Advective Flow Research Articles

Multiscale modeling of coupled thermo-hydro-mechanical analysis of heterogeneous porous media

This paper presents a numerical multiscale formulation for analysis of the transient heat and fluid flow in deformable heterogeneous porous media. Due to the heterogeneity of the media, the direct numerical simulation of the micro-structures leads to high computational costs. Hence, the multi-scale method can provide an efficient computational procedure. To this end, the first-order computational homogenization is adopted for two-scale simulation of THM problems. The governing equations of the problem contain a stress equilibrium equation, a mass continuity equation and an advection–diffusion equation in a fully coupled manner. Accordingly, the proper virtual power relations are defined as a bridging stage between the scales. The boundary conditions of the microscopic domain are specified in terms of the macroscopic information to fulfill the averaging constraints. In order to consider the transient effects, the micro-inertial terms are included in the micro-scale simulation, and then the microscopic boundary conditions are enhanced to assess the efficiency and accuracy. The macroscopic properties including the homogenized tangent operators and equivalent quantities are determined from the microscopic solution through the appropriate mathematical procedures. Moreover, an upwind finite element squared method is employed for highly advective heat transfer to achieve the accurate spatial results. Finally, several comprehensive examples are simulated to study the effectiveness and validity of the proposed computational model, particularly the boundary conditions; moreover, different case studies are discussed such as the highly advective flow, pure conduction heat transfer and a combined conduction–convection heat transfer.

A reactive transport modeling perspective on the dynamics of interface-coupled dissolution-precipitation

In interface coupled dissolution-precipitation systems, the dynamics of the mineral-fluid interface depends on two intertwining processes: the dissolution of the primary mineral that is needed for subsequent precipitation and the passivation of the dissolution reaction as a result of secondary mineral precipitation. The resulting thickness and texture of the precipitating coating layer will affect the progression of geochemical reactions, flow and transport processes at the macroscopic scale. Understanding the interplay between macroscopic flow regimes and microscopic reaction mechanisms (e.g., nucleation and crystal growth pathways) in controlling the dynamics of the mineral-fluid interface has important implications for predicting natural weathering processes, scaling in the subsurface energy production systems, etc. In this study, we use a micro-continuum pore-scale reactive transport model to investigate the feedback loop between reaction rate and solute transport with explicit consideration of the surface passivation and the diffusion process through the coating layer, as well as the impacts of saturation-dependent nucleation rate on the textures of precipitates that will largely dictate the diffusion properties of the coating layer. Our model results highlight that the drastically different coating behaviors at the macroscopic scale and their dependence on solution supersaturation observed in previous column experiments are primarily controlled by the interplay between mineral reaction rates, advective flow, and diffusion through the dynamically forming coating layer. The diffusion properties of the coating layer also play a secondary but non-negligible role in shaping the evolution of the co-dissolution and precipitation system. The probabilistic nucleation model building on the framework of classical nucleation theory highlights the complex dependence of precipitates’ texture on solution chemistry and substrate properties, which can affect the diffusion process within the precipitates. The modeling observations also underscore the necessity of further investigations to better characterize the properties of the coating layer and to improve modeling descriptions of the nucleation processes.

Assimilation of GOSAT Methane in the Hemispheric CMAQ; Part I: Design of the Assimilation System

We present a parametric Kalman filter data assimilation system using GOSAT methane observations within the hemispheric CMAQ model. The assimilation system produces forecasts and analyses of concentrations and explicitly computes its evolving error variance while remaining computationally competitive with other data assimilation schemes such as 4-dimensional variational (4D-Var) and ensemble Kalman filter (EnKF). The error variance in this system is advected using the native advection scheme of the CMAQ model and updated at each analysis while the error correlations are kept fixed. We discuss extensions to the CMAQ model to include methane transport and emissions (both anthropogenic and natural) and perform a bias correction for the GOSAT observations. The results using synthetic observations show that the analysis error and analysis increments follow the advective flow while conserving the information content (i.e., total variance). We also demonstrate that the vertical error correlation contributes to the inference of variables down to the surface. In a companion paper, we use this assimilation system to obtain optimal assimilation of GOSAT observations.

