Assumed Model Parameters Research Articles

Numerical analysis of matric suction effects of tree roots

The use of native vegetation in the coastal regions of Australia has become increasingly popular for stabilising railway corridors built over expansive clays and compressive soft soils. The tree roots provide three stabilising functions: (a) they reinforce the soil; (b) they dissipate excess pore pressures; and (c) they establish sufficient matric suction to increase the shear strength. The matric suction generated within the tree root zone propagates radially into the soil matrix, as a function of the moisture content change. Considering soil conditions, the type of vegetation and atmospheric conditions, a mathematical model for the rate of root water uptake is developed. A conical shape is considered to represent the geometry of the tree root zone. Based on this model for the rate of root water uptake, the pore water pressure distribution and the movement of the ground adjacent to the tree are numerically analysed. Field measurements taken from the previously published literature are compared with the authors’ numerical predictions. It is found that, given the approximation of the assumed model parameters, the agreement between the predicted results and field data is still promising. The study indicates that native vegetation improves the shear strength of the soil by increasing the matric suction, and also curtails soil movements.

Monte Carlo estimation of uncertainties in BEIS3 emission outputs and their effects on uncertainties in chemical transport model predictions

A Monte Carlo (MC) probabilistic approach is used to estimate uncertainties in the emissions outputs of the Biogenics Emission Inventory System Version 3 (BEIS3) model and subsequent ozone outputs of three Chemical Transport Models (CTMs) due to uncertainties in many key BEIS3 biogenics emissions model parameters and inputs. BEIS3 was developed by the Environmental Protection Agency to estimate emissions of biogenic substances such as isoprene, monoterpenes, oxygenated and other volatile organic compounds (OVOCs), and biogenic nitric oxide (BNO). Uncertainties are investigated for three time periods, 24–29 May, 11–15 July, and 4–9 September 1995, which are ozone episodes that have been extensively studied by others as part of emissions control planning exercises. In the MC approach, 1000 samples are drawn randomly and independently from seventeen BEIS3 parameters and inputs, whose distribution shapes and variances are determined from literature reviews. The 95% confidence range on the calculated uncertainties in isoprene and BNO emissions cover approximately an order of magnitude. On the other hand, the 95% confidence ranges on the calculated uncertainties in monoterpenes and OVOC emissions are much smaller: about ±20%. Correlations are calculated between the 1000 MC samples of pairs of variations in model parameters or inputs and variations in BEIS3 daily emissions estimates for individual grid squares. A few significant correlations are found for some of the assumed model parameters. The MC uncertainties in the CTM‐predicted 1‐ and 8‐hour averaged ozone concentrations were studied by drawing 20 random samples from the 1000 sets of BEIS3 outputs and running each CTM (MAQSIP, UAM‐V, and URM) 20 times for the three episodes. The estimated total uncertainties of ±15 to 20% are found to be nearly the same for the three CTMs over the three time periods, for 1‐ and 8‐hour averages.

Magnetic relaxation and dipole-coupling-induced magnetization in nanostructured thin films during growth: A cluster Monte Carlo study

For growing inhomogeneous thin films with an island nanostructure similar to that observed in experiment we determine the nonequilibrium and the equilibrium remanent magnetization. The single-island magnetic anisotropy, the dipole coupling, and the exchange interaction between magnetic islands are taken into account within a micromagnetic model. A cluster Monte Carlo method is developed which includes coherent magnetization changes of connected islands. This causes a fast relaxation towards equilibrium for irregularly connected systems. We analyze the transition from dipole coupled islands at low coverages to a strongly connected ferromagnetic film at high coverages during film growth. For coverages below the percolation threshold the dipole interaction induces a collective magnetic order with ordering temperatures of 1--10 K for the assumed model parameters. Anisotropy causes blocking temperatures of 10--100 K and thus pronounced nonequilibrium effects. The dipole coupling leads to a somewhat slower magnetic relaxation.

