Semi-quantitative Models Research Articles

On the Investigation of Field-Induced Director Dynamics: A Novel Esr Experiment

The rotational viscosity coefficient of a nematic liquid crystal with a positive diamagnetic susceptibility anisotropy can be determined by monitoring the time dependence of the director orientation as it is rotated by a field from a non-equilibrium to the equilibrium state parallel to the field. A variety of techniques is available using different properties to monitor the director orientation as a function of time. Normally these experiments are designed so that the property used to determine the director orientation does not change during the time taken for its measurement. Here using ESR spectroscopy, we explore the benefits of exploiting exactly the opposite situation. That is during the time taken to record the ESR spectrum the director orientation is allowed to change. We have developed both semi-quantitative and quantitative models to allow us to simulate how the form of the spectrum depends on experimental conditions such as the field scan rate. These models have also proved to be valuable in designing the experiment and in analysing the spectra. It seems that this novel ESR experiment provides a valuable route to the field-induced relaxation time and hence to the rotational viscosity coefficient.

Thermodynamic properties of highly undercooled liquid TiAl alloy

The specific heat of an undercooled liquid TiAl alloy was measured by an electromagnetic levitation drop calorimeter and the related thermodynamic properties, difference in Gibbs free energy and change in entropy are calculated based on the experimental results and some well known models. Under the containerless processing condition, the liquid alloy was undercooled by up to 219 K (0.122TL). The change in enthalpy shows a linear relationship with the temperature and a nearly constant specific heat was determined even though this alloy was undercooled far below its liquidus temperature. The calculated results of the thermodynamic properties reveal that even though semiquantitative models fit with the experimental curve of the difference in Gibbs free energy well, they lead to different degrees of deviation in the entropy difference calculation.

Energy isosbestic points in third-row transition metal alloys.

The total electronic energies of the six electrons per atom (e per atom) alloys W, TaRe, HfOs, and YIr and the seven electrons per atom alloys Re, WOs, TaIr, HfPt, and YAu have been calculated in the local density approximation of density functional theory. When one considers common alloy structures such as atomically ordered variants of the body-centered cubic, face-centered cubic, or hexagonally closest packed structures and plots the total electronic energy as a function of the unit cell parameter, one finds for both the six and seven electrons per atom series energetic isosbestic points. An energetic isosbestic point corresponds to a critical value of the size parameter for which all members of the 6 or 7 e per atom series of compounds have nearly identical total electronic energy. Just as in spectroscopy, where the existence of such isosbestic points is the hallmark of two compounds present in the mixture, an energy isosbestic point implies there are just two separate energy curves. For both series it is found that the total electronic energy can be viewed as the weighted sum of a purely covalent term and a purely ionic term. Two semi-quantitative models are proposed to account for these two separate energies. In the first model the total energy is viewed as the sum of the elemental structural energy plus an ionic energy based on the Born-Mayer ionic model. In the second model one considers within the confines of mu2-Hückel theory the evolution of the total electronic energy as the Coulombic Hii integrals change in value.

Theoretical investigation of substrate specificity for cytochromes P450 IA2, P450 IID6 and P450 IIIA4.

Three-dimensional models of the cytochromes P450 IA2, P450 IID6 and P450 IIIA4 were built by means of comparative modeling using the X-ray crystallographic structures of P450 CAM, P450 BM-3, P450 TERP and P450 ERYF as templates. The three cytochromes were analyzed both in their intrinsic structural features and in their interaction properties with fifty specific and non-specific substrates. Substrate/enzyme complexes were obtained by means of both automated rigid and flexible body docking. The comparative analysis of the three cytochromes and the selected substrates, in their free and bound forms, allowed for the building of semi-quantitative models of substrate specificity based on both molecular and intermolecular interaction descriptors. The results of this study provide new insights into the molecular determinants of substrate specificity for the three different eukaryotic P450 isozymes and constitute a useful tool for predicting the specificity of new compounds.

