Exchange Flow Research Articles

Climatic Change of Summer Wind Direction and Its Impact on Hydrodynamic Circulation in the Pearl River Estuary

Assessing the trend of sea surface wind is important for understanding the response of the marine environment to climate change. Analysis of wind data reveals that the summer wind direction in the Pearl River Estuary (PRE) shifts anticlockwise at a rate of −0.36°yr−1 over the past 42 years (1979–2020). The mean wind direction in July shifts from 183.6° (in 1979) to 169.3° (in 2020) and is predicted as 142.1° by 2100. How this long-term wind direction change affects the PRE hydrodynamic circulation structure has not been examined yet. A fully calibrated high resolution 3D hydrodynamic model is used to evaluate the response of local hydrodynamics to wind direction shifting in this study. The model results indicate that both the cross-channel wind-driven transport and along-channel seaward flow are weakened as wind direction shifts. Consequently, the lateral circulation is slowed down significantly while the longitudinal exchange flow is weakened slightly. A remarkable increase in stratification occurs in the coastal sea adjacent to the Modaomen where hypoxia has been frequently reported. The residence time of Lingding Bay increases slightly. The Momentum budget indicates the wind direction shifting can cause major changes in the barotropic pressure term, which is mainly balanced by the baroclinic pressure term and diffusion term.

A World Within a Block

St. James Town in Tkaronto (Toronto) was one of the biggest housing investments in so-called Canada in the 1960s. Each of the modernist high-rise buildings was named after a Canadian city, suggesting a vision of unity and coherence indicative of the country’s national imaginary, while also reflecting the nation-building aspirations of the settler-colonial state. In this article, the historical development of St. James Town is analyzed using Henri Lefebvre’s concept of production of space to depict how this site of nation-building and capitalism has been negotiated through an influx of im/migrant communities. Specifically, the term "cornerism" is used to convey the process by which St. James Town residents utilize spaces of interaction to facilitate exchange and foster information flows, thereby linking everyday life experiences with the formulation of a collective identity – as symbolized by the neighborhood’s motto: “A World Within a Block”. Two key points of discussion are considered. First, how the neoliberalized practices of urban development over the last few decades, namely the dominance of condo-towers in downtown Tkaronto, imperil “cornerism” as a practice of resistance by im/migrant residents in St. James Town; and second, the possible negative consequences of a large condominium investment planned in the central point of St. James Town on the socio-economic flows and rhythms of the neighborhood.

Misunderstanding and Offense in the Digital World as Impacts of Community Low Level of Language Literacy and Communication Skills

AbstractCommunication is part of social interaction that acts as a means of exchanging information, meeting individual needs and development. However, such social interaction activities may fail and create interaction problems, including misunderstanding and offense. Misunderstanding occurs when the listener thinks he or she has understood or misinterpreted what the speaker meant while offense occurs when the listener is unable to accept, is annoyed and disturbed by what the speaker is saying. Since entering the digital era which began about 10 years ago, misunderstandings and offenses have become increasingly common. The rapid flow of information exchange where everyone is almost free to speak and respond to speech makes these two issues even more unstoppable. Responding to these, as the main concern of this paper, the author invites the readers to discuss about misunderstanding and offense as results of individual low level of literacy and communication skills.Keywords: Misunderstanding and Offense, Communication, Language Literacy

Riverbed Temperature and 4D ERT Monitoring Reveals Heterogenous Horizontal and Vertical Groundwater-Surface Water Exchange Flows Under Dynamic Stage Conditions

Groundwater surface water exchange plays a critical role in physical, biological, and geochemical function of coastal and riverine systems. Observing exchange flow behavior in heterogeneous systems is a primary challenge, particularly when flows are governed by dynamic river stage or tidal variations. In this paper we demonstrate a novel application of time-lapse 3D electrical resistivity tomography and temperature monitoring where an array of thermistors installed beneath a riverbed double as resistivity electrodes. We use the array to monitor stage driven exchange flows over a 6-day period in a dynamic, stage-driven high order stream. We present a method for addressing the otherwise confounding effects of the moving river-surface boundary on the raw resistivity data, thereby enabling successful tomographic imaging. Temperature time-series at each thermistor location and time-lapse 3D images of changes in bulk electrical conductivity together provide a detailed description of exchange dynamics over a 10-meter by 45-meter section of the riverbed, to a depth of approximately 5 m. Results reveal highly variable flux behavior throughout the monitoring domain including both horizontal and vertical exchange flows.

