[MaL] Add KelvinVector to symmetric tensor cnv.

This is giving a shortened version of the matrix
in a vector form.

MathLib/KelvinVector.cpp

@@ -9,6 +9,8 @@
 
 #include "KelvinVector.h"
 
+#include "BaseLib/Error.h"
+
 namespace MathLib
 {
 namespace KelvinVector
@@ -78,5 +80,50 @@ Eigen::Matrix<double, 3, 3> kelvinVectorToTensor(
     return m;
 }
 
+template <>
+Eigen::Matrix<double, 4, 1> kelvinVectorToSymmetricTensor(
+    Eigen::Matrix<double, 4, 1, Eigen::ColMajor, 4, 1> const& v)
+{
+    Eigen::Matrix<double, 4, 1> m;
+    m << v[0], v[1], v[2], v[3] / std::sqrt(2.);
+    return m;
+}
+
+template <>
+Eigen::Matrix<double, 6, 1> kelvinVectorToSymmetricTensor(
+    Eigen::Matrix<double, 6, 1, Eigen::ColMajor, 6, 1> const& v)
+{
+    Eigen::Matrix<double, 6, 1> m;
+    m << v[0], v[1], v[2], v[3] / std::sqrt(2.), v[4] / std::sqrt(2.),
+        v[5] / std::sqrt(2.);
+    return m;
+}
+
+template <>
+Eigen::Matrix<double, Eigen::Dynamic, 1, Eigen::ColMajor, Eigen::Dynamic, 1>
+kelvinVectorToSymmetricTensor(Eigen::Matrix<double,
+                                            Eigen::Dynamic,
+                                            1,
+                                            Eigen::ColMajor,
+                                            Eigen::Dynamic,
+                                            1> const& v)
+{
+    if (v.size() == 4)
+    {
+        return kelvinVectorToSymmetricTensor<4>(v);
+    }
+    else if (v.size() == 6)
+    {
+        return kelvinVectorToSymmetricTensor<6>(v);
+    }
+    else
+    {
+        OGS_FATAL(
+            "Kelvin vector to tensor conversion expected an input vector of "
+            "size 4 or 6, but a vector of size %d was given.",
+            v.size());
+    }
+}
+
 }  // namespace KelvinVector
 }  // namespace MathLib

MathLib/KelvinVector.h

@@ -125,6 +125,24 @@ Eigen::Matrix<double, 3, 3> kelvinVectorToTensor(Eigen::Matrix<double,
                                                                KelvinVectorSize,
                                                                1> const& v);
 
+/// Conversion of a Kelvin vector to a short vector representation of a
+/// symmetric 3x3 matrix.
+///
+/// In the 2D case the entries for the xx, yy, zz, and xy components are stored.
+/// In the 3D case the entries for the xx, yy, zz, xy, yz, and xz components in
+/// that particular order are stored.
+///
+/// Only implementations for KelvinVectorSize 4 and 6, and dynamic size vectors
+/// are provided.
+template <int KelvinVectorSize>
+Eigen::Matrix<double, KelvinVectorSize, 1, Eigen::ColMajor, KelvinVectorSize, 1>
+kelvinVectorToSymmetricTensor(Eigen::Matrix<double,
+                                            KelvinVectorSize,
+                                            1,
+                                            Eigen::ColMajor,
+                                            KelvinVectorSize,
+                                            1> const& v);
+
 }  // namespace KelvinVector
 }  // namespace MathLib