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Reflection Coefficient (reflection + coefficient)
Selected AbstractsHyperglycemia Stimulates a Sustained Increase in Hydraulic Conductivity In Vivo without Any Change in Reflection CoefficientMICROCIRCULATION, Issue 7 2007RACHEL M. PERRIN ABSTRACT Objective: Increased microvascular permeability contributes to the development of diabetic microvascular complications and diabetic vasculopathy is correlated with blood glucose levels. The mechanisms underlying increased permeability, however, are poorly understood. Methods: The Landis-Michel technique was used to measure water permeability (hydraulic conductivity, Lp) and macromolecular permeability (reflection coefficient, ,) of exchange capillaries in frogs and rats. Results: Dialysed normoglycemic plasma from diabetic patients had no effect on Lp. The same plasma with 20 mM glucose increased hydraulic conductivity from (mean ± SEM × 10,7 cm · s,1· cm H2O,1) 5.73 ± 2.01 to 13.09 ± 2.67 (P < .01). Nondiabetic control plasma did not affect Lp, but addition of 20 mM glucose increased Lp to a similar degree. The effect of glucose alone was examined. Glucose at 20 mM increased Lp, from 2.82 ± 0.61 to 4.71 ± 1.35 × 10, 7 cm · s, 1· cm H2O,1 (P = .002, n = 13). A similar increase was seen in rat mesenteric microvessels, from 1.04 ± 0.40 in control perfusions to 2.18 ± 0.56, P < .05. The microvascular macromolecular reflection coefficient in all the above experiments was unaltered. The use of specific inhibitors indicated that the glucose-induced increased Lp did not appear to be mediated through protein kinase C (PKC), free radical generation, glucose metabolism, or albumin glycation. Conclusions: These data suggest that hyperglycemia induced increased apparent protein permeability may be secondary to a glucose-mediated change in macromolecular convective flux rather than any change in protein permeability per se. The authors speculate that the increased microvascular permeability to water in vivo is mediated by direct interaction of glucose with the endothelial cells (perhaps with the glycocalyx). [source] The lateral intercellular space as osmotic coupling compartment in isotonic transportACTA PHYSIOLOGICA, Issue 1 2009E. H. Larsen Abstract Solute-coupled water transport and isotonic transport are basic functions of low- and high-resistance epithelia. These functions are studied with the epithelium bathed on the two sides with physiological saline of similar composition. Hence, at transepithelial equilibrium water enters the epithelial cells from both sides, and with the reflection coefficient of tight junction being larger than that of the interspace basement membrane, all of the water leaves the epithelium through the interspace basement membrane. The common design of transporting epithelia leads to the theory that an osmotic coupling of water absorption to ion flow is energized by lateral Na+/K+ pumps. We show that the theory accounts quantitatively for steady- and time dependent states of solute-coupled fluid uptake by toad skin epithelium. Our experimental results exclude definitively three alternative theories of epithelial solute,water coupling: stoichiometric coupling at the molecular level by transport proteins like SGLT1, electro-osmosis and a ,junctional fluid transfer mechanism'. Convection-diffusion out of the lateral space constitutes the fundamental problem of isotonic transport by making the emerging fluid hypertonic relative to the fluid in the lateral intercellular space. In the Na+ recirculation theory the ,surplus of solutes' is returned to the lateral space via the cells energized by the lateral Na+/K+ pumps. We show that this theory accounts quantitatively for isotonic and hypotonic transport at transepithelial osmotic equilibrium as observed in toad skin epithelium in vitro. Our conclusions are further developed for discussing their application to solute,solvent coupling in other vertebrate epithelia such as small intestine, proximal tubule of glomerular kidney and gallbladder. Evidence is discussed that the Na+ recirculation theory is not irreconcilable with the wide range of metabolic cost of Na+ transport observed in fluid-transporting epithelia. [source] Mechanisms of intercellular hypertonicity and isotonic fluid absorption in proximal tubules of mammalian kidneysACTA PHYSIOLOGICA, Issue 1 2002F. KIILArticle first published online: 30 APR 200 ABSTRACT The main purpose of this theoretical analysis (second of two articles) is to examine whether transjunctional diffusion of NaCl causes intercellular hypertonicity, which permits transcellular water transport across solute-impermeable lateral cell membranes until osmotic equilibration. In the S2 segment with tubular NaCl concentration 140 mM, the calculated apical intercellular NaCl concentration is c0 , 132 mM, which exceeds peritubular NaCl concentration by 12 mM or 22 mOsm kg,1. Variations in volume flow, junctional reflection coefficient (,NaCl=0.25,0.50), gap distance (g=6,8 Å), junctional depth (d=18,100 Å), intercellular diffusion coefficient (DLIS=500,1500 ,m2 s,1) and hypothetical active NaCl transport alter c0 only by a fraction of 1 mM. However, dilution and back-leakage