Home About us Contact
|
Home About us Contact | ||
|
|
||
Pressure Difference (pressure + difference)
Selected AbstractsFast measurement of intracardiac pressure differences with 2D breath-hold phase-contrast MRI,MAGNETIC RESONANCE IN MEDICINE, Issue 6 2003Richard B. Thompson Abstract Intracardiovascular blood pressure differences can be derived from velocity images acquired with phase-contrast (PC) MRI by evaluating the Navier-Stokes equations. Pressure differences within a slice of interest can be calculated using only the in-plane velocity components from that slice. This rapid exam is proposed as an alternative to the lengthy 3D velocity imaging exams. Despite their good spatial coverage, the 3D exams are prone to artifacts and errors from respiratory motion and insufficient temporal resolution, and are unattractive in the clinical setting due to their excessive scan times (>10 min of free breathing). The proposed single-slice approach requires only one or two breath-holds of acquisition time, and the velocity data can be processed for the calculation of pressure differences online with immediate feedback. The impact of reducing the pressure difference calculation to two dimensions is quantified by comparison with 3D data sets for the case of blood flow within the cardiac chambers. The calculated pressure differences are validated using high-fidelity pressure transducers both in a pulsatile flow phantom and in vivo in a dog model. There was excellent agreement between the transducer and PC-MRI results in all of the studies. Magn Reson Med 49:1056,1066, 2003. Published 2003 Wiley-Liss, Inc. [source] Successful Transcatheter Closure of an Aorto-Left Atrial FistulaCONGENITAL HEART DISEASE, Issue 6 2007Malek M. El Yaman MD ABSTRACT Aorto-left atrial fistula is a rare entity in which the integrity of the aortic root bordering the left atrium is disrupted. The clinical presentation is highly variable, depending predominantly on the size of the fistula and the pressure difference between the aorta and the left atrium. Surgical repair was the standard treatment. Recently, however, there have been reports of successful transcatheter closure. We report a 32-year-old male with Shone's syndrome who had multiple prior surgical procedures including aortic and mitral valve replacements. He presented with an aorto-left atrial fistula that was successfully closed percutaneously using an Amplatzer atrial septal defect device. [source] Does limb angular motion raise limb arterial pressure?ACTA PHYSIOLOGICA, Issue 3 2009D. D. Sheriff Abstract Aim:, Mechanical factors such as the muscle pump have been proposed to augment flow by several mechanisms. The potential for limb angular motion to augment local perfusion pressure (pressure = ½,r2,2, where , is the fluid density, r the radius and , the angular velocity) has been overlooked. We sought to test the hypothesis that limb angular motion augments limb arterial pressure. Methods:, Nine human subjects performed horizontal shoulder flexion (,±90° at 0.75 Hz for 30 s). We measured finger arterial pressure (photoplethysmography) in the moving (Trial 1) and non-moving arm (Trial 2) in separate trials along with the pressure (strain gauge) generated at the fingers within a length of water-filled tubing mounted on the moving arm in both trials. Results:, Arm swinging raised (P < 0.05) the mean pressure measured in the tubing by 11 ± 2 and 14 ± 2 mmHg (Trials 1 and 2 respectively). In response to exercise, the rise in mean finger arterial pressure in the swinging limb (18 ± 3 mmHg, Trial 1) exceeded (P < 0.05) the rise in the resting limb (8 ± 2 mmHg, Trial 2) by an amount similar to the 11 mmHg rise in pressure generated in the tubing in Trial 1. Conclusions:, We conclude that the swinging of a limb creates centrifugal force (a biomechanical centrifuge) which imparts additional pressure to the arteries, but not the veins owing to the venous valves, which further widens the arterial,venous pressure difference. [source] 5-Hydroxytryptamine-induced microvascular pressure transients in lungs of anaesthetized rabbitsACTA PHYSIOLOGICA, Issue 2 2001N. Sen We determined lung microvascular pressure transients induced by 5-hydroxytryptamine (5HT), by the micropuncture technique. We mechanically ventilated anaesthetized (halothane 0.8%), open-chested rabbits, in which we recorded pulmonary artery (PA), left atrial (LA) and carotid artery pressures and cardiac output. For 4-min periods of stopped ventilation, we constantly inflated the lung with airway pressure of 7 cmH2O, then micropunctured the lung to determine pressures in arterioles and venules of 20,25 ,m diameter. An intravenous bolus infusion of 5HT (100 ,g), increased total pulmonary vascular resistance by 59%. Prior to 5HT infusion, the arterial, microvascular and venous segments comprised 30, 50 and 19% of the total pulmonary vascular pressure drop, respectively. However 14 s after 5HT infusion, the PA-arteriole pressure difference (arterial pressure drop) increased 46%, while the venule-LA pressure difference (venous pressure drop) increased >100%. The arteriole,venule pressure difference (microvascular pressure drop) was abolished. The increase in the arterial pressure drop was maintained for 4.8 min, whereas the increased venous pressure drop reverted to baseline in <1 min. We conclude that in the rabbit lung in situ, a 5HT bolus causes sustained arterial constriction and a strong but transient venous constriction. [source] Study on Crack-like Pores of Al Foams Made via the Powder-Metallurgy Route,ADVANCED ENGINEERING MATERIALS, Issue 1-2 2010Lei Wang Mechanisms for the formation and disappearance of the crack-like pores generated during the early stage of Al foaming are investigated. A model for their disappearance process is proposed for the first time. The stress, perpendicular to the compaction direction in uniaxial cold compaction, is caused by the interaction of the Al powder under a high compaction pressure and is the main reason for the formation of the crack-like pores. The results of the model analysis and theoretical calculations suggest that