Tracer flux ratios: a phenomenological approach

The kinetic behavior of tracer flows across epithelial membranes is examined and attention is called to the condition under which unidirectional tracer flows may be described by first order rate equations. It is shown that the first order nature of the tracer rate equation when combined with simple thermodynamic constraints on tracer flow yields a relation between the ratio of the unidirectional rate coefficients and thermodynamic driving forces. The form of this relation is examined for the case of simple diffusion and in the presence of coupled process.     

Characteristics of Velocity and Excess Density Profiles of Saline Underflows and Turbidity Currents Flowing over a Mobile Bed

Turbidity currents in the ocean and lakes are driven by suspended sediment. The vertical profiles of velocity and excess density are shaped by the interaction between the current and the bed as well as between the current and the ambient water above. We present results of a set of 74 experiments that focus on the characteristics of velocity and fractional excess density profiles of saline density and turbidity currents flowing over a mobile bed. The gravity flows include saline density flows, hybrid saline/turbidity currents and a pure turbidity current. The use of dissolved salt is a surrogate for suspended mud that is so fine that it does not settle out readily. Thus, all the currents can be considered to be model turbidity currents. The data cover both Froude-subcritical and Froude-supercritical regimes. Depending on flow conditions, the bed remains flat or bed forms develop over time, which in turn affect vertical profiles. For plane bed experiments, subcritical flow profiles have velocity peaks located higher up in the flow, and display a sharper interface at the top of the current, than their supercritical counterparts. The latter have excess density profiles that decline exponentially upward from the bed, whereas subcritical flows show profiles with a region near the bed where excess density varies little. Wherever bed forms are present, they have a significant effect on the profiles. Especially for Froude-supercritical flow, bed forms push the location of peak velocity upward, and render the near-bed fractional excess density more uniform. In the case of subcritical flow, bed forms do not significantly affect fractional excess density profiles; velocity profiles are pushed farther upward from the bed than in the case of a plane bed, but to a lesser extent than for supercritical bed forms. Overall, the relative position of the velocity peak above the bed shows a dependence upon flow regime, being lowered for increasing Froude number Fd. Gradient Richardson numbers Rig in the near-bed region increase with increasing Fd, but are lower than the critical value of 0.25, indicating that near-bed turbulent structures are not notably suppressed. At the top interface, values of Rig are above the critical value for subcritical and mildly supercritical Fd, effectively damping turbulence. However as Fd increases, Rig goes below the critical value. Shape factors calculated from the profiles for use in the depth-averaged equation of motion are evaluated for different flow and bed conditions. Normalized experimental profiles for supercritical currents scale up well with observations of field-scale turbidity currents in the Monterey Canyon, and the range of average bed slopes and Froude numbers also compare favorably with estimated field-scale flow conditions for the Amazon canyon and fan. This suggests that the experimental results can be used to interpret the kinds of flows that are responsible for the shaping of major submarine canyon-fan systems.     

Effects of Salt Tracer Amount,Concentration and Kind on the Fluid Flow Behavior in a Hydrodynamic Model of Continuous Casting Tundish

The hydrodynamic modeling method that widely used to simulate the fluid flow was reconsidered and discussed in this paper. The effects of injected salt tracer amount, concentration and kind on the fluid flow behavior in a hydrodynamic model tundish were investigated. The results were compared with the mathematical modeling calculation results, that the tracer density effect was eliminated. The residence time distribution (RTD) curve of tracer introduced deviated to the left side of the calculated curve, besides the deviation was increased as dimensionless tracer amount (the ratio of tracer amount to hydrodynamic model tundish volume) increased from 0.202 × 10^?3 to 1.008 × 10^?3. The results of tracer concentration study were similar, namely the deviation was increased with concentration increased; on the other hand, the deformation of a “stair‐shape” RTD curve occurred when tracer concentration was much lower (at dimensionless tracer amount of 0.168 × 10^?3 with converting to saturated solution). Besides, the effect of tracer kind on the accuracy of hydrodynamic modeling was also studied; the measurements of KCl solution with lower density than that of NaCl solution exhibited more of accuracy. Finally, the optimized tracer in hydrodynamic model tundish of present work is saturated KCl solution with dimensionless tracer amount of 0.202 × 10^?3.     

