Home About us Contact | |||

## Velocity Profile (velocity + profile)
## Selected Abstracts## Influence of the Orifice Inlet Angle on the Velocity Profile Across a Flow Convergence Region by Color Doppler In Vitro ECHOCARDIOGRAPHY, Issue 5 2000Martin Giesler M.D.The converging flow field proximal to a leaking valve is determined among other things by the orifice inlet angle formed by the leaflets. Thus, the inlet angle affects the determination of regurgitant flow rate by the flow convergence method. Based on the hypothesis of spheric isovelocity surfaces, others had postulated that a local velocity within the flow convergence should change inversely proportional to changes in the three-dimensional inlet angle. This concept would allow correction of the determination of regurgitant flow for nonplanar orifice inlet angles. We tested this concept in vitro. In a flow model, the flow convergence region proximal to different orifice plates was imaged by color Doppler: funnel-shaped, planar and tip-shaped (inverted funnels) orifice plates, with circular orifices of 2- and 7-mm diameter. Velocity profiles across the flow convergence along the flow centerline were read from the color maps. As predicted, the local velocities were inversely related to the inlet angle, but only at the 2-mm funnel orifices, this effect was inversely proportional to the three-dimensional inlet angle (i.e., in agreement with the mentioned concept). However, for any 7-mm orifice and/or inlet angle of > 180°, the effect of the inlet angle was considerably less than predicted by the aforementioned concept. With increasing orifice diameter and with decreasing distance to the orifice, the effect of the orifice inlet angle was reduced. The effect of the orifice inlet angle on the flow convergence region is modulated by orifice size and the distance to the orifice. Therefore, correction of flow estimates in proportion to the three-dimensional inlet angle will lead to considerable errors in most situations of clinical relevance, namely to massive overcorrection when analyzing velocities located close to wide orifices. [source] ## In Vitro Validation of a New Approach for Quantitating Regurgitations Using Proximal Isovelocity Surface Area ECHOCARDIOGRAPHY, Issue 7 2000A. DeloucheThe present work has been designed to validate the calculation of the effective regurgitant orifice (ERO) area with the use of a new formula that takes into account the velocity profile (Vr vs r) and that is insensitive to errors in the determination of the position of the orifice. Assuming a hemispheric model, ERO = 2,r2· Vr/Vo (with Vo= velocity at the orifice) and (Vo/Vr)0.5= (2,/ERO)0.5r. Thus, the slope of the corresponding linear regression allows ERO to be calculated as: ERO = 2,/slope2. This approach was tested in vitro in pulsatile conditions on circular, conical, and slit-like orifices. The calculated ERO was compared with the actual jet cross sectional area derived from the transverse velocity profile at the jet origin. For the purpose of comparison, the "classical" ERO was calculated for all the configurations, angulations, and threshold velocities. The relationship between (Vo/Vr)0.5 was linear (r > 0.98) over a wide range of velocities. The nonhemispheric components were found to modify the constant and not the slope. The mean variation of the calculated ERO was 6.5%. The correlation between the calculated and the actual ERO was very close (>0.97) with slope equal to 0.96. By comparison with the new method, the classical formula gave an underestimation of the ERO that dramatically increased when studying the flow closer to the orifice or in the case of error on the measurement of r. In conclusion, a method using velocity profiles instead of isolated values improves the accuracy of the proximal isovelocity surface area (PISA) method for measuring the ERO. [source] ## New insight into suction and dilution effects in CE coupled to MS via an ESI interface. ELECTROPHORESIS, Issue 10 2009Dilution effectAbstract The hyphenation of CE with MS is nowadays accepted as a powerful analytical approach. As far as ESI, the most common interface, is concerned, one challenge is to provide the most sensitive as well as quantitative information, which is quite a difficult task, as it is linked, among other factors, to suction and dilution effects. In the coaxial ESI configuration, it has been previously demonstrated that suction effect depends on many parameters inherent to the ESI interface geometry, the prevailing ones being the CE capillary protrusion from the interface needle, the sheath liquid (SL) and the overall BGE flow rates and velocity profile. In this paper, dilution effect is studied, as the CE electrolyte is mixed with SL at the interface. Considering peak intensity and efficiency, this effect was studied as a function of the various parameters of the interface (capillary protrusion from the SL tube, nebulizing gas, SL and CE electrolyte flow rates) or of the source (skimmer and ESI voltages, drying gas flow rate and temperature). It appears that the dilution effect seems slightly lower than what can be anticipated from the proportions of the liquid flow rates. This study also indicates that suction effect has to be considered first to better understand the dilution phenomenon, as suction effect leads to an increase in peak intensity, before