Fluid Dynamic Considerations Research Articles

Where is the common sense in aortic valve replacement? A review of hemodynamics and sizing of stented tissue valves

Heated debates revolve around the hemodynamic performance of stented aortic tissue valves. Because the opening area strongly influences the generation of a pressure gradient over the prosthesis, and the outer diameter determines which valve actually fits into the aortic root, it would seem logical that the valve with the greatest opening area in relation to its outer diameter should allow the best hemodynamic performance. Interestingly, neither of these 2 parameters is reflected by the manufacturing companies' size labels or suggested sizing strategies. In addition, it is known that valves with the same size label from different companies may differ significantly in their actual dimension (outer diameter). Finally, the manufacturer-suggested sizing strategies differ so much that expected differences from valve design may get lost because of differences in sizing. These size and sizing differences and the lack of information on the geometric opening area complicate true hemodynamic comparisons significantly. Furthermore, some fluid dynamic considerations regarding the determination of opening area by echocardiography (the effective orifice area) introduce additional obscuring factors in the attempt to compare hemodynamic performance data of different stented tissue valves. We analyzed the true dimensions of different tissue prostheses and the manufacturer-suggested sizing strategies in relation to published effective orifice areas. We have demonstrated how sizing and implantation strategy have much greater impact on postoperative valve hemodynamics than valve brand or type. In addition, our findings may explain the different opinions regarding valve hemodynamics of different tissue valves.

Urea Sensor Based on Chemiluminescence Using a Micro-Reactor

As urea is an index material of the hemodialysis efficiency in hemodialysis therapy, we propose a compact and accurate urea sensor based on chemiluminescence (CL) emitted by the reaction of urea with a hypobromite solution. We designed a new micro-reactor comprised of seven micro-cells based on the investigation of the CL reaction process and fluid dynamical considerations. The sensor has (1) high accuracy of 4%, (2) short response time of 1s, (3) linear characteristics for urea concentration between 1 to 16mM, (4) no interference by uric acid, creatinine, ammonia, and (5) long-life.

Fluid Dynamic Considerations for Realistic Odor Presentation Using Olfactory Display

This paper describes some fluid dynamic considerations for attaining realistic odor presentation using an olfactory display. Molecular diffusion is an extremely slow process and odor molecules released from their source are spread by being carried off by airflow. Therefore, we propose to use a computational fluid dynamics (CFD) simulation in conjunction with the olfactory display. The CFD solver is employed to calculate the turbulent airflow field in the given environment and the dispersal of odor molecules from their source. The simulation result is used to reproduce at the nose the realistic change in the odor concentration with time and space. However, our initial sensory test for evaluating the proposed method was not completely successful, and we also found some discrepancies between our real-life olfactory sensation and the experience of the CFD-based olfactory display. Here we report some insights to overcome these problems. In the initial sensory test, a nontrivial portion of the subjects did not properly recognize the spatial variation in the odor intensity. The result of our recent sensory test is presented in this paper to show that better contrast in the perceived odor intensity can be provided when the concentration range of the released odor is adjusted for the variation in the olfactory sensitivity of individual subjects. We noted that olfactory adaptation occurred more quickly in the initial sensory test of the CFD-based olfactory display than in real environments. In this paper, we show that olfactory adaptation can be alleviated by modulating the odor concentration randomly to mimic the random fluctuations of the turbulent flow fields in real environments. We also noted in our initial sensory test that there were sometimes discrepancies between our olfactory sensation in real environments and the simulated odor distribution. We show in this paper that the discrepancy can be attributed to the convection caused by the human body temperature that brings an odor vapor that is drifting around our feet up to our noses.

