Add conservative flux divergence operator (FV::ConservativeFluxDiv) by pressatojump · Pull Request #3122 · boutproject/BOUT-dev

pressatojump · 2025-06-13T00:36:03Z

Summary

This PR adds a conservative flux divergence operator for finite-volume discretizations in BOUT++. The operator computes the divergence of face-centered fluxes stored in FaceField3D objects.

Motivation

The conservative flux divergence operator is essential for finite-volume methods that ensure exact conservation properties. This is particularly important for:

Transport equations where conservation is critical
Shock-capturing schemes
Coupled multi-physics simulations requiring consistent fluxes

Implementation Details

New functions: FV::ConservativeFluxDiv with two overloads:
- Basic version with boolean flag for boundary flux from field
- Extended version with explicit boundary flux pointers for each dimension
Finite-volume discretization: Properly accounts for metric factors (J, dx, dy, dz)
Boundary handling: Complete support for X, Y, and Z boundary fluxes
Parallel support: Includes ghost cell communication via mesh_facefield_comm

Changes

Added include/bout/conservative_flux_div.hxx - header with function declarations
Added src/mesh/conservative_flux_div.cxx - implementation
Fixed FaceField3D virtual method overrides for C++17 compatibility
Added unit tests (compile test + framework for functional tests)
Updated build system files

Testing

Compile test verifies the operator builds and links correctly
Functional tests are implemented but disabled due to FakeMeshFixture size constraints
All existing tests continue to pass

API Addition

namespace FV {
  // Basic version - uses field values at boundaries if bndry_flux=true
  Field3D ConservativeFluxDiv(const FaceField3D& F, bool bndry_flux = false);

  // Extended version - explicit boundary fluxes
  Field3D ConservativeFluxDiv(const FaceField3D& F,
                             const Field3D* bndry_flux_x,
                             const Field3D* bndry_flux_y,
                             const Field3D* bndry_flux_z);
}

Implements FaceField3D<T> class to store field data on cell faces in all three directions (X, Y, Z). This is the foundation for the conservative flux divergence operator. Key features: - Templated class with default type BoutReal - Three Field3D components with appropriate staggered locations - x component: CELL_XLOW - y component: CELL_YLOW - z component: CELL_ZLOW - Inherits from FieldData for solver integration - Complete arithmetic operators (component-wise) - Unit tests for all operations - CMake build system integration This implementation follows the established BOUT++ patterns used in Vector3D and integrates cleanly with the existing field infrastructure. Part of conservative flux divergence feature implementation. Reference: Plan 01-data-structures.md 🤖 Generated with [Claude Code](https://claude.ai/code) Co-Authored-By: Claude <noreply@anthropic.com>

Implements halo exchange for face-centered fields leveraging BOUT++'s existing staggered grid support. The implementation provides convenience methods for communicating FaceField3D across processor boundaries. Key features: - Convenience methods communicate(FaceField3D&) and variants - Leverages existing Field3D staggered location support - Each component (x, y, z) communicated with correct location - Works with existing MPI infrastructure - Documentation explains reliance on StaggerGrids mechanism Implementation notes: - No core Mesh modifications needed - BOUT++ already handles CELL_XLOW/YLOW/ZLOW locations - Face ownership conventions maintained by existing code - Ghost cells exchanged to preserve flux continuity The approach is minimal and non-invasive, relying on BOUT++'s mature staggered grid infrastructure rather than reimplementing communication. Part of conservative flux divergence feature implementation. Reference: Plan 02-communication.md 🤖 Generated with [Claude Code](https://claude.ai/code) Co-Authored-By: Claude <noreply@anthropic.com>

Implements FV::ConservativeFluxDiv for finite-volume discretization of flux divergence. - Add ConservativeFluxDiv with two overloads for boundary flux handling - Implement complete boundary handling for X, Y, and Z dimensions - Add parallel communication support via mesh_facefield_comm - Fix FaceField3D compilation issues with virtual method overrides - Add unit tests (compile test + disabled functional tests due to mesh constraints) - Update CMakeLists.txt to include new source files The operator correctly handles: - Interior divergence calculation using finite-volume method - Optional boundary fluxes for all three dimensions - Ghost cell synchronization for parallel runs - Proper metric factors (J, dx, dy, dz) for curvilinear coordinates

ZedThree · 2025-06-13T13:08:28Z

This looks like it's built off next but going into master -- you probably just need to switch the base branch of the PR :)

pressatojump · 2025-06-20T05:56:24Z

Done @ZedThree :)

ZedThree · 2025-06-20T08:52:12Z

+// These tests are disabled because they require a larger mesh than FakeMeshFixture provides
+// The conservative flux div operator has been implemented and compiles correctly


Ah, @tomc271 encountered this as well -- Tom, please could you pull out your FakeMeshFixture generalisation into its own PR so that it can be used here as well?

