Adjustment Of Constants Research Articles

Appropriateness of equilibrium assumptions for determining metal distribution and transport in bacteria-bearing porous media

The fate and transport of strongly-sorbed contaminants in aquifers is tied to the mobility of the surfaces to which they are adsorbed. Mobile particulates, including mineral colloids, natural organic matter, or microbes, can enhance the mobility of contaminants that would otherwise be adsorbed to stationary surfaces. In this study, we compare numerical transport simulations of systems involving dissolved metals and microbial suspensions with previously published experimental results. The transport simulations utilize a finite difference solute transport scheme, a colloid attachment term, and a Newton–Raphson chemical speciation solver assuming thermodynamic equilibrium in the metal distribution between solution, mobile surfaces, and stationary surfaces. While model predictions of metal breakthrough are of mixed quality, adjustment of model equilibrium constants is adequate to provide accurate fits to observed breakthrough curves. Therefore, the idealized behaviors apparent in experimental breakthrough curves indicate metal transport under equilibrium sorption conditions, and hence the assumption of chemical equilibrium used in the model is appropriate. Since flow rates in the column experiments are fast compared to most flow rates in aquifers, we conclude that the kinetics of metal redistribution are fast enough that an assumption of chemical equilibrium would be appropriate for modeling coupled solute–colloid transport in pore environments where water flow is controlled by advective transport under water velocity conditions typical for most aquifers.

Modelling of strongly swirling flows in a complex geometry using unstructured meshes

PurposeSeeks to examine the performance of conventional turbulence models modelling strongly swirling flows within a Symmetrical Turn up Vortex Amplifier, with adjustment of the turbulence model constants to improve agreement with experimental data.Design/methodology/approachFirst, the standard k‐ε model and the Reynolds Stress Model (RSM) were used with standard values of model constants, using both the first order upwind and the quadratic upstream interpolation for convective kinetics (QUICK) schemes. Then, the swirling effect was corrected by adjusting the model coefficients.FindingsThe standard RSM with the QUICK did produce better predictions but still significantly overestimated the experimental data. Much improved simulation was obtained with the systematic adjustment of the model constants in the standard k‐ε model using the QUICK. The physical significance of the model constants accounted for changes of the eddy viscosity, and the production and destruction of k and ε.Research limitations/implicationsMore industrial cases could benefit from this simple and useful approach.Originality/valueThe constant adjustment is regular and directed, based on the eddy viscosity and the production and destruction of k and ε. The regularity of the effect of the model constants on the solutions makes it easier to quickly adjust them for other industrial applications.

Large eddy simulation of hydrogen auto-ignition with a probability density function method

Large eddy simulation is applied to the auto-ignition of a hydrogen jet issuing into a turbulent co-flowing air stream. A 19 step, nine species detailed mechanism is used for reaction. The influence of sub-grid species concentration and temperature fluctuations are accounted for by a sub-grid joint PDF for the reactive scalars, obtained from solution of a modelled transport equation. The method is able to reproduce ignition lengths and different regimes observed experimentally without any adjustment or calibration of the model constants.

Efficiently Estimating Nested Logit Models with Choice-Based Samples Example Applications: Example Applications

Choice-based samples oversample infrequently chosen alternatives to obtain an effective representation of the behavior of people who select these alternatives. However, the use of choice-based samples requires recognition of the sampling process in formulating the estimation procedure. In general, this procedure can be accomplished by applying weights to the observed choices in the estimation process. Unfortunately, the use of such weighted estimation procedures for choice models does not yield efficient estimators. However, for the special case of the multinomial logit model with a full set of alternative-specific constants, the standard maximum likelihood estimator–-which is efficient–-can be used with adjustment of the alternative-specific constants. The same maximum likelihood estimator can also be used with adjustment to estimate nested logit models with choice-based samples. The proof of this property is qualitatively described, and examples demonstrate how to apply the adjustment procedure.

