[navierstokes] Improve diffusive fluxes in case of boundaries (helpful for two-eq rans models)

dumux/freeflow/navierstokes/staggered/fluxvariables.hh

@@ -432,7 +432,9 @@ private:
         const auto& outsideVolVars = elemVolVars[normalFace.outsideScvIdx()];
 
         // Get the averaged viscosity at the staggered face normal to the current scvf.
-        const Scalar muAvg = (insideVolVars.effectiveViscosity() + outsideVolVars.effectiveViscosity()) * 0.5;
+        const Scalar muAvg = normalFace.boundary()
+                             ? insideVolVars.effectiveViscosity()
+                             : (insideVolVars.effectiveViscosity() + outsideVolVars.effectiveViscosity()) * 0.5;
 
         // For the normal gradient, get the velocities perpendicular to the velocity at the current scvf.
         // The inner one is located at staggered face within the own element,

test/freeflow/rans/CMakeLists.txt

@@ -7,7 +7,7 @@ dune_add_test(NAME test_pipe_laufer_zeroeq
               COMMAND ${CMAKE_SOURCE_DIR}/bin/testing/runtest.py
               CMD_ARGS  --script fuzzy
                         --files ${CMAKE_SOURCE_DIR}/test/references/pipe_laufer_zeroeq.vtu
-                                ${CMAKE_CURRENT_BINARY_DIR}/pipe_laufer_zeroeq_reference-00020.vtu
+                                ${CMAKE_CURRENT_BINARY_DIR}/pipe_laufer_zeroeq_reference-00018.vtu
                         --command "${CMAKE_CURRENT_BINARY_DIR}/test_pipe_laufer_zeroeq test_pipe_laufer_reference.input")
 
 dune_add_test(NAME test_pipe_zeroeqni

test/freeflow/ransnc/CMakeLists.txt

@@ -15,6 +15,6 @@ dune_add_test(NAME test_channel_zeroeq2cni
               COMMAND ${CMAKE_SOURCE_DIR}/bin/testing/runtest.py
               CMD_ARGS       --script fuzzy
                              --files ${CMAKE_SOURCE_DIR}/test/references/test_channel_zeroeq2cni.vtu
-                                     ${CMAKE_CURRENT_BINARY_DIR}/test_channel_zeroeq2cni-00018.vtu
+                                     ${CMAKE_CURRENT_BINARY_DIR}/test_channel_zeroeq2cni-00017.vtu
                              --command "${CMAKE_CURRENT_BINARY_DIR}/test_channel_zeroeq2cni test_channel_zeroeq2cni.input")
 target_compile_definitions(test_channel_zeroeq2cni PUBLIC "NONISOTHERMAL=1")