of NaHCO3 lower apical intercellular hyperosmolality to ,18 mOsm kg,1. Water transport through solute-impermeable lateral cell membranes continues until intercellular and cellular osmolalities are equal. Transcellular and transjunctional volume flow are of similar magnitude (2 nL min,1 mm,1 tubule length) in the S2 segment. Thus, diffusion ensures isotonic absorption of NaCl. Two-thirds of NaHCO3 and other actively transported sodium salts are extruded into the last third of the exponentially widening intercellular space where the exposure time is only 0.9 s. Osmotic equilibration is dependent on aquaporins in the cell membranes. If permeability to water is low, transcellular water transport stops; tubular fluid becomes hypotonic; NaCl diffusion diminishes, but transjunctional water transport remains unaltered as long as transcellular transport of NaHCO3 and other solutes provides the osmotic force. [source] Numerical simulation of reservoir sediment and effects on hydro-dynamic response of arch damsEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 12 2001Chuhan Zhang Abstract Based on the dynamic theory for saturated porous media by Biot (Journal of the Acoustical Society of America 1956; 28: 168,178), a numerical model is presented to analyse the reflection behaviours of reservoir sediment and compared with those from the visco-elastic model. It is concluded that the two models give very similar results of reflection coefficient , within the frequency range of interest. Then, using the two models, the change of the reflection coefficients , with various sedimentation parameters and excitation frequencies are studied in detail. The results are further used in the analysis of response functions of hydro-dynamic pressures on, and structural displacements of the Xiang Hong Dian arch dam, for which some results from a field vibration test are available. It appears that effects of water compressibility with sediment reflection on hydro-dynamic pressures and structural response are not significant for this specific case. Copyright © 2001 John Wiley & Sons, Ltd. [source] The influence of fluid-sensitive dispersion and attenuation on AVO analysisGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2006Mark Chapman SUMMARY Analysis of seismic data suggests that hydrocarbon deposits are often associated with higher than usual values of attenuation, but this is generally ignored during amplitude-versus-offset (AVO) analysis. The effect can be modelled with equivalent medium theory based on the squirt flow concept, but the excess attenuation is associated with strong velocity dispersion. Consequently, when we study reflections from the interface between such an equivalent medium and an elastic overburden we find that the reflection coefficient varies with frequency. The impact of this variation depends on the AVO behaviour at the interface; class I reflections tend to be shifted to higher frequency while class III reflections have their lower frequencies amplified. We calculate synthetic seismograms for typical models using the reflectivity method for materials with frequency dependent velocities and attenuations, and find that these effects are predicted to be detectable on stacked data. Two field data sets show frequency anomalies similar to those predicted by the analysis, and we suggest that our modelling provides a plausible explanation of the observations. [source] Seismic reflection coefficients of faults at low frequencies: a model studyGEOPHYSICAL PROSPECTING, Issue 3 2008Joost Van Der Neut ABSTRACT We use linear slip theory to evaluate seismic reflections at non-welded interfaces, such as faults or fractures, sandwiched between general anisotropic media and show that at low frequencies the real parts of the reflection coefficients can be approximated by the responses of equivalent welded interfaces, whereas the imaginary parts can be related directly to the interface compliances. The imaginary parts of low frequency seismic reflection coefficients at fault zones can be used to estimate the interface compliances, which can be related to fault properties upon using a fault model. At normal incidence the expressions uncouple and the complex-valued P-wave reflection coefficient can be related linearly to the normal compliance. As the normal compliance is highly sensitive to the infill of the interface, it can be used for gas/fluid identification in the fault plane. Alternatively, the tangential compliance of a fault can be estimated from the complex-valued S-wave reflection coefficient. The tangential compliance can provide information on the crack density in a fault zone. Coupling compliances can be identified and quantified by the observation of PS conversion at normal incidence, with a comparable linear relationship. [source] Vector attenuation: elliptical polarization, raypaths and the Rayleigh-window effectGEOPHYSICAL PROSPECTING, Issue 4 2006José M. Carcione ABSTRACT Waves in dissipative media exhibit elliptical polarization. The direction of the major axis of the ellipse deviates from the propagation direction. In addition, Snell's law does not give the raypath, since the propagation (wavevector) direction does not coincide with the energy-flux