Efficiently Estimating Nested Logit Models with Choice-Based Samples

Choice-based samples oversample infrequently chosen alternatives to obtain an effective representation of the behavior of people who select these alternatives. However, the use of choice-based samples requires recognition of the sampling process in formulating the estimation procedure. In general, this procedure can be accomplished by applying weights to the observed choices in the estimation process. Unfortunately, the use of such weighted estimation procedures for choice models does not yield efficient estimators. However, for the special case of the multinomial logit model with a full set of alternative-specific constants, the standard maximum likelihood estimator–-which is efficient–-can be used with adjustment of the alternative-specific constants. The same maximum likelihood estimator can also be used with adjustment to estimate nested logit models with choice-based samples. The proof of this property is qualitatively described, and examples demonstrate how to apply the adjustment procedure.

Experimental and theoretical studies of the vibrational spectra of cis-1-bromo-2-fluoroethene

The gas-phase infrared spectrum of cis-1-bromo-2-fluoroethene has been studied at low resolution in the range 200–6500 cm −1, leading to a complete assignment of the fundamentals, except the lowest vibrational mode ν 9 predicted at 167 cm −1. The remaining vibrational structure has been mainly interpreted in terms of first overtone or two quanta combination bands. Isotopic 79/81Br shift has been observed only in the ν 8 fundamental. The equilibrium structure and the quadratic force field have been investigated theoretically at CCSD(T) level of theory employing Dunning’s correlation consistent triple-zeta basis set. Cubic and semidiagonal quartic force field have been calculated using second-order Møller–Plesset perturbation theory and Ahlrich’ split valence (SV) contracted basis set. After a minor scaled quantum mechanical (SQM) adjustment of the quadratic force constants, the vibrational analysis, based on the second-order perturbation theory, has been carried out with the calculated force constants.

Regulation of nuclear functions – nucleocytoplasmic transport in context

A recent meeting of cell biologists in Cleveland, Ohio (April 22/23, 2004) highlighted the value of juxtaposing discussions of the functional organization of the nuclear periphery with themechanisms bywhich the nucleus receives signals, both from the cell surface and from defined sites in the cytoplasm. The event was jointly organized by theCell Biology Program of Case Western Reserve University School of Medicine and the Cell Biology Department and Lerner Research Institute of the Cleveland Clinic Foundation. In introductory remarks, Alan Tartakoff (Case Western Reserve University) highlighted the breadth of importance of dynamic nucleocytoplasmic communication. He described contemporary molecular understanding of TMsignaling∫ to the nucleus as one of the recent TMconquests∫ of cell biology, which characteristically insists on mechanistic questions which involve topologic or topographic issues. He also called attention to the realization that many examples of nuclear import and export do not directly involve the TMclassic∫ transport pathways which are driven by theRanGTPase andmembers of the family of importins/exportins (also known as TMkaryopherins∫) (Gˆrlich and Kutay, 1999; Xu and Massague, 2004). He further emphasized that net relocalization of proteins from the nucleus to the cytoplasm (and vice versa) can ± in many cases ± result from adjustment of the rate constants of ongoing shuttling, rather that being due to TMde novo∫ unidirectional transport (Fig. 1) (Tartakoff et al., 2000). As a prelude for subsequent lectures, he then enumerated additional circumstances which can cause individual transport cargoes to distribute asymmetrically across the nuclear envelope: unmasking of transport signals and protein-protein interaction domains (due to covalent changes or displacement of inhibitory peptide domains), tethering of cargo proteins to immobile structures (either covalently or non-covalently), etc.

Numerical simulation of axi-symmetric fire plumes: accuracy and limitations

Abstract The objective of this work was to determine the accuracy and limitations of a new version of Fire Dynamics Simulator (FDS), developed by McGrattan et al., on axi-symmetric fire plumes. The current version uses LES for turbulence, a mixture-fraction-based infinitely fast chemistry model for combustion, and a constant radiative loss fraction. These sub-models have been tested for unconfined fires of different sizes, based on a dimensionless heat release rate QD* in the range of 0.1 to 10.0, which covers most natural fire scenarios. No adjustment of constants or algorithms in the model FDS2.0 have been made. An examination of plume theory is made first to find the benchmark correlations. This shows a generalization for a collection of correlations based on theory, and which might be “the best”. Using the characteristic length as the scaling factor, it is found that the optimum resolution of a pool fire simulation is around 0.05. With this resolution, the flame height prediction is found to fit well with flame height correlations. Some other parameters such as temperature and mixture fraction are found to be close to the empirical estimations at flame tips. The Froude number, which describes the relative strength of momentum and buoyancy, falls within the measurement range of many researchers. The simulation also reveals that the temperature near the burner is over-predicted, while the centerline temperature and velocity in the non-combusting region is predicted well.

