[doc] Add citation

doc/handbook/dumux-handbook.bib

@@ -784,6 +784,23 @@ url = {http://www.sciencedirect.com/science/article/pii/S0169772204001160}
   pages = {255--293}
 }
 
+@Article{Kunz2016,
+author={{Kunz, P.
+and Zarikos, I. M.
+and Karadimitriou, N. K.
+and Huber, M.
+and Nieken, U.
+and Hassanizadeh, S. M.}},
+title={{Study of Multi-phase Flow in Porous Media: Comparison of SPH Simulations with Micro-model Experiments}},
+journal={{Transport in Porous Media}},
+year={2016},
+volume={114},
+number={2},
+pages={581--600},
+doi={10.1007/s11242-015-0599-1},
+url={http://dx.doi.org/10.1007/s11242-015-0599-1}
+}
+
 @BOOK{A3:lancaster:1969,
   title = {{Theory of Matrices}},
   publisher = {Academic Press, Inc.\ (London) Ltd.},

dumux/freeflow/stokes/localresidual.hh

@@ -171,6 +171,8 @@ protected:
      *
      * \param flux The advective flux over the sub-control-volume face for each component
      * \param fluxVars The flux variables at the current SCV/boundary face
+     *
+     * An additional wall friction term can be added to account for a dimensional reduction from 3d to 2d (Kunz et al., 2016) \cite Kunz2016 <BR>
      */
     void computeAdvectiveFlux(PrimaryVariables &flux,
                               const FluxVariables &fluxVars) const