Pressure Gradient Research Papers - Academia.edu (original) (raw)

The theory-based closure relations for the wall and interfacial shear stresses obtained previously for laminar stratified flow, are extended to be applicable also to turbulent flows in either or both of the phases. The closure relations... more

The theory-based closure relations for the wall and interfacial shear stresses obtained previously for laminar stratified flow, are extended to be applicable also to turbulent flows in either or both of the phases. The closure relations are formulated in terms of the single-phase-based expressions, which are augmented by two-phase interaction factors, due to the flow of the two phases in the same channel. These closure relations, which are valid for smooth stratified flow in horizontal or inclined pipes, were used as a platform for introducing necessary empirical corrections required in the stratified wavy flow regime. Based on experimental data available from the literature, new empirical correlations for the wave effect on the interface curvature, on the interfacial shear and on the liquid wall shear were obtained. The predictions of the two-fluid model for the pressure gradient and holdup are tested against extensive data bank and some analytical solutions for stratified flows. T...

Four sets of thin-section scale, Mode I (open mode), cemented microfractures are present in sandstone from the Eocene Misoa Formation, Maracaibo basin, Venezuela. The first set of microfractures is intragranular (F1), formed early during... more

Four sets of thin-section scale, Mode I (open mode), cemented microfractures are present in sandstone from the Eocene Misoa Formation, Maracaibo basin, Venezuela. The first set of microfractures is intragranular (F1), formed early during compaction and are filled with quartz cement precipitated at temperatures equal to or higher than 100 8C. The second set of microfractures (F2) is cemented by bituminite-pyrite, formed at temperatures between 60 and 100 8C, and are associated with kerogen maturation and hydrocarbon migration from underlying overpressured source rocks. The third set of microfractures (F3) is fully cemented by either quartz cement or calcite cement. The former has fluid inclusion homogenization temperatures between 149 and 175 8C. These temperatures are mostly higher than maximum burial temperatures (~160 8C), suggesting that upward flow, caused by a pressure gradient, transported silica vertically which crystallized into the fractures. Upward decompression may have also caused a P CO 2 drop, which, at constant temperature, allowed simultaneous carbonate precipitation into the third microfracture set. The fourth set of thin-section scale microfractures (F4) is open or partially cemented by sideritehematite and other iron oxides. The presence of hematite and iron oxides in microfractures is evidence for oxidizing conditions that may be associated with the uplift of the Misoa formation. In order to time and place constraints on the depth of formation of the fourth set of microfractures, we have coupled published quartz cementation kinetic algorithms with uniaxial strain equations and determined if, in fact, they could be associated with the uplift of the formation. Our results suggest that thermoelastic contraction, caused by the formation's uplift, erosion, and consequent cooling is a feasible mechanism for the origin of the last fracture set. Hence, we infer that meteoric water invasion into the fractures, at the end of the uplift, cause the precipitation of oxides and the transformation of siderite to hematite. D

The spatial resolution of numerical weather prediction and climate models is generally determined by their grid spacing (∆x) or spectral truncation and the numerical implementation of dynamical core and model parametrisations. For example... more

The spatial resolution of numerical weather prediction and climate models is generally determined by their grid spacing (∆x) or spectral truncation and the numerical implementation of dynamical core and model parametrisations. For example features of the scale 2∆x and 3∆x are smoothed to avoid numerical instabilities (e.g., aliasing effects) and parameterisations in connection to advection, pressure gradient force, and subgrid-scale diffusion can only be well represented at dimensions of at least four times the grid spacing. Some parametrisations, however, generate energy at the grid-spacing scale. These multiple effects on the effective resolution of models are investigated in this study for three high resolution regional climate models (RCMs) in dependence of their grid spacing.

The present study attempts to analyze the average ring current density and pressure in the magnetospheric equatoria• plane (as a function of geocentric distance and local time) based on the AMPTE/CCE-CHEM proton distributions according to... more

The present study attempts to analyze the average ring current density and pressure in the magnetospheric equatoria• plane (as a function of geocentric distance and local time) based on the AMPTE/CCE-CHEM proton distributions according to two different AE ranges, corresponding to a low (AE<100 nT) and a moderate (100 nT<AE<600 nT) geomagnetic or auroral activity. In order to obtain a statistically significant database for a high spatial resolution, the data were averaged over 9. years. The results provide a trend of the ring current and show its general average features as a function of geomagnetic activity. In particular, the total ring current is dominated by the pressure gradient term, resulting in an eastward current in the •nner magnetosphere (oe<4.5) and an extended westward current at higher altitudes. The westward current is always stronger than the eastward current. Moreover, the ring current system is a function of magnetic local time, exhibiting a noon-midnight asymmetry in general, as well as a function of geomagnetic •ctivity exhibiting •n enhancement of •bout 9,0% during •ctive times (in midnight-dusk region only). The location of the total current peak in the midnight sector is at about MLT=23'00, and its changes with magnetic activity are interpreted as an effect of the particles injection from the tail during the storms/substorms. The derived current values are compared to previous model predictions and observations. of the charged particles is equivalent to a m 106A intensity electric current which forms a ring that encircles the Earth. This ring current is always present because it arises from the perpetual presence of trapped charged particles in the magnetosphere and their ensuing drift motions in the geomagnetic field. The effect of this ring current observed on the Earth's surface is a magnetic field directed southward [Dessler et al.,1959]. Since a northward field is measured posi-Copyright 1997 by the American Geophysical Union. Paper number 96JA03743. 0148-0227/97/96JA-03743 $09.00 five, the southward directed field is registered as depression of the north-south component. A simple theoretical relationship, known as Dessler-Parker-Sckopke relation [Sckopke, 1966], relates the Earth's magnetic field changes to the energy of the ring current particles: AB 2E Bs 3U where AB is the fractional change in the surface field B•, U is the total energy in the dipole field exterior to Earth's surface and E is the total energy of all the ring current particles. During geomagnetic storms the intensity of the ring current increases as a consequence of the injection of particles from various geospace regions. Consequently, the horizontal component H of the Earth's magnetic field decreases. This rapid decrease, which occurs after the sudden storm commencement (SSC), is generally used to mark the onset of magnetic storms [Chapman et a/.Energy oe[eV] Energy oe[eV] ß ß ß ß ß ß ß ß ß

In presence of aortic stenosis, a jet is produced downstream of the aortic valve annulus during systole. The vena contracta corresponds to the location where the cross-sectional area of the flow jet is minimal. The maximal transvalvular... more

In presence of aortic stenosis, a jet is produced downstream of the aortic valve annulus during systole. The vena contracta corresponds to the location where the cross-sectional area of the flow jet is minimal. The maximal transvalvular pressure gradient (TPG(max)) is the difference between the static pressure in the left ventricle and that in the vena contracta. TPG(max) is highly time-dependent over systole and is known to depend upon the transvalvular flow rate, the effective orifice area (EOA) of the aortic valve and the cross-sectional area of the left ventricular outflow tract. However, it is still unclear how these parameters modify the TPG(max) waveform. We thus derived an explicit analytical model to describe the instantaneous TPG(max) across the aortic valve during systole. This theoretical model was validated with in vivo experiments obtained in 19 pigs with supravalvular aortic stenosis. Instantaneous TPG(max) was measured by catheter and its waveform was compared with the one determined from the derived equation. Our results showed a very good concordance between the measured and predicted instantaneous TPG(max). Total relative error and mean absolute error were on average 9.4+/-4.9% and 2.1+/-1.1 mmHg, respectively. The analytical model proposed and validated in this study provides new insight into the behaviour of the TPG(max) and thus of the aortic pressure at the level of vena contracta. Because the static pressure at the coronary inlet is similar to that at the vena contracta, the proposed equation will permit to further examine the impact of aortic stenosis on coronary blood flow.