Investigation on the Pressure Fluctuation Characteristics for a Regenerative Flow Pump Under Different Blade Arrangements

Abstract The flow in the regenerative flow pump (RFP) is usually featured by unsteady flow fluctuation and complex interference efficacy. Two pump models with various blade arrangements were investigated in this study to explore the transient flow and the pressure fluctuation characteristics by computational fluids dynamics (CFD) simulation and experimental validation. The results illustrate that the average pressure variation of the impeller and channel across the impeller's rotating direction is in consistence with the mass exchange flow and the circulation number in the pump. Furthermore, the inlet and outlet pressures are analyzed, respectively, in the time and frequency domain, showing that the head coefficient fluctuates periodically with the fluctuating number equal to the blade number. The dominant frequencies of the two pump models are primarily blade passing frequency (BPF) and its harmonics. Additionally, the pressure fluctuation spectrum and the fluctuating amplitude at dominant frequency in each component of RFP are extracted to reveal the effects of the blade arrangement on the pressure fluctuation characteristics, where the fluctuating amplitude in the model with staggered blades was reduced significantly compared to the model with symmetrical blades. This could be attributed to the more uniform distribution of the internal flow, the less stronger pressure fluctuation, and the better modulation pattern of the root-mean-square (RMS) pressure. This work reveals the pressure fluctuation characteristics inside the RFP, and the design approach of increasing the circumferential uniformity could serve as a reference for the pressure fluctuation reduction and vibration and noise improvement of RFPs.

Diagnostic Model of Aircraft Turbine Engine Governor Pump

Abstract This paper presents a mathematical model for a hydromechanical fuel governor pump, to be used in parametric diagnostics. The design and operation of the governor are described. The main requirements of the model are formulated, its structure is determined, corresponding to the specifics of the diagnostic task, and assumptions to make the model simpler are presented (single-dimensional flow and absence of heat exchange). The presented model consists of idealized elements with lumped parameters (such as pressure and mass consumption of the working fluid), accounting for the compressibility of the substance and the design arrangement of the governor (presence of mechanical rests, metering orifices of complex shapes, relay switchers, etc.). Equations of elements with lumped parameters, linked by hydraulic channels in one node, are presented. The model – a system of first-order differential-algebraic equations – is solved and the parameters of the governor pump are determined for different steady-state and transient operation modes. We compare our results to the requirements for the corresponding parameters outlined in the Engineering Specifications. The model is matched to the specifications by correcting setting parameters (tightening of elastic springs, areas of throttles, etc.), and a method of initial model linearization is developed. Based on the results, we conclude that our model can be used as a diagnostic algorithm for a governor pump, at the testing and development stages, during manufacturing, repair and maintenance.

Disrupt the upper or the lower conduit? The dual role of gas exsolution in the conduits of persistently active volcanoes

Many volcanoes emit a significant portion of the gas they transport to the atmosphere during continual passive degassing rather than during eruptions. To maintain a high gas and thermal flux without erupting magma, the flow field in the volcanic conduit must be approximately balanced with gas-rich, buoyant magma ascending and degassed, heavy magma descending. In vertical conduits, this exchange flow takes the form of core–annular flow, where the gas-rich magma forms a core enclosed by an annulus of degassed magma. The flow dynamics of core–annular flow have been studied extensively in fluid dynamics, but mostly for constant material properties. Our study aims to advance our understanding of how core–annular flow responds to volatile exsolution – a simple, yet ubiquitous disruption in volcanic conduits, which alters both the density and the viscosity of the core fluid. By deriving an evolution equation for the core–annular interface based on a generalized exchange-flow condition using a lubrication approximation, we find that the response of the system to volatile exsolution depends on the conduit flow regime. The same nucleation event can lead to a flow adjustment only in the upper, only in the lower or in both portions of the volcanic conduit. Our results emphasize that the thermodynamic evolution of magma properties and volcanic conduit flow are intricately linked, which may help understand the observed variability of eruptive behaviour at persistently degassing volcanoes.

