Remove gear3 integrator (#1221)

Description
The Gear3 integrator was not used anywhere in the code in any example or in any tests. This integrator never gave good performance and was actually not tested for years. I believe it is now time to fully delete it. I want to work on adding new integrators this year and this will require some more unification. I want to start by cleaning things before adding new things.

Testing
There were actually no test for gear3...

Documentation
I adapted the documentation but the gear3 integrator was never mentioned in the documentation.

CHANGELOG.md

@@ -3,6 +3,12 @@
 All notable changes to the Lethe project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/).
 
+## [Master] - 2024-08-05
+
+### Removed
+
+- MINOR Removed the gear3 DEM integrator. It was never used, it did not possess any unit test and it was not even clear if it still worked. [#1221](https://github.com/chaos-polymtl/lethe/pull/1211)
+
 ## [Master] - 2024-08-01
 
 ### Added

include/core/parameters_lagrangian.h

@@ -297,8 +297,7 @@ namespace Parameters
       enum class IntegrationMethod
       {
         velocity_verlet,
-        explicit_euler,
-        gear3
+        explicit_euler
       } integration_method;
 
       // Disable particle contacts to optimize performance

source/core/parameters_lagrangian.cc

@@ -699,12 +699,11 @@ namespace Parameters
           "Choosing rolling resistance torque model"
           "Choices are <no_resistance|constant_resistance|viscous_resistance>.");
 
-        prm.declare_entry(
-          "integration method",
-          "velocity_verlet",
-          Patterns::Selection("velocity_verlet|explicit_euler|gear3"),
-          "Choosing integration method"
-          "Choices are <velocity_verlet|explicit_euler|gear3>.");
+        prm.declare_entry("integration method",
+                          "velocity_verlet",
+                          Patterns::Selection("velocity_verlet|explicit_euler"),
+                          "Choosing integration method"
+                          "Choices are <velocity_verlet|explicit_euler>.");
 
         prm.enter_subsection("adaptive sparse contacts");
         {
@@ -912,8 +911,6 @@ namespace Parameters
           integration_method = IntegrationMethod::velocity_verlet;
         else if (integration == "explicit_euler")
           integration_method = IntegrationMethod::explicit_euler;
-        else if (integration == "gear3")
-          integration_method = IntegrationMethod::gear3;
         else
           {
             throw(std::runtime_error("Invalid integration method "));

source/dem/CMakeLists.txt

@@ -12,7 +12,6 @@ add_library(lethe-dem
   find_cell_neighbors.cc
   find_contact_detection_step.cc
   force_chains_visualization.cc
-  gear3_integrator.cc
   grid_motion.cc
   input_parameter_inspection.cc
   insertion.cc
@@ -63,7 +62,6 @@ add_library(lethe-dem
   ../../include/dem/find_cell_neighbors.h
   ../../include/dem/find_contact_detection_step.h
   ../../include/dem/force_chains_visualization.h
-  ../../include/dem/gear3_integrator.h
   ../../include/dem/grid_motion.h
   ../../include/dem/input_parameter_inspection.h
   ../../include/dem/insertion.h

source/dem/dem.cc

@@ -6,7 +6,6 @@
 #include <dem/distributions.h>
 #include <dem/explicit_euler_integrator.h>
 #include <dem/find_contact_detection_step.h>
-#include <dem/gear3_integrator.h>
 #include <dem/input_parameter_inspection.h>
 #include <dem/insertion_file.h>
 #include <dem/insertion_list.h>
@@ -314,8 +313,6 @@ DEMSolver<dim>::set_integrator_type()
         return std::make_shared<VelocityVerletIntegrator<dim>>();
       case ModelParameters::IntegrationMethod::explicit_euler:
         return std::make_shared<ExplicitEulerIntegrator<dim>>();
-      case ModelParameters::IntegrationMethod::gear3:
-        return std::make_shared<Gear3Integrator<dim>>();
       default:
         throw(std::runtime_error("Invalid integration method."));
     }

source/fem-dem/cfd_dem_coupling.cc

@@ -2,7 +2,6 @@
 #include <core/solutions_output.h>
 
 #include <dem/explicit_euler_integrator.h>
-#include <dem/gear3_integrator.h>
 #include <dem/post_processing.h>
 #include <dem/set_particle_particle_contact_force_model.h>
 #include <dem/set_particle_wall_contact_force_model.h>
@@ -200,8 +199,6 @@ CFDDEMSolver<dim>::set_integrator_type()
         return std::make_shared<VelocityVerletIntegrator<dim>>();
       case ModelParameters::IntegrationMethod::explicit_euler:
         return std::make_shared<ExplicitEulerIntegrator<dim>>();
-      case ModelParameters::IntegrationMethod::gear3:
-        return std::make_shared<Gear3Integrator<dim>>();
       default:
         throw(std::runtime_error("Invalid integration method."));
     }