Modeling blood flow heterogeneity

It has been known for some time that regional blood flows within an organ are not uniform. Useful measures of heterogeneity of regional blood flows are the standard deviation and coefficient of variation or relative dispersion of the probability density function (PDF) of regional flows obtained from the regional concentrations of tracers that are deposited in proportion to blood flow. When a mathematical model is used to analyze dilution curves after tracer solute administration, for many solutes it is important to account for flow heterogeneity and the wide range of transit times through multiple pathways in parallel. Failure to do so leads to bias in the estimates of volumes of distribution and membrane conductances. Since in practice the number of paths used should be relatively small, the analysis is sensitive to the choice of the individual elements used to approximate the distribution of flows or transit times. Presented here is a method for modeling heterogeneous flow through an organ using a scheme that covers both the high flow and long transit time extremes of the flow distribution. With this method, numerical experiments are performed to determine the errors made in estimating parameters when flow heterogeneity is ignored, in both the absence and presence of noise. The magnitude of the errors in the estimates depends upon the system parameters, the amount of flow heterogeneity present, and whether the shape of the input function is known. In some cases, some parameters may be estimated to within 10% when heterogeneity is ignored (homogeneous model), but errors of 15-20% may result, even when the level of heterogeneity is modest. In repeated trials in the presence of 5% noise, the mean of the estimates was always closer to the true value with the heterogeneous model than when heterogeneity was ignored, but the distributions of the estimates from the homogeneous and heterogeneous models overlapped for some parameters when outflow dilution curves were analyzed. The separation between the distributions was further reduced when tissue content curves were analyzed. It is concluded that multipath models accounting for flow heterogeneity are a vehicle for assessing the effects of flow heterogeneity under the conditions applicable to specific laboratory protocols, that efforts should be made to assess the actual level of flow heterogeneity in the organ being studied, and that the errors in parameter estimates are generally smaller when the input function is known rather than estimated by deconvolution.     

Effects of flow on morphological stability during directional solidification

Research involving the interaction of flow with morphological instability during directional solidification of binary alloys is reviewed. In general, flow may arise during the solidification process from thermal and solutal buoyancy, changes in density upon solidification, thermocapillary forces at free boundaries, or external forcing of the system. We focus primarily on the last of these, giving details of the influence of various forced flows on the critical conditions for morphological instability. These flows include the asymptotic suction profile, stagnation-point flow, and periodically driven shear flows. Parallel shear flows are unable to stabilize morphological instabilities in three dimensions but may lead to new long-wave, traveling instabilities. Flow-induced, long-wave instabilities are also encountered in the presence of both steady and modulated stagnation-point flows. Unsteady, nonparallel shear flows may stabilize morphological instability if the flow parameters are adjusted properly. This article is based on a presentation made at the “Analysis and Modeling of Solidification” symposium as part of the 1994 Fall meeting of TMS in Rosemont, Illinois, October 2–6, 1994, under the auspices of the TMS Solidification Committee.     

Experimental Studies on Vertical Dense Jets in a Flowing Current

Experiments were performed using three-dimensional laser-induced fluorescence on turbulent vertical dense jets in flowing currents typical of brine disposal from seawater desalination plants. The flows are complex and different phenomena can dominate at different locations and at different current speeds, indicating that predicting these flows numerically will be quite challenging. At low current speeds, the rising and falling flows are almost vertical with some interference between them and the bottom flow spreads upstream as a wedge. At higher current speeds the wedge is expelled; the ascending flow is still almost vertical, but the descending flow is more gradual so the jet impacts the lower boundary farther downstream. Dilutions at the terminal rise height and impact point increase with increasing current speeds. Cross-sectional profiles of tracer concentration are neither axially or self-similar. In the descending flow, at low or intermediate current speeds, the plume is much taller than it is wide, the peak concentration occurs much closer to the top, and fluid can detrain from the jet. At higher current speeds, the profiles initially approach radial symmetry, but develop a kidney shape due to formation of two counter-rotating vortices farther downstream. These vortices cause the jet to almost completely bifurcate after impacting the bottom.     

Submerged Hydraulic Jumps below Abrupt Expansions

In submerged flows below an expanding outlet, both symmetric and asymmetric flows are observed. The hydraulic conditions required to form a symmetric submerged jump are necessary for design purposes. In this study, the flow conditions are presented for the submerged flows at an abrupt-symmetrical expansion. Also, the hydraulic conditions for the formation of a symmetric submerged jump are clarified, and an expression is developed for the transition between symmetric and asymmetric flows. Further, an equation on the length of submerged jumps is derived.     

Flow Characteristics of Skimming Flows in Stepped Channels

Skimming flows in stepped channels are systematically investigated under a wide range of channel slopes (5.7°?θ?55°). The flow conditions of skimming flows are classified into two flow regimes, and the hydraulic conditions required to form a quasi-uniform flow are determined. An aerated flow depth of a skimming flow is estimated from the assumption that the residual energy at the end of a stepped channel coincides with the energy at the toe of the jump formed immediately downstream of the stepped channel. In a quasi-uniform flow region, the friction factor of skimming flows is represented by the relative step height and the channel slope. The friction factor for the channel slope of θ=19° appears to have a maximum. The residual energy of skimming flows is formulated for both nonuniform and quasi-uniform flow regions. Further, a hydraulic-design chart for a stepped channel is presented.     

