2 error from true solution = %g\n rel. err from previous cycle = %g\n num. cycles = %d" %
(enorm, a.relative_error, a.num_cycles))
# plot the solution
if make_plot:
plt.clf()
plt.figure(figsize=(10.0, 4.0), dpi=100, facecolor='w')
plt.subplot(121)
img1 = plt.imshow(np.transpose(v.v()),
interpolation="nearest", origin="lower",
extent=[a.xmin, a.xmax, a.ymin, a.ymax])
plt.xlabel("x")
plt.ylabel("y")
plt.title(f"nx = {nx}")
plt.colorbar(img1)
plt.subplot(122)
img2 = plt.imshow(np.transpose(e.v()),
interpolation="nearest", origin="lower",
extent=[a.xmin, a.xmax, a.ymin, a.ymax])
plt.xlabel("x")
plt.ylabel("y")
plt.title("error")
plt.colorbar(img2)
plt.tight_layout()
plt.savefig("mg_general_dirichlet_test.png")
# store the output for later comparison
bench = "mg_general_poisson_dirichlet"
my_data = a.get_solution_object()
if store_bench:
my_data.write(f"{bench_dir}/{bench}")
# do we do a comparison?
if comp_bench:
compare_file = f"{bench_dir}/{bench}"
msg.warning("comparing to: %s " % (compare_file))
bench = io.read(compare_file)
result = compare.compare(my_data, bench, rtol)
if result == 0:
msg.success(f"results match benchmark to within relative tolerance of {rtol}\n")
else:
msg.warning("ERROR: " + compare.errors[result] + "\n")
return result
# normal return -- error wrt true solution
return enorm
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def main():
N = [16, 32, 64, 128, 256, 512]
err = []
plot = False
store = False
do_compare = False
for nx in N:
if nx == max(N):
plot = True
# store = True
# do_compare = True
enorm = test_general_poisson_dirichlet(nx, make_plot=plot,
store_bench=store, comp_bench=do_compare)
err.append(enorm)
# plot the convergence
N = np.array(N, dtype=np.float64)
err = np.array(err)
plt.clf()
plt.loglog(N, err, "x", color="r")
plt.loglog(N, err[0]*(N[0]/N)**2, "--", color="k")
plt.xlabel("N")
plt.ylabel("error")
fig = plt.gcf()
fig.set_size_inches(7.0, 6.0)
plt.tight_layout()
plt.savefig("mg_general_dirichlet_converge.png")
if __name__ == "__main__":
main()
