.. DO NOT EDIT. .. THIS FILE WAS AUTOMATICALLY GENERATED BY SPHINX-GALLERY. .. TO MAKE CHANGES, EDIT THE SOURCE PYTHON FILE: .. "examples/plot_cahn_hilliard.py" .. LINE NUMBERS ARE GIVEN BELOW. .. only:: html .. note:: :class: sphx-glr-download-link-note :ref:`Go to the end ` to download the full example code. .. rst-class:: sphx-glr-example-title .. _sphx_glr_examples_plot_cahn_hilliard.py: Operators: Cahn Hilliard ======================== This example is based on the example in the FiPy_ library. Please see their documentation for more information about the Cahn-Hilliard equation. The "Cahn-Hilliard" equation separates a field :math:`\phi` into 0 and 1 with smooth transitions. .. math:: \frac{\partial \phi}{\partial t} = \nabla \cdot D \nabla \left( \frac{\partial f}{\partial \phi} - \epsilon^2 \nabla^2 \phi \right) Where :math:`f` is the energy function :math:`f = ( a^2 / 2 )\phi^2(1 - \phi)^2` which drives :math:`\phi` towards either 0 or 1, this competes with the term :math:`\epsilon^2 \nabla^2 \phi` which is a diffusion term that creates smooth changes in :math:`\phi`. The equation can be factored: .. math:: \frac{\partial \phi}{\partial t} = \nabla \cdot D \nabla \psi \\ \psi = \frac{\partial^2 f}{\partial \phi^2} (\phi - \phi^{\text{old}}) + \frac{\partial f}{\partial \phi} - \epsilon^2 \nabla^2 \phi Here we will need the derivatives of :math:`f`: .. math:: \frac{\partial f}{\partial \phi} = (a^2/2)2\phi(1-\phi)(1-2\phi) \frac{\partial^2 f}{\partial \phi^2} = (a^2/2)2[1-6\phi(1-\phi)] The implementation below uses backwards Euler in time with an exponentially increasing time step. The initial :math:`\phi` is a normally distributed field with a standard deviation of 0.1 and mean of 0.5. The grid is 60x60 and takes a few seconds to solve ~130 times. The results are seen below, and you can see the field separating as the time increases. .. _FiPy: https://github.com/usnistgov/fipy .. http://www.ctcms.nist.gov/fipy/examples/cahnHilliard/generated/examples.cahnHilliard.mesh2DCoupled.html .. GENERATED FROM PYTHON SOURCE LINES 45-117 .. image-sg:: /examples/images/sphx_glr_plot_cahn_hilliard_001.png :alt: Elapsed Time: 0.1, Elapsed Time: 0.3, Elapsed Time: 0.7, Elapsed Time: 1.7, Elapsed Time: 4.4, Elapsed Time: 11.7, Elapsed Time: 30.5, Elapsed Time: 75.1 :srcset: /examples/images/sphx_glr_plot_cahn_hilliard_001.png :class: sphx-glr-single-img .. rst-class:: sphx-glr-script-out .. code-block:: none 0 0.006737946999085467 10 0.09636267449939614 20 0.24412886910986079 30 0.4877541372545481 40 0.8894242989247158 50 1.5516664382758794 60 2.643519139778099 70 4.443679913216204 80 7.411643271063606 90 12.304987589805194 100 20.37274845297408 110 33.67423739500265 120 55.604685145707705 | .. code-block:: Python import discretize import numpy as np import matplotlib.pyplot as plt from scipy.sparse.linalg import spsolve def run(plotIt=True, n=60): np.random.seed(5) # Here we are going to rearrange the equations: # (phi_ - phi)/dt = A*(d2fdphi2*(phi_ - phi) + dfdphi - L*phi_) # (phi_ - phi)/dt = A*(d2fdphi2*phi_ - d2fdphi2*phi + dfdphi - L*phi_) # (phi_ - phi)/dt = A*d2fdphi2*phi_ + A*( - d2fdphi2*phi + dfdphi - L*phi_) # phi_ - phi = dt*A*d2fdphi2*phi_ + dt*A*(- d2fdphi2*phi + dfdphi - L*phi_) # phi_ - dt*A*d2fdphi2 * phi_ = dt*A*(- d2fdphi2*phi + dfdphi - L*phi_) + phi # (I - dt*A*d2fdphi2) * phi_ = dt*A*(- d2fdphi2*phi + dfdphi - L*phi_) + phi # (I - dt*A*d2fdphi2) * phi_ = dt*A*dfdphi - dt*A*d2fdphi2*phi - dt*A*L*phi_ + phi # (dt*A*d2fdphi2 - I) * phi_ = dt*A*d2fdphi2*phi + dt*A*L*phi_ - phi - dt*A*dfdphi # (dt*A*d2fdphi2 - I - dt*A*L) * phi_ = (dt*A*d2fdphi2 - I)*phi - dt*A*dfdphi h = [(0.25, n)] M = discretize.TensorMesh([h, h]) # Constants D = a = epsilon = 1.0 I = discretize.utils.speye(M.nC) # Operators A = D * M.face_divergence * M.cell_gradient L = epsilon**2 * M.face_divergence * M.cell_gradient duration = 75 elapsed = 0.0 dexp = -5 phi = np.random.normal(loc=0.5, scale=0.01, size=M.nC) ii, jj = 0, 0 PHIS = [] capture = np.logspace(-1, np.log10(duration), 8) while elapsed < duration: dt = min(100, np.exp(dexp)) elapsed += dt dexp += 0.05 dfdphi = a**2 * 2 * phi * (1 - phi) * (1 - 2 * phi) d2fdphi2 = discretize.utils.sdiag(a**2 * 2 * (1 - 6 * phi * (1 - phi))) MAT = dt * A * d2fdphi2 - I - dt * A * L rhs = (dt * A * d2fdphi2 - I) * phi - dt * A * dfdphi phi = spsolve(MAT, rhs) if elapsed > capture[jj]: PHIS += [(elapsed, phi.copy())] jj += 1 if ii % 10 == 0: print(ii, elapsed) ii += 1 if plotIt: fig, axes = plt.subplots(2, 4, figsize=(14, 6)) axes = np.array(axes).flatten().tolist() for ii, ax in zip(np.linspace(0, len(PHIS) - 1, len(axes)), axes): ii = int(ii) M.plot_image(PHIS[ii][1], ax=ax) ax.axis("off") ax.set_title("Elapsed Time: {0:4.1f}".format(PHIS[ii][0])) if __name__ == "__main__": run() plt.show() .. rst-class:: sphx-glr-timing **Total running time of the script:** (0 minutes 6.706 seconds) .. _sphx_glr_download_examples_plot_cahn_hilliard.py: .. only:: html .. container:: sphx-glr-footer sphx-glr-footer-example .. container:: sphx-glr-download sphx-glr-download-jupyter :download:`Download Jupyter notebook: plot_cahn_hilliard.ipynb ` .. container:: sphx-glr-download sphx-glr-download-python :download:`Download Python source code: plot_cahn_hilliard.py ` .. container:: sphx-glr-download sphx-glr-download-zip :download:`Download zipped: plot_cahn_hilliard.zip ` .. only:: html .. rst-class:: sphx-glr-signature `Gallery generated by Sphinx-Gallery `_