Shallow-water lee-side waves at obstacles: Experimental characterization and turbulent non-hydrostatic modeling using weighted-averaged residual equations

The flow over uneven topography is a problem of interest in environmental fluid flow modeling, including flows over river bedforms, exchange flows over oceanic sills or the airflow over mountains. The common experimental procedure to investigate these flows, moving a small obstacle in a laboratory flume, yields experimental difficulties, whereas modeling using non-linear shallow flow equations does not explain all the flow phenomena. Novel alternative procedures are presented for the experimentation and shallow water representation of flow interaction with obstacles. A large-scale obstacle model is constructed in a dam-break set-up, and used to generate flow phenomena over topography, including dispersive and broken surges, wave reflection, hydraulic jumps and non-hydrostatic sill overflows. Simulations are conducted with a shallow-water weighted-averaged residual flow software for turbulent flows. The proposed software reproduces the experiments satisfactorily, supporting its use in modeling, whereas the new experimental database can be used by modelers to test their software.

Colloidal exchange flow in ducts

Buoyancy-driven exchange flow, i.e. drainage of a heavy colloidal suspension by an immiscible light pure fluid in a vertical duct, is studied analytically. A lubrication model is developed for two-dimensional channel and axisymmetric pipe geometries. The outcome transport equations are solved numerically to capture the effect of important governing dimensionless parameters: Péclet number, precursor film thickness, Reynolds number, mixtures’ viscosity ratio, initial volume fraction of hard-sphere particles, particles concentration within the precursor film, and capillary number. Recent study of colloidal film flow over a flat surface reveals interface thinning over time due to diffusion impact on suspension viscosity. However, the current model suggests that there is no such thinning phenomenon observed for colloidal flows within confined geometry unless in the absence of surface tension. The extent of the mixed particle zone along the duct grows as the Péclet number is decreased. Precursor film thickness is found to decrease the capillary ridge height forming at heavy suspension front. Either an increase in the Reynolds number or a decrease in light-to-heavy-fluid viscosity ratio extends the interpenetration rate of mixtures. As the bulk volume fraction of particles is increased close to 50 %, interesting secondary fronts emerge within the flow that coincide with the range where the diffusion coefficient is minimized. The heavy suspension layer drains slightly faster when the gradient in particle concentration between the bulk and precursor film regions is maximum. Finally, the non-monotonic impact of surface tension on the extent of mixtures’ exchange zone is uncovered via simulations conducted at various capillary numbers.

Generation of Anatomically Inspired Human Airway Tree Using Electrical Impedance Tomography: A Method to Estimate Regional Lung Filling Characteristics.

The purpose of lung recruitment is to improve and optimize the air exchange flow in the lungs by adjusting the respiratory settings during mechanical ventilation. Electrical impedance tomography (EIT) is a monitoring tool that permits measurement of regional pulmonary filling characteristics or filling index (FI) during ventilation. The conventional EIT system has limitations which compromise the accuracy of the FI. This paper proposes a novel and automated methodology for accurate FI estimation based on EIT images of recruitable regional collapse and hyperdistension during incremental positive end-expiratory pressure. It identifies details of the airway tree (AT) to generate a correction factor to the FIs providing an accurate measurement. Multi-scale image enhancement followed by identification of the AT skeleton with a robust and self-exploratory tracing algorithm is used to automatically estimate the FI. AT tracing was validated using phantom data on a ground-truth lung. Based on generated phantom EIT images, including an established reference, the proposed method results in more accurate FI estimation of 65% in all quadrants compared with the current state-of-the-art. Measured regional filling characteristics were also examined by comparing regional and global impedance variations in clinically recorded data from ten different subjects. Clinical tests on filling characteristics based on extraction of the AT from the resolution enhanced EIT images indicated a more accurate result compared with the standard EIT images.