Computational Materials Science: The Era of Applied Quantum Mechanics

For many centuries, materials were discovered, mined, and processed in a largely serendipitous way. However, the characterization of the atom and the progress made in x-ray diffraction during the early years of this century started a quest for a theory of materials in terms of their atomic constituents. Later decades saw scientists developing many qualitative and semi-quantitative models that explained the principles of atomic cohesion and the basic properties of semiconductors, metals, and salts. Considering their simplicity, some of the models were surprisingly accurate and led to remarkable progress. However, for most materials of current interest, the interatomic interactions are intricate enough to require fairly elaborate models. Fortunately, we are entering an era in which high-performance computing is coming into its own, allowing true predictive simulations of complex materials to be made from information on their individual atoms.

Turbidite systems, Part 1: Sequence and seismic stratigraphy

With the increased emphasis on deepwater exploration and development globally, considerable work has been done to understand turbidite systems. These are defined as sediments initially deposited in deep water, and which may now occur in the deeper parts of sedimentary basins (sometimes in great water depths). These deposits have been studied in terms of the lithofacies distribution, reservoir architecture, 2-D and 3-D seismic appearance, and their production capabilities. There are considerable stratigraphic variabilities in turbidite systems that affect reservoir performance and development plans. The petroleum industry’s increased understanding of these variabilities and their effects has occurred through the integration of a number of disciplines: 2-D and 3-D seismic, wireline logs, biostratigraphy, cores, petrophysics and rock properties, reservoir simulation, and outcrop studies for building semiquantitative reservoir models.

The perennial cup anemometer

A short version of the history of the cup anemometer precedes a more technical discussion of the special features of this instrument. These include its extremely linear calibration and the non-linearity of its response to wind speed changes. A simple conceptual model by Schrenk is used to demonstrate this and to explain why the cup anemometer is able to start from a zero rotation rate at zero wind to one corresponding to a sudden change in the ambient wind speed to a finite value. The same model is used to show that the cup anemometer should be characterized by a distance constant rather than by a time constant. The bias in the measured mean wind speed due to the random variations in the three velocity components is discussed in terms of standard, semiquantitative turbulence models, and the main thesis is that this bias is overwhelmingly dominated by the fluctuations of the lateral wind velocity component, i.e. the wind component perpendicular to the mean wind direction, and not, as is often assumed, by the longitudinal wind velocity component. It is shown theoretically and tested experimentally that the bias due to lateral wind velocity fluctuations can be significantly reduced by means of a special data processing of the simultaneous signals from a cup anemometer and a wind vane. This means that, with care, the overall overspeeding can be reduced to less than 1%. Copyright © 1999 John Wiley & Sons, Ltd.

Ab initio calculation of the NMR spin-lattice relaxation time and the diffusion coefficient of 21Ne in liquid and supercritical states

Molecular dynamics simulations are performed with ab initio calculated potentials and electric field gradient curves on different levels of approximation, to study their influence on the relaxation time and diffusion coefficient of fluid 21Ne. Additional properties, such as quadrupole couplings, effective electric field gradient correlation times, and diffusion correlation times, are studied to get a better understanding of the underlying mechanisms. Semiquantitative models for the total, self and cross correlation functions of the electric field gradient are developed.

An algorithm for complete enumeration of the mechanisms of supraventricular tachycardias that use multiple atrioventricular, AV nodal, and/or Mahaim pathways

The EINTHOVEN system is a model-based expert system that interprets the cardiac rhythm from the electrocardiogram. It simulates the expected behavior of realistic semi-quantitative cardiac models constructed by heuristic rules to generate interpretations that include both text descriptions and event-by-event causal explanations in the form of ladder diagrams. The simulation has been limited by an inability to predict all possible behaviors of hearts with more than one reentrant circuit. We now describe an algorithm that overcomes this limitation. Its output has been validated by an independent possibility-tree analysis. Timing and storage measurements are presented for models with up to three slow atrioventricular nodal pathways, four atrioventricular pathways, and a single atriofascicular (Mahaim) pathway. This is the first report in the literature of an algorithm that enumerates all possible mechanisms for reentrant supraventricular tachycardias that use atrioventricular, atrioventricular nodal, and/or atriofascicular pathways in humans.

