chaos-polymtl · blaisb · Feb 8, 2026 · Jan 16, 2026 · Jan 30, 2026 · Jan 30, 2026
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -10,6 +10,10 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/).
 
 - MAJOR This PR updates the project version to the current release tag. [#1904](https://github.com/chaos-polymtl/lethe/pull/1904)
 
+### Fixed
+
+- MAJOR This PR fixes the Boussinesq buoyancy term that was miscalculated in VOF. Contrary to the Navier-Stokes equations from the `PSPGSUPGNavierStokesAssemblerCore`, when using VOF, the density term appears explicitely in both left and right had sides of the assembly from `GLSNavierStokesVOFAssemblerCore`. As such, the BuoyancyAssembler from the base Navier-Stokes code did not compute the proper buoyancy term. This was fixed by adding a new buoyancy assembler specific for VOF `navier_stokes_vof_assemblers` and changed the setup assembler in both `fluid_dynamic_matrix_based` and `fluid_dynamic_block` to compute buoyancy properly. All tests affected by this change have been updated and a new VOF specific buoyancy test has been added to the application test suite.
+
 ## [Master] - 2026/01/29
 
 ### Changed

diff --git a/applications_tests/lethe-fluid/buoyant_flow_between_parallel_plates.output b/applications_tests/lethe-fluid/buoyant_flow_between_parallel_plates.output
@@ -5,35 +5,23 @@ Running on 1 MPI rank(s)...
    Number of thermal degrees of freedom: 81
 
 *****************************
-Steady iteration:        1/3
+Steady iteration:        1/2
 *****************************
-L2 error velocity : 2.97964e-05
+L2 error velocity: 2.97964e-05
 L2 error temperature : 6.63477e-15
 
 *****************************
-Steady iteration:        2/3
+Steady iteration:        2/2
 *****************************
    Number of active cells:       256
    Number of degrees of freedom: 867
    Volume of triangulation:      0.05
    Number of thermal degrees of freedom: 289
-L2 error velocity : 6.87211e-06
+L2 error velocity: 6.87211e-06
 L2 error temperature : 7.97464e-13
-
-*****************************
-Steady iteration:        3/3
-*****************************
-   Number of active cells:       1024
-   Number of degrees of freedom: 3267
-   Volume of triangulation:      0.05
-   Number of thermal degrees of freedom: 1089
-L2 error velocity : 6.84085e-06
-L2 error temperature : 1.45997e-12
-cells  error_velocity     error_pressure   
-   64 2.979638e-05    - 6.154347e-01     - 
-  256 6.872113e-06 2.12 3.166238e-01  0.96 
- 1024 6.840850e-06 0.01 3.166244e-01 0.00 
+cells  error_velocity    error_pressure   
+   64 2.979638e-05    - 6.154347e-01    - 
+  256 6.872113e-06 2.12 3.166238e-01 0.96 
 cells error_temperature  
    64 6.634769e-15     - 
   256 7.974639e-13 -6.91 
- 1024 1.459965e-12 -0.87 
diff --git a/applications_tests/lethe-fluid/buoyant_flow_between_parallel_plates.prm b/applications_tests/lethe-fluid/buoyant_flow_between_parallel_plates.prm
@@ -12,7 +12,7 @@ set dimension = 2
 
 subsection simulation control
   set method            = steady
-  set number mesh adapt = 2
+  set number mesh adapt = 1
   set output name       = mms_22
   set output frequency  = 0
   set subdivision       = 1

diff --git a/applications_tests/lethe-fluid/gls_droplet_marangoni_effect.output b/applications_tests/lethe-fluid/gls_droplet_marangoni_effect.output
@@ -8,7 +8,7 @@ Enstrophy: 0
 Kinetic energy: 0
 
 *******************************************************************************
-Transient iteration: 1        Time: 0.001    Time step: 0.001    CFL: 0
+Transient iteration: 1        Time: 0.001    Time step: 0.001    CFL: 0       
 *******************************************************************************
 Enstrophy: 0.42658
 Kinetic energy: 1.5988e-05
@@ -20,19 +20,19 @@ Enstrophy: 97.103
 Kinetic energy: 0.001011
 
 *******************************************************************************
-Transient iteration: 3        Time: 0.003    Time step: 0.001    CFL: 0.17531
+Transient iteration: 3        Time: 0.003    Time step: 0.001    CFL: 0.17531 
 *******************************************************************************
 Enstrophy: 299.47
 Kinetic energy: 0.0039228
 
 *******************************************************************************
-Transient iteration: 4        Time: 0.004    Time step: 0.001    CFL: 0.31002
+Transient iteration: 4        Time: 0.004    Time step: 0.001    CFL: 0.31002 
 *******************************************************************************
 Enstrophy: 513.9
 Kinetic energy: 0.0079503
 
