# Solving incompressible flow with adaptive mixed finite elements in FEniCS

This Jupyter notebook shows how to solve the lid-driven cavity benchmark for incompressible flow by solving the Navier-Stokes equations using mixed finite elements in FEniCS with goal-oriented adaptive mesh refinement (AMR).

Nomenclature
$\mathbf{x}$ point in the spatial domain
$\mathbf{u} = \mathbf{u}(\mathbf{x})$ velocity vector field
$p = p(\mathbf{x})$ pressure field
$\mu$ constant dynamic viscosity of the fluid
$\Omega$ spatial domain
$\mathbf{w} = \begin{pmatrix} \mathbf{u} \\ p \end{pmatrix}$ system solution
$\mathbf{W}$ mixed finite element function space
$\boldsymbol{\psi} = \begin{pmatrix} \boldsymbol{\psi}_u \\ \psi_p \end{pmatrix}$ mixed finite element basis functions
$\Omega_h$ discrete spatial domain, i.e. the mesh
$M$ goal functional
$\epsilon_M$ error tolerance for goal-oriented AMR

## The Navier-stokes equations for incompressible flow

The steady incompressible Navier-Stokes equations can be written as

\begin{align*} \left( \mathbf{u}\cdot\nabla \right)\mathbf{u} + \nabla p - 2\mu \nabla \cdot \mathbf{D}(\mathbf{u}) &= 0 \quad \\ \nabla \cdot \mathbf{u} &= 0 \quad \\ \end{align*}

where $\mathbf{D}(\mathbf{u}) = \mathrm{sym}(\nabla\mathbf{u}) = \frac{1}{2}\left(\nabla \mathbf{u} + \left( \nabla \mathbf{u} \right)^{\mathrm{T}} \right)$ is the Newtonian fluid's rate of strain tensor. [3]

## Python packages

Import the Python packages for use in this notebook.

We need the finite element method library FEniCS.



In [1]:

import fenics


Note
This Jupyter notebook server is using FEniCS 2017.2.0 from ppa:fenics-packages/fenics, installed via apt on Ubuntu 16.04.

FEniCS has convenient plotting features that don't require us to import matplotlib; but using matplotlib directly will allow us to annotate the plots.



In [2]:

import matplotlib



Tell this notebook to embed graphical outputs from matplotlib, includings those made by fenics.plot.



In [3]:

%matplotlib inline



## Coarse mesh

Now, define a coarse mesh on the unit square.



In [4]:

N = 4

mesh = fenics.UnitSquareMesh(N, N)



Let's look at the mesh.



In [5]:

fenics.plot(mesh)

matplotlib.pyplot.title("Coarse Mesh")

matplotlib.pyplot.xlabel("$x$")

matplotlib.pyplot.ylabel("$y$")




Out[5]:

<matplotlib.text.Text at 0x7fe0fb476cf8>



## Mixed finite element function space, test functions, and solution functions

Make the Taylor-Hood (i.e. P2P1) mixed finite element, which has been shown to be stable for the incompressible Navier-Stokes equations [3].



In [6]:

P2 = fenics.VectorElement('P', mesh.ufl_cell(), 2)

P1 = fenics.FiniteElement('P', mesh.ufl_cell(), 1)

P2P1 = fenics.MixedElement([P2, P1])


Note
fenics.FiniteElement requires the mesh.ufl_cell() argument to determine some aspects of the domain (e.g. that the spatial domain is two-dimensional).

Make the mixed finite element function space $\mathbf{W}$, which enumerates the finite element basis functions on each cell of the mesh.



In [7]:

W = fenics.FunctionSpace(mesh, P2P1)



Make the test functions $\boldsymbol{\psi}_u$ and $\psi_p$.



In [8]:

psi_u, psi_p = fenics.TestFunctions(W)



Make the system solution function $\mathbf{w} \in \mathbf{W}$ and obtain references to its components $\mathbf{u}$ and $p$.



