The viscosity of a fluid is a measure of its tendency to resist an internal shearing force.
Viscosity = Shear Stress / Shear Rate
Shear Stress = Force / Area = Mass(M) x Acceleration(Ft/sec/sec) / Ft x Ft
In other words the Shear Stress can be increased or decreased by either changing the velocity of the liquid (acceleration) and/or the pipe size.
Shear Rate = Velocity / Film Thickness = Ft/sec / Ft = 1/Sec (Reciprocal Seconds)
The rate is a function of the flow and the pipe size measured in reciprocal seconds shown in the following graph.