Determinação e análise das constantes de tempo de um sensor anelar de temperatura em leito fixo

A theoretical and experimental methodology was developed and made possible to evaluate and analyse the time constants of five thermocouples of a ring-shaped sensor installed in a fixed bed. In this sense, step disturbances were applied to the temperature of the air that flows across the sensor and the step responses were registered by periodic measures of temperature until the sensor reached steadystate. The time constants were evaluated by linear regressions of the sensor mathematical model. Both the mathematical model and the technique used for the adjustment of the time constants were satisfactory. Each thermocouple from the ring-shaped sensor has presented distinct time constants. The external ring-shaped thermocouple, r = 0,027m, and the central thermocouple, r = 0m, has presented the higher and the lower value of time constant, respectively, independent of the volumetric gas flow.

Metrology of the hydrogen and deuterium atoms: Determination of the Rydberg constant and Lamb shifts

We present a detailed description of several experiments which have been previously reported in several letters: the determination of the 1S Lamb shift in hydrogen by a comparison of the frequencies of the 1S-3S and 2S-6S or 2S-6D two-photon transitions, and the measurement of the 2S-8S/D and 2S-12D optical frequencies. Following a complete study of the lineshape of the two-photon transitions, we provide the updated values of these frequencies which have been used in the 1998 adjustment of the fundamental constants. From an analysis taking into account these results and several other precise measurements by other authors, we show that the optical frequency measurements have superseded the microwave determination of the 2S Lamb shift and we deduce optimized values for the Rydberg constant, = 109 737.315 685 50(84) cm-1 (relative uncertainty of ) and for the 1S and 2S Lamb shifts L(1S) = 8 172.840(22) MHz and L(2S-2P) = 1 057.8450(29) MHz (respectively, 8 183.970(22) MHz and 1 059.2341(29) MHz for deuterium). These are now the most accurate values available.

Identical independent sites for dye ligand on bovine serum albumin demonstrated by multivariate analysis

The most fundamental parameters concerning an interaction between a ligand and a protein are equilibrium constants and the number of binding sites. The Scatchard plot has for a long time been widely used to obtain those parameters. However, controversy in 1982–1983 over the reliability of this plot (the graphical estimation of the number of identical independent sites from the x-intercept) indicated that some methodologies other than the Scatchard plot are expected. Over the past decade, we have developed a method for applying multivariate analysis to the problem of determining spectral features of a ligand associated with a protein molecule. In principle, this method is based mainly on the computer-assisted adjustment of dissociation constants to an assumed reaction model. We discovered in this process that an n-parameter, introduced into an equation for calculating the amount of dye ligand bound to a protein, coincided with the number of identical independent sites, under a certain condition in principal factor analysis calculation. In this study, we established a new methodology for determining the number of identical independent sites using synthesized spectral series, and we then applied this method to a simple reaction system composed of bovine serum albumin (BSA) and bromocresol purple (BCP) anions. BSA was found to have two identical independent sites for BCP anions at pH 8.8.

Measuring Gas-Phase Chlorine Atom Concentrations: Rate Constants for Cl + HN3, CF3I, and C2F5I

A convenient method is reported for the measurement of gas-phase Cl atom concentrations in the 1011−1013 atoms cm-3 range in a flow reactor. The titration reaction with vinyl bromide is used to measure the absolute Cl atom concentration, and the relative Cl atom concentration is monitored by the HCl(v) infrared emission intensity produced by the reaction of Cl atoms with hydrogen sulfide. A microwave discharge through dilute flows of Cl2, CCl4, CFCl3, and CF2Cl2 in Ar were characterized as sources of Cl atoms in the flow reactor. The elementary rate constants for reaction of Cl atoms with HN3, CF3I, and C2F5I were measured as (1.1 ± 0.3) × 10-12, (5.1 ± 1.5) × 10-13, and (3.9 ± 0.8) × 10-12 cm3 molecule-1 s-1, respectively, at 300 K. These rate constants were obtained after adjustment of the apparent rate constants for secondary reactions.

