# Camassa-Holm equation
# =====================
#
# .. rst-class:: emphasis
#
# This tutorial was contributed by `Colin Cotter
# `__.
#
# The Camassa Holm equation :cite:`CH1993` is an integrable 1+1 PDE
# which may be written in the form
#
# .. math::
# m_t + mu_x + (mu)_x = 0, \quad u - \alpha^2u_{xx} = m,
#
# solved in the interval :math:`[a,b]` either with periodic boundary
# conditions or with boundary conditions `u(a)=u(b)=0`; :math:`\alpha>0`
# is a constant that sets a lengthscale for the solution. The solution
# is entirely composed of peaked solitons corresponding to Dirac delta
# functions in :math:`m`. Further, the solution has a conserved energy,
# given by
#
# .. math::
# \int_a^b \frac{1}{2} u^2 + \frac{\alpha^2}{2} u_x^2\, \mathrm{d}x.
#
# In this example we will concentrate on the periodic boundary
# conditions case.
#
# A weak form of these equations is given by
#
# .. math::
# \int pm_t + pmu_x - p_xmu\, \mathrm{d}x=0, \quad \forall p,
#
# \int qu + \alpha^2q_xu_x - qm\, \mathrm{d}x=0, \quad \forall q.
#
# Energy conservation then follows from substituting the second equation
# into the first, and then setting :math:`p=u`,
#
# .. math::
# \dot{E} &= \frac{\mathrm{d}}{\mathrm{d}t}\int_a^b \frac{1}{2}u^2 + \frac{\alpha^2}{2}u_x^2\, \mathrm{d}x \\
# &= \int_a^b uu_t + \alpha^2 u_xu_{xt}\, \mathrm{d}x, \\
# &= \int_a^b um_t\, \mathrm{d}x, \\
# &= \int_a^b -umu_x + u_xmu\, \mathrm{d}x = 0.
#
# If we choose the same continuous finite element spaces for :math:`m` and :math:`u`
# then this proof immediately extends to the spatial discretisation, as
# noted by :cite:`Ma2010`. Further, it is a property of the implicit midpoint
# rule time discretisation that any quadratic conserved quantities of an
# ODE are also conserved by the time discretisation (see :cite:`Is2009`, for
# example). Hence, the fully discrete scheme,
#
# .. math::
# \int p(m^{n+1}-m^n) + \Delta t(pm^{n+1/2}u^{n+1/2}_x - p_xm^{n+1/2}u^{n+1/2})\,\mathrm{d}x=0, \quad \forall p\in V,
#
# \int qu^{n+1/2} + \alpha^2q_xu^{n+1/2}_x - qm^{n+1/2}\, \mathrm{d}x=0, \quad \forall q \in V,
#
# where :math:`u^{n+1/2}=(u^{n+1}+u^n)/2`,
# :math:`m^{n+1/2}=(m^{n+1}+m^n)/2`, conserves the energy exactly. This
# is a useful property since the energy is the square of the :math:`H^1`
# norm, which guarantees regularity of the numerical solution.
#
# As usual, to implement this problem, we start by importing the
# Firedrake namespace. ::
from firedrake import *
# To visualise the output, we also need to import matplotlib.pyplot to display
# the visual output ::
try:
import matplotlib.pyplot as plt
except:
warning("Matplotlib not imported")
# We then set the parameters for the scheme. ::
alpha = 1.0
alphasq = Constant(alpha**2)
dt = 0.1
Dt = Constant(dt)
# These are set with type :class:`~.Constant` so that the values can be
# changed without needing to regenerate code.
#
# We use a :func:`periodic mesh <.PeriodicIntervalMesh>` of width 40
# with 100 cells, ::
n = 100
mesh = PeriodicIntervalMesh(n, 40.0)
# and build a :class:`mixed function space <.MixedFunctionSpace>` for the
# two variables. ::
V = FunctionSpace(mesh, "CG", 1)
W = MixedFunctionSpace((V, V))
# We construct a :class:`~.Function` to store the two variables at time
# level ``n``, and :attr:`~.Function.subfunctions` it so that we can
# interpolate the initial condition into the two components. ::
w0 = Function(W)
m0, u0 = w0.subfunctions
# Then we interpolate the initial condition,
#
# .. math::
#
# u^0 = 0.2\text{sech}(x-403/15) + 0.5\text{sech}(x-203/15),
#
# into u, ::
x, = SpatialCoordinate(mesh)
u0.interpolate(0.2*2/(exp(x-403./15.) + exp(-x+403./15.))
