Partial Residence Time Research Articles

Exchange Flow and Material Transport Along the Salinity Gradient of a Long Estuary

AbstractMost estuaries are characterized by non‐uniform axial topography with shallow shoals near the mouth. Previous studies have addressed the impacts of the axial topographic variations on mixing and estuarine circulations yet seldom on material transport and retention. This study investigates the longitudinal structure and mechanisms of exchange flow and material transport of Chesapeake Bay (CB), featuring a shallow sill in the lower bay, by applying total exchange flow (TEF) algorithm, tracer experiments, and partial residence time (PRT) using a validated 32‐years numerical model simulation. A retention coefficient was adopted to quantify the material retention rate using two characteristic PRTs: with and without incorporating water parcels returning to a concerned region. It is found that shoaling from the Rappahannock Shoal to the mouth causes persistent downwelling, strong reflux, and the highest material retention rate in the middle of the bay. The gravitational circulation and the river outflow dominate the transport of salt and riverine dissolved materials (RDMs), whereas the contribution of the tidal oscillatory process is localized near the mouth. The dominance of river outflow over the gravitational circulation for transporting RDMs is confined within the upper bay, where PRTs exhibit distinct seasonality. PRTs show small seasonality in the middle to the lower bay controlled by the exchange flow. The present analysis combining TEF, efflux/reflux theory, and PRT is applicable to other coastal aquatic ecosystems to characterize the water exchange and renewal efficiency along the salinity gradient and understand the contributions of transport to biogeochemical processes.

Partial residence times: determining residence time composition in different subregions

Residence time (RT) is a diagnosis widely used to quantify the water exchange rate and mass transport timescale in semi-enclosed systems. The RT focuses on only the total time a water parcel spends in a system (i.e., control region). However, for a system that consists of several subregions (e.g., sub-bays or functional zones), the RT will not include information about the time spent in different subregions. To determine the RT compositions in different subregions, partial residence times (PRTs) are proposed and defined as the amount of time a water parcel spends in different subregions until leaving the control region. The equations for PRTs are derived using the adjoint method, which can quickly determine the variation of PRTs in time and space. To validate the PRT diagnostic equation and numerical model, a test is conducted in an idealized 1D channel with idealized tidal currents. The numerical results are in excellent agreement with the analytical solution. Finally, the PRT method is applied to tide-dominated Jiaozhou Bay. The PRTs in six functional subregions of Jiaozhou Bay are derived, and the detailed RT compositions of the different functional subregions are presented. The application indicates that PRTs could provide detailed information and new insights into the water exchange process.

Dynamic Stabilization of Metal Oxide-Water Interfaces.

The interaction of water with metal oxide surfaces plays a crucial role in the catalytic and geochemical behavior of metal oxides. In a vast majority of studies, the interfacial structure is assumed to arise from a relatively static lowest energy configuration of atoms, even at room temperature. Using hematite (α-Fe2O3) as a model oxide, we show through a direct comparison of in situ synchrotron X-ray scattering with density functional theory-based molecular dynamics simulations that the structure of the (11̅02) termination is dynamically stabilized by picosecond water exchange. Simulations show frequent exchanges between terminal aquo groups and adsorbed water in locations and with partial residence times consistent with experimentally determined atomic sites and fractional occupancies. Frequent water exchange occurs even for an ultrathin adsorbed water film persisting on the surface under a dry atmosphere. The resulting time-averaged interfacial structure consists of a ridged lateral arrangement of adsorbed water molecules hydrogen bonded to terminal aquo groups. Surface pKa prediction based on bond valence analysis suggests that water exchange will influence the proton-transfer reactions underlying the acid/base reactivity at the interface. Our findings provide important new insights for understanding complex interfacial chemical processes at metal oxide-water interfaces.

Numerical simulation of flow characteristics in new co-rotating triangle arrayed triple screw extruders

Triangle arrayed triple screw extruder (TATSE) is a new equipment of polymer processing, which is different from traditional single and twin-screw extruder in flow and mixing characteristics. The studies about TATSE are very limited and all have been in the conditions of qualitative analysis due to its complicated geometry. In order to understand the flow rule of polymer melt and optimise the design of TATSE, in this paper, 3D flow of polymer melt in TATSE was simulated by using finite element method (FEM). A special study of flow in particular central region of TATSE was carried out by the analysis of the velocity distribution, pressure distribution and partial residence time distribution (RTD). The extrusion characteristics of TATSE, namely the abilities of pumping capability, distributive mixing, dispersive mixing, power consumption and productivity ratio, were compared with those of twin-screw and four-screw extruder. The results showed that there was circumfluence in central region but no stagnation in it. Although the velocity and pressure gradients were small relatively, the ability of pressure generation for TATSE was not evidently lower. TATSE had better operative abilities and high productivity ratio than intermeshing twin-screw extruder in the capacities of pumping capability and mixing.