Internal Effects Model for Spacecraft Modules Perforated by Orbital Debris

A model is developed to predict the effect of a perforating orbital debris particle impact on the pressure and temperature within a pressurized habitable spacecraft module. The model is developed such that it sequentially characterizes the phenomena comprising the impact event, beginning with the initial impact on the outer module wall. The model then considers the creation and motion of a debris cloud within the module wall system and the impact of the debris cloud on the inner module wall. The e nal phase of the model is concerned with the creation and motion of the debris cloud that enters the module interior and its effect within the module on module pressure and temperature levels. This characterization is accomplished through the application of elementary shock physics theory and fundamental thermodynamic principles. The predictions of the analytical model are compared with experimental pressure and temperature data from a series of instrumented high-speed impact tests. Several modie cations are made to thebasicmodel to bring its predictions closerin line with theexperimental results. Following the adjustment of several empirical constants, the predictions of the modie ed internal effects model are shown to be in close agreement with the experimental results.

On the Dangers of Adjusting the Parameter Values of Mechanism-Based Mathematical Models

Mechanism-based mathematical models describe systems in terms of identifiable physical processes, and the parameters are assumed to have fundamental physical significance. Ideally, the parameter values are measured independent of the system being modeled, but these values are often adjusted to give the best fit of model predictions to experimental data. A systematic investigation of the effects of such parameter adjustment was conducted by developing a model system comprising a known reaction mechanism and known rate constants. Simulations of experiments were run, and then attempts were made to model the system under a variety of problematic, but realistic, conditions. (1) When one rate constant was seriously in error, adjustment of a different rate constant gave the greatest improvement in the model fit. (2) When a contaminant was present in the experiment, the effects could be hidden by the adjustment of the rate constants. (3) When an incorrect reaction mechanism was assumed, the error could be hidden by parameter adjustment if the concentrations of only one of the reacting species were considered or if an unweighted fit was used for the optimization. (4) Parameter values adjusted for one set of experimental conditions gave a poorer fit than did the unadjusted parameter values when attempting to model a new set of experimental conditions (addition of an inhibitor). These results show the potential dangers of adjusting parameter values and the importance of measuring as many variables as possible in a complex system.

The temperature dependence of H + OH recombination in phosphorus oxide containing post-combustion gases

The temperature dependence of the rate of H + OH recombination is experimentally investigated using laser photolysis of high-temperature (1500–2500 K) water vapor. Parametric adjustment of reaction rate constants to fit a simulated OH signal with the experimental OH decay data gives a three-body reaction rate constant for H + OH + M → H 2O + M that is larger than expected at low temperature and lower at high temperatures. This result could have negative consequences for hypersonic propulsion systems since much of the thrust producing radical recombination occurs at high temperature. When PH 3 combustion products are present in the photolyzed gas mixture at low concentration, the OH decay is much faster and dominated by catalytic reactions that include phosphorus oxides and acids. Parametric reaction rate analysis indicates that two reactions, H + PO 2 → HOPO and H + HOPO → H 2 + PO 2 are responsible for the increase in the overall rate of H + OH recombination. The temperature dependence of the phosphine catalyzed recombination reaction is adequately described by the rate constant estimates previously reported.

An alternative algorithm for adjusting the fundamental physical constants

The adjustment of the fundamental physical constants according to least-squares appears to imply considerable dependence on the modelling of the random and (unknown) systematic errors. In contrast to the error formalism proposed and used before, the suggested alternative algorithm treats random errors on the basis of a so-called compound probability postulate, and the systematic errors as constant quantities to be assessed exclusively by symmetric intervals. The operational differences and the consequences of the various error formalisms are illustrated on the basis of the 1990 Status Report: Recommended Values of the Fundamental Physical Constants.

