Add a r-test for Springs to test out the springs forces under axial d…

…isplacement

src/system/springs/calculate_force_coefficients.hpp

@@ -20,7 +20,7 @@ struct CalculateForceCoefficients {
     KOKKOS_FUNCTION
     void operator()(int i_elem) const {
         // c1 = k * (l_ref/l - 1)
-        c1_(i_elem) = k_(i_elem) * (l_ref_(i_elem) / l_(i_elem) - 1.0);
+        c1_(i_elem) = k_(i_elem) * (l_ref_(i_elem) / l_(i_elem) - 1.);
         // c2 = k * l_ref/(l^3)
         c2_(i_elem) = k_(i_elem) * l_ref_(i_elem) / (l_(i_elem) * l_(i_elem) * l_(i_elem));
     }

tests/regression_tests/regression/CMakeLists.txt

@@ -11,5 +11,6 @@ target_sources(
     test_rotor.cpp
     test_solver.cpp
     test_springs.cpp
+    test_springs_force.cpp
     test_utilities.cpp
 )

tests/regression_tests/regression/test_precession.cpp

@@ -72,7 +72,7 @@ inline auto SetUpPrecessionTest() {
     create_element_freedom_table(elements, state);
     create_constraint_freedom_table(constraints, state);
 
-    [[maybe_unused]] auto solver = Solver(
+    auto solver = Solver(
         state.ID, state.node_freedom_allocation_table, state.node_freedom_map_table,
         elements.NumberOfNodesPerElement(), elements.NodeStateIndices(), constraints.num_dofs,
         constraints.type, constraints.base_node_freedom_table, constraints.target_node_freedom_table,

tests/regression_tests/regression/test_springs_force.cpp

@@ -0,0 +1,97 @@
+#include <gtest/gtest.h>
+
+#include "test_utilities.hpp"
+
+#include "src/elements/springs/create_springs.hpp"
+#include "src/elements/springs/springs.hpp"
+#include "src/model/model.hpp"
+#include "src/step/update_system_variables_springs.hpp"
+
+namespace openturbine::tests {
+
+inline auto SetUpSpringsForceTest() {
+    auto model = Model();
+
+    // Add two nodes for the spring element
+    model.AddNode(
+        {0, 0, 0, 1, 0, 0, 0},  // First node at origin
+        {0, 0, 0, 1, 0, 0, 0}, {0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0}
+    );
+    model.AddNode(
+        {1, 0, 0, 1, 0, 0, 0},  // Second node at (1,0,0)
+        {0, 0, 0, 1, 0, 0, 0}, {0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0}
+    );
+
+    const auto springs_input = SpringsInput({SpringElement(
+        std::array{model.GetNode(0), model.GetNode(1)},
+        10.,  // Spring stiffness coeff.
+        1.    // Undeformed length
+    )});
+
+    auto springs = CreateSprings(springs_input);
+    auto state = model.CreateState();
+
+    return std::make_tuple(springs, state);
+}
+
+TEST(SpringsForceTest, ZeroDisplacement) {
+    auto [springs, state] = SetUpSpringsForceTest();
+
+    UpdateSystemVariablesSprings(springs, state);
+
+    auto l_ref_host = Kokkos::create_mirror(springs.l_ref);
+    auto l_host = Kokkos::create_mirror(springs.l);
+    auto c1_host = Kokkos::create_mirror(springs.c1);
+    auto f_host = Kokkos::create_mirror(springs.f);
+    auto a_host = Kokkos::create_mirror(springs.a);
+
+    Kokkos::deep_copy(l_ref_host, springs.l_ref);
+    Kokkos::deep_copy(l_host, springs.l);
+    Kokkos::deep_copy(c1_host, springs.c1);
+    Kokkos::deep_copy(f_host, springs.f);
+    Kokkos::deep_copy(a_host, springs.a);
+
+    EXPECT_DOUBLE_EQ(l_ref_host(0), 1.);  // Undeformed length = 1.
+    EXPECT_DOUBLE_EQ(l_host(0), 1.);      // Current length = 1.
+    EXPECT_DOUBLE_EQ(c1_host(0), 0.);     // c1 = 0.
+    expect_kokkos_view_1D_equal(Kokkos::subview(f_host, 0, Kokkos::ALL), {0., 0., 0.});  // No force
+    expect_kokkos_view_2D_equal(
+        Kokkos::subview(a_host, 0, Kokkos::ALL, Kokkos::ALL),
+        {{-10., 0., 0.}, {0., 0., 0.}, {0., 0., 0.}}
+    );
+}
+
+TEST(SpringsForceTest, UnitDisplacement) {
+    auto [springs, state] = SetUpSpringsForceTest();
+
+    state.q(1, 0) = 1.;  // Displace second node in x direction
+    UpdateSystemVariablesSprings(springs, state);
+
+    auto l_ref_host = Kokkos::create_mirror(springs.l_ref);
+    auto l_host = Kokkos::create_mirror(springs.l);
+    auto c1_host = Kokkos::create_mirror(springs.c1);
+    auto c2_host = Kokkos::create_mirror(springs.c2);
+    auto f_host = Kokkos::create_mirror(springs.f);
+    auto a_host = Kokkos::create_mirror(springs.a);
+
+    Kokkos::deep_copy(l_ref_host, springs.l_ref);
+    Kokkos::deep_copy(l_host, springs.l);
+    Kokkos::deep_copy(c1_host, springs.c1);
+    Kokkos::deep_copy(c2_host, springs.c2);
+    Kokkos::deep_copy(f_host, springs.f);
+    Kokkos::deep_copy(a_host, springs.a);
+
+    EXPECT_DOUBLE_EQ(l_ref_host(0), 1.);                       // Undeformed length = 1.
+    EXPECT_DOUBLE_EQ(l_host(0), 2.);                           // Current length = 2.
+    EXPECT_DOUBLE_EQ(c1_host(0), -5.);                         // c1 = -5.
+    EXPECT_DOUBLE_EQ(c2_host(0), 10. * 1. / std::pow(2., 3));  // c2 = 10. / 8.
+    expect_kokkos_view_1D_equal(
+        Kokkos::subview(f_host, 0, Kokkos::ALL), {-10., 0., 0.}
+    );  // Force = c1 * {r}
+    expect_kokkos_view_2D_equal(
+        Kokkos::subview(a_host, 0, Kokkos::ALL, Kokkos::ALL),
+        {{-10., 0., 0.}, {0., -5., 0.}, {0., 0., -5.}}
+    );
+}
+
+}  // namespace openturbine::tests