a dilution effect appears. [source] ## A new type of migrating zone boundary in electrophoresis: 1. ELECTROPHORESIS, Issue 2 2005General description of boundary behavior based on electromigration dispersion velocity profilesAbstract Till now two principal types of electrophoretic boundaries have been recognized: a steady-state one showing self-sharpening properties and an unsteady-state one showing electromigration dispersion (EMD). In this contribution, the existence of a third (hybrid) type of electrophoretic boundary is revealed, that shows simultaneously: (i) steady-state character with self-sharpening properties in a certain part of the boundary, demarcated by a certain range of its composition; (ii) unsteady-state character with EMD in the resting part. A new theoretical approach to the study of the structure and properties of electrophoretic boundaries is presented, based on EMD velocity profiles representing the dependence of the EMD velocity on the composition in any point of the transition region across the boundary. According to this approach, the linearity or nonlinearity of the EMD velocity profile is recommended as the criterion for distinguishing the actual type of the boundary in question. It is shown that the new type of electrophoretic boundary is not an exotic case but may be observed even in simple systems as, e.g., formed by adjacent zones containing mixtures of current species such as picrate and acetate. Computer simulations are presented which confirm the theoretical conclusions and illustrate the three types of electrophoretic boundaries. [source] ## Numerical Modelling of Flow Boiling Heat Transfer in Horizontal Metal-Foam Tubes, ADVANCED ENGINEERING MATERIALS, Issue 10 2009Wei LuAbstract The flow boiling heat transfer performance in horizontal metal-foam tubes is numerically investigated based on the flow pattern map retrieved from experimental investigations. The flow pattern and velocity profile are generally governed by vapour quality and mass flow rate of the fluid. The porous media non-equilibrium heat transfer model is employed for modelling both vapour and liquid phase zones. The modelling predictions have been compared with experimental results. The effects of metal-foam morphological parameters, heat flux and mass flux on heat transfer have been examined. The numerical predictions show that the overall heat transfer coefficient of the metal-foam filled tube increases with the relative density (1-porosity), pore density (ppi), mass and heat flux. [source] ## Temperature effects on the mass flow rate in the SBI and similar heat-release rate test equipment FIRE AND MATERIALS, Issue 1 2007Bart J. G. SetteAbstract In various medium-to-large-scale fire test equipments like the ISO room corner test (RC), and more recently, the single burning item test (SBI) the mass flow rate measurement of the combustion gases plays a key role in the determination of the heat-release rate and smoke-production rate. With the knowledge of the velocity profile and the temperature of the flow, the mass flow rate is obtained by measuring the velocity on the axis of the duct. This is done by means of a bi-directional probe based on the pitot principle. However, due to the variation of the mean temperature and the temperature gradient in any cross section of the duct, introduced by ever changing combustion gas temperatures, the velocity nor the density profile are constant in time. This paper examines the resulting uncertainty on the mass flow rate. Copyright © 2006 John Wiley & Sons, Ltd. [source] ## Earth's magnetic field: ocean current contributions to vertical profiles in deep oceans GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2001F. E. M. (Ted) LilleySUMMARY The Earth's main magnetic field, arising in the core, should, in the ocean, have a well-defined uniform gradient with depth. Superimposed upon this uniform gradient may be magnetic signals due to a variety of sources. These include crustal magnetization, the transient fluctuations arising external to the Earth and causing secondary induced fields within it; and, the focus of the present paper, magnetic signals arising from the motional induction of seawater moving in the steady main magnetic field of Earth. There are circumstances where theory predicts such motionally-induced magnetic fields to be of order 102 nT, and to vary with depth in a way which is directly related to the velocity profile. Exploratory soundings of the magnetic field with depth have been made in the oceans around Australia, both to test these predictions, and to investigate the practicability of measuring such profiles. The magnetic field parameter observed has been that of the ,total field', which should sense the component of the ocean velocity which lies in the magnetic meridian. The magnetometer has been lowered by cable from a ship and also operated free-fall to the seafloor (and return). The observations appear both to confirm the theoretical gradient of the main field where there is no ocean current and, where ocean currents exist, to give evidence of their profiles resolved in the direction of magnetic north. In particular, observations taken in an eddy of the East Australian Current show the correct contrast in sign for north and south flowing streams. [source] ## Addressing non-uniqueness in linearized multichannel