The flow is incompressible and Newtonian. Coordinates are orthogonal. Flow is 2D: u3 = ∂u1/∂x3 = ∂u2/∂x3 = 0 The first two scale factors of the coordinate system are independent of the last coordinate: ∂h1/∂x3 = ∂h2/∂x3 = 0, otherwise extra terms appear.