bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow Control Using Rotating Cylinders: Effect of Gap
Authors: Mittal S.
Published: October 2003

The flow past a bluff body can be controlled significantly by placing small rotating cylinders at appropriate locations. Computational results for control of Re=104 flow past a circular cylinder are presented. Two control cylinders of one-twentieth the diameter of the main cylinder rotate at a rate such that their tip speed is five times the free-stream speed of the flow. Computations are carried out for various values of the gap between the main and control cylinders. A stabilized finite element method is utilized to solve the incompressible Navier-Stokes equations in the primitive variables formulation. A gap value of one-tenth the diameter of the main cylinder is found to be close to the optimal value. Compared to the flow past an isolated cylinder a very significant reduction in the drag and unsteady forces is observed for the flow with control.
Numerical Analysis of Cavitating Flow of Liquid Helium in a Converging-Diverging Nozzle
Authors: Ishimoto Jun , Kamijo Kenjiro
Published: October 2003

The fundamental characteristics of the two-dimensional cavitating flow of liquid helium through a horizontal converging-diverging nozzle near the lambda point are numerically investigated to realize the further development and high performance of new multiphase superfluid cooling systems. First, the governing equations of the cavitating flow of liquid helium based on the unsteady thermal nonequilibrium multifluid model with generalized curvilinear coordinates system are presented, and several flow characteristics are numerically calculated, taking into account the effect of superfluidity. Based on the numerical results, the two-dimensional structure of the cavitating flow of liquid helium though a horizontal converging-diverging nozzle is shown in detail, and it is also found that the generation of superfluid counterflow against normal fluid flow based on the thermomechanical effect is conspicuous in the large gas phase volume fraction region where the liquid to gas phase change actively occurs. Furthermore, it is clarified that the mechanism of the He I to He II phase transition caused by the temperature decrease is due to the deprivation of latent heat for vaporization from the liquid phase.
Measurements of Resistance of Individual Square-Mesh Screens to Oscillating Flow at Low and Intermediate Reynolds Numbers
Authors: Wakeland Ray Scott , Keolian Robert M.
Published: October 2003

Measurements are reported of pressure losses across single screens subjected to low-frequency oscillating flow for 0.002≲Red≲400, where Red is Reynolds number based on wire diameter and peak approach velocity. Several correlation methods are examined. Extensive comparisons are made between present oscillating-flow results and previous reports of the resistance of screens to steady flow. Defining oscillating results in terms of peak amplitudes, the oscillating and steady-flow resistances are found to be the same, including behavior in the intermediate Reynolds number region that departs from correlations of the form ARe−1+B. The friction factor is also found to depend on Reynolds number, but not independently on oscillation amplitude, over the range of conditions measured.
Modeling of the Onset of Gas Entrainment Through a Finite-Side Branch
Authors: Ahmed M. , Hassan I. , Esmail N.
Published: October 2003

A theoretical investigation has been conducted for the prediction of the critical height at the onset of gas entrainment during single discharge from a stratified, two-phase region through a side branch with a finite diameter. Two different models have been developed, a simplified point-sink model and a three-dimensional finite-branch model. The two models are based on a new criterion for the onset of gas entrainment. The results of the predicted critical heights at the onset of gas entrainment showed that the finite-branch model approaches the physical limits at low Froude numbers. However, as the values of the Froude number increased, the predictions of both models eventually converged to the same value. Based on the results of the models, the critical height corresponding to the onset of gas entrainment was found to be a function of Froude number and fluid densities. The results of both models are compared with available experimental data. The comparisons illustrate a very good agreement between the measured and predicted values.
Supersonic Through-Flow Fan Blade Cascade Studies
Authors: Chesnakas Christopher J. , Ng Wing F.
Published: October 2003

