Skip to main content
F
FDS13-Aquamarine
13-Aquamarine
September 12, 2017
Solved

What could I have been overlooking

  • September 12, 2017
  • 4 replies
  • 2961 views

I am trying to solve the main equation of the attached article. I can easily reproduce table 3 of the article but am not able to reproduce table 4 and 5. Would someone be so kind what to look what could cause this behavior? Thanks in advance!

Best answer by terryhendicott

Hi,

Forgot to enclose.

4 replies

F
Fred_Kohlhepp23-Emerald I
23-Emerald I
September 12, 2017

First, you're carrying units, Mathcad 15 won't deal with units in ode solvers.

Second, you have an undefined variable "D" in the definition of the function D(t,R); that's likely to be a problem too.

Third, D(t,R) needs to be a vector function defining derivatives for rkfixed to work.  I'm pretty sure that yours is a program, not a vector.

F
FDS13-AquamarineAuthor
13-Aquamarine
September 13, 2017

Dear Fred, My excuses I attached the wrong worksheet. But even without units I cannot reproduce table 4 and 5 while I can reproduce table 3

T
21-Topaz II
terryhendicott21-Topaz II
21-Topaz II
September 13, 2017

Hi

 

The problem is using the units.  Odesolvers only operate on numbers they are not unit aware.

You need to reduce the variables with units to a value only by multiplying by the correct units to do this.

 

I have used the gm/cm/s unit system so you can see what has been done.

 

Formula (1) is not unit consistent raising e to a power, the power LambdaDt must be dimensionless quantity in Mathcad

Formula (2) for Lambda has units. D has units and t has units.

function erf with parameters Lambda,D, and t called "error function" is undefined as it is an errer fuinction set it to zero.

I have chosen the gm,cm,s unit system but it depends how the author did it and there is a mix of units

Shown is how to get the solver to work. It is not getting the 2.3 minutes answer and expect this is a unit thing.

Have enclosed Mathcad 14/15 file, and a pdf.

 

Regards

T
21-Topaz II
terryhendicott21-Topaz II
21-Topaz II
September 13, 2017

Hi,

 

Needs one more change in equation for f_RT with pressure supplied in torr but needs to be in gm*g/cm^2.

 

Article answer 4.6 minutes or 276 seconds, this worksheet 282 seconds.

F
FDS13-AquamarineAuthor
13-Aquamarine
September 13, 2017

Dear Terry, thanks for your comments. I made a mistake in my original mail by sending the wrong version. In attachment you will find a version without units. I agree that table 3 where the N2 concentration is changed gives the same results but I cannot obtain the same results for table 4 and 5 where flow and solubility are changed. So I assume I made somewhere a mistake.

T
21-Topaz II
terryhendicott21-Topaz II
21-Topaz II
September 13, 2017

Hi,

 

Table 4 Line 18  Temp = 27, Q = 0.40, adjusting all values for 27 deg,  I get the right answer.

Have enclosed the erf errorfunction but i am right it is an insignificant quantity.

 

Terms & ConditionsCookie settingsAccessibility statement