the pressure difference ,P between the initial, round bubbles and the crack-like pores is the driving force for their disappearance. The rapid reduction of ,P is attributed to the decomposition characteristics of the TiH2 powder. [source] Acute Hypervolaemia Improves Arterial Oxygen Pressure in Athletes with Exercise-Induced HypoxaemiaEXPERIMENTAL PHYSIOLOGY, Issue 4 2003Gerald S. Zavorsky The aim of this study was to determine the effect of acute plasma volume expansion on arterial blood-gas status during 6.5 min strenuous cycling exercise comparing six athletes with and six athletes without exercise-induced arterial hypoxaemia (EIAH). We hypothesized that plasma volume expansion could improve arterial oxygen pressure in a homogeneous sample of athletes - those with EIAH. In this paper we have extended the analysis and results of our recently published surprising findings that lengthening cardiopulmonary transit time did not improve arterial blood-gas status in a heterogeneous sample of endurance cyclists. One 500 ml bag of 10% Pentastarch (infusion condition) or 60 ml 0.9% saline (placebo) was infused prior to exercise in a randomized, double-blind fashion on two different days. Power output, cardiac output, oxygen consumption and arterial blood gases were measured during strenuous exercise. Cardiac output and oxygen consumption were not affected by acute hypervolaemia. There were group × condition interaction effects for arterial oxygen pressure and alveolar-arterial oxygen pressure difference, suggesting that those with hypoxaemia experienced improved arterial oxygen pressure (+4 mmHg) and lower alveolar-arterial oxygen pressure difference (-2 mmHg) with infusion. In conclusion, acute hypervolaemia improves blood-gas status in athletes with EIAH. The impairment of gas exchange occurs within the first minute of exercise, and is not impaired further throughout the remaining duration of exercise. This suggests that arterial oxygen pressure is only minimally mediated by cardiac output. [source] Experimental study of fire compartment with door opening and roof openingFIRE AND MATERIALS, Issue 5 2005E. H. Yii Abstract A series of reduced-scale experimental fires was conducted to study the characteristics of fire induced vent flows in a reduced-scale post-flashover fire compartment with a door opening and a roof opening. The fire source was a heptane pool fire near the wall furthest from the door vent. In the study, the roof vent opening area was systematically varied between experiments and the characteristics of vent flows through the door opening are presented as a function of the roof vent opening area. The experimental results show that the mass flow rate of air into the compartment increases linearly as the size of roof vent opening increases. Analytical vent flow calculations based on the hydrostatic pressure difference between two quiescent environments are presented for a post-flashover fire compartment with both horizontal and vertical openings. The calculated results are in good agreement with the experimental measurements. Copyright © 2005 John Wiley & Sons, Ltd. [source] Numerical simulation of flapping-wing insect hovering flight at unsteady flowINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2007Decan Zuo Abstract A computational fluid dynamics (CFD) analysis was conducted to study the unsteady aerodynamics of a virtual flying bumblebee during hovering flight. The integrated geometry of bumblebee was established to define the shape of a three-dimensional virtual bumblebee model with beating its wings, accurately mimicking the three-dimensional movements of wings during hovering flight. The kinematics data of wings documented from the measurement to the bumblebee in normal hovering flight aided by the high-speed video. The Navier,Stokes equations are solved numerically. The solution provides the flow and pressure fields, from which the aerodynamic forces and vorticity wake structure are obtained. Insights into the unsteady aerodynamic force generation process are gained from the force and flow-structure information. The CFD analysis has established an overall understanding of the viscous and unsteady flow around the virtual flying bumblebee and of the time course of instantaneous force production, which reveals that hovering flight is dominated by the unsteady aerodynamics of both the instantaneous dynamics and also the past history of the wing. A coherent leading-edge vortex with axial flow and the attached wingtip vortex and trailing edge vortex were detected. The leading edge vortex, wing tip vortex and trailing edge vortex, which caused by the pressure difference between the upper and the lower surface of wings. The axial flow, which include the spanwise flow and chordwise flow, is derived from the spanwise pressure gradient and chordwise pressure gradient, will stabilize the vortex and gives it a characteristic spiral conical shape. Copyright © 2006 John Wiley & Sons, Ltd. [source] HISTALP,historical instrumental climatological surface time series of the Greater Alpine RegionINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 1 2007Ingeborg Auer This paper describes the HISTALP database, consisting of monthly homogenised records of temperature, pressure, precipitation, sunshine and cloudiness for the ,Greater Alpine Region' (GAR, 4,19°E, 43,49°N, 0,3500m asl). The longest temperature and air pressure series extend back to 1760, precipitation to 1800, cloudiness to the 1840s and sunshine to the 1880s. A systematic QC procedure has been applied to the series and a high number of inhomogeneities (more than 2500) and outliers (more than 5000) have been detected and removed. The 557 HISTALP series are kept in different data modes: original and homogenised, gap-filled and outlier corrected station mode series, grid-1 series (anomaly fields at 1° × 1°, lat × long) and Coarse Resolution Subregional (CRS) mean series according to an EOF-based regionalisation. The leading climate variability features within the GAR are discussed through selected examples and a concluding linear trend analysis for 100, 50 and 25-year