surface wave inversion GEOPHYSICAL PROSPECTING, Issue 1 2009Michele CercatoABSTRACT The multichannel analysis of the surface waves method is based on the inversion of observed Rayleigh-wave phase-velocity dispersion curves to estimate the shear-wave velocity profile of the site under investigation. This inverse problem is nonlinear and it is often solved using ,local' or linearized inversion strategies. Among linearized inversion algorithms, least-squares methods are widely used in research and prevailing in commercial software; the main drawback of this class of methods is their limited capability to explore the model parameter space. The possibility for the estimated solution to be trapped in local minima of the objective function strongly depends on the degree of nonuniqueness of the problem, which can be reduced by an adequate model parameterization and/or imposing constraints on the solution. In this article, a linearized algorithm based on inequality constraints is introduced for the inversion of observed dispersion curves; this provides a flexible way to insert a priori information as well as physical constraints into the inversion process. As linearized inversion methods are strongly dependent on the choice of the initial model and on the accuracy of partial derivative calculations, these factors are carefully reviewed. Attention is also focused on the appraisal of the inverted solution, using resolution analysis and uncertainty estimation together with a posteriori effective-velocity modelling. Efficiency and stability of the proposed approach are demonstrated using both synthetic and real data; in the latter case, cross-hole S-wave velocity measurements are blind-compared with the results of the inversion process. [source] ## Fluid flow and heat transfer characteristics of cone orifice jet (effects of cone angle) HEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 4 2009Mizuki KitoAbstract The use of a jet from an orifice nozzle with a saddle-backed-shape velocity profile and a contracted flow at the nozzle exit may improve the heat transfer characteristics on an impingement plate because of its larger centerline velocity. However, it requires more power to operate than a common nozzle because of its higher flow resistance. We therefore initially considered the use of a cone orifice nozzle to obtain better heat transfer performance as well as to decrease the flow resistance. We examined the effects of the cone angle , on the cone orifice free jet flow and heat transfer characteristics of the impinging jet. We compared two nozzles: a pipe nozzle and a quadrant nozzle. The first one provides a velocity profile of a fully developed turbulent pipe flow, and the second has a uniform velocity profile at the nozzle exit. We observed a significant enhancement of the heat transfer characteristics of the cone orifice jets at Re=1.5×104. Using the cone orifice impinging jets enhanced the heat transfer rates as compared to the quadrant jet, even when the jets were supplied with the same operational power as the pipe jet. For instance, a maximum enhancement up to approximately 22% at r/do,0.5 is observed for ,=15°. In addition, an increase of approximately 7% is attained as compared to when the pipe jet was used. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20243 [source] ## Numerical analysis of turbulent flow separation in a rectangular duct with a sharp 180-degree turn by algebraic Reynolds stress model INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2008Hitoshi SugiyamaAbstract Turbulent flow in a rectangular duct with a sharp 180-degree turn is difficult to predict numerically because the flow behavior is influenced by several types of forces, including centrifugal force, pressure-driven force, and shear stress generated by anisotropic turbulence. In particular, this type of flow is characterized by a large-scale separated flow, and it is difficult to predict the reattachment point of a separated flow. Numerical analysis has been performed for a turbulent flow in a rectangular duct with a sharp 180-degree turn using the algebraic Reynolds stress model. A boundary-fitted coordinate system is introduced as a method for coordinate transformation to set the boundary conditions next to complicated shapes. The calculated results are compared with the experimental data, as measured by a laser-Doppler anemometer, in order to examine the validity of the proposed numerical method and turbulent model. In addition, the possibility of improving the wall function method in the separated flow region is examined by replacing the log-law velocity profile for a smooth wall with that for a rough wall. The analysis results indicated that the proposed algebraic Reynolds stress model can be used to reasonably predict the turbulent flow in a rectangular duct with a sharp 180-degree turn. In particular, the calculated reattachment point of a separated flow, which is difficult to predict in a turbulent flow, agrees well with the experimental results. In addition, the calculation results suggest that the wall function method using the log-law velocity profile for a rough wall over a separated flow region has some potential for improving the prediction accuracy. Copyright © 2007 John Wiley & Sons, Ltd. [source] ## Görtler vortices in Falkner,Skan flows with suction and blowing INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 3 2008O. John E. MatssonArticle first published online: 15 MAY 200Abstract In this paper, we use nonlinear calculations to study curved boundary-layer flows with pressure gradients and self-similar suction or blowing. For an accelerated outer flow, stabilization occurs in the linear region while the saturation amplitude of vortices is larger than for flows with a decelerating outer flow. The combined effects of boundary-layer suction and a favourable pressure gradient can give a significant stabilization of the flow. Streamwise vortices can be amplified on both concave and convex walls for decelerated Falkner,Skan flow with an overshoot in the velocity profile. The disturbance amplitude is generally lower far downstream compared with profiles without overshoot. Copyright © 2007 John Wiley & Sons, Ltd. [source] ## Empirical slip and viscosity model performance for microscale gas flow INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 11 2005Matthew J. McNenlyAbstract For the simple geometries of Couette and Poiseuille flows, the velocity profile maintains a similar shape from continuum to free molecular flow. Therefore, modifications to the fluid viscosity and slip boundary conditions can improve the continuum based Navier,Stokes solution in the non-continuum non-equilibrium regime. In this investigation, the optimal modifications are found by a linear least-squares fit of the Navier,Stokes solution to the non-equilibrium solution obtained using the direct simulation Monte Carlo (DSMC) method. Models are then constructed for the Knudsen number dependence of the viscosity correction and the slip model from a database of DSMC solutions for Couette and Poiseuille flows of argon and nitrogen gas, with Knudsen numbers ranging from 0.01 to 10. Finally, the accuracy of the models is measured for non-equilibrium cases both in and outside the DSMC database. Flows outside the database include: combined Couette and Poiseuille flow, partial wall accommodation, helium gas, and non-zero convective acceleration. The models reproduce the velocity profiles in the DSMC database within an L2 error norm of 3% for Couette flows and 7% for Poiseuille flows. However, the errors in the model predictions outside the database are up to five times larger. Copyright © 2005 John Wiley & Sons, Ltd. [source] ## The performance of natural draft dry cooling towers under crosswind: CFD study INTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 2 2004Rafat Al-WakedAbstract The thermal performance of a natural draft dry cooling tower (NDDCT) under a crosswind has been investigated using a general-purpose CFD code. A three-dimensional study using the standard k,, turbulence model to simulate airflow in and around an NDDCT has been conducted. A parametric study has been carried out to examine the effect of crosswind velocity profile and air dry-bulb temperature on the thermal performance of an NDDCT. Two approaches have been considered in this study to quantify the crosswind effect. Firstly, simulations have been conducted at the nominal conditions and crosswind effect has been represented by thermal effectiveness parameter. Secondly, the ejected heat from the NDDCT has been maintained at a constant value (285 MW) and the crosswind effect has been represented by the change in the cooling tower approach parameter. After quantifying the effect of the crosswind on the thermal performance, windbreak walls have been introduced as a means of reducing this effect. The results in this paper show the importance of considering the crosswind velocity profile. Moreover, the introduction of windbreak walls has indicated an improvement in reducing the thermal performance losses due to the crosswind. Copyright © 2004 John Wiley & Sons, Ltd. [source] ## Simplified solution of developing laminar forced flow between parallel plates INTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 5 2002Esmail M. A. MokheimerAbstract A simplified simulation for developing laminar forced flow in the entrance region between two parallel plates is presented. This simulation is based on an implicit finite difference numerical representation of a boundary layer model describing the flow in the entry region. This boundary layer model comprises the two conservation equations of mass and momentum. A non-iterative implicit numerical scheme is developed to convert the partial differential form of these governing equations into algebraic equations. The resultant algebraic equations have been solved simultaneously via a simplified simulation using spreadsheet programs as well as a Fortran code for the sake of comparison. The numerically obtained developing axial velocity profile at large distance downstream of the entrance shows excellent agreement with the available fully developed analytical profile. Comparison between the abilities of the spreadsheet simulation with other high-level programming languages is outlined. Copyright © 2002 John Wiley & Sons, Ltd. [source] ## Bio-mimetic trajectory generation using a neural time-base generator JOURNAL OF FIELD ROBOTICS (FORMERLY JOURNAL OF ROBOTIC SYSTEMS), Issue 11 2005Yoshiyuki TanakaThis paper presents a neural time-base generator (TBG) that can generate a family of neural control signals with a controllable finite duration and bell-shaped velocity