Document Type

Thesis - Open Access

Award Date

2024

Degree Name

Master of Science (MS)

Department / School

Civil and Environmental Engineering

First Advisor

Francis Ting

Abstract

The study of flows over sudden change in bed roughness in open channels is important in understanding the flow dynamics of a developing boundary layer. Parameters such as shear velocity, mean flow velocity, turbulence intensity and the Reynolds stress distribution are critical in channel design optimization and predicting sediment transport. In this thesis, a detailed examination of the distribution of these parameters is undertaken in a rough-to-smooth and smooth-to-rough transition. Laboratory experiments were conducted on rough-to-smooth transition (Case A) and smooth-to-rough transition (Case B) in an open channel flume. Measurements of the fluid velocity fields at the centerline of the flume were conducted using a Particle Image Velocimetry (PIV) system. Five different methods were used to determine the bed shear velocity. This study had two main objectives. The first objective was to investigate the development of the flow velocity, shear velocity and turbulence intensities due to a sudden change in bed roughness for a rough-to-smooth-transition and a smooth-to-rough transition. The second objective was to evaluate the different methods for determining the bed shear stress near a roughness transition. The study found that in flows through roughness transitions in open channels, the transition had an immediate impact on the flow velocity near the bed and the shear velocity. In both case A and B, the flow velocity adjacent to the bed and the shear velocity adjusted quickly to the new bed conditions even though the flow velocity away from the bed was still evolving. The turbulence intensities and the Reynolds shear stress did not adjust quickly to the change in bed roughness. The velocity gradient method, the log law method (when existed) and the Spalding wall function gave comparable results of shear velocity on the smooth bed in a rough-to-smooth transition (Case A). The study also found that the log law method can be used to determine the friction velocity on the rough bed in a smooth-to-rough transition (Case B) if the equivalent roughness is independently determined. The results obtained were consistent with the findings of previous studies in closed conduit flows and flows without a free surface. The Reynolds stress extrapolation method overestimated the bed shear stress after the transitions in both cases and the results were also susceptible to the presence of secondary currents.

Publisher

South Dakota State University

Download

Share

COinS
 

Rights Statement

In Copyright