Dynamics of flow in river bends
Date
2018
Authors
Aseperi, Oladapo, author
Venayagamoorthy, Subhas K., advisor
Julien, Pierre Y., committee member
Ramirez, Jorge A., committee member
Barnes, Elizabeth, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Water is indispensable to life and the means by which it is conveyed is equally important. Natural rivers and manmade channels play a critical role in this respect because they are vital for water supply, navigation, transport of sediments, pollutants and nutrients. Most natural rivers typically have meandering (curved) geometries which make a direct study of their flow dynamics cumbersome. In order to reduce this complexity, natural rivers are usually idealized as open channel bends with rigid boundaries in order to gain insights into the flow dynamics. As such, this research examines the dynamics of flow in open channel bends with rigid boundaries, using computational fluid dynamics (CFD). The particular computational fluid dynamics code used in this research, discretizes the equations of fluid motion (i.e. the Navier-Stokes equations) using a finite volume scheme while tracking the free surface with the volume of fluid method. Turbulence was incorporated into the solution of the equations using large eddy simulation techniques. Even though the general aim is to improve current understanding of natural river bend physics, the specific aims of this research are threefold. These are: (1) to study the effects of radius of curvature on the flow physics of an idealized river bend; (2) to study in detail the effect of a variation in curvature length on the flow structure and dynamics of an open channel bend; and (3) examine in detail the effect of inertial forces on the flow dynamics of an idealized river bend by varying the inflow Froude number. While some of the findings in this research confirm some of the results that has already appeared in literature, a significant amount of results highlight new insights into dynamic events in an open channel bend. As a concrete example on the effect of curvature on the flow structure, simulation results show that the maximum bed and wall shear stress are exerted on the inner wall at the entrance to the curve regardless of curvature. However, further into the bend, the maximum shear stress shifts to the outer bend and wall region. Furthermore, the angular distance into the bend at which this occurs is found to depend on the curvature of the channel. Thus, for a mild channel, the maximum shear stress shifts to the outer bend and wall region a short angular distance from the entrance. This distance increases with a decrease in radius of curvature (i.e. as the channel gets tighter) with the maximum shear stress in the tightest channel (that was simulated in this study) always occurring on the inner side of the bend for the entire channel length. Another key finding comes from an investigation of the effect of the variation of curvature length on the flow structure and dynamics of open channel bends. It was found that the flow circulation pattern depends on the curvature length. Simulation results showed that shorter channel bends reached fully developed vortical states faster than similar channels with longer lengths. Furthermore, new results from this study provide a clear explanation for the emergence of a three-cell circulation structure in tight channel bends that occurs as a result of the splitting of the main cell circulation due to the enhanced vorticity in tight bends. Finally, the study on the effects of Froude number on flow structure clearly shows that an increase in the inertia of the fluid does not affect the radial pressure gradient force (a very important force that plays a critical role in shaping the bend channel dynamics) in a mild channel. Remarkably in the tight channels, there seems to be a positive correlation between the magnitudes of the fluid inertia (as measured by the velocity) and the radial pressure gradient force. This finding has important implications for the modeling of river bends since geometric factors are not sufficient to adequately parameterize the flow structure under certain circumstances in reduced order models. These and more results not mentioned in this abstract are detailed in this dissertation. The overall aim of this research is to provide better insights into bend channel flow dynamics so as to enable engineers to carry out more accurate river modeling and training works.
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Subject
large eddy simulation
river mechanics
computational river modelling
smooth bend dynamics
river bends