Implementing variable density for unsteady incompressible flow by tkiymaz · Pull Request #2641 · su2code/SU2

tkiymaz · 2025-12-10T13:45:17Z

Proposed Changes

Implements variable density treatment for unsteady incompressible flow simulations. Currently, SU2 uses constant density in transient simulations, which is inaccurate for combustion cases where density varies significantly due to heat release and species composition changes. This contribution enables proper density updates during time-stepping for flamelet-based combustion modeling. For now, only 1st order time marching is implemented

Related Work

Related to incompressible flow solver and flamelet combustion modeling. No specific issue linked yet.

PR Checklist

I am submitting my contribution to the develop branch.
My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
I have added a test case that demonstrates my contribution, if necessary.
I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.

pcarruscag · 2025-12-10T16:14:23Z

@Cristopher-Morales can you review this pr please?

Removed commented-out code for setting density at local point.

Updated the comment for density retrieval to reflect changes for transient density handling.

Removed unnecessary blank lines in CIncEulerVariable.hpp

Removed unnecessary blank lines in CVariable.hpp

Removed unnecessary blank lines to improve code readability.

Cristopher-Morales · 2025-12-10T17:24:32Z

@Cristopher-Morales can you review this pr please?

Hi @pcarruscag !

Yes, I can help reviewing this PR.

Please let me know if you need something else from my side

Removed commented-out code and updated density calculation.

bigfooted · 2025-12-10T17:31:59Z

@Cristopher-Morales I guess the changes to the preconditioner can be removed from this PR since we now have your implementation

Removed unnecessary blank lines in SetPrimVar function.

Removed unnecessary blank lines in CVariable.cpp.

Removed debug print statement from SetPrimVar function.

Cristopher-Morales · 2025-12-10T17:58:13Z

+
+  inline void Set_Density_time_n(unsigned long iPoint, su2double val) {
+    Density_time_n[iPoint] = val;
+  }


Documentation must be added, similar to the function above SetDensity, Same for the two functions below Set_Density_unsteady and GetDensity_time_n

bigfooted · 2026-05-04T08:00:22Z

@pcarruscag can you have a quick look at this PR? The primal works as expected, the adjoint unsteady will be finalized in another PR.

Co-authored-by: Pedro Gomes <38071223+pcarruscag@users.noreply.github.com>

pcarruscag · 2026-07-03T16:20:58Z

        if (incompressible) {
-          /*--- This is temporary and only valid for constant-density problems:
-          density could also be temperature dependent, but as it is not a part
-          of the solution vector it's neither stored for previous time steps
-          nor updated with the solution at the end of each iteration. */
-          Density_nM1 = flowNodes->GetDensity(iPoint);
-          Density_n = flowNodes->GetDensity(iPoint);
-          Density_nP1 = flowNodes->GetDensity(iPoint);
+          Density_nM1 = incFlowNodes->GetDensity_time_n1(iPoint);
+          Density_n = incFlowNodes->GetDensity_time_n(iPoint);
+          Density_nP1 = incFlowNodes->GetDensity(iPoint);
        } else {
          Density_nM1 = flowNodes->GetSolution_time_n1(iPoint)[0];
          Density_n = flowNodes->GetSolution_time_n(iPoint, 0);
          Density_nP1 = flowNodes->GetSolution(iPoint, 0);
        }


We should use CFlowVariable to abstract this away.
Declare the GetDensity_time_n functions in CFlowVariable as virtual, the default implementation grabs density from the solution containers, whereas CIncEulerVariable overrides and uses its own containers for density.
Then the scalar solver does not need to carry logic about whether the flow is compressible or incompressible.

pcarruscag · 2026-07-03T16:21:59Z

+  /*!
+   * \brief Register the density at time n as an AD input (no-op for non-incompressible nodes).
+   */
+  inline virtual void RegisterDensity_time_n() {}
+
+  /*!
+   * \brief Register the density at time n-1 as an AD input (no-op for non-incompressible nodes).
+   */
+  inline virtual void RegisterDensity_time_n1() {}


Can't this be part of registering the time_n solutions?

pcarruscag · 2026-07-03T16:24:44Z

  SetVolumeOutputValue("PRESSURE_COEFF", iPoint, (Node_Flow->GetPressure(iPoint) - solver[FLOW_SOL]->GetPressure_Inf())/factor);
  SetVolumeOutputValue("DENSITY", iPoint, Node_Flow->GetDensity(iPoint));

+  // Load density at previous timesteps for restart (unsteady with non-constant density)
+  if (config->GetTime_Domain() && config->GetKind_FluidModel() != CONSTANT_DENSITY) {
+    SetVolumeOutputValue("DENSITY_TIME_N", iPoint, Node_Flow_Inc->GetDensity_time_n(iPoint));
+    if (config->GetTime_Marching() == TIME_MARCHING::DT_STEPPING_2ND) {
+      SetVolumeOutputValue("DENSITY_TIME_N1", iPoint, Node_Flow_Inc->GetDensity_time_n1(iPoint));
+    }
+  }


We have density in the restart file, we load more than one restart file for transient to get the previous solutions.
Why do we need each of the multiple files we write and load to also carry old information? This way we are storing and loading repeated information.