We investigate the critical speeding up of heat equilibration by the piston effect (PE) in a nearly supercritical van der Waals (vdW) fluid confined in a homogeneous porous medium. We perform an asymptotic analysis of the averaged... more

We investigate the critical speeding up of heat equilibration by the piston effect (PE) in a nearly supercritical van der Waals (vdW) fluid confined in a homogeneous porous medium. We perform an asymptotic analysis of the averaged linearized mass, momentum and energy equations to describe the response of the medium to a boundary heat flux. While nearing the critical point (CP), we find two universal crossovers depending on porosity, intrinsic permeability and viscosity. Closer to the CP than the first crossover, a pressure gradient appears in the bulk due to viscous effects, the PE characteristic time scale stops decreasing and tends to a constant. In infinitly long samples the temperature penetration depth is larger than the diffusion one indicating that the PE in porous media is not a finite size effect as it is in pure fluids. Closer to the CP, a second cross over appears which is characterized by a pressure gradient in the thermal boundary layer (BL). Beyond this second crossover, the PE time remains constant, the expansion of the fluid in the BL drops down and the PE ultimately fades away.

Patients and Methods Cirrhotic patients with acute esophageal variceal bleeding that was initially controlled with emergency sclerotherapy and in whom a hemodynamic evaluation could be performed within the first 24 hours of admission,... more

Patients and Methods Cirrhotic patients with acute esophageal variceal bleeding that was initially controlled with emergency sclerotherapy and in whom a hemodynamic evaluation could be performed within the first 24 hours of admission, were considered for inclusion in this study. All patients were admitted to our hospital with hematemesis and/or melena. Diagnostic endoscopy was performed between 1 and 5 hours later, as soon as safely possible after initial resuscitation. Variceal hemorrhage was defined when endoscopy showed active hemorrhage

Metamorphic segregation is defined here as rhe formation and growth of bands or domains of different bulk compositions within an originally unbanded rock. It can result from an instability arising in some deforming rocks when diffusion... more

Metamorphic segregation is defined here as rhe formation and growth of bands or domains of different bulk compositions within an originally unbanded rock. It can result from an instability arising in some deforming rocks when diffusion transfer is significant. The nature of this instability is demonstrated separately for a differentiation associated with a crenulation (Type C) and for one without crenulation (Type L). However, the continuous gradation between the two types observed in nature is also expected from the models.

The so called Upper Río Negro Valley in Argentina is one of the most important fruit and vegetable production regions of the country. It comprises the lower valleys of the Limay and Neuquén rivers and the upper Negro river valley. Out of... more

The so called Upper Río Negro Valley in Argentina is one of the most important fruit and vegetable production regions of the country. It comprises the lower valleys of the Limay and Neuquén rivers and the upper Negro river valley. Out of the 41,671 cultivated hectares, 84.6% are cultivated with fruit trees, especially apple, pear and stone fruit trees. Late frosts occurring when trees are sensitive to low temperatures have a significant impact on the regional production. This study presents an analysis of air flow characteristics in the Upper Río Negro Valley and its relationship with ambient air flow. To such effect, observations made when synoptic-scale weather patterns were favorable for radiative frosts (light wind and clear sky) or nocturnal temperature inversion in the lower layer were used. In the Negro river valley, both wind channeling and downward horizontal momentum transport from ambient wind were observed; in nighttime, very light wind events occurred, possibly associated with drainage winds from the nearby higher levels of the barda. In the Neuquén river valley, the prevailing effect appeared to be forced channeling, consistent with the results obtained in valleys where the synoptic scale wind crossed the axis of the valley. In the Limay river valley, the flow was observed to blow parallel to the longitudinal valley axis, possibly influenced by pressure gradient and forced channeling.

This work presents an experimental investigation of the ester oil ISO VG10refrigerant R134a mixture flashing flow with foam formation through a straight horizontal 3.22 mm-diameter-6.0 m-long tube. An experimental apparatus was designed... more

This work presents an experimental investigation of the ester oil ISO VG10refrigerant R134a mixture flashing flow with foam formation through a straight horizontal 3.22 mm-diameter-6.0 m-long tube. An experimental apparatus was designed to allow the measurement of both pressure and temperature profiles along the tube as well as the visualization of the flow patterns. Tests were performed at different mass flow rates, several refrigerant mass fractions at the inlet of the flow, and inlet mixture temperatures around 28 and 39 °C. A liquid mixture flow with constant temperature and pressure gradient could be noticed at the inlet of the tube. As the flow proceeded towards the exit of the tube the pressure drop produced a reduction of the refrigerant solubility in the oil yielding to formation of the first bubbles. Initially, small and few bubbles could be noticed and the flow behaved as a conventional two-phase flow. Eventually, the bubble population increased and foam flow was observed at the exit of the tube. Due to the great formation of bubbles, both the temperature and pressure gradient of the mixture were greatly reduced in this region of the flow.

VOL. 8. (I) The Laminar Boundary Layer (LBL) over a flat plate is a member of the family of similar flows over a wedge, which is famously known as Falkner-Skan Flows (FSF). Based on the available numerical results, this paper gives... more

VOL. 8. (I) The Laminar Boundary Layer (LBL) over a flat plate is a member of the family of similar flows over a wedge, which is famously known as Falkner-Skan Flows (FSF). Based on the available numerical results, this paper gives velocity distribution equations for LBL over a flat plate and FSF, which exhibit the influence of viscosity and external stream flow all over the depth of LBL. In the case of a Turbulent Boundary Layer (TBL), viscosity and boundary roughness affect lhe velocity distribution in the inner region, while the external stream flow influences the ollter region. The majority of TBL consists of the outer region in which the velocity saturates to external stream value. Available outer region models consist of superimposition of a wake function over the log-layer equation. The superimposed equation does not satisfy the boundary condition of free-stream velocity at infinite distance from the boundary. Presented herein is a generalized velocity distribution equation for both the inner and outer regions of the TBL. The equation is valid for both negative and positive pressure gradients. As the equation contains wall shear velocity, a generalized equation for wall shear velocity has also been given.

Simvastatin improves liver generation of nitric oxide and hepatic endothelial dysfunction in patients with cirrhosis, so it could be an effective therapy for portal hypertension. This randomized controlled trial evaluated the effects of... more

Simvastatin improves liver generation of nitric oxide and hepatic endothelial dysfunction in patients with cirrhosis, so it could be an effective therapy for portal hypertension. This randomized controlled trial evaluated the effects of continuous simvastatin administration on the hepatic venous pressure gradient (HVPG) and its safety in patients with cirrhosis and portal hypertension. Methods: Fifty-nine patients with cirrhosis and portal hypertension (HVPG >12 mm Hg) were randomized to groups that were given simvastatin 20 mg/day for 1 month (increased to 40 mg/day at day 15) or placebo in a double-blind clinical trial. Randomization was stratified according to whether the patient was being treated with ␤-adrenergic blockers. We studied splanchnic and systemic hemodynamics and variables of liver function and safety before and after 1 month of treatment. Results: Simvastatin significantly decreased HVPG (؊8.3%) without deleterious effects in systemic hemodynamics. HVPG decreases were observed in patients who were receiving ␤-adrenergic blockers (؊11.0%; P ‫؍‬ .033) and in those who were not (؊5.9%; P ‫؍‬ .013). Simvastatin improved hepatic, fractional, and intrinsic clearance of indocyanine green, showing an improvement in effective liver perfusion and function. No significant changes in HVPG and liver function were observed in patients receiving placebo. The number of patients with adverse events did not differ significantly between groups. No patient was withdrawn from the study based on adverse events. Conclusions: Simvastatin decreased HVPG and improved liver perfusion in patients with cirrhosis. These effects were additive with those of ␤-adrenergic blockers. The beneficial effects of simvastatin should be confirmed in longterm clinical trials for portal hypertension.

