# Specifying boundary conditions at $\infty$

I am trying to solve the coupled Ekman layer solutions numerically, but I am not sure how to enter the boundary conditions or begin to define the equations and write my code. These are my equations and boundary conditions:

$$u''(z) + c [ v(z) - v_g ] = 0$$,

$$v''(z) + c [ u(z) - u_g ] = 0$$,

$$u = 0, \, v = 0$$ at $$z = 0$$,

$$u\to u_g, v\to v_g\ {\rm as}\ z\to\infty$$.

This is the code I am using:

vg = 0;
ug = 5;
k = 0.1;
f = 0.006;
e =
NDSolve[
{u''[z] == -(f/k)*(v[z] - vg]),
v''[z] == (f/k)*(u[z] - ug),
u[0] == 0, v[0] == 0,
u[Infinity] -> ug, v[Infinity] -> vg},
{u, v}, {z, 0, 1000}]


I am getting errors because of the boundary/limit at infinity, but I am not sure how to address this.

I have values for k, f, ug and vg that I define earlier in the code.

The specific error is

Equation or list of equations expected instead of u[[Infinity]]->10 in the first argument

• You need to clean up your syntax. On the rhs of each eq, u[z] and v[z] need to be expressed as functions of z just like on the left. We cannot test your code numerically without knowing f,k,ug and vg. The equations can be solved analytically, but it is tough satisfying infinite boundary conditions. – Bill Watts Oct 23 '18 at 22:08
• Thank you. That was a dumb mistake on my part. I've solved it analytically by hand, but I need to now solve it numerically for comparison to that solution. I'll update the code with example values of the variables. – vernigan Oct 23 '18 at 23:54
• – xzczd Oct 24 '18 at 7:36

As you have correctly pointed out that the problem is due the boundary conditions at the infinity. Lets make these conditions to be on a large finite value and see which large enough values gives a reasonable result.

vg = 0;
ug = 5;
k = 0.1;
f = 0.006;
N1 = 100;

sol = NDSolve[{u''[z] == -(f/k)*(v[z] - vg),
v''[z] == (f/k)*(u[z] - ug), u[0] == 0, v[0] == 0, u[N1] == ug,
v[N1] == vg}, {u, v}, {z, 0, N1}]

Plot[Evaluate[u[z] /. (sol)], {z, 0, N1}, PlotRange -> All, Frame -> True]


Just experiment with different values of N1 and see what happens.

• This isn't a comment, it's a valid answer :) . – xzczd Oct 24 '18 at 7:38
• @xzczd thanks for confidence. I wasn’t sure. – zhk Oct 24 '18 at 7:39
• @xzczd please have a look at my problem in the chat area pdetoae`. – zhk Oct 25 '18 at 3:22
• Thank you! I think I have it now. – vernigan Nov 8 '18 at 16:57