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A
AH_105277164-Participant
4-Participant
August 26, 2024
Solved

using Solve in a loop

  • August 26, 2024
  • 2 replies
  • 3846 views

I am having trouble understanding why I can't get a solution using the defined variables but am able to get one when I put in a value for the variable.  My issue arises because the values X, Y and Z depend on Twall.  I want to be able to loop this with an update to Twall at each dt.  Any help would be appreciated.

 

mathcad prime9

 

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AH_10527716_0-1724695810465.png

 

 

 

 

Best answer by Werner_E
@AH_10527716 wrote:

I figured it out.  I can't have the "=" after the X1 and Z1.  I was just checking units, but for some reason it didn't like it,

 

thanks

This is because you cant numerically evaluate a function definition!

After all, which result would you expect from f(x):=x^2= ?

Even if you already had defined a value for a variable x before - the x in the function definition is a formal function argument and has nothing to do with that variable x.

 

2 replies

L
LucMeekes23-Emerald IV
23-Emerald IV
August 26, 2024

Please attach the Prime worksheet (.mcdx file).

 

Other than that: I guess you're not interested in the complex answers for T, nor the negative one, so only the first answer is needed.

And you only need a numeric answer. In that case it's better to use the root() function, instead of solve. You can set it to not give you negative or complex answers by supplying two extra parameters that give limits of the range within which you expect your answer.

Look it up in the help information of the program.

 

Success!
Luc

A
AH_105277164-ParticipantAuthor
4-Participant
August 26, 2024

Thanks Luc.  What I am doing is a transient heat transfer case.  Yes, I only need the positive root.  But, I need to use that root and resolve for all the variables after a time-step and resolve for Twall.  Repeat until Twall is equal to Thot.  I know how to do the programming, I just can't get the initial part to work.  My worksheet has some other stuff at the bottom that I was trying to get to work as well.

 

thanks

L
LucMeekes23-Emerald IV
23-Emerald IV
August 26, 2024

This is what I'm talking about:

LucMeekes_0-1724711393006.png

Now, I don't see you calculating new values of A,B,C,D,E,F,X,Y or Z based on this temperature value. You're using the same inputs over and again. Then there's no point in iterating.

 

Success,

Luc

 

    ttokoro
    21-Topaz I
    ttokoro21-Topaz I
    21-Topaz I
    August 26, 2024

    image.png

    t.t.
    A
    AH_105277164-ParticipantAuthor
    4-Participant
    August 27, 2024

    thanks.. 

    A
    AH_105277164-ParticipantAuthor
    4-Participant
    October 11, 2024

    thanks for that,.  Ai, as you can tell, an area.  Basically, what it represents is a column filled with sand that is being drained at 1" per second.  So, at time=0 Ai is small, and at the end of the time it is Ai*1..  So, as z approaches L it should get closer to 1, right?  that is what I was trying to program.  Thanks again for the help.

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