Sophisticated Numerical Models Research Articles

Dynamic mechanisms of the gulf surge

The gulf surge is a summertime phenomenon associated with the North American Monsoon that transports moisture from the Gulf of California into western portions of the United States. This phenomenon has been acknowledged operationally and by researchers for over 30 years. The surge is typically linked to the presence of some sort of low‐pressure system such as an easterly wave, mesoscale convective system, or hurricane located near the southern end of the Gulf of California. Proposed mechanisms for the gulf surge are gravity currents, ageostrophic flows, Kelvin, or Rossby edge waves. The purpose of this paper is to describe the fundamental dynamics of each of these mechanisms and assess their plausibility through scale analyses and idealized numerical simulations and to suggest signatures that may appear in data and output of more sophisticated numerical models.

Nickel based superalloy welding practices for industrial gas turbine applications

The continued drive for increased efficiency, performance and reduced costs for industrial gas turbine engines demands extended use of high strength-high temperature capability materials, such as nickel based superalloys. To satisfy the requirements of the component design and manufacturing engineers, these materials must be capable of being welded in a satisfactory manner. The present paper describes the characteristic defects found as a result of welding the more difficult, highly alloyed materials and reviews a number of welding processes used in the manufacture and repair of nickel alloy components. These include gas tungsten arc (GTA) and electron beam (EB) welding, laser powder deposition and friction welding. Many of the more dilute nickel based alloys are readily weldable using conventional GTA processes; however, high strength, precipitation hardened materials are prone to heat affected zone and strain age cracking defect formation. A number of factors are found to affect the propensity for defects: composition (aluminium and titanium content), grain size, pre- and post-weld heat treatment, as well as the welding process itself (control of heat input and traverse speed). Process parameter identification is still largely empirical and a fuller understanding of the joining processes is dependent upon the development and application of more sophisticated numerical modelling techniques.

Hydrologic flood routing in branched river systems via nonlinear optimization

The Muskingum method is one of the common hydrologic flood routing methods for rivers. The method is simple but its application is restricted to single reach problems. In order to apply this method for a certain reach two basic parameters should be determined first. These parameters are usually estimated by using trial and error combined with a curve-fitting technique. In this study, a method is proposed in which the calibration is performed by using a nonlinear optimization technique, which is capable to solve problems with more parameters. The proposed method is applied for (1) one river reach, (2) three branches in which six parameters must be determined, and (3) a river with an unknown intermediate catchment hydrograph. These problems were also solved by a sophisticated numerical hydraulic model applying the de SaintVenant equations. The comparison of the results indicated good agreement between the methods.

Description of a coupled atmosphere - fire model

This paper describes a coupled fire–atmosphere model that uses a sophisticated high-resolution non-hydrostatic numerical mesoscale model to predict the local winds which are then used as input to the prediction of fire spread. The heat and moisture fluxes from the fire are then fed back to the dynamics, allowing the fire to influence its own mesoscale winds that in turn affect the fire behavior. This model is viewed as a research model and as such requires a fireline propagation scheme that systematically converges with increasing spatial and temporal resolution. To achieve this, a local contour advection scheme was developed to track the fireline using four tracer particles per fuel cell, which define the area of burning fuel. Using the dynamically predicted winds along with the terrain slope and fuel characteristics, algorithms from the BEHAVE system are used to predict the spread rates. A mass loss rate calculation, based on results of the BURNUP fuel burnout model, is used to treat heat exchange between the fire and atmosphere. Tests were conducted with the uncoupled model to test the fire-spread algorithm under specified wind conditions for both spot and line fires. Using tall grass and chaparral, line fires were simulated employing the full fire–atmosphere coupling. Results from two of these experiments show the effects of fire propagation over a small hill. As with previous coupled experiments, the present results show a number of features common to real fires. For example, we show how the well-recognized elliptical fireline shape is a direct result of fire–atmosphere interactions that produce the ‘heading’, ‘flanking’, and ‘backing’ regions of a wind-driven fire with their expected behavior. And, we see how perturbations upon this shape sometimes amplify to become fire whirls along the flanks, which are transported to the head of the fire where they may interact to produce erratic fire behavior.