An investigation has been performed of the flow in a supersonic through-flow fan blade cascade. The blade shapes are those of the baseline supersonic through-flow fan (STF). Measurements were made at an inlet Mach number of 2.36 over a 15 deg range of incidence. Flowfield wave patterns were recorded using spark shadowgraph photography and steady-state instrumentation was used to measure blade surface pressure distributions and downstream flowfield. From these measurements, the integrated loss coefficients are presented as a function of incidence angle along with analysis indicating the source of losses in the STF cascade. The results are compared with calculations made using a two-dimensional, cell-centered, finite-volume, Navier-Stokes code with upwind options. Good general agreement is found at design conditions, with lesser agreement at off-design conditions. Analysis of the leading edge shock shows that the leading edge radius is a major source of losses in STF blades. Losses from the leading edge bluntness are convected downstream into the blade wake, and are difficult to distinguish from viscous losses. Shock losses are estimated to account for 70% to 80% of the losses in the STF cascade.
Elastic Field in a Semi-Infinite Solid due to Thermal Expansion or a Coherently Misfitting Inclusion
Authors: Davies J. H.
Published: October 2003

It is shown that the elastic field due to nonuniform temperature or a coherently misfitting inclusion in a semi-infinite region can be derived simply from the corresponding field in an infinite region. This follows from the work of Mindlin and Cheng [J. Appl. Phys. 21, 931 (1950)] but it is not necessary to calculate the thermoelastic potential itself. In particular, the displacement of the free surface is the same as that of the equivalent plane in an infinite solid, increased by a factor of 4(1−ν). The change in volume associated with the distortion of the surface is reduced by a factor of 2(1+ν)/3 from the free expansion of the inclusion. A rectangular inclusion is used to illustrate the theory.
Unsteady Gust Response of Road Vehicles
Authors: Filippone Antonio
Published: October 2003

A theoretical model based on an indicial method is proposed to simulate the unsteady response of a series of road vehicles, including high-speed trains, sports utility vehicles, sports cars, caravans, and pick-up trucks. The response is described in the frequency domain by the aerodynamic admittance for both side force and yawing moment. The properties of the admittance function are discussed for basic two-dimensional geometries, and the existence of critical damping is shown for a number of cases. The vehicles are undergoing aerodynamic forcing in the form of a gust. Systems with one degree-of-freedom were considered. The results show that the main parameters affecting the vehicle’s aerodynamic response are the mean vehicle length compared to the wave length of the gust, and the inclination of the nose.
Experimental and Numerical Study of Unsteady Flow in a Diffuser Pump at Off-Design Conditions
Authors: Wang Hong , Tsukamoto Hiroshi
Published: October 2003

An experimental and numerical study was developed for the unsteady phenomena at off-design conditions of a diffuser pump. Unsteady pressure measurements were made downstream of the impeller, and the pressure fluctuations were analyzed using the ensemble averaging technique as well as the statistical and chaotic time series analysis. The unsteady flow was classified into five ranges as a result of the statistical and chaotic time series analysis. And a two-dimensional vortex method was employed to investigate the unsteady flow structure due to the interaction between impeller and diffuser vanes in a diffuser pump at various off-design conditions. The numerical results of unsteady flow at a partial discharge range (approximately 83% of the rated flow rate) show an asymmetrical separation bubble near the pressure surface of the impeller vane. The intermittence of the separation bubble may be the main factor to cause the unstable characteristics of the test diffuser pump. The calculated unsteady flow at the lower partial discharge range (50% of the rated flow rate) presents a rotating stall in the impeller passage as well as in the diffuser passage, which can be main cause of unstable characteristics there.
Solidification of a Finite Medium Subject to a Periodic Variation of Boundary Temperature
Authors: Dursunkaya Z. , Nair S.
Published: October 2003

The motion of a solid-liquid interface in a finite one-dimensional medium, subject to a fluctuating boundary temperature, is analyzed. The fluctuations are assumed to be periodic. The solution method involves a semi-analytic approach in which, at any given time, the spatial temperature distributions are represented in infinite series. The effect of the solid, liquid Stefan numbers and the unsteady boundary temperature variation is investigated. The results showed a retrograde motion of the solidification front for large liquid Stefan numbers.
Jørgen Lund: A Perspective on His Contributions to Modern Rotor Bearing Dynamics
Authors: Smalley Anthony J.
Published: October 2003

