# How to solve this type of differential equations?

1. Oct 11, 2012

### svenki7

dε/dt=d(uε)/dZ+[(e^(-k1t) - e^(-k2t)]

where ε=% area opening, u= velocity, Z=length , k1, k2= constants, t= time

Last edited: Oct 11, 2012
2. Oct 11, 2012

### Simon Bridge

You have not said if u and Z are constant - are they?

"dε/dt=d(uε)/dZ-[(e^(-k1t) - e^(-k2t)]" translates into $\LaTeX$ as:

$$\frac{d\varepsilon}{dt}-\frac{d(u\varepsilon)}{dZ} = e^{-k_1t} - e^{-k_2t}$$
Appears to be a coupled differential equation - so there must be another one for $\frac{d\varepsilon}{dZ}$ or some other information to help you out.

3. Oct 11, 2012

### Staff: Mentor

If u is a constant or a function only of t, then this problem can be solved easily using the method of characteristics. I assume those are partial derivatives with respect to t and Z. Just factor out the u from the partial with respect to z.

4. Oct 15, 2012

### svenki7

Q=u*ε;
Q= flow rate , u= velocity, ε=area
Q=flow rate is constant;

boundary conditions are
Z=0, t=0, ε=1 and u=uo
Z=0, t>0, ε=1 and u=uo

where uo= initial velocity

5. Oct 15, 2012

### haruspex

I guess you mean Q is constant wrt to t, but not wrt Z. Seems that u and epsilon are functions of both. So it would be natural to use Q in the equation instead of u.
Can you describe the physical system? It would help ensure we're all on the same page.

6. Oct 16, 2012

### Staff: Mentor

In my judgement, there is something wrong with this formulation. If the problem were truly as stated, then the throughput rate Q would be constant with z and t, and the PDE would reduce to an ODE.

This looks like the equation for the void fraction variation in some type of fixed bed operation, where the porosity is changing as a result of say dissolution or chemical reaction at the interface. Also, in my judgement, almost certainly, the d(εu)/dz term on the right had side has the wrong sign. Please provide a detailed description of the physical problem being solved so that we can check the formulation. The first step in any math modeling of a physical system is to articulate the physical mechanisms involved, and to correctly translate these physical mechanisms into the language of mathematics.