Numerical model of heat transfer in the human eye with consideration of fluid dynamics of the aqueous humour

In this work we present a new 3D numerical model for heat transfer in the human eye, which takes into account the aqueous humour flow in the anterior chamber. We show that consideration of this phenomenon in the calculations alters the temperature distribution on the corneal and lens surfaces, without, however, noticeably changing their absolute values. The most notable effect is that the coolest area of the cornea moves at a point of 2 mm inferior to its geometric centre. The maximum velocity of the fluid in the anterior chamber was found to be 3.36 × 10−4 m s−1. The effect of the flow on displacing the cool area of the corneal surface temperature is counterbalanced by assuming anisotropic thermal conductivity. The model was implemented in the case of an artificial intraocular lens to show the resulting temperature variations.

A low shear stress modular bioreactor for connected cell culture under high flow rates

A generic "system on a plate" modular multicompartmental bioreactor array which enables microwell protocols to be transferred directly to the bioreactor modules, without redesign of cell culture experiments or protocols is described. The modular bioreactors are simple to assemble and use and can be easily compared with standard controls since cell numbers and medium volumes are quite similar. Starting from fluid dynamic and mass transport considerations, a modular bioreactor chamber was first modeled and then fabricated using "milli-molding," a technique adapted from soft lithography. After confirming that the shear stress was extremely low in the system in the range of useful flow rates, the bioreactor chambers were tested using hepatocytes. The results show that the bioreactor chambers can increase or maintain cell viability and function when the flow rates are below 500 microL/min, corresponding to wall shear stresses of 10(-5) Pa or less at the cell culture surface.

Circulation over a submarine canyon in the NW Mediterranean

The circulation in Blanes canyon, an interruption in the NW Mediterranean continental shelf north of Barcelona, was investigated. The study employs data from oceanographic surveys carried out in the summer and fall of 2003. Velocity data show that in the vicinity of the shelf break the flow is deflected along the canyon walls. A cyclonic mean flow can be seen over the canyon mouth owing to vortex stretching of fluid parcels advected across the shelf break. Field observations are in qualitative agreement with fundamental fluid dynamic considerations based on potential vorticity conservation and friction effects at lateral boundaries. Evidence is given that upwelling is found near the shelf break inside the canyon in the two field experiments. This upwelling extends vertically from the seasonal thermocline (at about 100 m) to the shelf‐slope front (at about 200 m). There is no evidence that upwelled water can reach the continental shelf.

Electroosmotic Flow-Based Pump for Liquid Chromatography on a Planar Microchip

An electroosmotic flow (EOF)-based pump, integrated with a sol-gel stationary phase located in the electric field-free region of a microchip, enabled the separation of six nitroaromatic and nitramine explosives and their degradation products via liquid chromatography (LC). The integrated pump and LC system were fabricated within a single quartz substrate. The pump region consisted of a straight channel (3.0 cm x 230 microm x 100 microm) packed with 5-microm porous silica beads. The sol-gel stationary phase was derived from a precursor mixture of methyltrimethoxy- and phenethyltrimethoxysilanes and was synthesized in the downstream, field-free region of the microchip, resulting in a stationary-phase monolith with dimensions of 2.6 cm x 230 microm x 100 microm. Fluid dynamic design considerations are discussed, especially as they relate to integrating the EOF pump with the LC system. Pump and separation performance, as characterized by flow rate measurements, injection, elution, separation, and detection, point to a viable analytical chemistry platform that encompasses all of the benefits expected of portable, laboratory-on-chip systems, including reduced sample requirements and small packaging.

