Low Pressure Gradient Research Articles

Immediate Outcomes of Aortic Valve Neocuspidization with Glutaraldehyde-treated Autologous Pericardium: a Multicenter Study.

ObjectiveTo determine the feasibility of aortic valve neocuspidization (AVNeo) with glutaraldehyde-treated autologous pericardium.MethodsOne hundred and seventy (170) AVNeo (84 males/86 females) were performed from January 2017 through March 2019 in three centers. All the records were prospectively collected and retrospectively reviewed.ResultsMost of the patients were older than 60 years and over 95% were operated for aortic stenosis. Preoperatively, pressure gradients were 69.9±21.3 mmHg for patients with aortic stenosis, and the surgical annular diameter was 21.0±2.0 mm for all patients. Effective orifice area (EOA) and indexed EOA (iEOA) averaged 0.7±0.3 cm2 and 0.4±0.2 cm2/m2 for patients with aortic stenosis before surgery, respectively. There was no conversion to prosthetic aortic valve replacement. Eight patients needed reoperation for bleeding, but no patient needed reoperation due to early infective endocarditis. There were five in-hospital deaths due to noncardiac cause. Compared to preoperative echocardiographic measurements, postoperative peak pressure gradient decreased significantly (-58.7±1.7 mmHg; P<0.001) and reached 11.2±5.6 mmHg, and mean pressure gradient also decreased significantly (-36.8±1.1 mmHg; P<0.001) and reached 6.0±3.5 mmHg. Accordingly, EOA and iEOA increased significantly 2.0 cm2 and 1.0 cm2/m2 (both P<0.001) to reach 2.7±0.6 cm2 and 1.4±0.3 cm2/m2 after surgery, respectively, with minimal significant aortic regurgitation (0.6% > mild).ConclusionAVNeo is feasible and reproducible with good clinical results. Hemodynamically, AVNeo produces immediate postoperative low-pressure gradients, large EOA, and minimal regurgitation of the aortic valve. Further studies are necessary to evaluate mid- and long-term evolution.

Foldable multiple-energies harvester consisting of a thin ion-exchange membrane prepared by a two-step interfacial polymerization

This study reports on the fabrication of a multiple-energies harvester capable of producing electrical energy from membrane potential induced by mechanical and thermal energy sources. A two-step interfacial polymerization method was introduced to synthesize a thin ion-exchange membrane, in which membrane potential can be induced and maintained under low pressure and temperature gradients. To increase the potential applications of this work, a flexible, compressible, and foldable energy harvester was developed by incorporating the as-synthesized membranes. The investigation into the mechanical energy output of the harvester found it to have a sensitivity of 1.74 V/N and a power density of 4.26 mW/m2. Equally, when a thermal gradient was applied to the harvester, results showed a Seebeck coefficient of approximately 1 mV/K and a power density of 0.16 mW/m2.

Dynamic Characteristics of Offshore Natural Gas Hydrate Dissociation by Depressurization in Marine Sediments

Dynamic characteristics of offshore natural gas hydrate (NGH) dissociation will provide the theoretical basis to analyze technical issues of oceanic hydrate exploitation. A mathematical model is developed to simulate offshore NGH dissociation by depressurization in marine sediments. Different phase combination statuses are involved in the process of NGH dissociation by taking ice melting and water freezing into account. The proposed methodology can analyze the processes of hydrate and water phase transitions, decomposition kinetics and thermodynamics, viscosity and permeability, ice-water phase equilibrium, and natural gas and water production. A set of an experimental system is built and consists of one 3-D visual reactor vessel, one isothermal seawater vessel, one natural gas and water separator, and one data acquisition unit. The experiments on offshore NGH dissociation by depressurization in 3-D marine sediments are carried out, and this methodology is validated against the full-scale experimental data measured. The results show that during the prophase, natural gas flow is preceded by water flow into the production wellbore and natural gas occupies more continuous flow channels than water under a large pressure gradient. Then, the natural gas flow rate begins to decline accompanied by an increase of water production. During the second phase, natural gas flow rate decreases slowly because of the decreased temperature of hydrate-bearing formation and low pressure gradient. The lower the intrinsic permeability in marine sediments, the later the water flow rate reaches the peak production. And the space interval of the production wellbore should be enlarged by an increase of the intrinsic permeability. The stable period of natural gas production enhances, and the water flow rate reduces with the increase of bottom-hole pressure in production wellbores. The main reason is the slow offshore NGH dissociation under the low producing pressure and the restriction of heat conductivity under the low temperature.

