Integrating over all appropriate velocities within the constraint { v > 0 0 < θ θ < π π / 2 0 < ϕ ϕ < 2 π π yields the forward momentum transfer per unit time per unit area (also known as shear stress): τ τ ± ± = 1 4 v ¯ ¯ n ⋅ ⋅ m ( u 0 ± ± 2 3 l d u d y )