Устойчивость адвективного течения во вращающемся слое проводящей жидкости, помещенной в постоянное однородное магнитное поле

An exact solution of the Navier-Stokes equations in the Boussinesq approximation is presented. It describes advective flow in a flat rotating layer of an incompressible conducting fluid, on the horizontal boundaries of which a linear temperature distribution is set. The rotation axis is perpendicular to the fluid layer. The layer is placed in a constant homogeneous magnetic field oriented opposite to the force of gravity. The behavior of the flow velocity and temperature depending on the Taylor and Hartmann numbers is investigated. The stability of advective flow under normal perturbations is investigated within the framework of linear theory using a new numerical method. The problem is reduced to a one-dimensional system of partial differential equations with unknown functions depending on time and the vertical coordinate, which is solved using the grid method. Calculations have shown that a weak magnetic field increases the stability of the flow without changing the oscillatory character of instability. For the nonlinear formulation of the problem, the finite-amplitude perturbations in the supercritical region near the minima of neutral curves are studied numerically. Calculations have shown that thermal spots moving in opposite directions appear near the upper and lower boundaries of the layer. Near the horizontal boundaries of the layer forms a system of running helical vortices, arising in the region with unstable temperature stratification. The magnetic field strength perturbation has a spiral shape.

Mechanisms Controlling the Distribution of Net Water Transport in Estuarine Networks

AbstractNet water transport (NWT) in estuaries is important for, for example, salt intrusion and sediment dynamics. While NWT is only determined by river runoff in single channels, in estuarine networks, it results from a complex interplay between tides and residual flows. This study aims to disentangle the various contributions of these physical drivers to NWT in estuarine networks and investigate the sensitivities of NWT to variable forcing conditions, interventions, and sea level rise (SLR). To this end, a processes‐based perturbative network model is developed, which accounts for the vertical flow structure to resolve density‐driven flow driven by a vertically uniform along‐channel salinity gradient. Other identified drivers are river discharge and three tidal rectification processes: Stokes transport and its return flow, momentum advection, and velocity‐depth asymmetry. The model is applied to the Yangtze Estuary. NWT due to tidal rectifications and density‐driven flow can be comparable to river discharge. Specifically in the North Branch, the direction of NWT may differ from the direction of river discharge. Varying river discharge mainly affects NWT as tide‐river interaction is weak and density‐driven flow is shown to be insensitive to salt intrusion. Conversely, variations in tidal amplitude strongly affect NWT related to tidal rectification and density‐driven flow. The deepening (narrowing) of one channel (Deep Waterway Project), affected the NWT mostly through the density‐driven flow (advection). Furthermore, NWT distribution in the Yangtze is insensitive to SLR up to 2 m because the effects of SLR on transport due to different drivers compensate each other.

Interplay of high zeta potential and steric factor on the slip thermofluidics of power law fluids through narrow confinements

Thermal transport in a non-Newtonian fluid flow through a parallel plate microchannel is investigated with an emphasis on the effects of high zeta potential and steric factor. The fluid viscosity is modelled using the power law and the flow is driven by a combination of electric field and pressure gradient in the form of a dimensionless flow actuation coefficient. The effects of joule heating, viscous dissipation and slip length at the wall are considered. A semi-analytical formulation for the temperature profile is developed which is solved numerically using the Galerkin Finite Element method. We find that the Nusselt number increases as the flow behaviour index is increased. Velocity slip is also found to have a positive impact on convective heat transfer. However, as the zeta potential and steric factor increase, the Nusselt number begins to decline as there is an increase in viscous dissipation. We highlight the importance of considering variable electrical conductivity in evaluating the effect of joule heating at high zeta potential and moderate steric factor. In this regime, joule heating dominates advective transport and heat flow is reversed which may be detrimental or even damaging to microfluidic devices. A comprehensive thermal map is developed for a wide range of zeta potentials and steric factors.

