Values Of Tr Research Articles

Diffusion Coefficients for Binary, Ternary, and Polydisperse Solutions from Peak-Width Analysis of Taylor Dispersion Profiles

Binary mutual diffusion coefficients D can be estimated from the width at half height W 1/2 of Taylor dispersion profiles using D=(ln 2)r 2 t R/(3W 2 h) and values of the retention time t R and dispersion tube radius r. The generalized expression D h=−(ln h)r 2 t R/(3W 2 h ) is derived to evaluate diffusion coefficients from peak widths W h measured at other fractional heights (e.g., (h = 0.1, 0.2,…,0.9). Tests show that averaging the D h values from binary profiles gives mutual diffusion coefficients that are as accurate and precise as those obtained by more elaborate nonlinear least-squares analysis. Dispersion profiles for ternary solutions usually consist of two superimposed pseudo-binary profiles. Consequently, D h values for ternary profiles generally vary with the fractional peak height h. Ternary profiles with constant D h values can however be constructed by taking appropriate linear combinations of profiles generated using different initial concentration differences. The invariant D h values and corresponding initial concentration differences give the eigenvalues and eigenvectors for the evaluation of the ternary diffusion coefficient matrix. Dispersion profiles for polymer samples of N i-mers consist of N superimposed pseudo-binary profiles. The edges of these profiles are enriched in the heavier polymers owing to the decrease in polymer diffusion coefficients with increasing polymer molecular weight. The resulting drop in D h with decreasing fractional peak height provides a signature of the polymer molecular weight distribution. These features are illustrated by measuring the dispersion of mixed polyethylene glycols.

Heparine perfusiondoes not improve mechanical recovery of intervertebral discs in vitro

Many fold-and-thrust belts display multi-storied thrust sequences, characterizing a composite architecture of the thrust wedges. Despite dramatic progress in sandbox modelling over the last three decades, our understanding of such composite thrust-wedge mechanics is limited and demands a re-visit to the problem of sequential thrusting in mechanically layered systems. This study offers a new approach to sandbox modelling, designed with a two-layered sandpack simulating a mechanically weak Coulomb layer, resting coherently upon a stronger Coulomb layer. Our experimental models reproduce strikingly similar styles of the multi-storied frontal thrust sequences observed in natural fold-and- thrust belts. The upper weak horizon undergoes sequential thrusting at a high spatial frequency, forming numerous, closely spaced frontal thrusts, whereas the lower strong horizon produces widely spaced thrusts with progressive horizontal shortening. This contrasting thrust progression behaviour gives rise to composite thrust architecture in the layered sandpack. We show the evolution of such composite thrust sequences as a function of frictional strength (μb) at the basal detachment and thickness ratio (Tr) between the weak and strong layers. For any given values of Tr and μb, the two thrust sequences progress at different rates; the closely-spaced, upper thrust sequence advances forelandward at a faster rate than the widely-spaced, lower thrust sequence. Basal friction (μb) has little effects on the vergence of thrusts in the upper weak layer; they verge always towards foreland, irrespective of Tr values. But, the lower strong layer develops back-vergent thrusts when μb is low (∼0.36). In our experiments, closely spaced thrusts in the upper sequence experience intense reactivation due to their interaction with widely spaced thrusts in the lower sequence. The interaction eventually affects the wedge topography, leading to two distinct parts: inner and outer wedges, characterised by steep and gentle surface slopes, respectively.

Magnetization recovery for signal enhancement: a fast imaging DEFT-based technique

This paper describes the development and application of a new fast MRI technique based on the DEFT principle. The sequence named MAgnetization RecoverY for Signal Enhancement (MARYSE) is composed of two completely symmetric gradient echoes separated by a 180° refocusing pulse. The RF pulse scheme, 90° x –180° y –90° − x enables restoration of the transverse magnetization along the longitudinal axis, and consequently artificially increases R 1 relaxation rate. In this sequence, the period between the excitation pulse and the restoring pulse (Tem: transverse magnetization evolution time) is very short (<10 ms). This makes possible a significant increase in signal-to-noise ratio, even with a relatively short repetition time (20 ms). Simulations were performed for different values of Tem and TR at definite T 1 and T 2 and for different values of T 1 and T 2 at constant Tem and TR. Relevant signal enhancement for species with long relaxation time constants as compared to classical gradient echo and fast spin-echo imaging was expected. In vitro studies on a fat/water phantom confirmed this simulation. Application of MARYSE to mouse brain imaging permitted to visualize almost completely cerebrospinal fluid of the ventricles, a signal usually partially saturated in fast gradient echo imaging.

