Physical and numerical modelling of negative surges in open channels

Negative surges are caused by a sudden change in flow resulting from a decrease in water depth. New experiments were conducted in a horizontal channel (L = 12 m, W = 0.5 m) to record the unsteady water depth and turbulence in negative surges propagating upstream against an initiallysteady flow. The data were collected using video-imagery, acoustic displacement meters and acoustic Doppler velocimetry (ADV). The physical observations showed that the leading edge of negative surge propagated upstream with a celerity which varied with time. During the first initial instants following the surge formation, the negative surge leading edge accelerated ad its celerity increased with time up to xGate-x = 4do. After the acceleration phase, the negative surge propagation was more gradual: the surge leading edge was very flat and barely perceptible, and its celerity tended to decrease slowly with increasing distance from the gate. The data implied some deceleration in a manner which is contrary to theoretical considerations. The physical measurements highlighted that the negative surges were associated with some flow acceleration. The turbulent velocity data highlighted some increased turbulence occurring beneath the negative surge with large velocity fluctuations and large Reynolds stress components. The velocity fluctuations and turbulent stresses were significantly larger than in the initially steady flow and in the final flow motion. The physical data were used to test an analytical solution of the Saint-Venant equations (the simple wave solution) and some 1-D and 2-D numerical model results. The findings showed that the negative surge propagation was relatively little affected by the boundary friction. For a relatively simple geometry such as the prismatic rectangular flume used in the present study, the physical data were best modelled by the simple wave theory, although the numerical model results were qualitatively in agreement with the experimental observations. The present results suggested that the negative surge remains a challenging topic for the computational modellers

Unsteady turbulent properties in negative waves in open channels

In an open channel, a sudden drop in free-surface elevation is associated with the development of a negative wave. While some simple analytical solution is widely described in textbooks, little research was conducted to date on the unsteady turbulence properties beneath negative waves. A series of new physical experiments were conducted in a rectangular channel. The unsteady free-surface profile and turbulence characteristics were measured in a negative wave propagating upstream against an initially steady flow using non-intrusive acoustic displacement meters, video imagery and acoustic Doppler velocimetry (ADV). For one set of flow conditions, the experiments were repeated 25 times at two longitudinal locations and four vertical elevations to yield ensemble-averaged data. The wave leading edge propagated upstream with a speed which was a function of time and space. The velocity data showed that the upstream propagation of the negative wave was linked with a gentle drop in water elevation associated with an acceleration of the flow, while some increased turbulence occurred beneath the wave associated with large velocity fluctuations and large Reynolds stress components. The velocity fluctuations and turbulent stresses were significantly larger than in the initially steady flow and in the final flow motion. (C) 2012 Elsevier Masson SAS. All rights reserved

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

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

Bottom Reflectance in Ocean Color Satellite Remote Sensing for Coral Reef Environments

Most ocean color algorithms are designed for optically deep waters, where the seafloor has little or no effect on remote sensing reflectance. This can lead to inaccurate retrievals of inherent optical properties (IOPs) in optically shallow water environments. Here, we investigate in situ hyperspectral bottom reflectance signatures and their separability for coral reef waters, when observed at the spectral resolutions of MODIS and SeaWiFS sensors. We use radiative transfer modeling to calculate the effects of bottom reflectance on the remote sensing reflectance signal, and assess detectability and discrimination of common coral reef bottom classes by clustering modeled remote sensing reflectance signals. We assess 8280 scenarios, including four IOPs, 23 depths and 45 bottom classes at MODIS and SeaWiFS bands. Our results show: (i) no significant contamination (Rrscorr 17 m for MODIS and >19 m for SeaWiFS for the brightest spectral reflectance substrate (light sand) in clear reef waters; and (ii) bottom cover classes can be combined into two distinct groups, “light” and “dark”, based on the modeled surface reflectance signals. This study establishes that it is possible to efficiently improve parameterization of bottom reflectance and water-column IOP retrievals in shallow water ocean color models for coral reef environments

Negative surges in open channels: physical and numerical modeling

Negative surges can be caused by a sudden change in flow resulting from a decrease in water depth. In the present study, some physical experiments were conducted in a rectangular channel to characterize the unsteady free-surface profile and longitudinal velocity beneath a negative surge propagating upstream. The physical observations showed that, during the first initial instants, the celerity of the surge leading edge increased rapidly with time, while later the negative surge propagated upstream in a more gradual manner with a celerity decreasing slowly with increasing distance. The velocity data highlighted some relatively large turbulent fluctuations beneath the negative surge. The physical results were used to test the analytical solution of the Saint-Venant equations and some numerical models. The findings suggested that the negative surge propagation appeared relatively little affected by the boundary friction within the investigated flow conditions. DOI: 10.1061/(ASCE)HY.1943-7900.0000674. (C) 2013 American Society of Civil Engineers

Optically Induced Avoided Crossing in Graphene

Degenerate states in condensed matter are frequently the cause of unwanted fluctuations, which prevent the formation of ordered phases and reduce their functionalities. Removing these degeneracies has been a common theme in materials design, pursued for example by strain engineering at interfaces. Here, we explore a non-equilibrium approach to lift degeneracies in solids. We show that coherent driving of the crystal lattice in bi- and multilayer graphene, boosts the coupling between two doubly-degenerate modes of E1u and E2g symmetry, which are virtually uncoupled at equilibrium. New vibronic states result from anharmonic driving of the E1u mode to large amplitdues, boosting its coupling to the E2g mode. The vibrational structure of the driven state is probed with time-resolved Raman scattering, which reveals laser-field dependent mode splitting and enhanced lifetimes. We expect this phenomenon to be generally observable in many materials systems, affecting the non-equilibrium emergent phases in matter.Comment: 13 pages, 4 figure