# Laplace's equation & Fourier series - I can use cos or sin?

1. Nov 11, 2012

### Silversonic

1. The problem statement, all variables and given/known data

This is a question related to finding the velocity field of an incompressible fluid in a square pipe with sides at y = ±(a/2) and x = ± (a/2).

It comes down to solving a homogenous equation which is also Laplace's equation

$\frac {δ^2 w(x,y)^H}{δ x^2} + \frac {δ^2 w(x,y)^H}{δy^2} = 0$

using separation of variables, and combining this with the particular solution (which is only a function of y!) and using the boundary conditions;

w(a/2,y) = w(-a/2, y) = 0
w(x, a/2) = w(x, -a/2) = 0

And

$w(x,y) = w(y)^P + w(x,y)^H = w(y)^P + X(x)Y(y)$

representing the homogenous and particular solutions.

I let Y(y) be the trigonemtric function (this is mandatory) from Laplace's equation, and the boundary conditions impose that Y(a/2) = Y(-a/2) = 0.

$Y(y) = Csin(Kx) + Dcos(Kx)$

Where C, D, K are constants (K arises from the constant used in Laplace's equation).

This implies.

$Csin(Ka/2) + Dcos(Ka/2) = 0$
$-Csin(Ka/2) + Dcos(Ka/2) = 0$

My issue is that K can be resolved to having two values, either;

$K = \frac{(2m+1)\pi}{a}$

m runs from 0 to infinity

or

$K = \frac{2m\pi}{a}$

m runs from 1 to infinity.

And in the first case, C = 0. In the second case D = 0.

My confusion is the simple question: Which is the right one? I go on to add to w(x,y) a linear superposition of solutions to Laplace's equation which uses the value of K throughout, however does this produce the same answer? How can I know?

If I follow through with C = 0 or D = 0, then I get two very different answers for each - and I can't tell if they're the same or not. (The mathematics is long and complicated that I would rather not post it here).

However, if I were to change my co-ordinate system (by shifting the y-axis down by a/2), the boundary conditions become w(x,0) = w(x,a). This is exactly the same situation, except that I find that D = 0 is a mandatory requirement, and that

$K = \frac {n\pi}{a}$

So I get a definite solution. This is caused by changing the co-ordinate system, so surely either of the ways above (D = 0, or C = 0) produce the same answer?

Last edited: Nov 11, 2012