profile. Then, a bio-mimetic trajectory generation method using the neural TBG model is explained. Using the proposed model, the generation ability of human-like trajectories is examined through comparisons between computer simulations and human arm trajectories during reaching movements according to the curvature of constrained trajectories. © 2005 Wiley Periodicals, Inc. [source] ## Optimal Velocity Planning of Wheeled Mobile Robots on Specific Paths in Static and Dynamic Environments JOURNAL OF FIELD ROBOTICS (FORMERLY JOURNAL OF ROBOTIC SYSTEMS), Issue 12 2003María PradoThis paper deals with optimal temporal-planning of wheeled mobile robots (WMRs) when navigating on predefined spatial paths. A method is proposed to generate a time-optimal velocity profile for any spatial path in static environments or when mobile obstacles are present. The method generates a feasible trajectory to be tracked by fully exploiting velocity, acceleration and deceleration boundaries of the WMR, and by ensuring the continuity of the velocity and acceleration functions. As an additional benefit for the tracking process the jerk is also bounded. The algorithm is not time consuming, since it mostly uses closed mathematical expressions, nonetheless iteration strategies are presented to solve specific situations. However, such situations are not expected to occur when the spatial paths are planned as smooth curves. The success of the algorithm was tested by experimental and simulation results on the WMR "RAM." © 2003 Wiley Periodicals, Inc. [source] ## Study on factors influencing stagnation point offset of turbulent opposed jets AICHE JOURNAL, Issue 10 2010Wei-Feng LiAbstract Turbulent opposed jets were experimentally studied by the hot-wire anemometer measurement, the smoke-wire flow visualization, and the CFD simulation at L = 1,20D (where L is the nozzle separation and D is the nozzle diameter) and Re > 4500. The instability pattern of turbulent opposed jets was identified by investigating the smoke-wire photos recorded by a high-speed camera. The factors affecting stagnation point offset, such as the bulk velocity, the velocity profile, and the turbulence intensity at the nozzle exits were investigated. Results show that the stagnation point offset is the main instability regime of turbulent opposed jets. Uniform exit velocity profile and increasing exit turbulence intensity will decrease the stagnation point offset of turbulent opposed jets. © 2010 American Institute of Chemical Engineers AIChE J, 2010 [source] ## Simulation of particle migration in free-surface flows AICHE JOURNAL, Issue 10 2010Kyung Hoon MinAbstract The migration of particles in free surface flows using the diffusive flux model was investigated. As the free-surface flows, a planar jet flow and a slot coating flow were chosen. The suspension was assumed to be a Newtonian fluid with a particle concentration dependent viscosity. The governing equations were solved numerically by the finite volume method, and the free-surface problem was handled by the volume of the fraction model. The result shows that even though the velocity profile is fully developed and becomes flat, the particle distribution never reaches the uniform distribution for both of the cases. It is also shown that the die swell of the free jet is reduced compared to the Newtonian fluid and when the particle loading is 0.5, die contraction is observed. The change in die swell characteristics is purely due to particle migration since the suspension model does not show normal stress differences. © 2010 American Institute of Chemical Engineers AIChE J, 2010 [source] ## Effect of Taylor vortices on mass transfer from a rotating cylinder AICHE JOURNAL, Issue 11 2005R. SrinivasanAbstract Mass transfer from solids, which has important applications in a number of chemical and pharmaceutical industries, has been studied experimentally and semiempirically under turbulent flow conditions, and correlations are available in the literature to calculate the mass-transfer coefficients from pellets, rotating cylinders and disks etc. However, mass transfer under laminar flow has not been sufficiently addressed. One of the difficulties here is the strong Reynolds number dependence of the flow pattern, for example, due to the onset of Taylor vortices for the case of a rotating cylinder. This problem is circumvented by using a computational fluid dynamics (CFD)-based solution of the governing equations for the case of a cylinder rotating inside a stationary cylindrical outer vessel filled with liquid. The parameters cover a range of Reynolds number (based on the cylinder diameter, and the tangential speed of the cylinder), Schmidt number and the ratio of the outer to inner cylinder diameters. The results confirm that the circumferential velocity profile is a strong function of the Reynolds number and varies from a nearly Couette-type flow at very low Reynolds numbers to a boundary layer-like profile at high Reynolds numbers. The onset of Taylor vortices has a strong effect on the flow field and the mass-transfer mode. The calculations show that the Sherwood number has a linear dependence on the Reynolds number in the Couette-flow regime, and roughly square-root dependence after the onset of Taylor vortices. Correlations have been proposed to calculate the Sherwood number taking account of