I've imported a 2D element mesh from another program as a list of triangle coordinates {{x1,y1},{x2,y2}....} ("nodes") and element connections {{n1,n2,n3},...} ("elements"). Then I create an ElementMesh as follows:
mesh = ToElementMesh["Coordinates" -> nodes, "MeshElements" -> {TriangleElement[elements]}, "BoundaryMarkerFunction" -> boundaryMarkerFunction]; I was hoping this would automatically assign ElementMarkers to each boundary edge, using the boundaryMarkerFunction specified in the documentation for ToBoundaryMesh:
boundaryMarkerFunction = Compile[{{boundaryElementCoords, _Real, 3}, {pointMarkres, _Integer, 2}}, Range[Length[boundaryElementCoords]]]; However, no boundary edges actually get ElementMarkers assigned to them: 
How can I assign ElementMarkers to these boundary edges? (Even a randomized assignment would be okay, as long as each edge gets a unique marker). I just need a way to reference particular boundary edges for a NeumannValue condition.
Any help would be greatly appreciated. Thanks!
Edit: Here is what I get from running
Show[ mesh["Wireframe"], mesh["Wireframe"["MeshElement" -> "BoundaryElements", "MeshElementMarkerStyle" -> Red]]]:
after trying the suggestions from @user21 : 
