Mixed multibody/fluid model initialization error

[Bruno Scaglioni]

The attached model is a mixed fluid/multibody model build with MSL 3.2.1 components, OMC fails to initialize the model if the initial conditions for the body are not specified and stucks in infinite loop during initialization of simulation (see -lv LOG_INIT) if initial conditions are specified.
The issue happens only if both multibody and fluid components are present because the state variables are in both domains.

[Martin Sjölund]

Postponed until 1.9.1

[Lennart Ochel]

This model has not fully specified initial conditions. This is reported by Dymola and OpenModelica:

Dymola:

Assuming fixed start value for the continuous states:
two_Port_Valve.body_mass.frame_a.r_0[1](start = two_Port_Valve.body_r_start[1])
two_Port_Valve.body_mass.frame_a.r_0[2](start = two_Port_Valve.body_r_start[2])
two_Port_Valve.body_mass.frame_a.r_0[3](start = two_Port_Valve.body_r_start[3])
two_Port_Valve.body_mass.Q[2](start = two_Port_Valve.body_mass.Q_start[2])
two_Port_Valve.body_mass.Q[3](start = two_Port_Valve.body_mass.Q_start[3])
two_Port_Valve.body_mass.Q[4](start = two_Port_Valve.body_mass.Q_start[4])
two_Port_Valve.body_mass.v_0[1](start = two_Port_Valve.body_v_start[1])
two_Port_Valve.body_mass.v_0[2](start = two_Port_Valve.body_v_start[2])
two_Port_Valve.body_mass.v_0[3](start = two_Port_Valve.body_v_start[3])
two_Port_Valve.body_mass.w_a[1](start = two_Port_Valve.body_mass.R_start.T[1, 1] *two_Port_Valve.body_mass.w_0_start[1]+two_Port_Valve.body_mass.R_start.T[1, 2] *two_Port_Valve.body_mass.w_0_start[2]+two_Port_Valve.body_mass.R_start.T[1, 3] *two_Port_Valve.body_mass.w_0_start[3])
two_Port_Valve.body_mass.w_a[2](start = two_Port_Valve.body_mass.R_start.T[2, 1] *two_Port_Valve.body_mass.w_0_start[1]+two_Port_Valve.body_mass.R_start.T[2, 2] *two_Port_Valve.body_mass.w_0_start[2]+two_Port_Valve.body_mass.R_start.T[2, 3] *two_Port_Valve.body_mass.w_0_start[3])
two_Port_Valve.body_mass.w_a[3](start = two_Port_Valve.body_mass.R_start.T[3, 1] *two_Port_Valve.body_mass.w_0_start[1]+two_Port_Valve.body_mass.R_start.T[3, 2] *two_Port_Valve.body_mass.w_0_start[2]+two_Port_Valve.body_mass.R_start.T[3, 3] *two_Port_Valve.body_mass.w_0_start[3])

OpenModelica:

