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Pressure Variation (pressure + variation)
Kinds of Pressure Variation Selected AbstractsThe effect of contact load reduction on the fatigue life of pearlitic rail steel in lubricated rolling,sliding contactFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 8 2000D. I. Fletcher Twin-disc contact simulation tests were carried out to investigate the influence of contact pressure variation on rail steel fatigue life. Both a colloidal suspension of molybdenum disulphide in an oil carrier fluid (similar to a commercial flange lubrication product) and water were used as lubricants. It was found that the reduction from 1500 to 900 MPa of the maximum Hertzian contact pressure (at which a molybdenum,disulphide-lubricated and previously worn rail sample was tested) extended the fatigue life of the rail steel by over five times. For water lubrication a similar reduction in contact pressure produced only a marginal increase in fatigue life. The results were found to be in qualitative agreement with the predictions of the newly developed Three Mechanism (TM) model of rolling contact fatigue, which is introduced here. This model combines the mechanisms of ratcheting and the fracture mechanics-based mechanisms of both shear stress- and tensile stress-driven, fluid-assisted, crack growth. [source] Numerical analysis of Rayleigh,Plesset equation for cavitating water jetsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 7 2007H. Alehossein Abstract High-pressure water jets are used to cut and drill into rocks by generating cavitating water bubbles in the jet which collapse on the surface of the rock target material. The dynamics of submerged bubbles depends strongly on the surrounding pressure, temperature and liquid surface tension. The Rayleigh,Plesset (RF) equation governs the dynamic growth and collapse of a bubble under various pressure and temperature conditions. A numerical finite difference model is established for simulating the process of growth, collapse and rebound of a cavitation bubble travelling along the flow through a nozzle producing a cavitating water jet. A variable time-step technique is applied to solve the highly non-linear second-order differential equation. This technique, which emerged after testing four finite difference schemes (Euler, central, modified Euler and Runge,Kutta,Fehlberg (RKF)), successfully solves the Rayleigh,Plesset (RP) equation for wide ranges of pressure variation and bubble initial sizes and saves considerable computing time. Inputs for this model are the pressure and velocity data obtained from a CFD (computational fluid dynamics) analysis of the jet. Copyright © 2007 John Wiley & Sons, Ltd. [source] Liquid vorticity computation in non-spherical bubble dynamicsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 2 2005A. A. Aganin Abstract The purpose of this work is to compare efficiency of a number of numerical techniques of computation of liquid vorticity from non-spherical bubble oscillations. The techniques based on the finite-difference method (FDM), the collocation method (one with differentiating (CMd) the integral boundary condition and another without it (CM)) and the Galerkin method (GM) have been considered. The central-difference approximations are used in FDM. Sinus functions are chosen as the basis in GM. Problems of decaying a small distortion of the spherical shape of a bubble and dynamics of a bubble under harmonic liquid pressure variation with various parameters are used for comparison. The FDM technique has been found to be most efficient in all the cases. Copyright © 2004 John Wiley & Sons, Ltd. [source] Automated pre-ejection period variation predicts fluid responsiveness in low tidal volume ventilated pigsACTA ANAESTHESIOLOGICA SCANDINAVICA, Issue 2 2010S. T. VISTISEN Introduction: The respiratory variation in the pre-ejection period (,PEP) has been used to predict fluid responsiveness in mechanically ventilated patients. Recently, we automated this parameter and indexed it to tidal volume (PEPV) and showed that it was a reliable predictor for post-cardiac surgery, mainly paced, patients ventilated with low tidal volumes. The aims of the present animal study were to investigate PEPV's ability to predict fluid responsiveness under different fluid loading conditions and natural heart rates during low tidal volume ventilation (6 ml/kg) and to compare the performance of PEPV with other markers of fluid responsiveness. Methods: Eight prone, anesthetized piglets (23,27 kg) ventilated with tidal volumes of 6 ml/kg were subjected to a sequence of 25% hypovolemia, normovolemia, and 25% and 50% hypervolemia. PEPV, ,PEP, pulse pressure variation (PPV), central venous pressure (CVP), and pulmonary artery occlusion pressure (PAOP) were measured before each volume expansion. Results: Sensitivity was 89% and specificity was 93% for PEPV, 78% and 93% for ,PEP, 89% and 100% for PPV, 78% and 93% for CVP, and 89% and 87% for PAOP. Conclusion: PEPV predicts fluid responsiveness in low tidal volume ventilated piglets. [source] Automated pre-ejection period variation indexed to tidal volume predicts fluid responsiveness after cardiac surgeryACTA ANAESTHESIOLOGICA SCANDINAVICA, Issue 4 2009S. T. VISTISEN Background: Reliable continuous monitoring of fluid responsiveness is an unsolved issue in patients ventilated with low tidal volume. We