these effects. © 2005 American Institute of Chemical Engineers AIChE J, 2005 [source] ## Masticatory jaw movement optimization after introduction of occlusal interference JOURNAL OF ORAL REHABILITATION, Issue 3 2010K. YASHIROSummary, How ,control' characteristics of masticatory jaw movement, such as skilfulness of the movement, change after alteration in occlusion remains uncertain. For each of 10 healthy adults with good occlusion, an occlusal interference with artificial ,tooth-cusp' was introduced to the crown of the upper molar tooth on the non-working side of unilateral chewing. Mandibular incisor-point movements were then recorded by a 3D tracking device. The introduction of the occlusal interference induced a remarkable increase in the normalized jerk-cost (NJC), prolonged duration of the decelerative phase and lowered peak velocity for jaw closing movement during chewing. Overall, the NJC and velocity profile showed significant recoveries during the course of about 90 repetitive chewing cycles performed under the altered occlusal condition. These findings suggest that acute adaptive changes of jaw motion after introduction of occlusal interference might be characterized as the recovery process of movement skilfulness in terms of movement smoothness and velocity profile. [source] ## Effect of filament drawdown on aerodynamic drag and heat transfer in fiber spinning AICHE JOURNAL, Issue 5 2004C. MillerAbstract The momentum and thermal boundary layer equations are solved numerically to assess the effects of filament drawdown on aerodynamic drag and heat transfer in melt fiber spinning. It is found that, relative to the case without drawdown (constant filament velocity and diameter), the aerodynamic drag on the filament increases substantially, but the heat transfer rate is suppressed. Moreover, the air velocity profile eventually becomes fully developed (unlike the zero drawdown case), and the (velocity) boundary layer thickness, rather than continuing to grow with distance beneath the spinneret, eventually becomes proportional to the filament diameter, and thus decreases with increasing axial distance. Quantitative results are presented for the dimensionless drag force per unit length along the filament (the Drag number) and the Nusselt number as functions of the dimensionless axial distance and a new dimensionless parameter, the Drawdown Reynolds number. © 2004 American Institute of Chemical Engineers AIChE J, 50: 898,905, 2004 [source] ## Characterization of downflowing high velocity fluidized beds AICHE JOURNAL, Issue 3 2000Chunshe CaoA downer-riser circulating high velocity fluidization apparatus was developed to study the fundamentals of downflowing gas-solid particle mixtures. The acceleration and deceleration of solids due to the influences of the entrance and exit sections result in a relatively uniform axial solids distribution. Radial solid density profiles detected with an X-ray imaging system in the downer show the existence of a core-annulus flow with a dilute core surrounded by a denser wall region. Local solids flux profiles were obtained with an aspirating probe device and the solid velocity profile obtained from the two measured quantities. These confirm that the majority of solids segregates in a wall region that flows faster than the dilute core region. Thus, the shorter residence time in the high-speed downer wall region is coupled with faster reaction rates due to the accompanying high concentration of catalyst, while the dilute core has slower reaction rates with longer residence time due to the lower catalyst concentration and flow velocity. This results in much more uniform reaction extent over the cross-sectional area of the downer and, therefore, should improve the product selectivity. [source] ## Phase-contrast velocimetry with hyperpolarized 3He for in vitro and in vivo characterization of airflow MAGNETIC RESONANCE IN MEDICINE, Issue 6 2006Ludovic de RochefortAbstract This paper describes a technique that combines radial MRI and phase contrast (PC) to map the velocities of hyperpolarized gases (3He) in respiratory airways. The method was evaluated on well known geometries (straight and U-shaped pipes) before it was applied in vivo. Dynamic 2D maps of the three velocity components were obtained from a 10-mm slice with an in-plane spatial resolution of 1.6 mm within 1 s. Integration of the in vitro through-plane velocity over the slice matched the input flow within a relative precision of 6.4%. As expected for the given Reynolds number, a parabolic velocity profile was obtained in the straight pipe. In the U-shaped pipe the three velocity components were measured and compared to a fluid-dynamics simulation so the precision was evaluated as fine as 0.025 m s,1. The technique also demonstrated its ability to visualize vortices and localize characteristic points, such as the maximum velocity and vortex-center positions. Finally, in vivo feasibility was demonstrated in the human trachea during inhalation. