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Strange behavior of NDSolve function (Error NDSolve::ntdvdae)

Posted 2 years ago

Hi there,
i discovered a weird behavior using NDSolve to solve a set of differential equations.

Let me outline my problem:
I need to solve a set of differential equations. These are equations of motion where the number of equations is depending on the number of shape functions i use.
If i use 8 shape functions i'll get a set of 8 differential equations which NDSolve is evaluating. So there are 8 generalized coordinates (from Subscript[qq, 1][t] to Subscript[qq, 8][t]) which need to be solved via NDSolve.
If i switch the number of shape functions to 9 which means i'll get 9 equations to solve, the NDSolve function is reporting the error message NDSolve::ntdvdae: Cannot solve to find an explicit formula for the derivatives. NDSolve will try solving the system as differential-algebraic equations.
(Please find a .nb file attached with my code. For 8 and 9 differential equations respectively)
(I use Version 12.1.1.0)

Since i need to solve it for 9 (or more) shape functions/equations i wanted to ask you guys if anybody can please help me fixing my problem? I looked it up on the internet but have no idea on how to fix it and i really appreciate any hint of you guys ;-)

POSTED BY: Paul Müller
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