direction. Each of these physical characteristics depends on the properties of the medium and on the inhomogeneity angle of the wave. The calculations are relevant for multicomponent surveys, where the receivers are placed on the ocean-floor. An example of the role played by inhomogeneous waves is given by the Rayleigh-window effect, which implies a significant amplitude reduction of the reflection coefficient of the ocean-bottom. [source] Modelling of GPR waves for lossy media obeying a complex power law of frequency for dielectric permittivityGEOPHYSICAL PROSPECTING, Issue 1 2004Maksim Bano ABSTRACT The attenuation of ground-penetrating radar (GPR) energy in the subsurface decreases and shifts the amplitude spectrum of the radar pulse to lower frequencies (absorption) with increasing traveltime and causes also a distortion of wavelet phase (dispersion). The attenuation is often expressed by the quality factor Q. For GPR studies, Q can be estimated from the ratio of the real part to the imaginary part of the dielectric permittivity. We consider a complex power function of frequency for the dielectric permittivity, and show that this dielectric response corresponds to a frequency-independent- Q or simply a constant- Q model. The phase velocity (dispersion relationship) and the absorption coefficient of electromagnetic waves also obey a frequency power law. This approach is easy to use in the frequency domain and the wave propagation can be described by two parameters only, for example Q and the phase velocity at an arbitrary reference frequency. This simplicity makes it practical for any inversion technique. Furthermore, by using the Hilbert transform relating the velocity and the absorption coefficient (which obeys a frequency power law), we find the same dispersion relationship for the phase velocity. Both approaches are valid for a constant value of Q over a restricted frequency-bandwidth, and are applicable in a material that is assumed to have no instantaneous dielectric response. Many GPR profiles acquired in a dry aeolian environment have shown a strong reflectivity inside dunes. Changes in water content are believed to be the origin of this reflectivity. We model the radar reflections from the bottom of a dry aeolian dune using the 1D wavelet modelling method. We discuss the choice of the reference wavelet in this modelling approach. A trial-and-error match of modelled and observed data was performed to estimate the optimum set of parameters characterizing the materials composing the site. Additionally, by combining the complex refractive index method (CRIM) and/or Topp equations for the bulk permittivity (dielectric constant) of moist sandy soils with a frequency power law for the dielectric response, we introduce them into the expression for the reflection coefficient. Using this method, we can estimate the water content and explain its effect on the reflection coefficient and on wavelet modelling. [source] AVO investigations of shallow marine sedimentsGEOPHYSICAL PROSPECTING, Issue 2 2001M. Riedel Amplitude-variation-with-offset (AVO) analysis is based on the Zoeppritz equations, which enable the computation of reflection and transmission coefficients as a function of offset or angle of incidence. High-frequency (up to 700 Hz) AVO studies, presented here, have been used to determine the physical properties of sediments in a shallow marine environment (20 m water depth). The properties that can be constrained are P- and S-wave velocities, bulk density and acoustic attenuation. The use of higher frequencies requires special analysis including careful geometry and source and receiver directivity corrections. In the past, marine sediments have been modelled as elastic materials. However, viscoelastic models which include absorption are more realistic. At angles of incidence greater than 40°, AVO functions derived from viscoelastic models differ from those with purely elastic properties in the absence of a critical angle of incidence. The influence of S-wave velocity on the reflection coefficient is small (especially for low S-wave velocities encountered at the sea-floor). Thus, it is difficult to extract the S-wave parameter from AVO trends. On the other hand, P-wave velocity and density show a considerably stronger effect. Attenuation (described by the quality factor Q) influences the reflection coefficient but could not be determined uniquely from the AVO functions. In order to measure the reflection coefficient in a seismogram, the amplitudes of the direct wave and the sea-floor reflection in a common-midpoint (CMP) gather are determined and corrected for spherical divergence as well as source and streamer directivity. At CMP locations showing the different AVO characteristics of a mud and a boulder clay, the sediment physical properties are determined by using a sequential-quadratic-programming (SQP) inversion technique. The inverted sediment physical properties for the mud are: P-wave velocity ,=1450±25 m/s, S-wave velocity ,=90±35 m/s, density ,=1220±45 kg/m3, quality factor for P-wave QP=15±200, quality factor for S-wave QS=10±30. The inverted sediment physical properties for the boulder clay are: ,=1620±45 m/s,,=360±200 m/s,,=1380±85 