Sequential invasions by fruit flies (Diptera: Tephritidae) in Pacific and Indian Ocean islands: A systematic review.

The aim of our review was to examine the cases of Tephritidae invasions across island systems in order to determine whether they follow a hierarchical mode of invasion. We reviewed the literature on factors and mechanisms driving invasion sequences in Pacific and Southwest Indian Ocean islands and gathered every record of invasion by a polyphagous tephritid in island groups. From invasion date or period, we defined an invasion link when a new fruit fly established on an island where another polyphagous tephritid is already resident (that was indigenous or a previous invader). Across surveyed islands, we documented 67 invasion links, involving 24 tephritid species. All invasion links were directional, i.e., they involved a series of invasions by invaders that were closely related to a resident species but were increasingly more competitive. These sequential establishments of species are driven by interspecific competition between resident and exotic species but are also influenced by history, routes, and flows of commercial exchanges and the bridgehead effect. This information should be used to improve biosecurity measures. Interactions between trade flow, invasive routes, and the presence of invasive and resident species should be integrated into large‐scale studies.

Investigation on Thermal Behavior During Pressurization Discharge of Liquid Oxygen Under Different Acceleration Levels

Cryogenic pressurization discharge involves on complex heat exchange and fluid flow issues, and the related thermal physical performance should be comprehensively investigated. In this study, a two-dimensional axisymmetric numerical model is adopted to research the outflow characteristic from a cylindrical liquid oxygen storage tank with the gas injection. The VOF method is utilized to predict the pressurization discharge with 360 K high-temperature gaseous oxygen as the pressurant gas. Validated against the liquid hydrogen discharge experiments, the numerical model is turned out to be proper and acceptable with the calculation errors limited within 20%. On the basis of the numerical model, effect of the flight acceleration level on the tank pressurization and liquid outflow performance are numerically simulated and analyzed, with the gas injection rate of 0.18 kg/s and the liquid outflow rate of 36.0 kg/s. Some valuable conclusions are obtained finally. The present study is significant to the safety flight of launch vehicle and may supply some technical supports for the design of cryogenic propellant system.

Using Ensemble Data Assimilation to Estimate Transient Hydrologic Exchange Flow Under Highly Dynamic Flow Conditions

AbstractQuantifying dynamic hydrologic exchange flows (HEFs) within river corridors that experience high‐frequency flow variations caused by dam regulations is important for understanding the biogeochemical processes at the river water and groundwater interfaces. Heat has been widely used as a tracer to infer steady‐state flow velocities through analytical solutions of heat transport defined by the diurnal temperature signals. Under sub‐daily dynamic flow conditions, however, such analytical solutions are not applicable due to the violation of their fundamental assumptions. In this study, we developed a data assimilation‐based approach to estimate the sub‐daily flux under highly dynamic flow conditions using multi‐depth temperature observations at a 5‐min resolution. If the hydraulic gradient is measured, Darcy's law was used to calculate the flux with permeability estimated from temperature responses below the riverbed. Otherwise, flux was estimated directly by assimilating multi‐depth temperature data at 1‐ or 2‐hr time intervals assuming one‐dimensional flow and heat transport governing equation. By comparing estimated fluxes with model‐generated synthetic truth, we demonstrated that both schemes have robust performance in estimating fluxes under highly dynamic flow conditions. This data assimilation‐based flux estimation method was able to capture the vertical sub‐daily fluxes using multi‐depth high‐resolution temperature data alone, even in the presence of multi‐dimensional flow. This approach has been successfully applied to real field temperature data collected at the Hanford site, which experiences highly dynamic HEFs. Our study shows the promise of adopting distributed 1‐D temperature monitoring to capture spatial and temporal exchange dynamics in river corridors at a watershed scale or beyond.

Modeling Contaminant Microbes in Rivers During Both Baseflow and Stormflow.