Observation of inverted-molecular compression in boron carbide.

High-pressure neutron diffraction studies of boron carbide, ${\mathrm{B}}_{4}$C, to 11 GPa show that the icosahedral structural units are 23(4)% more compressible than the structure between them. This inverted-molecular compression is in accordance with qualitative predictions based on models of the bonding, but had not previously been observed directly. The results show the effect of the structural compression on the electrical resistivity to be more complex than present, semiquantitative models suggest.

901-36 Computer-based Interpretation of Intracardiac Electrograms: Identifying Rhythm Mechanisms from Ventricular Extrastimulus Tests

A computer-based system for analyzing complex cardiac rhythms is being developed in our laboratory. The system analyzes rhythms by adapting and simulating one or more semi-quantitative electrophysiologic models based on a sequence of events (P waves, QRS complexes, His bundle potential, etc.) on an event-by-event basis. The electrophysiologic model developed for the system can represent anatomies consisting of up to 4 accessory pathways, up to 3 AV nodal slow pathways, and 1 Mahaim pathway. We hypothesized that this system can (1) generate the correct electrophysiologic model(s) based on the observed events, (2) correctly differentiate between AV Nodal Reentrant Tachycardia (AVNRT), AV Reentrant Tachycardia (AVRT), and Atrial Tachycardia (ATACH) based on intracardiac electrogram data obtained during ventricular extrastimulus pacing protocols, and (3) generate ladder diagrams describing the identified rhythm mechanisms. Typical cases were identified for each of the desired rhythms (AVNRT, AVRT, and ATACH). Using software written in our lab, the cases were annotated to extract wave timing and morphology from the body-surface electrocardiogram and wave timing of the high right atrial. His bundle, and right ventricular intracardiac electrograms. Due to the anatomic complexity underlying reentrant tachyarrhythmias, we developed novel algorithms for enumerating the complete set of conduction paths through complex electrophysiologic models. We also developed the logic necessary for reasoning with multiple model paths and simultaneous activation of anatomically distinct model regions. The system was developed in C on a UNIX workstation and runs on UNIX workstations or IBM PC-compatible computers. In all cases, the system (1) generated the correct set of electrophysiologic models, (2) eliminated or supported the appropriate models (i.e. rhythm mechanisms) based on the ventricular extrastimulus, and (3) generated ladder diagrams describing the rhythm mechanisms. This is the first report of a computer-based system capable of deriving differential diagnoses with supporting ladder diagrams from intracardiac electrogram recordings. This demonstrates the potential for automatic analysis of complex rhythms by computer-assisted analysis of intracardiac electrograms. This system may be useful in the clinical electrophysiology laboratory by assisting physicians with the enormous amount of data generated during the typical electrophysiology study.

Molybdotungstovanadates [V2MonW(4–n)O19]4–

The complete series of hexametalate anions [V2MonW(4–n)O19]4–, with the two vanadiums cis and n= 0–4, has been identified in aqueous solution by 51V NMR spectroscopy. The vanadium shifts and the pKa values both increase with increasing substitution of Mo for W. They correlate well with semiquantitative models, showing that somewhat greater changes arise from substitutions taking place in the V2O9 plane. A consistent preference is also found for like metal atoms to lie mutually cis.

