Bulle-Effect and its implications for morphodynamics of river diversions

Bifurcations are one of the fundamental elements of a fluvial (river) system, and diversions are a special type of asymmetrical bifurcation where one of the channels after bifurcation continues along the original channel. Diversions can be found in nature, though many of them are built to divert water and sediment from the river for various purposes. Historically, diversions were built to divert water for irrigational and navigational purposes. Recently the importance of diversions has increased, as building diversions to divert sediment (and water) have been put forth as a method to rebuild deltas that have been losing land due to rapid rise in sea-level, subsidence etc. One of the prime examples is the plan under consideration by United States Army Corps of Engineers (USACE) to re-build the Mississippi river delta through diverting water and sediment from the lower Mississippi river. Design of aforementioned diversions would be immensely benefited by a better understanding of the sediment distribution at diversions, and the hydrodynamics that drive it. One of the first and most extensive experimental studies to understand the dynamics at a diversion was conducted by Bulle in 1926, at Karlsruhe, Germany. Bulle found that a disproportionate percentage of bedload went into the lateral-channel, compared to the percentage of water entering the lateral-channel. This non-linear distribution of near-bed sediment between the two channels at a diversion is known as the Bulle-Effect; and since the seminal work of Bulle, multiple experimental studies have corroborated the phenomenon. Despite the importance of this phenomenon, till date the exact mechanism behind the Bulle-Effect is not clear. This thesis first unravels the mechanism behind the phenomenon, and then explores how Bulle-Effect might impact the morphodynamics of a diversion. This thesis can be divided into two major parts: 1) First the mechanism behind the Bulle-Effect phenomenon is explored using high-resolution numerical simulation (Direct Numerical Simulations, Large Eddy Simulations) of flow and sediment transport for a configuration and at the scale similar to Bulle’s experiment. The simulations were conducted using the highly-scalable spectral-element based incompressible Navier-Stokes solver Nek5000, on which a Lagrangian point particle submodel was developed and implemented to model the transport of sediment. The simulations were computationally very expensive (∼ 240 million computational points), thus they required the use of the peta-scale supercomputer Blue Waters for conducting them. The simulation results showed that the phenomenon is caused by the mechanism, where most of the flow near the bottom entered the lateral-channel, even when the percentage of the total water discharge entering lateral-channel is relatively smaller. The phenomenon was found to be at play not only for sediment transported as bed- load, but also for suspended sediment that travels in the lower 25-35 percent of the water-column. These findings were found to hold across a range of Reynolds number (10 − 25000) of the flow, and for different diversion angles. 2) In the second part of the thesis, a Reynolds Averaged Navier-Stokes (RANS) based 3D hydrodynamics and sediment transport model was developed for Bulles experiments using the open-source solver Telemac-Mascaret. This model was found to capture the phenomenon satisfactorily but at a relatively lower computational cost. The substantial reduction in computational cost is important, because at this point it impossible to conduct accurate Large Eddy Simulations (LES) of flows at the scale of real rivers. Thus it becomes important to evaluate if RANS based models can capture a complex phenomenon, and if this is the case to what extent? The RANS model was then used to study the impact of Bulle-Effect on morphodynamics of the diversion. The separation of the flow from the left-bank of the lateral-channel was found to result in formation of a scour-hole under the high-flow zone and subsequent deposition sediment under the flow-recirculation. The impact of the change in morphology of the channel on Bulle-Effect was also analyzed. The findings of this dissertation not only add to the fundamental understanding of an important phenomenon in nature, these also provide insights that will help in optimal design of engineered diversions and other facilities where vorticity and secondary-flow driven sediment/particle transport occurs. Based on disproportionately high sediment transported into the diversions of the Yellow River, China, Canal del Dique on the Magdalena River, Columbia etc., it can be conjectured that the Bulle-Effect plays a major role at the aforementioned diversions. Thus, in the future numerical simulations of real-world diversions should be conducted ( in conjunction with field measurements) in order to study the flow-structure and sediment distribution pattern at diversions, and to understand the extent to which the Bulle-Effect impacts real-world diversions

Multirate Timestepping for the Incompressible Navier-Stokes Equations in Overlapping Grids

We develop a multirate timestepper for semi-implicit solutions of the unsteady incompressible Navier-Stokes equations (INSE) based on a recently-developed multidomain spectral element method (SEM). For {\em incompressible} flows, multirate timestepping (MTS) is particularly challenging because of the tight coupling implied by the incompressibility constraint, which manifests as an elliptic subproblem for the pressure at each timestep. The novelty of our approach stems from the development of a stable overlapping Schwarz method applied directly to the Navier-Stokes equations, rather than to the convective, viscous, and pressure substeps that are at the heart of most INSE solvers. Our MTS approach is based on a predictor-corrector (PC) strategy that preserves the temporal convergence of the underlying semi-implicit timestepper. We present numerical results demonstrating that this approach scales to an arbitrary number of overlapping grids, accurately models complex turbulent flow phenomenon, and improves computational efficiency in comparison to singlerate timestepping-based calculations.Comment: 40 pages, 13 figure

