how to write derivative of two variables in a Modelica function?

Question

I started coding in modelica recently and I am writing a modelica function. In one of the equation in the alogrithm, I need to write an equation in which the left hand side is taking derivate of two variables but dymola gives me error "Unexpected assignment left-hand side der(rho_c*h_c). left hand side of assignment must be indexed component references." I am providing the modelica function code as

function SuctionSide
  input Modelica.Units.SI.Temperature T_c;
  input Modelica.Units.SI.Temperature T_env;
  input Modelica.Units.SI.Pressure P_suc;
  input Modelica.Units.SI.Volume V_f;
  input Modelica.Units.SI.MassFlowRate mdot_suc;
  input Modelica.Units.SI.MassFlowRate mdot_2;
  input Modelica.Units.SI.Mass m_oil; 
  input Modelica.Units.SI.Mass m_wall; 
  input Modelica.Units.SI.Density rho_c;
  input Modelica.Units.SI.SpecificEnthalpy h_c;
  input Modelica.Units.SI.SpecificEnthalpy h_2;
  input Modelica.Units.SI.CoefficientOfHeatTransfer h_rc;
  input Modelica.Units.SI.CoefficientOfHeatTransfer h_env;
  input Modelica.Units.SI.HeatFlowRate Qdot_ch; //crankcase heat
  input Modelica.Units.SI.Area A_c;
  input Modelica.Units.SI.Area A_env;
  input Modelica.Units.SI.SpecificHeatCapacityAtConstantPressure Cp_oil;
  input Modelica.Units.SI.SpecificHeatCapacityAtConstantPressure Cp_wall;
  input Modelica.Units.SI.Power Wdot_c;
  input Modelica.Units.SI.Power Wdot_th;
  input Modelica.Units.SI.HeatFlowRate Qdot_discharge;
  output Modelica.Units.SI.HeatFlowRate Qdot_c;
  output Modelica.Units.SI.HeatFlowRate Qdot_m;
  output Modelica.Units.SI.HeatFlowRate Qdot_env;
  output Modelica.Units.SI.Temperature T_wall;
algorithm 
  der(rho_c) := (mdot_suc -mdot_2)/V_f;
  der(rho_c * h_c):= ((mdot_suc*h_c) - (mdot_2*h_2) + Qdot_c+Qdot_ch + (der(P_suc)*V_f))/V_f;
  Qdot_c:= h_rc * A_c * (T_wall - T_c);
  der(T_wall) := (Qdot_m + Qdot_discharge -Qdot_c -Qdot_env)/(m_oil * Cp_oil + m_wall *   Cp_wall);
  Qdot_m := Wdot_c + mdot_2 * Wdot_th;
  Qdot_env := A_env * h_env*( T_wall - T_env);
  end SuctionSide;

whereas the errors are shown here as,

I tried to replace the * with comma to (make it like der(a,b) ) see if it is resolved but was not successful. I would be grateful, if you help me correct this function.

Answer 1

I think you, as many beginners, need to come out of the procedural thinking since Modelica is mainly a declarative language. In the algorithm part you write procedural code, as you do, but here you cannot use the language concept "der()". The der() concept belongs to an equation-section, not an algorithm-section.

So to correct your code you just replace der() with a descriptive variable names as follows

der_rho_c := (mdot_suc - mdot_d)/V_f;

For better understanding you could look up the concept of der() in a Modelica text book, for instance https://mbe.modelica.university.

Answer 2

Not necessarily an answer, since @janpeter's is quite complete.

However, your code seems to compute heat and mass balance equations. Wouldn't it be more convenient to put it in an equation section? Of course, you could keep your causal inputs and outputs. Just a thought ;)