Rivers transport contaminant microorganisms (including fecal indicator bacteria and human pathogens) long distances downstream of diffuse and point sources, posing a human health risk. We present a mobile‐immobile model that incorporates transport as well as immobilization and remobilization of contaminant microbes and other fine particles during baseflow and stormflow. During baseflow conditions, hyporheic exchange flow causes particles to accumulate in streambed sediments. Remobilization of stored particles from streambed sediments occurs slowly during baseflow via hyporheic exchange flow, while remobilization is vastly increased during stormflow. Model predictions are compared to observations over a range of artificial and natural flood events in the dairy contaminated Topehaehae Stream, New Zealand. The model outputs closely matched timing and magnitude of E. coli and turbidity observations through multiple high‐flow events. By accounting for both state‐of‐flow and hyporheic exchange processes, the model provides a valuable framework for predicting particle and contaminant microbe behavior in streams.

Assessing the potential impacts of the Canal Istanbul on the physical oceanography of the Turkish Straits System

The Turkish Straits System (TSS) is a region of intense mixing between the Black Sea and the Mediterranean Sea exerting crucial control on the water & substance exchange through the Bosphorus and Dardanelles straits between these two sea basins. A waterway project, namely the Canal Istanbul, was proposed to be built within this system parallel to the Bosphorus. Even tough environmental impacts of the project have been of concern, a few studies that address these scientific and public concerns under the light of scientific methods are available. In order to fill this gap in the literature, a three-dimensional hydrodynamic model with realistic forcing has been set up for long-term integration. The study includes simulating of two scenarios for the adjacent basins; (i) existing configuration of the TSS, and (ii) the proposed extension with the recent planned route of Canal Istanbul that was added to the validated model. The results reveals that the canal project would increase the exchange flow between the Black Sea and the Mediterranean Sea in both directions while causing slight changes in temperature & salinity along the TSS. Findings of this study can be used in further research that would analyze the impacts of the proposed Canal Istanbul on the water quality and ecosystem of the TSS.

Impacts of Storm Surge Barriers on Drag, Mixing, and Exchange Flow in a Partially Mixed Estuary

AbstractStorm surge barriers are increasingly being considered as risk mitigation measures for coastal population centers. During non‐storm periods, permanent barrier infrastructure reduces the flow cross‐sectional area and affects tidal exchange. Effects of barrier structures on estuarine tidal and salinity dynamics have not been extensively examined, particularly for partially mixed estuaries. A nested, high‐resolution model is used to characterize impacts of a potential storm surge barrier near the mouth of the Hudson River estuary. Maximum tidal velocities through barrier openings are more than double those in the base case. Landward of the barrier, tidal amplitude decreases on average by about 6% due to increased drag. The drag coefficient with the barrier is about 5 times greater than the base case due primarily to form drag from flow separation at barrier structures rather than increased bottom friction. The form drag scales with barrier geometry similar to previous studies of flow around headlands. Tidal water levels are reduced particularly during spring tides, such that marsh inundation frequency is reduced up to 25%. Strong tidal velocities through barrier openings enhance salinity mixing locally, but overall mixing in the estuary decreases due to reduced tidal velocities. Correspondingly, stratification decreases near the barrier and increases landward in the estuary. The salinity intrusion length increases by 5%–15% depending on discharge due to the decreased mixing and increased exchange flow. Exchange flow increases near the barrier due reflux into the lower layer with the increased mixing, which has the potential to increase estuarine residence times.

Diurnal Thermally Driven Cross‐Shore Exchange in Steady Alongshore Currents

AbstractIdealized numerical modeling of thermally driven baroclinic exchange is performed to understand how cross‐shore flow is modulated by steady alongshore currents and associated shear‐generated turbulence. In general, we find that shear‐driven vertical mixing reduces the temperature gradients responsible for establishing the baroclinic flow, such that cross‐shore thermal exchange diminishes with alongshore current speed. Circulation in a base‐case simulation of thermal exchange with no alongshore forcing contains a cooling response consisting of a midday flow in the form of a downslope current with a compensating onshore near‐surface flow driving cross‐shore exchange, followed by an afternoon warming response flow via an offshore‐directed surface warm front, with a compensating return flow at the bottom. Nighttime convective cooling enhances vertical mixing and decelerates the warming response, and the diurnal cycle is renewed. In this base‐case scenario, representative of tropical reef environments with optically clear water and weak alongshore flow, surface heating and cooling can drive cross‐shore circulation with O(1) cm s−1 velocities. Alongshore flow forcing is implemented to induce upwelling‐ and downwelling‐favorable cross‐shore circulation. For mild alongshore forcing, the baroclinic cross‐shore exchange flow is enhanced due to an increase in the horizontal temperature gradient. Stronger alongshore flow leads to diminished thermally driven exchange, ultimately reaching a regime where the cross‐shore exchange is due predominantly to Ekman dynamics. Though exchange velocities are relatively small (O(1) cm s−1), these persistent exchange flows are capable of flushing the nearshore region multiple times per day, with important implications for water properties of nearshore ecosystems.