High pressure electroosmotic pump based on a packed bed planar microchip

Abstract A high pressure electroosmotic flow (EOF) pump was fabricated within a glass substrate and tested to characterize its fluid dynamic performance. The EOF pump was constructed on a compact, planar microchip platform with (L × W × D) dimensions of 10 cm × 3.6 cm × 0.3 cm. The pumping region itself consisted of a straight channel (3 cm × 160 μm × 62 μm) uniformly packed with 3.38 μm silica microspheres held in place by a microfabricated weir structure. Characterization of the microchip EOF pump included pressure and flow rate measurements as a function of voltage and system backpressure for a buffered aqueous fluid containing 10 mM cyclohexylamino alkyl sulfonate CHES. Measurements indicate a maximum pressure of 25 atm and a maximum measured flow rate of 85 nL/min. Linear relationships of pressure and flow rate with voltage were confirmed. Current measurements show linear profiles with voltage for flow rate and maximum pressure measurements, indicating minimal effects due to Joule heating under either test condition. Design and performance considerations relating to fluid dynamic considerations are discussed. The large pressures generated electrokinetically by the microchip EOF pump show the potential for a compact motive source that allows for easy integration with applications requiring high pressures.

Model problems for the tear film in a blink cycle: single-equation models

We consider model problems for the tear film over multiple blink cycles in limits that yield a single equation for the tear film; the single nonlinear partial differential equation that governs the film thickness arises from lubrication theory. The two models arise from considering the absence of naturally occurring surfactant and the case when the surfactant strongly affects the surface tension. The film is considered on a sinusoidally varying domain length with specified film thickness and volume flux at each end; only one end of the domain is moving, which is analogous to the upper eyelid moving with each blink. A main contribution of this article is computation of solutions for multiple complete blink cycles; the results of these non-trivial computations show a distinct similarity to quantitative in vivo observations of the tear film under partial blink conditions. A transition between periodic and non-periodic solutions has been estimated and this may be a criterion for what is effectively a full blink according to fluid dynamic considerations.

Fluid Dynamic Considerations on Internal Polishing for Tubes Utilizing Magnetic Responsive Functional Fluid

Fluid Dynamic Considerations on Internal Polishing for Tubes Utilizing Magnetic Responsive Functional Fluid

Numerical modeling of micro single-chamber ceria-based SOFC

Performance of intermediate temperature SOFC system of micro scale single-chamber with composite electrodes alternately placed on the ceria-based electrolyte has been successfully evaluated by numerical modeling. The polarization curve was obtained through a series of calculations for ohmic loss, activation loss and concentration loss. The calculation of each loss was based on the consideration of fluid dynamics, convection of fuel-air gas mixture, diffusion and oxygen ion conduction. The performance characteristics of SOFC were quantitatively investigated for various structural parameters such as distance between electrodes and thickness of electrolyte and electrode. The effect of the inflow direction of fuel-air mixture on the performance of the fuel cell was analyzed.

Premixed swirl combustion modes emerging for a burner tube with converging entrance

The fluid dynamics underlying swirl combustion was experimentally investigated by observing the subsequent behavior of a conical premixed flame which was formed at the nozzle exit when the air flow rate was increased stepwise for each fixed fuel flow rate. In the experiment, the swirling flow of a methane–air mixture was produced by a vane swirler, resulting in a nearly unit swirl number at the nozzle exit for any net gas flow rate. Another characteristic feature of this experiment is that the nozzle with a diameter of 21.5 mm and a length of 100 mm was connected by a tapered connector to a 38 mm diameter duct into which the swirling flow flowed. Hence, the converging connector produces a large axial velocity peak near the nozzle entrance by the contraction of the vortex core, and creates a barrier for flame flashback. Thus, together with the occurrence of turbulent transition, the presence of the converging connector results in a variety of premixed flame behaviors depending on the nozzle flow velocity, stoichiometric ratio and turbulence intensity. The stable combustion mode was either a lifted flame or flame holding in the nozzle tube. Their mode map was experimentally determined on the plots of nozzle flow velocity as a function of fuel flow rate. The flame behaviors in seven distinguished regimes were characterized on the basis of fluid dynamics considerations and supplemental cold flow experiment results. The results are useful for the development of flame control techniques.

