Cross-Channel Flow in a rectangular geometry

I am simulating the flow in a single screw extruder. I simplified the geometry by unwrapping the material from the screw channel. Therefore the geometry of my simulation is rectangular (Inlet and outlet are in the zy plane, geometry is attached). The upper wall of the rectangle moves in both the x- and y-direction and the other walls have a no-slip boundary condition. The stationary study of the velocity shows that the velocity behaves like in the screenshots attached (Down channel flow and cross channel flow).

Regarding the Cross-Channel Flow: Why is the flow not changing its flow direction although it is moving straight ahead towards the wall (because of the y velocity of the upper moving wall)? How are positive and negative flows distinguished in COMSOL?