The effect of lower sea level on geostrophic transport through the Florida Straits during the last glacial maximum

We investigate the effect of a 120 meter sea level drop on transport through the Caribbean Sea and the Florida Straits during the Last Glacial Maximum (LGM) relative to the present, using the Regional Ocean Modeling System (ROMS). A geostrophic transport estimate for the Florida Straits suggests the LGM Florida Current was weaker than today by one third, inferring a likely decrease in the North Atlantic overturning circulation by 12-15 Sv. A possible impact of a shallower LGM Florida Straits sill depth on the Florida Current has been suggested. Our model results show that the volume transport through the Florida Straits is slightly reduced in a lower sea level model simulation when compared to a control sea level simulation (34.8 ± 2.0 Sv vs. 39.8 ± 2.3 Sv). The difference in transport is of the order of 5 Sv, representing a maximum limit to the LGM flow reduction due to sea level change. Therefore the change in sill depth between the LGM and the present is unlikely to have been a cause of the entire observed flow reduction.M.S.Committee Chair: Jean Lynch-Stieglitz; Committee Co-Chair: Emanuele Di Lorenzo; Committee Member: Annalisa Bracco; Committee Member: Robert Blac

Reliability-based economic model predictive control for generalized flow-based networks including actuators' health-aware capabilities

This paper proposes a reliability-based economic model predictive control (MPC) strategy for the management of generalized flow-based networks, integrating some ideas on network service reliability, dynamic safety stock planning, and degradation of equipment health. The proposed strategy is based on a single-layer economic optimisation problem with dynamic constraints, which includes two enhancements with respect to existing approaches. The first enhancement considers chance-constraint programming to compute an optimal inventory replenishment policy based on a desired risk acceptability level, leading to dynamically allocate safety stocks in flow-based networks to satisfy non-stationary flow demands. The second enhancement computes a smart distribution of the control effort and maximises actuators’ availability by estimating their degradation and reliability. The proposed approach is illustrated with an application of water transport networks using the Barcelona network as the considered case study.Peer ReviewedPostprint (author's final draft

Modeling of Ti-W Solidification Microstructures Under Additive Manufacturing Conditions

Additive manufacturing (AM) processes have many benefits for the fabrication of alloy parts, including the potential for greater microstructural control and targeted properties than traditional metallurgy processes. To accelerate utilization of this process to produce such parts, an effective computational modeling approach to identify the relationships between material and process parameters, microstructure, and part properties is essential. Development of such a model requires accounting for the many factors in play during this process, including laser absorption, material addition and melting, fluid flow, various modes of heat transport, and solidification. In this paper, we start with a more modest goal, to create a multiscale model for a specific AM process, Laser Engineered Net Shaping (LENS™), which couples a continuum-level description of a simplified beam melting problem (coupling heat absorption, heat transport, and fluid flow) with a Lattice Boltzmann-cellular automata (LB-CA) microscale model of combined fluid flow, solute transport, and solidification. We apply this model to a binary Ti-5.5 wt pct W alloy and compare calculated quantities, such as dendrite arm spacing, with experimental results reported in a companion paper

Predicting bulk powder flow dynamics in a continuous mixer operating in transitory regimes

Over recent years there has been increasing interest in continuous powder mixing processes, due mainly to the development of on-line measurement techniques. However, our understanding of these processes remains limited, particularly with regard to their flow and mixing dynamics. In the present work, we study the behaviour of a pilot-scale continuous mixer during transitory regimes, in terms of hold-up weight and outflow changes. We present and discuss experimental results concerning the start-up dynamics of a Gericke GCM 500 mixer, for which a specific experimental protocol has been developed to determine the evolution of the hold-up in the mixer and the real outflow. Empirical relationships are derived so as to link hold-up weight variations with operating conditions. A simple stochastic approach, based on a non-homogeneous Markov chain, is developed to simulate the bulk particle flow and transport in the continuous mixer at a macroscopic level. Although this simple model is only based on the start-up behaviour, it provides a full description of the mixer dynamics in response to strong perturbations on the flow rate or on the rotational speed of the stirring device, such as negative or positive steps. This model is validated experimentally for a wide range of operating conditions, and constitutes a first approach to process control

3D layer-integrated modelling of morphodynamic processes near river regulated structures