Evolution of sea fog has been investigated using three-dimensional Mesoscale Model 5 (MM5) simulations. The study focused on widespread fog-cloud layers advected along the California coastal waters during 14-16 April 1999. According to... more

Evolution of sea fog has been investigated using three-dimensional Mesoscale Model 5 (MM5) simulations. The study focused on widespread fog-cloud layers advected along the California coastal waters during 14-16 April 1999. According to analysis of the simulated trajectories, the intensity of air mass modification during this advection significantly depended on whether there were clouds along the trajectories and whether the modification took place over the land or ocean. The air mass, with its trajectory endpoint in the area where the fog was observed and simulated, gradually cooled despite the gradual increase in sea-surface temperature along the trajectory. Modelling results identified cloud-top cooling as a major determinant of marine-layer cooling and turbulence generation along the trajectories. Scale analysis showed that the radiative cooling term in the thermodynamic equation overpowered surface sensible and latent heat fluxes, and entrainment terms in cases of the transformation of marine clouds along the trajectories. Transformation of air masses along the trajectories without clouds and associated cloud-top cooling led to fog-free conditions at the endpoints of the trajectories over the ocean. The final impact on cloud-fog transition was determined by the interaction of synoptic and boundary-layer processes. Dissipation of sea fog was a consequence of a complex interplay between advection, synoptic evolution, and development of local circulations. Movement of the high-pressure system over land induced weakening of the along-shore advection and synoptic-pressure gradients, and allowed development of offshore flows that facilitated fog dissipation.

An integrated thermofluidic analysis of DNA hybridization, in the presence of combined electrokinetically and/or pressure-driven microchannel flows, is presented in this work. A comprehensive model is developed that combines bulk and... more

An integrated thermofluidic analysis of DNA hybridization, in the presence of combined electrokinetically and/or pressure-driven microchannel flows, is presented in this work. A comprehensive model is developed that combines bulk and surface transport of momentum, heat and solute with the pertinent hybridization kinetics, in a detailed manner. Results confirm that electrokinetic accumulation of DNA occurs within a few seconds or minutes, as compared to passive hybridization that could sometimes take several hours. Further, it is observed that by increasing the accumulation time, significantly higher concentration of DNA can be achieved at the capture probes. However, this eventually tends to attain a saturation state, due to a lesser probability of successful hybridization on account of a prior accumulation of target DNA molecules on the capture probe strands. While favorable pressure gradients augment DNA hybridization rates that are otherwise established by the electro-osmotic transport, adverse pressure gradients of comparable magnitude may turn out to be much less consequential in retarding the same. Such effects can be of potential significance in the designing of a microfluidic arrangement to achieve the fastest rate of DNA hybridization.

Rhinologists are often faced with the challenge of assessing nasal breathing from a functional point of view to derive effective therapeutic interventions. While the complex nasal anatomy can be revealed by visual inspection and medical... more

Rhinologists are often faced with the challenge of assessing nasal breathing from a functional point of view to derive effective therapeutic interventions. While the complex nasal anatomy can be revealed by visual inspection and medical imaging, only vague information is available regarding the nasal airflow itself: Rhinomanometry delivers rather unspecific integral information on the pressure gradient as well as on total flow and nasal flow resistance. In this article we demonstrate how the understanding of physiological nasal breathing can be improved by simulating and visually analyzing nasal airflow, based on an anatomically correct model of the upper human respiratory tract. In particular we demonstrate how various Information Visualization (InfoVis) techniques, such as a highly scalable implementation of parallel coordinates, time series visualizations, as well as unstructured grid multi-volume rendering, all integrated within a multiple linked views framework, can be utilized to gain a deeper understanding of nasal breathing. Evaluation is accomplished by visual exploration of spatio-temporal airflow characteristics that include not only information on flow features but also on accompanying quantities such as temperature and humidity. To our knowledge, this is the first in-depth visual exploration of the physiological function of the nose over several simulated breathing cycles under consideration of a complete model of the nasal airways, realistic boundary conditions, and all physically relevant time-varying quantities.

When bone is mechanically loaded, bone #uid #ow induces shear stresses on bone cells that have been proposed to be involved in bone's mechanosensory system. To investigate bone #uid #ow and strain-generated potentials, several theoretical... more

When bone is mechanically loaded, bone #uid #ow induces shear stresses on bone cells that have been proposed to be involved in bone's mechanosensory system. To investigate bone #uid #ow and strain-generated potentials, several theoretical models have been proposed to mimic oscillatory four-point bending experiments performed on thin bone specimens. While these previous models assume that the bone #uid relaxes across the specimen thickness, we hypothesize that the bone #uid relaxes primarily through the vascular porosity (osteonal canals) instead and develop a new poroelastic model that integrates the microstructural details of the lacunar}canalicular porosity, osteonal canals, and the osteonal cement lines. Local #uid pressure pro"les are obtained from the model, and we "nd two di!erent #uid relaxation behaviors in the bone specimen, depending on its microstructure: one associated with the connected osteonal canal system, through which bone #uid relaxes to the nearby osteonal canals; and one associated with the thickness of a homogeneous porous bone specimen (&1 mm in our model), through which bone #uid relaxes between the external surfaces of the bone specimen at relatively lower loading frequencies. Our results suggest that in osteonal bone specimens the #uid pressure response to cyclic loading is not sensitive to the permeability of the osteonal cement lines, while it is sensitive to the applied loading frequency. Our results also reveal that the #uid pressure gradients near the osteonal canals (and thus the #uid shear stresses acting on the nearby osteocytes) are signi"cantly ampli"ed at higher loading frequencies.

This work is focused on the analysis of non-linear flows of slightly compressible fluids in porous media not adequately described by Darcy's law. We study a class of generalized nonlinear momentum equations which covers all three... more

This work is focused on the analysis of non-linear flows of slightly compressible fluids in porous media not adequately described by Darcy's law. We study a class of generalized nonlinear momentum equations which covers all three well-known Forchheimer equations, the so-called two-term, power, and three-term laws. The non-linear Forchheimer equation is inverted to a non-linear Darcy equation with implicit permeability tensor depending on the pressure gradient. This results in a degenerate parabolic equation for the pressure. Two classes of boundary conditions are considered, given pressure and given total flux. In both cases they are allowed to be unbounded in time. The uniqueness, Lyapunov and asymptotic stabilities, and other long-time dynamical features of the corresponding initial boundary value problems are analyzed. The results obtained in this paper have clear hydrodynamic interpretations and can be used for quantitative evaluation of engineering parameters. Some numerical simulations are also included.

The effect of a drag-reducing polymer (DRP) in the water phase during horizontal oil-water flow was investigated in a 14 mm ID acrylic pipe. Oil (5.5 mPa s, 828 kg/m 3 ) and a co-polymer (Magnafloc 1011) of polyacrylamide and sodium... more

The effect of a drag-reducing polymer (DRP) in the water phase during horizontal oil-water flow was investigated in a 14 mm ID acrylic pipe. Oil (5.5 mPa s, 828 kg/m 3 ) and a co-polymer (Magnafloc 1011) of polyacrylamide and sodium acrylate were used. Two polymer concentrations were tested, 20 ppm and 50 ppm, made from a 1000 ppm master solution. The results showed a strong effect of DRP on flow patterns. The presence of DRP extended the region of stratified flow and delayed transition to slug flow. The addition of the polymer clearly damped interfacial waves. Annular flow changed in all cases investigated to stratified or dual continuous flow, while slug flow changed in most cases to stratified flow. In the cases where the slug and bubble flow patterns still appeared after the addition of the polymer, the oil slugs and bubbles were seen to flow closer together than in the flow without the polymer. The DRP caused a decrease in pressure gradient and a maximum drag reduction of about 50% was found when the polymer was introduced into annular flow. The height of the interface and the water hold up increased with DRP. There were no large differences on pressure gradient and hold up between the two DRP concentrations. Using a two-fluid model it was found that the addition of the polymer results in a decrease in both the interfacial and the water wall shear stresses.

