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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!
Solved! Go to Solution.
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.
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
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
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.
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.
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.