Quantitative Appraisal Through MRI of Breast Tissue Thickness and Implant Projection After Breast Augmentation with High- and Extra-High-Profile Round Implants.

Changes in breast tissue thickness and in implant projection 5years after augmentation with high- and extra-high-profile round implants were measured through MRI with a DICOM standard viewer. Twenty-four females with small-volume breast asymmetry without hypertrophy or ptosis underwent subfascial breast augmentation for cosmetic purposes, by using micro-textured soft cohesive silicone gel-filled round implants, from a single manufacturer. MRI measured the linear antero-posterior dimension of breast tissue thickness and projection of the implants. Statistical analysis of data was performed by Pearson correlation coefficient, line graph, and scatter diagram. The "r" of Pearson for right and left breasts indicated a significant correlation between the breast tissue thickness before and 5years after augmentation. Closeness of the lines displayed in the line graph indicated strong linear positive correlation between the breast tissue thicknesses. The "r" values for projection of right and left implants indicated a significant correlation between the projection standardized by the manufacturer and that encountered 5years after augmentation with high- and extra-high-profile round implants. A scatter diagram of data indicated a strong positive correlation between implant projection standardized by the manufacturer and that encountered 5 years after augmentation, on both breasts. Soft cohesive silicone gel-filled high- and extra-high-profile round implants supported breast tissue compressing without significant loss of the implant projection. Despite the consistency of the soft cohesive silicone gel, the implant softness and flexibility were preserved, resulting in low-pressure gradient over the mammary parenchyma without significant changes of the breast tissue thickness. This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266.

Pore network model for retrograde gas flow in porous media

Gas well deliverability in retrograde gas reservoirs is affected by the appearance of a liquid phase near the wellbore when the bottom-hole pressure is below the dew point. At low enough pressure gradients, the liquid phase may bridge the pore throats, blocking the gas phase flow, leading to what is known as condensate blockage. If the pressure gradient is above a critical value, the capillary forces cannot block the gas flow and the condensate bridges are displaced. Gas production can be estimated using reservoir-scale simulation of the retrograde gas flow near producing wells using relative permeability curves that take into account the different physical phenomena and their dependence on local pressure, temperature and pressure gradient (capillary number). Pore-scale simulators can be used to model retrograde gas flow through porous media and to determine the gas and liquid phases relative permeability curves as a function of different flow parameters. Here, a fully-implicit isothermal compositional pore-scale network model is presented for retrograde gas flow in porous media at high capillary number, at which capillary forces are neglected. The flow pattern in each capillary changes from a single-phase gas flow to a two-phase core-annular flow regime when the pressure and the fluid composition reach a critical condition based on the thermodynamics of the fluid mixture. Newton's method is used to solve the non-linear flow and thermodynamics equations to calculate the pressure distribution and the flow rate of each component through the network. The model was used to predict gas-liquid relative permeability at high capillary number from a depletion process in a 2D network. The results show the evolution of the condensate saturation at different conditions and how the gas permeability falls abruptly at a certain condensate saturation.

Entropy generation in peristalsis with iron oxide

Entropy generation in peristaltic transport of nanomaterial with iron oxide is discussed. MHD and Joule heating are analyzed. Energy equation further consists of heat source/sink and viscous dissipation. Velocity slip and temperature jump conditions are also accounted. Large wavelength analysis is carried out. Results for velocity, temperature, pressure and entropy generation are presented graphically. Temperature decreases by increasing nanomaterials’ volume fraction. Larger velocity slip parameter yields lower pressure gradient. Entropy generation is increased for Hartmann number and nanoparticle volume fraction.