In vivo (1)H double quantum filtered MRI of the human wrist and ankle.

Proton double quantum filtered (DQF) NMR imaging was applied in vivo to the human wrist and ankle with a clinical 1.5 T MR scanner. Water molecules having anisotropic motion were detected from tendons and ligaments. Images of Achilles tendon were obtained for a voxel size of 1.25 x 1.25 x 20 mm with three values of TR = 1.0, 0.5, and 0.2 sec, resulting in total acquisitions time of 17, 8.5, and 3.4 mins, respectively. Some degradation of the signal-to-noise ratio was observed at the shortest TR value and the contrast was significantly reduced due to SQ coherence leakage. The in vivo DQF images showed structure within the tendon that is otherwise not visible by conventional gradient-recalled echo (GRE) methods. Published 2000 Wiley-Liss, Inc.

Physiological Responses of Diverse Tall Fescue Cultivars to Drought Stress

Drought is among the most limiting factors for turfgrass growth. Understanding genetic variations and physiological mechanisms in turfgrass drought resistance would facilitate breeding and management programs to improve drought resistance. The experiment was designed to investigate shoot physiological responses of six tall fescue (Festuca arundinacea Schreb.) cultivars representing several generations of turfgrass improvement to drought stress. Grasses were grown in well-watered or drying (nonirrigated) soil for 35 days in the greenhouse. Net photosynthetic rate (Pn), stomatal conductance (gs), transpiration rate (Tr), relative water content (RWC), and photochemical efficiency (Fv/Fm) declined during drought progression in all cultivars, but the time and the severity of reductions varied with cultivar and physiological factors. The values of Pn, RWC, gs, and Tr decreased significantly for `Rebel Jr', `Bonsai', and `Phoenix' when soil water content declined to 20% after 9 days of treatment (DOT) and for `Houndog V', `Kentucky-31', and `Falcon II' when soil water content dropped to 10% at 15 DOT. A significant decrease in Fv/Fm was not observed in drought-stressed plants until 21 DOT for `Rebel Jr', `Bonsai', and `Phoenix' and 28 DOT for `Houndog V', `Kentucky-31', and `Falcon II'. The decline in Pn resulted mainly from internal water deficit and stomatal closure under mild drought-stress conditions. After a prolonged period of drought stress (35 DOT), `Falcon II', `Houndog V', and `Kentucky-31' maintained higher Pn than did `Rebel Jr', `Bonsai', and `Phoenix', which could be attributed to their higher Fv/Fm. This study demonstrated variation in drought resistance among tall fescue cultivars, which was related to their differential responses in photosynthetic capacity and water relations.

The effect of fringing field on the transmission of the prefiltered quadrupole mass spectrometers

The transmission properties of prefiltered quadrupole mass spectrometers (QMS) are significantly modified by the fringing field between the prefilter and the analyser. This effect depends on the length of the fringing field and parameters of the incoming ions as it has been revealed by the systematic study using a mass selective ion source coupled to a prefiltered QMS. The transmission of the prefiltered QMS has been determined experimentally. It depends on the distance (df ) between the prefilter and analyser, however, it is not unambiguously characterised by df expressed, as usually, in terms of the number of RF cycles (nRF) that the ions spend in this fringing field. At a fixed nRF the transmission of the system depends on the energy of ions (E) and df. Plotting the relative transmission ( Tr : the ratio of the transmission of prefiltered QMS and that of QMS without prefilter) as a function of E and df there are favourable (Tr has local maximum) and not favourable (Tr has local minimum) areas which are alternating periodically as a function of E. The difference of maximum and minimum values of Tr is more pronounced at higher nRF. The transmission is decreasing by increasing df.