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc. [source] ## A novel all-fiber high velocity interferometer for any reflector MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 8 2007Guang-Wei HongAbstract A novel all-fiber high velocity interferometer system for any reflector (AFHVISAR) is proposed and demonstrated. The system mainly consists of two 3 × 3 SM fiber couplers. A quasi-direct current (QDC) signal directly from the moving surface is obtained and used to offset the QDC component in the interfering signals for more precise phase demodulation. The use of the diffuse surface solves the fringe loss problem induced by the nonparallel movements of the surface. The experiment with a one-stage light gas gun shows that the AFHVISAR can successfully achieve the surface velocity profile of LY12 aluminum specimen. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2032,2035, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22606 [source] ## A comprehensive set of simulations studying the influence of gas expulsion on star cluster evolution MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007H. BaumgardtABSTRACT We have carried out a large set of N -body simulations studying the effect of residual-gas expulsion on the survival rate, and final properties of star clusters. We have varied the star formation efficiency (SFE), gas expulsion time-scale and strength of the external tidal field, obtaining a three-dimensional grid of models which can be used to predict the evolution of individual star clusters or whole star cluster systems by interpolating between our runs. The complete data of these simulations are made available on the internet. Our simulations show that cluster sizes, bound mass fraction and velocity profile are strongly influenced by the details of the gas expulsion. Although star clusters can survive SFEs as low as 10 per cent if the tidal field is weak and the gas is removed only slowly, our simulations indicate that most star clusters are destroyed or suffer dramatic loss of stars during the gas removal phase. Surviving clusters have typically expanded by a factor of 3 or 4 due to gas removal, implying that star clusters formed more concentrated than as we see them today. Maximum expansion factors seen in our runs are around 10. If gas is removed on time-scales smaller than the initial crossing time, star clusters acquire strongly radially anisotropic velocity dispersions outside their half-mass radii. Observed velocity profiles of star clusters can therefore be used as a constraint on the physics of cluster formation. [source] ## Hiding cusps in cores: kinematics of disc galaxies in triaxial dark matter haloes MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2006Eric HayashiABSTRACT We study the kinematics of gaseous discs in triaxial dark matter haloes using the closed-loop orbit solutions in non-axisymmetric potentials. The orbits are in general non-circular and, for a given triaxiality, their ellipticity depends on the ratio of escape to circular velocities, V2esc/V2c. This ratio increases steeply towards the centre for cold dark matter (CDM) halo density profiles, implying that even minor deviations from spherical symmetry may induce large deviations from circular orbits in the velocity field of a gaseous disc, especially near the centre. This result suggests that caution should be exercised when interpreting constraints on the presence of density cusps in the dark halo derived from the innermost velocity profile. Simulated long-slit rotation curves vary greatly in shape, depending primarily on the viewing angle of the disc and on its orientation relative to the principal axes of the potential. ,Solid-body' rotation curves , typically interpreted as a signature of a constant density core in the dark matter distribution , are often obtained when the slit samples velocities near the major axis of the closed-loop orbits. Triaxial potentials imprint specific symmetries in 2D velocity fields, generally inducing ,twists' in the isovelocity contours and antisymmetric patterns in opposite quadrants. We suggest that triaxial haloes may be responsible for the variety of shapes of long-slit rotation curves of low surface brightness (LSB) galaxies, as well as for the complex central kinematics of LSBs, which are sometimes ascribed to the presence of ,radial motions' in the gas. We argue that LSB rotation curves might be reconciled with the structure of CDM haloes once the effects of halo triaxiality on the dynamics of gaseous discs are properly taken into account. [source] ## Effect of non-isothermal oriented crystallization on the velocity and elongational viscosity profiles during the melt spinning of high density polyethylene fibers POLYMER ENGINEERING & SCIENCE, Issue 7 2001A. MakradiBased on the experimental data of spine line temperature and percent crystallization, a time-integral constitutive equation has been used together with the degree of phase transformation theory to predict the velocity and elongational viscosity profiles. For the velocity profile, our predicted results are compared to experimental data and good agreement is found. Under a drawing force, the elongational viscosity profile shows a stress softening due to the molecular alignment; then the fiber hardens close to the take-up point, owing to filament crystallization. [source] ## Positron Imaging Studies of Rotating Drums THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2005David J. ParkerAbstract The potential of the radioisotope tracer technique of positron emission tomography (PET) and the related techniques of positron emission projection imaging (PEPI) and positron emission particle tracking (PEPT) is