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## Re: solve partial differential equation

@arnair81 wrote:

Hi LucMeekes,

I am a little confused by how you got square of d/dz ϕ(z,t,V,a) in the pde. Shouldn't it just be d/dz ϕ(z,t,V,a) as per the original equation. Kindly clarify.

That's simple.

The original equation given (by you) was NOT (something like):

But:

Which means that when the

d/dphi  of  f(phi)

changed to

V * d/dz of phi(z,t)

the square of      d/dz of phi(z,t)       emerged.

If I look at the article that you took the formulae from, the situation doesn't get much clearer.

Considering that for the derivative of f(phi) the notation f'(phi) was used, one could even argue that it should have read:

d/dt  of  f(phi)

d/dphi  of  f(phi).

I think you are more an expert on the subject at hand, so you should guide as to what the PDE should be. After that, I can help you solve it.

To get the (eventually resulting) formulae into Prime, you'll just have to retype them, or use the Mathcad to Prime converter. Note that you might need to do some rework after conversion.

Success!
Luc

## Re: solve partial differential equation

Hi LucMeekes,

Sorry, I am still a bit confused. The original equation is given by dϕ/dt +df(ϕ)/dz=0.  In the equivalent form, we can write dϕ/dt +(df(ϕ)/dϕ)*dϕ/dz=0. Now f(ϕ)=ϕ*V. So shouldn't the final PDE be dϕ/dt +V*dϕ/dz=0 since df(ϕ)/dϕ=V.

I am relatively new to Mathcad. So far I have only used prime (version 2.0 and up). I do have Mathcad 15 and I find it to be bit different (slightly confusing) to prime. That is why I asked for a way to convert it to prime. Thanks for the help.

Regards

Highlighted

## Re: solve partial differential equation

Sorry, I made a mistake: read one thing, interpreted as another.

You're right. The PDE, simplified by knowing that f(phi)=phi*V, should read:

d/dt of phi(z,t) + V * d/dz of phi(z,t).

I'll see if I can make this thing work.

Luc

Thanks

## Re: solve partial differential equation

Hi,

I believe the boundary condition should be

ϕ(z,t0,V,a) = ϕ0 (initial condition) - uniform volume fraction throughout the mixture initially

ϕ(z0,t,V,a) = ϕinfinity for t >0 (BC1) - sedimentation starts and volume fraction at the base starts to increase to final value.

ϕ(L,t,V,a) = 0 for t>0 (BC2) - concentration at surface z=L is 0 as sedimentation starts (clarification).

I think you have it slightly mixed up.

The settling plot should like what you see in the attachment. Any ideas?

Regards

## Re: solve partial differential equation

Hmm. I can only hope this helps:

But none of these phi solve your PDE anymore !

Success!
Luc

## Re: solve partial differential equation

Hi,

So if none of the phi values solve the PDE, then it means that it is probably not the solution. That's a bummer. That kind of leaves me a bit stuck.

Regards

## Re: solve partial differential equation

Sure, no problem.