A model for unsteady mixed flows in non uniform closed water pipes and a well-balanced finite volume

Full Title: A model for unsteady mixed flows in non uniform closed water pipes and a well-balanced finite volume

Abstract:

We present the derivation of a new unidirectional model for unsteady mixed flows in non uniform closed water pipes. We introduce a local reference frame to take into account the local perturbation caused by the changes of section and slope. Then an asymptotic analysis is performed to obtain a model for free surface flows and another one for pressurized flows.

By coupling these models through the transition points using a common set of variables and a suitable pressure law, we obtain a simple formulation called PFS-model close to the shallow water equations with source terms. It takes into account the changes of section and the slope variation in a continuous way through transition points. Transition points between the two types of flows are treated as a free boundary associated to a discontinuity of the gradient of pressure.

The numerical simulation is performed by making use of a VFRoe scheme that we adapted to take into account geometrical source terms in the convection matrix. By a suitable definition of the convection matrix, we obtain an exactly well-balanced scheme.

Finally some numerical tests are presented.