The effect of support morphology on the reaction of oxidative dehydrogenation of ethane to ethylene at short contact times

The oxidative dehydrogenation of ethane is carried out in short contact time reactors over Pt and LaMnO 3 based catalysts supported on a large number of different ceramic substrates (45, 60 and 80 ppi foam monoliths and 200, 400, 600, 900 and 1200 cpsi honeycomb monoliths). Experimental results, obtained under the same conditions at varying the C 2H 6/O 2 ratio, showed that the highest performance in terms of ethylene selectivity and yield is always attained on LaMnO 3 catalysts. Furthermore, the results are significantly influenced by the morphology and cell density of the support, with 45 and 60 ppi foams and 400 and 600 cpsi honeycombs giving the best performance. The experimental results are explained by means of geometrical and fluid dynamic considerations on the support, and by means of a 2D mathematical model, which clearly indicates an optimal intermediate cell density for maximising ethylene selectivity and yield.

Differential susceptibility of horizontal and vertical swimming activity to cadmium exposure in a gammaridean amphipod ( Gammarus lawrencianus)

In this study two indices of swimming behavior (horizontal and vertical swimming activity) in a gammaridean amphipod ( Gammarus lawrencianus) were examined for their sensitivity to Cd exposure. G. lawrencianus were exposed for 72 h to a variety of Cd concentrations [background (∼12), 62, 125, 250 and 500 μg l −1] at 20 ppt. Subsequent to exposure, video surveillance of survivors held within grooved rings or clear boxes was used to assess horizontal swimming activity (percentage of time mobile) and vertical swimming activity (number of surfacings), respectively. Results show that control amphipods were quite active, being mobile ∼61% of the time, with horizontal swimming activity decreasing ( P < 0.01) to ∼0.3% between a Cd exposure concentration of 125 and 500 μg l −1. Vertical swimming activity in amphipods was also impacted by Cd exposure ( P < 0.001), with the greatest decrease occurring between background (∼12 μg l −1) and 62 μg l −1 (60 versus ∼26 surfacings, respectively), which is approximately four-fold lower than the estimated 72 h LC 50 (250 μg l −1) for G. lawrencianus. Based on fluid dynamic considerations, it is speculated that of the two behaviors, vertical swimming activity is more sensitive to Cd exposure because of the presumed greater energetic costs associated with producing enough thrust to attain the lift required to make a vertical ascent into the water.

Box modeling the chemical evolution of geophysical systems: Case study of the Earth's mantle

Geochemical measurements have been widely used for understanding geophysical dynamic systems. Fluid mechanics and box models are quantitative tools for testing the reliability of these interpretations. We present here the connection between these two methods, especially in the case of the Earth's mantle. We show that box models implicitly assume a chemical diffusivity inversely proportional to the number of boxes. From fluid dynamics considerations we suggest that at least 15 boxes should be used to model the mantle. Then, we compare the results of a simple convective geochemical model with box models to illustrate a way to incorporate dynamical constraints in them.

Fluid dynamical considerations on heat exchanger networks

The synthesis and analysis of heat exchanger networks are issues of great industrial interest due to the possibilities of decreasing plant costs, through the reduction of the utilities consumption and/or the number of equipments, in a grassroot design or retrofitting an existent network. The present paper explores a new design algorithm based on the Total Annual Cost (TAC) optimization for a thermal equipment, with mean tubeside and shellside flow velocities constraints, studying also the influence of pumping cost in the network’s final cost.

Mathematical modelling tools for the optimisation of direct smelting processes

Direct smelting operations involve the strong interaction of a wide range of complex physico-chemical processes. Moreover, for such processes to be efficient, these interactions have to be optimised to yield the desired set of chemical reactions and exchanges of heat and mass amongst the variety of gaseous, liquid and solid phases. This paper focuses upon a consideration of Computational Fluid Dynamics (CFD) based models developed to represent the HIsmelt ® direct smelting process. The models are extremely sophisticated and at every stage have challenged the limits of CFD technology, as well as the adequacy of constitutive sub-models to represent the chemical reaction/combustion phenomena. From an unprecedented synergy amongst process metallurgists, experimental scientists and CFD modellers, sophisticated, comprehensive and well-validated models of the process have evolved. The paper highlights some of the key state-of-the-art CFD techniques developed for the models and the role of specially designed experiments in parameter estimation and validation against plant measurements.