Autologous pericardial aortic valve reconstruction: early results and comparison with mechanical valve replacement

Autologous pericardial aortic valve reconstruction is an attractive option compared with prosthetic valve replacement due to the absence of anticoagulation, lower pressure gradient across the valve and excellent valve haemodynamics. We wanted to share the early results of autologous pericardial aortic valve reconstruction from our centre. The outcomes were compared with that of mechanical valve replacement. Between August 2016 to July 2018, 20 patients underwent autologous aortic valve reconstruction as per the techniques described by Ozaki et al. The surgery was done for aortic stenosis or regurgitation or a combination of both. All the surgeries were done by a single experienced surgeon. The results were compared to that of mechanical valve replacement. A comprehensive echocardiographic evaluation was done pre-discharge and at 6months after surgery. The postoperative echocardiographic parameters that were evaluated include aortic valve pressure gradient, aortic valve orifice area, ejection fraction, left ventricular diameters etc. All the parameters were measured by a single expert. Other perioperative parameters were also evaluated like cardiopulmonary bypass and cross-clamp time, need for anticoagulation etc. Statistical analysis was done using chi-square test/Mann-Whitney U test/independent sample t test. Autologous pericardial aortic valve reconstruction had several favourable results including lower aortic valve pressure gradient, bigger aortic valve orifice area etc. None of the patients required anticoagulation. There were no conversions to prosthetic valve replacement or reinterventions in the follow-up period. Autologous pericardial aortic valve reconstruction is a feasible alternative to prosthetic valve replacement with several advantages.

Concentration Characteristics of PM2.5 and the Causes of Heavy Air Pollution Events in Beijing During Autumn and Winter

To study the changing of characteristics and formation mechanisms of PM2.5 in Beijing during the last two years, particulate matter concentrations, weather conditions, and air-mass trajectories were analyzed during severe pollution episodes in fall and winter 2016-2017 using routine observations and the TrajStat model. Results showed that 13 heavy pollution events, each lasting at least two days, occurred in Beijing. Of these, approximately 61.5% occurred in winter, characterized by heavier pollution concentrations and longer durations than those occurring in autumn. A low-pressure gradient, high humidity, low surface wind speed, low boundary layer, and particular terrain (i. e., being surrounded by mountains on three sides) all contributed to the high occurrence frequency of severe pollution episodes in autumn and winter. During the pollution episodes, the average ratio of PM2.5 to PM10 reached 0.86. The air-masses during the accumulation stage were mainly transported from the northwest, west, southwest, and southeast of Beijing. The southwestern and southeastern transmission paths accounted for 21.6% of the total pollution load. In addition, the WRF-CAMx model was used to quantitatively analyze the contributions of local and external sources to the concentrations of PM2.5 in Beijing during 16-22 December 2016. Based on this analysis, PM2.5 contributions notably varied with different air-masses; in the case of southern air-masses, external sources dominated the PM2.5 concentrations in Beijing and local contributions decreased rapidly; in contrast, in the case of northwestern air-masses, the opposite pattern occurred. Overall, the contribution of local sources to PM2.5 concentrations in Beijing varied from 16.5% to 69.3% during the monitored pollution episodes.

Pore-network modelling of non-Darcy flow through heterogeneous porous media

A pore-network model (PNM) was developed to simulate non-Darcy flow through porous media. This paper investigates the impact of micro-scale heterogeneity of porous media on the inertial flow using pore-network modelling based on micro X-ray Computed Tomography (XCT) data. Laboratory experiments were carried out on a packed glass spheres sample at flow rates from 0.001 to 0.1 l/s. A pore-network was extracted from the 3D XCT scanned volume of the 50 mm diameter sample to verify the reliability of the model. The validated model was used to evaluate the role of micro-heterogeneity in natural rocks samples. The model was also used to investigate the effect of pore heterogeneity on the onset of the non-Darcy flow regime, and to estimate values of the Darcy permeability, Forchheimer coefficient and apparent permeability of the porous media. The numerical results show that the Reynold's number at which nonlinear flow occurs, is up to several orders of magnitude smaller for the heterogeneous porous domain in comparison with that for the homogeneous porous media. For the Estaillades carbonate rock sample, which has a high degree of heterogeneity, the resulting pressure distribution showed that the sample is composed of different zones, poorly connected to each other. The pressure values within each zone are nearly equal and this creates a number of stagnant zones within the sample and reduces the effective area for fluid flow. Consequently, the velocity distribution within the sample ranges from low, in stagnant zones, to high, at the connection between zones, where the inertial effects can be observed at a low pressure gradient.

