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Abstract

Experiments show that pipe flows with large expansion angle and area ratio always accompany with significant amount of energy loss, actually energy loss ratios over 70% of incoming flow kinetic energy are commonplace. It is generally believed that all the energy loss is entirely due to viscous dissipation effect. In this paper, several interesting demonstrations have been given to prove the existence and importance of impact energy loss. Literally, impact energy loss is usually more important than viscous energy loss in many cases. In fact, the pipe expansion flow can be configured as high-speed flow ramming into low-speed flow and then moving together, this situation is similar to rope-lifting problem in system dynamics as illustrated herein as continuous inelastic impact process with unavoidable impact energy loss. Virtually the intrinsic negligence of impact energy loss is the original major cause for many weird paradoxes and disaster solutions in fluid dynamics. Moreover, in CFD practice, many kinds of ad hoc treatments such as artificial viscosity, numerical dissipation, upwinding and so forth are devised awkwardly in an attempt to somehow manipulate a certain amount of disguised energy dissipation that is largely due to impact. These treatments are done as usual without recognizing what an important role the impact energy loss is playing in the game. More surprisingly, this intrinsic negligence of impact energy loss in deriving process renders the Euler equation as a non-genuine momentum equation which is hardly solvable, and in turn unveils the even more stunning role of impact energy loss as the triggering concept for a full-scale historical revolution in FD/CFD as deliberated in the related paper [10].

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