Blood in the splanchnic vasculature can be transferred to the extremities. We quantified such blood shifts in normal subjects by measuring trunk volume by optoelectronic plethysmography, simultaneously with changes in body volume by whole... more

Blood in the splanchnic vasculature can be transferred to the extremities. We quantified such blood shifts in normal subjects by measuring trunk volume by optoelectronic plethysmography, simultaneously with changes in body volume by whole body plethysmography during contractions of the diaphragm and abdominal muscles. Trunk volume changes with blood shifts, but body volume does not so that the blood volume shifted between trunk and extremities (Vbs) is the difference between changes in trunk and body volume. This is so because both trunk and body volume change identically with breathing and gas expansion or compression. During tidal breathing Vbs was 50-75 ml with an ejection fraction of 4-6% and an output of 750-1500 ml/min. Step increases in abdominal pressure resulted in rapid emptying presumably from the liver with a time constant of 0.6160.1SE sec. followed by slower flow from non-hepatic viscera. The filling time constant was 0.5760.09SE sec. Splanchnic emptying shifted up to 650 ml blood. With emptying, the increased hepatic vein flow increases the blood pressure at its entry into the inferior vena cava (IVC) and abolishes the pressure gradient producing flow between the femoral vein and the IVC inducing blood pooling in the legs. The findings are important for exercise because the larger the Vbs the greater the perfusion of locomotor muscles. During asystolic cardiac arrest we calculate that appropriate timing of abdominal compression could produce an output of 6 L/min. so that the abdominal circulatory pump might act as an auxiliary heart.

We have compared 14 different sediment incubation chambers, most of them were used on bottom landers. Measurements of mixing time, pressure gradients at the bottom and Diffusive Boundary Layer thickness (DBL) were used to describe the... more

We have compared 14 different sediment incubation chambers, most of them were used on bottom landers. Measurements of mixing time, pressure gradients at the bottom and Diffusive Boundary Layer thickness (DBL) were used to describe the hydrodynamic properties of the chambers and sediment-water solute fluxes of silicate (34 replicates) and oxygen (23 replicates) during three subsequently repeated incubation experiments on a homogenized, macrofauna-free sediment. The silicate fluxes ranged from 0.24 to 1.01 mmol m−2 day−1 and the oxygen fluxes from 9.3 to 22.6 mmol m−2 day−1. There was no statistically significant correlation between measured fluxes and the chamber design or between measured fluxes and hydrodynamic settings suggesting that type of chamber was not important in these flux measurements. For verification of sediment homogeneity, 61 samples of meiofauna were taken and identified to major taxa. In addition, 13 sediment cores were collected, sectioned into 5-10-mm slices and separated into pore water and solid phase. The pore water profiles of dissolved silicate were used to calculate diffusive fluxes of silicate. These fluxes ranged from 0.63 to 0.87 mmol m−2 day−1. All of the collected sediment parameters indicated that the sediment homogenization process had been satisfactorily accomplished. Hydrodynamic variations inside and between chambers are a reflection of the chamber design and the stirring device. In general, pump stirrers with diffusers give a more even distribution of bottom currents and DBL thicknesses than paddle wheel-type stirrers. Most chambers display no or low static differential pressures when the water is mixed at rates of normal use. Consequently, there is a low risk of creating stirrer induced pressure effects on the measured fluxes. Centrally placed stirrers are preferable to off-center placed stirrers which are more difficult to map and do not seem to give any hydrodynamic advantages. A vertically rotating stirrer gives about five times lower static differential pressures at the same stirring speed as the same stirrer mounted horizontally. If the aim is to simulate or mimic resuspension at high flow velocities, it cannot be satisfactorily done in a chamber using a horizontal (standing) rotating impeller (as is the case for most chambers in use) due to the creation of unnatural conditions, i.e. large static differential pressures and pre-mature resuspension at certain locations in the chamber.

Turbulent flow over a sinusoidal solid wavy surface was investigated by a direct numerical simulation using a spectral element technique. The train of waves has an amplitude to wavelength ratio of 0.05. For the flow conditions (Re=hU... more

Turbulent flow over a sinusoidal solid wavy surface was investigated by a direct numerical simulation using a spectral element technique. The train of waves has an amplitude to wavelength ratio of 0.05. For the flow conditions (Re=hU b/2ν= 3460) considered, adverse pressure gradients were large enough to cause flow separation. Numerical results compare favorably with those of Hudson's (1993) measurements. Instantaneous flow fields show a large variation of the flow pattern in the spanwise direction in the separated bubble at a given time. A surprising result is the discovery of occasional velocity bursts which originate in the separated region and extend over large distances away from the wavy wall. Turbulence in this region is very different from that near a flat wall in that it is associated with a shear layer which is formed by flow separation.

Water removal from the gas diffusion layer (GDL) is crucial for the efficient operation of proton exchange membrane (PEM) fuel cell. Static pressure gradient caused by the fast reactant flow in the flow channel is one of the main... more

Water removal from the gas diffusion layer (GDL) is crucial for the efficient operation of proton exchange membrane (PEM) fuel cell. Static pressure gradient caused by the fast reactant flow in the flow channel is one of the main mechanisms of water removal from GDL. Reactant can leak or cross directly to the neighboring channel via the porous GDL in the cells with serpentine flow channel and many of its modifications. Such cross flow plays an important role for the removal of liquid water accumulated in the GDL especially under land area. To investigate the characteristics of liquid water behavior in the GDL under pressure gradient, the fibrous porous structure of the carbon paper is modeled by three dimensional impermeable cylinders randomly distributed in the in-plane directions and unsteady two-phase simulations are conducted. It is shown that the permeability from the numerical model matches well the experimental measurements of the common GDLs in the literature. The contact angle and pressure gradient are the key parameters that determine the initiation and the process of liquid water transport in the GDL which is initially wet with stagnant liquid water. It has been observed that the larger contact angle results in faster water removal from the GDL. Numerical simulations are performed for a wide range of pressure gradient with different contact angles to determine the minimum pressure gradient that initiates the liquid water transport in the GDL. It is found that the amount of pressure gradient caused by the cross flow is sufficient and effective to get rid of the liquid water accumulated in the GDL. The simulation results are also compared with experimental data in literature showing a good agreement. The characteristics of liquid water discharging from the gas diffusion layer are also described.

The extended Darcy's law is a commonly used equation for the description of immiscible two-phase flow in porous media. It dates back to the 1940s and is essentially an empirical relationship. According to the extended Darcy's law,... more

The extended Darcy's law is a commonly used equation for the description of immiscible two-phase flow in porous media. It dates back to the 1940s and is essentially an empirical relationship. According to the extended Darcy's law, pressure gradient and gravity are the only driving forces for the flow of each fluid. Within the last two decades, more advanced and physically based descriptions for multiphase flow in porous media have been developed. In this work, the extended Darcy's law is compared to a thermodynamically consistent approach which explicitly takes the important role of phase interfaces into account, both as entities and as parameters. In this theoretically derived approach, forces related to capillarity and interfaces appear as driving/resisting forces, in addition to the traditional terms. It turns out that the extended Darcy's law and the thermodynamically based approach are compatible if either (i) relative permeabilities are a function of saturation only, but capillary pressure is a function of saturation and specific interfacial area or (ii) relative permeabilities are a function of saturation and saturation gradients. Theoretical considerations suggest that the former alternative is only valid in case of reversible displacement while in the general case (irreversible displacement), the latter alternative is relevant.