Assessing the impact of Bulle-Effect on the morphodynamics of a laboratory-scale diversion using 3D numerical simulations

Water Qualit

On the Threshold of Drop Fragmentation under Impulsive Acceleration

We examine the complete landscape of parameters which affect secondary breakup of a Newtonian droplet under impulsive acceleration. A Buckingham-Pi analysis reveals that the critical Weber number $\mathit{We}_\mathit{cr}$ for a non-vibrational breakup depends on the density ratio $(\rho)$, the drop $(\mathit{Oh}_d)$ and the ambient $(\mathit{Oh}_o)$ Ohnesorge numbers. Volume of fluid (VOF) multiphase flow simulations are performed using Basilisk to conduct a reasonably complete parametric sweep of the non-dimensional parameters involved. It is found that, contrary to current consensus, even for $\mathit{Oh}_d \leq 0.1$, a decrease in $\mathit{Oh}_d$ has a substantial impact on the breakup morphology, motivating plume formation. In addition to $\rho$, $\mathit{Oh}_o$ also affects the balance between pressure differences between a droplet's pole and its periphery, and the shear stresses on its upstream surface, which ultimately dictates the flow inside the droplet. This behavior manifests in simulations through the observed pancake shapes and ultimately the breakup morphology (forward or backward bag). All these factors affecting droplet deformation process are specified and theories explaining the observed results are provided. A $\mathit{We}_\mathit{cr}-\mathit{Oh}_d$ plot is provided to summarize all variations in $\mathit{We}_\mathit{cr}$ observed due to changes in the involved non-dimensional parameters. All observed critical pancake and breakup morphologies are summarized using a phase diagram illustrating all deformation paths a droplet might take under impulsive acceleration. Finally, based on the understanding of process of bag breakup gained from this work, a non-dimensional parameter to predict droplet breakup threshold is derived and tested on all simulation data obtained from this work and all experimental data gathered from existing literature

Predicting Flow Through the Causeway of the Great Salt Lake Using Hydrodynamic Simulations and Artificial Neural Networks

At the Great Salt Lake, the northern and southern portions of the lake are divided by an east-to-west causeway that disrupts natural lake currents and significantly increases salt concentrations in the norther portion. To support management efforts to address rising environmental and economic concerns, the causeway was recently modified to include a new breach that typically exhibits a strong density-driven bidirectional flow pattern. To obtain much needed insights into the hydraulic performance of this hydraulic structure and the exchange between the two sections of the lake, a field campaign coupled with computational fluid dynamics (CFD) modeling and an artificial neural network (ANN) model were undertaken

Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application

Most recently, organic-inorganic semiconductor light harvester materials, have arisen as a new class of functional element and attracts the research community due to its outstanding optoelectronic properties. Organic-inorganic perovskites are solution process that is easy for the fabrication of devices at low temperature. Additionally, up to date, perovskite quantum dots have emerged as the most efficient light harvester for LEDs and display applications, with high color purity, color tunability, and photoluminescence quantum yield up to 100%. However, the presence of lead in organic-inorganic perovskites and the stability issue of perovskite materials are the significant challenges for the research community. To date, some lead substitute materials have been tried to enhance the film morphology and production of the less toxic light harvester. In this chapter, we focus on the lead substitution on B sight with homovalent cations like Sn2+, Mn2+, Cd2+, and Zn2+ cations. These lead substitutions not only reduce the toxicity of perovskite material while these dopants also enhance the optical and performance of LEDs. We also included the LEDs application of lead substituted perovskite quantum dots (PQDs) that may be useful for the environmental friendly and highly performing perovskite quantum dot LEDs (PQ-LEDs) shortly

Energy Expenditure and Nutritional Status of Sailors and Submarine Crew of the Indian Navy

Nutritional requirements of sailors and submariners are different from those of ground forces as they work under confined environment as well as due to logistic constraints of cooking and storage of food. Study was conducted for a period of three months at Eastern Naval Command to evaluate nutrient requirements, nutritional status of Indian Navy personnel and adequacy of the existing ration scales. The study volunteers were from crew of two warships (n = 35) and submariners (n = 20) who were attached to their mother ships during time of data collection and offshore during rest of the period. Energy expenditure, nutrient intake, level of nutrients in body, and urinary excretion were measured along with changes in body composition. All variables were analysed before and after three months of nutritional monitoring. Energy expenditure at ship was in the range of 2449-4907 kcal/day with a mean of 3313 + 578 kcal/day, while in the case of submariners, it was 3168 + 282 (2606-3907) kcal/day. The energy intake in the case of sailors and submariners was not different either on shore establishment or at ship. Energy intake was found to be 3518 + 286 kcal/day. The energy contribution from carbohydrates, fats, and protein was 59.9 per cent, 27.8 per cent and 12.3 per cent, respectively. No sign and symptoms of any nutritional deficiency were observed either initially or after three months. Status of micronutrients present in their blood and in their urinary excretions along with body composition were maintained, indicating adequacy of existing ration scales of Indian Navy.Defence Science Journal, 2011, 61(6), pp.540-544, DOI:http://dx.doi.org/10.14429/dsj.61.93