Probing geodynamic processes beneath the sea floor

A new initiative in oceanic mantle dynamics is being designed to test geodynamic models of plate tectonics and convection with a new generation of higher‐resolution observations in the oceans. The plate tectonic revolution provided a quantitative kinematic framework for describing geodynamic processes in the solid Earth. Three and one‐half decades of geological, geophysical, and geochemical observations since this revolution, coupled with increasingly sophisticated numerical models of mantle dynamics, have led to an increased understanding of how plate tectonics works and how convection stirs the interior of the Earth. However, within this broad framework, many fundamental questions remain regarding the structure, composition, and dynamics of the sub‐oceanic mantle.

Optimizing imbalance and loss in 2 x 2 3-db multimode interference couplers via access waveguide width

The imbalance and excess loss in multimode interference couplers with fabrication errors are examined. Remarkably, there exists a number of optimum access waveguide widths which give a minimum imbalance. Furthermore, quite low excess loss can be simultaneously achieved by choosing one particular optimum width. It is also shown that paired interference couplers have better imbalance performance than general interference couplers, given the same fabrication tolerances and length constraint for both coupler types. In particular, a paired interference coupler with an access waveguide width approximately 0.3 times the coupler width has the best imbalance performance. These results are obtained using a simple model and are supported by more sophisticated numerical models and experimental data from couplers fabricated in InP/InGaAsP.

Cosmic dust physical properties and theICAPS facility on board the ISS

Cometary comae, cometary tails, and the interplanetary dust cloud, are low density dust clouds built of cosmic dust particles. Light scattering observations, from in-situ space probes and remote observatories, are a key to their physical properties. This presentation updates results on cometary and interplanetary dust derived from such observations (with emphasis on polarization), and compares them with results on asteroidal regoliths. The polarization phase curves follow similar trends, with parameters that may vary from one object to another. The wavelength dependence is highly variable, although it is usually linear in the visible domain. It may be suggested (from observations, modeling and laboratory measurements) that these dust particles are irregular, with a size greater than the wavelength, and that cometary dust is highly porous, as compared to asteroidal or interplanetary dust. Sophisticated numerical models and laboratory measurements on dust analogues are indeed required to interpret without any ambiguity the ensemble of results. The opportunity offered by the ICAPS facility (an ESA project selected for the ISS, now in phase B) to deduce the physical properties of cosmic dust particles from their optical properties, as well as their evolution (breaking-off and agglomeration, ices condensation and evaporation), is presented.

A Simple Model forr‐Process Scatter and Halo Evolution

Recent halo-star observations of heavy elements produced by rapid neutron capture (r-process) show a striking behavior: within a single star, the relative abundances of r-process elements heavier than Ba are the same as those of solar system matter, while across stars with similar metallicity Fe/H, the r/Fe ratio varies by more than 2 orders of magnitude. In this paper we present a simple analytic model that describes a star's abundances in terms of its ancestry, i.e., the number of nucleosynthesis events (e.g., supernova explosions) that contributed to the star's composition. This model leads to a very simple analytic expression for the abundance scatter versus Fe/H, which is in good agreement with the data and with more sophisticated numerical models. We investigate two classes of scenarios for r-process nucleosynthesis, one in which r-process synthesis events occur in only ~4% of supernovae but iron synthesis is ubiquitous, and one in which iron nucleosynthesis occurs in only about 9% of supernovae (the Wasserburg-Qian model). We find that the predictions in these scenarios are similar for [Fe/H] -2.5, but that these models can be readily distinguished observationally by measuring the dispersion in r/Fe at [Fe/H] -3.

Plasma sheet access to geosynchronous orbit: Generalization to numerical global field models

A previous statistical analysis of the geosynchronous particle environment by Korth et al. [1999] showed that the particle fluxes are well organized by local time and the geomagnetic activity as measured by the Kp index. Regions of high and low fluxes at geosynchronous orbit are separated from each other by distinct boundaries, approximately matching the Alfvén boundary crossings of the geosynchronous satellites calculated analytically from a Volland‐Stern electric potential and a dipole magnetic field. Expanding the analysis technique from the previous work to arbitrary, numerical electric and magnetic field models, we reevaluate the Alfvén boundary crossings using several available global electric potential models and considering external magnetic field contributions. The more sophisticated numerical models do not do a better job of explaining the observed average access of plasma sheet material to the geosynchronous region than does the simple analytical model.