Uptake of seven myocardial tracers during increased myocardial blood flow by dobutamine infusion

RATIONALE AND OBJECTIVES: Direct comparison of myocardial perfusion tracers has been made difficult by variability in experimental models, and by a virtual absence of data comparing tracer uptake to myocardial blood flow under conditions of increased myocardial oxygen consumption, similar to what occurs with dynamic exercise. METHODS: Tracer uptake versus myocardial blood flow was evaluated for thallium-201 (201TI) and six technetium-99m (99mTc) myocardial-imaging agents in 24 open-chest canines with an occluded left-anterior descending coronary artery during dobutamine infusion. Data were fitted to the exponential model y = ax(1 - exp[-PSc/x]), where y is the tissue tracer/g normalized to normal (activity at 1 mL/minute/g) and x is the blood flow measured by the radioactive microsphere method. RESULTS: With dobutamine, myocardial tracer uptake was linear across a wide range of ischemic and hyperemic flows for each tracer. Based on the permeability surface area product, 201TI and 99mTc Q3 provided the best tracer estimate of myocardial blood flow (5.30+/-0.86 mL/minute/g, r = 0.91; 5.46+/-0.58 mL/minute/g, r = 0.94, respectively). Correlation coefficient (r) values for other tracers studied were 99mTc Q4 (r =0.93), 99mTc Q12 (r = 0.93), 99mTc sestamibi (r = 0.90), 99mTc tetrofosmin (r = 0.96), and 99mTc-N-Noet (r = 0.82). CONCLUSIONS: Of the 99mTc tracers examined under conditions of dobutamine-altered myocardial contractility, the myocardial uptake properties of 99mTc Q3 were most similar to those of 201TI.     

Ladle Shroud as a Flow Control Device for Tundish Operations

The performance characteristics of a tundish, such as the flotation of inclusions and slag entrainment, are largely influenced by the fluid-flow phenomena. Physical modeling in water is widely used to understand the fluid flows in a tundish and as a tool to improve, control, and design procedures for high-quality steel processing operations. These approaches were used to study the performance of fluid flow for a new design of ladle shroud. The new design for a dissipative ladle shroud (DLS) was studied, using a one-third scale, delta shaped, four-strand tundish. The results were compared with those achieved with the conventional ladle shroud. Different cases have been analyzed, including a conventional ladle shroud (LS) with a bare tundish and a tundish furnished with an impact pad. Similarly, the new design of the shroud (DLS) was studied under equivalent conditions. The physical experiments included the use of particle image velocimetry (PIV) and conductivity tracer techniques. The PIV measured the instantaneous velocities at the outlet of the DLS and the LS at different flow rates, showing the detailed jetting characteristics of water leaving the two types of ladle shroud. Residence time distribution (RTD) curves were also obtained for the different flow arrangements previously mentioned, and the dispersion of a colored dye tracer was observed at different intervals of time during tundish operation and analyzed using the video visualization technique.     

Strongly Advected Jet in a Coflow

We present the results of an experimental investigation into the behavior of a strongly advected jet (weak jet) in a coflowing ambient fluid. Laser-induced-fluorescence and image processing techniques are employed to provide quantitative information about the mean properties of the flow. Data are obtained at previously inaccessible distances from the flow source, enabling the mean and fluctuating behavior of the flow in the strongly advected region to be more clearly defined. The spread relationship, traditionally employed to model the mean behavior of these flows, is redefined based on this experimental information. Both the spread constant and the ratio of the tracer to the velocity spread are shown to change in the strongly advected region. The results presented are discussed in the context of recent observations about the importance of large-scale eddy motions in defining the mean behavior of weakly advected jets and plumes. In addition the experiments provide information on the contrasts between the mixing processes in the weakly advected and strongly advected coflowing jet regions, and their influence on the mean behavior of these flows. Concentration fluctuation statistics from both regions are presented and implications for the application of the entrainment assumption in the strongly advected region are also discussed.     