An NMR line shape and relaxation analysis of heavy water powder spectra of the Lα, Lβ′and Pβ′phases in the DPPC/water system

The 2H2O NMR powder line shapes and relaxation times, T1 and T2, of the liquid crystal Lα, the intermediate Pβ′ and the gel Lβ′ phases of dipalmitoylphosphatidylcholine (DPPC)/2H2O-system are analysed. The water structure and dynamics of the lipid/water interfaces of DPPC in the hydration regime, where all water molecules are associated to the interface, are described in terms of orientational order parameters and correlation times. The line shape of the ripple phase (Pβ′) is analysed assuming model parameters of the gel or liquid crystalline phase. The narrow line shape of the ripple phase is partly due to an extra average of the quadrupole interaction because of lateral diffusion along the curved surface, reducing the splitting with a factor 0.5–0.2 depending on the nature of the curved ripple surface. However, more importantly, an extra reduction of the quadrupole splitting may be due to the same reorganization of water, among bound sites with different signs of the order parameter, which also explains the increase in the quadrupole splitting with temperature observed in the liquid crystalline phase. The linewidths in 14N MAS NMR spectra clearly indicate slow dynamics of the polar headgroup in the ripple phase. The results indicate that the headgroup hydrations of the ripple and liquid crystalline phases are similar, while the acyl chains are still in their gel state in the ripple phase. The increased headgroup area introduces a stress, as confirmed by the slow headgroup dynamics, which causes the bilayer to curve in the ripple phase.

A consistent set of single-scattering properties for cirrus cloud: tests using radiance measurements from a dual-viewing multi-wavelength satellite-based instrument

In this paper a consistent set of single-scattering properties is presented for radiative transfer calculations and remote sensing of cirrus cloud. The single-scattering properties consist of the extinction coefficient, single-scattering albedo and phase function. A randomly oriented randomized hexagonal ice aggregate is assumed to derive the extinction coefficient and single-scattering albedo. The phase function is an extension of the Henyey–Greenstein model called the “analytic” phase function, which is generated from the asymmetry parameter at non-absorbing and absorbing wavelengths. The satellite-based dual-view along track scanning radiometer (ATSR-2) instrument is utilized to test the single-scattering properties for consistency at scattering angles between about 60° and 170°, using a method of Optimal Estimation. Optimal Estimation is applied to a set of cloud parameters and radiance measurements, which are made simultaneously at the wavelengths of 0.87, 1.6, 3.7, 11.0 and 12.0 μm , over cases of cirrus cloud located in the tropics and mid-latitudes. If the single-scattering properties and assumed model parameters were a perfect representation of the radiative properties of cirrus then the measurement residuals (i.e., differences between measurements and simulated measurements) would be identically equal to zero at each of the wavelengths for all scattering angles. It is found that the randomized ice aggregate combined with the analytic phase function minimizes the measurement residuals to generally well within ±1% (reflectance) and ±1 K (brightness temperature) at 0.87, 11.0 and 12.0 μm and to within ±3% and ±3 K at 1.6 and 3.7 μm , respectively. This compares to measurement residuals of about 8% and 10 K if the single-scattering properties are based on the randomly oriented hexagonal ice column. It is recommended that single-scattering properties based on the randomized ice aggregate combined with the analytic phase function (or a very similar phase function) should be applied to remote sensing and radiative transfer studies of cirrus cloud.

Effect of frictional heating on pre-seismic sliding: a numerical simulation using a rate-, state- and temperature-dependent friction law

Summary Using a simple spring-block model and a rate- and state-dependent friction law incorporating the effect of temperature variation, I perform a numerical simulation of aseismic sliding prior to unstable (seismic) faulting. I compare the simulation result of pre-seismic sliding in the case where the temperature effect due to frictional heating on frictional strength is taken into consideration (heated fault) with the result in the case where the temperature effect is neglected (unheated fault), assuming model parameters appropriate for the nucleation site of a large interplate earthquake. It is found that an earthquake occurrence time is a little earlier on the heated fault than on the unheated fault. This is because the frictional strength is weakened by temperature rise due to frictional heating during pre-seismic sliding. The amplitude of precursory crustal deformation caused by pre-seismic sliding on the heated fault is estimated to be about 20 per cent smaller than that on the unheated fault.