Comparative study of Mössbauer spectra of iron(III) complexes of para-substituted tetraphenylporphyrins. Electronic effects of substituents and axial ligands

The Mossbauer spectra of high-spin iron(III) complexes of tetra-para-substituted derivatives of 5,10,15,20-tetraphenylporphyrin [FeL(X)](R = OMe, H, F or Cl; X = Cl, Br or I), have been compared with each other to clarify the electronic effects of the phenyl substituents and the axial ligands. They show a variety of patterns depending on the temperature: asymmetric broadening of the high-velocity line at higher temperatures, a symmetric doublet at a temperature Tr, and reversal in asymmetry below Tr. The temperature dependence results from the temperature-dependent spin–spin relaxation of high-spin iron(III) ion and the off-diagonal terms of the hyperfine operator, and reflects the ionic zero-field splitting. The reversal temperature, Tr, and the quadrupole splitting, ΔEQ, show two kinds of dependence on R. This is explained by considering the σ- and π-bonding characteristics of the porphyrins under the influence of the substituents. With a given R, the values of both Tr and ΔEQ increase in the order X = Cl < Br < I. This is interpreted as due to the predominantly ionic character of the bond between the halide and iron ions, except for R = OMe.

Another way of deriving the ring current decay time during disturbed periods

Coupling parameter, E, and the total energy dissipated by the magnetosphere, UT, are determined for six disturbed periods, following three known criteria for UT computation. It is observed that UT exceeds E for Dst &lt; -90 nT, for alI models. Differences between models reside on the estimated valnes for the particles' life time il1 the equatorial ring current. The values of TR, used in the models, are small during the main phase of the di."turbance, in disagreement with the charge exchange life time of the majority species, H+ and O'-. Based on this conclusion, a different criterion to calculate TR is proposed, differentiating the different stages of the perturbation. TR is calculated, for the main phase of the storm, from the rate of energy deposition estimation, Q, in the ring current. For Dst recovery phase, the vallles are obtained from a ring current decay law computation. The UTvu calculated, physically more coherent with the processes occurring during the event, is now smaller than expected. In this sense, it is understood that the power generated by the solar wind-magnetosphere dy- namo, should also be distributed in the inner magnetosphere, auroral zones and equatorial ring current, as in the outer magnetosphere, plasmoids in the tail shot in antisolar direction. A further adjustment of E, with the Chapman-Ferraro distance, 10' variable, has been made. Although the reslllts, improve the estimation of E, they are sti!l smaller than UT, except UTNU, for some disturbed periods. This result indicates the uncertainty in the computation of the input energy, by using the many expressions proposed in the literature, which are always presented as laws proportional to a given group of parameters, with an unknown factor of proportionality, which deserves more detailed physical analysis.

Acute changes in oxygen consumption and body temperature after burn injury.

This study describes the pattern of oxygen consumption (VO2), rectal temperature (Tr), and acral skin temperature (Tac) in sleeping and resting (awake) burned children nursed in a thermoneutral environment. Measurements of respiratory gas exchange (VO2 and carbon dioxide production (VCO2)) were made using an open circuit, flow through system of indirect calorimetry. Tr and Tac were monitored continuously. Sixteen patients were studied during the first 18 hours after being burned. Three phases of change in VO2, Tr, and Tac are described. The first was a stable period and there was little change from admission values. The second (7-10 hours after burn) was a phase of rapid heat storage. It started with a fall in Tac. Peak values of Tr (38.8-41.1, median 40.0 degrees C) and VO2 (8.5-11.8 ml/min/kg) occurred either in phase 2 or in the later phase 3. At its peak VO2 was 12-61% above values in phase 1. In phase 3, Tac returned towards admission values but Tr and VO2 were variable. These changes suggest that both an increase in metabolic heat production as well as heat conservation at the extremities may be involved in the generation of early fever after a burn.

Nuclear magnetic resonance imaging as a probe of the freezing—thawing phenomena of liquids in heterogeneous systems

AbstractNMR microscopic imaging is shown to be an excellent probe of the freezing and thawing phenomena of liquids in heterogeneous chemical systems, and, in addition, is capable of monitoring the freezing‐thawing of mixed solvent systems. In the model mixed solvent system Me2SO–water, the chemical shift specific slice selective (C4S) imaging technique is used to obtain the spatial distributions of the concentrations and mobilities of the two individual solvents in test samples. Measurements of relaxation parameters make it possible to choose values of imaging parameters TE and TR to ensure quantitative reliability of the images. Images of test samples generated at various temperatures demonstrate the qualitative relationship between image intensity and the Me2SO–water phase diagram. This series of experiments verifies that NMR microscopic imaging will be a powerful tool for the study of liquids within solid matrices, and in particular of cryoprotectant solvents in biological systems.

