Practical Capacities and Challenges of 3D CFD Modelling: Feedback Experience in Engineering Projects

Abstract

Until recent years, detailed studies of hydraulic structures were made almost systematically by hydraulic physical models offering great capabilities to understand the hydraulic behaviour of the studied flows. On the other hand, physical models are in general rather expensive and time consuming, and can only represent some hydraulic phenomenon of interest. This depending on the similarity law for which they have been calibrated. Nowadays, given the increasing computational capabilities and further development of user-friendly interfaces, 3D CFD modelling can balance these issues and provide a flexible and powerful tool to support physical modelling, or even replace it in certain cases. The numerical modelling of complex hydraulic flows is becoming more and more efficient and cost effective, providing a powerful tool for hydraulic analysis, from preliminary to detailed studies. This article describes how the 3D CFD tool has been first validated on a large range of hydraulic phenomena, then how it is exploited by Tractebel on several large projects to analyse and optimize the design or operation of hydraulic structures. Several way to exploit 3D CFD models are developed, either at early stages of the design process, ahead or together with a physical model in order to optimize it exploitation. Application examples are presented, among which the modelling of large gated overflow and orifice spillways … Possibilities and limitations of the 3D CFD tool on these various topics is discussed. Simplified modelling methods has been developed to address specific issues, in an optimized way, among which applications on vertical 2D, and hybrid 2D planar/3D models are presented, the advantages provided by these approaches are highlighted with respect to the addressed issues. The procedures developed for each particular case is described so as to provide indicative frameworks for deploying 3D CFD tools in engineering projects, and provide added value. Based on Tractebel experience, the challenges that have still to be overcome are discussed, with an emphasis put on the modelling of air entrainment. Physical phenomena involved in monophasic flows, however complex, were proven to be well simulated by 3D CFD software. This as long as the modeller has sufficient hydraulic knowledge and experience to adapt a priori the model parameters, mesh and boundary conditions and to carry out an a posteriori critical analysis of the results. In spillway design, this statement applies in particular to the control and conveyance structures, such as channels, weirs, spillway chutes, or flip buckets. When it comes to energy dissipation structures, such as stepped spillways, stilling basins or plunge pools, careful calibration tests of the air entrainment parameters must be performed for each case, questioning the practical and standalone use of current 3D biphasic models in hydraulic engineering. Nevertheless, once the air entrainment parameters have been calibrated, hybrid approach for such complex analyses, combining physical and numerical models, is of main interest, leading to time savings, better physical understanding and enhanced flexibility in the design activities.