re-generate particles in initial adaptive refinement

Author: jdannberg

doc/modules/changes/20250129_jdannberg

@@ -0,0 +1,6 @@
+Changed: ASPECT now re-generates particles in each adaptive
+refinement cycle instead of only once after global refinement.
+This means that particle locations during adaptive refinement
+are chosen according to the generator instead of randomly.
+<br>
+(Juliane Dannberg, 2025/01/29)

source/particle/manager.cc

@@ -574,9 +574,14 @@ namespace aspect
     void
     Manager<dim>::setup_initial_state ()
     {
-      // If we are in the first adaptive refinement cycle generate particles
-      if (this->get_pre_refinement_step() == 0)
-        generate_particles();
+      // We want to generate a new set of particles in each adaptive refinement
+      // cycle to get the right number of particles per cell and to accurately
+      // initialize their properties. Delete existing particles beforehand.
+      if (this->get_pre_refinement_step() > 0)
+        particle_handler->clear();
+
+      // Generate particles in each adaptive refinement cycle
+      generate_particles();
 
       // And initialize the particle properties according to the initial
       // conditions on the current mesh

tests/particle_generator_reference_cell_adaptive.prm

@@ -0,0 +1,21 @@
+# Like particle_generator_reference_cell.prm but with initial adaptive
+# refinement to test correct particle initialization.
+# There should be 4 particles per direction within each cell.
+
+set Dimension = 2
+
+include $ASPECT_SOURCE_DIR/tests/particle_generator_reference_cell.prm
+
+subsection Mesh refinement
+  set Initial adaptive refinement        = 1
+  set Strategy                           = composition gradient
+  set Initial global refinement          = 3
+  set Coarsening fraction                = 0.6
+  set Refinement fraction                = 0.15
+  set Adapt by fraction of cells         = true
+end
+
+subsection Particles
+  set Load balancing strategy = remove and add particles
+  set Minimum particles per cell = 16
+end