subperiods for the four horizontal and two altitudinal CRSs. Among the key findings of the trend analysis is the parallel centennial decrease/increase of both temperature and air pressure in the 19th/20th century. The 20th century increase (+1.2 °C/+ 1.1 hPa for annual GAR-means) evolved stepwise with a first peak near 1950 and the second increase (1.3 °C/0.6hPa per 25 years) starting in the 1970s. Centennial and decadal scale temperature trends were identical for all subregions. Air pressure, sunshine and cloudiness show significant differences between low versus high elevations. A long-term increase of the high-elevation series relative to the low-elevation series is given for sunshine and air pressure. Of special interest is the exceptional high correlation near 0.9 between the series on mean temperature and air pressure difference (high-minus low-elevation). This, further developed via some atmospheric statics and thermodynamics, allows the creation of ,barometric temperature series' without use of the measures of temperature. They support the measured temperature trends in the region. Precipitation shows the most significant regional and seasonal differences with, e.g., remarkable opposite 20th century evolution for NW (9% increase) versus SE (9% decrease). Other long- and short-term features are discussed and indicate the promising potential of the new database for further analyses and applications. Copyright © 2006 Royal Meteorological Society. [source] Wavelet analysis of the Lisbon and Gibraltar North Atlantic Oscillation winter indicesINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 5 2006S. Barbosa Abstract The North Atlantic Oscillation (NAO) is one of the most important climatic patterns in the Northern Hemisphere. Indices based on the normalised pressure difference between Iceland and a southern station, such as Lisbon or Gibraltar, have been defined in order to describe NAO temporal evolution. Although exhibiting interannual and decadal variability, the signals are statistically rather featureless and therefore it is difficult to discriminate between different types of stochastic models. In this study, Lisbon and Gibraltar NAO winter indices are analysed using the discrete wavelet transform discrete wavelet transform(DWT). A multi-resolution analysis (MRA) is carried out for a scale-based description of the indices and the wavelet spectrum is used to identify and estimate long-range dependence. The degree of association of the two NAO indices is assessed by estimating the wavelet covariance for the two signals. The scale-based approach inherent to the discrete wavelet methodology allows a scale-by-scale comparison of the signals and shows that although the short-term temporal pattern is very similar for both indices, the long-term temporal structure is distinct. Furthermore, the degree of persistence or ,memory' is also distinct: the Lisbon index is best described by a long-range dependent (LRD) process, while the Gibraltar index is adequately described by a short-range process. Therefore, while trend features in the Lisbon NAO index may be explainable by long-range dependence alone, with no need to invoke external factors, for the Gibraltar index such features cannot be interpreted as resulting only from internal variability through long-range dependence. Copyright © 2006 Royal Meteorological Society. [source] Instrumental pressure observations and atmospheric circulation from the 17th and 18th centuries: London and ParisINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2001V.C. Slonosky Abstract Daily pressure observations recorded by William Derham (1657,1735) at Upminster, Essex (near London), from 1697 to 1706 and 1708 have been corrected, converted to modern units and the Gregorian calendar, and adjusted for homogeneity. These pressure readings have been compared with previously published contemporary observations from Paris, and the two sets of early instrumental data used to calculate a daily series of the pressure difference between Paris and London. Frequency analysis of the daily series reveals that reversals of the south,north pressure gradient and easterly winds were more common from 1697 to 1708 than during the 1990s. Monthly mean values of Paris,London pressure differences have been compared with previously published monthly mean reconstructed surface pressure maps and to a reconstructed North Atlantic Oscillation (NAO) index. There is a good agreement between the strength and direction of monthly mean flow between London and Paris estimated from the circulation maps and the sign and magnitude of the Paris,London westerly flow index, but the correlation between the Paris,London index, known to be a good proxy for European zonal circulation, and the reconstructed NAO index, is low (0.2). Correlations between the monthly mean Paris,London zonal circulation index and central England temperatures suggest a strong relationship during winter and late summer from 1697 to 1708. The meticulous daily instrumental observations and the monthly and seasonal climate descriptions of Derham, his collection of instrumental observations and climatic descriptions from contemporary observers throughout Europe, and his early theories on the causes of climate change make his publications a valuable source of information for studies on climate during the early instrumental period. It is hoped that more of Derham's papers related to weather and climate may eventually come to light. Copyright © 2001 Royal Meteorological Society. [source] Mathematical modelling of hydrogen storage in a LaNi5 hydride bedINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 11 2003Yuksel Kaplan Abstract This paper presents a numerical investigation of hydrogen storage in a metal hydrid bed. For this purpose, a two-dimensional mathematical model which considers complex heat and mass transfer and fluid flow during the hydriding process is accomplished in this study. The coupled differential equations are solved with numerical method based on integrations of governing equation over finite control volumes. The driving force for fluid flow