illustrated with reference to laboratory scale studies of particulate motion in rotating drums, operating either in batch or continuous flow modes. Sand grains, glass beads and TiO2 granules down to 0.5mm diameter were labelled. Using PEPT the transition between rolling and slumping modes has been identified and the velocity profile within the active layer has been determined for a range of drum diameters. PEPI has been used to measure and explain residence time distributions, while all three techniques have been used to study segregation based on particle size, both radially and axially within the drum. Data on particle motion within a novel baffled drum is also presented. Le potentiel de la technique de traçage par radio-isotopes en tomographie par émission de positrons (PET) et les techniques associées de l'imagerie par projection des émissions de positrons (PEPI) ou le traçage des particules par émission de positrons (PEPT), est illustré en référence à des études à l'échelle de laboratoire du déplacement de particules dans des tambours rotatifs, fonctionnant soit en mode d'écoulement discontinu ou continu. Des grains de sable, des billes de verre et des granules de TiO2 aussi petits que 0,5 mm de diamètre ont été marqués. À l'aide de la technique PEPT, la transition entre les modes roulant et glissant a été identifiée et le profil de vitesse à l'intérieur de la couche active a été déterminé pour une gamme de diamètres de tambours. La technique PEPI a été utilisée pour mesurer et expliquer les distributions de temps de séjour, tandis que les trois techniques ont servi pour l'analyse de la ségrégation d'après la taille des particules, à la fois radialement et axialement dans le tambour. Des données sur le déplacement des particules dans un nouveau tambour à chicanes sont également présentées. [source] ## Mechanism of Local Fluidization in Converging Packed Beds THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2003Baoquan ZhangAbstract The velocity profile on the bed surface of two-dimensional linear-converging beds with 15° and 30° wall angles was measured at different superficial velocities using hot-wire anemometry. The results of the velocity measurements indicate that local fluidization in the corners is caused by the geometry-induced maldistribution of fluid flow, and it occurs when the velocity in the corners exceeds the minimum fluidization velocity of particles. The results of pressure measurements within the bed show the same trend as the velocity profile, providing a qualitative verification of the velocity profile measurement. It is shown that the variation of measured pressure drop over the bed with velocity does not agree with the Ergun equation at high superficial velocities due to the severe maldistribution of fluid flow. Le profil de vitesse à la surface du lit de lits linéaires-convergents bidimensionnels avec des angles de paroi de 15 et 30∞ a été mesuré à différentes vitesses superficielles par anémométrie à fil chaud. Les résultats des mesures de vitesse indiquent que la fluidisation locale dans les coins provient de la mauvaise distribution causée par la géométrie de l'écoulement du fluide, et qu'elle survient lorsque la vitesse dans les coins excède la vitesse de fluidisation minimale des particules. Les résultats des mesures de pression dans le lit montrent la même tendance que le profil de vitesse, permettant ainsi une vérification qualitative de la mesure des profils de vitesse. On montre que la variation de la perte de charge mesurée dans le lit en fonction de la vitesse ne concorde pas avec l'équation d'Ergun à des vitesses superficielles élevées en raison de la très mauvaise distribution de l'écoulement du fluide. [source] ## Liquid Dispersion in Large Diameter Bubble Columns, with and without Internals THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3-4 2003Ann ForretAbstract Liquid mixing has been studied in a 1 m diameter bubble column, with and without internals (vertical cooling tubes). The presence of internals significantly affects both large scale recirculation and local dispersion. The most common approach to model liquid mixing is the one-dimensional axial dispersion model, validated many times in small bubble columns without internals. This paper shows that this model is still appropriate to large columns, but without internals. A two-dimensional model, taking into account a radially dependent axial velocity profile, and both axial and radial dispersion, is required to account for the internals on liquid mixing. Le mélange du liquide dans une colonne à bulles de 1 m de diamètre a été étudié, avec et sans internes (tubes verticaux simulant des échangeurs de chaleur). La présence d'internes affecte de manière significative à la fois la recirculation globale du liquide ainsi que la dispersion locale. L'approche la plus couramment employée pour modéliser le mélange du liquide est le modèle de dispersion axiale mono dimensionnel, validé maintes fois pour les petites colonnes à bulles sans internes. Cet article montre que ce modèle reste valable pour les colonnes de grande taille, sans internes. Par contre, la prise en compte des effets des internes sur le mélange liquide passe par l'utilisation d'un modèle bidimensionnel, prenant en compte le profile radiale de la vitesse axiale ainsi que les dispersions axiale et radiale. [source] |