# The Crank-Nicolson Method for Convection-Diffusion Systems

Here we extend our discussion and implementation of the Crank- Nicolson (CN) method to convection-diffusion systems.

To clarify nomenclature, there is a physically important difference between convection and advection. Since we are interested in the transport of a protein suspended in a (semi)liquid medium, we will use the term advection (as opposed to convection) in the following discussion.

We study the following advection-diffusion equation:

with concentration $u$, diffusion coefficient $D$, advection velocity $a$ (velocity of the medium), reaction term $f$, and domain length $L$. Our Neumann boundary conditions make certain that no amount of $u$ enters or leaves our domain.

## Crank-Nicolson Stencil for the Advection Term

We use the same grid as before. Also see our earlier discussion of finite difference stencils.

As described, for instance, in Trefethen Table 3.2.1 the Crank-Nicolson stencil for the advection term takes the following form (assuming that $a$ is constant)

Extending our previous CN stencil with this expression and applying the resulting, extended stencil to our advection-diffusion equation in grid point $(j,n)$ we obtain:

## Extended Linear System with Advection Term

As before, we define $\sigma = \frac{D \Delta t}{2 \Delta x^2}$ and $\rho = \frac{a \Delta t}{4 \Delta x}$ and rearrange the above approximation of our advection-diffusion equation:

This equation holds for all grid points with spatial indices $j=1,\ldots,J-2$. On the two boundaries of our spatial grid we apply the above Neumann boundary conditions that dictate

resulting in the following amended expressions for $j=0$ and $j=J-1$:

We use the same vector notation, $\mathbf{U}^n$ and $\mathbf{f}^n$, and matrix names $A$ and $B$ as before and can now write the above equations as a linear system in matrix notation:

where the tridiagonal matrix $A$ has the following vector of length $J$ on its diagonal

the following vector of length $J-1$ on its first superdiagonal

and the following vector of length $J-1$ on its first subdiagonal:

The tridiagonal matrix $B$ has the following vector of length $J$ on its diagonal

the following vector of length $J-1$ on its first superdiagonal

the following vector of length $J-1$ on its first subdiagonal

## An Example in Python

Let us implement the above numerical scheme for the same example we discussed previously.

Construction of the linear system we solve to integrate our advection-diffusion system numerically over one time step is very similar to our previous linear system.

We therefore expect, that we can reuse our previous code and amend merely matrices A_u, B_u, A_v, and B_v as described above.

### Our Python Code

Here is just a copy-and-paste of our previous code. Refer to that post for explanations.

All that changes in this code segment compared with before are our definition of rho and construction of matrices A_u, B_u, A_v, B_v. (We actually also change J and N - see comment below)

Note that rho is the same for both protein species U and V.

As advection velocity a we choose a negative value so that the fluid carrying proteins U and V flows from right to left.

import numpy
from matplotlib import pyplot
numpy.set_printoptions(precision=3)

L = 1.
J = 500
dx = float(L)/float(J-1)
x_grid = numpy.array([j*dx for j in range(J)])

T = 200
N = 2000
dt = float(T)/float(N-1)
t_grid = numpy.array([n*dt for n in range(N)])

D_v = float(10.)/float(100.)
D_u = 0.01 * D_v

k0 = 0.067
f_vec = lambda U, V: numpy.multiply(dt, numpy.subtract(numpy.multiply(V,

sigma_u = float(D_u*dt)/float(2.*dx*dx)
sigma_v = float(D_v*dt)/float(2.*dx*dx)

a = -0.0003
rho = float(a*dt)/float(4.*dx)

total_protein = 2.26

no_high = 10
U = numpy.array([0.1 for i in range(no_high,J)] + [2. for i in range(0,no_high)])
V = numpy.array([float(total_protein-dx*sum(U))/float(J*dx) for i in range(0,J)])

A_u = numpy.diagflat([-sigma_u+rho for i in range(J-1)], -1) +\
numpy.diagflat([1.+sigma_u+rho]+[1.+2.*sigma_u for i in range(J-2)]+[1.+sigma_u-rho]) +\
numpy.diagflat([-(sigma_u+rho) for i in range(J-1)], 1)

B_u = numpy.diagflat([sigma_u-rho for i in range(J-1)], -1) +\
numpy.diagflat([1.-sigma_u-rho]+[1.-2.*sigma_u for i in range(J-2)]+[1.-sigma_u+rho]) +\
numpy.diagflat([sigma_u+rho for i in range(J-1)], 1)

A_v = numpy.diagflat([-sigma_v+rho for i in range(J-1)], -1) +\
numpy.diagflat([1.+sigma_v+rho]+[1.+2.*sigma_v for i in range(J-2)]+[1.+sigma_v-rho]) +\
numpy.diagflat([-(sigma_v+rho) for i in range(J-1)], 1)

B_v = numpy.diagflat([sigma_v-rho for i in range(J-1)], -1) +\
numpy.diagflat([1.-sigma_v-rho]+[1.-2.*sigma_v for i in range(J-2)]+[1.-sigma_v+rho]) +\
numpy.diagflat([sigma_v+rho for i in range(J-1)], 1)

U_record = []
V_record = []

U_record.append(U)
V_record.append(V)

for ti in range(1,N):
U_new = numpy.linalg.solve(A_u, B_u.dot(U) + f_vec(U,V))
V_new = numpy.linalg.solve(A_v, B_v.dot(V) - f_vec(U,V))

U = U_new
V = V_new

U_record.append(U)
V_record.append(V)


We notice that the numerical stability of this scheme is greatly dependent on the magnitude of a.

As was discussed by Walther et al., preservation of total protein mass in a certain regime near total_protein = 2.26 is important to preserve the pattern-forming pattern of the original system (excluding the advection term).

Notice that even though we increased J = 500 and N = 2000 for a much finer numerical grid compared with before (J = 100, N = 1000), we still lose some protein mass to numerical errors.

Initial protein mass:

sum(numpy.multiply(dx,U_record[0]) + numpy.multiply(dx,V_record[0]))

2.259999999999998


Protein mass at the end of the simulation:

sum(numpy.multiply(dx,U_record[-1]) + numpy.multiply(dx,V_record[-1]))

2.2064888602225103


We can reduce the loss of protein mass by reducing the magnitude of a or refining our grid even further by increasing both J and N.

An important question to answer would be how much numerical accuracy we gain by increasing J and N.

And here is a kymograph showing the time and space behaviour of U.

U_record = numpy.array(U_record)
V_record = numpy.array(V_record)

fig, ax = pyplot.subplots()
pyplot.xlabel('x'); pyplot.ylabel('t')
heatmap = ax.pcolor(x_grid, t_grid, U_record, vmin=0., vmax=1.2)
colorbar = pyplot.colorbar(heatmap)
colorbar.set_label('concentration U')