[WIP] Pressure Based Solver by thijsaalbers · Pull Request #2812 · su2code/SU2

thijsaalbers · 2026-05-07T13:05:07Z

Proposed Changes

The current work aims at getting a working version of the pressure based incompressible flow solver into the development branch.

Progress:

Pressure-based solver added as alternative to density based solver for the incompressible equations.
The pressure-based solver has been tested for constant density cases only for basic Navier-Stokes and Euler flow.
The pressure-based solver is implemented based on a pseudo time-stepping approach to remain consistent with the other solvers in SU2, as described in the thesis of Aksay. The pressure-based solver is currently set to the SIMPLE algorithm by default, with options for SIMPLEC and PISO available.
The Poisson solver is a major bottleneck in the computation speed. To account for this, an option was added to use a different linear solver and preconditioner for the poisson solver.
The current architecture is split between the CPBIncEulerSolver which solves the momentum equations only, and the CPoissonSolver which solves the pressure correction equation. The momentum solver is thus only responsible for the momentum solution variables and stores pressure only as a primitive and thus not as solution. The Poisson solver is subsequently initialized using information from CPBIncEulerSolver, after which the Poisson equation is solved and corrections to the momentum and pressure are computed, these are then communicated back to the CPBIncEulerSolver.

Current performance issues:

Time step size is very limited due to pressure-velocity coupling. (This issue is carried over from old pressure based versions). Attempts have been made with multiple PISO corrections but to no avail
Convergence issues with RANS
- At high reynolds numbers the solver becomes difficult to resolve and needs a very large number of linear solver iterations for the poisson equation. Possible fixes include adding multigrid support or a DIC preconditioner (less efficient). Multigrid support is tricky as SU2 currently only considers multigrid for the main (flow) solver and not for auxiliary solvers.
- SA and SST work but diverge under larger Reynolds numbers, cause has not yet been identified
Periodic boundary conditions have not been implemented/tested at all as of yet.
Any code related to adjoints has not been considered at all either.

Current code issues:

Failing regression tests due to a different value for the RMS[W] for incompressible density based simulations due to unknown reason.
when running OMP with more than 1 thread the momentum solve gives different (very wrong) results, there is maybe an issue with race conditions somewhere?
The constructor, nonDimensionalization and setReferenceValues functions of the CPBIncEulerSolver are largely a direct duplicate of CIncEulerSolver except for the nVar=... definition in the constructor and different fluid models which are not yet supported by the PB solver.
The CPBIncNSSolver is almost entirely duplicate to CIncNSSolver. They cannot be unified trivially as the underlying Euler solvers which they derive from are very different.

TODO list

Fix issues mentioned above
Validation and Verification
Documentation, Tutorials etc.

Related Work

This work is based on earlier attempts by Nitish Anand (2024) and Akshay Koodly (2021), see feature branches feature_PBFlow_V8 and feature_Pressure_based respectively. Also see PR #2210

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.

github-advanced-security

CodeQL found more than 20 potential problems in the proposed changes. Check the Files changed tab for more details.

Implements the full class structure required for the pressure-based solver in a minimal form. The code compiles and runs but does not yet contain any numerical/physical implementation. Future work will focus on implementing solver logic. Note: In the previous attempts (see related work) there is noticeable code duplication between CIncEuler and CPBIncEuler. The final architecture may be revised depending on how the implementation of CPBIncEuler evolves.

Centered residual is not yet implemented as the old code did not have a working version. Upwind residual is functional but requires cleanup and move to more appropiate file.

Variables are a work in progress and still include some commented out code related to energy and pressure

The current viscous residual of the poisson equation uses an isotropic diffusion coefficient, this should not be the case and should thus be replaced

thijsaalbers · 2026-07-03T12:44:22Z

I will have to review the code myself first, it is not ready for review as of yet.

thijsaalbers

The current version of the solver is ready for review. It is however not yet finished and there is a list of known issues that I am still working through. I have updated the PR description with the current status and known limitations; please read that before reviewing the implementation.

thijsaalbers · 2026-07-21T11:43:40Z

  su2double SemiSpan;                   /*!< \brief Wing Semi span. */
  su2double MSW_Alpha;                  /*!< \brief Coefficient for blending states in the MSW scheme. */
  su2double Roe_Kappa;                  /*!< \brief Relaxation of the Roe scheme. */
+  su2double RCFactor;                   /*!< \brief Relaxation for the Rhie-Chow interpolation contribution. */


Should this variable still be named RCfactor? This factor is named RC to remain consistent with the thesis of Akshay, it however has no relation to Rhie-Chow at all and instead functions as a means of controlling the time dependent term on the momentum coefficients for the poisson equation... I do not know why he called it RC in his thesis, nor if we should keep this same name or change it.

thijsaalbers · 2026-07-21T12:09:52Z

+//TODO: almost everything in this file is a direct or indirect copy of the code in 
+// the CIncNSSolver class. this will need to be refactored.


