Question

The angle between viscous force and the motion of fluid is

Answer 1

 this is a basic fluid mechanics problem but I have one thing I do not understand.

I am about to solve a problem with steady, incompressible, parallel, laminar flow of viscous fluid falling between two infinite walls, inclined at an angle

θ

θ

. There is no applied pressure driving the flow, it falls by gravity alone. And I want to calculate the velocity field.

In order to solve it I use Navier-Stokes