This contribution from Livesey and Turner is of considerable interest. Following work by Horlock and Lewis(1) on the flow of an inviscid fluid through a diffuser from which an analytic solution was obtained, the present authors show that such a solution is only significant if the boundary layer at entry to the diffuser is in equilibrium. Non-equilibrium profiles in pipes, such as those produced using short inlet lengths or gauzes, will continue to change their state beyond the plane of measurement and so will influence the state of flow through whatever pipe component succeeds them. Our work at the University of Leicester accords with this.

Most investigators into the characteristics of fluid flow through diffusers have used methods of analysis based on the Karman Momentum Equation (e.g. Spence and Norbury; Ross and Fraser; Schlichting). The pressure rise which would accompany the tendency of an inlet profile to achieve its equilibrium state is not considered part of the diffusion process, although the corresponding pressure rise occurring in a downstream pipe length, as the outlet boundary layer velocity profile regains its equilibrium state, is recognised and is calculable.