kg/m3,QP=790±660,QS=25±10. [source] Multiresolution model of electromagnetic wave propagation in dispersive materialsINTERNATIONAL JOURNAL OF NUMERICAL MODELLING: ELECTRONIC NETWORKS, DEVICES AND FIELDS, Issue 3 2003Ismael Barba Abstract A multiresolution time-domain (MRTD) technique based on Haar wavelets has been applied to model the propagation of waves in dispersive materials, using a discretization of the convolution equation that governs the behaviour of the media. To validate the method, the reflection coefficient at the interface between air and several different linear dispersive media, as well as the propagation of a plane wave towards a non-linear dispersive medium have been calculated and compared with other numerical techniques and theoretical results. Copyright © 2003 John Wiley & Sons, Ltd. [source] Reverse osmosis of nonaqueous solutions through porous silica-zirconia membranesAICHE JOURNAL, Issue 2 2006Toshinori Tsuru Abstract Porous silica-zirconia membranes with pore diameters from 0.8 to 2 nm were prepared by a sol-gel process, and applied to the separation of alcohols (hexanol, octanol, decanol) and alkanes (hexane, decane, tetradecane) in ethanol solutions by reverse osmosis over the temperature range from 25 to 60° C. A silica-zirconia membrane with a pore diameter of 1 nm showed a molecular weight-cut-off (MWCO) of 200 in ethanol solutions. Rejection increased with the applied pressure, for both alcohol and alkane solutes. However, the rejection of alcohols was found to decrease with temperature, while that for alkanes remained nearly constant. The separation characteristics were examined for the following membrane parameters: solvent permeability, Lp, reflection coefficient, ,, and solute permeability, P, based on the Spiegler-Kedem equation. The viscosity of solutions and the diffusivity of alkanes and alcohol solutes in nano-sized pores were found to show a larger temperature dependency than in bulk. The diffusivity of alkane solutes showed the same temperature dependency as the viscosity of ethanol in nano-sized pores, while the diffusivity of alcohol solutes showed a larger temperature dependency than the viscosity of ethanol, probably because of a larger interaction between alcohol solutes and the hydrophilic surface of silica-zirconia membranes. Diffusion experiments were carried out to confirm the temperature dependency of the diffusivities in nano-sized pores. A bilayer model verified that solute permeabilities by reverse osmosis and diffusion experiments were consistent with each other. © 2005 American Institute of Chemical Engineers AIChE J, 2006 [source] Subtraction of scattering parameters for small-signal modulation characteristics of laser diodeLASER PHYSICS LETTERS, Issue 4 2005S. J. Zhang Abstract An extended subtraction method of scattering parameters for characterizing laser diode is proposed in this paper. The intrinsic response is extracted from the measured transmission coefficients of laser diode, and the parasitics of packaging network and laser chip are determined from the measured reflection coefficient of laser diode simultaneously. It is shown that the theories agree well with the experimental results. (© 2005 by Astro, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] Hyperglycemia Stimulates a Sustained Increase in Hydraulic Conductivity In Vivo without Any Change in Reflection CoefficientMICROCIRCULATION, Issue 7 2007RACHEL M. PERRIN ABSTRACT Objective: Increased microvascular permeability contributes to the development of diabetic microvascular complications and diabetic vasculopathy is correlated with blood glucose levels. The mechanisms underlying increased permeability, however, are poorly understood. Methods: The Landis-Michel technique was used to measure water permeability (hydraulic conductivity, Lp) and macromolecular permeability (reflection coefficient, ,) of exchange capillaries in frogs and rats. Results: Dialysed normoglycemic plasma from diabetic patients had no effect on Lp. The same plasma with 20 mM glucose increased hydraulic conductivity from (mean ± SEM × 10,7 cm · s,1· cm H2O,1) 5.73 ± 2.01 to 13.09 ± 2.67 (P < .01). Nondiabetic control plasma did not affect Lp, but addition of 20 mM glucose increased Lp to a similar degree. The effect of glucose alone was examined. Glucose at 20 mM increased Lp, from 2.82 ± 0.61 to 4.71 ± 1.35 × 10, 7 cm · s, 1· cm H2O,1 (P = .002, n = 13). A similar increase was seen in rat mesenteric microvessels, from 1.04 ± 0.40 in control perfusions to 2.18 ± 0.56, P < .05. The microvascular macromolecular reflection coefficient in all the above experiments was unaltered. The use of specific inhibitors indicated that the glucose-induced increased Lp did not appear to be mediated through protein kinase C (PKC), free radical generation, glucose metabolism, or albumin glycation. Conclusions: These data suggest that hyperglycemia induced increased apparent protein permeability may be secondary to a glucose-mediated change in macromolecular convective flux rather than any change in protein permeability per se. The authors speculate that the increased microvascular permeability to water in vivo is mediated by direct interaction of glucose with the