Simulated Effects of Seawater Desalination on Persian/Arabian Gulf Exchange Flow

Simulated Effects of Seawater Desalination on Persian/Arabian Gulf Exchange Flow

A Dynamic Model for Estimating the Interaction of ROS-PUFA-Antioxidants in Rabbit.

Defining optimal nutrition in animals and humans remains a main scientific challenge. The objective of the work was to develop a dynamic model of reactive oxygen species (ROS)–polyunsaturated fatty acid (PUFA)–antioxidant homeostasis using the rabbit as a model. The problem entity was to evaluate the main metabolites generated from interactions between traits included in the conceptual model and identified by three main sub–models: (i) ROS generation, (ii) PUFA oxidation and (iii) antioxidant defence. A mathematical model (VENSIM software) that consisted of molecular stocks (INPUTs, OUTPUTs), exchange flows (intermediate OUTPUTs) and process rates was developed. The calibration was performed by using standard experimental data (Experiment 1), whereas the validation was carried out in Experiments 2 and 3 by using supra–nutritional dietary inputs (VIT E+ and PUFA+). The accuracy of the models was measured using 95% confidence intervals. Analytical OUTPUTs (ROS, PUFA, Vit E, Ascorbic acid, Iso–/NeuroProstanes, Aldehydes) were well described by the standard model. There was also good accuracy for the VIT E+ scenario, whereas some compensatory rates (Kc1–Kc4) were added to assess body compensation when high levels of dietary PUFA were administered (Experiment 3). In conclusion, the model can be very useful for predicting the effects of dietary treatments on the redox homeostasis of rabbits.

Compound flood modeling framework for surface–subsurface water interactions

Abstract. Compound floods are an active area of research in which the complex interaction between pluvial, fluvial, coastal and groundwater flooding are analyzed. A number of studies have simulated the compound flooding impacts of precipitation, river discharge and storm surge variables with different numerical models and linking techniques. However, groundwater flooding is often neglected in flood risk assessments due to its sporadic frequency (as most regions have water tables sufficiently low that do not exacerbate flooding conditions), isolated impacts and considerably lower severity with respect to other types of flooding. This paper presents a physics-based, loosely coupled modeling framework using FLO-2D and MODFLOW-2005 that is capable of simulating surface–subsurface water interactions. FLO-2D, responsible for the surface hydrology and infiltration processes, transfers the infiltration volume as recharge to MODFLOW-2005 until the soil absorption capacity is exceeded, while MODFLOW-2005 returns exchange flow to the surface when the groundwater heads are higher than the surface depth. Three events characterized by short-duration intense precipitation, average tide levels and unusually high water table levels are used to assess the relevance of groundwater flooding in the Arch Creek Basin, a locality in North Miami particularly prone to flooding conditions. Due to limitations in water level observations, the model was calibrated based on properties that have experienced repetitive flooding losses and validated using image-based volunteer geographic information (VGI). Results suggest that groundwater-induced flooding is localized, and high groundwater heads influence pluvial flooding as the shallow water table undermines the soil infiltration capacity. Understanding groundwater flood risk is of particular interest to low-elevation coastal karst environments as the sudden emergence of the water table at ground surface can result in social disruption, adverse effects to essential services and damage to infrastructure. Further research should assess the exacerbated impacts of high tides and sea level rise on water tables under current and future climate projections.