ENH: added references to topology optimisation headers

src/optimisation/adjointOptimisation/adjoint/optimisation/designVariables/levelSet/levelSetDesignVariables.H

@@ -29,7 +29,32 @@ Class
 
 Description
     Signed distance field design variables for level-set based topology
-    optimization.
+    optimization (topO).
+
+    A great reference for various aspects of level-set-based topO can be found
+    in
+
+    Reference:
+    \verbatim
+        van Dijk, N.P., Maute, K., Langelaar, M., & & van Keulen, F. (2013).
+        Level-set methods for structural topology optimization: a review.
+        Structural and Multidisciplinary Optimization, 48, 437–472.
+        https://doi.org/10.1007/s00158-013-0912-y
+    \endverbatim
+
+    The implemented approach borrows from the references therein, but does not
+    follow any of them exactly. The underlaying field of the design variables
+    is not the level-set field but affects the latter through:
+    a) a regularisation approach similar to that used in porosity-based topO
+    (see the regularisation class and references therein) and
+    b) a re-initialisation strategy. Since solving a modified Hamilton-Jacobi
+    equation poses a number of numerical challenges, the signed distance field
+    is obtained by identifying the zero level-set contour using the
+    cutFaceIso, cutCellIso infrastucture (see references therein for the
+    methodology) and using these "cut-faces" as seeds for meshWave.
+
+SourceFiles
+    levelSetDesignVariables.C
 
 \*---------------------------------------------------------------------------*/
 

src/optimisation/adjointOptimisation/adjoint/optimisation/designVariables/topODesignVariables/betaMax/Darcy/betaMaxDarcy.H

@@ -37,6 +37,15 @@ Description
     - 2D: area of the inlet patches divided by the span in the empty direction
     - 3D: the hydraulic diameter of the inlet patches
 
+    Reference:
+    \verbatim
+        Olesen, L. H., Okkels, F., & Bruus H. (2006).
+        A high-level programming-language implementation of topology
+        optimization applied to steady-state Navier–Stokes flow.
+        International Journal for Numerical Methods in Engineering,
+        65(7), 975-1001.
+        https://doi.org/10.1002/nme.1468
+    \endverbatim
 
 SourceFiles
     betaMaxDarcy.C

src/optimisation/adjointOptimisation/adjoint/optimisation/designVariables/topODesignVariables/regularisation/fieldRegularisation.H

@@ -33,6 +33,18 @@ Description
     optimisation to regularise the designed geometries and obtain 'grid
     independent' solutions.
 
+    Reference:
+    \verbatim
+        Largely follows the PDE-based smoothing of
+
+            Lazarov, B. S., & Sigmund O. (2010).
+            Filters in topology optimization based on Helmholtz-type
+            differential equations.
+            International Journal for Numerical Methods in Engineering,
+            86(6), 765-781.
+            https://doi.org/10.1002/nme.3072
+    \endverbatim
+
 SourceFiles
     fieldRegularisation.C
 

src/optimisation/adjointOptimisation/adjoint/optimisation/designVariables/topODesignVariables/regularisation/regularisationPDE/Helmoltz/Helmholtz.C

@@ -82,7 +82,7 @@ void Foam::Helmholtz::solveEqn
             zeroGradientFvPatchScalarField::typeName
         )
     );
-    // If solution corresponds to the topology density field, modify boundary
+    // If solution corresponds to the topology porosity field, modify boundary
     // conditions accordingly
     if (isTopoField && growFromWalls_)
     {

src/optimisation/adjointOptimisation/adjoint/optimisation/designVariables/topODesignVariables/topODesignVariables.H

@@ -29,7 +29,27 @@ Class
     Foam::topODesignVariables
 
 Description
-    Design variables for density-based topology optimisation (topO) problems
+    Design variables for porosity-based topology optimisation (topO) problems
+
+    Reference:
+    \verbatim
+        The main idea behind porosity-based topO can be found in
+
+            Papoutsis-Kiachagias, E. M., & Giannakoglou, K. C. (2014).
+            Continuous Adjoint Methods for Turbulent Flows, Applied to Shape
+            and Topology Optimization: Industrial Applications.
+            Archives of Computational Methods in Engineering, 23(2), 255-299.
+            http://doi.org/10.1007/s11831-014-9141-9
+
+        with regularisation and projection approaches following
+
+            Lazarov, B. S., & Sigmund O. (2010).
+            Filters in topology optimization based on Helmholtz-type
+            differential equations.
+            International Journal for Numerical Methods in Engineering,
+            86(6), 765-781.
+            https://doi.org/10.1002/nme.3072
+    \endverbatim
 
 SourceFiles
     topODesignVariables.C

src/optimisation/adjointOptimisation/adjoint/optimisation/designVariables/topODesignVariables/topOVariablesBase/topOVariablesBase.H

@@ -205,7 +205,7 @@ public:
         //  The interface is computed as an isoValue contour of the indicator
         //  field
         //  - 0 distance contour for levelSet or
-        //  - 0.5 beta contour for density-based topO.
+        //  - 0.5 beta contour for porosity-based topO.
         //  For levelSet topO, the process of identitying the contour sets also
         //  the seeds for computing the distance field in the entire domain
         void writeFluidSolidInterface