endothelial cells (perhaps with the glycocalyx). [source] Transient Osmotic Absorption of Fluid in Microvessels Exposed to Low Concentrations of Dimethyl SulfoxideMICROCIRCULATION, Issue 1 2006CATHERINE A. GLASS ABSTRACT Dimethyl Sulfoxide (DMSO) is a common solvent for pharmacological agents. It is a small, lipophilic molecule thought to be relatively highly permeable through the cell membrane. While measuring the effect of low concentrations of DMSO (0.05,0.5% v/v) on capillary hydraulic conductivity as a vehicle control for pharmacological agents, the authors noticed what appeared to be an unusual transient absorption of fluid across the vessel wall. This absorption occurred during occlusion of the vessel, but dissipated quickly (1.7,8.6 s). The transient reabsorption reappeared upon each successive occlusion. To determine the nature of this transient absorption, the authors have measured the effect of increasing the pressure of the perfusing solution, of the concentration and time of perfusion of DMSO, and of superfusing the DMSO. They found that the absorption rate, but not the filtration rate, was concentration dependent, and was significantly correlated with the osmotic pressure of the DMSO. Moreover, the time taken for completion of the transient, i.e., time to reversal of flow, was inversely proportional to the hydraulic conductivity of the vessel. Furthermore, the transient absorption could be reduced and eventually abolished by increasing the hydrostatic pressure. These results strongly suggested that perfusion with low concentrations of DMSO could set up a significant osmotic pressure gradient across the vessel wall. This proposed mechanism for the absorption was confirmed by the measurement of a significant osmotic reflection coefficient of the vessel wall to DMSO (0.11 ± 0.01). Relatively low concentrations (0.05,0.5%) of DMSO were therefore able to stimulate a significant osmotic transient across the blood vessel walls. [source] Broadband microwave gas sensor: A coaxial designMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 8 2010J. Rossignol Abstract The method of gas detection by a broadband microwave gas sensor is presented in this article. The principle of this method is to use a sensitive material as substrate of a transmission coaxial line. The sensor represents an open line with an extreme surface in contact with gas. The interaction with the gas and the surface induces a dielectric variation of the substrate. From experimental results, the interaction between the sensitive material and each tested gas present a specific frequency spectrum of the reflection coefficient at the input of the line. The proposed method has the advantage of a compact size, wide bandwidth, simple structure, and capable with all sensitive materials. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1739,1741, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25359 [source] Study of the passive microwave RFID tag range using distance-dependent reflection coefficient over multipath channelMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 10 2009Fu Wei Abstract The distance-dependent reflection coefficients are investigated based on a multi-ray model at microwave frequency bands. Tag range of passive microwave radio-frequency identification (RFID) systems is then calculated taking into account the tagged object material characteristic. Obtained values are verified through the precise simulations based on a 3-D physical finite element model. The results shown that the proposed method provides a good agreement than that obtained by existing methods. The analytical method can be used for the empirical tag range value to avoid reader collision while ensuring system reliability. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2266,2268, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24602 [source] Magnitude of the voltage reflection coefficient of terminated reciprocal uniform lossy transmission linesMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 7 2007D. Torrungrueng Abstract In this paper, lossy transmission lines with arbitrary loss are analyzed in detail. Interestingly, analytical results show that the magnitude of the voltage reflection coefficient could be greater than unity for some passive load terminations. However, it can be shown rigorously that the input power is always greater than or equal to the power at the load; i.e., it still satisfied the law of power conservation even when the magnitude of the voltage reflection coefficient is greater than unity. In addition, the Smith chart is also employed to graphically illustrate that the magnitude of the voltage reflection coefficient at some points along lossy transmission lines could be greater than unity for proper load terminations. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1516,1519, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22490 [source] A simple K-band waveguide-to-microstrip probe transitionMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 7 2007Alessandro Navarrini Abstract We describe the design, construction, and test of a simple K-band waveguide-to-microstrip transition based on a radial-shaped probe patterned on a quartz substrate. From 16 to 28 GHz, the measured reflection coefficient at the coaxial SMA input, used for test purposes of the adapter, was less than ,14 dB, and the dissipative loss was ,0.30 dB. The estimated loss of the waveguide to microstrip transition only, with coaxial connector effects removed, is ,0.08 dB. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1597,1600, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22558 [source] Development of ultra wideband planar stepped-fat dipole antennaMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 9 2006Young-Jin Park Abstract An ultra wideband (UWB) planar stepped-fat dipole antenna is developed. The antenna consists of two stepped-fat arms used for improving the bandwidth of a fat dipole antenna using only two square arms. A prototype antenna is designed and fabricated for high-resolution UWB image radars. For verification of the performance, reflection coefficient and radiation patterns are measured. Also, the impulse response of the antenna is obtained. It is shown that return loss of the antenna between 0.5 and 2.7 GHz is better than 10 dB. From the impulse response, ringing and dispersion are observed. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1698,1701, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21806 [source] Combination of electric and magnetic dipoles with single-element feeding for broadband applicationsMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 1 2006Karlo Q. da Costa Abstract This work presents four types of broadband compound antennas. The antennas are a combination of an electric dipole and small square or circular loops. The feeding of the electric dipole only is realized. The input impedance, reflection coefficient, and gain of the antennas with different geometries are analyzed numerically using the method of moments (MoM). It is shown that for the level of the reflection coefficient |,| < ,10 dB, 80% to 90% bandwidth can theoretically be achieved for the proposed antennas. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 8,12, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21248 [source] Polarization-dependent electromagnetic band gap (PDEBG) structures: Designs and applicationsMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 6 2004Fan Yang Abstract EBG structures exhibit an in-phase reflection coefficient, which makes them desirable for low-profile antenna designs. However, a conventional EBG structure has an identical reflection phase for a normally incident plane wave in spite of its polarization state. This paper presents novel polarization-dependent EBG (PDEBG) structures whose reflection phases are different, depending upon the polarization state of the incident plane wave. This polarization-dependent reflection-phase feature is realized by changing the unit geometries, for example, by using a rectangular patch to replace the square patch, by cutting slots into the patch, or by offsetting the vias. By properly tailoring the phase difference between different polarizations, a useful EBG reflector is introduced, which can control the polarization state of the reflected wave. One attractive application of this reflector is that the reflected wave of a circularly polarized incident plane wave can maintain its polarization sense. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 41: 439,444, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20164 [source] Three transmission-line transformers for phase retrieval from scalar reflection coefficientsMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 3 2004Ahmed M. Attiya Abstract In this paper, a new approach to obtain the phase angle of an unknown complex load using scalar reflection coefficients is presented. This approach is based on measuring the scalar reflection coefficient using three different transmission-line transformers. The relation between the two scalar reflection coefficients is presented as a nonlinear equation of the unknown phase angle. The value of the unknown phase angle represents a common root of these nonlinear phase equations. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 40: 231,235, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.11338 [source] Temperature-related fluid extravasation during cardiopulmonary bypass: An analysis of filtration coefficients and transcapillary pressuresACTA ANAESTHESIOLOGICA SCANDINAVICA, Issue 1 2002J. K. Heltne Background: Cardiopulmonary bypass (CPB) as used for cardiac surgery and for rewarming individuals suffering deep accidental hypothermia is held responsible for changes in microvascular fluid exchange often leading to edema and organ dysfunction. The purpose of this work is to improve our understanding of fluid pathophysiology and to explore the implications of the changes in determinants of transcapillary fluid exchange during CPB with and without hypothermia. This investigation might give indications on where to focus attention to reduce fluid extravasation during CPB. Methods: Published data on "Starling variables" as well as reported changes in fluid extravasation, tissue fluid contents and lymph flow were analyzed together with assumed/estimated values for variables not measured. The analysis was based on the