Sedimentation and erosion can significantly affect the performance of river regulated reservoirs. In the vicinity of flow control structures, the interaction between the hydrodynamics and sediment transport often induces complex morphological processes. It is generally very challenging to accurately predict these morphological processes in real applications. Details are given of the refinement and application of a three-dimensional (3-D) layer integrated model to predict the morphological processes in a river regulated reservoir. The model employs an Alternating Direction Implicit finite difference algorithm to solve the mass, momentum and suspended sediment transport conservation equations, and an explicit finite difference scheme for the bed sediment mass conservation equation for calculating bed level changes. The model is verified against experimental data reported in the literature. It is then applied to a scaled physical model of a regulated reservoir, including the associated intakes and sluice gates, to predict the velocity distributions, sediment transport rates and bed level changes in the vicinity of the hydraulic structures. It is found that the velocity distribution near an intake is non-uniform, resulting in a reduction in the suspended sediment flux through the intake and the formation of a sedimentation zone inside the reservoir

Physical controls on water flow and solute transport in coastal aquifers

Groundwater flow and associated subsurface solute fates have a significant impact on the structure and productivity of near-shore coastal ecosystems. For proper assessment and management of these coastal groundwater resources, it is quite essential to investigate the key factors (tides, waves, evaporation, and freshwater recharge etc.) affecting coastal groundwater systems. The main objective of this study is to examine and quantify two important physical control factors, oceanic waves and evaporation, on the groundwater flow and solute transport in near-shore aquifers. For the investigation of wave effects, a Computational Fluid Dynamics (CFD) modeling tool, Fluent, is used to simulate wave- induced sea level oscillations. A flow-averaged approach is developed to generalize wave motions acting onto the beach for the sake of the feasibility of numerical computation. A two-dimensional numerical model MA RU N is used to simulate variably saturated, variable density groundwater flow and subsurface solute transport in coastal aquifers. To investigate evaporation effects, a classic bulk aerodynamic formulation is adopted as a module to the model MARUN for simulating groundwater flow and subsurface solute transport in bare saline soils subjected to transient evaporation. The simulation results reveal that these two factors significantly impact beach hydrodynamics. Wave forcing induces pore water circulations in the swash zone of the near-shore aquifers; wave forcing also modifies the pathways of solute transport in the beach prior to discharge into the ocean, and subsequently impacts plume’s residence time, migration speed, discharge location, and discharge rate. The evaporation decreases the moisture at shallow layer of the beach and subsequently impacts the subsurface salinity distribution

Health-aware economic MPC for operational management of flow-based networks using bayesian networks

This paper presents a health-aware economic Model Predictive Control (EMPC) approach for the Prognostics and Health Management (PHM) of generalized flow-based networks. The proposed approach consists of the integration of the network reliability model obtained from a Bayesian network in the control model. The controller is then able to optimally manage the supply taking into consideration the distribution of the control effort, to extend the life of the actuators by delaying the network reliability decay as much as possible. It also considers an optimal inventory replenishment policy based on a desired risk acceptability level, leading to the availability of safety stocks for unexpected excess demand in networks. The proposed implementation is illustrated with a real case study corresponding to an aggregate model of the Drinking Water transport Network (DWN) of Barcelona.Peer ReviewedPostprint (published version

A numerical study of the water exchange through the Danish Straits

The free surface version of the GFDL model is used to study inflow and outflow through the Danish Straits, which connect the Baltic with the North Sea. Three problems are addressed: (i) the piling up of inflowing water in the Arkona basin; (ii) the transport ratios between Belt and Sound; (iii) the dominance of hydraulic or geostrophic control. Model results show that a cyclonic eddy (dome) is formed by the inflowing saline water that prevents this water from passing rapidly into the Bornholm basin. This eddy is enforced with increasing inflow due to a sea level difference between Kattegat and western Baltic. If density gradients along the straits are weak and the flow is dominantly driven by sea level differences between Kattegat and Baltic, the well-known ratio of 70% : 30% for the transports through Belt and Sound are confirmed. Strong density gradients can change this ratio considerably, especially in the outflow case, when the light water of the Baltic flows against the heavier water of the Kattegat. Under variable wind conditions, no fixed ratio is found. The flow in the Straits is geostrophically controlled; however, the strong baroclinic density field does not allow us to derive the transport simply from sea level inclination

Learning-Based Predictive Control with Gaussian Processes: An Application to Urban Drainage Networks

© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting /republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksMany traditional control solutions in urban drainage networks suffer from unmodelled nonlinear effects such as rain and wastewater infiltrating the system. These effects are challenging and often too complex to capture through physical modelling without using a high number of flow sensors. In this article, we use level sensors and design a stochastic model predictive controller by combining nominal dynamics (hydraulics) with unknown nonlinearities (hydrology) modelled as Gaussian processes. The Gaussian process model provides residual uncertainties trained via the level measurements and captures the effect of the hydrologic load and the transport dynamics in the network. To show the practical effectiveness of the approach, we present the improvement of the closed-loop control performance on an experimental laboratory setup using real rain and wastewater flow data.Peer ReviewedPostprint (author's final draft