Earthquake slip on oceanic transform faults.

Oceanic transform faults are one of the main types of plate boundary, but the manner in which they slip remains poorly understood. Early studies suggested that relatively slow earthquake rupture might be common; moreover, it has been reported that very slow slip precedes some oceanic transform earthquakes, including the 1994 Romanche earthquake. The presence of such detectable precursors would have obvious implications for earthquake prediction. Here we model broadband seismograms of body waves to obtain well-resolved depths and rupture mechanisms for 14 earthquakes on the Romanche and Chain transform faults in the equatorial Atlantic Ocean. We found that earthquakes on the longer Romanche transform are systematically deeper than those on the neighbouring Chain transform. These depths indicate that the maximum depth of brittle failure is at a temperature of approximately 600 degrees C in oceanic lithosphere. We find that the body waves from the Romanche 1994 earthquake can be well modelled with relatively deep slip on a single fault, and we use the mechanism and depth of this earthquake to recalculate its source spectrum. The previously reported slow precursor can be explained as an artefact of uncertainties in the assumed model parameters.

The Nonstellar Infrared Continuum of Seyfert Galaxies

JHKL'M (1 - 5 micrometers) imaging of a sample of Seyfert 2 galaxies is presented. We have performed an accurate estimate of the near-infrared non-stellar nuclear fluxes. We confirm that the near-infrared nuclear continuum between 1 and 2.2microns of some Seyfert 2s is dominated by stellar emission, whereas the continuum emission at longer wavelengths (lambda = 3 - 5 micrometers) is almost entirely non-stellar in origin. The non-stellar spectral energy distributions (SED) in the infrared (up to 15 micrometers) of Seyfert galaxies show a variety of shapes, and they are well reproduced with the tapered disk models of Efstathiou & Rowan-Robinson (1995). We have used two models, one including an optically thin cone component found to fit the SED of NGC 1068, and a coneless model. Although our modelling of the SEDs does not allow us to favor either model to account for all the observed SEDs, we find that the viewing angle towards the central source is well constrained by both models. The galaxies in our sample have fitted values of the viewing angle in the range Theta(sub V) = 0 deg - 64 deg, for the assumed model parameters. We have also investigated non-stellar color-color diagrams (L' - M vs. H - M and L' - M vs. H - L'). The colors of the Seyfert galaxies with viewing angles Theta(sub v) less than 30 deg are better reproduced with the cone model. These diagrams provide a good means to separate Seyfert 2s with moderate obscuration (A(sub V) approx. less than 20 mag from hard X-ray observations) from those with high obscuration. The ground-based 4.8 microns and ISO 9.6 microns luminosities are well correlated with the hard X-ray luminosities of Seyfert ls and 2s. These continuum emissions appear as a good indicator of the AGN luminosity, at least in the cases of hard X-ray Compton-thin Seyfert galaxies (N(sub H) less than or = 10(exp 24)/sq cm). We finally stress the finding that some Compton thick galaxies show bright non-stellar emission at 5 microns This suggests that the near-infrared emission in Seyfert galaxies is produced in an extended component illuminated by the central source, that is more visible from all viewing angles, providing a good explanation for the differing N(sub H)/A(sub V) ratios found in some Seyfert 2s. We discuss possible implications of mid-infrared surveys for the search of counterparts of highly obscured hard X-ray sources.

Capacitance transient spectroscopy models of coupled trapping kinetics among multiple defect states: Application to the study of trapping kinetics of defects in heavy-ion-damaged silicon

We have considered five different models of charge transfer among coupled defect states in semiconductors where the free-carrier density is limited by the density of unoccupied trap levels, as in the case of defect-dominated materials. To determine the time dependence of the trap occupancy features, we formulate a set of coupled differential equations that govern charge capture and emission processes for two defect states. A numerical solution assuming model parameters for traps provides features of the trap occupancy as a function of time. A critical comparison is made in occupancy features for different models, primarily categorized as serial (hierarchical) and parallel mechanisms of charge transfer. The model predictions are successfully applied to a study of trapping kinetics of defects observed in heavily damaged n-type silicon. We show that, in addition to the occurrence of charge redistribution among multiple traps, the major trap in the damaged silicon exists in two metastable configurations, perhaps with negative U (Hubbard correlation energy), and the stable configuration refers to a midgap compensating center related to a small cluster of self-interstitials. The applicability of our model simulations can be extended to more complex defect systems using a combination of these simple models.

