Fix LINELET with mpi, add regression case for laminar NACA0012

Common/include/geometry/CGeometry.hpp

@@ -238,6 +238,8 @@ class CGeometry {
     std::vector<unsigned long> colorOffsets;
 
     std::vector<uint8_t> lineletColor; /*!< \brief Coloring transfered to points, for visualization. */
+
+    bool computed = false; /*!< \brief Set once the (collective) computation is done, even if a rank has no linelets. */
   };
 
  protected:

Common/src/geometry/CGeometry.cpp

@@ -4339,7 +4339,10 @@ void CGeometry::ColorMGLevels(unsigned short nMGLevels, const CGeometry* const*
 
 const CGeometry::CLineletInfo& CGeometry::GetLineletInfo(const CConfig* config) const {
   auto& li = lineletInfo;
-  if (!li.linelets.empty() || nPoint == 0) return li;
+  /*--- Do not use "linelets.empty()" as the check, some ranks may have no linelets, and this
+   * function contains collective communication that all ranks must call the same number of times. ---*/
+  if (li.computed) return li;
+  li.computed = true;
 
   li.lineletIdx.resize(nPoint, CLineletInfo::NO_LINELET);
 
@@ -4444,7 +4447,7 @@ const CGeometry::CLineletInfo& CGeometry::GetLineletInfo(const CConfig* config)
   SU2_MPI::Allreduce(&sumNPoints, &globalNPoints, 1, MPI_UNSIGNED_LONG, MPI_SUM, SU2_MPI::GetComm());
   SU2_MPI::Allreduce(&nLinelet, &globalNLineLets, 1, MPI_UNSIGNED_LONG, MPI_SUM, SU2_MPI::GetComm());
 
-  if (rank == MASTER_NODE) {
+  if (rank == MASTER_NODE && globalNLineLets > 0) {
     std::cout << "Computed linelet structure, "
               << static_cast<unsigned long>(passivedouble(globalNPoints) / globalNLineLets)
               << " points in each line (average)." << std::endl;
@@ -4470,7 +4473,8 @@ const CGeometry::CLineletInfo& CGeometry::GetLineletInfo(const CConfig* config)
 
   const auto coloring = colorSparsePattern<uint8_t, std::numeric_limits<uint8_t>::max()>(
       CCompressedSparsePatternUL(adjacency), 1, false, true);
-  const auto nColors = coloring.getOuterSize();
+  /*--- unsigned long because it is communicated as MPI_UNSIGNED_LONG below. ---*/
+  const unsigned long nColors = coloring.getOuterSize();
 
   /*--- Sort linelets by color. ---*/
   std::vector<std::vector<unsigned long>> sortedLinelets;

Common/src/linear_algebra/CSysMatrix.cpp

@@ -1460,13 +1460,13 @@ void CSysMatrix<ScalarType>::BuildLineletPreconditioner(const CGeometry* geometr
 
   const auto nThreads = omp_get_max_threads();
 
+  /*--- The linelet info must be computed on all ranks (it is collective), and the working vectors
+   * must be allocated even if this rank has no linelets (e.g. partitions away from walls). ---*/
   BEGIN_SU2_OMP_SAFE_GLOBAL_ACCESS {
-    const auto& li = geometry->GetLineletInfo(config);
-    if (!li.linelets.empty()) {
-      LineletUpper.resize(nThreads);
-      LineletVector.resize(nThreads);
-      LineletInvDiag.resize(nThreads);
-    }
+    geometry->GetLineletInfo(config);
+    LineletUpper.resize(nThreads);
+    LineletVector.resize(nThreads);
+    LineletInvDiag.resize(nThreads);
   }
   END_SU2_OMP_SAFE_GLOBAL_ACCESS
 