Pressure gradients and the corresponding mass¯ow rates of ®ve dierent non-Newtonian¯uid foods: 1% solutions of sodium alginate and CMC, 1.5% CMC solution, two dierent tomato ketchups, oyster sauce, in four dierent diameter stainless steel... more

Pressure gradients and the corresponding mass¯ow rates of ®ve dierent non-Newtonian¯uid foods: 1% solutions of sodium alginate and CMC, 1.5% CMC solution, two dierent tomato ketchups, oyster sauce, in four dierent diameter stainless steel tubes ranging from 7.51 to 16.34 mm i.d. were recorded using a continuous recording type tube¯ow viscometer capable of operating in both transient and continuous¯ow modes. The¯uids were pseudoplastic in nature and followed the power law model. The¯ow was con®ned to the laminar¯ow regime and appreciable slippage occurred in all cases. Commercially available arti®cial neural networks based on back-propagation and generalized regression algorithm were applied to predict the pressure gradients in tube¯ow providing mass¯ow rate, consistency coecients and¯ow behavior indices obtained from a low shear rate rotational viscometer, mass density and tube diameters as inputs. The net predicted values closely followed the experimental ones with an average absolute error below 5.44%.

A dynamic model of extension in the Baltimore Canyon trough region of the central North Atlantic indicates that rifting proceeded in two stages and was controlled by laterally offset preexisting weaknesses within the middle crust and... more

A dynamic model of extension in the Baltimore Canyon trough region of the central North Atlantic indicates that rifting proceeded in two stages and was controlled by laterally offset preexisting weaknesses within the middle crust and upper mantle beneath the Appalachian orogen. The crustal weakness is hypothesized to result from thin-skinned thrust faulting in the Appalachian foreland, and the upper mantle weakness results from crustal thickening beneath the Appalachian hinterland. During the early stages of rifting (225-185 Ma), the loci of extension in the crust and mantle lie within the respective preweakened regions and are connected by a shear zone in the lower crust in a manner similar to whole lithosphere simple shear. Extension and thinning of the upper crust at this time results in the formation of Late Triassic-Early Jurassic rift basins in the Appalachian Piedmont and in the development of a horizontal pressure gradient which drives widespread ductile flow in the lower crust. The pressure gradient may also have influenced magma transport, moving melt generated beneath the eastern Baltimore Canyon trough into the more westerly Triassic basin province. During the late stages of rifting (185-175 Ma), extension in the crust shifts to a position overlying the preexisting upper mantle weakness as a result of thermal weakening by the upwelling asthenosphere in this region. The loci of extension in the crust and mantle are then coincident, and the style of rifting becomes similar to whole lithosphere pure shear. The shift in the locus of crustal extension is accompanied by the cessation of subsidence and volcanism in the Triassic basins, the onset of rapid subsidence in the Baltimore Canyon trough, and the formation of a basement hinge zone separating the highly thinned crust beneath the Baltimore Canyon trough from moderately thinned crust beneath the Triassic basin province. Extension becomes increasingly focused beneath the eastern edge of the Baltimore Canyon trough until the onset of seafloor spreading, approximately 50 m.y. after rifting began. The resulting conjugate margins are asymmetric, with most of the region encompassing the Triassic basins stranded on the North American plate. This paper presents a dynamic model of rifting in the Baltimore Canyon trough region of the central North Atlantic, focusing on the processes that controlled the timing of events and the formation of the structural, stratigraphic, and volcanic Paper number 91TC03012. 0278-7407/92/91TC-03012510.00 features of the margin. Because the Baltimore Canyon trough is one of the best studied rifted margins in the world, the model is well constrained by observations. The Baltimore Canyon trough region has many features common to the neighboring Carolina trough and Georges Bank basins, and in fact to many other rifted margins, so the model is generally applicable to much of the U.S. Atlantic margin and provides important constraints on the dynamics of rifting processes in general. Previous papers have examined the postrift subsidence of the Baltimore Canyon trough [Sawyer et al., 1982; Watts et al., 1982]. This paper examines the synrift development of the margin, emphasizing the role that preexisting weaknesses within the lithosphere played in determining the style of rifting. Preexisting weaknesses have previously been recognized as a controlling factor in the evolution of rifts [Braun and Beaumont, 1987; Bassi and Bonin, 1988a, b; Dunbar and Sawyer, 1988, 1989a], and the tendency of rifts to develop along orogenic zones indicates that orogeny is a major cause of lithospheric weakening. For example, extension in the North Atlantic focused within the Caledonian and Acadian orogens [Ziegler, 1989], the Iberian and Moroccan margins exploited preexisting Hercynian trends [Manspeizer, 1988], and the Rio Grande rift developed within the Laramide belt [Eaton, 1979, 1980]. At the U.S. Atlantic margin, extension focused within the Paleozoic Appalachian orogen. A finite element model presented below demonstrates that preexisting weaknesses within the orogen controlled the development of major structural features of the margin in the Baltimore Canyon trough region, including the early development of onshore Triassic basins, later formation of the deep offshore trough and basement hinge zone, coastal onlap, and crustal thickness variations across the margin. The model also accounts for the distribution of volcanism, the location of seafloor spreading initiation, and the duration of rifting. Features of the model that are broadly applicable to other rifted margins include asymmetric conjugate margins, abrupt seaward thinning of the crust beneath the basement hinge zone, a change in the locus of extension with time, and a transition from a simple shear style of deformation in the early stages of rifting to pure shear in the later stages. THE U.S. ATLANTIC MARGIN The prerift structure of the U.S. Atlantic margin was dominated by the Appalachian orogen as a consequence of intermittent orogenic activity throughout the Paleozoic. The most recent event was the Alleghanian orogeny, which resulted from the collision of North America and Africa during the late Carbonifereous to Early Permian [Hatcher, 1989; Taylor, 1989; Williams and Hatcher, 1982]. The Alleghanian foreland is characterized by thin-skinned thrusting, and encompasses the present Inner Piedmont, Blue Ridge, and Valley and Ridge provinces [Cooke et al., 1979; Harris and Bayer, 1979; Hatcher and Odom, 1980]. The hinterland encompasses the more easterly Avalon terrane and extends beneath the coastal plain and continental margin. Deep-seated thrusting within the hinterland is inferred from high-grade metamorphic facies in the easternmost Avalon exposures [Dallmeyer, 1989; Hatcher, 1989]. Rifting began 225-230 Ma with the formation of Triassic basins in the Appalachian Piedmont and beneath the present

The research work period covered herein lasted from 15. Nov 1990 to 14. Nov 1991 and presents the final one after several successive sponsoring periods. The Institut fur Raumfahrtsysteme (Space Systems) of the University Stuttgart is... more

The research work period covered herein lasted from 15. Nov 1990 to 14. Nov 1991 and presents the final one after several successive sponsoring periods. The Institut fur Raumfahrtsysteme (Space Systems) of the University Stuttgart is therefore indepted and would like to thank for the longyears support on the investigation of Basic Processes of Plasma Propulsion. We also hope to have contributed helped in understanding more profoundly the fundamental processes in and assessing more deeply the performance limits of coaxial magneto plasmadynamic arc thrusters.

Numerical simulation of fluid flow and transport processes in the subsurface must account for the presence of wells. The pressure at a gridblock that contains a well is different from the average pressure in that block and different from... more

Numerical simulation of fluid flow and transport processes in the subsurface must account for the presence of wells. The pressure at a gridblock that contains a well is different from the average pressure in that block and different from the flowing bottom hole pressure for the well (17). Various finite difference well models have been developed to account for the difference. This paper presents a systematical derivation of well models for other numerical methods such as standard finite element, control volume finite element, and mixed finite element methods. Numerical results for a simple well example illustrating local grid refinement effects are given to validate these well models. The well models have particular applications to groundwater hydrology and petroleum reservoirs. Numerical simulation of fluid flow and transport processes in the subsurface must account for the presence of wells. The pressure at a gridblock that contains a well is different from the average pressure in...