Starling hypothesis where the transcapillary fluid filtration rate is given by: JV=Kf[Pc,Pi,,(COPp,COPi)]. Here Kf is the capillary filtration coefficient, , the reflection coefficient, P and COP are hydrostatic and colloid osmotic pressures, and subscript ,c' refers to capillary, ,i` to the interstitium and `p' to plasma. Results and conclusion: The analysis indicates that attempts to limit fluid extravasation during normothermic CPB should address primarily changes in Kf, while changes in both Kf and Pc must be considered during hypothermic CPB. [source] Oxidative gating of water channels (aquaporins) in corn rootsPLANT CELL & ENVIRONMENT, Issue 4 2006QING YE ABSTRACT An oxidative gating of water channels (aquaporins: AQPs) was observed in roots of corn seedlings as already found for the green alga Chara corallina. In the presence of 35 m m hydrogen peroxide (H2O2) , a precursor of hydroxyl radicals (*OH) , half times of water flow (as measured with the aid of pressure probes) increased at the level of both entire roots and individual cortical cells by factors of three and nine, respectively. This indicated decreases in the hydrostatic hydraulic conductivity of roots (Lphr) and of cells (Lph) by the same factors. Unlike other stresses, the plant hormone abscisic acid (ABA) had no ameliorative effect either on root Lphr or on cell Lph when AQPs were inhibited by oxidative stress. Closure of AQPs reduced the permeability of acetone by factors of two in roots and 1.5 in cells. This indicated that AQPs were not ideally selective for water but allowed the passage of the organic solute acetone. In the presence of H2O2, channel closure caused anomalous (negative) osmosis at both the root and the cell level. This was interpreted by the fact that in the case of the rapidly permeating solute acetone, channel closure caused the solute to move faster than the water and the reflection coefficient (,s) reversed its sign. When H2O2 was removed from the medium, the effects were reversible, again at both the root and the cell level. The results provide evidence of oxidative gating of AQPs, which leads on to inhibition of water uptake by the roots. Possible mechanisms of the oxidative gating of AQPs induced by H2O2 (*OH) are discussed. [source] The determination of membrane transport parameters with the cell pressure probe: theory suggests that unstirred layers have significant impactPLANT CELL & ENVIRONMENT, Issue 12 2005MELVIN T. TYREE ABSTRACT A simulation model was written to compute the time-kinetics of turgor pressure, P, change in Chara corallina during cell pressure probe experiments. The model allowed for the contribution of a membrane plus zero, one, or two unstirred layers of any desired thickness. The hypothesis that a cell with an unstirred layer is a composite membrane that will follow the same kind of kinetics with or without unstirred layers was tested. Typical ,osmotic pulse' experiments yield biphasic curves with minimum or maximum pressures, Pmin(max), at time tmin(max) and a solute exponential decay with halftime . These observed data were then used to compute composite membrane properties, namely the parameters Lp = the hydraulic conductance, , = reflection coefficient and Ps = solute permeability using theoretical equations. Using the simulation model, it was possible to fit an experimental data set to the same values of Pmin(max), tmin(max) and incorporating different, likely values of unstirred layer thickness, where each thickness requires a unique set of plasmalemma membrane values of Lp, , and Ps. We conclude that it is not possible to compute plasmalemma membrane properties from cell pressure probe experiments without independent knowledge of the unstirred layer thickness. [source] A fiber optic thermoset cure monitoring sensorPOLYMER COMPOSITES, Issue 4 2000Michele Giordano Curing determines the chemical and physical properties of a reacting resin. Among these, the optical properties strongly correlate with the structural features of the developing polymeric network. By monitoring changes of the refractive index, it is possible to analyze the polymerization of thermoset resin. In this work, a fiber optic sensor system has been designed and developed (based on the optical time domain reflectometry principle) to measure the reflection coefficient at the interface between the fiber optic and the resin during a curing process. Correlation between the sensor output and conversion has been proposed, following the Lorentz-Lorenz law. Isothermal data from the sensor have been compared with calorimetric analysis of an epoxy-based resin. [source] Mechanisms of transjunctional transport of NaCl and water in proximal tubules of mammalian kidneysACTA PHYSIOLOGICA, Issue 1 2002F. KIILArticle first published online: 30 APR 200 ABSTRACT Tight junctions and the intercellular space of proximal tubules are not accessible to direct measurements of fluid composition and transport rates, but morphological and functional data permit analysis of diffusion and osmosis causing transjunctional NaCl and water transport. In the S2 segment NaCl diffuses through tight junctions along a chloride gradient, but against a sodium gradient. Calculation in terms of modified