hypothesised that variations in the pre-ejection period (PEP) defined as the time interval between electrocardiogram (ECG) R-wave and onset of systolic upstroke in arterial blood pressure could reliably predict fluid responsiveness in patients ventilated with moderately low tidal volume. Furthermore, we hypothesised that indexing dynamic parameters to tidal volume would improve their prediction. The aim was to refine and automate a previously suggested algorithm for PEP variation (,PEP) and to test this new parameter indexed to tidal volume (PEPV), as a marker of fluid responsiveness along with central venous pressure (CVP), pulse pressure variation (PPV) and ,PEP. Additionally, the aim was to evaluate the concept of indexing dynamic parameters to tidal volume. Methods: Arterial pressure, CVP, ECG and cardiac index (CI) were acquired from 23 mechanically ventilated post-cardiac surgery patients scheduled for volume expansion. PEPV, PPV and ,PEP were extracted. Results: Using responder/non-responder classification (response=change in CI>+15%), sensitivity and specificity were 100% and 83%, respectively, for PEPV, 94% and 83% for ,PEP, and 94% and 83% for PPV. CVP offered no relevant information. Tidal volume indexing improved sensitivity for ,PEP to 100%. Conclusion: In this study in post-cardiac surgery patients, a refined parameter, PEPV, predicted fluid responsiveness better than PPV and ,PEP. Our results suggest that dynamic parameters using variations in PEP should be indexed to tidal volume. [source] Fluid therapy in acute myocardial infarction: evaluation of predictors of volume responsivenessACTA ANAESTHESIOLOGICA SCANDINAVICA, Issue 1 2009J. SNYGG Background: Static vascular filling pressures suffer from poor predictive power in identifying the volume-responsive heart. The use of dynamic arterial pressure variables, including pulse pressure variation (PPV) has instead been suggested to guide volume therapy. The aim of the present study was to evaluate the performance of several clinically applicable haemodynamic parameters to predict volume responsiveness in a pig closed chest model of acute left ventricular myocardial infarction. Methods: Fifteen anaesthetized, mechanically ventilated pigs were studied following acute left myocardial infarction by temporary coronary occlusion. Animals were instrumented to monitor central venous (CVP) and pulmonary artery occlusion (PAOP) pressures and arterial systolic variations (SPV) and PPV. Cardiac output (CO) was measured using the pulmonary artery catheter and by using the PiCCO® monitor also giving stroke volume variation (SVV). Variations in the velocity time integral by pulsed-wave Doppler echocardiography were determined in the left (,VTILV) and right (,VTIRV) ventricular outflow tracts. Consecutive boluses of 4 ml/kg hydroxyethyl starch were administered and volume responsiveness was defined as a 10% increase in CO. Results: Receiver,operator characteristics (ROC) demonstrated the largest area under the curve for ,VTIRV [0.81 (0.70,0.93)] followed by PPV [0.76 (0.64,0.88)] [mean (and 95% CI)]. SPV, ,VTILV and SVV did not change significantly during volume loading. CVP and PAOP increased but did not demonstrate significant ROC. Conclusion: PPV may be used to predict the response to volume administration in the setting of acute left ventricular myocardial infarction. [source] Pulse pressure variation and stroke volume variation during different loading conditions in a paediatric animal modelACTA ANAESTHESIOLOGICA SCANDINAVICA, Issue 3 2008J. RENNER Background: Previous studies in adult patients and animal models have demonstrated that pulse pressure variation (PPV) and stroke volume variation (SVV) can be used to predict the response to fluid administration. Currently, little information is available on the performance of these variables in infants and neonates. The aim of our study was to assess whether PPV and SVV can predict fluid responsiveness in an animal model and to investigate the influence of different tidal volumes applied. Methods: PPV and SVV were monitored by pulse contour analysis in 19 anaesthetized and paralysed piglets during ventilation with tidal volumes (VT) of 5, 10 and 15 ml/kg both before and after fluid loading with 25 ml/kg of hydroxy-ethyl starch 6% (HES). Cardiac output was measured by pulmonary artery thermodilution and a positive response to HES infusion was defined as ,20% increase in the stroke volume index (SVI). Results: Before HES infusion, PPV and SVV were significantly greater during ventilation with a VT of 10 and 15 ml/kg than during ventilation with a VT of 5 ml/kg (P<0.05). After HES infusion, only ventilation with VT 15 ml/kg resulted in a significant increase in PPV and SVV. As assessed by receiver operating characteristic curve analysis, SVV during ventilation with VT 10 ml/kg was the best predictor of a positive response to fluid loading (AUC=0.87). Conclusions: In this paediatric animal model, we found that SVV during ventilation with 10 ml/kg was a sensitive and specific predictor of the response to fluid loading. [source] Carrier recombination processes in 1.3 ,m and 1.5 ,m InGaAs(P)-based lasers at cryogenic temperatures and high pressuresPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 14 2004S. J. Sweeney Abstract We describe measurements of the threshold current of 1.3 ,m and 1.5 ,m InGaAs(P)-based quantum-well lasers measured at cryogenic temperatures and at high pressures. At low temperatures (,100 K), we find that the threshold current of the devices increases with increasing pressure consistent with the calculated pressure variation of the radiative current. This is in sharp contrast with their pressure dependence at room temperature (RT), where the threshold current decreases with increasing pressure due to the decrease in importance of Auger recombination. These low-temperature, high-pressure results agree well with previous temperature dependence measurements on the same devices, which show a transition from radiative to non-radiative Auger recombination dominated behaviour as the laser temperature is increased from ,100 K to room temperature. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Turbulent flows on forested hilly terrain: the recirculation regionTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 625 2007D. Poggi Abstract A number of analytical and numerical studies employing first-order closure principles have suggested that canopy flows on gentle sinusoidal hills feature a recirculation region, situated on the lee side, that can dramatically affect scalar transfer between the biosphere and the atmosphere. To date, the onset of this region, and its effects on bulk flow properties, have not been experimentally investigated. We study the applicability of first-order closure schemes jointly with the properties of this recirculation region, using detailed laser Doppler anemometry (LDA) measurements. These experiments are conducted in a neutrally stratified boundary-layer flow within a large flume over a train of gentle and narrow hills. The canopy is composed of an array of vertical cylinders with a frontal-area index concentrated in the upper third, to resemble a tall hardwood forest at maximum leaf area. The LDA measurements are recorded for both sparse and dense canopies. We find that, while the onset of a recirculation region is ambiguous in the sparse-canopy case, it is well delineated in the dense-canopy case. This finding constitutes the first experimental evidence confirming the analytical and numerical model predictions concerning this region in dense canopies on gentle hills. Moreover, we show that the presence of the recirculation region can explain the anomalous pressure variation across the hill (first reported in numerical simulations) using an ,effective hill shape' function. Detailed momentum-flux measurements show, surprisingly, that the effective mixing length leff within the canopy and in the inner layer is not significantly affected by the recirculation region. We expected leff to be comparable to the size of the vortex responsible for the recirculation zone, but the measurements show that leff maintains its canonical canopy turbulence shape. Using laser-induced fluorescence (LIF) measurements, we find that the recirculation region is not characterized by a classical ,rotor', but by a highly intermittent zone with alternating positive and negative velocity values in the lower layers of the canopy. These LIF measurements may explain why leff maintains its canonical canopy turbulence shape in the recirculation region. The LIF measurements also show that the main mechanism for scalar transfer within the recirculation region is a sequence of accumulation,ejection episodes that are quasi-periodic in nature. Copyright © 2007 Royal Meteorological Society [source] The value of pulse pressure and stroke volume variation as predictors of fluid responsiveness during open chest surgeryANAESTHESIA, Issue 7 2010P. A. H. Wyffels Summary We investigated the ability of pulse pressure variation and stroke volume variation to predict fluid responsiveness during mechanical ventilation in patients undergoing open chest surgery by comparing their respective correlations with cardiac output changes induced by leg elevation. Serial leg elevation manoeuvres were performed before and after sternotomy in 15 patients scheduled for elective off-pump coronary bypass surgery. Under closed chest conditions, both pulse pressure variation and stroke volume variation correlated well with the induced cardiac output changes (r = 0.856, p = 0.002 and r = 0.897, p = 0.0012, respectively). These correlations were lost for both parameters following sternotomy. Our data show that pulse pressure variation and stroke volume variation are valid predictors of fluid responsiveness under closed chest conditions but that this property no longer holds when the chest is open. [source] Estimation of Joule heating effect on temperature and pressure distribution in electrokinetic-driven microchannel flowsELECTROPHORESIS, Issue 3 2006Reiyu Chein Professor Abstract In this study we present simple analytical models that predict the temperature and pressure variations in electrokinetic-driven microchannel flow under the Joule heating effect. For temperature prediction, a simple model shows that the temperature is related to the Joule heating parameter, autothermal Joule heating parameter, external cooling parameter, Peclet number, and the channel length to channel hydraulic diameter ratio. The simple model overpredicted the thermally developed temperature compared with the full numerical simulation, but in good agreement with the experimental measurements. The factors that affect the external cooling parameters, such as the heat transfer coefficient, channel