Nature of the boundary between open and closed magnetic field line regions at the Sun revealed by composition data and numerical models

Recently, Fisk et al. [1999] have presented a theory that describes a number of features of the large‐scale coronal and heliospheric magnetic field. This theory predicts large‐scale transport of magnetic flux across the boundaries of the polar coronal holes, which leads to reconnection processes of open field lines with preliminary closed magnetic structures. Reconnection processes reveal themselves in solar wind composition data: Plasma released out of previously closed magnetic field structures exhibits hotter charge state distributions and has a tendency to be enriched by elements with low first ionization potentials. The idea of reconnection at the boundaries of coronal holes is not new. For example, Wang and Sheeley [1993] and Luhmann et al. [1999] found evidence for that mechanism by comparison of observations of the rotation and evolution of coronal holes with potential field models of the solar corona. We use Ulysses Solar Wind Ion Composition Spectrometer composition measurements and sophisticated numerical models [Linker et al., 1999; Riley et al., 1999] to accurately map these observations back to the solar surface. We then constrain the thickness of the stream interface at the Sun and compare the location of the source region with SOHO observations of the low corona. The results are discussed in the context of the global structure of the heliospheric magnetic field.

Experimental and theoretical analysis of phase singularity dynamics in cardiac tissue.

Quantitative analysis of complex self-excitatory wave patterns, such as cardiac fibrillation and other high-order reentry, requires the development of new tools for identifying and tracking the most important features of the activation, such as phase singularities. Image processing operations can be used to detect the phase singularity at the tip of a spiral wave. The phase space behavior of a spatiotemporal sequence of data may be reconstructed using time-series analysis. The phase singularities then are localized efficiently by computing the topologic charge density as the curl of the spatial phase gradient. We analyzed the singularity interaction dynamics of both experimentally observed and numerically simulated instances of quatrefoil reentry and found that the singularity behavior in the experimental preparations can be classified into three categories on the basis of how their separation changes with time. Topologic charge densities can be calculated easily and efficiently to reveal phase singularity behavior. However, the differences between theoretical and experimental observations of singularity separation distances indicate the need for more sophisticated numerical models.

Gear Hobbing Cutting Process Simulation and Tool Wear Prediction Models

Gear hobbing is an efficient method to manufacture high quality and performance toothed wheels, although it is associated with complicated process kinematics, chip formation and tool wear mechanisms. The variant cutting contribution of each hob tooth to the gear gaps formation might lead to an uneven wear distribution on the successive cutting teeth and to an overall poor tool utilization. To study quantitatively the tool wear progress in gear hobbing, experimental-analytical methods have been established. Gear hobbing experiments and sophisticated numerical models are used to simulate the cutting process and to correlate the undeformed chip geometry and other process parameters to the expected tool wear. Herewith the wear development on the individual hob teeth can be predicted and the cutting process optimized, among others, through appropriate tool tangential shifts, in order to obtain a uniform wear distribution on the hob teeth. To determine the constants of the equations used in the tool wear calculations, fly hobbing experiments were conducted. Hereby, it was necessary to modify the fly hobbing kinematics, applying instead of a continuous tangential feed, a continuous axial one. The experimental data with uncoated and coated high speed steel (HSS) tools were evaluated, and correlated to analytical ones, elaborated with the aid of the numerical simulation of gear hobbing. By means of the procedures described in this paper, tool wear prediction as well as the optimization of various magnitudes, as the hob tangential shift parameters can be carried out.

Performance modeling of an airborne Raman water-vapor lidar

We have developed a sophisticated Raman lidar numerical model to simulate the performance of two ground-based Raman water-vapor lidar systems. After verifying the model using these ground-based measurements, we then used the model to simulate the water-vapor measurement capability of an airborne Raman lidar under both daytime and nighttime conditions for a wide range of water-vapor conditions. The results indicate that, under many circumstances, the daytime measurements possess comparable quality to an existing airborne differential absorption water-vapor lidar whereas the nighttime measurements have improved spatial and temporal resolution. In addition, an airborne Raman lidar can offer measurements that are difficult or impossible with the differential absorption lidar technique.