1] Reference evapotranspiration (RET), an indicator of atmospheric evaporating capability over a hypothetical reference surface, was calculated using the Penman-Monteith method for 75 stations across the Qinghai-Tibetan Plateau between... more

1] Reference evapotranspiration (RET), an indicator of atmospheric evaporating capability over a hypothetical reference surface, was calculated using the Penman-Monteith method for 75 stations across the Qinghai-Tibetan Plateau between 1971 and 2004. Generally, both annual and seasonal RET decreased for most part of the plateau during the study period. Multivariate linear models were used to determine the contributions of climate factors to RET change, including air temperature, air humidity, solar radiation, and wind speed. Spatial differences in the causes of RET change were detected by K-means clustering analysis. It indicates that wind speed predominated the changes of RET almost throughout the year, especially in the north of the study region, whereas radiation was the leading factor in the southeast, especially during the summertime. Although the recent warming trend over the plateau would have increased RET, the combined effect of the reduced wind speed and shortened sunshine duration negated the effect of rising temperature and caused RET to decrease in general. The significant decrease in surface wind speed corresponded to the decreasing trends of upper-air zonal wind and the decline of pressure gradient, possibly as a result of the recent warming.

a b s t r a c t r-coordinate ocean models are currently used in large basin and coastal seas simulations. When there is a strongly varying bathymetry, r-coordinate system solutions exhibit an intrinsic error in the horizontal pressure... more

a b s t r a c t r-coordinate ocean models are currently used in large basin and coastal seas simulations. When there is a strongly varying bathymetry, r-coordinate system solutions exhibit an intrinsic error in the horizontal pressure gradient (HPG) term. In this paper, a new, linear programming (LP) procedure has been proposed to deal with this problem. The success of the procedure in alleviating HPG error is gauged in comparison with two implementations of the Shapiro filter, the Mellor-Ezer-Oey (1994) algorithm (MEO), and the recently proposed procedure of Martinho and Batteen (MB). All are applied to three idealised cases and then to the Adriatic Sea, as modelled with the Rutgers version of ROMS. The comparisons reveal that even the improved Shapiro filter creates very large modifications to the bathymetry and should be avoided. The MB procedure performance is comparable to that of LP when the slope factor is larger than approximately 0.2, and inferior to it at smaller slope factor values. In cases where volume conservation is required, the MB method is inferior to both MEO and LP. The difference between the above-mentioned solutions is more pronounced for coarser grids.

Numerical simulation of an air and water two-phase flow in a micro tube is carried out. The focus is laid upon bubbly and slug flows with dry-out. An axisymmetric two-dimensional flow is assumed. The Phase-Field method is adopted to... more

Numerical simulation of an air and water two-phase flow in a micro tube is carried out. The focus is laid upon bubbly and slug flows with dry-out. An axisymmetric two-dimensional flow is assumed. The Phase-Field method is adopted to capture the interface, as well as to resolve the singularities arising at the rupture of liquid film and the consequent contact

In this study, analytical solutions are obtained to predict laminar heat-convection in a Couette-Poiseuille flow between two plane parallel plates with a simultaneous pressure gradient and an axial movement of the upper plate. A Newtonian... more

In this study, analytical solutions are obtained to predict laminar heat-convection in a Couette-Poiseuille flow between two plane parallel plates with a simultaneous pressure gradient and an axial movement of the upper plate. A Newtonian fluid with constant properties is considered with an emphasis on the viscous-dissipation effect. Both hydrodynamically and thermally fully-developed flow cases are investigated. The axial heat-conduction in the fluid is neglected. Two different orientations of the thermal boundary-conditions are considered: the constant heat-flux at the upper plate with an adiabatic lower plate (Case A) and the constant heat-flux at the lower plate with an adiabatic upper plate (Case B). For different values of the relative velocity of the upper plate, the effect of the modified Brinkman number on the temperature distribution and the Nusselt number are discussed. Comparison of the present analytical results for a special case with those available in the literature indicates an excellent agreement.

Water loss from the skin of term and preterm babies, nursed naked in incubators under neutral thermal conditions, was measured by a method based on estimating the water vapour pressure gradient close to the skin surface. 199 sets of... more

Water loss from the skin of term and preterm babies, nursed naked in incubators under neutral thermal conditions, was measured by a method based on estimating the water vapour pressure gradient close to the skin surface. 199 sets of measurements were made on 78 babies whose gestational ages ranged from 26 to 41 weeks, during the first 4 weeks of life. Babies of 34 to 41 weeks' gestation had high water losses in the first 4 hours after birth, which then fell to low levels averaging

Fracture gradient of formation is a key to determine the casing setting depth in drilling oil/gas wells. In addition, for projects, such as hydraulic fracturing and enhanced oil recovery injection, knowing the fracture gradient of the... more

Fracture gradient of formation is a key to determine the casing setting depth in drilling oil/gas wells. In addition, for projects, such as hydraulic fracturing and enhanced oil recovery injection, knowing the fracture gradient of the injection zone is necessary. Also, the pressure integrity of the exposed open hole dictates the maximum allowed wellbore pressure. Several theoretical and operational methods for predicting fracture pressures have been developed and refined. A Leack-off test, which is the most reliable and common method for evaluating fracture pressure gradient, is performed by too much cost and time and also this test cannot be performed at several points. In the present article, a novel technique is presented to obtain an estimation of fracture pressure gradient from drilling operation data reports. This method is based on the effect of pore pressure and confining pressure on compressive strength of rock and, consequently, on drilling speed. Artificial neural networks were implemented to build a simulator for the rate of penetration and analyze the effect of hydrostatic pressure of wellbore on the rate of penetration. The presented method was performed on field data of an Iranian southern field and the results were satisfactorily close to the actual measured fracture pressure by an average error of about 1%.

i n t e r n a t i o n a l j o u r n a l o f g r e e n h o u s e g a s c o n t r o l 2 ( 2 0 0 8 ) 5 7 1 -5 8 1 a b s t r a c t CO 2 capture and geological storage (CCS) is considered as a viable option to mitigate greenhouse gas emissions... more

i n t e r n a t i o n a l j o u r n a l o f g r e e n h o u s e g a s c o n t r o l 2 ( 2 0 0 8 ) 5 7 1 -5 8 1 a b s t r a c t CO 2 capture and geological storage (CCS) is considered as a viable option to mitigate greenhouse gas emissions during the transition phase towards the use of clean and renewable energy. This paper concentrates on the transport of CO 2 between source (CO 2 capture at plants) and sink (geological storage reservoirs). In the cost estimation of CO 2 transport, the pipeline diameter plays an important role. In this respect, the paper reviews equations that were used in several reports on CO 2 pipeline transport. As some parameters are not taken into account in these equations, alternative formulas are proposed which calculate the proper inner diameter size based on flow rate, pressure drop per unit length, CO 2 density, CO 2 viscosity, pipeline material roughness and topographic height differences (the Darcy-Weisbach solution) and, in addition, on the amount and type of bends (the Manning solution). Comparison between calculated diameters using the reviewed and the proposed equations demonstrate the important influence of elevation difference (which is not considered in the reviewed equations) and pipeline material roughness-related factor on the calculated diameter. Concerning the latter, it is suggested that a Darcy-Weisbach roughness height of 0.045 mm better corresponds to a Manning factor of 0.009 than higher Manning values previously proposed in literature. Comparison with the actual diameter of the Weyburn pipeline confirms the accuracy of the proposed equations. Comparison with other existing CO 2 pipelines (without pressure information) indicate that the pipelines are designed for lower pressure gradients than 25 Pa/m or for (future) higher flow rates. The proposed Manning equation is implemented in an economic least-cost route planner in order to obtain the best economic solution for pipeline trajectory and corresponding diameter.

