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Mathcad Prime Solve Blocks Not Working!

ptc-5079124
1-Newbie

Mathcad Prime Solve Blocks Not Working!

I'm running into major problems with Mathcad Prime not wanting to solve my equations. They work in the old versions but not in this one. The find wasn't working and now it's saying my Velocities arn't defined. Any help would be greatly appreciated.

16 REPLIES 16

The answers should be:

8.408

-4.345

12.753

This is again a problem with mathcad prime solve block........

the labeling

in your solve block

for the definitions, all Vs are labeled as "variable"

then everywhere else they are labeled as "none"

try changing all Vs label, (definitions, equations , and fin() ) to "variable"

or change all of them to "none".

Changing the labels did not work for me. But deleting the three guess values and retyping them will let the error vanish. Try it yourself - type V_3x= right after the definition of the guess und you get the error of the variable not known. This happens even if they are labeled the same.

But the solutions now are just the guess values, so obviously there is still something wrong.

Seth Erickson wrote:

The answers should be:

8.408

-4.345

12.753

If you substitute these 'solutions' into your equations you find that the RHS of the third equation does not match that of the LHS.

Alan

Agree with Alan...I just spotted that too

and the RHS of the second and third equation are the same...so he has only 2 equation with 3 unknowns?

looks like the second two equation is energy balance (bernoulli equation) why there is a negative for the kinectic energy term on the LHS? I am confused. Do they come from the RHS originally? I do not understand the physics. Do you have a split? is your control volume enclosing the junction?

I think : Energy into junction = Energy out from split A + Energy out from split B

and the RHS of the second and third equation are the same...so he has only 2 equation with 3 unknowns?

Why? This would only be true if the third equation would consist of setting equal the two LHSs which is not the case.

You are right. what was I thinking...um...

I think a got the main probem - it was not the labelling: It looks like the guess values and equations were typed outside the solve block and then dragged inside. So they look like they are in the solve block template but for Prime they aren't. Prime does not "see" these definitions. Thats the reason is seemed to work when I retyped the guess values.

Copy and paste should work (there was an error in the first release of Prime2 with copied expressions in a solve block) but moving it does not.

You may try it yourself: if you are able to drag/move a region outside of the solve block template, then there is something wrong.

Addiotionally the multiplication signs after pi were missing in the first equation.

In the attached all should be corrected, nevertheless Prime finds no solution which could be because, as Alan pointed out, there seems to be something wrong with the equations as the supposed solutions don't fit.

EDIT: If you change L_2h in the third equation to L_3, you get the expected results.

Ok I did what you tried and also changed one more thing. But now I got 2 more problems after going a bit more in depth into the problem I'm already working on.

1) for part 1.2) Vxb is supposed to be positive not negative and can't see where I'm going wrong.

2) for part 1.3) Vx3, Vxb, Vxr are supposed to be 9.967, 4.808, 10.981. It got the solutions in MathCad 15 but not in Prime and once again I can't see my problem.

Thanks for the help!

Seth Erickson wrote:

.... It got the solutions in MathCad 15 but not in Prime and once again I can't see my problem.

Thanks for the help!

Upload the M15 worksheet where you got the desired result, so we can do a direct comparison.

Alan

Here is the other one. The names for stuff are just slightly varied.

1.2) Your first equation is wrong, compared to the MC15 sheet (exchange = and +)

1.3) Different guess values (especially for V.xb) will give you different results. I don't get the values from your MC15 sheet though, so you may check the values of all the constants involved if they really are the same (and have the same meaning)

The attached corrects your problem 1.3 to correspond with your MC15 version, giving the same results. I've not bothered to look at problems 1.1 and 1.2.

Alan

Hi,

Can you help me with my problem please, I am not sure why im getting an error. It look pretty simple, 2 equations and 2 unknowns but not sure I get an error.

Screenshot 2016-05-02 14.41.39.png

I didn't check your equations so I can't tell why you don't get the results you expected. So I'm not sure if the change of L3 for L2h was correct - I just noticed that it yields the results you expected.

According the results of 1.3 - post the Mathcad 15 worksheet. I'm sure someone will spot the difference or will convert it to Prime.

I see in your sheet pi(D)^2 (a function) but must be pi*D^2 (multiplication).

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