Computer simulation of subchondral bone adaptation to mechanical loading in an incongruous joint.

We hypothesized that the typically bicentric distribution of subchondral bone density (i.e., two maxima) in incongruous joints with deeper sockets could be predicted by a computer simulation employing a concavely incongruous finite-element model and current bone remodeling theory. Additional objectives were to assess the uniqueness of the solution with respect to assumed model parameters and initial conditions and to determine the relationship between contact areas, subchondral bone stress, and subchondral bone density patterns. An idealized model of the humeroulnar joint was constructed with a quantitatively realistic representation of its natural incongruity. A currently accepted remodeling theory was implemented with a finite-element code using a node-based approach. The simulation predicted a dense subchondral bone plate after application of 3,000 daily load cycles for 300 days. The pronounced bicentric distribution of subchondral mineralization emerged. The solution was virtually independent of the initial density distribution and other assumed model parameters. Furthermore, the model predicted high tensile stresses in the subchondral bone, when the joint socket was spread apart during loading. Therefore, the locations of maximal strain energy density in the subchondral bone did not correspond with areas of joint contact. The results of the bone remodeling simulations are consistent with patterns of subchondral bone density determined experimentally. Furthermore, the solutions exhibited a high degree of uniqueness, were rather insensitive to changes in cartilage stiffness, moderately sensitive to number of applied loading cycles, and highly sensitive to loading magnitude. Tensile stresses seem to play a dominant role in subchondral bone remodeling due to bending in the subchondral bone plate. Thus, we conclude that, in the case of an incongruous joints with deeper sockets, the density of the subchondral bone cannot be regarded as a direct measure of the adjacent articular pressure.

Actuator with Built-In Viscous Damping for Isolation and Structural Control

The development and experimental application of an actuator with built-in viscous damping are described. A passive damper was modified to create a novel actuation device for two applications : isolation and structural control. The device uses the same fluid for damping and as a hydraulic lever for a voice coil actuator. For vibration suppression, active enhancement of the inherent passive damping can optimize broadband damping or target certain frequencies. For vibration isolation applications, the passive stiffness can be chosen for small gravity sag and the active control designed to provide reduced mount transmissibility over a broad- or narrow-frequency range. Lumped parameter models capturing structural and fluid effects are presented. Component tests of free stroke, blocked force, and passive complex stiffness are used to update assumed model parameters. The structural damping effectiveness of the new actuator is shown to be that of a typical passive viscous damper and that of an actively damped piezoelectric strut with load cell feedback in a complex truss structure. Open- and closed-loop results are presented for a force isolation application showing an 8-dB passive and a 20-dB active improvement over an undamped mount.

Ferroelectric behavior in microtubule dipole lattices: Implications for information processing, signaling and assembly/disassembly

Cytoskeletal microtubules structurally organize interiors of living eukaryotic cells. As polymers of subunit proteins (“tubulin”), which are each dipoles, microtubules are thus lattices of oriented dipoles. In general, three types of arrangements of dipoles in lattices may occur: (i) random, (ii) ferroelectric (parallel-aligned) and (iii) an intermediate weakly ferroelectric phase, which is length-dependent. Because of involvement in dynamical cell activities (movement, growth, mitosis, differentiation, etc.), models of microtubule signaling and information processing have been proposed. In these, tubulin units are assumed to represent informational “bit states” and to be coupled to intra-tubulin dipoles. In the present paper, we consider microtubules as lattice arrays of coupled local dipole states that interact with their immediate neighbors. Depending on the values of assumed model parameters, the system may exhibit “frustration”: conflict in satisfying all dipole couplings. Such systems have properties suitable for efficient information processing and computation. By slightly altering temperature and external field (both within physiological conditions), microtubule dipole lattices may assume a ferroelectric phase with long-range order and alignment with capabilities to propagate kink-like excitations. The ferroelectric phase appears to be optimal for microtubule signaling and assembly/disassembly. Microtubules may organize cell activities by operating in different modes suitable for information processing and computation (intermediate phase) or signaling and assembly/disassembly (ferroelectric phase).