is considered to be pressure difference due to the temperature distribution in the system. It is found that fluid flow enhances the local hydriding rate in the system by driving the hot fluid to the colder regions. The numerical results were found to agree satisfactorily with the experimental data available in the literature. Copyright © 2003 John Wiley & Sons, Ltd. [source] Monitoring struturants of fat blends with ultrasound based in-line rheometry (ultrasonic velocity profiling with pressure difference)INTERNATIONAL JOURNAL OF FOOD SCIENCE & TECHNOLOGY, Issue 11 2008Niall W. G. Young Summary Ultrasonic velocity profiling with pressure difference (UVP-PD) was demonstrated to be a successful, non-invasive, in-line measurement system for instantaneous velocity and rheological flow profiling of complex, opaque fat blends. Model systems of 25% Akomic, 75% rapeseed oil; and 25% Akomic, 74% rapeseed oil and 1% Grindsted® Crystalliser 110 were compared under real process conditions with UVP-PD. Results indicated that the sample containing the crystalliser had twice the viscosity of the control. These in-line results are in agreement with previous off-line results, and offer the chance to probe the mechanics of fat blend physics under real, dynamic conditions. [source] CLARIFICATION OF WATERMELON (CITRULLUS LANATUS) JUICE BY MICROFILTRATIONJOURNAL OF FOOD PROCESS ENGINEERING, Issue 6 2008Ch. CHHAYA ABSTRACT Microfiltration of watermelon juice was conducted using stirred membrane cell in continuous mode. The experiments were conducted at operating pressures of 136.5, 204.7 and 276 kPa, and stirring speeds 1,200 rpm (Re = 1.40 × 105), 1,400 rpm (Re = 1.63 × 105) and 1,600 rpm (Re = 1.87 × 105). Permeate flux decline was analyzed using a first-order kinetic model, and correlations were developed for the steady-state polarized layer resistance with the operating conditions, e.g., transmembrane pressure difference, Reynolds number and membrane resistance. The permeate flux was calculated based on the developed correlation, and found to be in good agreement with the actual experimental flux. The change in quality parameters of clarified juice was marginal compared to that of original juice. PRACTICAL APPLICATIONS The application of membrane technology is one of the emerging areas in food industry. The major application includes fruit juice clarification and concentration. Because of the nonthermal nature of membrane separation, the juice can be clarified at room temperature and packed aseptically for a longer shelf life without the loss of its initial quality parameters. The major problem during clarification is decline in permeate flux of fruit juice with time. Identification of causes for flux decline is essential for designing of membrane modules to make the clarification process commercially viable. The quality of juice during storage is vital, and therefore, determination of variation of its physicochemical properties during storage is also important. [source] QUANTIFICATION OF FLUX DECLINE OF DEPECTINIZED MOSAMBI (CITRUS SINENSIS[L.] OSBECK) JUICE USING UNSTIRRED BATCH ULTRAFILTRATIONJOURNAL OF FOOD PROCESS ENGINEERING, Issue 4 2005P. RAI ABSTRACT Ultrafiltration of enzymatically treated mosambi (Citrus sinensis [L.] Osbeck) juice was performed in a batch, unstirred membrane cell. Thin film composite polyamide membrane of molecular weight cut-off 50,000 was used. The flux-decline mechanism was identified by the growth of a gel-type layer over the membrane surface. The flux decline, the gel resistance, the gel-layer thickness and various domains of resistances were calculated at a constant pressure difference in the range of 276,552 kPa. The conventional gel-filtration theory was employed to analyze the flux-decline behavior. The calculated permeate flux and gel-layer resistances were consistent with the experimental results. The calculated gel-layer thickness varied from 4.12 to 74.1 µm for different operating condition (pressure) and time. The time at which the gel resistance becomes equal to the membrane-hydraulic resistance ranged from 13 to 31 s for the pressure range studied herein. Thus, the deposited layer resistance to permeate flow was substantial, throughout the whole operation, except in the first few seconds. [source] The applicability of the GRIPS geobarometry in metapelitic assemblagesJOURNAL OF METAMORPHIC GEOLOGY, Issue 4 2006C.-M. WU Abstract Although the garnet-rutile-ilmenite-plagioclase-silica (quartz) (GRIPS) geobarometer has been experimentally calibrated and widely applied, its applicability to metapelitic rocks has not yet been discussed carefully. In this paper, this barometer was recalibrated by fitting the available reversed-phase equilibrium data incorporating different combinations of activity models of garnet, plagioclase and ilmenite. The resultant GRIPS barometer formalisms reproduce the experimental pressures well within ±0.2 kbar. The GRIPS and garnet-aluminium silicate-plagioclase-quartz (GASP) barometer values are concordant within ±1 kbar for pressures above ,6 kbar for natural metapelites, but the difference of pressure determinations between these two barometers becomes larger when pressure and/or the grossular content of garnet decrease. However, the pressure difference is independent of either temperature, or almandine in garnet, or anorthite in plagioclase, or iron content in ilmenite. After testing and application of the GRIPS barometer to aluminosilicate-bearing metapelites and metapelitic assemblages within limited geographical areas as well as within contact thermal aureoles, it is concluded that this barometer may be applied to low- to high-grade, medium- to high-pressure metapelites. The application of the GRIPS barometer to metapelites is not advocated in situations where calcium is deficient in garnet ( < 0.05) or plagioclase ( < 0.17), or for pressures below ,6 kbar. [source] Experimental and theoretical study of the influence of pressure on SCWOAICHE JOURNAL, Issue 11 2006M. D. Bermejo Abstract It is well known that pressure is an important parameter in the SCWO process, because it