METHOD DEVELOPMENT AND VALIDATION OF SELECTIVE AND HIGHLY SENSITIVE METHOD FOR DETERMINATION OF APIXABAN IN HUMAN PLASMA USING LIQUID CHROMATOGRAPHY-TANDEM MASS SPECTROMETRY

Objective: The present research work aims to develop and validate a selective and highly sensitive method for the determination of apixaban in human plasma using liquid chromatography-tandem mass spectrometry (LC-MS/MS).&#x0D; Methods: 200 µl of sodium heparin plasma samples were acidified and clean-up was performed by using solid-phase extraction (SPE). Apixaban 13C D3 was used as an internal standard (deuterated) to lower the relative matrix effects and a single step SPE was employed for sample clean up. 10 µl of SPE eluent was loaded onto Hypersil Beta Basic C18, 100×4.6 mm, 5 µ column for highly selective chromatographic separation using an isocratic mobile phase. 2 mmol ammonium acetate in water and acetonitrile were delivered by using a quaternary low-pressure gradient pump without premixing at a minimum flow rate of 0.50 ml/min.&#x0D; Results: LC-MS/MS method was successfully developed and validated to demonstrate the lowest detection limit of 0.05 ng/ml and a linear dynamic range from 1-250 ng/ml with r2&gt;0.99. Method development and validation results proved that the method is selective and highly sensitive for the determination of apixaban in human plasma using LC-MS/MS.&#x0D; Conclusion: Current method can be applied for both therapeutic drug monitoring (TDM) and pharmacokinetic (PK) study analysis.

A Note on the Forchheimer Approach for Estimating Soil Air Permeability

Important soil pore characteristics may be revealed from air permeability data. Recent research has quantified significant bias in estimates of the true Darcian permeability when frequently reported pneumatic pressure differences to drive the convective flow are used. An alternative to measurement at infinitesimal pressure differences is the Forchheimer approach, including a polynomial regression of corresponding values of the superficial air velocity (v) and the pressure gradient (G) applied. However, in situations with Darcian flow at low pressure gradients, this procedure may theoretically give an overestimation of the Darcian permeability. We constructed sample plastic cores (∼3.5 cm high) with different numbers (n = 1–19) of drilled tubes (holes of 1, 2, 4.5, and 5.8 mm diameter). Gas diffusivity was measured with a transient‐state method. Air permeability was measured at four pneumatic pressure differences (0.5, 1, 2, and 5 hPa), and the Darcian permeability was estimated with the Forchheimer approach. The ratio of apparent permeability at 5 hPa pneumatic pressure difference to the Forchheimer‐estimated Darcian permeability was significantly lower than unity (0.06–0.52) for all test samples. For all 1‐ and 2‐mm hole samples and for all four levels of G, the Reynolds number indicated nonturbulent flow, whereas turbulence was predicted for samples with 4.5‐ and 5.8‐mm diameters. A model combining relative gas diffusivity, air permeability, and the space available for gas transport indicated that the Forchheimer estimates of Darcian permeability were correct in situations with nonturbulent flow, whereas erroneous estimates were generated for the larger tubes with turbulence.Core Ideas We tested the Forchheimer approach for simple samples with straight tube pores. For nonturbulent flow conditions, the Forchheimer approach provides accurate results. Air flow at 5 hPa pressure difference implies significant underestimation of permeability.