The Full Multi: An open-source framework for modelling the transport and fate of nano- and microplastics in aquatic systems

We present a freely available open-source modelling framework to explore environmental transport and fate of nano-and microplastics in aquatic systems. The “Full Multi” models 1) fragmentation of plastic into a predefined set of size classes, 2) speciation of plastic particles between pristine, heteroaggregated, biofouled, and biofouled & heteroaggregated species, 3) dynamic vertical exchange between water layers and sediment of a freshwater system, and 4) horizontal particle transport by eddy diffusion and advective flow. The Full Multi framework relates emission rates to environmental exposure concentrations while considering the intrinsic properties of plastic particles and variable environmental system properties. The model can be applied to analyse scenarios with different process descriptions, plastic types, emission routes and environmental parameters for hypothesis generation, to identify dominant fate processes, and in hazard and risk assessment. Here, we introduce and illustrate the framework by modelling plastic particles with a range of densities in a generic flowing river system.

A comprehensive study on the characterization of chaotic advection flow and heat transfer in a twisted pipe with elliptical cross-section

Chaotic advection flow is one of the most useful methods to enhance mixing and heat transfer simultaneously in the laminar flow which is generated by spatially chaotic trajectories of the fluid particles. The present study aims to characterize the behavior of chaotic advection flow and heat transfer in a twisted pipe. The considered geometry consists of three 90° bends with an elliptical cross-section; the angle between curvature planes of consecutive bends is 90°. Numerical simulations are performed for both steady-state and pulsatile (unsteady) inlet velocity conditions with the Reynolds numbers in the range of 200 ≤ Re ≤ 450, wall temperatures 310 ≤ Tw(K) ≤ 340, velocity amplitude ratios (the ratio of peak oscillatory velocity component to the mean flow velocity) 1 ≤ β ≤ 3, and Womersley numbers 2.1118 ≤ α ≤ 7.9018. Mixing and heat transfer are analyzed both qualitatively and quantitatively for various considered conditions. The results revealed that the elliptical cross-section has a considerable impact besides the chaotic configuration on mixing and heat transfer enhancement. In the steady-state flow, the desirable system efficiency can be achieved either by increasing the wall temperature or decreasing the Reynolds number. Moreover, in the pulsatile flow, the Womersley numbers in the range of 2.1118 ≤ α ≤ 3.9509 provide a favorable condition to augment the mixing and heat transfer in the twisted pipe.

Transit‐Time and Temperature Control the Spatial Patterns of Aerobic Respiration and Denitrification in the Riparian Zone

AbstractDuring the flow of stream water from losing reaches through aquifer sediments, aerobic and anaerobic respiration (denitrification) can deplete dissolved oxygen and nitrate (NO3−), impacting water quality in the floodplain and downstream gaining reaches. Such processes, which vary in time with short and long‐term changes in stream flow and temperature, need to be assessed at the stream corridor scale to fully capture their effects on net turnover, but this has rarely been done. To address this gap, we combine a fully‐integrated 3D transient numerical flow model with temperature‐dependent reactive transport along advective subsurface flow paths to assess aerobic and anaerobic respiration dynamics at the stream corridor scale in a predominantly losing stream. Our results suggest that given carbon availability (as an electron donor), complete NO3− removal occurred further away from the stream after complete oxygen depletion and was relatively insensitive to variations in temperature and transit‐times. Conversely, transit‐times and oxygen concentrations constrained nitrate removal along short hyporheic flow paths. Even under limited carbon availability and low‐temperatures, NO3− removal fractions () will be greater at locations further from the stream than along shorter hyporheic flow paths ( = 0.4 and = 0.1, respectively). With increasing temperature, the relative effects of stream flow and solute concentrations on biogeochemical turnover and the redox zonation around the stream decreased. The study highlights the importance of seasonal variations of stream flow and temperature for water quality at the stream‐corridor scale. It also provides an adaptive framework to assess and quantify reach‐scale biogeochemical turnover around dynamic streams.

