[Prime 7] SDoF nonlin spring oscillator: How to make "odesolve" work with a substitute functition ?

[PR_9168273]

Hello,

"odesolve" works with A*y''(t) + B*y'(t) + 3*y(t) / (y(t)² + 1) = 0 but it doesn't work with the substitution term

C(y(t)) = 3*y(t) / (y(t)² + 1)

A*y''(t) + B*y'(t) + C(y(t)) = 0

I tried in several ways to define C, e.g.  C:= 3*y(t) / (y(t)² + 1); C(t, y) = 3*y(t) / (y(t)² + 1) ... etc, but in vain!

I am attaching my file to this post and would appreciate your help very much ! 

Thank you in advance, Peter

[Werner_E]

Haven't yet installed Prime 7 and so I can't open your sheet.
Can it be that you are looking for something like this?

[PR_9168273]

Dear Werner,

Thank you so much for your valuable help!

I fixed my sheet and extended the content. Now, it works without units.

First, I tried to reproduce an example from the old Biggs' book. Key scalar results look pretty good.

However, the form of the time-dependent deflection curve is quite different: after the first peak the next minima are much smaller compared to my results. I guess, this is to be attributed to energy dissipation...

Anyway, there is some bug that I could not figure out yet ... or "odesolve" doesn't  provide correct results.

Maybe I should try to improve the integration time step ... have to figure it out how to do it.... 

Next, I introduced the units and it doesn't work! I couldn't figure out the reason for this as well.

I was not aware of the compatibility issues between P7 and older versions. I am providing my results, for those who are interested to use the sheet. But still have to do some homework... 🙂

Best wishes, Peter

P.S. For some reason could not upload the P7 file! I received the error message: 

• The attachment's 2021-05-27-nonlinear-sdof-ptc-forum.mcdx content type (application/x-mathcad-mcdx) does not match its file extension and has been removed.

[LucMeekes]

Hi Peter,

Prime introduces a new file format with every major release, which causes that an older version of Prime (like 6 or less) cannot read an .MCDX file created by a newer version (like 7 or up). So far most releases of Prime have been major releases....And PTC doesn't appear to see any reason in allowing Prime to save sheets to a lower version, or they are just incapable...

I also sometimes run into the problem that the forum refuses a Prime ( .MCDX) file, one that Prime is happy to open. It somehow 'thinks' it's smarter than Prime itself.

Generally opening the file in Prime and saving it, overwriting the existing file, will help solving that problem.

Success!
Luc

[PR_9168273]

Hi Luc,

Thanks for your feedback about the backward compatibility issue and the tip about overwriting an existing ".mcdx" file!

Best wishes, Peter