A unifying computational fluid dynamics investigation on the river-like to river-reversed secondary circulation in submarine channel bends

A numerical model of saline density currents across a triple-bend sinuous submerged channel enclosed by vertical sidewalls is developed. The unsteady, non-Boussinesq, turbulent form of the Reynolds Averaged Navier-Stokes equations is employed to study the flow structure in a quasi-steady state. Recursive tests are performed with axial slopes of 0.08°, 0.43°, 1.5°, and 2.5°. For each numerical experiment, the downstream and vertical components of the fluid velocity, density, and turbulent kinetic energy are presented at four distinct locations within the channel cross section. It is observed that a crucial change in the flow pattern at the channel bends is observed as the axial slope is increased. At low values of the axial slope a typical river-like pattern is found. At an inclination of 1.5°a transition starts to occur. When the numerical test is repeated with an axial slope of 2.5°, a clearly visible river-reversed secondary circulation is achieved. The change in the cross-sectional flow pattern appears to be associated with the spatial displacement of the core of the maximum downstream fluid velocity. Therefore, the axial slope in this series of experiments is linked to the velocity structure of the currents, with the height of the velocity maximum decreasing as a function of increasing slope. As such, the axial slope should be regarded also as a surrogate for flows with enhanced density or sediment stratification and higher Froude numbers. The work unifies the apparently paradoxical experimental and numerical results on secondary circulation in submarine channels

A numerical study of the triggering mechanism of a lock-release density current

A numerical study on the effects induced by the impulsive vertical removal of a lock-gate at the interface between two fluids of different densities is presented. This configuration represents the typical setup of those experiments commonly employed for investigating density currents in the laboratory. Experimentally induced effects resulting from opening the lock-gate are expected to occur, but the evaluation of these dynamics and their impact on the evolution of the laboratory density current produced in such a manner are not easy to estimate. Despite the fact that numerical studies are often concerned with lock-release density currents, the triggering mechanism which occurs in the early stages of the evolution of the fluid flow is commonly neglected. Here a comparison is established between the case when the triggering mechanism is completely neglected and a series of cases where, in contrast, this effect is taken into account. The withdrawal of the lock-gate is modeled either by employing a zero-thickness lock-gate or by accounting for the volumetric nature of the lock-gate. Subsequently the influence of speed on the withdrawal of the lock-gate is assessed. The numerical results suggest that the density current is mainly affected by the constraining effect of the lock-gate on the flow and by the responses of the submerged fluid and the free surface to the displacement of the lock-gate. These differences lead to improved physical modeling and numerical simulation validation in the case where the physics of the lock-gate is accounted for. Such differences can be very important particularly in particulate-laden flows, where small changes in initial conditions may lead to longer-term divergence as a result of positive feedback effects. The work has significant implications for physical modeling of density currents and a series of recommendations are made for the standardization of experimental protocols. Finally, the approach adopted here for the moving gate is applicable to civil and environmental engineering problems including dam-break flows and sluice gate modeling

Geographic range limits: achieving synthesis

Understanding of the determinants of species' geographic range limits remains poorly integrated. In part, this is because of the diversity of perspectives on the issue, and because empirical studies have lagged substantially behind developments in theory. Here, I provide a broad overview, drawing together many of the disparate threads, considering, in turn, how influences on the terms of a simple single-population equation can determine range limits. There is theoretical and empirical evidence for systematic changes towards range limits under some circumstances in each of the demographic parameters. However, under other circumstances, no such changes may take place in particular parameters, or they may occur in a different direction, with limitation still occurring. This suggests that (i) little about range limitation can categorically be inferred from many empirical studies, which document change in only one demographic parameter, (ii) there is a need for studies that document variation in all of the parameters, and (iii) in agreement with theoretical evidence that range limits can be formed in the presence or absence of hard boundaries, environmental gradients or biotic interactions, there may be few general patterns as to the determinants of these limits, with most claimed generalities at least having many exceptions