 The frictional effect is modeled as a shear stress at the wall acting on the fluid with uniform properties over any cross section of the duct.For a flow with an upstream Mach number greater than 1.0 in a sufficiently long enough duct, deceleration occurs and the flow can become choked.On the other hand, for a flow with an upstream Mach number less than 1.0, acceleration occurs and the flow can become choked in a sufficiently long duct.