This paper presents a perspective on the work of Dr. J. W. Lund, in the field of rotor-bearing dynamics. It traces his most influential work in published journals, software, data published in various manuals, government reports, and course notes. It addresses his work in oil bearings, gas bearings, unbalance response of flexible rotors, stability of flexible rotors, squeeze-film dampers, rotor balancing, and modal testing of rotors. It provides a road map for those who would revisit the knowledge base. It shows how Dr. Lund’s work has permeated the practice of today’s suppliers and users of turbomachinery, and industry consultants.
Application of Lund’s Stability Analysis Program in Design of Modern Turbomachinery
Authors: De Choudhury Pranabesh
Published: October 2003

The analytical tool developed by Lund has aided in the design and manufacture of modern turbomachinery operating at relatively high speeds, under severe operating conditions, with high gas pressure, and gas density. The purpose of this paper is to show, by means of selected problems, as a practicing engineer in the design of high speed turbomachinery, how the methods developed by Lund aided in the design and problem diagnostics of such turbomachinery.
Some Unusual Cases of Rotor Instability
Authors: Olsson Karl-Olof
Published: October 2003

In the paper presented here three peculiar cases of rotor instability are presented. The first is a tilting-pad bearing used as an intershaft bearing in a two-spool gas turbine. It was initially chosen for its good stability properties. However, in a deeper study it was found that those benefits are greatly reduced when both inner shaft and outer bearing are rotating. The second case comprises a turbine which showed a very strange performance with vibrations alternating between heavy and mild amplitudes. The periods were a couple of minutes long. After a more elaborate instrumentation was installed it was found that vibrations were synchronous and that the rotor experienced continuous changes of vibrations interrupted by sudden changes of amplitude. This revealed that nonlinearity and thermal effects coupled to the rotor vibrations must be incorporated to understand the behavior. Many hypotheses were tested and the one coming closest to explaining the case was one incorporating thermal bending caused by heat conduction through an annulus partly filled with oil. The third case is a simple one of more pure curiosity, where a rotor was provided with a substantial amount of inner damping, and just according to the school book showed dynamic instability.
Approximate Model for a Viscoelastic Oscillator
Authors: Ketema Y.
Published: October 2003

An oscillator where the restoring force is furnished by a viscoelastic bar and therefore depends on the history of the motion is considered. The history-dependent force is characterized by a relaxation modulus and a relaxation time. Assuming that the relaxation time is small, an approximate model for the oscillator is derived. This model is then linearized for the study of small vibrations. It is shown that the viscoelastic force, in addition to viscous damping, effects an apparent decrease in mass that modifies the natural frequency of the linear oscillator. The temperature dependence of the relaxation time, and consequently the frequency shift, is studied.
Hydrostatic Pressurization and Depletion of Trapped Lubricant Pool During Creep Contact of a Rippled Indenter Against a Biphasic Articular Cartilage Layer
Authors: Soltz Michael A. , Basalo Ines M. , Ateshian Gerard A.
Published: October 2003

This study presents an analysis of the contact of a rippled rigid impermeable indenter against a cartilage layer, which represents a first simulation of the contact of rough cartilage surfaces with lubricant entrapment. Cartilage was modeled with the biphasic theory for hydrated soft tissues, to account for fluid flow into or out of the lubricant pool. The findings of this study demonstrate that under contact creep, the trapped lubricant pool gets depleted within a time period on the order of seconds or minutes as a result of lubricant flow into the articular cartilage. Prior to depletion, hydrostatic fluid load support across the contact interface may be enhanced by the presence of the trapped lubricant pool, depending on the initial geometry of the lubricant pool. According to friction models based on the biphasic nature of the tissue, this enhancement in fluid load support produces a smaller minimum friction coefficient than would otherwise be predicted without a lubricant pool. The results of this study support the hypothesis that trapped lubricant decreases the initial friction coefficient following load application, independently of squeeze-film lubrication effects.
Influence of Stress Magnitude on Water Loss and Chondrocyte Viability in Impacted Articular Cartilage
Authors: Milentijevic Dejan , Helfet David L. , Torzilli Peter A.
Published: October 2003

