Numerical simulations of air-water flow and energy dissipation for safe spillway discharge

QC 20260506

Abstract

Spillway is a critical hydraulic structure designed to regulate water level, prevent overtopping, and ensure a safe outlet for water passage from the reservoir to downstream. A chute spillway may experience cavitation if the flow velocity exceeds from 20 m/s, requiring its design to be optimized. Many existing energy dissipation structures were designed using lower flood standards and are inadequate under current flood conditions. Cavitation damage in chute spillways of high-head dams poses a risk to structural integrity and dam safety. Aeration in high-velocity spillway flow mitigates bubble collapse and sub-atmospheric pressure conditions, increasing bulk flow depth and reducing the potential for cavitation damage to spillway surfaces and sidewalls. The investigation of such flow behaviour is complex due to high turbulence and the unsteady nature of air-water interactions. 

To evaluate such scenarios, physical modeling is commonly used to examine spillway capacity and energy dissipation. However, accurately measuring air entrainment under high-flow conditions remains challenging. Computational Fluid Dynamics (CFD) simulations complement physical experiments and enable independent hydraulic investigations, providing valuable insight into air-water flow behaviour and energy dissipation mechanisms.This thesis presents a comprehensive numerical investigation of two-phase flow and energy dissipation in a spillway, focusing on high-velocity discharge conditions that are crucial to dam safety. The primary objective is to use advanced numerical modelling capabilities to assess spillway hydraulic performance and to provide practical guidance for design and operation. The methodology employs advanced CFD techniques, including Volume of Fluid (VOF) and Mixture models for multiphase flow, alongside a hierarchy of turbulence models, including Reynolds-Averaged Navier-Stokes (RANS), Detached Eddy Simulation (DES), Delayed Detached Eddy Simulation (DDES), and Large Eddy Simulation (LES). The numerical models are rigorously validated against experimental data.

Paper I presents an investigation of four geometrical configurations flat and pooled stepped spillways. Key findings of Paper I reveal that pooled stepped spillway configurations enhance energy dissipation and reduce cavitation risk compared to flat stepped designs. Additionally, pooled steps demonstrated superior overall hydraulic performance and improved flow behavior, achieving the lowest downstream velocities and effectively minimizing cavitation potential. Furthermore, Paper II investigates the comparative performance of 2D and 3D modelling approaches, along with advanced turbulence models, for simulating two-phase air-water flow. The results indicate that 2D Eulerian models are suitable for shallow water flows with negligible vertical velocity components and are appropriate for preliminary analyses due to their computational efficiency and reasonable accuracy. In contrast, 3D models are better suited for strongly accelerated, highly turbulent flow, offering improved accuracy, particularly in representing complex flow characteristics. Advanced turbulence-resolving approaches, such as DES and DDES, provide enhanced flow field resolution and more reliable prediction of air entrainment. Additionally, the application of 3D turbulence modeling improves the representation of air-water flow behavior and enhances the prediction of air concentration within the cavity in the impact zone.

Subsequently, Paper III investigates the sensitivity of aerated spillway flow predictions to grid resolution with Sub-Grid Scale (SGS) modelling. The findings indicate that grid resolution with Locally Refined Structured Meshes (LRSM) shows improved accuracy in capturing complex air-water interactions compared to Unstructured Meshes (USM). Moreover, aerated spillway flow predictions are highly sensitive to grid resolution rather than to the SGS modelling approach.

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