Bubble-Population-Balance Modeling for Supercritical Carbon Dioxide Foam Enhanced-Oil-Recovery Processes: From Pore-Scale to Core-Scale and Field-Scale Events

Summary A bubble-population-balance foam-modeling technique is developed to investigate how carbon dioxide (CO2) foam behaves rheologically and propagates in a field-scale radial system. The modeling technique is based on pore-scale events and honors three different foam states (weak, strong, and intermediate) and two steady-state strong-foam-flow regimes (high- and low-quality) measured in core-scale experiments. The model parameters are first obtained from a fit to laboratory–coreflood experimental data, and then the mechanistic model is applied to different types of CO2 foams, ranging from gaseous to supercritical–CO2 foams, represented by various mobilization pressure gradients. The results from the fit to existing coreflood data show that a reasonable match can be made satisfying multiple constraints, such as hysteresis exerted by three foam states, non–Newtonian flow behavior caused by gas trapping and shear–thinning rheology, and bubble stability in different capillary pressure environments. When applied to field–scale scenarios, supercritical–CO2 foams requiring low mobilization pressure gradients propagate much farther than gaseous–CO2 foams, far enough to make use of promising supercritical–CO2 foams in the field. This study, for the first time, theoretically demonstrates why supercritical–CO2 foams should be preferred in the field compared with gaseous N2 or CO2 foams. The companion paper to extend this study to full–field–scale foam propagation in conjunction with gravity segregation is Izadi and Kam (2018).

Pore network modelling of fluid flow in tight formations considering boundary layer effect and media deformation

Fluid flow in tight formations deviates significantly from that in conventional reservoirs due to the micro/nano pores and high confining pressure; non-linear flow occurs at low displacement pressure gradients. The effects of boundary layer and media deformation on fluid flow cannot be ignored. In this paper, a stochastic network model with anisotropic topology is firstly constructed to represent the complex structures of tight formations. Subsequently, a series of network models with different physical scales are generated to obtain the representative elementary volume (REV). Based on the representative network model, a pore network flow model coupling the boundary layer effect with media deformation is developed. Iterative methods are proposed to solve the strong nonlinear mathematical model. Microtube experiments with different radii and three corresponding representative networks with different distributions of throat radii are employed to verify the accuracy and reliability of the numerical results. After the validation, pore scale flow regularity considering the boundary layer effect and media deformation of tight formations is obtained. Furthermore, the factors influencing pore scale flow, such as the effective stress, aspect ratio, coordination number and fluid viscosity, are studied. The results show that pore scale flow velocity considering the boundary layer effect and media deformation is smaller than that without considering the two mechanisms. Nonlinear flow occurs at low displacement pressure gradients because of the boundary layer effect, and media deformation aggravates the nonlinear phenomenon. Similarly, the absolute permeability also decreases when the two mechanisms are considered. It is no longer constant when boundary layer effect is taken into consideration, which increases with increasing displacement pressure gradient and reaches a plateau when the displacement pressure gradient is large enough. The effects of different factors on pore scale flow considering boundary layer effect and media deformation differ from their roles in conventional flow network models. Although the absolute permeability increases with decreasing aspect ratio and increasing coordination number, the value is far smaller than that without considering the two mechanisms because they result in the compression of the effective flow space. Additionally, the absolute permeability no longer remains unchanged when the variation of fluid viscosity is included. This indicates that the absolute permeability considering the boundary layer effect is not only affected by the pore structures but also the fluid viscosity. The methodology developed in this work provides a novel way to study the effects of boundary layer and media deformation on pore scale flow. The results provide new insights into fluid flow in tight formations.

Experimental investigation of heat transfer and fluid flow in octet-truss lattice geometry

