[FEM] fix material task dialog for fluids

- this fixes a mistake from commit 821dc915
- it also consistently use the thermal expansion coefficient
- add code comments

now the material task dialog works with all fluid materials without errors or warnings

src/Mod/Fem/femtaskpanels/task_material_common.py

@@ -141,8 +141,8 @@ class _TaskPanel:
         self.parameterWidget.label_category.setText(self.obj.Category)
         if self.obj.Category == "Fluid":
             self.parameterWidget.groupBox_mechanical.setVisible(0)
-            self.parameterWidget.label_expansion_coefficient.setVisible(0)
-            self.parameterWidget.input_fd_expansion_coefficient.setVisible(0)
+            self.parameterWidget.label_vol_expansion_coefficient.setVisible(0)
+            self.parameterWidget.input_fd_vol_expansion_coefficient.setVisible(0)
         else:
             self.parameterWidget.groupBox_fluidic.setVisible(0)
             self.parameterWidget.label_vol_expansion_coefficient.setVisible(0)
@@ -265,10 +265,9 @@ class _TaskPanel:
     # choose material ****************************************************************************
     def get_material_card(self, material):
         for a_mat in self.materials:
+            # check if every item of the current material fits to a known material card
+            # if all items were found we know it is the right card
             unmatched_items = set(self.materials[a_mat].items()) ^ set(material.items())
-            # print(a_mat + "  -->  unmatched_items = " + str(len(unmatched_items)))
-            # if len(unmatched_items) < 4:
-            #     print(unmatched_items)
             if len(unmatched_items) == 0:
                 return a_mat
         return ""
@@ -475,7 +474,7 @@ class _TaskPanel:
                     "ThermalExpansionCoefficient not found in {}\n"
                     .format(self.material["Name"])
                 )
-                self.material["VolumetricThermalExpansionCoefficient"] = "0 1/K"
+                self.material["ThermalExpansionCoefficient"] = "0 1/K"
             if "VolumetricThermalExpansionCoefficient" in self.material:
                 # unit type VolumetricThermalExpansionCoefficient is ThermalExpansionCoefficient
                 vol_ther_ex_co = self.material["VolumetricThermalExpansionCoefficient"]
@@ -487,10 +486,9 @@ class _TaskPanel:
                     )
                     self.material["VolumetricThermalExpansionCoefficient"] = "0 1/K"
             else:
-                if "ThermalExpansionCoefficient" in self.material:
-                    the_index = "VolumetricThermalExpansionCoefficient"  # line was to long
-                    self.material[the_index] = self.material["ThermalExpansionCoefficient"]
-                else:
+                # as fallback only add VolumetricThermalExpansionCoefficient if there is no
+                # ThermalExpansionCoefficient
+                if "ThermalExpansionCoefficient" not in self.material:
                     self.material["VolumetricThermalExpansionCoefficient"] = "0 1/K"
         # Thermal properties
         if "ThermalConductivity" in self.material:

src/Mod/Material/FluidMaterial/Water.FCMat

@@ -17,7 +17,7 @@ PrandtlNumber = 7.56
 SpecificHeat = 4182 J/kg/K
 ThermalConductivity = 0.591 W/m/K
 ; https://en.wikipedia.org/wiki/Water
-VolumetricThermalExpansionCoefficient = 2.07e-4 m/m/K
+ThermalExpansionCoefficient = 2.07e-4 m/m/K
 
 [Electrical]
 RelativePermittivity = 80.0