Nernst,Fick diffusion equation after eliminating electrical terms shows that transport rates (300,500 pmol min,1 mm,1 tubule length) and transepithelial voltage of +2 mV are in agreement with observations. Diffusion coefficients are Dtj=1500 ,m2 s,1 in the S1 segment, and Dtj=90,100 ,m2 s,1 in the S2 segment where apical intercellular NaCl concentration is 132 mM, 1 mM below complete stop (Dtj=0 and Donnan equilibrium). Tight junctions with gap distance 6 Å are impermeable to mannitol (effective molecular radius 4 Å); reflection coefficients are ,=0.92 for NaHCO3 and ,=0.28 for NaCl, because of difference in anion size. The osmotic force is provided by a difference in effective transjunctional osmolality of 10 mOsm kg,1 in the S1 segment and 30 mOsm kg,1 in the S2 segment, where differences in transjunctional concentration contribute with 21 mOsm kg,1 for NaHCO3 and ,4 mOsm kg,1 for NaCl. Transjunctional difference of 30 mOsm kg,1 causes a volume flow of 2 nL min,1 mm,1 tubule length. Luminal mannitol concentration of 30 mM stops all volume flow and diffusive and convective transport of NaCl. In conclusion, transjunctional diffusion and osmosis along gradients generated by transcellular transport of other solutes account for all NaCl transport in proximal tubules. [source] Numerical simulation of reservoir sediment and effects on hydro-dynamic response of arch damsEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 12 2001Chuhan Zhang Abstract Based on the dynamic theory for saturated porous media by Biot (Journal of the Acoustical Society of America 1956; 28: 168,178), a numerical model is presented to analyse the reflection behaviours of reservoir sediment and compared with those from the visco-elastic model. It is concluded that the two models give very similar results of reflection coefficient , within the frequency range of interest. Then, using the two models, the change of the reflection coefficients , with various sedimentation parameters and excitation frequencies are studied in detail. The results are further used in the analysis of response functions of hydro-dynamic pressures on, and structural displacements of the Xiang Hong Dian arch dam, for which some results from a field vibration test are available. It appears that effects of water compressibility with sediment reflection on hydro-dynamic pressures and structural response are not significant for this specific case. Copyright © 2001 John Wiley & Sons, Ltd. [source] Seismic reflection coefficients of faults at low frequencies: a model studyGEOPHYSICAL PROSPECTING, Issue 3 2008Joost Van Der Neut ABSTRACT We use linear slip theory to evaluate seismic reflections at non-welded interfaces, such as faults or fractures, sandwiched between general anisotropic media and show that at low frequencies the real parts of the reflection coefficients can be approximated by the responses of equivalent welded interfaces, whereas the imaginary parts can be related directly to the interface compliances. The imaginary parts of low frequency seismic reflection coefficients at fault zones can be used to estimate the interface compliances, which can be related to fault properties upon using a fault model. At normal incidence the expressions uncouple and the complex-valued P-wave reflection coefficient can be related linearly to the normal compliance. As the normal compliance is highly sensitive to the infill of the interface, it can be used for gas/fluid identification in the fault plane. Alternatively, the tangential compliance of a fault can be estimated from the complex-valued S-wave reflection coefficient. The tangential compliance can provide information on the crack density in a fault zone. Coupling compliances can be identified and quantified by the observation of PS conversion at normal incidence, with a comparable linear relationship. [source] Seismic modelling study of a subglacial lakeGEOPHYSICAL PROSPECTING, Issue 6 2003José M. Carcione ABSTRACT We characterize the seismic response of Lake Vostok, an Antarctic subglacial lake located at nearly 4 km depth below the ice sheet. This study is relevant for the determination of the location and morphology of subglacial lakes. The characterization requires the design of a methodology based on rock physics and numerical modelling of wave propagation. The methodology involves rock-physics models of the shallow layer (firn), the ice sheet and the lake sediments, numerical simulation of synthetic seismograms, ray tracing, ,,p transforms, and AVA analysis, based on the theoretical reflection coefficients. The modelled reflection seismograms show a set of straight events (refractions through the firn and top-ice layer) and the two reflection events associated with the top and bottom of the lake. Theoretical AVA analysis of these reflections indicates that, at near offsets, the PP-wave anomaly is negative for the ice/water interface and constant for the water/sediment interface. This behaviour is shown by AVA analysis of the synthetic data set. This study shows that subglacial lakes can be identified by using seismic methods. Moreover, the methodology provides a tool for designing suitable seismic surveys. 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