Emplacement of Hebridean Tertiary flood basalts: evidence from an inflated pahoehoe lava flow on Mull, Scotland

The lower 200 m of the Tertiary lava pile on Mull, western Scotland, consists mainly of small volume (0.01–1 km 3 ), high-magnesian basaltic lava flows. Eruption of these flows probably occurred from fissures or point-source vents, producing pahoehoe-textured sheets averaging about 5 m in thickness. Certain lavas cropping out in northwest Mull greatly exceed this average thickness and may represent inflated pahoehoe flows akin to those described at Kilauea, Hawai’i. Structural features of a 16–30 m thick lava at Port Haunn, northwest Mull, that conform to this proposal include sub-horizontal sheets of amygdales up to 14 cm thick separated by amygdale-poor zones, pipe amygdales at the base of the flow and a low-relief flow top. Alternating olivine-rich and olivine-poor bands in the Port Haunn lava are suggestive of lava pulses during a continuous eruption, also consistent with inflation. Elsewhere in western Mull, there is evidence for filled lava tubes and pahoehoe toes, both of which are characteristic of inflated sheet flows formed on gentle ground slopes (&lt;4° and perhaps &lt;0.5°). Fluid dynamical considerations suggest that each sheet flow in the lower part of the Mull lava sequence probably was active for a short period, permitting the build up of small lava fields over several months or years. This is commensurate with the small volume and limited areal extent of the Tertiary Mull lavas when compared to basalts in igneous provinces such as the Columbia River and Deccan.

97/01602 Axial flow reversal and its significance in airsparged hydrocyclone (ASH) flotation

In recent years the potential of air-sparged hydrocyclone (ASH) flotation for fine coal cleaning has been demonstrated both in pilot plant testing and in a plant-site demonstration program. Further improvements in the ASH technology will depend, to some extent, on improved understanding of the complex multiphase fluid flow. Froth transport plays a very important role in determining the efficiency of fine coal cleaning by ASH flotation. It should be noted that the surface of zero axial velocity is of particular significance in froth transport because the location of this surface actually accounts for the amount of froth being transported to the overflow. In this regard, the axial flow reversal has been examined based on specially designed tracer experiments. On the basis of these experimental results, modeling efforts were made to characterize the flow pattern in the ASH. The theoretical predictions based on turbulent fluid dynamic considerations were found to describe experimental observations regarding the surface of zero axial velocity. These results that define the surface of zero axial velocity together with froth phase features established from X-ray CT measurements provide an excellent description of the flow characteristics in ASH flotation and explain the effect of various process variables, such asmore » dimensionless area (A*), dimensionless flowrate (Q*), inlet pressure, percent solids, etc., on flotation recovery. On this basis it is expected that further advances in the design and operation of the ASH system can be made, leading to more efficient use of the ASH technology for fine coal cleaning.« less

Some design considerations for spiral separators

There are a number of design considerations which influence the selection of a suitable pitch and trough shape for a new spiral separator. The separation requirements impose certain constraints on the total design envelope and mutual interaction between some of the requirements and the conflict between metallurgy and materials handling requirements often defines a narrow range of permissible geometries. Capacity requirements determine the diameter of a spiral trough but fundamental fluid dynamic considerations play a major role in the selection of pitch and trough shape and the paper considers some of the issues in detail. Where the dynamics of the separation lead to excessive accumulation of solids at high slurry densities, various types of flow deflectors have been adopted either to restore fluidity to the material or to assist in securing a better quality of product. Apart from the trough, the design of suitable ancillaries such as feed boxes, auxiliary splitters and product collection facilities can exert a significant influence on both the operation of the spiral and the quality of separation achieved. These features are discussed in the paper and illustrated with examples.