Modeling spatial patterns in Dictyostelium

The life cycle of Dictyostelium discoideum provides a striking example of the transition from single cell behavior to multicellular cooperativity. In this paper the status of the attempts at making semiquantitative models of the aggregation phase of this cycle is reviewed. Specifically, it is discussed how the propagation of cAMP waves is a typical example of excitable signaling, which is then rendered unstable by coupling to cell chemotaxis. To investigate the streaming pattern that emerges from this clumping instability, we next turn to a new simulation strategy, which couples dynamical cell-like entities ("bions") to continuum chemical concentration fields. Finally, we discuss two directions for further research: One is the study of the robustness with respect to the variation of system parameters (such as the cell density) exhibited by the biological system, but not by any simple model. The other concerns going beyond the aggregation phase to tackle the three-dimensional problem of slug formation and motion.

Controller Verification using Qualitative Reasoning

The performance of a heterogeneous (gain scheduling) controller is verified, for the control of a CSTR with a reversible exothermic reaction, in the face of sizable disturbances and under uncertainty. A qualitative reasoning technique. NSIM, is used to build and simulate semi-quantitative models with both parametric and non-parametric uncertainty. Bounds on the state variables are determined that are guaranteed to include all of the solutions to the uncertain model, and hence, this technique can be used for automatic proof construction.

Controller Verification Under Non-Parametric Uncertainty

This paper describes two methodologies for the verification of controllers for nonlinear processes with incompletely known parameters and functions (parametric and non-parametric uncertainty), one using symbolic manipulation to build bounding equations (NSIM) and one using a non-parametric Monte-Carlo method. NSIM builds and simulates semi-quantitative models and determines bounds on the state variables that are guaranteed to include all of the solutions to the uncertain model, but are conservative. A Monte-Carlo technique is introduced to handle non-parametric uncertainty: functions (rather man parameters) are randomly generated for simulation. The method produces bounds that are not conservative, but it is computationally expensive and its results are guaranteed to include all solutions only in the limit. This is illustrated for a CSTR with a reversible exothermic reaction and a heterogeneous (gain scheduling) controller.

Bulk activity measurements of organic compounds in aqueous electrolyte solutions and their implication in electrosorption studies

Bulk activity measurements of the slightly soluble aliphatic alcohols 1-butanol, 2-butanol, 1-pentanol, 3-methyl-1-butanol, and 1-hexanol in various aqueous electrolyte solutions were made using the gas chromatographic method of Mohilner et al. a[sub A], x[sub A], m[sub A], and f[sub A] are the alcohol activity, mole fraction, molality, and activity coefficient with f[sub A] defined by a[sub A] = f[sub A]x[sub A] and standard state defined by a[sub A] = 1 when x[sub A] = 1. Representation of data by lnf[sub A] = ln f[degrees][sub A]-k[prime][sub A]m[sub A] + k[double prime][sub A]m[sub A][sup 2], with k[prime][sub A] and k[double prime][sub A] adjustable parameters expected to depend on both alcohol and electrolyte, was investigated. Plots of lnf[sub A] vs m[sub A] were found to be linear within experimental error up to saturation value of m[sub A] in all cases investigated; i.e., the k[double prime][sub A]m[sub A][sup 2] contributed negligibly in all cases and concentrations investigated and could be omitted. Semiquantitative models, based on the principle of independent surface action (for water-alcohol interactions) and dependence of electrolyte activities on dielectric constant, were developed and found to provide reasonable qualitative and semiquantitative interpretation of parameter trends and basis for semiquantitative estimation of parameters formore » other similar systems. In electrosorption studies involving measurement of the dependence of interfacial tension on organic solute concentration at fixed polarization with respect to a fixed reference electrode (e.g., a saturated calomel electrode), the activity coefficients of both alcohol and electrolyte will change as the organic solute concentration changes. Bases for calculating these changes and their effect on the estimation of organic solute surface excess are given and discussed. 23 refs., 3 figs., 1 tab.« less

