A mathematical model for unsteady mixed flows in closed water pipes

TL;DR

This paper introduces a new mathematical model for unsteady mixed flows in closed water pipes, capturing free surface and pressurized states, with a focus on transitions and non-uniform geometries.

Contribution

It derives a unified model combining free surface and pressurized flow dynamics using asymptotic analysis for the first time.

Findings

Derivation of the FS-model for free surface flows

Development of the Pres-model for pressurized flows

Introduction of the PFS-model for mixed flow transitions

Abstract

We present the formal derivation of a new unidirectional model for unsteady mixed flows in non uniform closed water pipe. In the case of free surface incompressible flows, the \FS-model is formally obtained, using formal asymptotic analysis, which is an extension to more classical shallow water models. In the same way, when the pipe is full, we propose the \Pres-model, which describes the evolution of a compressible inviscid flow, close to gas dynamics equations in a nozzle. In order to cope the transition between a free surface state and a pressured (i.e. compressible) state, we propose a mixed model, the \PFS-model, taking into account changes of section and slope variation.