In [9]:

w = fenics.Function(W)

u, p = fenics.split(w)



## Benchmark parameters

Set a constant dynamic viscosity $\mu$. Here we choose $\mu = 0.01$ corresponding to a Reynolds Number of $\mathrm{Re} = 100$. We define a fenics.Constant for use in the variational form so that FEniCS can more efficiently compile the finite element code.



In [10]:

dynamic_viscosity = 0.01

mu = fenics.Constant(dynamic_viscosity)



Now we have everything we need to define the nonlinear variational form. First let's define aliases for some of the build-in math functions from fenics to improve readability. We also define an alias for the integration measure $d\mathbf{x}$.

## Nonlinear variational form

We can write the nonlinear system of equations as

\begin{align*} \mathbf{F}(\mathbf{w}) = \mathbf{0} \end{align*}

To obtain the finite element weak form, we follow the standard Ritz-Galerkin method extended for mixed finite elements [1]. Therefore, we multiply the system from the left by test functions $\boldsymbol{\psi}$ from the mixed finite element function space $\mathbf{W}$ and integrate over the spatial domain $\Omega$. This gives us the variational problem: Find $\mathbf{w} \in \mathbf{W}$ such that

\begin{align*} \mathcal{F}(\boldsymbol{\psi};\mathbf{w}) = \int_\Omega \boldsymbol{\psi}^\mathrm{T} \mathbf{F}(\mathbf{w}) d\mathbf{x} = 0 \quad \forall \boldsymbol{\psi} \in \mathbf{W} \end{align*}

Integrating $\mathcal{F}$ by parts yields

\begin{align*} \mathcal{F}(\boldsymbol{\psi};\mathbf{w}) = (\boldsymbol{\psi}_u, \nabla\mathbf{u}\cdot\mathbf{u}) -(\nabla\cdot\boldsymbol{\psi}_u,p) + 2\mu(\mathbf{D}(\boldsymbol{\psi}_u),\mathbf{D}(\mathbf{u})) \\ -(\psi_p,\nabla\cdot\mathbf{u}) \end{align*}
Note
We denote integrating inner products over the domain as $(v,u) = \int_\Omega v u d \mathbf{x}$ or $(\mathbf{v},\mathbf{u}) = \int_\Omega \mathbf{v} \cdot \mathbf{u} d \mathbf{x}$.

Define the nonlinear variational form $\mathcal{F}$.



In [11]:

inner, dot, grad, div, sym = \

momentum = dot(psi_u, dot(grad(u), u)) - div(psi_u)*p \

mass = -psi_p*div(u)

F = (momentum + mass)*fenics.dx



## Linearization

Notice that $\mathcal{F}$ is a nonlinear variational form. FEniCS will solve the nonlinear problem using Newton's method. This requires computing the Jacobian (formally the Gâteaux derivative) of the nonlinear variational form, yielding a a sequence of linearized problems whose solutions may converge to approximate the nonlinear solution.

We could manually define the Jacobian; but thankfully FEniCS can do this for us.



In [12]:

JF = fenics.derivative(F, w, fenics.TrialFunction(W))


Note
When solving linear variational problems in FEniCS, one defines the linear variational form using fenics.TrialFunction instead of fenics.Function (while both approaches will need fenics.TestFunction). When solving nonlinear variational problems with FEniCS, we only need fenics.TrialFunction to define the linearized problem, since it is the linearized problem which will be assembled into a linear system and solved.

## Boundary conditions

We need boundary conditions before we can define a nonlinear variational problem (i.e. in this case a boundary value problem).

For the lid-driven cavity, we physically consider no slip velocity boundary conditions for all boundaries. These manifest as homogeneous Dirichlet boundary conditions on the fixed walls, but as a non-homogeneous Dirichlet boundary condition on the lid. Because the problem's geometry is simple, we can identify the boundaries with the following piece-wise function.

\begin{align*} \mathbf{u}(\mathbf{x}) &= \begin{cases} \begin{pmatrix} 1 \\ 0 \end{pmatrix}, && x_1 = 1 \\ \begin{pmatrix} 0 \\ 0 \end{pmatrix}, && x_0 = 0 \hspace{2mm} \mathrm{or} \hspace{2mm} x_0 = 1 \hspace{2mm} \mathrm{or} \hspace{2mm} x_1 = 0 \end{cases} \end{align*}


In [13]:

lid_velocity = (1., 0.)

lid_location = "near(x[1],  1.)"

fixed_wall_velocity = (0., 0.)

fixed_wall_locations = "near(x[0], 0.) | near(x[0], 1.) | near(x[1], 0.)"



