Bernard Brachet
Expert hydraulique, Egis
Published on August 03, 2018

Reading time : 3 min

Digital design of hydraulic structures

Three-dimensional calculations applied to fluid dynamics are today taking on an increasingly important role in engineering. We take a closer look at this innovative method.

Modélisation numérique d'un ouvrage hydraulique

- Crédits : © B. Brachet - Egis

Very often, modelling is necessary to design hydraulic infrastructure and understand the movements that happen inside it. As the liquid flows in these structures are overwhelmingly made up of chaotic movements called “turbulences”, engineers must be able to forecast their effects when designing a structure or a system. Unfortunately, turbulent movement is very complex, most often displaying three dimensional and unstable characteristics.

This is why digital simulation can be particularly useful to the engineer, as analytical calculation would be inefficient here. One of the main advantages of digital calculation is its propensity to produce virtual variations of the geometric, dynamic and physical parameters of the problem addressed, thereby avoiding the need to conduct and repeat time-consuming experiments.

This method, known as computational fluid dynamics, or CFD (see below), contributes more detailed knowledge of complex hydraulic phenomena and as a result helps to optimise the structure’s design. Quicker to implement than a physical model approach, it is also cheaper and thus helps reduce the construction costs of the structure. All these reasons have led Egis to make this methodology one of its production standards.

A classic example: the upgrade of the SIAAP plant in Clichy

The Paris region sanitation authority SIAAP, which in 2012 commenced a project to upgrade its Clichy plant (a massive combined sewerage pre-treatment and distribution facility), appointed us to carry out most of the hydraulic studies for the project in its design phase. This assignment was complex in view of the structures to be built, their massive scale and the similarly exceptional flows involved.

The project comprises the total overhaul of the infrastructure used to receive and distribute wastewater to the various parts of the plant, the construction of a new pre-treatment unit and the construction of a 70,000m3 storage facility and its feeding system (down shaft, gallery, etc). All these systems have highly complex geometrical characteristics and simulated shape.

While the modelling approach should never be considered as the only possible method, in this particular case it is both advantageous and complementary to traditional methods which use analytical calculations and a physical model. It furthermore proved its reliability on several occasions, in particular when it very clearly described the hydraulic and airflow workings of the down shaft, a structure subject to high pressure. These workings were later substantiated through verifications on a physical model.

CFD at a glance

Cette méthode de simulation consiste à étudier, dans une géométrie donnée, les mouvements d'un fluide ou leurs effets, par la résolution numérique des équations régissant le fluide. Cette résolution passe par trois étapes :

  1. Mise en données du problème : définition d'une géométrie, d'un maillage discrétisant le domaine de calcul, choix des modèles et des méthodes numériques employés.
  2. Résolution numérique du problème : intégration des données dans un logiciel de traitement appelé "solveur", dont la fonction première est de résoudre les équations régissant la physique des fluides.
  3. Exploitation des résultats : analyse de la cohérence, extraction et représentation sous forme graphique des données physiques caractérisant le fonctionnement de l'ouvrage.

L'exploitation des résultats passe le plus souvent par des logiciels de CFD. Egis utilise, en particulier, la suite complète ANSYS 17 (solveur CFX).

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