Artificial intelligence in process engineering—current state and future trends

Recent advances in artificial intelligence have changed the fundamental assumptions upon which the progress of computer-aided process engineering (modeling and methodologies) during the last 30 yr has been founded. Thus, in certain instances, numerical computations today constitute inferior alternatives to qualitative and/or semi-quantitative models and procedures which can capture and utilize more broadly- based sources of knowledge. In this paper it will be shown how process development and design, as well as planning, scheduling, monitoring, analysis and control of process operations can benefit from improved knowledge-representation schemes and advanced reasoning control strategies. It will also be argued that the central challenge coming from research advances in artificial intelligence is “modeling the knowledge”, i.e. modeling: (a) physical phenomena and the systems in which they occur; (b) information handling and processing systems; and (c) problem-solving strategies in design, operations and control. Thus, different strategies require different forms of declarative knowledge, and the success or failure of various design, planning, diagnostic and control systems depends on the extent of actively utilizable knowledge. Furthermore, this paper will outline the theoretical scope of important contributions from AI and what their impact has been and will be on the formulation and solution of process engineering problems.

Equilibrium state of diopside-bearing harzburgites from ophiolites: Geobarometric and geodynamic implications

The bulk compositions of coexisting enstatite and diopside in basal lherzolites and clinopyroxene-bearing harzburgites from ophiolitic complexes are typical of solidus/subsolidus equilibria, but for a few texturally distinct “magmatic” diopsides. They would presumably reflect the state of equilibrium at the time they last coexisted with liquid as the rocks reentered subsolidus conditions. The total lack of correlation between Al and Ca concentrations shows that the compositional scatter observed for any given massif, results from analytical errors related to extensive exsolution and serpentinization, rather than from differences in equilibrium conditions. However, significant differences are found between the residual ophiolitic lherzolites from Hare Bay, Newfoundland, and from Xigaze, Tibet, two massifs selected for their distinct structural and textural features. As for thermobarometry techniques relevant to these rocks, the best barometer found is an empirical relation for the expression of pressure as a virtually temperature-independent function of the ratioKf=(XDiopx)/(1 −XDicpx), in agreement with semi-quantitative models based on natural solid solutions. Temperatures are then simply derived from a surface-fitting expression relating pressure, temperature and diopside-solvus compositions, according to a regularXEncpx solution model (CMS) corrected for the effect of Al in the spinel facies. Application of these techniques yield pressures of 0.4 and 1.4 GPa, i.e. depths from sea-bottom of about 13 and 43 km, for temperatures of 1,170 and 1,300° C for the ophiolitic lherzolites of Tibet and New-foundland, respectively, in good agreement with dry-solidus data by radioactive tracing and with geothermal-model estimates for ridges.

Thermal diffusivity in mesophases: A systematic study in 4-4′-di-(n-alkoxy) azoxy benzenes

We present a systematic study of thermal diffusivity in the nematic liquid crystalline phase of 4-4′-di-(n-alkoxy) azoxy benzenes and several other materials. The data allow to separate the respective contributions of the rigid central core and of the flexible aliphatic chains. For the component of the diffusivity tensor parallel to the long molecular axis, it is found that they differ by as much as a factor of 6 in favor of the core contribution. Combining the present observations with our earlier results, we are now able to draw general conclusions on the thermal diffusivity in a nematic and smetic A, B, and C phases. We have also developed two semiquantitative models based, one on a static network of distributed resistances, and the other on an extension of the Eyring kinetic model for the thermal conductivity of simple liquids. Only the dynamic model yields a satisfactory description of our data. The distance over which the thermal energy is not transferred instantaneously upon collision between two neighboring molecules is found to correspond to the aliphatic chain length (calculated for a freely rotating chain). Using this formalism, a priori calculations of the thermal diffusivities can be performed, to better than 20%, for any rodlike liquid crystalline material.

Dynamics of multiple-photon excitation of CF 3Br studied in a supersonic molecular beam

The dynamics of multiple-photon absorption and dissociation of CF 3Br have been experimentally investigated in the collisional and collision-free regimes. Results are interpreted in terms of current semiquantitative models.