[FEM] Elmer: fix initial flow velocity

- we must allow must be several velocities because there can be several fluid bodies
- don't output flow velocity for solid bodies

src/Mod/Fem/femsolver/elmer/writer.py

@@ -1,6 +1,7 @@
 # ***************************************************************************
 # *   Copyright (c) 2017 Markus Hovorka <m.hovorka@live.de>                 *
 # *   Copyright (c) 2020 Bernd Hahnebach <bernd@bimstatik.org>              *
+# *   Copyright (c) 2022 Uwe Stöhr <uwestoehr@lyx.org>                      *
 # *                                                                         *
 # *   This file is part of the FreeCAD CAx development system.              *
 # *                                                                         *
@@ -23,7 +24,7 @@
 # ***************************************************************************
 
 __title__ = "FreeCAD FEM solver Elmer writer"
-__author__ = "Markus Hovorka"
+__author__ = "Markus Hovorka, Uwe Stöhr"
 __url__ = "https://www.freecadweb.org"
 
 ## \addtogroup FEM
@@ -1442,19 +1443,37 @@ class Writer(object):
                         name, "Compressibility Model",
                         m["CompressibilityModel"])
 
+    def _outputInitialVelocity(self, obj, name):
+        # flow only makes sense for fluid material
+        if self._isBodyMaterialFluid(name):
+            if obj.VelocityXEnabled:
+                velocity = self._getFromUi(obj.VelocityX, "m/s", "L/T")
+                self._initial(name, "Velocity 1", velocity)
+            if obj.VelocityYEnabled:
+                velocity = self._getFromUi(obj.VelocityY, "m/s", "L/T")
+                self._initial(name, "Velocity 2", velocity)
+            if obj.VelocityZEnabled:
+               velocity = self._getFromUi(obj.VelocityZ, "m/s", "L/T")
+               self._initial(name, "Velocity 3", velocity)
+
     def _handleFlowInitialVelocity(self, bodies):
-        obj = self._getSingleMember("Fem::ConstraintInitialFlowVelocity")
-        if obj is not None:
-            for name in bodies:
-                if obj.VelocityXEnabled:
-                    velocity = self._getFromUi(obj.VelocityX, "m/s", "L/T")
-                    self._initial(name, "Velocity 1", velocity)
-                if obj.VelocityYEnabled:
-                    velocity = self._getFromUi(obj.VelocityY, "m/s", "L/T")
-                    self._initial(name, "Velocity 2", velocity)
-                if obj.VelocityZEnabled:
-                    velocity = self._getFromUi(obj.VelocityZ, "m/s", "L/T")
-                    self._initial(name, "Velocity 3", velocity)
+        initialVelocities = self._getMember("Fem::ConstraintInitialFlowVelocity")
+        for obj in initialVelocities:
+            if obj.References:
+                for name in obj.References[0][1]:
+                    self._outputInitialVelocity(obj, name)
+                self._handled(obj)
+            else:
+                # if there is only one initial velocity without a reference
+                # add it to all fluid bodies
+                if len(initialVelocities) == 1:
+                    for name in bodies:
+                        self._outputInitialVelocity(obj, name)
+                else:
+                    raise WriteError(
+                        "Several initial velocities found without reference to a body.\n"
+                        "Please set a body for each initial velocity."
+                    )
             self._handled(obj)
 
     def _handleFlowBndConditions(self):

src/Mod/Fem/femtaskpanels/task_constraint_initialflowvelocity.py

@@ -34,6 +34,7 @@ import FreeCAD
 import FreeCADGui
 from FreeCAD import Units
 
+from femguiutils import selection_widgets
 from femtools import femutils
 from femtools import membertools
 
@@ -42,10 +43,23 @@ class _TaskPanel(object):
 
     def __init__(self, obj):
         self._obj = obj
+
         self._paramWidget = FreeCADGui.PySideUic.loadUi(
             FreeCAD.getHomePath() + "Mod/Fem/Resources/ui/InitialFlowVelocity.ui")
         self._initParamWidget()
-        self.form = [self._paramWidget]
+
+        # geometry selection widget
+        # start with Solid in list!
+        self._selectionWidget = selection_widgets.GeometryElementsSelection(
+            obj.References,
+            ["Solid", "Face"],
+            True,
+            False
+        )
+
+        # form made from param and selection widget
+        self.form = [self._paramWidget, self._selectionWidget]
+
         analysis = obj.getParentGroup()
         self._mesh = None
         self._part = None
@@ -69,6 +83,8 @@ class _TaskPanel(object):
         return True
 
     def accept(self):
+        if self._obj.References != self._selectionWidget.references:
+            self._obj.References = self._selectionWidget.references
         self._applyWidgetChanges()
         self._obj.Document.recompute()
         FreeCADGui.ActiveDocument.resetEdit()