François Kerger

An exact Riemann solver and a Godunov scheme for simulating highly transient mixed flows

The current research aims at deriving a one-dimensional numerical model for describing highly transient mixed flows. In particular, this paper focuses on the development and assessment of a unified numerical scheme adapted to describe free-surface flow, pressurized flow and mixed flow (characterized by the simultaneous occurrence of free-surface and pressurized flows). The methodology includes three steps. First, the authors derived a unified mathematical model based on the Preissmann slot model. Second, a first-order explicit finite volume Godunov-type scheme is used to solve the set of equations. Third, the numerical model is assessed by comparison with analytical, experimental and numerical results. The key results of the paper are the development of an original negative Preissmann slot for simulating sub-atmospheric pressurized flow and the derivation of an exact Riemann solver for the Saint-Venant equations coupled with the Preissmann slot.

1D unified mathematical model for environmental flow applied to steady aerated mixed flows

Hydraulic models available in literature are unsuccessful in simulating accurately and efficiently environmental flows characterized by the presence of both air-water interactions and free-surface/pressurized transitions (aka mixed flows). The purpose of this paper is thus to fill this knowledge gap by developing a unified one-dimensional mathematical model describing free-surface, pressurized and mixed flows with air-water interactions. This work is part of a general research project which aims at establishing a unified mathematical model suitable to describe the vast majority of flows likely to appear in civil and environmental engineering (pure water flows, sediment transport, pollutant transport, aerated flows...). In order to tackle this problem, our original methodology consists in both time- and space-averaging the Local Instant Formulation, which includes field equations for each phase taken separately and jump conditions, over a flow cross-section involving a free-surface. Subsequently, applicability of the model is extended to pressurized flows as well. The first key result is an original 1D Homogeneous Equilibrium Model which describes two-phase free-surface flows. It is proven to be fundamentally multiphase, to take into account scale heterogeneities of environmental flow and to be very easy to solve. Next, applicability of this free-surface model is extended to pressurized flows by using the classical Preissmann slot concept. A second key result here is the introduction of an original negative Preissmann slot to simulate sub-atmospheric pressurized flows. The model is then closed by using constitutive equations suitable for air-water flows. Finally, this mathematical model is discretised by means of a finite volume scheme and validated by comparison with experimental results from a physical model in the case of a steady flow in a large scale gallery.

Three-phase bi-layer model for simulating mixed flows

Mixed flows characterized by a simultaneous occurrence of free surface and pressurized flows are often encountered in hydraulic engineering. Numerous researches have been dedicated to unify the mathematical description of both flows. Herein, shock-capturing models succeed in giving a unique set of equations. However, no method accounts for both air-entrapment and air-entrainment. This study proposes an original model to simulate air–water interactions in mixed flows. The new approach relies on the area-integration of a three-phase model over two layers. The applicability of this free surface model is extended to pressurized flows by a modified pressure term accounting for the dispersed air. The derived modelling system WOLF IMPack is then validated. The code successfully simulates open channel flows, mixed flows and water hammer in a unified framework, including air–water interactions, in structures like the drainage network.

Experimental and Numerical Investigation of Mixed Flow in a Gallery

Experimental investigations on a physical model of a gallery performed in the Laboratory of Structures Hydraulics at the University of Liege are presented. The study focuses on the influence of the gallery aeration rate on the mixed flow pattern and the pressure distribution. In particular, the effect of air vents on the flow patterns and the release capacity of the gallery are assessed. An unusual mechanism leading to a two-phase instability is also pointed out when the aeration rate is not sufficient. Experimentations are completed with numerical computations performed with an original 1D model developed by the authors to give new insight into the mechanisms involved.

Numerical Simulation Of One-dimensionalMixed Flow With Air/water Interaction

An original one-dimensional unified numerical model dealing with aerated mixed flow, characterized by a simultaneous occurrence of free-surface and pressurized flow, is derived and applied to the case of a gallery. The mathematical model is based on a new integration of the Homogeneous Equilibrium Model (HEM) over the cross section of a free-surface flow and consists of a simple set of equations analogous to the Saint-Venant equations. In addition, both free-surface and pressurized flows are mathematically modeled by means of the free-surface set of equations (Preissmann slot model). The original concept of the negative Preissmann slot is proposed to simulate sub-atmospheric pressure. This model is shown to be particularly well suited for the simulation of bubbly and intermittent flows.

Hydrodynamic forces acting on vertically translating bodies in free surface water

The dynamics of free surface flow induced by a partially or totally submerged moving body generates non-straightforward effort distribution. In the design process of moving hydraulic structures and their operation device, the pressure field acting on them is of prime interest. A fundamental example of such a phenomenon, namely the vertical uplift of a submerged horizontal rigid sheet, is hereafter experimentally analyzed. The present study includes the description of the experimental apparatus built in the HACH laboratory and the description of the rigorous experimental procedure used. The purpose of the analysis is to identify the fundamental hydrodynamic mechanisms involved.