Modelling of the power dissipation and rovibrational heating and cooling in SiH4-H2 RF glow discharges

The electrical power dissipation and the vibrational (v) and rotational-translational (RT) heating and cooling mechanisms in SiH4-H2 radio-frequency (RF) glow discharges are analysed. Detailed balance of the various contributions leads to determination of the steady state v and RT temperatures TV and TR depending on the discharge geometry, total and partial pressures, wall temperature and power density. In low-power SiH4-dominated discharges (0.1-0.2 Torr range), v excitation is mostly due to low-energy electron-SiH4 collisions, but in high-power discharges, dominated by H2 (0.1-1 Torr), v excitation proceeds via collisional relaxation of hot H atoms and exothermic chemical reactions. v cooling is mostly controlled by v-RT transfer and thermal diffusion to the walls and by electron impact dissociation of v excited molecules. In any case the contribution of spontaneous decomposition highly v excited SiH4 molecules (pyrolysis) remains negligible. The model is tested on various experimental situations and the computed values of TV and TR are in fair agreement with available measurements by IR emission spectroscopy and CARS. It is also shown that the spatial temperature profiles may induce thermophoresis on particulates generated within the discharge.

Time domain information from resonant Raman excitation profiles: A direct inversion by maximum entropy

A direct method for the inversion of resonant Raman excitation profiles to the time domain is proposed. The inversion procedure is implemented within the maximum entropy (ME) formalism. The constraints used in the ME procedure are the values of the Fourier transform of the Raman excitation profile at a given set of values of times tr. It is shown that the ME functional form of the Raman cross section can be expressed in terms of a Raman amplitude, depending on the Lagrange multipliers and on the constraints. The Fourier transform of the Raman amplitude yields the time dependent cross correlation function. Another route to direct inversion, using a Fourier series expansion of the dispersion relation, is also considered. The analytical requirements that must be satisfied by the input excitation profile for a successful inversion to be possible are discussed. The optimum values of the times tr and of the Lagrange multipliers which determine the Raman amplitude are computed using a new algorithm (the min–max algorithm). The proposed ME numerical procedure is implemented for computed resonant Raman excitation profiles of the B̃ electronic state of the iodobenzene molecule and of a model anharmonic system. In addition, the analytical implications of the ME functional form of the excitation profile are discussed with special reference to the separation of time scales in the dynamics.

Effect of oxygen reduction on the retention of passivity by 430 stainless steel in naturally aerated dilute sulphuric acid

AbstractThe role of the cathodic reaction of oxygen reduction in extending the passivity retention time (tr) of conventional 430 stainless steel (Fe–17Cr) in naturally aerated dilute sulphuric acid has been demonstrated. Theoretically evaluated and experimentally obtained values of tr after passivation at a potential corresponding to the middle of the passive region of the anodic polarisation curve and experimental values of tr after passivation at a potential within the cathodic loop region are compared. The effect of oxygen reduction on maintenance of passivity, following passivation at the potential corresponding to the middle of the passive region, is small and diminished with an increase in the passivation time. In contrast, this effect is particularly significant if the passivation potential is within the cathodic loop region. The cathodic loop region is thus proposed as a preferable region for the implementation of anodic protection in the studied system as it provides a prolonged aftereffect and, ...

Ultra-low-noise 100 GHz receiver based on parallel biased SIS arrays

The possibility of overcoming the main drawbacks of the traditional series arrays by using a parallel connection of SIS junctions for DC and IF current has been experimentally demonstrated in full height 70-116 GHz waveguide mixers. This new array concept has been successfully developed to: (i) avoid the influence of junction non-uniformity in the array; (ii) provide optimal matching at both RF input and IF output even in the case of high gain, and (iii) tune out junction capacitance without any DC blocks. The experimentally tested arrays contain 10 or 11 trilayer 2.5 mu m2 Nb-AlOx-Nb junctions. Best values of Tr(DSB) as low as 20+or-2 K have been measured at the receiver room temperature horn in the 104-108 GHz LO range with a 4 GHz IF and at a mixer ambient temperature of 4.5-5.5 K. An SSB mixer noise temperature as low as 6+or-3 K was estimated. This value is very close to the quantum noise limit hf/k approximately=5 K.