Relative Buoyancy Dominates Thermal-Like Flow Interaction along an Incline

This paper describes laboratory investigations of the motion between two fixed volumes of dense fluid (surge-type gravity currents) with different salt concentrations that interact above an incline in the presence of ambient stratification. The experiments include both large and small density contrasts between the interacting surges. Initially, the propagation of each fluid mass assumes a thermal-like nature, but then the lower density surge is quickly caught up by the denser fluid flow because of its higher velocity. There are two key process regarding the surge interaction. With a large density contrasting the fluid volumes, the denser flow moves to the front of the current as an intrusion with no mixing. With a small density difference, pronounced mixing occurs between the surges with the development of a homogeneous underflow. A simple energy parameterization is developed to evaluate the source conditions under which the different flow dynamics develop.     

Cytotoxicity of the anticancer agents cisplatin and taxol during cell proliferation and the cell cycle

The cerebral extraction and retention of three radioiodinatéd SPECT perfusion tracers were measured using residue detection in a baboon. A permeability-surface area product PS' with special relevance to SPECT was calculated from the retention of tracer in the brain after 10 min. PS' differs from the traditional PS value, which is calculated from the tracer clearance curve at 2 min. The PS' values ranged from 50 to 95 mL/min/100 g, decreased in the order [123I]IMP > [123I]iodoperidol approximately [123I]HIPDM, and did not differ for specific activities of 10 MBq/mmol to 74 TBq/mmol. These radioiodinated compounds exhibited extraction characteristics superior to those of [99mTc]HMPAO but underestimated cerebral blood flow when flows were above 20-30 mL/min/100 g, underscoring the need for development of a more ideal SPECT perfusion tracer.     

Effects of Electromagnetic Brake,Mold Curvature and Slide Gate on Fluid Flow of Steel in a Slab Mold

The influence of mold curvature, slide gate and magnetic forces on the steel flow in a slab mold was studied with a 3‐D mathematical model. The slide gate application induces a biased flow toward the mold side where its opening is located in the submerged entry nozzle (SEN). Turbulence and asymmetry of flows are more intense in a curved mold than in a straight mold. The effect of an electromagnetic brake (EMBr), located in the discharging ports to control flow turbulence, is only appreciable when magnetic flux density is higher than 0.1 T. The magnetic flux density does not affect the velocity profile in the discharging ports in the SEN because its construction material is insulated. Increasing the magnetic flux density leads to a decrease of the discharging jets angle and to the elimination of the two upper roll flows. The use of the EMBr in a curved mold equipped with a slide gate eliminates the meniscus velocity spikes observed in the mold corners. These results help to demonstrate that EMBr eliminates the asymmetry in a curved mold even under the excessive turbulent conditions existing in curved continuous casting molds.     

Mass Transport in Shallow Turbulent Wake Flow by Planar Concentration Analysis Technique

A planar concentration analysis (PCA) system is used for observing the transport and mixing of a tracer mass in a shallow turbulent free-surface wake flow of a large cylindrical obstacle. The nonintrusive, fieldwise PCA measuring technique is applied to evaluate depth-averaged mass concentrations by making use of light attenuation due to absorption and scattering processes related to a dissolved tracer mass. The scalar fields are decomposed into a low-frequency quasiperiodic part, the coherent flow, and a randomly fluctuating part. From accompanying near-surface velocity measurements, large-scale coherent structures are identified and related to the coherent mass fields. This allows one to assess the role of the large-scale vortices for advection and diffusion in shallow wake flows. The time–mean wake flow displays a self-similar spanwise distribution both for mass and velocity. The longitudinal development of shallow wakes initially shows the growth of unbounded wakes; in the wake far field an attenuated behavior applies.     

Three-Dimensional Modeling of Negatively Buoyant Flow in Diverging Channels

Diverging channels, also known as diffusers, represent common natural and industrial outlets to lakes, reservoirs, and rivers. If the outflow in a diffuser has a larger density than the ambient water, the inflow may plunge and form a density underflow. In this paper, a three-dimensional numerical study was conducted to gain insight into the mechanism of negatively buoyant flows in diffusers with a sloping bottom. Of particular interest is the formation of separated flows such as wall-jet and free-jet flows. Various cases of plunging and the associated density current in a diffuser with different divergence angles and inflow densimetric Froude numbers are considered. The model successfully simulates the formation of attached flow, wall jets, and free jets in a negatively buoyant environment. The onset, evolution, and stabilization of a stall and the subsequent development of a wall jet in a negatively buoyant flow are investigated in detail. Computed results also show favorable agreement with some published experimental data on density current generated by the plunging of cold water in ambient warm water in a diverging channel.     

