topic Re: Help for iteration in Mathcad

Help for iteration

NandaV — Tue, 11 Dec 2018 18:42:26 GMT

Hey guys,

I'm new in Mathcad and I'm doing my thesis now. I have some trouble finding how to do iteration in Mathcad.

So in my file, I have a reactor of 250 m. And I want to know how the concentration changes.

But, I want the simulation to run for >1 cycle. Which means I have to define a way to make the Cout (concentration out) from previous cycle, as a value for the Cin(2) concentration in of the next cycle. These are for both liquid and gas concentration.

So in summary I want to make a loop of :

COlin (x+1) = COlout (x)

COgin (x+1) = COgout (x) + COgin (x)

Help guys 😞

Thank you in advance

Re: Help for iteration

Fred_Kohlhepp — Tue, 11 Dec 2018 20:11:23 GMT

What you have is two coupled first order differential equations. Your "Given" block is missing its closure.

Given needs to be a closed block, in your case with Odesolve. Review help on solving differential equations--there should be an example that is directly applicable.

Re: Help for iteration

LucMeekes — Tue, 11 Dec 2018 21:15:39 GMT

Maybe you should look here.

https://community.ptc.com/t5/PTC-Mathcad/Integration-Solver/m-p/578494#M183033

It deals with (what appears to be/almost?) the exact same problem.

Success!
Luc

Re: Help for iteration

SavitriN — Wed, 12 Dec 2018 08:42:51 GMT

Hey Luc, unfortunately, that is my other account which I forgot the password..

So that is still my case haha :'')

Re: Help for iteration

SavitriN — Wed, 12 Dec 2018 08:45:23 GMT

Hey Fred,

Thanks for your reply. What do you mean by closed solve block?

Because in that file i'm using the "Given" which means I have the solve block already?

Now i'm just confused in trying to figure out how to loop the result as an input of the next derivation.

Re: Help for iteration

Fred_Kohlhepp — Wed, 12 Dec 2018 15:07:28 GMT

There's an old method in Mathcad that mimics Euler's approximation to ODE's. This might work for you.

(set ORIGIN to 1)

Re: Help for iteration

Fred_Kohlhepp — Wed, 12 Dec 2018 15:10:20 GMT

@SavitriN wrote:

Hey Fred,

Thanks for your reply. What do you mean by closed solve block?

Because in that file i'm using the "Given" which means I have the solve block already?

Now i'm just confused in trying to figure out how to loop the result as an input of the next derivation.

A solve block opens with "Given".

It closes with "Find" or with "Minerr" or with "Minimize", or with :Maximize," or with "ODEsolve" (maybe I've missed a couple.)

Yours doesn't close.

Re: Help for iteration

NandaV — Wed, 12 Dec 2018 16:30:17 GMT

Re: Help for iteration

NandaV — Wed, 12 Dec 2018 16:31:24 GMT

Yes I did closed it with the Odesolver…

As you can see in my attached picture.

Re: Help for iteration

Fred_Kohlhepp — Wed, 12 Dec 2018 19:04:31 GMT

So you should have two functions of the two parameters. What happens if you type C.O2lSS(2m), or try to plot C.02lSS(x) vs x?

Re: Help for iteration

Werner_E — Wed, 12 Dec 2018 22:16:55 GMT

Fred, in case you can't open and view the OPs worksheet, here is a pdf print of it.

He had already solved the ODE system and I have no clue which iteration he is asking for.

Re: Help for iteration

NandaV — Thu, 13 Dec 2018 08:43:04 GMT

Hey Werner & Fred, What i meant by iteration is a for loop.

I have a system with a fix inlet and outlet which has a length of 250 m.

1 cycle means that the system run from inlet into outlet for 1 time.

When i run the derivation, i use intial value of Clin (concentration liquid in) and Cgin (concentration gas in) which will give a result of Clout (concentration liquid out) and Cgout (concentration gas out).

For the 2nd cycle, i want to use Clout for the initial value instead of Clin. And for the gas, i will always want to use Cgin as intial value (fresh air).

Clin1cycle = Clin & Clin2cycle = Clout1cycle & Clin3cycle = Clout2cycle, etc.

Cgin1cycle = Cgin2cycle = Cgin2cycle = Cgin

However, i dont know how to translate in into for loop command in mathcad.

That's what i was trying to ask.

Thank you

Re: Help for iteration

AlanStevens — Thu, 13 Dec 2018 09:20:03 GMT

Is the attached what you are looking for?

Re: Help for iteration

AlanStevens — Thu, 13 Dec 2018 09:20:38 GMT

Is the attached what you are looking for?

Alan

Re: Help for iteration

NandaV — Thu, 13 Dec 2018 09:47:54 GMT

Hey Alan,

I think yes but i do not understand the outliq (L.total) and outgas(L.total) term and do i have to add inside the ODE solver the determination of cycles ?

Re: Help for iteration

AlanStevens — Thu, 13 Dec 2018 15:01:53 GMT

@NandaV wrote:

Hey Alan,

I think yes but i do not understand the outliq (L.total) and outgas(L.total) term and do i have to add inside the ODE solver the determination of cycles ?

The number of cycles is set by the variable ncycles that is assigned immediately before the program structure. I've arbitrarily set ncycles to 5. You will need to set it to whatever is appropriate for your problem.

I've assumed that for given inlet values of gas and liquid, the calculations are done for every section (x) from 1 to Ltotal. Within the program I've used dummy variables outliq and outgas to calculate the values of CO2l and CO2g for every section (x) (via the Odesolve block, which is now a function of the inlet values). The inlets for the next cycle, inliq and ingas are then set to the outlet values of outliq and outgas for the previous cycle, hence they are set to the last section values, namely outliq(Ltotal) and inliq(Ltotal). The final values of outliq and outgas each a vector of length Ltotal, are then returned to CO2lSS and CO2gSS respectively.

Alan

Edit: My description above is not completely correct! outliq and outgas are not vectors, but functions (of x), so that CO2lSS and CO2gSS are also functions.

Alan

(For some reason my previous reply was posted twice!!)

Re: Help for iteration

Fred_Kohlhepp — Thu, 13 Dec 2018 14:15:34 GMT

@Werner_E wrote:

Fred, in case you can't open and view the OPs worksheet, here is a pdf print of it.

He had already solved the ODE system and I have no clue which iteration he is asking for.

Thanks Werner,

Smath will open parts of xmcd drawings (not Prime), but it just skips what it doesn't understand (things like "Find" and graphs. So I can speak to some of the problems, but not what I can't see. . .

Re: Help for iteration

NandaV — Thu, 13 Dec 2018 15:38:06 GMT

Ah.. okay I understand. But is it possible to do the derivation only once?

I mean now I have the for loop cycle after the 1st cycle derivation.

Also, isn't now the system taking the value from previous x point, so the COlin (x) = COlout (X-1) instead of per cycle? (I hope you understand what I meant).

Re: Help for iteration

AlanStevens — Thu, 13 Dec 2018 15:51:09 GMT

@NandaV wrote:

I mean now I have the for loop cycle after the 1st cycle derivation.

No. I just left an example of a single cycle so you could see that changing the Odesolve block to a function didn't alter the calculation. In the attached I've removed this!

@NandaV wrote:

Also, isn't now the system taking the value from previous x point, so the COlin (x) = COlout (X-1) instead of per cycle?

No. Each cycle covers all values of x before updating the inlet values for the next cycle. This is because all values of x are considered by the Odesolve function which is called just once per cycle.

Alan

Re: Help for iteration

NandaV — Fri, 14 Dec 2018 09:40:08 GMT

Yes. I understand now. Thank you so much Alan!