Compressibility charts for gases with different acentric factors and evaluation of the Redlich-Kwong-Soave equation of state

The Redlich-Kwong-Soave generalized equation of state is transformed into a dimensionless form, which permits plotting of one compressibility chart for each value of the acentric factor ω. Such charts are provided for various values of ω in the range ω=0–0.6. A systematic evaluation of the Redlich-Kwong-Soave equation is made by comparison with the latest version of the generalized compressibility chart given by Nelson and Obert. It is found that for ω=0, the Redlich-Kwong-Soave compressibility chart agrees well with the Nelson-Obert chart, apart from a small region near the critical point: For values of the reduced pressure pr<1, the relative ercompressibility factor z is less than about 1%. For reduced pressures and temperatures in the ranges 1≤pr≤8 and 1≤Tr≤3.5, respectively, it is found e≤1% at the higher values of Tr, e≤5% at the intermediate Tr and e≤10% at the lower Tr. The highest disagreement is observed near the critical point (pr=Tr=1). Moreover, the exact compressibility chart of the real air is developed, based on its exact equation of state, and compared with the Redlich-Kwong-Soave compressibility chart for ω=0.

Characterization of two active site mutations of thioredoxin reductase from Escherichia coli

Thioredoxin reductase (TRR), a member of the pyridine nucleotide-disulfide oxidoreductase family of flavoenzymes, undergoes two sequential thiol-disulfide interchange reactions with thioredoxin during catalysis. In order to assess the catalytic role of each nascent thiol of the active site disulfide of thioredoxin reductase, the 2 cysteines (Cys-136 and Cys-139) forming this disulfide have been individually changed to serines by site-directed mutageneses of the cloned trxB gene of Escherichia coli. Spectral analyses of TRR(Ser-136,Cys-139) as a function of pH and ionic strength have revealed two pKa values associated with the epsilon 456, one of which increases from 7.0 to 8.3 as the ionic strength is increased, and a second at 4.4 which is seen only at high ionic strength. epsilon 458 of wild type TRR(Cys-136,Cys-139) and epsilon 453 of TRR(Cys-136,Ser-139) are pH-independent. A charge transfer complex (epsilon 530 = 1300 M-1 cm-1), unique to TRR(Ser-136,Cys-139), has been observed under conditions of high ammonium cation concentration (apparent Kd = 54 microM) at pH 7.6. These results suggest the assignment of Cys-139 as the FAD-interacting thiol in the reduction of thioredoxin by NADPH via thioredoxin reductase. If, as with other members of this enzyme family, the two distinct catalytic functions are each carried out by a different nascent thiol, then Cys-136 would perform the initial thiol-disulfide interchange with thioredoxin. Steady state kinetic analyses of the proteins have revealed turnover numbers of 10 and 50% of the value of the wild type enzyme for TRR(Ser-136,Cys-139) and TRR(Cys-136,Ser-139), respectively, and no changes in the apparent Km values of TR(S2) or NADPH. The finding of activity in the mutants indicates that the remaining thiol can carry out interchange with the disulfide of thioredoxin, and the resulting mixed disulfide can be reduced by NADPH via the flavin.

Variations in flow and intraalveolar pressure during jet ventilation: theoretical and experimental analysis

Alteration in flow and pressure in a chest-lung model during jet ventilation was studied from a theoretical and experimental standpoint. The model consisted of a stiff box, with an inserted cast of the bronchial tree, simulating either normal or low compliance of the human chest-lung. Airway resistances in the model could be altered. A constant driving flow (V̇d) was applied through the opening of the trachea at various rates. The driving flow drew in a constant flow ( entrained flow = V ̇ e ) and increased the box pressure (P a), which in turn induced an increasing back flow (V̇r). V̇d, V̇e - V̇r and P a were measured. Theoretical analysis of time variation in box pressure showed a monoexponential pattern which was confirmed experimentally. Tr, the time at which back flow and entrained flow become equal could be predicted taking the physical characteristics of the experimental set-up and the values of V̇d and V̇e into account. Tr values depended mainly on the compliance of the experimental set-up. Increasing the resistances was found to reduce V̇e. In practice, the efficiency of jet ventilation for gas exchange at a frequency below 4 Hz is known to be mainly related to the volume of gas flowing in the trachea towards the alveoli. This efficiency generally depends on the values of Tr and the inspiratory period (T i). A value of T i less than or equal to Tr will thus improve the jet ventilation efficiency.