produces changes in the phase behavior and thermodynamics properties of the system. The influence of pressure on the oxidation efficiency of feeds containing isopropyl alcohol as fuel and acetic acid as waste was studied in an autothermal adiabatic transpiring wall reactor at pilot-plant scale, at pressures from 13 to 25 Mpa, and temperatures from 600 to 750°C. It was found that reaction temperature decreased between 40 and 100°C, when pressure was lowered, and, subsequently TOC removal decreased. At higher-temperatures, the decreasing of TOC removal due to pressure difference was reduced. The process was simulated at different pressures using a mathematical model previously developed for the reactor. The Anderko-Pitzer EoS, especially developed for aqueous systems at high-temperatures and pressures, was used to have accurate values of the heat capacities of the reaction mixture. It was found that having into account only the effect of pressure in the thermodynamical properties and in the concentration of the reagents, the experimental results could not be explained. Thus, the influence of the pressure in the reaction rate equation had to be considered. Results were qualitatively reproduced considering the reaction rate constant variable with pressure, using a constant volume of activation of ,1400 cm3/mol. © 2006 American Institute of Chemical Engineers AIChE J, 2006 [source] The characterization of fluidization behavior using a novel multichamber microscale fluid bedJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 3 2004Eetu Räsänen Abstract In the preformulation stage, there is a special need to determine the process behavior of materials with smaller amounts of samples. The purpose of this study was to assemble a novel automated multichamber microscale fluid bed module with a process air control unit for the characterization of fluidization behavior in variable conditions. The results were evaluated on the basis of two common computational methods, the minimum fluidization velocity, and the Geldart classification. The materials studied were different particle sizes of glass beads, microcrystalline cellulose, and silicified microcrystalline cellulose. During processing, the different characteristic fluidization phases (e.g., plugging, bubbling, slugging, and turbulent fluidization) of the materials were observed by the pressure difference over the bed. When the moisture content of the process air was increased, the amount of free charge carriers increased and the fine glass beads fluidized on the limited range of velocity. The silicification was demonstrated to improve the fluidization behavior with two different particle sizes of cellulose powders. Due to the interparticle (e.g., electrostatic) forces of the fine solids, the utilization of the computational predictions was restricted. The presented setup is a novel approach for studying process behavior with only a few grams of materials. © 2004 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 93: 780,791, 2004 [source] Dehydration studies using a novel multichamber microscale fluid bed dryer with in-line near-infrared measurementJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 10 2003Eetu Räsänen Abstract The purpose of this research was to study the effect of two process parameters (temperature and moisture content) on dehydration behavior of different materials using a novel multichamber microscale fluid bed dryer with a process air control unit and in-line near-infrared (NIR) spectroscopy. The materials studied were disodium hydrogen phosphates with three different levels of hydrate water and wet theophylline granules. Measured process parameters of fluid bed drying were logged, including in-line NIR signals. Off-line analyses consisted of X-ray powder diffraction patterns, Fourier transform NIR spectra and moisture contents of studied materials. During fluid bed drying, the stepwise dehydration of materials was observed by the water content difference of inlet and outlet air, the pressure difference over the bed, and the in-line NIR spectroscopy. The off-line analysis confirmed the state of solid materials. The temperature and the moisture content of the process air were demonstrated to be significant factors for the solid-state stability of theophylline. The presented setup is a material and cost-saving approach for studying the influence of different process parameters on dehydration behavior during pharmaceutical processing. © 2003 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 92:2074,2081, 2003 [source] A remark on the pressure for the Navier,Stokes flows in 2-D straight channel with an obstacleMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 8 2004H. Morimoto Abstract Let T=,×(-1,1) and &ℴ,,2 be a smoothly bounded open set, closure of which is contained in T. We consider the stationary Navier,Stokes flows in . In general, the pressure is determined up to a constant. Since , has two extremities, we want to know if we can choose the constant same. We study the behaviour of the pressure at the infinity in , and give a relation between the velocity and the pressure difference. Copyright © 2004 John Wiley & Sons, Ltd. [source] Stomatal sensitivity to vapour pressure difference over a subambient to elevated CO2 gradient in a C3/C4 grasslandPLANT CELL & ENVIRONMENT, Issue 8 2003H. MAHERALI ABSTRACT In the present study the response of stomatal conductance (gs) to increasing leaf-to-air vapour pressure difference (D) in early season C3 (Bromus japonicus) and late season C4 (Bothriochloa ischaemum) grasses grown in the field across a range of CO2 (200,550 µmol mol,1) was examined. Stomatal sensitivity to D was calculated as the slope of the response of gs to the natural log of externally manipulated D (dgs/dlnD). Increasing D and CO2 significantly reduced gs in both species. Increasing CO2 caused a significant decrease in stomatal sensitivity to D in Br. japonicus, but not in Bo. ischaemum. The decrease in stomatal sensitivity to D at high CO2 for Br. japonicus fit theoretical expectations of a hydraulic model of stomatal regulation, in which gs varies