Events Calendar

CEE Oral Dissertation Defense

Speaker: Ziyi Huang, Ph.D. Candidate, Astani Departmentof Civil and Environmental Engineering, USC

Talk Title: OPEN CHANNEL FLOW INSTABILITIES: MODELING THE SPATIAL EVOLUTION OF ROLL WAVES

Abstract:
This study is concerned with "roll waves", which are a series of intermittent shock-like waves occurring in turbulent water flows down a wide, rectangular open channel due to the natural instability. Most literatures on the subject of roll waves have concentrated on theoretical investigations. Only two of them made efforts on computationally simulating real roll-wave flows, but no validation tests were implemented.

A computational model capable of simulating the spatial evolution of roll waves is constructed in the present work. The equations of motion are the one-dimensional viscous Saint Venant equations, incorporating the turbulently viscous force to describe the momentum diffusion resulting from tremendous gradient and curvature of roll waves. The equations are solved by a high-resolution, shock-capturing numerical scheme. The scheme is based on the conservative finite volume formulation and adopts the second-order TVD Lax-Wendroff Riemann solver. The computational model, written in FORTRAN 95, is named as "Shallow Water Analyzed by a TVD Scheme at University of Southern California'' with an acronym "USC SWAT''. It can be applied to analyze a wide range of shallow water flows.

The numerical scheme is tested through comparisons with both transient linear and quasi-steady nonlinear theories. The numerical solutions of maximum amplitude of progressing roll waves match the linear theory at time levels in which the wave amplitude is small. The numerical solution of converged progressing roll waves match a mathematical solution of roll waves deduced from the governing equations in a progressing coordinate.

The constructed computational model is utilized to simulate real roll-wave flows in a constant-slope channel. In the validation test, the spatial development curves of time-averaged wave crest and trough depths achieved from the simulation are compared with those from experimental data. It is shown that the value of the turbulent viscosity needs to be appropriately selected so as to agree with the experimental data. The comparison results manifest the simulation satisfactorily predicts the spatial evolution of wave crest and trough depths.

Some of important roll-wave properties are investigated. The simulation discovers three types of wave-wave interactions, which impact characteristics of the roll-wave evolution. The spatial evolution of roll waves obeys a generality despite different hydraulic conditions. The nonperiodicity of roll waves increases from upstream to downstream channel locations.

The extent of the flow instability is weakened in channels with rough bottom. A concept of the breaking-slope open channel is tested through a computational experiment. This concept is found to be useful in suppressing growth of the roll-wave amplitude.

This computational study covering the comparison with experimental data enriches the knowledge of computational mechanics regarding free-surface water flows, and provides a complete insight on simulating the spatial evolution of roll waves using the Saint Venant equations. Findings from the computational experiments on mitigating the roll-wave evolution could help overcome difficulties caused by open channel flow instabilities.


Advisor: Prof. J.J. Lee

Location: Kaprielian Hall (KAP) - 209 Conference Room

Audiences: Everyone Is Invited

Contact: Evangeline Reyes