We use quality controlled in-situ meteorological observations collected at Antarctic research stations over the last 60 years to carry out the first continent-wide investigation into the occurrence, variability and trend in strong wind... more

We use quality controlled in-situ meteorological observations collected at Antarctic research stations over the last 60 years to carry out the first continent-wide investigation into the occurrence, variability and trend in strong wind events (SWEs). Reanalysis/analysis fields are used to examine the synoptic background in which such events take place. SWEs are a feature of the extended winter season and involve a complex interaction between the downslope buoyancy forcing and the pressure gradient force from synoptic-scale cyclones. Around the coast of East Antarctica the significant majority of SWEs are associated with enhancement of the downslope katabatic flow by the broadscale synoptic circulation, involving a deepening of pressure off the coast and an increase of pressure inland. Orientation of the valleys in relation to the cyclone track is critical in enabling enhancement of the katabatic winds. Casey, Mawson and Dumont d'Urville stations report the greatest number of winds of storm force and stronger. Inter-annual variability of SWE numbers is large. Trends in the number of winter strong wind reports are small. The greatest statistically significant increase in wind speed since the 1950s has been at Faraday/Vernadsky and Syowa 2 stations. The largest wind speed increases since 1979 have been at Davis and Mawson stations. Comparison with high resolution numerical simulations showed that the reanalysis/analysis fields are able to capture the large scale synoptic features and the associated enhancement of the katabatic flow, but underestimated the observed wind speed if it was strongly influenced by local topographical conditions.

Oil-water two-phase flow experiments were conducted in a 15 m long, 8.28 cm diameter, inclinable steel pipe using mineral oil (density of 830 kg/m 3 and viscosity of 7.5 mPa s) and brine (density of 1060 kg/m 3 and viscosity of 0.8 mPa... more

Oil-water two-phase flow experiments were conducted in a 15 m long, 8.28 cm diameter, inclinable steel pipe using mineral oil (density of 830 kg/m 3 and viscosity of 7.5 mPa s) and brine (density of 1060 kg/m 3 and viscosity of 0.8 mPa s). Steady-state data on flow patterns, two-phase pressure gradient and holdup were obtained over the entire range of flow rates for pipe inclinations of À5°, À2°, À1.5°, 0°, 1°, 2°and 5°. The characterization of flow patterns and identification of their boundaries was achieved via observation of recorded movies and by analysis of the relative deviation from the homogeneous behavior. A stratified wavy flow pattern with no mixing at the interface was identified in downward and upward flow. Two gamma-ray densitometers allowed for accurate measurement of the absolute in situ volumetric fraction (holdup) of each phase for all flow patterns. Extensive results of holdup and two-phase pressure gradient as a function of the superficial velocities, flow pattern and inclinations are reported. The new experimental data are compared with results of a flow pattern dependent prediction model, which uses the area-averaged steady-state two-fluid model for stratified flow and the homogeneous model for dispersed flow. Prediction accuracies for oil/water holdups and pressure gradients are presented as function of pipe inclination for all flow patterns observed. There is scope for improvement for in particular dual-continuous flow patterns.

Air pollution episodes in urban coastal areas follow certain pre-determined patterns, being associated with certain local meteorological conditions and emission of primary pollutants. In this study, the synoptic and local scale... more

Air pollution episodes in urban coastal areas follow certain pre-determined patterns, being associated with certain local meteorological conditions and emission of primary pollutants. In this study, the synoptic and local scale atmospheric circulation that prevails during air pollution episodes in a coastal major city in Greece, Thessaloniki, is examined for a period of 15 years (1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004). The study signifies the importance of studying air pollution meteorological patterns between coastal areas with different terrain characteristics. For Thessaloniki, it was found that the episodes occur mainly during the cold period of the year, while four types of synoptic scale circulation were recognized (I, II, III, IV) and five patterns of the local scale circulation (A1, A2, B1, B2 and B3). The highest percentage of episodes is associated with the presence of an anticyclone over the northern Greece (types I and IV), being characterized by weak or very weak surface pressure gradient intensity, according to the position and extension of the anticyclone. Moreover, a temperature increase of at least 1°C during the previous 3 days is required in the lower troposphere. Consistent with the synoptic conditions, the development of the sea breeze plays a crucial role in the occurrence of the episodes, even in the cold period of the year, when the sea breeze can still develop with smaller frequency and intensity. Finally, it was found that a small number of episodes is related with the advection of polluted air masses from the industrial area in the northwest of the city and from the Eordaia area in the west, which is the largest lignite producing area of Balkans.

The gas lift is a widely used method of artificial lift, for which there are different options. Previous work in this regard to optimize the continuous gas lift (CGL) of Aghajari oil field located in the south of Iran was promising. A new... more

The gas lift is a widely used method of artificial lift, for which there are different options. Previous work in this regard to optimize the continuous gas lift (CGL) of Aghajari oil field located in the south of Iran was promising. A new intermittent gas lift (IGL) simulator is developed considering all aspects of this artificial lift method including the temperature differences between the injected and produced fluids. In order to check the validity of the mathematical model, the intermittent gas lift experimental data, i.e. pressure gradient as a function of time, of Brown [Gas Lift Theory and Practice, Prentice-Hall, Englewood Cliffs, NJ (1967)] were compared with the results obtained by this simulator. The calculated pressure gradient matched well with the experimental data. For the same system, the pressure under the liquid slug as a function of opening pressure of gas lift valve was also calculated and compared with the experimental data. It was found that ignoring the heat transfer between the injected gas and the liquid slug shifts away the calculated results from the experimental points and therefore, decreasing the accuracy of the model. The model then was used to study the Aghajari oil field. The results showed that the production of this oil field will increase significantly using intermittent gas lift method. D

By using spatial averaging methods, in this work we derive a Darcy's-type law from a fractional Newton's law of viscosity, which is intended to describe shear stress phenomena in non-homogeneous porous media. As a prerequisite towards... more

By using spatial averaging methods, in this work we derive a Darcy's-type law from a fractional Newton's law of viscosity, which is intended to describe shear stress phenomena in non-homogeneous porous media. As a prerequisite towards this end, we derive an extension of the spatial averaging theorem for fractional-order gradients. The usage of this tool for averaging continuity and momentum equations yields a Darcy's law with three contributions: (i) similar to the classical Darcy's law, a term depending on macroscopic pressure gradients and gravitational forces; (ii) a fractional convective term induced by spatial porosity gradients; and (iii) a fractional Brinkman-type correction. In the three cases, the corresponding permeability tensors should be computed from a fractional boundary-value problem within a representative cell. Consistency of the resulting Darcy's-type law is demonstrated by showing that it is reduced to the classical one in the case of integer-order velocity gradients and homogeneous porous media. r 2006 Published by Elsevier B.V. (J. Alvarez-Ramirez).

The control of boundary layer separation on the suction side of an airfoil at high angle of attack has been renewed by the possibilities of active control. Nevertheless, such an active control needs a deep understanding of the flow to... more

The control of boundary layer separation on the suction side of an airfoil at high angle of attack has been renewed by the possibilities of active control. Nevertheless, such an active control needs a deep understanding of the flow to manipulate and of the actuating flow, both being 3D and unsteady. For that purpose, a model experiment has been designed in the frame of a coordinated European project called AEROMEMS, with a simpler (2D) geometry and with a dilatation of the scales in order to be able to characterize the actuation flow. This model is a bump in a boundary layer wind tunnel, which mimics the adverse pressure gradient on the suction side of an airfoil at the verge of separation. The present contribution describes preliminary tests done to optimize standard passive devices before testing active systems. The optimization was done with hot film shear stress probes, the characterization with hot wire anemometry and PIV. The results show quantitatively the improvement brought by the passive devices in terms of skin friction. They also show the mechanism which is at the origin of this improvement. The next step of the project is to replace passive devices by synthetic jets.