The objective of this study was to assess mechano-biological response of articular cartilage when subjected to a single impact stress. Mature bovine cartilage explants were impacted with peak stresses ranging from 10 to 60 MPa at a stress rate of 350 MPa/s. Water loss, matrix axial deformation, dynamic impact modulus (DIM), and cell viability were measured immediately after impaction. The water loss through the articular surface (AS) was small and ranged from 1% to 6% with increasing peak stress. The corresponding axial strains ranged from 2.5% to 25%, respectively, while the DIM was 455.9±111.9 MPa. Chondrocyte death started at the articular surface and increased in depth to a maximum of 6% (70 μm) of the cartilage thickness at the highest stress. We found that the volumetric (axial) strain was more than twice the amount of water loss at the highest peak stress. Furthermore, specimens impacted such that the interstitial water was forced through the deep zone (DZ) had less water loss, a higher DIM, and no cell death. These findings appear to be due to matrix compaction in the superficial region causing higher compressive strains to occur at the surface rather than in the deeper zones.
Comparison of Thermal Insulation Performance of Fibrous Materials for the Advanced Space Suit
Authors: Paul Heather L. , Diller Kenneth R.
Published: October 2003

The current multi-layer insulation used in the extravehicular mobility unit (EMU) will not be effective in the atmosphere of Mars due to the presence of interstitial gases. Alternative thermal insulation means have been subjected to preliminary evaluation by NASA to attempt to identify a material that will meet the target conductivity of 0.005 W/m-K. This study analyzes numerically the thermal conductivity performance for three of these candidate insulating fiber materials in terms of various denier (size), interstitial void fractions, interstitial void media, and orientations to the applied temperature gradient to evaluate their applicability for the new Mars suit insulation. The results demonstrate that the best conductive insulation is achieved for a high-void-fraction configuration with a grooved fiber cross section, aerogel void medium, and the fibers oriented normal to the heat flux vector. However, this configuration still exceeds the target thermal conductivity by a factor of 1.5.
Numerical Simulation of the Flow and Concentration Fields for Oxygen Delivery Systems
Authors: Hangan H. , Bekele S.
Published: October 2003

The flow and concentration fields for various medical oxygen delivery devices are numerically investigated. Simulations are performed for a classical Venturi mask and two new OxyArm portable devices. The velocity and oxygen concentration fields are investigated for: (i) a constant (steady-state) inhalation and (ii) a complete respiratory cycle (unsteady). The numerical results are qualitatively compared with clinical trials. It is found that the optimal functioning of these medical devices implies a balance between oxygen delivery by advection and the mixing process that allows for reliable CO2 monitoring (capnographic capability). Also, at the typical scales associated with these devices the flow is found to be Reynolds number dependent.
Thermomechanical Analysis of Soft-Tissue Thermotherapy
Authors: Aksan Alptekin , McGrath John J.
Published: October 2003

Soft-tissue thermotherapy based on sub-ablative heating of collagenous tissues finds widespread application in medicine such as tissue welding, thermokeratoplasty, skin resurfacing, elimination of discogenic pain in the spine and treatment of joint instability. In this paper, heat-induced thermomechanical response characteristics of collagenous tissues are quantified by means of in vitro experimentation with a representative model tissue (New Zealand white rabbit patellar tendon). Three distinct heat-induced thermomechanical response regimes (defined by the rate of deformation and the variation of material properties) are identified. Arrhenius damage integral representation of collagenous tissue thermal history is shown to be adequate in establishing the master response curves for quantification of thermomechanical response for modeling purposes. The trade-off between the improved kinematical stability and compromised mechanical stability of the heated collagenous tissue is shown to be the major challenge hindering the success of subablative thermotherapies.
Compressive and Shear Properties of Commercially Available Polyurethane Foams
Authors: Thompson Mark S. , McCarthy Ian D. , Lidgren Lars , Ryd Leif
Published: October 2003

