Runga Kutta Solver

[dcs0014]

Hi,

Fairly new to mathcad.

Can anyone tell me why the Runga kutta is giving me an error of "this function cannot be used here"?

Attached as PDF

[ValeryOchkov]

Attach please the mcdx file too

[dcs0014]

I tried but it is giving me an error "The contents of the attachment doesn't match its file type." I have some images in there and a readExcel command. Could one of those be causing the issue?

[ValeryOchkov]

---

@dcs0014 wrote:

I tried but it is giving me an error "The contents of the attachment doesn't match its file type." I have some images in there and a readExcel command. Could one of those be causing the issue?

---

Create please the rar file and send it us!

[dcs0014]

I was able to attach it as a zip.

[LucMeekes]

That's the way to do it.

Your definition of Dt(t,Tt) contains a J1 in the first (top) expression which is undefined....

Or is that just because I'm using Express, so the solve block that determines the value of J1 (through J6) does not run?

Looking more closely at that solve block...(now I'm taking advantage of the fact that Prime saves the results in the sheet) All J's ( J1 through J6) come out at the same value of 367.4419 kg/s^3. Is that to be expected?

Luc

[dcs0014]

I am using mathcad prime 4.0 and the solve block outputs values for J1-J6

[LucMeekes]

But they're all the same. Is that to be expected?

Luc

[dcs0014]

All of the surfaces in question start off at the same temp, so technically yes. I am trying to get the runga kutta working for constant radiosities before I try to make them functions of temperature (in which case they would change). The surface temps (represented by Tt) should still change as a result of the change in Tinf and the change in htop and hside.

[LucMeekes]

What is the result if you evaluate outside of the solve block:

Dt(0,Tt)=

?

[dcs0014]

It says that Dt is undefined

[dcs0014]

Actually, it says that "the array index is invalid, the index must be  an integer, not less than origin..."

Sorry for the last response

[LucMeekes]

About which array is it complaining?

Attach the file (with this evaluation in it)...

Luc

[LucMeekes · Accepted solution]

I already see it. You're using a Tt[6 in the definition of Dt(), its last, bottom equation, while Tt only has 5 elements....

See if it helps to add another element to Tt.

Success!
Luc

[dcs0014]

That was it

[RichardJ]

Please post the Excel spreadsheet with the air properties.

[RichardJ]

Where is T defined (it may be defined somewhere, but I can't find it)?

[dcs0014]

I believed I defined it at the beginning of the code on page 1.