Buckingham Pi / Dimensional analysis

[beth92]

Homework Statement

A capillary filled with water is placed in a container filled with a chemical of concentration $C_{0}$, measured in number of molecules per unit volume. The chemical diffuses into the capillary of water according to the following relation (where x is distance along capillary):

$\frac{\partial C}{\partial t} = D \frac{\partial^{2}C}{\partial x^{2}} ~~~~ C(0,t) = C_{0} ~,~ C(x,0)=0$

a) Find dimensions of diffusion coefficient D
b) For capillary with cross sectional area A, the number of molecules entering the capillary N in a fixed time T is measured. We can assume that there is a law relating the 5 quantities $D, C_{0}, N, T, A$. Use dimensional analysis to find the general form of this law.
c) If experiments show that $N$ is proportional to $\sqrt{T}$, then give the simplest law which expresses N as a function of the 4 other quantities.

Homework Equations

Buckingham Pi theorem says that the law will have the form $F(\pi_{1},\pi_{2},...) = 0$ where the $\pi_{i}$ are dimensionless quantities created using the 5 given physical quantities.

The Attempt at a Solution

a) I calculate the dimensions of D to be L²/T

b) I calculate my dimensionless quantities as:

$<br /> \pi_{1} = N \\<br /> <br /> \pi_{2} = A^{3}C_{0}^{2} \\<br /> <br /> \pi_{3} = \frac{T D}{A}<br />$

c) This is the part I'm not sure of. What does it mean by the 'simplest' law? From the buckingham Pi theorem I get that:

$N = g( A^{3}C_{0}^{2},\frac{TD}{A} )$

Where g is some unknown function.

But I'm not sure what to do with the fact that N varies with the square root of T. Can I take that term out and say:

$N = \sqrt{\frac{TD}{A}} f(A^{3}C_{0}^{2})$

This doesn't seem right/complete to me but I can't think of anything else.

 

[Orodruin]

But I'm not sure what to do with the fact that N varies with the square root of T. Can I take that term out and say:

N=TDA‾‾‾√f(A3C20) N = \sqrt{\frac{TD}{A}} f(A^{3}C_{0}^{2})

This doesn't seem right/complete to me but I can't think of anything else.

Yes, you can do this. If $f$ would depend on $\pi_3$, then $N$ would not vary as $\sqrt{T}$.