Modeling of soot formation and oxidation in turbulent diffusion flames

Soot concentration in a turbulent diffusion flame burning propane was predicted using several formation and oxidation models. The Favre-averaged equations governing conservation of mass, momentum, and energy and transport equations for turbulent quantities, mixture fraction and its variance were solved using the k-t model and the laminar flamelet approach. Calculation of soot concentration was performed using the results of the flame field model as input data. The soot formation models of Khan and Greeves, and Moss and co-workers were used together with the soot oxidation models of Magnussen and Hjertager, Lee et al., and Nagle and Strickland-Constable. Comparison of the results with available measurements, and with the predictions obtained by Fairweather et al. using their own soot formation model, shows that reasonable predictions of soot concentration requires an adjustment of the constants of the model of Moss and co-workers. The soot formation model of Fairweather et al. appears to be less sensitive to the constants. The model of Khan and Greeves yields correct orders of magnitude, but fails to predict some important features of the data. Hence, there is a need to improve presently available soot formation and oxidation models to achieve a satisfactory predictive capability.

Identification of elastic constants of alloys with sheet and fibre textures based on resonance measurements and finite element analysis

This paper presents a method for identifying the elastic constants of alloys of cubic crystal structure with sheet and fibre textures. It is based on conventional resonance measurements and finite element (FE) analysis. It includes a non-linear optimizer, an FE program for solving eigenvalue equations in which the exact stress-strain constitutive law for orthorhombic or hexagonal symmetry is considered, and a frequency assignment algorithm based on the orthogonality of natural modes. The identification strategy consists in the adjustment of the elastic constants in an optimization procedure by minimizing the differences between calculated and measured natural frequencies of several specimens. The determination of the elastic single-crystal constants from textured material measurements is also discussed. The results of four practical applications to Ni-based alloys demonstrate the efficiency and the economy of this method.

Massive strings in higher-dimensional inhomogeneous spacetime

An inhomogeneous cosmological model with massive strings as source term is developed in a Kaluza-Klein type of spacetime where the usual space exhibits spherical symmetry and the extra dimension is taken in the form of a circle. When an arbitrary constant in our solution vanishes we get an inhomogeneous cosmological model for pure dust, which further reduces to some particular forms of well known homogeneous models under suitable adjustment of arbitrary constants. The dynamical behaviour of the model is studied and it is found that the extra space admits of dimensional reduction via the successive appearances of one minimum and one maximum throughout its evolution.

Vibrational analysis of benitoite (BaTiSi3O9) and theSi3O9ring

The normal modes of vibration and their frequencies are calculated for benitoite, a mineral whose crystal structure (space group ${\mathit{D}}_{3\mathit{h}}^{2}$) consists of three-membered silicate rings (${\mathrm{Si}}_{3}$${\mathrm{O}}_{9}$) linked by ${\mathrm{Ba}}^{2+}$ and ${\mathrm{Ti}}^{4+}$ ions. Factor-group analysis dictates that certain normal modes involve the motion of only the ring atoms. On the assumption that mode mixings and splittings due to inter-ring interactions are small, the normal frequencies of the isolated ring of ${\mathit{C}}_{3\mathit{h}}$ symmetry are determined by fitting to suitable averages of selected frequencies in the Raman spectra. A valence force potential consisting of only central interactions between nearest neighbors and bond-bending interactions centered at the silicon atoms is used. This potential is then extended to the full crystal structure by including interactions involving the ${\mathrm{Ba}}^{2+}$ and ${\mathrm{Ti}}^{4+}$ ions. The frequencies obtained are in excellent agreement with the infrared and Raman spectra, requiring only minor adjustment of the force constants obtained for the isolated ring. The identification of normal modes characteristic of three-membered silicate rings may prove to be a valuable guide in the interpretation of the infrared and Raman spectra of amorphous silicates, potentially leading to new information on the ring statistics of these materials.