Background: The shear properties of rigid polyurethane (PU-R) foams, routinely used to simulate cancellous bone, are not well characterized. Method of approach: The present assessment of the shear and compressive properties of four grades of Sawbones “Rigid cellular” PU-R foam tested 20 mm gauge diameter dumb-bell specimens in torsion and under axial loading. Results: Shear moduli ranged from 13.3 to 99.7 MPa, shear strengths from 0.7 MPa to 4.2 MPa. Compressive yield strains varied little with density while shear yield strains had peak values with “200 kgm−3” grade. Conclusions: PU-R foams may be used to simulate the elastic but not failure properties of cancellous bone.
Geometrical Stress-Reducing Factors in the Anisotropic Porcine Heart Valves
Authors: Luo X. Y. , Li W. G. , Li J.
Published: October 2003

This study carries out a detailed parameter study based on a nonlinear anisotropic finite-element model published previously. The aim of this study is to identify the stress-reducing influences from geometrical parameters such as stent height, valve diameter, and the nonuniform thickness of porcine aortic valves under static loading condition. The anisotropy of the valve is considered to be transversely isotropic with fibers oriented along the circumferential directions, which enables us to use a simple anisotropic constitutive model using uniaxial experimental data. The results showed that in general, higher stent height and smaller diameter combined with nonuniform thickness give rise to a much more reduced overall stress level. Although the absolute values of the peak stresses may be influenced by the detailed orientations of fibers, the trends of the stress variation with the geometrical factors seem to be qualitatively consistent within the parameter ranges considered.
Three-Dimensional CFD Rotordynamic Analysis of Gas Labyrinth Seals
Authors: Moore J. Jeffrey
Published: October 2003

Labyrinth seals are utilized inside turbomachinery to provide noncontacting control of internal leakage. These seals can also play an important role in determining the rotordynamic stability of the machine. Traditional labyrinth seal models are based on bulk-flow assumptions where the fluid is assumed to behave as a rigid body affected by shear stress at the interfaces. To model the labyrinth seal cavity, a single, driven vortex is assumed and relationships for the shear stress and divergence angle of the through flow jet are developed. These models, while efficient to compute, typically show poor prediction for seals with small clearances, high running speed, and high pressure.* In an effort to improve the prediction of these components, this work utilizes three-dimensional computational fluid dynamics (CFD) to model the labyrinth seal flow path by solving the Reynolds Averaged Navier Stokes equations. Unlike bulk-flow techniques, CFD makes no fundamental assumptions on geometry, shear stress at the walls, as well as internal flow structure. The method allows modeling of any arbitrarily shaped domain including stepped and interlocking labyrinths with straight or angled teeth. When only leakage prediction is required, an axisymmetric model is created. To calculate rotordynamic forces, a full 3D, eccentric model is solved. The results demonstrate improved leakage and rotordynamic prediction over bulk-flow approaches compared to experimental measurements.
Direct demonstration of the transverse Stern–Gerlach effect
Authors: Porter J. , Pettifer R. F. , Leadley D. R.
Published: October 2003

A Stern–Gerlach experiment constructed in the spirit of the original experiment is reported that demonstrates spatial quantization for a beam of silver atoms directly to the unaided eye. Splitting of the atomic beam is seen both parallel to the magnetic field direction, as in the “usual” experiment, and in a direction mutually perpendicular to the field and atomic trajectory, which is a new transverse Stern–Gerlach effect. Splitting of up to 4 mm can be recorded visually. Calculations based on the force components acting on the atomic beams reproduce the experimental results within the experimental accuracy of ±0.3 mm.
Torque and the rate of change of angular momentum at an arbitrary point
Authors: Rodrı́guez V Luis
Published: October 2003

In this article we show that the general relation between torque and rate of change of the angular momentum for an arbitrary point lets us obtain explicit solutions of many problems in plane motion of a rigid body when the motion of one of its points is given. This method involves considerably less calculation than the standard procedure.
Laser interferometric characterization of a vibrating speaker system
Authors: Freschi A. A. , Caetano N. R. , Santarine G. A. , Hessel R.
Published: October 2003