The present work analyzes the effects of a solid boundary and the inertial forces on flow and heat transfer in porous media. Specific attention is given to flow through a porous medium in the vicinity of an impermeable boundary. The local... more

The present work analyzes the effects of a solid boundary and the inertial forces on flow and heat transfer in porous media. Specific attention is given to flow through a porous medium in the vicinity of an impermeable boundary. The local volume-averaging technique has been utilized to establish the governing equations, along with an indication of physical limitations and assumptions made in the course of this development. A numerical scheme for the governing equations has been developed to investigate the velocity and temperature fields inside a porous medium near an impermeable boundary, and a new concept of the momentum boundary layer central to the numerical routine is presented. The boundary and inertial effects are characterized in terms of three dimensionless groups, and these effects are shown to be more pronounced in highly permeable media, high Prandtl-number fluids, large pressure gradients, and in the region close to the leading edge of the flow boundary layer.On analyse les effets d'une frontière solide et des forces d'inertie sur l'écoulement et le transfert thermique dans les milieux poreux. Une attention particulière est portée à l'écoulement à travers un milieu poreux au voisinage d'une frontière imperméable. La technique de la moyenne locale en volume a été utilisée pour établir les équations avec une indication sur les limitations physiques et sur les hypothèses. Un schéma numérique a été développé pour déterminer les champs de vitesse et de température dans un milieu poreux près d'une frontière imperméable et un nouveau concept de couche limite placé au centre de la routine numérique est présenté. Les effets de limite et d'inertie sont caractérisés par trois groupements sans dimension et ces effets sont montrés plus prononcés dans les milieux fortement imperméables, pour des fluides à grand nombre de Prandtl, pour des forts gradients de pression et dans la région proche du bord d'attaque de la couche limite de l'écoulement.Die vorliegende Arbeit untersucht die Einflüsse einer festen Begrenzung und der Trägheitskräfte auf die Strömung und den Wärmeübergang in porösen Medien.Besondere Aufmerksamkeit wird der Strömung durch ein poröses Medium in der Nähe einer undurchlässigen Begrenzung gewidmet. Das Verfahren der örtlichen Volumen-Mittelbildung wurde bei der Aufstellung der beschreibenden Gleichungen und der Angabe der physikalischen Vereinfachungen und Näherungen verwendet, die im Verlauf der Herleitung getroffen wurden. Für die beschreibenden Gleichungen wurde ein numerisches Lösungsverfahren entwickelt, um die Geschwindigkeits- und Temperaturfelder innerhalb des porösen Mediums nahe einer undurchlässigen Begrenzung zu untersuchen. Für die Impulsgrenzschicht wird ein neues Konzept innerhalb des numerischen Verfahrens vorgestellt. Die Rand- und Trägheitseinflüsse werden durch drei dimensionslose Gruppen ausgedrückt. Diese Einflüsse erweisen sich als stärker ausgeprägt in gut durchlässigen Medien, in Fluiden mit groβer Prandtl-Zahl, bei groβen Druckgradienten und im Bereich nahe der Vorderkante der Strömungsgrenzschicht.AнAлизиpyeтcя Bлияниe тBepдoй гpAницы и cил инepции нA тeчeниe и тeплoпepeнoc B пopиcтыч cpeдAч. ocoбoe BнимAниe yдeляeтcя тeчeнию чepeз пopиcтyю cpeдy Bблизи нeпpoницAeмoй гpAницы. Для BыBoдA ocнoBныч ypABнeний иcпoльзyeтcя мeтoд лoкAльнoгo ycpeднeния пo oбъeмy. yкAзыBAeтcя нA физичecкиe oгpAничeния и пpинятыe дoпyщeния. c цeлью иccлeдoBAния пoлeй cкopocти и тeмпepAтypы Bнyтpи пopиcтoй cpeды y нeпpoницAeмoй гpAницы pAзpAбoтAнA чиcлeннAя cчeмA для peшeния ocнoBныч ypABнeний и BBeдeнo нoBoe пoнятиe пoгpAничнoгo cлoя кoличecтBA дBизeния, яBляющeгocя ocнoBным B чиcлeннoй cчeмe peшeния. Bлияниe гpAницы и cил инepции BыpAзAeтcя c пoмoщью тpeч бeзpAзмepныч кoмплeкcoB. ПoкAзAнo, чтo Bлияниe бoлee зAмeтнo B cpeдAч c Bыcoкoй пpoницAeмocтью, зидкocтяч c бoльшим чиcлoм ПpAндтля, бoльшими гpAдиeнтAми дABлeния и B oблAcти пoгpAничнoгo cлoя y пepeднeй кpoмки.

A new method is given for the calculation of the drag force, pressure gradient and friction factor for sedimenting suspensions and fluidised beds of uniform spherical particles in liquids. A comparison is made with previous methods of... more

A new method is given for the calculation of the drag force, pressure gradient and friction factor for sedimenting suspensions and fluidised beds of uniform spherical particles in liquids. A comparison is made with previous methods of correlation. In addition, the relationship between the various drag coefficients for particles in concentrated suspensions is clarified. The importance of the particle-to-vessel diameter ratio in determining the value of the drag coefficient is established.

In the case of high overexpansion, the exhaust jet of the supersonic nozzle of rocket engines separates from nozzle wall because of the large adverse pressure gradient. Correspondingly, to match the pressure of the separated flow region,... more

In the case of high overexpansion, the exhaust jet of the supersonic nozzle of rocket engines separates from nozzle wall because of the large adverse pressure gradient. Correspondingly, to match the pressure of the separated flow region, an oblique shock is generated which evolves through the supersonic jet starting approximately at the separation point. This shock reflects on the nozzle axis with a Mach reflection. Thus, a peculiar Mach reflection takes place whose features depend on the upstream flow conditions, which are usually not uniform. The expected features of Mach reflection may become much difficult to predict, depending on the nozzle shape and the position of the separation point along the divergent section of the nozzle.

This thesis begins with a study of the origin of cosmological fluctuations with special attention to those cases in which the non-Gaussian correlation functions are large. The analysis shows that perturbations from an almost massless... more

This thesis begins with a study of the origin of cosmological fluctuations with special attention to those cases in which the non-Gaussian correlation functions are large. The analysis shows that perturbations from an almost massless auxiliary field generically produce large values of the non-linear parameter f_NL. The effects of including non-Gaussian correlation functions in the statistics of cosmological structure are explored by constructing a non-Gaussian probability distribution function (PDF). Such PDF is derived for the comoving curvature perturbation from first principles in the context of quantum field theory, with n-point correlation functions as the only input. The non-Gaussian PDF is then used to explore two important problems in the physics of primordial black holes (PBHs): First, to compute non-Gaussian corrections to the number of PBHs generated from the primordial curvature fluctuations. The second application concerns new cosmological observables. The formation of PBHs is known to depend on two main physical characteristics: the strength of the gravitational field produced by the initial curvature inhomogeneity and the pressure gradient at the edge of the curvature configuration. We account for the probability of finding these configurations by using two parameters: The amplitude of the inhomogeneity and its second radial derivative, evaluated at the centre of the configuration. The implications of the derived probability for the fraction of mass in the universe in the form of PBHs are discussed.

Numerical simulations were performed for laminar fluid flow in a porous tube with variable wall suction, a model of a crossflow filtration tubular membrane. The variable wall suction is described by Darcy's law, which relates the pressure... more

Numerical simulations were performed for laminar fluid flow in a porous tube with variable wall suction, a model of a crossflow filtration tubular membrane. The variable wall suction is described by Darcy's law, which relates the pressure gradient within a flow stream to the flow rates through the permeable wall. The feed stream in the tube, which flows mainly tangentially to the porous wall, is modelled by the Navier-Stokes equations. A method of coupling the Navier-Stokes and the Darcy equations in a solution scheme was considered to develop a fluid dynamic model of crossflow filtration. The governing equations were solved numerically using a finite difference scheme. The solution depends on both the Reynolds axial and filtration number. Some assumptions adopted in simplified models are discussed.