configuration, and channel material are also examined based on this simple model. Based on the mass conservation, a simple model is developed that predicts the pressure variations, including the temperature effect. An adverse pressure gradient is required to satisfy the mass conservation requirement. The temperature effect on the pressure gradient is via the temperature-dependent fluid viscosity and electroosmotic velocity. [source] Cushioning the pressure vibration of a zeolite concentrator system using a decoupled balancing duct systemENVIRONMENTAL PROGRESS & SUSTAINABLE ENERGY, Issue 2 2007Feng-Tang Chang Abstract A honeycomb Zeolite Rotor Concentrator (HZRC) is the main air pollution control device utilized by many semiconductor and optoelectronics manufacturers. Various plant exhaust streams are collected and then transferred to the HZRC for decontamination. In a conventional HZRC, the exhaust fan movement and the switching between different air ducts can cause significant duct pressure variations resulting in production interruption. The minimization of pressure fluctuations to ensure continuous operation of production lines while maintaining a high volatile organic compounds (VOCs) removal efficiency is essential for exhaust treatment in these high technology manufactures. The article introduces a decoupled balancing duct system (DBDS) for controlling the airflows to achieve a balanced pressure in the HZRC system by adding a flow rate control device to the VOCs loaded stream bypass duct of a conventional system. Performance comparisons of HZRC with DBDS and other air flow control systems used by the wafer manufacturers in Hsinchu Science Park, Taiwan are presented. DBDS system had been proved effectively to stabilize the pressure in the airflow ducts, and thus avoided pressure fluctuations; it helped to achieve a high VOCs removal efficiency while ensuring the stability of the HZRC. © 2007 American Institute of Chemical Engineers Environ Prog, 2007 [source] Sea surface shape derivation above the seismic streamerGEOPHYSICAL PROSPECTING, Issue 6 2006Robert Laws ABSTRACT The rough sea surface causes perturbations in the seismic data that can be significant for time-lapse studies. The perturbations arise because the reflection response of the non-flat sea perturbs the seismic wavelet. In order to remove these perturbations from the received seismic data, special deconvolution methods can be used, but these methods require, as input, the time varying wave elevation above each hydrophone in the streamer. In addition, the vertical displacement of the streamer itself must also be known at the position of each hydrophone and at all times. This information is not available in conventional seismic acquisition. However, it can be obtained from the hydrophone measurements provided that the hydrophones are recorded individually (not grouped), that the recording bandwidth is extended down to 0.05 Hz and that data are recorded without gaps between the shot records. The sea surface elevation, and also the wave-induced vertical displacement of the streamer, can be determined from the time-varying pressure that the sea waves cause in the hydrophone measurements. When this was done experimentally, using a single sensor seismic streamer without a conventional low cut filter, the wave induced pressure variations were easily detected. The inversion of these experimental data gives results for the sea surface elevation that are consistent with the weather and sea state at the time of acquisition. A high tension approximation allows a simplified solution of the equations that does not demand a knowledge of the streamer tension. However, best results at the tail end of the streamer are obtained using the general equation. [source] Pressure relaxation procedures for multiphase compressible flowsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 1 2005M.-H. Lallemand Abstract This paper deals with pressure relaxation procedures for multiphase compressible flow models. Such models have nice mathematical properties (hyperbolicity) and are able to solve a wide range of applications: interface problems, detonation physics, shock waves in mixtures, cavitating flows, etc. The numerical solution of such models involves several ingredients. One of those ingredients is the instantaneous pressure relaxation process and is of particular importance. In this article, we present and compare existing and new pressure relaxation procedures in terms of both accuracy and computational efficiency. Among these procedures we enhance an exact one in the particular case of fluids governed by the stiffened gas equation of state, and approximate procedures for general equations of state, which are particularly well suited for problems with large pressure variations. We also present some generalizations of these procedures in the context of multiphase flows with an arbitrary number of fluids. Some tests are provided to illustrate these comparisons. Copyright © 2005 John Wiley & Sons, Ltd. [source] Numerical simulation of cavitating flow in 2D and 3D inducer geometriesINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 2 2005O. Coutier-Delgosha Abstract A computational method is proposed to simulate 3D unsteady cavitating flows in spatial turbopump inducers. It is based on the code FineTurbo, adapted to take into account two-phase flow phenomena. The initial