Numerical studies of the advective flow of heptadecane in a horizontal layer with a longitudinal temperature gradient at the lower boundary

Numerical studies of the convective flow of heptadecane in a horizontal layer with a suddenly applied longitudinal temperature gradient at the lower high-thermal conductivity boundary have been carried out by the finite element method. A system of nonstationary dimensionless equations of free convection containing stream function, velocity vortex, and temperature as variables was solved. The calculations were carried out with a free upper boundary with and without taking into account the influence of the thermocapillary effect.

Enhanced dissipation, hypoellipticity for passive scalar equations with fractional dissipation

We consider the passive scalar equations subject to shear flow advection and fractional dissipation. The enhanced dissipation estimates are derived. For the classical passive scalar equation (γ=1), our result agrees with the sharp one obtained in [46].

The challenge in measuring low-permeability materials: a comparison of different approaches

Abstract. The German repository site selection procedure calls for a radioactive waste containment zone with a low-permeability host rock (kf<10-10 m s−1, StandAG §23, 5) and long-term sealing by barrier materials (EndlSiAnfV, 2020; ESK, 2019). The potential host rocks, clay and rock salt, as well as the considered barrier materials, bentonite and compacted crushed salt, show permeability in the range of kf∼10-16 m s−1 (K∼10-21 m2). These low values suggest that advective flow is as slow as diffusive mass flux. Measuring such low permeability with adequate accuracy challenges measurement setups and respective error evaluation. Methodologies. Several low-permeability measurements are carried out by transient tests, e.g. by monitoring controlled fluid pressure changes in: (1) pressure decay and (2) oscillating pulse tests. The first method (1) deviates permeability from the time needed to compensate pressure differences through the sample. The latter (2) monitors phase shift and amplitude attenuation of controlled pressure pulses passing through the sample. Any permeability measurement needs to be post-processed, e.g. for: (1) material-intrinsic controls (saturation state, storativity, the fluids' compressibility, etc.), (2) environmental controls (temperature, confining pressures, etc.) and (3) theoretical considerations (Klinkenberg correction, multi-phase wetting angles, etc.). Salts. A porosity-permeability relation was found down to K=10-19 m2 (e.g., Popp et al., 2007). Testing fluids were NaCl brine, oils, He and N2 as a fluid. As a matter of current research, a critical, low-permeability value might be associated with the so-called “percolation threshold” that defines the minimal requirements for an interconnected pore system (e.g., DAEF, 2016). Clays. A major challenge is the long duration of sample saturation (up to several months) and pressure equilibration (often days), as well as precise, temperature-compensated measuring and the determination of the samples' storativity (e.g., Winhausen et al., 2021). Testing fluids are commonly designed mixtures mimicking the rocks' pore waters. Geotechnical barrier materials. The permeability testing performed is similar to that of salt and clays mentioned above. However, both barrier materials, crushed salt and bentonite, have significant permeability early after emplacement. This is beneficial, as it allows the outflow of unwanted canister corrosion gases. Eventually, the permeability drops by orders of magnitude and the barriers become tight seals in the long-term. Here, identifying the gas entry/breakthrough pressure has been valuable (e.g., Rothfuchs et al., 2007). Figure 1 shows a preliminary sensitivity analysis as an example of pressure decay measurements. It suggests that the pressure equilibration term (c), and hence the test duration, is most sensitive to the calculation of low permeability. However, the large variation of (representative) material and environmental controls make permeability measurements complex. This workshop aims to encourage discussions on uncertainty and sensitivity of the influencing controls, such that it may lead to a “best-practice” guide for permeability measurements.