The longitudinal distribution and community dynamics of zooplankton in a regulated large river: a case study of the Nakdong River (Korea)

The longitudinal distribution and seasonal dynamics of zooplankton were examined along a 200-km section of the middle to lower Nakdong River, Korea. Zooplankton was sampled twice a month from January 1995 to December 1997 at five sites in the main river channel. There was considerable longitudinal variation in total zooplankton abundance (ANOVA, p < 0.001). All major zooplankton groups (rotifers, cladocerans, copepodids and nauplii) increased significantly with distance downstream along the river. There also were statistically significant seasonal differences in zooplankton abundance at the sampling sites (ANOVA, p < 0.01). Zooplankton abundance was high in spring and fall and low in summer and winter. The seasonal pattern of rotifers was similar to that of total zooplankton. This reflected the fact that rotifers (Brachionus calyciflorus, B. rubens, Keratella cochlearis and Polyarthra spp.) strongly dominated the zooplankton community at all locations. Among the macrozooplankton, small-bodied cladocerans (e.g. Bosmina spp.) dominated; the abundance of large-bodied cladocerans (e.g. Daphnia) was negligible (0–5 ind. l−1). Among the environmental variables considered, partial residence time seemed to play the most important role in determining characteristics of the river zooplankton community.

Flow patterns in twin-screw extruders

Flow visualisation studies in an intermeshing co-rotating twin-screw extruder operated under starve-fed conditions are presented. Different numbers of kneading discs and relative staggering angles were used in the definition of the mixing blocks. Conveying and left-handed elements were also utilised. The images recorded provided information on flow patterns, on the degree of filling of the screw channels and on the number of filled turns. Local flow times were estimated. Partial residence time distribution (RTD) functions were determined by inserting especially designed collecting devices along the barrel, and measuring the evolution with time of the concentration of a tracer.

Application of PLTMG to a stream function problem

The finite-element package PLTMG was designed to solve a wide variety of non-linear, scalar elliptic PDEs. In this paper we examine the application of PLTMG to the solution of a potential flow problem using the stream function formulation. The flow problem is that of irrotational steady flow through a shallow lake with prescribed inflows and outflows. A two-dimensional (vertically integrated) model is used. One or more islands may be present. The paper focuses on the advantages of the stream function formulation for obtaining accurate streamlines and partial residence times as opposed to using a potential function formulation. If islands are present the stream values at the island boundaries are unknowns which must be determined to fulfil a condition of zero circulation round each island. Thus, if N islands are present, an N-fold linear continuation must be performed to determine the solution.

Oceanic consumption of CH3CCl3: Implications for tropospheric OH

Strong and extensive negative saturation anomalies for CH3CCl3 were observed in the mid‐Pacific Ocean during the spring of 1990. Anomalies were largest at the equator, where upwelling and biological activity were greatest, but present virtually everywhere in our sampling range of 15°S to 20°N. The mean saturation anomaly for all of our data, corrected for physical effects, was −11.0%, implying that the ocean is a significant sink for CH3CCl3. The loss rate to maintain this anomaly is supported roughly by known hydrolysis rates, but contributions from other processes cannot be ruled out at this time. If hydrolysis and mixing are the only processes removing CH3CCl3 from seawater, then about 6% of atmospheric CH3CCl3 is removed by consumption in the oceans. Our data show that losses to the ocean most probably would range from 5 to 11% of the CH3CCl3 removed from the atmosphere. This range translates to a partial residence time of 59–128 years for atmospheric CH3CCl3 relative to oceanic consumption. Although these data indicate that the ocean is not an overwhelmingly large sink, its neglect does represent a systematic error in the calculation of tropospheric OH concentration from CH3CCl3 production, growth, and distribution in the atmosphere.

Pu Residence Times in Freshwaters and Accumulation in Shield Lake Sediments

Activity balances for 239,240Pu in seven oligotrophic lakes were constructed from measurements of 239,240Pu inputs and measurements of 239,240Pu accumulated in the lake sediments. 239,240Pu eroded from the catchment was a significant input to lakes with rapid hydrologic flushing rates. From 28 to 100% of the 239,240Pu input to the lakes accumulated in the lake sediments. The fraction of 239,240Pu inputs accumulated in the lake sediments was inversely correlated with the hydrologic flushing rate of the lake. 239,240Pu concentrations in the sediments were simulated using a single reservoir input–output model. Partial residence times for 239,240Pu transfer from the water to the sediments ranged from 0.09 to 2.7 yr and were shorter in lakes with more rapid flushing. Partial residence times of 239,240Pu in these small shield lakes were very similar to the residence times of 239,240Pu in the Laurentian Great Lakes and to those of 137Cs and 210Pb in other small oligotrophic lakes.