Pumping effects in models of periodically forced flow configurations

A periodically forced system of differential equations is defined to be a pump, if there exists an asymptotically periodic solution with non-equilibrium mean. It is proved that such systems exist. The definition is based on physical and numerical observations of pumping in (models of) asymmetric flow configurations. For models with rigid pipes and tanks, physical explanations for the pumping effects are derived. One of the pumps is an internally forced linear system. For externally forced nonlinear rigid pipe models, necessary and sufficient conditions for pumping are given. It is then demonstrated in a general setting that no externally forced linear pump exists. (c) 2006 Elsevier B.V. All rights reserved.