Nitrogen isotopic constraints on nutrient transport to the upper ocean

Ocean circulation supplies the surface ocean with the nutrients that fuel global ocean productivity. However, the mechanisms and rates of water and nutrient transport from the deep ocean to the upper ocean are poorly known. Here, we use the nitrogen isotopic composition of nitrate to place observational constraints on nutrient transport from the Southern Ocean surface into the global pycnocline (roughly the upper 1.2 km), as opposed to directly from the deep ocean. We estimate that 62 ± 5% of the pycnocline nitrate and phosphate originate from the Southern Ocean. Mixing, as opposed to advection, accounts for most of the gross nutrient input to the pycnocline. However, in net, mixing carries nutrients away from the pycnocline. Despite the quantitative dominance of mixing in the gross nutrient transport, the nutrient richness of the pycnocline relies on the large-scale advective flow, through which nutrient-rich water is converted to nutrient-poor surface water that eventually flows to the North Atlantic.

Competing thermal and solutal advection decelerates droplet evaporation on heated surfaces

We report the complex evaporation kinetics of saline sessile droplets on surfaces with elevated temperatures. Our previous studies show that on non-heated substrates, saline sessile droplets evaporate faster compared to the water counterparts. In the present study, we discover that on heated surfaces, the saline droplets evaporate slower than the water counterpart, thereby posing a counter-intuitive phenomenon. The reduction in the evaporation rates is directly dependent on the salt concentration and the surface wettability. Natural convection around the droplet and thermal modulation of surface tension is found to be inadequate to explain the mechanisms. Flow visualizations using particle image velocimetry (PIV) reveal that the morphed advection within the saline droplets is a probable reason behind the arrested evaporation. Infrared thermography is employed to map the thermal state of the droplets. A thermo-solutal Marangoni based scaling analysis is put forward and the major governing non-dimensional numbers have been accounted for in the analysis. It is observed that the Marangoni flow and internal advection borne of thermal and solutal gradients are competitive, thereby leading to the overall decay of internal circulation velocity compared to the equivalent pure water case, which reduces the evaporation rates. The theoretically proposed advection velocities conform to the experimental results. This study sheds rich insight on a novel species transport behaviour in saline droplets.

Simulating Autoconvective Flow to Improve the Rate Performance of High Energy Density Lithium-Ion Electrodes

There are many design tradeoffs in lithium-ion batteries, the most notable being the loss of energy density at high power densities. New design requirements, such as low battery cost, further exacerbate these classic tradeoffs because they require thick electrodes with high loading. The power density in these dense electrodes is typically limited by Li+ depletion in the cathode electrolyte, which shuts off the cell before the full capacity can be extracted. In most industrial processes, concentration depletion can be overcome by introducing advection, but the small-scale pores and solid current collectors in batteries prevent the electrolyte from sustaining any form of bulk motion. Strategies to improve power density in thick electrodes, therefore, include increasing the electrode porosity (which reduces energy density) or increasing the electrolyte transference number (which is a difficult materials challenge).This work uses finite-element simulations to understand how battery performance would be influenced by internal advection of liquid electrolyte. We also propose a mechanism to achieve this through electro-osmotic flow of the electrolyte through charged separators. Our simulations demonstrate that the addition of a channel that circulates the electrolyte between the two ends of the cell can improve the battery’s volumetric energy density by 69% when discharged at a 4 C rate. Performance gains can be further amplified to 104% if an advective current can be maintained at 1 um/s.Figures 1(a) and 1(b) depict the simulation layout, which includes a standard lithium-ion battery with an 80-um thick lithium cobalt oxide (LCO) cathode, 50 um separator region, and 152 um LiC6 anode with 1 M LiPF6 in 1:1 EC/DEC. A modification introduces a thin channel, 1.2X the width of the cell (~340 um), that allows for electrolyte flow between the current collector ends of the cathode and anode. The multiphysics model follows a Newman approach and captures Li+ diffusion, migration, and advection in the electrolyte. Lithium diffusion and electron conduction are modeled in the electrodes, and Butler-Volmer kinetics govern the reaction kinetics. In both geometries, the LCO cathode is discharged from 4.3 V to 2.5 V.We perform simulations of the following geometries to understand the isolated effects of electrolyte continuity and advection: (1) a standard cell with no channel, (2) a cell with a channel but no flow, and (3) a cell with bulk motion of electrolyte through a channel. Figure 1(c) shows the electrolyte salt concentration within each battery at the end of discharge. The standard cell geometry notably features depletion of lithium ions ~15 um into the cathode, rationalizing the sharp drop in the cell’s discharge curve at 2.85 V (Figure 2). The addition of a channel results in migration and diffusion of lithium ions along the concentration gradient between the two current collector ends, significantly delaying ion depletion in the cathode during discharge. Introducing bulk motion further minimizes the depletion of electrolyte in the cathode and, in the extreme case of 100 um/s flow, completely eliminates ion depletion limitations.Figure 2 shows the voltage vs. energy density graph of each design, demonstrating a 104% increase in energy density for a channel-added geometry with 1 um/s of continuous bulk motion. Higher flow rates have marginal performance benefits, as a two order of magnitude increase in velocity leads to a <1% increase in energy density. Figure 2 further illustrates that, without bulk motion, the introduction of a channel results in an energy density increase of 69% relative to a normal, channel-less cell. Because the channel’s contribution to the battery size varies depending on the design of the modified cell, the discharge curves in Figure 2 do not account for volumetric changes and thus represent the maximum performance benefit that can be realized. Taking into account the channel’s volume in this particular design, however, only drops the performance gains to 82% for 1 um/s flow and 52% without any flow, demonstrating significant improvements despite the lack of robust design optimization.This simulation shows that advective flow of liquid electrolyte can improve the high rate performance of classic lithium-ion battery designs, as well as future designs with thick and dense electrodes. We believe that this can be rationally done by using porous current collectors and allowing flow through individual cell layers until the electrolyte is recirculated at cylindrical cell walls. To drive the flow internally, a charged membrane can function as an electro-osmotic pump that harnesses the inherent electric field of the electrolyte, allowing for massive gains in energy density within compact energy storage systems. Figure 1

