CODEWITHSUNDEEP
functionmodelica
kernstock
kernstock

Reputation: 234

Derivative of a Function in Modelica

First, excuse me for not providing a minimal working example, it is that I just can't think of one, really. I'll just give some pieces of code and ask my question "in principle".

I'm doing thermophysical properties calculation with a real gas model (Peng-Robinson) and here I am having problems when translating a model, where I use pressure p and specific enthalpy h as inputs to calculate all other properties. When it comes to calculating the temperature T, it is linked to the enthalpy h via an equation called departure function, which is itself a function of T. In Modelica it looks like this:

Dh_real = R_m*T*(Z - 1) + (T*dadT - a)/(sqrt(8)*b)*log((Z + (1 + sqrt(2))*B)/(Z + (1 - sqrt(2))*B));

Here a, dadT and Z are also temperature-dependent scalars and partly calculated using matrix operations (dadT) or polynomial-root-calculation (Z) in functions, b and B are parameters.

Calculating the enthalpy from an input temperature (in another model) is straightforward and working fine, the solver can solve the departure function analytically. The other direction has to be solved numerically and this is, I think, why Dymola gives me this error, when translating.

Cannot find differentiation function:
DadT_Unique2([some parameters and T])
with respect to time

Failed to differentiate the equation
dadT = DadT_Unique2([some parameters and T]);

in order to reduce the DAE index.

Failed to reduce the DAE index.

Now DadT is a function within the model, where I use some simple matrix operations to calculate dadT from some parameters and the temperature T. Obviously, Dymola is in need of the derivative of some internal _Unique2-function. I couldn't find anything in the specification nor in the web about this. Can I provide a derivative of the functions somehow? I tried the smoothOrder-annotation, but without effect. How can I deal with this?

Upvotes: 2

Views: 2756

Answers (1)

matth
matth

Reputation: 2696

This is not a full answer, but a list of interesting links that you should read:

Upvotes: 4

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