Warning: Assuming fixed start value for the following 12 variables:
         two_Port_Valve.body_mass.frame_a.r_0[1]:VARIABLE(flow=false start = two_Port_Valve.body_r_start[1] unit = "m" )  "Position vector from world frame to the connector frame origin, resolved in world frame".IncompressibleValveExample.Test_Two_Port_Valve, .IncompressibleValveExample.Two_Port_Valve$two_Port_Valve, .Modelica.Mechanics.MultiBody.Parts.Body$two_Port_Valve$body_mass, .Modelica.Mechanics.MultiBody.Interfaces.Frame_a, .Modelica.SIunits.Position type: Real 
         two_Port_Valve.body_mass.frame_a.r_0[2]:VARIABLE(flow=false start = two_Port_Valve.body_r_start[2] unit = "m" )  "Position vector from world frame to the connector frame origin, resolved in world frame".IncompressibleValveExample.Test_Two_Port_Valve, .IncompressibleValveExample.Two_Port_Valve$two_Port_Valve, .Modelica.Mechanics.MultiBody.Parts.Body$two_Port_Valve$body_mass, .Modelica.Mechanics.MultiBody.Interfaces.Frame_a, .Modelica.SIunits.Position type: Real 
         two_Port_Valve.body_mass.frame_a.r_0[3]:VARIABLE(flow=false start = two_Port_Valve.body_r_start[3] unit = "m" )  "Position vector from world frame to the connector frame origin, resolved in world frame".IncompressibleValveExample.Test_Two_Port_Valve, .IncompressibleValveExample.Two_Port_Valve$two_Port_Valve, .Modelica.Mechanics.MultiBody.Parts.Body$two_Port_Valve$body_mass, .Modelica.Mechanics.MultiBody.Interfaces.Frame_a, .Modelica.SIunits.Position type: Real 
         $STATESET1.x[1]:VARIABLE(start = /*Real*/($STATESET1.A[1,1]) * $_start(two_Port_Valve.body_mass.Q[4]) + /*Real*/($STATESET1.A[1,2]) * $_start(two_Port_Valve.body_mass.Q[3]) + /*Real*/($STATESET1.A[1,3]) * $_start(two_Port_Valve.body_mass.Q[2]) + /*Real*/($STATESET1.A[1,4]) * $_start(two_Port_Valve.body_mass.Q[1]) fixed = false )  type: Real 
         $STATESET1.x[2]:VARIABLE(start = /*Real*/($STATESET1.A[2,1]) * $_start(two_Port_Valve.body_mass.Q[4]) + /*Real*/($STATESET1.A[2,2]) * $_start(two_Port_Valve.body_mass.Q[3]) + /*Real*/($STATESET1.A[2,3]) * $_start(two_Port_Valve.body_mass.Q[2]) + /*Real*/($STATESET1.A[2,4]) * $_start(two_Port_Valve.body_mass.Q[1]) fixed = false )  type: Real 
         $STATESET1.x[3]:VARIABLE(start = /*Real*/($STATESET1.A[3,1]) * $_start(two_Port_Valve.body_mass.Q[4]) + /*Real*/($STATESET1.A[3,2]) * $_start(two_Port_Valve.body_mass.Q[3]) + /*Real*/($STATESET1.A[3,3]) * $_start(two_Port_Valve.body_mass.Q[2]) + /*Real*/($STATESET1.A[3,4]) * $_start(two_Port_Valve.body_mass.Q[1]) fixed = false )  type: Real 
         two_Port_Valve.body_mass.v_0[1]:VARIABLE(start = two_Port_Valve.body_v_start[1] unit = "m/s" stateSelect=StateSelect.avoid )  "Absolute velocity of frame_a, resolved in world frame (= der(r_0))".IncompressibleValveExample.Test_Two_Port_Valve, .IncompressibleValveExample.Two_Port_Valve$two_Port_Valve, .Modelica.Mechanics.MultiBody.Parts.Body$two_Port_Valve$body_mass, .Modelica.SIunits.Velocity$two_Port_Valve$body_mass$v_0 type: Real 
         two_Port_Valve.body_mass.v_0[2]:VARIABLE(start = two_Port_Valve.body_v_start[2] unit = "m/s" stateSelect=StateSelect.avoid )  "Absolute velocity of frame_a, resolved in world frame (= der(r_0))".IncompressibleValveExample.Test_Two_Port_Valve, .IncompressibleValveExample.Two_Port_Valve$two_Port_Valve, .Modelica.Mechanics.MultiBody.Parts.Body$two_Port_Valve$body_mass, .Modelica.SIunits.Velocity$two_Port_Valve$body_mass$v_0 type: Real 
         two_Port_Valve.body_mass.v_0[3]:VARIABLE(start = two_Port_Valve.body_v_start[3] unit = "m/s" stateSelect=StateSelect.avoid )  "Absolute velocity of frame_a, resolved in world frame (= der(r_0))".IncompressibleValveExample.Test_Two_Port_Valve, .IncompressibleValveExample.Two_Port_Valve$two_Port_Valve, .Modelica.Mechanics.MultiBody.Parts.Body$two_Port_Valve$body_mass, .Modelica.SIunits.Velocity$two_Port_Valve$body_mass$v_0 type: Real
         two_Port_Valve.body_mass.w_a[1]:VARIABLE(start = two_Port_Valve.body_mass.w_0_start[1] unit = "rad/s" fixed = false stateSelect=StateSelect.avoid )  "Absolute angular velocity of frame_a resolved in frame_a".IncompressibleValveExample.Test_Two_Port_Valve, .IncompressibleValveExample.Two_Port_Valve$two_Port_Valve, .Modelica.Mechanics.MultiBody.Parts.Body$two_Port_Valve$body_mass, .Modelica.SIunits.AngularVelocity$two_Port_Valve$body_mass$w_a type: Real 
         two_Port_Valve.body_mass.w_a[2]:VARIABLE(start = two_Port_Valve.body_mass.w_0_start[2] unit = "rad/s" fixed = false stateSelect=StateSelect.avoid )  "Absolute angular velocity of frame_a resolved in frame_a".IncompressibleValveExample.Test_Two_Port_Valve, .IncompressibleValveExample.Two_Port_Valve$two_Port_Valve, .Modelica.Mechanics.MultiBody.Parts.Body$two_Port_Valve$body_mass, .Modelica.SIunits.AngularVelocity$two_Port_Valve$body_mass$w_a type: Real 
         two_Port_Valve.body_mass.w_a[3]:VARIABLE(start = two_Port_Valve.body_mass.w_0_start[3] unit = "rad/s" fixed = false stateSelect=StateSelect.avoid )  "Absolute angular velocity of frame_a resolved in frame_a".IncompressibleValveExample.Test_Two_Port_Valve, .IncompressibleValveExample.Two_Port_Valve$two_Port_Valve, .Modelica.Mechanics.MultiBody.Parts.Body$two_Port_Valve$body_mass, .Modelica.SIunits.AngularVelocity$two_Port_Valve$body_mass$w_a type: Real

The simulation reports the following:

Cannot initialize the dynamic state selection in an unique way. Use -lv LOG_DSS to see the switching state set.

I will further investigate this issue.

[Martin Sjölund]

This ticket was not closed for 1.9.1, which has now been released. It was batch modified for milestone 1.9.2 (but maybe an empty milestone was more appropriate; feel free to change it).

[Lennart Ochel]

This was fixed in the meantime.