`Modelica.Fluid.Examples.Tanks.ThreeTanks` fails structural lowering with structurally singular system

[hjxy2012]

Summary

The standard MSL 4.1.0 Modelica.Fluid.Examples.Tanks.ThreeTanks example fails during rumoca's DAE structural lowering with a structurally singular system error: 121 equations are matched against 121 unknowns, leaving 6 unknowns in the StaticPipe.flowModel sub-components unmatched. This blocks simulation of a canonical Modelica.Fluid example.

Context

• rumoca version: 0.9.20 (also tested 0.9.18)

• OS: Windows

• Modelica Standard Library: 4.1.0, downloaded via cargo xtask repo modelica-deps ensure

• Model: Modelica.Fluid.Examples.Tanks.ThreeTanks from Modelica.Fluid.Examples.Tanks

• Command used:

  cargo run -p rumoca --release -- sim -c examples/simulation/rumoca-scenario.tanks_three_tanks_msl.toml

• Scenario config: examples/simulation/rumoca-scenario.tanks_three_tanks_msl.toml

  source_roots = [
      "../../target/msl/ModelicaStandardLibrary_v4.1.0",
  ]
  
  [rumoca]
  version = "1"
  task = "simulate"
  
  [model]
  file = "../../target/msl/ModelicaStandardLibrary_v4.1.0/Modelica 4.1.0/Fluid/Examples/Tanks.mo"
  name = "Modelica.Fluid.Examples.Tanks.ThreeTanks"
  
  [viewer]
  mode = "results_panel"
  
  [sim]
  solver = "auto"
  t_end = 200.0
  
  [[plot.views]]
  id = "tank_levels"
  title = "Tank Levels vs Time"
  type = "timeseries"
  x = "time"
  y = ["tank1.level", "tank2.level", "tank3.level"]

• Full error log:

  Simulating Modelica.Fluid.Examples.Tanks.ThreeTanks to t=200...
     structural lowering failed: structurally singular system: 121 matched out of 121 equations and 127 unknowns;
    │ unmatched equations: -; unmatched unknowns: pipe1.flowModel.vs[2], pipe1.flowModel.mus_act, pipe2.flowModel.vs[2],
    │ pipe2.flowModel.mus_act, pipe3.flowModel.vs[2], pipe3.flowModel.rhos_act
  
  error: process didn't exit successfully: `target\release\rumoca.exe sim -c examples/simulation/rumoca-scenario.tanks_three_tanks_msl.toml` (exit code: 1)
  

• Unmatched unknowns:

  • pipe1.flowModel.vs[2]
  • pipe1.flowModel.mus_act
  • pipe2.flowModel.vs[2]
  • pipe2.flowModel.mus_act
  • pipe3.flowModel.vs[2]
  • pipe3.flowModel.rhos_act

  These variables come from Modelica.Fluid.Pipes.BaseClasses.FlowModels inside each StaticPipe component (pipe1, pipe2, pipe3).

• The same structural lowering failure also occurs for TanksWithOverflow and EmptyTanks in the same Modelica.Fluid.Examples.Tanks package.

• Source-root resolution and parsing issues were resolved before hitting this structural lowering failure; the problem is now in the DAE index/matching phase.

Relevant Specs

• spec/README.md
• spec/SPEC_0007_IR_PIPELINE.md — IR stage contracts and structural transformation scope, where DAE structural lowering/index reduction belongs
• spec/SPEC_0022_MLS_COMPILER_COMPLIANCE.md — Modelica semantics and MSL-backed validation for standard Modelica.Fluid examples
• spec/SPEC_0032_RANGE_PRESERVING_TENSORS.md — array/tensor range preservation, relevant because vs[2] is an array element among the unmatched unknowns
• spec/SPEC_0001_DEFID.md / spec/SPEC_0002_SCOPE_TREE.md — name lookup and scoping for nested flowModel component variables

Expected Behavior

Modelica.Fluid.Examples.Tanks.ThreeTanks should pass rumoca's structural DAE analysis and proceed to the numerical solver, producing a simulation trace. This model simulates successfully in OpenModelica and Dymola.

Verification

• Run cargo run -p rumoca --release -- sim -c examples/simulation/rumoca-scenario.tanks_three_tanks_msl.toml and confirm it no longer fails with a structurally singular system error.
• Optionally add a regression test that compiles (or fully simulates) Modelica.Fluid.Examples.Tanks.ThreeTanks against target/msl/ModelicaStandardLibrary_v4.1.0.

Notes

• The failure is in the structural DAE matching / index reduction step, not parsing or source-root loading.
• The unmatched unknowns involve array indices (vs[2]) and auxiliary variables (mus_act, rhos_act) from Modelica.Fluid pipe flow models, which may exercise rumoca's handling of array element variables, auxiliary variables, or balance equations in reusable flow-model components.
• This issue is a blocker for using rumoca to simulate standard Modelica.Fluid example models.

[jgoppert]

Thanks for reporting. I think we will get this is the next big MSL support push. #38