This paper undertakes an experimental measurement of effective thermal conductivity, permeability, inertial coefficient, friction factor and wall convective heat transfer from octet truss lattice (OTL) structures as air-cooled compressive load-bearing light-weight heat sinks. Three aluminium alloy (AlSi10Mg) OTL structures of porosities 70.6%, 86.9% and 91.3% were fabricated using an additive manufacturing technique. Experimentally measured effective thermal conductivity, permeability, inertial coefficient, friction factor and wall Nusselt number for OTL structures are reported. Owing to high solid-to-fluid thermal conductivity ratio (∼4250), heat flows through the metal ligament and air independently and establishes steady-state, i.e. peripheral conduction coupling between solid metal and air is negligible. This is conferred from various theoretical and semi-empirical models from the literature. Friction factor and wall Nusselt number are characterised using a single length scale as against different length-scales often used in the literature. Flows through OTL structures seem to follow Forchheimer's flow law with negligible viscous contribution even at low pressure gradients. Further, the results are compared with previous analytical and experimental results for stochastic open-cellular foams. Owing to periodic nature of OTL structures, friction factor is lower than stochastic foams as mechanical dispersion effects are negligible. In conclusion, OTL structures are an excellent candidate for multifunctional applications where heat sinks are expected to bear compressive loads as well as dissipate heat while being lightweight.

Numerical investigation on wellbore temperature and pressure during carbon dioxide fracturing

In the present study, a transient 2D model considering the factors of CO2 thermal properties, viscous heating source, Joule-Thomson effect, axial and radial heat transfer is proposed to investigate the wellbore temperature and pressure. The temperature governing equations are solved numerically by using the method of Alternating Direction Implicit (ADI). Based on the law of conservation of energy, the viscous heating source is reasonably considered as the thermal energy converted by mechanical energy. The wellbore temperature and pressure are analyzed based on the investigation of the tubing and annulus injection well with the diameter of 50.3 mm and depth of 3600 m. The simulation results of our model are in good agreement with the field data. The analysis results show that the bottom hole temperature increases first and then decreases as injection time increases. Large injection rate results in low bottom hole pressure and negative axial pressure gradient. The critical depth increases as injection time increases. Compared with tubing injection, tubing and annulus injection decreases critical depth and increases the duration of wellbore unsteady flow and heat transfer process. The solid heat conduction in axial direction significantly affects the wellbore temperature and pressure distributions.

High-aspect-ratio single-crystalline AlN nanowires: Free-catalytic PVT growth and field-emission studies

Nanowires (NWs) with high aspect ratios (HARs) have great advantages for the fabrication of nanodevices. Herein, a high-efficiency and simple physical vapor transport (PVT) method is utilized to synthesize the uniform HAR aluminum nitride (AlN) NWs on a tungsten substrate without any catalysts. Synergistic effect of high surface energy of (0001), low saturated vapor pressure and large axial temperature gradient leads to the growth of HAR AlN NWs, which provides new insight for the growth of low-dimensional AlN nanostructures. The as-obtained AlN NWs with super HAR have hexagonal wurtzite structure, the diameters are about 100 nm and the lengths are over 200 μm. The AlN NWs have an intensive deep ultraviolet (DUV) absorption peak at 5.94 eV and exhibit a relatively high electrical conductivity (1.29 × 10−3 Ω−1 cm−1), low turn-on field (6.2 V μm−1) and threshold field (8.5 V μm−1). These results indicate that PVT method is efficient to fabricate HAR AlN NWs and the AlN NWs not only play an important role in DUV photoelectric devices but also have tremendous potential as a candidate for field-emission nanodevices.

1-Year Survival After TAVR of Patients With Low-Flow, Low-Gradient and High-Gradient Aortic Valve Stenosis in Matched Study Populations

ObjectivesThis study sought to examine whether the prognosis of patients with severe aortic stenosis (AS) having high versus low transvalvular mean pressure gradients (MPGs) is intrinsically different after transcatheter aortic valve replacement (TAVR), even after strict matching of baseline parameters. BackgroundPatients with low MPG are characterized by higher cardiovascular risk and more comorbidities than other AS patients are. MethodsIn this retrospective, single-center study involving 2,282 patients, 3 groups were derived according to the following criteria: 1) high-gradient AS (HG-AS) (MPG ≥40 mm Hg); 2) low-flow, low-gradient AS (LFLG-AS) (MPG <40 mm Hg, ejection fraction [EF] ≤40%, stroke volume index ≤35 ml/m2); 3) paradoxical LFLG-AS (pLFLG-AS) (similar to LFLG-AS but with EF ≥50%). Propensity score matching that included EF was used to compare 1-year survival. ResultsA total of 136 patients with HG-AS or LFLG-AS were identified. Kaplan-Meier survival curves were significantly different (p = 0.039), with death occurring in 11 versus 21 patients (hazard ratio: 2.12; 95% confidence interval: 1.02 to 4.39; p = 0.044), respectively. A total of 226 patients with HG-AS or pLFLG-AS were identified and here the curves were identical (p = 0.468), with death occurring in 18 versus 21 patients (hazard ratio: 1.26; 95% confidence interval: 0.67 to 2.38; p = 0.469). ConclusionsThis is the first study comparing survival after TAVR of patients with high versus low MPG in matched study populations. Mortality in patients with LFLG-AS was twice that of HG-AS patients. However, it appears that patients with pLFLG-AS might benefit from TAVR to the same extent as patients with HG-AS. There must be still unmasked factors that influence mortality of patients with LFLG-AS.