The only really different thing in this file from the CIncNSSolver is the wall boundary condition defintion (which is just massively simplified). I again do not know how this should be unified with the CIncNSSolver class as the C..IncEuler classes in between are massively distinct.

thijsaalbers · 2026-07-21T12:11:20Z


    unsigned short skipVars = nDim + solver[MESH_0][FLOW_SOL]->GetnVar();

+    if (config->GetKind_Incomp_System() == ENUM_INCOMP_SYSTEM::PRESSURE_BASED) skipVars+=2; // skip pressure


This works, although i do not know why +2 and not +1...

Initial configuration options and a test case

64d4213

thijsaalbers self-assigned this May 7, 2026

thijsaalbers added PR: draft changelog:feature GSoC Google Summer of Code labels May 7, 2026

Numerics and driver additions, WIP

1a7e82a

github-advanced-security AI found potential problems May 9, 2026

View reviewed changes

pcarruscag reviewed May 9, 2026

View reviewed changes

Comment thread SU2_CFD/src/numerics/poisson.cpp Outdated

Comment thread SU2_CFD/src/numerics/pbflow.cpp Outdated

Comment thread SU2_CFD/src/numerics/pbflow.cpp Outdated

thijsaalbers added 2 commits May 12, 2026 13:24

Remove outdated numerics

8d0bcf9

github-advanced-security AI found potential problems May 12, 2026

View reviewed changes

thijsaalbers added 2 commits June 3, 2026 11:30

Add numerics

a1805d4

Centered residual is not yet implemented as the old code did not have a working version. Upwind residual is functional but requires cleanup and move to more appropiate file.

Add variables and output

d28be3b

Variables are a work in progress and still include some commented out code related to energy and pressure

thijsaalbers closed this Jun 4, 2026

thijsaalbers added 12 commits June 10, 2026 14:16

MPI quantities

0b7f37b

Added momentum equations

e542979

First working version of pressure correction

ef875fb

The current viscous residual of the poisson equation uses an isotropic diffusion coefficient, this should not be the case and should thus be replaced

Added PISO and linear solver options

1931fa9

Altered some config parameters

81f47a3

Removed duplicate code in iteration

30933b8

Small fix for poisson preconditioners

425c39d

Small cleanup on numerics

51827d8

forgotten meson file

c43dbba

Workaround for symmetry BC

c7aadfa

Latest working version of the solver

e2ec0e2

small change

d588424

thijsaalbers reopened this Jul 3, 2026

thijsaalbers added 2 commits July 3, 2026 14:51

Merge branch 'develop' into feature_pressure_based_2026

9207743

Fixed loading of restart file

9ca3e00

thijsaalbers added 6 commits July 20, 2026 16:29

Clean up of old code and commenting

e421e68

updated test case

1531084

Merge branch 'develop' into feature_pressure_based_2026

4df1bd7

fix for new linalg logic

57639fd

fix compile issue with OMP

a8d8a8d

fixed compile issue with OMP

5ed114c

thijsaalbers commented Jul 21, 2026

View reviewed changes

thijsaalbers requested a review from pcarruscag July 21, 2026 12:29

Small cleanup and testcases

f3536fd

		//TODO: almost everything in this file is a direct or indirect copy of the code in
		// the CIncNSSolver class. this will need to be refactored.


		unsigned short skipVars = nDim + solver[MESH_0][FLOW_SOL]->GetnVar();

		if (config->GetKind_Incomp_System() == ENUM_INCOMP_SYSTEM::PRESSURE_BASED) skipVars+=2; // skip pressure

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Proposed Changes

Progress:

Current performance issues:

Current code issues:

TODO list

Related Work

PR Checklist

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thijsaalbers commented May 7, 2026 •

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