+ 0.5*2/(exp(x-203./15.)+exp(-x+203./15.)))
# before solving for the initial condition for ``m``. This is done by
# setting up the linear problem and solving it (here we use a direct
# solver since the problem is one dimensional). ::
p = TestFunction(V)
m = TrialFunction(V)
am = p*m*dx
Lm = (p*u0 + alphasq*p.dx(0)*u0.dx(0))*dx
solve(am == Lm, m0, solver_parameters={
'ksp_type': 'preonly',
'pc_type': 'lu'
}
)
# Next we build the weak form of the timestepping algorithm. This is expressed
# as a mixed nonlinear problem, which must be written as a bilinear form
# that is a function of the output :class:`~.Function` ``w1``. ::
p, q = TestFunctions(W)
w1 = Function(W)
w1.assign(w0)
m1, u1 = split(w1)
m0, u0 = split(w0)
# Note the use of :func:`split(w1) ` here, which splits up a
# :class:`~.Function` so that it may be inserted into a UFL
# expression. ::
mh = 0.5*(m1 + m0)
uh = 0.5*(u1 + u0)
L = (
(q*u1 + alphasq*q.dx(0)*u1.dx(0) - q*m1)*dx +
(p*(m1-m0) + Dt*(p*uh.dx(0)*mh -p.dx(0)*uh*mh))*dx
)
# Since we are in one dimension, we use a direct solver for the linear
# system within the Newton algorithm. To do this, we assemble a monolithic
# rather than blocked system. ::
uprob = NonlinearVariationalProblem(L, w1)
usolver = NonlinearVariationalSolver(uprob, solver_parameters=
{'mat_type': 'aij',
'ksp_type': 'preonly',
'pc_type': 'lu'})
# Next we use the other form of :attr:`~.Function.subfunctions`, ``w0.subfunctions``,
# which is the way to split up a Function in order to access its data
# e.g. for output. ::
m0, u0 = w0.subfunctions
m1, u1 = w1.subfunctions
# We choose a final time, and initialise a :class:`~.File` object for
# storing ``u``. as well as an array for storing the function to be visualised::
T = 100.0
ufile = File('u.pvd')
t = 0.0
ufile.write(u1, time=t)
all_us = []
# We also initialise a dump counter so we only dump every 10 timesteps. ::
ndump = 10
dumpn = 0
# Now we enter the timeloop. ::
while (t < T - 0.5*dt):
t += dt
# The energy can be computed and checked. ::
#
E = assemble((u0*u0 + alphasq*u0.dx(0)*u0.dx(0))*dx)
print("t = ", t, "E = ", E)
# To implement the timestepping algorithm, we just call the solver, and assign
# ``w1`` to ``w0``. ::
#
usolver.solve()
w0.assign(w1)
# Finally, we check if it is time to dump the data. The function will be appended
# to the array of functions to be plotted later::
#
dumpn += 1
if dumpn == ndump:
dumpn -= ndump
ufile.write(u1, time=t)
all_us.append(Function(u1))
# This solution leads to emergent peakons (peaked solitons); the left
# peakon is travelling faster than the right peakon, so they collide and
# momentum is transferred to the right peakon.
#
# At last, we call the function :func:`plot ` on the final
# value to visualize it::
try:
from firedrake.pyplot import plot
fig, axes = plt.subplots()
plot(all_us[-1], axes=axes)
except Exception as e:
warning("Cannot plot figure. Error msg: '%s'" % e)
# And finally show the figure::
try:
plt.show()
except Exception as e:
warning("Cannot show figure. Error msg: '%s'" % e)
# Images of the solution at shown below.
#
# .. figure:: ch0.png
# :align: center
#
# Solution at :math:`t = 0.`
#
# .. figure:: ch25.png
# :align: center
#
# Solution at :math:`t = 2.5.`
#
# .. figure:: ch53.png
# :align: center
#
# Solution at :math:`t = 5.3.`
#
# A python script version of this demo can be found :demo:`here `.
#
# .. rubric:: References
#
# .. bibliography:: demo_references.bib
# :filter: docname in docnames