Salinity evolution of aquitard porewater associated with transgression and regression in the coastal plain of Eastern China

Aquitards can preserve paleo-saltwater and have a long and profound influence on the aquifer groundwater, yet they have received less attention. Here, five one-dimensional vertical profiles of the natural tracers, Cl−, Br−, δ2H, and δ18O of aquitard porewater were measured and combined with eight existing profiles collected from previous studies in three Eastern China coastal plains, Bohai bay, Laizhou bay, and Jiangsu coastal plain, to show the salinity origin and the timescale of the transport of porewater across aquitards since the mid-Holocene. Evidence of paleo-seawater impacted by Holocene transgression is noticed as peak Cl− concentration, enriched water isotopes, and lower Cl/Br ratios are observed in the porewater at about the bottom depth of Holocene of 25 m. The shape of Cl− profiles and associated 1-D transport models confirms Holocene seawater's presence in the sediment and the alternating transgression and regression control the salinity evolution of coastal aquitard porewater. Diffusion is the dominant transport across aquitard with an upward velocity between 0.03 and 0.6 mm/yr. Reverse advective flow has affected the solute transport in areas of extensive pumping. Anthropogenic activities enhance the leaching of salty porewater into aquifers in coastal areas, necessitating the need for research focused on marine aquitard salty water.

Protein Transport upon Advection at the Air/Water Interface: When Charge Matters.

The formation of dense protein interfacial layers at a free air-water interface is known to result from both diffusion and advection. Furthermore, protein interactions in concentrated phases are strongly dependent on their overall positive or negative net charge, which is controlled by the solution pH. As a consequence, an interesting question is whether the presence of an advection flow of water toward the interface during protein adsorption produces different kinetics and interfacial structure of the adsorbed layer, depending on the net charge of the involved proteins and, possibly, on the sign of this charge. Here we test a combination of the following parameters using ovalbumin and lysozyme as model proteins: positive or negative net charge and the presence or absence of advection flow. The formation and the organization of the interfacial layers are studied by neutron reflectivity and null-ellipsometry measurements. We show that the combined effect of a positive charge of lysozyme and ovalbumin and the presence of advection flow does induce the formation of interfacial multilayers. Conversely, negatively charged ovalbumin forms monolayers, whether advection flow is present or not. We show that an advection/diffusion model cannot correctly describe the adsorption kinetics of multilayers, even in the hypothesis of a concentration-dependent diffusion coefficient as in colloidal filtration, for instance. Still, it is clear that advection is a necessary condition for making multilayers through a mechanism that remains to be determined, which paves the way for future research.