FORCED FLOW NEAR A HEATED ROTATING DISK: A SIMILARITY SOLUTION

This paper presents an analysis of the flow and heat transfer between two parallel infinite disks that are separated by a distance L. One of the disks is solid, heated, and rotated; the other disk is porous, unheated, and stationary. Fluid is injected through the porous disk normal to its surface and toward the rotating disk. The three-dimensional Navier-Stokes and energy equations have been reduced to a system of ordinary differential equations by means of a similarity transformation and have been solved over a range of values of the two Reynolds numbers Reωequals; L2ω/νL and Reν = LU/νL, for two values of the temperature ratio Tr = (Ts - TL)/TL, of 0.001 (constant property flow) and 2.33. For Tr = 0.001 the velocity profiles are independent of the temperature profile which is determined for Pr = 0.67; for Tr = 2.33, the velocity and temperature profiles are determined for helium. The assumptions made in the analysis are shown to be valid provided that the mixed convection parameter , is small. Velocity ...

Transverse relaxation (1/T2) of solvent protons induced by magnetized spheres and its relevance to contrast enhancement in MRI.

At typical imaging fields, the transverse relaxation rates 1/T2 of the protons of soft tissue are much greater than their longitudinal rates 1/T1. Because of this, clinical magnetic resonance images are generally collected using relatively short values of TR, an approach that both increases comfort for the patient and reduces medical costs. As a result, image contrast is dominated by the 1/T2 values of the tissue protons. Currently, small single-ion paramagnetic complexes--Gd-DTPA is the prime example--are being used to enhance contrast in clinical magnetic resonance imaging (MRI). However, such agents contribute comparably to 1/T1 and 1/T2 so that their utility is greatest when introduced into body fluids, for which 1/T1 and 1/T2 are also comparable; they are much less useful for enhancing contrast of soft tissue. For this, one must look elsewhere, to rather large aggregates of paramagnetic ions, which may either be paramagnetic or ferromagnetic. Iron in its many chemical and biochemical forms, both exogenous and endogenous, is important in this respect. Its presence in ferritin and hemosiderin--in excess in some diseases--is one example; deoxyhemoglobin in cells and methemoglobin in blood pools from trauma are others in which endogenous iron in several oxidation states is important. Magnetic particulates of various iron oxides are now being used as exogenous agents for enhancing 1/T2 preferentially at imaging fields. Predicting contrast enhancement under such circumstances can become rather complex, not because the theory is difficult, but because the underlying concepts are subtle.(ABSTRACT TRUNCATED AT 250 WORDS)

On the Representation of Rossby Wave Critical Layers and Wave Breaking in Zonally Truncated Models

A detailed comparison is made between the Stewartson-Warn-Warn analytical solution for a fully nonlinear, nondiffusive Rossby-wave critical layer and a new analytical solution for the corresponding zonally truncated, “wave-mean” or “quasi-linear” problem in which the motion is represented by the zonal mean and a single zonal harmonic only. The effect of adding harmonics one by one is also considered. The results illustrate the extent to which zonally truncated models, which inevitably miss certain aspects of the fluid behavior, nevertheless contrive to mimic some important dynamical features of the evolution of the nonlinear critical layer, particularly the absorption-reflection behavior. The zonally truncated and fully nonlinear models predict almost the same reflection coefficient up to the time Tr when a state of perfect reflection is first reached, and the predicted values of Tr itself differ by only 5%. The agreement deteriorates only subsequently, during the first overreflecting stage. The reasons for this behavior are clarified by visualizing the way in which the truncated model misrepresents the (potential) vorticity field. Solutions, analytical and numerical, are also presented for zonally truncated models of nonlinear critical layers in which Rayleigh friction, or viscous diffusion, play an important role in the vorticity balance, as they may do in some numerical models. These solutions are compared with the corresponding fully nonlinear solutions. Implications for the numerical modeling of dynamical and tracer-transport processes involving planetary or Rossby wave “breaking” are discussed.