to maintain constant transpiration and leaf water potential. The weaker stomatal sensitivity to D in Bo. ischaemum suggested that stomatal regulation of leaf water potential was poor in this species, or that non-hydraulic signals influenced guard cell behaviour. Photosynthesis (A) declined with increasing D in both species, but analyses of the ratio of intercellular to atmospheric CO2 (Ci/Ca) suggested that stomatal limitation of A occurred only in Br. japonicus. Rising CO2 had the greatest effect on gs and A in Br. japonicus at low D. In contrast, the strength of stomatal and photosynthetic responses to CO2 were not affected by D in Bo. ischaemum. Carbon and water dynamics in this grassland are dominated by a seasonal transition from C3 to C4 photosynthesis. Interspecific variation in the response of gs to D therefore has implications for predicting seasonal ecosystem responses to CO2. [source] Hydrodynamic Cell Model: General Formulation and Comparative Analysis of Different ApproachesTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2007Emiliy K. Zholkovskiy Abstract This paper is concerned with the Cell Model method of addressing hydrodynamic flow through system of solid particles. The starting point of the analysis is the general problem formulation intended for describing a pressure driven flow through a diaphragm which can be considered as a set of representative cells having arbitrary shape and containing any number of particles. Using the general problem formulation, the hydrodynamic field inside an individual representative cell is interrelated with the applied pressure difference and the external flow velocity. To this end, four relationships containing integrals over the outer boundary of a representative cell are derived in the paper. Assuming that the representative cell is a sphere containing a single particle in the centre, the derived general relationships are transformed into outer cell boundary conditions employed in the literature by different authors. The general number of the obtained outer boundary conditions is more than the required number. Accordingly, by choosing different sets of the outer boundary conditions, different models are considered and compared with each other and with the results obtained by others for regular particle arrays. The common and different features of the hydrodynamic and electrodynamic versions of the Cell Model approaches are analyzed. Finally, it is discussed which version of the cell model gives the best approximation while describing pressure and electrically driven flows through a diaphragm and sedimentation of particles. On s'intéresse dans cet article à la méthode du Modèle de Cellules pour traiter l'écoulement à travers un système de particules solides. Le point de départ de l'analyse consiste à formuler le problème général dans le but de décrire un écoulement sous pression dans un diaphragme qui peut être considéré comme un ensemble de cellules représentatives de forme arbitraire et contenant un nombre quelconque de particules. À l'aide de cette formulation générale du problème, l'hydrodynamique dans une cellule représentative donnée est reliée à la différence de pression appliquée et à la vitesse d'écoulement externe. À cette fin, quatre relations contenant des intégrales sur la frontière d'une cellule représentative sont établies dans cette étude. Si l'on suppose que la cellule représentative est une sphère contenant une particule unique en son centre, les relations générales calculées peuvent être transformées en conditions à la frontière des cellules semblables à celles employées dans la littérature scientifique par différents auteurs. Le nombre général de conditions limites obtenues dépasse le nombre requis. Par conséquent, en choisissant différents ensembles de conditions limites, différents modèles sont considérés et comparés entre eux ainsi qu'avec les résultats obtenus pour des arrangements réguliers de particules. Les caractéristiques des versions hydrodynamiques et électrodynamiques des approches du Modèle de Cellules sont analysées. Finalement, on examine quelle version de modèle de cellule donne la meilleure approximation des écoulements sous pression et des écoulements électrodynamiques à travers un diaphragme et pour la sédimentation des particules. [source] Identification and control of a riser-type FCC unitTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 6 2001Abdul-Alghasim Alaradi Abstract This paper addresses the use of feedforward neural networks for the steady-state and dynamic identification and control of a riser type fluid catalytic cracking unit (FCCU). The results are compared with a conventional PI controller and a model predictive control (MPC) using a state space subspace identification algorithm. A back propagation algorithm with momentum term and adaptive learning rate is used for training the identification networks. The back propagation algorithm is also used for the neuro-control of the process. It is shown that for a noise-free system the adaptive neuro-controller and the MPC are capable of maintaining the riser temperature, the pressure difference between the reactor vessel and the regenerator, and the catalyst bed level in the reactor vessel, in the presence of set-point and disturbance changes. The MPC performs better than the neuro controller that in turn is superior to the conventional multi-loop diagonal PI controller. On examine dans cet article l'utilisation de réseaux neuronaux à anticipation pour la détermination et la régulation en régimes dynamique et permanent d'une unité de craquage catalytique de fluide de type colonne montante (FCCU). Un algorithme de rétro-propagation avec un terme de quantité de mouvement et une vitesse d'apprentissage adaptative est utilisé pour l'entraînement des réseaux d'identification. L'algorithme de rétro-propagation est également utilisé pour le controle neuronal du procédé. On montre que pour un système non bruité le contôleur neuronal adaptatif est capable de maintenir la température de colonne, la différence de pression entre le réacteur et le régénerateur ainsi que le niveau de