Hydraulic Modelling of Unsteady Open Channel Flow: Physical and Analytical Validation of Numerical Models of Positive and Negative Surges

摘要

Positive and negative surges are generally observed in open channels. Positive surges that occur due to tidal origins are referred to as tidal bores. A positive surge occurs when a sudden change in flow leads to an increase of the water depth, while a negative surge occurs due to a sudden decrease in water depth. Positive and negative surges are commonly induced by control structures, such as the opening and closing of a gate. In this study, the free-surface properties and velocity characteristics of negative and positive surges were investigated physically under controlled conditions, as well as analytically and numerically. Unsteady open channel flow data were collected during the upstream propagation of negative and positive surges. Both, physical and numerical modelling, were performed. Some detailed measurements of free-surface fluctuations were recorded using non-intrusive techniques, including acoustic displacement meters and video recordings. Velocity measurements were sampled with high temporal and spatial resolution using an ADV (200 Hz) at four vertical elevations and two longitudinal locations. The velocity and water depth results were ensemble-averaged for both negative and positive surges. The results showed that the water curvature of the negative surge was steeper near the gate at x=10.5 m compared to further upstream at x=6 m. Both the instantaneous and ensemble-average data showed that in the negative surge the inflection point of the water surface and the longitudinal velocity Vx occurred simultaneously. Also, an increase in Vx was observed at all elevations during the surge passage. For the positive surge the propagation of the bore and the velocity characteristics supported earlier findings by Koch and Chanson (2009) and Docherty and Chanson (2010). The surge was a major discontinuity in terms of the free-surface elevations, and a deceleration of the longitudinal velocities Vx was observed during the surge passage. A number of analytical and numerical models were tested, including the analytical and numerical solutions of the Saint-Venant equations and a computational fluid dynamics (CFD) package. Overall, all models provided reasonable results for the negative surge. None of the models were able to provide a good agreement with the measured data for the positive surge. The study showed that theoretical models may be applied successfully to unsteady flow situations with simple channel geometry. Also, it was found that the selection of the appropriate mesh size for CFD simulations is essential in highly unsteady turbulent flows, such as a positive surge, where the surge front is a sharp discontinuity in terms of water elevation, velocity and pressure. It was concluded that the highly unsteady open channel flows remain a challenge for professional engineers and researchers.