Three-dimensional computational modeling of curved channel flow

Abstract

Investigating flow dynamics in curved channels is a challenging problem due to its complex three-dimensional flow structure. Despite the numerous investigations that have been performed on this important topic over the last several decades, there remains much to be understood. The focus of this dissertation is on flow around curved channel bends with an emphasis on the use of three-dimensional numerical simulations to provide insights on the flow dynamics in channel bends. In particular, the answers to the following two main questions are sought: 1) when is it appropriate to use the rigid lid assumption for simulating flow around bends?; and 2) what is (are) the most relevant parameters for quantifying the enhanced shear stress in channel bends from a practical standpoint? A computational fluid dynamics framework was developed using the ANSYS Fluent code and validated using experimental flume data. Following the validation study, a total of 26 simulations were performed and the results analysed in an attempt to answer the two main questions. In an attempt to answer the first question, a broad parametric study was conducted using both free surface resolving simulations as well as simulations that make use of the rigid lid assumption. It is shown that the two main parameters that appear to control the flow dynamics in a bend are the maximum bend angle, expressed as the ratio of the length of the channel bend Lc to its radius of curvature Rc, and the upstream Froude number. Analysis reveal when that Lc/Rc ≥ π/2, the curvature effects begin to dominate the dynamics and the error between the free surface model and the rigid lid model dramatically increases regardless of the value of the Froude number. The study calls for caution to be used when using the rigid lid assumption and indicates that this assumption should not be used for simulating flows when Lc/Rc ≥ π/2, especially for sharply curved channels with a radius of curvature to top width ratio Rc/Tw < 2. The increase in shear stress is commonly expressed as a Kb value, which is simply the ratio of shear stress in a bend of the channel to the averaged approach shear stress in a straight channel. The results from the parametric study show that the conventional approach for parameterizing Kb as a function of Rc/Tw, where Rc is the radius of curvature and Tw is the channel top width, appears to be inadequate because the distributions in the Kb values exhibit significant scatter for small changes in Rc/Tw i.e. for flow around sharply curved bends. Dimensional analysis reveals that for a given channel cross-section, constant flow rate, bed slope and channel bed roughness, Kb depends on both Lc/Rc and Rc/Tw. In this study, the combined effects of these two parameters were investigated. It is shown from the parametric study that the magnitude of the shear stress increases as a function of Lc/Rc and reaches an asymptotic limit as Lc/Rc > π/2, for Rc/Tw < 2. The study also highlights that the location of the maximum shear stress occurs in the inner (convex) side of the bend for Rc/Tw < 2 but shifts towards the outer bend for Rc/Tw > 2. While the emphasis (and in a sense a limitation) of this study has been mainly on sharp curved bends (Rc/Tw < 2), the analysis can be readily extended to curved bends with Rc/Tw > 2. It is envisaged that such an analysis will lead to a framework for parameterizing Kb in a comprehensive manner that would be useful for practical design guidelines.