 *******************************************************************************
-Transient iteration: 5        Time: 0.005    Time step: 0.001    CFL: 0.40314
+Transient iteration: 5        Time: 0.005    Time step: 0.001    CFL: 0.40314 
 *******************************************************************************
 Enstrophy: 705.51
 Kinetic energy: 0.012354
diff --git a/applications_tests/lethe-fluid/gls_droplet_marangoni_effect.prm b/applications_tests/lethe-fluid/gls_droplet_marangoni_effect.prm
@@ -92,13 +92,13 @@ subsection physical properties
     set density              = 100
     set kinematic viscosity  = 0.01
     set thermal conductivity = 60
-    set thermal expansion    = 1
+    set thermal expansion    = 1e-2
   end
   subsection fluid 1
     set density              = 1000
     set kinematic viscosity  = 0.01
     set thermal conductivity = 60
-    set thermal expansion    = 1
+    set thermal expansion    = 1e-3
   end
 
   set number of material interactions = 1

diff --git a/...ications_tests/lethe-fluid/gls_droplet_marangoni_effect_without_dcdd_stabilization.output b/...ications_tests/lethe-fluid/gls_droplet_marangoni_effect_without_dcdd_stabilization.output
@@ -4,35 +4,35 @@ Running on 1 MPI rank(s)...
    Volume of triangulation:      0.0064
    Number of thermal degrees of freedom: 4225
    Number of VOF degrees of freedom: 4225
-Enstrophy  : 0
-Kinetic energy : 0
+Enstrophy: 0
+Kinetic energy: 0
 
 *******************************************************************************
 Transient iteration: 1        Time: 0.001    Time step: 0.001    CFL: 0       
 *******************************************************************************
-Enstrophy  : 0.42658
-Kinetic energy : 1.5988e-05
+Enstrophy: 0.42658
+Kinetic energy: 1.5988e-05
 
 ********************************************************************************
 Transient iteration: 2        Time: 0.002    Time step: 0.001    CFL: 0.0080812
 ********************************************************************************
-Enstrophy  : 97.104
-Kinetic energy : 0.00101
+Enstrophy: 97.104
+Kinetic energy: 0.00101
 
 *******************************************************************************
 Transient iteration: 3        Time: 0.003    Time step: 0.001    CFL: 0.17532 
 *******************************************************************************
-Enstrophy  : 297.76
-Kinetic energy : 0.0038305
+Enstrophy: 297.76
+Kinetic energy: 0.0038305
 
 *******************************************************************************
 Transient iteration: 4        Time: 0.004    Time step: 0.001    CFL: 0.31092 
 *******************************************************************************
-Enstrophy  : 510.24
-Kinetic energy : 0.0077006
+Enstrophy: 510.24
+Kinetic energy: 0.0077006
 
 *******************************************************************************
 Transient iteration: 5        Time: 0.005    Time step: 0.001    CFL: 0.40673 
 *******************************************************************************
-Enstrophy  : 704.33
-Kinetic energy : 0.011957
+Enstrophy: 704.33
+Kinetic energy: 0.011957
diff --git a/applications_tests/lethe-fluid/gls_droplet_marangoni_effect_without_dcdd_stabilization.prm b/applications_tests/lethe-fluid/gls_droplet_marangoni_effect_without_dcdd_stabilization.prm
@@ -100,13 +100,13 @@ subsection physical properties
     set density              = 100
     set kinematic viscosity  = 0.01
     set thermal conductivity = 60
-    set thermal expansion    = 1
+    set thermal expansion    = 1e-2
   end
   subsection fluid 1
     set density              = 1000
     set kinematic viscosity  = 0.01
     set thermal conductivity = 60
-    set thermal expansion    = 1
+    set thermal expansion    = 1e-3
   end
 
   set number of material interactions = 1

diff --git a/applications_tests/lethe-fluid/gls_vof_buoyant_flow_between_parallel_plates.output b/applications_tests/lethe-fluid/gls_vof_buoyant_flow_between_parallel_plates.output
@@ -0,0 +1,34 @@
+Running on 1 MPI rank(s)...
+   Number of active cells:       64
+   Number of degrees of freedom: 243
+   Volume of triangulation:      0.05
+   Number of thermal degrees of freedom: 81
+   Number of VOF degrees of freedom: 81
+
+*****************************
+Steady iteration:        1/2
+*****************************
+L2 error velocity: 2.97905e-05
+L2 error temperature : 5.81828e-13
+L2 error phase : 0
+
+*****************************
+Steady iteration:        2/2
+*****************************
+   Number of active cells:       256
+   Number of degrees of freedom: 867
+   Volume of triangulation:      0.05
+   Number of thermal degrees of freedom: 289
+   Number of VOF degrees of freedom: 289
+L2 error velocity: 6.92009e-06
+L2 error temperature : 7.74873e-11
+L2 error phase : 0
+cells  error_velocity    error_pressure   
+   64 2.979046e-05    - 6.154162e+00    - 
+  256 6.920086e-06 2.11 3.166236e+00 0.96 
+cells error_temperature  
+   64 5.818282e-13     - 
+  256 7.748728e-11 -7.06 
+cells error_phase  
+   64 0.000000e+00 
+  256 0.000000e+00