lit de catalyseur dans le réacteur, en présence de changements dans les point de consigne et les perturbations. [source] Fully Autonomous Preload-Sensitive Control of Implantable Rotary Blood PumpsARTIFICIAL ORGANS, Issue 9 2010Andreas Arndt Abstract A pulsatility-based control algorithm with a self-adapting pulsatility reference value is proposed for an implantable rotary blood pump and is to be tested in computer simulations. The only input signal is the pressure difference across the pump, which is deduced from measurements of the pump's magnetic bearing. A pulsatility index (PI) is calculated as the mean absolute deviation from the mean pressure difference. As a second characteristic, the gradient of the PI with respect to the pump speed is derived. This pulsatility gradient (GPI) is used as the controlled variable to adjust the operating point of the pump when physiological variables such as the systemic arterial pressure, left ventricular contractility, or heart rate change. Depending on the selected mode of operation, the controller is either a linear controller or an extremum-seeking controller. A supervisory mechanism monitors the state of the system and projects the system into the region of convergence when necessary. The controller of the GPI continuously adjusts the reference value for PI. An underlying robust linear controller regulates the PI to the reference value in order to take into account changes in pulmonary venous return. As a means of reacting to sudden changes in the venous return, a suction detection mechanism was included. The control system is robustly stable within a wide range of physiological variables. All the clinician needs to do is to select between the two operating modes. No other adjustments are required. The algorithm showed promising results which encourage further testing in vitro and in vivo. [source] Prediction of Leakage Flow in a Shrouded Centrifugal Blood PumpARTIFICIAL ORGANS, Issue 9 2010Ji-Bin Teo Abstract This article proposes a phenomenological model to predict the leakage flow in the clearance gap of shrouded centrifugal blood pumps. A good washout in the gap clearance between the rotating impeller surfaces and volute casing is essential to avoid thrombosis. However, excessive leakage flow will result in higher fluid shear stress that may lead to hemolysis. Computational fluid dynamics (CFD) analysis was performed to investigate the leakage flow in a miniaturized shrouded centrifugal blood pump operating at a speed of 2000 rpm. Based on an analytical model derived earlier, a phenomenological model is proposed to predict the leakage flow. The leakage flow rate is found to be proportional to h,, where h is the gap size and the exponent , ranges from 2.955 to 3.15 for corresponding gap sizes of 0.2,0.5 mm. In addition, it is observed that , is a linear function of the gap size h. The exponent , compensates for the variation of pressure difference along the circumferential direction as well as inertia effects that are dominant for larger gap clearances. The proposed model displays good agreement with computational results. The CFD analysis also showed that for larger gap sizes, the total leakage flow rate is of the same order of magnitude as the operating flow rate, thus suggesting low volumetric efficiency. [source] Physiological Control of a Rotary Blood Pump With Selectable Therapeutic Options: Control of Pulsatility GradientARTIFICIAL ORGANS, Issue 10 2008Andreas Arndt Abstract A control strategy for rotary blood pumps meeting different user-selectable control objectives is proposed: maximum support with the highest feasible flow rate versus medium support with maximum ventricular washout and controlled opening of the aortic valve (AoV). A pulsatility index (PI) is calculated from the pressure difference, which is deduced from the axial thrust measured by the magnetic bearing of the pump. The gradient of PI with respect to pump speed (GPI) is estimated via online system identification. The outer loop of a cascaded controller regulates GPI to a reference value satisfying the selected control objective. The inner loop controls the PI to a reference value set by the outer loop. Adverse pumping states such as suction and regurgitation can be detected on the basis of the GPI estimates and corrected by the controller. A lumped-parameter computer model of the assisted circulation was used to simulate variations of ventricular contractility, pulmonary venous pressure, and aortic pressure. The performance of the outer control loop was demonstrated by transitions between the two control modes. Fast reaction of the inner loop was tested by stepwise reduction of venous return. For maximum support, a low PI was maintained without inducing ventricular collapse. For maximum washout, the pump worked at a high PI in the transition region between the opening and the permanently closed AoV. The cascaded control of GPI and PI is able to meet different control objectives and is worth testing in vitro and in vivo. [source] Physiological Control of Blood Pumps Using Intrinsic Pump Parameters: A Computer Simulation StudyARTIFICIAL ORGANS, Issue 4 2006Guruprasad A. Giridharan Abstract:, Implantable flow and pressure sensors, used to control rotary blood pumps, are unreliable in the long term. It is, therefore, desirable to develop a physiological control system that depends only on readily available measurements of the intrinsic pump parameters, such as measurements of the pump current, voltage, and speed (in revolutions per minute). A previously proposed ,P control method of ventricular assist devices (VADs) requires the implantation of two pressure sensors to measure the pressure difference between the left ventricle and aorta. In this article, we propose a model-based method for estimating ,P, which eliminates the need for implantable pressure sensors. The developed estimator consists of the extended Kalman filter in conjunction with the Golay,Savitzky filter. The performance of the combined estimator,VAD controller system was evaluated in computer simulations for a broad range of physical activities and