Yes, here you are:
#3132

ZedThree · 2025-06-20T08:53:36Z

+ * Field3D& fz = flux.z();
+ * \endcode
+ */
+template <typename T = BoutReal>


Is the idea that this could also be used with other types? I couldn't immediately see what T is actually being used for

I could see the use of templating it over Field3D - but what do you intend to use it for besides BoutReal?

Also, you have some implementation in the cxx file, so this can only be used for BoutReal. So remove the template?

dschwoerer

@pressatojump do you think this is still useful? It has been sitting here for quite some time without any progress, as far as I can tell.

If you still want this, I am happy to help you move this along ...

dschwoerer · 2026-04-29T06:08:16Z

+// Note: No additional implementation needed here as all methods are inline
+// in the header file. This file exists primarily for documentation and
+// to maintain consistency with BOUT++ structure.


Note that other only header files exist, so this is not needed.
Also, cxx files are not great places for docs.
They should either go to the header, or to the sphinx documentation

dschwoerer · 2026-04-29T06:11:01Z

+inline void communicate(Mesh* mesh, FaceField3D& field) {
+  // Communicate each component separately
+  // Each component already has the correct staggered location
+  mesh->communicate(field.x(), field.y(), field.z());
+}
+
+/*!
+ * \brief Communicate only XZ components of a FaceField3D
+ * 
+ * \param mesh The mesh to use for communication
+ * \param field The FaceField3D to communicate
+ */
+inline void communicateXZ(Mesh* mesh, FaceField3D& field) {
+  // Only communicate X and Z components
+  mesh->communicateXZ(field.x(), field.z());
+}
+
+/*!
+ * \brief Communicate only YZ components of a FaceField3D
+ * 
+ * \param mesh The mesh to use for communication
+ * \param field The FaceField3D to communicate
+ */
+inline void communicateYZ(Mesh* mesh, FaceField3D& field) {
+  // Only communicate Y and Z components
+  mesh->communicateYZ(field.y(), field.z());
+}
+
+// Convenience methods that use the mesh from the field
+inline void communicate(FaceField3D& field) {
+  communicate(field.getMesh(), field);
+}
+
+inline void communicateXZ(FaceField3D& field) {
+  communicateXZ(field.getMesh(), field);
+}
+
+inline void communicateYZ(FaceField3D& field) {
+  communicateYZ(field.getMesh(), field);
+}
+


They should probably go to where the other functions are defined, and also add appropriate overloads to the mesh class.

dschwoerer · 2026-04-29T06:12:46Z

+ * Field3D& fz = flux.z();
+ * \endcode
+ */
+template <typename T = BoutReal>


I could see the use of templating it over Field3D - but what do you intend to use it for besides BoutReal?

Also, you have some implementation in the cxx file, so this can only be used for BoutReal. So remove the template?

Anonymous and others added 3 commits June 12, 2025 18:39

pressatojump changed the base branch from master to next June 20, 2025 05:55

ZedThree reviewed Jun 20, 2025

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dschwoerer reviewed Apr 29, 2026

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Add conservative flux divergence operator (FV::ConservativeFluxDiv)#3122

Add conservative flux divergence operator (FV::ConservativeFluxDiv)#3122
pressatojump wants to merge 3 commits intoboutproject:nextfrom
pressatojump:feat/conservative-flux-div

pressatojump commented Jun 13, 2025

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		// These tests are disabled because they require a larger mesh than FakeMeshFixture provides
		// The conservative flux div operator has been implemented and compiles correctly

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pressatojump commented Jun 13, 2025

Summary

Motivation

Implementation Details

Changes

Testing

API Addition

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ZedThree commented Jun 13, 2025

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pressatojump commented Jun 20, 2025

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