model is a time-marching algorithm devoted to compressible flow, associated with a low-speed preconditioner to treat low Mach number flows. The presented work covers the 3D implementation of a physical model developed in LEGI for several years to simulate 2D unsteady cavitating flows. It is based on a barotropic state law that relates the fluid density to the pressure variations. A modification of the preconditioner is proposed to treat efficiently as well highly compressible two-phase flow areas as weakly compressible single-phase flow conditions. The numerical model is applied to time-accurate simulations of cavitating flow in spatial turbopump inducers. The first geometry is a 2D Venturi type section designed to simulate an inducer blade suction side. Results obtained with this simple test case, including the study of its general cavitating behaviour, numerical tests, and precise comparisons with previous experimental measurements inside the cavity, lead to a satisfactory validation of the model. A complete three-dimensional rotating inducer geometry is then considered, and its quasi-static behaviour in cavitating conditions is investigated. Numerical results are compared to experimental measurements and visualizations, and a promising agreement is obtained. Copyright © 2004 John Wiley & Sons, Ltd. [source] Effect of delayed supine positioning after induction of spinal anaesthesia for caesarean sectionACTA ANAESTHESIOLOGICA SCANDINAVICA, Issue 4 2002F. Køhler Background: The study tested the hypothesis that the incidence of hypotension during spinal anaesthesia for caesarean section is less in parturients who remain in the sitting position for 3 min compared with parturients who are placed in the modified supine position immediately after induction of spinal anesthesia. Methods: Spinal anaesthesia was induced with the woman in the sitting position using 2.8 ml hyperbaric bupivacaine 0.5% at the L3,4 or L2,3 interspace. Ninety-eight patients scheduled for elective caesarean section under spinal anaesthesia were randomised to assume the supine position on an operating table tilted 10° to the left (modified supine position) immediately after spinal injection (group 0, n=52) or to remain in the sitting position for 3 min before they also assumed the modified supine position (group 3, n=46). Isotonic saline 2,300 ml was given intravenously over 15 min before spinal injection followed by 15 ml/kg over 15,20 min after induction of spinal anaesthesia. If the systolic blood pressure decreased to less than 70% of baseline or to less than 100 mmHg or if there was any complaint of nausea, ephedrine was given in 5 mg boluses intravenously every 2 min. Results: The blood pressure decreased significantly in both groups following spinal injection (P<0.001). Blood pressure variations over time differed significantly between the two groups (P<0.05). However, the incidence of maternal hypotension before delivery was similar in the two groups. The difference was caused by the time to the blood pressure nadir being significantly shorter in group 0 compared with group 3 (9.1±4.5 min vs. 11.7±3.7 min, P<0.01). Similar numbers of patients received rescue with ephedrine before delivery: 35 (67%) in group 0 vs. 26 (57%) in group 3 (NS). The mean total dose of ephedrine before delivery was 10.9 mg in group 0 vs. 9.2 mg in group 3 (NS). There were no differences in neonatal outcome between the two groups. Conclusion: At elective caesarean section, a 3-min delay before supine positioning does not influence the incidence of maternal hypotension after induction of spinal anaesthesia in the sitting position with 2.8 ml of bupivacaine 0.5% with 8% dextrose. [source] Simulating the Dynamics of Spouted-Bed Nuclear Fuel Coaters,CHEMICAL VAPOR DEPOSITION, Issue 9 2007S. Pannala Abstract We describe simulation studies of the dynamics of spouted beds used for CVD coating of nuclear fuel particles. Our principal modeling tool is the Multiphase Flow with Interphase eXchanges (MFIX) code that was originally developed by the National Energy Technology Laboratory (NETL) for fossil energy process applications. In addition to standard MFIX features that allow coupling of transient hydrodynamics, heat and mass transfer, and chemical kinetics, we employ special post-processing tools to track particle mixing and circulation as functions of operating conditions and bed design. We describe in detail one major feature of the dynamics, which is the occurrence of very regular spontaneous pulsations of gas and particle flow in the spout. These pulsations appear to be critically linked to the entrainment and circulation of solids, and they produce readily accessible dynamic pressure variations that can be used for direct comparisons of model predictions with experiments. Spouted-bed dynamics are important from a CVD perspective because they directly determine the magnitude and variability of the concentration and species gradients in the zone where reactant gases first come into contact with hot particles. As this unsteady spouted-bed environment differs from other types of CVD reactors, the design and scale-up of such reactors is likely to involve unique modeling issues. Our primary goal here is to lay the groundwork for how computational simulation can be used to address these modeling issues in the specific context of nuclear fuel particle coating. [source] | |||||||||||||||||||