Modulation of Cosmic Rays in the Heliosphere from Solar Minimum to Maximum: A Theoretical Perspective

The modulation of galactic cosmic rays in the heliosphere seems to be dominated by four major mechanisms: convection, diffusion, drifts (gradient, curvature and current sheet), and adiabatic energy losses. In this regard the global structure of the solar wind, the heliospheric magnetic field (HMF), the current sheet (HCS), and that of the heliosphere itself play major roles. Individually, the four mechanisms are well understood, but in combination, the complexity increases significantly especially their evolvement with time - as a function of solar activity. The Ulysses observations contributed significantly during the past solar minimum modulation period to establish the relative importance of these major mechanisms, leading to renewed interest in developing more sophisticated numerical models, and in the underlying physics, e.g., what determines the diffusion tensor. With increased solar activity, the relative contributions of the mentioned mechanisms change, but how they change and what causes these changes over an 11-year solar cycle is not well understood. It can therefore be expected that present and forthcoming observations during solar maximum activity will again produce very important insights into the causes of long-term modulation. In this paper the basic theory of solar modulation is reviewed for galactic cosmic rays. The influence of the Ulysses observations on the development of the basic theory and numerical models are discussed, especially those that have challenged the theory and models. Model-based predictions are shown for what might be encountered during the next solar minimum. Lastly, modulation theory and modelling are discussed for periods of maximum solar activity when a global reorganization of the HMF, and the HCS, occurs.

Analytical Modeling of Contaminant Transport through Vadose and Saturated Soil Zones

Screening level analyses for risk-based corrective actions and preliminary remediation investigations and feasibility studies for contaminated sites require simple and conservative models for the transport of contaminants to potential receptor locations. A quasi-analytical method is presented to analyze the transport of contaminants originating at a source located near the ground surface separated from the water table by a relatively thick vadose zone. The method includes one segment to compute source decay rate, a second to simulate vertical transport through the vadose zone, and a third to simulate saturated zone transport to the receptor. Practical application of the method is demonstrated by an illustrative example. The method may be useful and convenient for situations where the objectives of the analysis do not warrant the use of sophisticated numerical models.

Numerical modelling for design and evaluation of groundwater remediation schemes

The growth in readily available computational resources has led to increasingly sophisticated numerical models of biologically mediated, reactive chemical transport in aquifers. Such models are able to both simulate and predict behaviour of contaminants over reasonably long time scales and under a variety of circumstances and conditions. Application of one such model is demonstrated in two contamination scenarios, each of which constitutes a significant environmental problem. The first concerns acid mine drainage (AMD), a widespread problem facing mining and other industries. The second concerns contamination of soil and groundwater by benzene, toluene, ethylbenzene, xylene (BTEX) compounds, resulting from spills or leaks of petroleum hydrocarbons. In each case, the model is used to simulate both the production and evolution of the contaminants as they migrate in groundwater, and also to evaluate remediation schemes.

Three‐dimensional Spherical Simulations of Solar Convection. I. Differential Rotation and Pattern Evolution Achieved with Laminar and Turbulent States

Rotationally constrained convection possesses velocity correlations that transport momentum and drive mean —ows such as diUerential rotation. The nature of this transport can be very complex in turbu- lent —ow regimes, where large-scale, coherent vorticity structures and mean —ows can be established by smaller scale turbulence through inverse cascades. The dynamics of the highly turbulent solar convection zone therefore may be quite diUerent than in early global-scale numerical models, which were limited by computational resources to nearly laminar —ows. Recent progress in high-performance computing tech- nology and ongoing helioseismic investigations of the dynamics of the solar interior have motivated us to develop more sophisticated numerical models of global-scale solar convection. Here we report three- dimensional simulations of compressible, penetrative convection in rotating spherical shells in both laminar and turbulent parameter regimes. The convective structure in the laminar case is dominated by ii banana cells,ˇˇ but the turbulent case is much more complex, with an intricate, rapidly evolving down- —ow network in the upper convection zone and an intermittent, plume-dominated structure in the lower convection zone and overshoot region. Convective patterns generally propagate prograde at low lati- tudes and retrograde at high latitudes relative to the local rotation. The diUerential rotation pro—les show some similarity with helioseismic determinations of the solar rotation but still exhibit signi—cantly more cylindrical alignment. Strong, intermittent, vortical down—ow lanes and plumes play an important dynamical role in turbulent —ow regimes and are responsible for signi—cant diUerences relative to laminar —ows with regard to momentum and energy transport and to the structure of the overshoot region at the base of the convection zone. Subject headings: convectionhydrodynamicsstars: interiorsSun: interiorSun: rotation ¨ turbulence