Deposition from the two-phase multicomponent flow of a vacuum-arc plasma containing droplets of an evaporated material

The interaction of a surface with the two-phase flow of a multicomponent vacuum-arc plasma containing droplets of an evaporated material along with multicharged ions and neutral atoms is studied. For the two-phase flow of a multicomponent plasma, the dependences of the ion-current density, the deposition rate, and the heat flow to a substrate on the bias voltage are obtained, and the conditions at which the coating deposition rate is inverted are determined. A unique probe technique for the plasma flow generated by an end-face Hall plasma accelerator with a “cold” eroded cathode is used to determine the volt-equivalent energy U** of the interaction of the two-phase plasma flows of Ti, Al, Cr, and an Ni-Cr-Al-Y alloy with a surface and the self-sputtering coefficient of the Ni-Cr-Al-Y alloy and its elements (Ni, Cr, Al) as a function of the bias voltage. Metallographic analysis is used to study the structure of thick (～100-μm) coatings deposited from the two-phase flow of a metallic multicomponent vacuum-arc plasma.     

Quantitation of lymphatic drainage of the peritoneal cavity in sheep: comparison of direct cannulation techniques with indirect methods to estimate lymph flow

OBJECTIVE: It has been suggested that lymphatics may contribute to ultrafiltration failure in patients on continuous ambulatory peritoneal dialysis (CAPD) by absorbing dialysate and ultrafiltrate from the peritoneal cavity. In most studies lymphatic drainage has been estimated from the disappearance of an instilled tracer from the peritoneal cavity or estimated from the appearance of an intraperitoneally administered tracer in the bloodstream. However, in sheep it is possible to cannulate several of the relevant lymphatics that drain the peritoneal cavity and assess lymph drainage parameters directly. The purpose of this study was to estimate lymph drainage from the peritoneal cavity in sheep using the disappearance of tracer from the cavity and the appearance of intraperitoneally instilled tracer in the bloodstream and to compare these results with those obtained from our previous studies using cannulation techniques. DESIGN: Experiments were performed in anesthetized and nonanesthetized animals. Volumes of 50 mL/kg of Dianeal 4.25% containing 25 microCi of 125I-albumin were infused into the peritoneal cavity. RESULTS: In anesthetized sheep the calculated peritoneal lymph drainage from monitoring the disappearance of tracer from the peritoneal cavity over 6 hours was 1.873 +/- 0.364 mL/kg/hour. Monitoring the appearance of tracer in the blood gave significantly lower peritoneal lymph flow rates of 1.094 +/- 0.241 mL/kg/hour. Directly measured lymph flow rates from our earlier publication were lower still and ranged from 0.156 +/- 0.028-0.265 +/- 0.049 mL/hour/kg, depending on how we estimated the right lymph duct contribution to peritoneal drainage, since we could not cannulate this vessel. We repeated these experiments in conscious sheep. The value for lymph flow estimated from the disappearance of tracer from the peritoneal cavity was 2.398 +/- 0.617 mL/hour/kg and from the appearance of tracer in the blood, 1.424 +/- 0.113 mL/hour/kg. The lymph flow rates monitored from indwelling lymphatic catheters ranged from 1.021 +/- 0.186-1.523 +/- 0.213 mL/hour/kg (again, depending on our estimates for the right lymph duct). CONCLUSIONS: Lymph flow rates measured from indwelling lymphatic catheters provided the most conservative values for lymphatic drainage of the peritoneal cavity under dialysis conditions. Estimates of lymphatic drainage based on the appearance of tracer in the blood gave values that were on average higher. The method using the disappearance of tracer from the cavity to estimate lymph flows overestimated peritoneal lymph drainage. Fluid was lost from the peritoneal cavity, and the estimated proportion of liquid lost through lymphatic drainage depended on the technique used to measure lymph flow rates.     

Hydraulic Models of the Flow Distribution in a Four Branch Open Channel Junction with Supercritical Flow

Intense rainfall on urban areas can generate severe flooding in the city, and if the conditions are right, the flow in the streets can be supercritical. The redistribution of the flow in street intersections determines the flow rates and water levels in the street network. We have investigated the flow that occurs when two supercritical flows collide in a 90° junction formed by streets of identical cross section. Several flow configurations within the intersection are possible, depending on the position of the hydraulic jumps that form in and upstream of the intersection. Previous work has identified three flow types, with Type II flows being further classified into three subregimes. Hydraulic models have been developed, based on the principles of the conservation of flow and momentum flux in the intersection, which predict the angles at which the jumps will form. These models can be used to determine the flow type that will occur. Moreover, additional models have been developed for computing the outflow discharge distribution. For Type I flows, it has not been possible to develop such a hydraulic model for the discharge distribution, but some data are provided for one configuration to indicate the influence of different parameters. For Type II and Type III flows, such models are developed, and their predictions agree with data obtained from the channel intersection facility at the Laboratory of Fluid Mechanics and Acoustics in Lyon.