varying cardiac conditions. The results show that there was no appreciable performance degradation of the estimator,controller system compared to the case when ,P is measured directly. The proposed approach effectively utilizes a VAD as both a pump and a differential pressure sensor, thus eliminating the need for dedicated implantable pressure and flow sensors. The simulation results show that different pump designs may not be equally effective at playing a dual role of a flow actuator and ,P sensor. [source] Effects of Left Ventricular Assist Device on Cardiac Function: Experimental Study of Relationship between Pump Flow and Left Ventricular Diastolic FunctionARTIFICIAL ORGANS, Issue 9 2001Akira Saito Abstract: The left ventricular assist device (LVAD) with centrifugal pump has two characteristics. One is a pump flow wave of the centrifugal pump, consisting of the pulsatile flow of the native heart and the nonpulsatile flow of the centrifugal pump. The other is that the centrifugal pump fills from the native heart not only in the systolic phase, but also in the diastolic phase. In the case of the apex outlet LVAD with centrifugal pump, blood flows from the left atrium through the left ventricle to the pump. Pump flow is regulated by preload, and preload is regulated by diastolic hemodynamics. The aim of this study is to analyze the relationship between pump flow and the diastolic hemodynamics of the native heart. Ten anesthetized intact pigs were studied after placement of an LVAD. Data were recorded with the LVAD off (control) and the LVAD on. The assist rate was changed to 25%, 50%, and 75%. The indexes of left ventricular (LV) diastolic function included LV myocardial relaxation (time constant of isovolumic pressure decay [Tau] and maximum negative dP/dt[LV dP/dt min]) and LV filling (peak filling rate [PFR], time to peak filling rate [tPFR], and diastolic filling time [DFT]). Stroke volume decreased significantly in 75% assist. LV end-systolic pressure decreased significantly in 50% and 75% assist. LV end-diastolic volume decreased as assist rate increased, but there were no significant changes. Stroke work decreased significantly in 50% and 75% assist. LV dP/dt min decreased significantly in 50% and 75% assist. Tau prolonged as assist rate increased, but there were no significant changes. DFT shortened significantly in 75% assist. PFR increased significantly in 75% assist. tPFR shortened significantly in 50% and 75% assist. In this study, LV relaxation delayed as an increasing of pump assist rate, but it suggested a result of reduction of cardiac work. Also, it was suggested that LVAD increases the pressure difference between the left atrium and the left ventricle in the diastolic phase. This phenomenon is due to the filling of the left ventricle. In this study it was suggested that as pump assist rate increases, it is more effective to keep cardiac function in the diastolic phase. [source] Dynamic simulation of kinetics, heat and mass transfer during hydrogen sorption by LaNi5ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 3 2010Dr Tarek Moustafa Abstract A two-dimensional transient heat and mass transfer models have been developed to investigate the dynamic phenomena of hydrogen absorption and desorption in metal hydride bed. LaNi5 has been chosen as the alloy used for hydrogen storage. The numerical simulation has been conducted to simulate the time,space evolution of temperature, fractional conversion, hydrogen pressure and velocity, in addition to metal density. A correlation for the volumetric reaction rate has been deduced. Also, comparisons have been done between various bed geometries and their influence on the average hydrogen desorbed mass. The simulation results showed that heat transfer controls the overall rate of absorption and desorption processes, and because the driving force for the hydrogen flow is the axial pressure difference; the challenge to get optimum bed geometry is in compromising between heat transfer and pressure drop limitations. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] A chromospheric dark-cored fibril in Ca II IR spectraASTRONOMISCHE NACHRICHTEN, Issue 6 2010C. Beck Abstract We investigate the thermodynamical and magnetic properties of a "dark-cored" fibril seen in the chromospheric Ca II IR line at 854.2 nm to determine the physical process behind its appearance. We analyse a time series of spectropolarimetric observations obtained in the Ca II IR line at 854.2 nm and the photospheric Fe I line at 630.25 nm. We simultaneously invert the spectra in both wavelength ranges with the SIR code to obtain the temperature and velocity stratification with height in the solar atmosphere and the magnetic field properties in the photosphere. The structure can be clearly traced in the line-of-sight (LOS) velocity and the temperature maps. It connects from a small pore with kG fields to a region with lower field strength. The flow velocity and the temperature indicate that the height of the structure increases with increasing distance from the inner footpoint. The Stokes V signal of 854.2 nm shows a Doppler-shifted polarization signal with the same displacement as in the intensity profile, indicating that the supersonic flow seen in the LOS velocity is located within magnetized plasma. We conclude that the chromospheric dark-cored fibril traces a siphon flow along magnetic field lines, driven by the gas pressure difference caused by the higher magnetic field strength at the inner footpoint. We suggest that fast flows guided by the magnetic field lead to the appearance of "dark-cored" fibrils in intensity images. Although the observations included the determination of the polarization signal in the chromospheric Ca II IR line, the signal could not be analysed quantitatively due to the low S/N. Chromospheric polarimetry will thus require telescopes of larger aperture able to collect a sufficient number of photons for a reliable determination of polarization in deep and only weakly polarized spectral lines (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||||||||||||||||||||||||||||||||