Spectral Signature of Mass Outflow in the Two Component Advective Flow Paradigm

Abstract Outflows are common in many astrophysical systems. In the Two Component Advective Flow (TCAF) paradigm, which is essentially a generalized Bondi flow including rotation, viscosity, and cooling effects, the outflow originates in the hot, puffed up, post-shock region at the inner edge of the accretion disk. We consider this region to be the base of the jet carrying away matter with a high velocity. In this paper, we study the spectral properties of black holes using TCAF that includes also a jet (JeTCAF) in the vertical direction of the disk plane. Soft photons from the Keplerian disk are up-scattered by the post-shock region as well as by the base of the jet and are emitted as hard radiation. We also include the bulk motion Comptonization effect by the diverging flow of the jet. Our self-consistent accretion-ejection solution shows how the spectrum from the base of the jet varies with accretion rates, geometry of the flow, and the collimation factor of the jet. We apply the solution to a jetted candidate GS 1354-64 to estimate its mass outflow rate and the geometric configuration of the flow during the 2015 outburst observed by NuSTAR. The estimated mass outflow to mass inflow rate is 0.12 − 0.03 + 0.02 . From the model-fitted accretion rates, shock compression ratio, and the energy spectral index, we identify the presence of hard and intermediate spectral states of the outburst. Our model-fitted jet collimation factor (f col) is found to be 0.47 − 0.09 + 0.09 .

Strong noise limit for population dynamics in incompressible advection.

Genetic diversity is at the basis of the evolution process of populations and it is responsible for the populations' degree of fitness to a particular ecosystem. In marine environments many factors play a role in determining the dynamics of a population, including the amount of nutrients, the temperature, and many other stressing factors. An important and yet rather unexplored challenge is to figure out the role of individuals' dispersion, due to flow advection, on population genetics. In this paper we focus on two populations, one of which has a slight selective advantage, advanced by an incompressible two-dimensional flow. In particular, we want to understand how this advective flow can modify the dynamics of the advantageous allele. We generalize, through a theoretical analysis, previous evidence according to which the fixation probability is independent of diffusivity, showing that this is also independent of fluid advection. These findings may have important implications in the understanding of the dynamics of a population of microorganism, such as plankton or bacteria, in marine environments under the influence of (turbulent) currents.

Spatiotemporal characteristics of hydraulic performance and contaminant transport in treatment wetlands

Wetlands have been proven to be efficient and cost-effective ecological treatment systems for municipal and domestic wastewater. It is essential to understand the hydrodynamic characteristics and the related contaminant transport process to optimize the treatment efficiency in free water surface wetlands. Thirty-six tracer experiments were conducted under different water depths and wetland configurations, such as installing static obstacles and dynamic disturbances. The particle image velocimetry and a novel color-concentration transform method were developed to reveal spatiotemporal flow velocity and residence time distribution. The flow fields are categorized into short-circuiting, circulation flow, dead zones, and the corresponding contaminant transport phenomena are flow advection, shear dispersion, and eddy diffusion. The flow circulation dominates the formation of the dead zone and decreases contaminant dissipation. The flow path could have effectively meandered, and the dead zone and short-circuiting could be reduced by increasing the length of the obstacles. The improved flow field is close to the plug flow, indicating enhanced hydraulic performance and treatment efficiency. The dynamic disturbance reflects the movement of fish in wetlands and provides momentum flux to promote the dissipation of pollutants in the circulation field and dead zone, alleviating the deterioration of water quality caused by pollutant accumulation. The findings of this study may provide a critical reference for the optimal design of wetlands.