pcarruscag · 2026-07-03T16:27:33Z

    direct_solver->GetNodes()->RegisterSolution_time_n();
+    /*--- Density is stored separately from the solution vector for incompressible flows. ---*/
+    if (config->GetKind_Regime() == ENUM_REGIME::INCOMPRESSIBLE)
+      direct_solver->GetNodes()->RegisterDensity_time_n();
+  }


So make RegisterSolution_time_n virtual and specialize it for the incompressible variable.
The purpose of abstractions like CSolver, CVariable, etc. is to keep physics/regime specific aspects contained in the "leaf classes" to allow the bigger ones like iteration, driver, etc. to deal with more general orchestration aspects

Tahsin Berk Kiymaz and others added 3 commits June 17, 2025 10:28

my changes

509a608

my changes

6aeb0a3

specieschanges

79e238c

tkiymaz added the PR: draft label Dec 10, 2025

Remove slurm files from repository

526b9a1

bigfooted added bug changelog:fix labels Dec 10, 2025

pcarruscag assigned Cristopher-Morales Dec 10, 2025

tkiymaz added 7 commits December 10, 2025 18:08

Remove unnecessary blank line in COutput.hpp

59979d0

Clean up commented-out density setting code

99abfa4

Removed commented-out code for setting density at local point.

Update density retrieval comment for transient handling

bee5a27

Updated the comment for density retrieval to reflect changes for transient density handling.

Remove empty line before temperature setter comment

cea69cc

Clean up whitespace in CIncEulerVariable.hpp

a721a2e

Removed unnecessary blank lines in CIncEulerVariable.hpp

Clean up blank lines in CVariable.hpp

02c03bb

Removed unnecessary blank lines in CVariable.hpp

Clean up CFlowIncOutput.cpp by removing blank lines

0449bb4

Removed unnecessary blank lines to improve code readability.

Clean up CompleteImplicitIteration and update dRhodT

572291f

Removed commented-out code and updated density calculation.

tkiymaz added 7 commits December 10, 2025 18:32

Revert back the preconditioning to original

c9fd0f6

Remove unnecessary whitespace and debug print statement

fc33c6a

Clean up blank lines in CIncNSVariable.cpp

644a543

Removed unnecessary blank lines in SetPrimVar function.

Clean up whitespace in CVariable.cpp

fed4892

Removed unnecessary blank lines in CVariable.cpp.

Fix constructor signature for CSpeciesVariable

8e09f7f

Remove unnecessary blank line in CSpeciesVariable.cpp

1e5008a

Fix constructor signature for CSpeciesFlameletVariable

2aebc54

Cristopher-Morales reviewed Dec 10, 2025

View reviewed changes

Comment thread SU2_CFD/include/variables/CVariable.hpp Outdated

Remove debug output from SetPrimVar method

3f19c55

Removed debug print statement from SetPrimVar function.

Cristopher-Morales reviewed Dec 10, 2025

View reviewed changes

Comment thread SU2_CFD/include/output/COutput.hpp Outdated

Cristopher-Morales reviewed Dec 10, 2025

View reviewed changes

bigfooted and others added 12 commits March 17, 2026 19:29

Merge branch 'develop' into fix_inc_unsteady_density

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Merge branch 'develop' into fix_inc_unsteady_density

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Merge branch 'develop' into fix_inc_unsteady_density

b8e840a

Merge branch 'develop' into fix_inc_unsteady_density

ec308a5

Merge branch 'develop' into fix_inc_unsteady_density

a7ab5b7

merge with develop

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update w. develop

0f538ea

update naming

d12f7b3

Merge branch 'develop' into fix_inc_unsteady_density

de4a95b

Merge branch 'develop' into fix_inc_unsteady_density

f7775f0

fix bounded scalar

7905679

fix restart, add regression

b2a9094

bigfooted removed the PR: draft label Apr 30, 2026

bigfooted changed the title ~~[WIP] Implementing variable density for unsteady incompressible flow~~ Implementing variable density for unsteady incompressible flow Apr 30, 2026

bigfooted and others added 4 commits April 30, 2026 14:41

point github workflow to temporary testcases branch

f10305e

Jacobian update for unsteady density

f5e0e34

extension of bounded scalar fix to RANS

b71c7ff

change OMP call

b359140

pcarruscag reviewed May 4, 2026

View reviewed changes

tkiymaz and others added 6 commits May 13, 2026 14:25

Update SU2_CFD/include/solvers/CSpeciesSolver.hpp

5f7e34d

Co-authored-by: Pedro Gomes <38071223+pcarruscag@users.noreply.github.com>

Update SU2_CFD/include/solvers/CTurbSolver.hpp

3a7553e

Co-authored-by: Pedro Gomes <38071223+pcarruscag@users.noreply.github.com>

formating and addition of bool functions

663105f

Merge branch 'develop' into fix_inc_unsteady_density

c25ac53

Merge branch 'develop' into fix_inc_unsteady_density

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Merge branch 'develop' into fix_inc_unsteady_density

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pcarruscag reviewed Jul 3, 2026

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pcarruscag commented Dec 10, 2025

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tkiymaz commented Dec 10, 2025 •

edited by bigfooted

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