A modeling procedure to evaluate the coherence of independently derived environmental quality objectives for air, water and soil

AbstractSets of independently derived environmental quality objectives (EQOs) for air, water, and soil may not be coherent in that maintaining the concentration at EQO level in one compartment may lead to exceeding EQO levels in other compartments. A methodology to evaluate this coherence is suggested. Starting from a steady concentration in the compartment of focus (the primary compartment), steady‐state concentrations in the adjacent (secondary) compartments are estimated using a multimedia fate model. If air is the primary compartment, steady‐state concentrations in water and soil close to the equilibrium concentrations can be expected, and coherence of EQOs can be evaluated easily by means of an extended equilibrium partitioning procedure. If water or soil is the primary compartment, the steady‐state concentration in air is usually well below the equilibrium concentration. Subequilibrium steady‐state concentrations are sensitive to assumed model parameters. The procedure is illustrated with the results of a coherence analysis for seven chemicals for The Netherlands.

Use of an Indoor Air Quality Model (IAQM) to Estimate Indoor Ozone Levels

Currently, outdoor ozone levels in many U.S. cities exceed the primary health-based national ambient air quality standard. While outdoor ozone levels are an important measure of the severity of those exceedances, people typically spend more than 80 percent of their time indoors, where ozone levels are lower. Indoor ozone levels range from 10 to 80 percent of outdoor levels, with many people receiving a substantial portion of their ozone exposure while indoors. This paper uses an indoor air quality model (IAQM) to estimate indoor ozone levels by microenvironment type (home, office, and vehicle) and configuration (windows open, windows closed, older construction, weatherized, and air conditioned). The formulation of IAQM is discussed, along with specification of model parameters for ozone. The multicompartment version of IAQM is described, with a single-compartment version used for the analyses. IAQM-calculated ozone indoor-outdoor ratios compare well with research-reported values. Results indicate that ozone peak-concentration indoor-outdoor ratios range as follows: home--0.65 (windows open), 0.36 (air conditioned), 0.23 (typical construction, windows closed), and 0.05 (energy-efficient construction, windows closed); office--0.82 (heating, ventilation and air conditioning systems supplying 100 percent outdoor air), 0.60 (typical HVAC), and 0.32 (energy-efficient HVAC); and vehicle--0.41 (85 mph), 0.33 (55 mph), and 0.21 (10 mph). Analysis results are presented to characterize IAQM's sensitivity to assumed model parameters.

Model of equatorial scintillations from in‐situ measurements

In‐situ measurements of F‐region irregularity amplitude and ambient electron density made by the retarding potential analyzer on OGO‐6 near the perigee altitude of 400 km have been utilized to derive the variation of electron density deviation over the equatorial region. Based on these measured electron density deviations and other assumed model parameters including a three‐dimensional power law form of irregularity spectrum with index 4, a model of equatorial scintillations is developed in the framework of diffraction theory. The percentage occurrence contours of estimated equatorial scintillations ≥ 4.5 dB at 140 MHz during 1900–2300 LMT for the period November–December 1969 and 1970 have been derived. The model is found to depict a pronounced longitude variation with the scintillation belt width and percentage occurrence being maximum over the African sector. The latitude extent of the spatial scintillation belt narrows over the American sector without much decrease in the scintillation occurrence whereas over the Indian and Far Eastern sectors both the extent and the occurrence are found to decrease. The percentage occurrence of scintillations estimated from this model is found to be consistent with VHF scintillation measurements at Ghana, Huancayo, and Calcutta. In addition, the model was found to be in qualitative agreement with GHz observations at various longitudes made by the COMSAT group. The effects of varying model parameters on scintillation estimates at VHF, UHF, and GHz are discussed. Implications of the observed longitudinal variation of scintillations on current theories of equatorial irregularity formation are indicated.