Modeling equator-crossing currents on the ocean bottom

Observations unambiguously show that deep ocean currents carry a significant amount of fluid across the equator. Away from the equator in either hemisphere, these currents are relatively quiescent so that planetary vorticity dominates relative vorticity within the fluid. Thus, the po- tentialvorticity of cross-equatorialflow changes sign en route. The breakdown of geostrophic balance at the equator because of the vanishing horizontalcomponent of the Coriol is force and the fact that potentialvorticity is not conserved in these flows constitute formidable challenges to modeling these cross- equatorialcurrents. Recent research points to friction as being crucialto the crossing process since it provides the mechanism by which potential vorticity can be altered. As well, since these flows are bottom-dwelling currents, the geometry of the bottom topography is an important factor in determining the portion of the current which successfully crosses the equator. We examine the dynamicalbalances within equator-crossing flows by studying a simplified model of the flow in the equato- rialregion. This modelretains the effects of friction and bot- tom topography. We compare the predictions of this model with the predictions of more sophisticated numericalmodel s and with observations. It is shown that, despite the simplicity of the model, it captures certain aspects of the flow quite well.

The coupling of solar convection and rotation - (Invited Review)

In recent years, helioseismology has provided an unprecedented look at the dynamics of the solar interior. These new insights have been accompanied by tremendous advances in high-performance computing technology, prompting increasingly sophisticated and realistic numerical models of solar convection. Among the most important helioseismic constraints on global-scale convection models is the mean differential rotation profile of the solar envelope, which is established by convection under the influence of rotation. The highly turbulent nature of solar convection makes this rotational influence difficult to determine and model. I will begin this review by discussing the solar rotation profile inferred from helioseismic measurements and various theoretical and numerical approaches to account for it. Computational constraints limited early numerical models to relatively laminar flow regimes but more recent investigations have begun to explore the distinct nature of turbulent convection. After a brief overview of empirical and numerical results on the related Rayleigh-Bernard system, I will outline the current state of numerical modeling of turbulent convection in rotating, stratified fluids, first in Cartesian and then in spherical geometries. The emphasis throughout will be on how rotation influences the structure, evolution, and transport processes of turbulent convection and what type of differential rotation can result.

Alis, a state-of-the-art optical observation network for the exploration of polar atmospheric processes

Abstract An optical group at the Swedish Institute of Space Physics in Kiruna, Sweden has been developing the ALIS (Auroral Large Imaging System) multi-station optical observing network which makes it possible to obtain composite monochromatic 2-D images over a fairly wide field-of-view (FOV), and more interestingly, a CT (Computed Tomography) image set for the retrieval of 3-D structure of aurora by adjusting vergence angles of cameras to a common volume. National Institute of Polar Research, Japan is collaborating in observation and analysis. At the moment, the network has 6 stations separated from each other by about 50 km. Each station houses a monochromatic CCD (Charge Coupled Device) imaging system mounted on the steerable azimuth/elevation drive along with a house keeping unit and supervising computer linked to the control center via a telephone line. Altitude profiles of luminosity for stable arc and aurora vortex at 557.7nm and recently at 427.8nm are analysed by the algebraic reconstruction technique and compared with sophisticated numerical modelling of auroral emission rate. Conjunctions with satellites and radars are now intensively explored towards comprehensive understanding of the formation and electrodynamics of aurora. Imaging of polar stratospheric clouds is also attempted in relation to arctic environmental studies.