TestCases/navierstokes/naca0012/lam_NACA0012_LINELET.cfg

@@ -0,0 +1,97 @@
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%                                                                              %
+% SU2 configuration file                                                       %
+% Case description: NACA0012 Laminar (Re 5000), LINELET preconditioner         %
+% Author: Francisco Palacios (base case), Syphonicc (LINELET variant)          %
+% Institution: Stanford University (base case)                                 %
+% Date: Oct 2, 2026                                                            %
+% File Version 8.5.0 "Harrier"                                                 %
+%                                                                              %
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+% ------------- DIRECT, ADJOINT, AND LINEARIZED PROBLEM DEFINITION ------------%
+%
+SOLVER= NAVIER_STOKES
+KIND_TURB_MODEL= NONE
+MATH_PROBLEM= DIRECT
+RESTART_SOL= NO
+
+% -------------------- COMPRESSIBLE FREE-STREAM DEFINITION --------------------%
+%
+MACH_NUMBER= 0.5
+AOA= 1.0
+FREESTREAM_TEMPERATURE= 288.15
+REYNOLDS_NUMBER= 5000.0
+REYNOLDS_LENGTH= 1.0
+
+% ---------------------- REFERENCE VALUE DEFINITION ---------------------------%
+%
+REF_ORIGIN_MOMENT_X = 0.25
+REF_ORIGIN_MOMENT_Y = 0.00
+REF_ORIGIN_MOMENT_Z = 0.00
+REF_LENGTH= 1.0
+REF_AREA= 1.0
+
+% -------------------- BOUNDARY CONDITION DEFINITION --------------------------%
+%
+MARKER_HEATFLUX= ( airfoil, 0.0 )
+MARKER_FAR= ( farfield )
+MARKER_PLOTTING= ( airfoil )
+MARKER_MONITORING= ( airfoil )
+
+% ------------- COMMON PARAMETERS DEFINING THE NUMERICAL METHOD ---------------%
+%
+NUM_METHOD_GRAD= WEIGHTED_LEAST_SQUARES
+CFL_NUMBER= 30.0
+CFL_ADAPT= NO
+CFL_ADAPT_PARAM= ( 0.5, 1.5, 1.0, 100.0 )
+RK_ALPHA_COEFF= ( 0.66667, 0.66667, 1.000000 )
+ITER= 10000
+LINEAR_SOLVER= FGMRES
+LINEAR_SOLVER_PREC= LINELET
+LINEAR_SOLVER_ERROR= 1E-1
+LINEAR_SOLVER_ITER= 3
+
+% -------------------------- MULTIGRID PARAMETERS -----------------------------%
+%
+MGLEVEL= 3
+MGCYCLE= V_CYCLE
+MG_PRE_SMOOTH= ( 4, 4, 4, 4 )
+MG_POST_SMOOTH= ( 4, 4, 4, 4 )
+MG_CORRECTION_SMOOTH= ( 1, 1, 1, 1 )
+MG_DAMP_RESTRICTION= 0.5
+MG_DAMP_PROLONGATION= 0.5
+
+% -------------------- FLOW NUMERICAL METHOD DEFINITION -----------------------%
+%
+CONV_NUM_METHOD_FLOW= ROE
+MUSCL_FLOW= YES
+SLOPE_LIMITER_FLOW= VENKATAKRISHNAN
+VENKAT_LIMITER_COEFF= 0.0001
+JST_SENSOR_COEFF= ( 0.5, 0.02 )
+TIME_DISCRE_FLOW= EULER_IMPLICIT
+
+% --------------------------- CONVERGENCE PARAMETERS --------------------------%
+%
+CONV_RESIDUAL_MINVAL= -12
+CONV_STARTITER= 10
+CONV_CAUCHY_ELEMS= 100
+CONV_CAUCHY_EPS= 1E-10
+
+% ------------------------- INPUT/OUTPUT INFORMATION --------------------------%
+%
+MESH_FILENAME= mesh_NACA0012_lam_hybrid_v1.su2
+MESH_FORMAT= SU2
+MESH_OUT_FILENAME= mesh_out
+SOLUTION_FILENAME= solution_flow
+SOLUTION_ADJ_FILENAME= solution_adj
+TABULAR_FORMAT= CSV
+CONV_FILENAME= history
+RESTART_FILENAME= restart_flow
+RESTART_ADJ_FILENAME= restart_adj
+VOLUME_FILENAME= flow
+VOLUME_ADJ_FILENAME= adjoint
+GRAD_OBJFUNC_FILENAME= of_grad
+SURFACE_FILENAME= surface_flow
+SURFACE_ADJ_FILENAME= surface_adjoint
+OUTPUT_WRT_FREQ= 250

TestCases/parallel_regression.py

@@ -402,6 +402,15 @@ def main():
 
     test_list.append(cylinder_lowmach)
 
+    # Laminar NACA0012 with linelet preconditioner (see #2877), 4 ranks so that some partitions have no linelets
+    lam_naca0012_linelet           = TestCase('lam_naca0012_linelet')
+    lam_naca0012_linelet.cfg_dir   = "navierstokes/naca0012"
+    lam_naca0012_linelet.cfg_file  = "lam_NACA0012_LINELET.cfg"
+    lam_naca0012_linelet.test_iter = 20
+    lam_naca0012_linelet.test_vals = [-5.335771, -2.936187, -2.943926, 0.102449]
+    lam_naca0012_linelet.command   = TestCase.Command("mpirun --oversubscribe -n 4", "SU2_CFD")
+    test_list.append(lam_naca0012_linelet)
+
     # 2D Poiseuille flow (body force driven with periodic inlet / outlet)
     poiseuille           = TestCase('poiseuille')
     poiseuille.cfg_dir   = "navierstokes/poiseuille"

TestCases/serial_regression.py

@@ -195,6 +195,14 @@ def main():
     cylinder_lowmach.test_vals = [-8.698687, -3.236566, 0.401347, 1.754864, 0.000000]
     test_list.append(cylinder_lowmach)
 
+    # Laminar NACA0012 with linelet preconditioner (see #2877)
+    lam_naca0012_linelet           = TestCase('lam_naca0012_linelet')
+    lam_naca0012_linelet.cfg_dir   = "navierstokes/naca0012"
+    lam_naca0012_linelet.cfg_file  = "lam_NACA0012_LINELET.cfg"
+    lam_naca0012_linelet.test_iter = 20
+    lam_naca0012_linelet.test_vals = [-5.345015, -2.936895, -2.943468, 0.096348]
+    test_list.append(lam_naca0012_linelet)
+
     # 2D Poiseuille flow (body force driven with periodic inlet / outlet)
     poiseuille           = TestCase('poiseuille')
     poiseuille.cfg_dir   = "navierstokes/poiseuille"