Define the boundary conditions on the velocity subspace.



In [14]:

V = W.sub(0)

boundary_conditions = [
fenics.DirichletBC(V, lid_velocity, lid_location),
fenics.DirichletBC(V, fixed_wall_velocity, fixed_wall_locations)]



## Nonlinear variational problem

Now we have everything we need to define the variational problem.



In [15]:

problem = fenics.NonlinearVariationalProblem(F, w, boundary_conditions, JF)



## Goal-oriented adaptive mesh refinement (AMR)

We wish to solve the problem with adaptive mesh refinement (AMR). For this it helps to explain that we have already defined the discrete nonlinear variational problem using FEniCS: Find $\mathbf{w}_h \in \mathbf{W}_h \subset \mathbf{W}(\Omega)$ such that

\begin{align*} \mathcal{F}(\boldsymbol{\psi}_h;\mathbf{w}_h) = 0 \quad \forall \boldsymbol{\psi}_h \in \mathbf{W}_h \subset \mathbf{W} \end{align*}

Given this, goal-oriented AMR poses the problem: Find $\mathbf{W}_h \subset \mathbf{W}(\Omega)$ and $\mathbf{w}_h \in \mathbf{W}_h$ such that

\begin{align*} \left| M(\mathbf{w}) - M(\mathbf{w}_h) \right| < \epsilon_M \end{align*}

where $M$ is some goal functional of the solution which we integrate over the domain, and where $\epsilon_M$ is a prescribed tolerance. Note that since we do not know the exact solution $\mathbf{w}$, this method requires an error estimator. This is detailed in [2]. For our purposes, we only need to define $M$ and $\epsilon_M$.

We choose a goal involving the horizontal velocity.

\begin{align*} M = \int_\Omega u_0^2 d\mathbf{x} \end{align*}


In [16]:

M = u[0]**2*fenics.dx



Let's set the tolerance somewhat arbitrarily. For real problems of scientific or engineering interest, one might have accuracy requirements which could help drive this decision.



In [17]:

epsilon_M = 1.e-4



## The benchmark solution

Finally we instantiate the adaptive solver with our problem and goal



In [18]:



and solve the problem to the prescribed tolerance.



In [19]:

solver.solve(epsilon_M)


Note
solver.solve will modify the solution w, which means that u and p will also be modified.

To post-process the solution, we must take an extra step calling fenics.Function.leaf_node() to access the solution on the refined mesh.



In [20]:

u, p = fenics.split(w.leaf_node())



Now plot the velocity vector field



In [21]:

fenics.plot(u)

matplotlib.pyplot.title("Velocity vector field")

matplotlib.pyplot.xlabel("$x$")

matplotlib.pyplot.ylabel("$y$")




Out[21]:

<matplotlib.text.Text at 0x7fe0f7a7e390>



and also plot the adapted mesh.



In [22]:

fenics.plot(mesh.leaf_node())

matplotlib.pyplot.xlabel("$x$")

matplotlib.pyplot.ylabel("$y$")




Out[22]:

<matplotlib.text.Text at 0x7fe0f7223240>



## References

[1] F Brezzi and M Fortin. Mixed and hybrid finite element methods, volume 15. Springer Science & Business Media, 1991.

[2] W Bangerth and R Rannacher. Adaptive Finite Element Methods for Differential Equations. Springer Science & Business Media, 2003.

[3] J Donea and A Huerta. Finite element methods for flow problems. John Wiley & Sons, 2003.

[4] A Logg, KA Mardal, and GN Wells. Automated Solution of Differential Equations by the Finite Element Method, Springer, 2012.