Trouble with ODESolve with System of Equations

[ptc-4929944]

Hello Community!

I have researched quite a bit and am having difficulty understanding the syntax of the ODESolve function. I cannot figure out how to solve the attached system of equations.

I have:

• Checked that the construction equations are all solveable and unitless.
• Checked that the boundary values are unitless.

I think I need a slight tweak to my ODESolve construct, but cannot figure it out. Please help.

[Fred_Kohlhepp]

You have a functions of pressure, P.ppt, and P.plt that are functions of time, but I don't see where any relationship between them and (say) flow rate is defined. So your problem may not be sufficiently constrained.

Typically pressure gets tied to flow rate and volume.

I can't see, since you've locked the areas . . .

[ptc-4929944]

I believe the system is correctly constrained.

P.ppt shows up in Constitutive Equations 1 (mechanical), 4 and 6 (hydraulic).

P.plt shows up in Constitutive Equations 2 (mechanical), 6 and 7 (hydraulic).

The locked areas of the worksheet only hide the calculation of constants in the equations (i.e. A.or.main.11).

Thanks for the feedback.

[HarveyHensley]

All of your A constants, e.g. A.or.plt etc. are zero. Thus, some of your variables (e.g. P.plt(t)) don't exist in the system as far as odesolve is concerned. Thus, you have a mismatch on the number of variables and the number of initial conditions.

[Werner_E]

All of your A constants, e.g. A.or.plt etc. are zero. Thus, some of your variables (e.g. P.plt(t)) don't

No, they are just to small to show up with three decimals and unit m^2. Set unit to mm^2 or let show more decimals to see.

[ptc-4929944]

To Werner's point - yes there are non-zero values in the constants of the constitutive equations. One check I did to help check unit integrity was to subsitute arbitrary, unitless, and non-trivial values for the functions in each equation. All seven equations gave me a unitless result without error.

I do suspect I have an overcontrained boundary condition.... I'll keep searching for it.

I am still suspecting my construct for the Odesolve is incorrect. Can someone confirm that the fundamental structure of the equation and have and time boundaries are in an acceptable format?

[HarveyHensley]

Okay about the non-zero values. I should have known that was the case when I saw a unit.

I'll try to help again. It appears to me that you only have three active equations but four unknowns when you go through some algebraic manipulations. See my attached worksheet.

I changed some of the variable names to eliminate the literal subscripts, but that wasn't the problem. I think your odesolve form is okay if you find another equation.

[ptc-4929944]

Thank you to all who have contributed!

I need to re-evaluation the system dynamics to try and find that missing equation. Any suggestions on the likely underconstrained variable are appreciated.

[HarveyHensley]

I realized that I eliminated one too many equations when I subtracted the two equations. However, when I included one of them, I still get the error as before. I've tried some other simplifications but no luck. Re-derivation of the system is the right thing to do.