An accurate data base on laminar-to-turbulent transition in variable pressure gradient flows

An experimental data base on attached and separated-flow transitional boundary layers, under various levels of free-stream turbulence, Reynolds number and pressure gradient has been composed, with the aim of providing high-accuracy data and boundary conditions for development and testing of laminar-to-turbulent transition models. Experiments were carried out on the boundary layer developing on a flat plate installed within a channel with a variable opening angle. The test section imposes acceleration to the boundary layer followed by deceleration, which can be varied. Time Resolved Particle Image Velocimetry (TR-PIV) and Laser Doppler Velocimetry (LDV) data have been acquired on the boundary layer evolution, as well as the free-stream properties in terms of mean velocity and fluctuating velocity components. The overall test matrix spans 3 Reynolds numbers (70,000;150,000;220,000), 4 free-stream turbulence intensity levels (Tu=1.5;2.5;3.5;5%) and 4 pressure gradients (opening angle α=0;5;9;12deg). This large variation of flow parameters allows a gradual shift of the mode of transition from a bypass process in attached flow, occurring with zero (Blasius like) and mild adverse pressure gradients at high free-stream turbulence, to separated-flow transition, occurring with low Reynolds number, low free-stream turbulence intensity and elevated adverse pressure gradient.

CORONARY ARTERY SUBCLAVIAN STEAL SYNDROME WITH DILATED LEFT INTERNAL MAMMARY ARTERY

Coronary artery subclavian steal syndrome has an incidence of 0.2-0.7%. First described in 1974 by Harjola and Valle, it results from >70% stenosis of the proximal subclavian artery, most commonly due to atherosclerotic disease. This results in lower pressure gradient in the distal subclavian artery

A Two-Parametric Model for Gas Flow in Low-Permeable Porous Materials

Capillary flow at low pressure gradients is determined by the pore structure of the matrix and the viscosity of the fluid. Traditionally, the conductivity of porous materials is characterized by a single parameter—the coefficient of permeability—which includes all geometric and structural resistances of the capillary structure. The fluid is described by its dynamic viscosity, which is independent of pressure in liquids. Gases, on the other hand, are highly compressible which means that their coefficient of permeability is becoming pressure dependent. The Klinkenberg effect takes account of this behaviour by introducing the notion of intrinsic permeability; nevertheless, it remains applicable only to a limited pressure range. At low- and high-pressure gradients, the Klinkenberg linearization deviates considerably from the real flow behaviour. This paper demonstrates that microporous capillary resistance to gas flow cannot be adequately described by a single material parameter. On the basis of ideal gas law conditions, the mass flow is derived by a two-parametric power law. Pore pressure decline is characterized by a permeability exponent n and a reference velocity v1, replacing both dynamic viscosity of the fluid and permeability. The actual pressure distribution over the flow path is expressed by means of an equation of motion which leads to a logarithmic linearization of the material parameters. Velocity and acceleration vectors are derived and discussed in relation to the actual pressure distribution. The two parameters n and v1 remain constant throughout and independent of pore pressure and are thus used to describe the flow rates over the entire pressure range. This model is then validated by means of flow analyses based on data given in the literature and compared to the classical theory of permeability.