An experiment that combines opto-mechanical and electrical measurements for the characterization of a loudspeaker is presented. We describe a very simple laser vibrometer for evaluating the amplitude of the vibration (displacement) of the speaker cone. The setup is essentially a Michelson-type interferometer operated by an inexpensive semiconductor laser (diode laser). It is shown that the simultaneous measurements of three amplitudes (displacement, electrical current, and applied voltage), as functions of the frequency of vibration, allow us to characterize the speaker system. The experiment is easy to perform, and it demonstrates several useful concepts of optics, mechanics, and electricity, allowing students to gain an intuitive physical insight into the relations between mathematical models and an actual speaker system.
Computer-controlled in-class feedback system for interactive lectures
Authors: Bensky T. J.
Published: October 2003

Instructors who wish to implement an interactive lecture style, such as peer instruction, have the need to collect feedback from students in a lecture environment. We present a computer-controlled electronic circuit that allows for quick, rigorous, and accurate measurement and reporting of student feedback in the lecture environment.
Faster-than-light speeds, tachyons, and the possibility of tachyonic neutrinos
Authors: Ehrlich Robert
Published: October 2003

Faster-than-light speeds and hypothetical FTL particles known as tachyons are exciting subjects for students, given their speculative and controversial nature. This article presents an overview of these subjects and their role in special relativity and examines the possibility that one or more of the three neutrinos is a tachyon. The paper also describes several low tech demonstrations useful for teaching about faster-than-light speeds and tachyons in intermediate and advanced introductory college-level physics courses.
Helping students develop an understanding of Archimedes’ principle. II. Development of research-based instructional materials
Authors: Heron Paula R. L. , Loverude Michael E. , Shaffer P. S. , McDermott L. C.
Published: October 2003

This paper is the second of two that describe how research on student understanding of Archimedes’ principle is being used to guide the development of instructional materials to supplement instruction on this topic in typical introductory courses. The instructional materials that resulted have proven to be effective. Also discussed are instructional materials for special courses and workshops for K–8 teachers. Evidence is presented that, on some tasks, teachers who have worked through these materials perform much better than introductory physics students.
Irreversible Adiabatic Compression of an Ideal Gas
Authors: Mungan Carl E.
Published: October 2003

Consider the following problem.1 A frictionless, massive piston partitions an insulated box vertically into two parts. The upper compartment is evacuated, while the lower contains an equilibrated ideal gas. A weight is suddenly placed on the piston. How much is the gas compressed when equilibrium is again restored, neglecting the heat capacity of the cylinder and piston?
The Hydraulic Analogy for Electric Current
Authors: Greenslade Thomas B.
Published: October 2003

About 1900 the British scientist Sir Oliver Lodge (1851–1940) described a hydraulic analogue for electrical circuits that was widely used in the first half of the century and is still with us. Although some physics teachers object to the use of analogy in their presentations, the hydraulic analogue has become firmly fixed in our consciousness, and it is interesting to see how some older textbooks used it. The mechanical model of the emf is a bit strained, but the other models are easy to accept as analogues.
Tycho Brahe's Stjerneborg
Authors: Manos Harry
Published: October 2003

Uraniborg and Stjerneborg are where Tycho Brahe lived, worked, and made many of his famous observations. Most of his original instruments have been destroyed, but replicas have been constructed according to Brahe's original plans, and two instruments still exist that are believed to have been built by Brahe.
Statistics of Measurements of Automobile Fuel Efficiency
Authors: Bartlett Albert A.
Published: October 2003

In these days, with the growing need to worry about petroleum and the efficiency of our automobiles, there should be an increased interest, especially among physics students, in the miles per gallon (MPG) of cars. There should also be interest in the accuracy of single and multiple measurements of the MPG of a car. These topics are covered here in this report of a series of measurements that span 16 years.
Checking Students' Symbolic Math on a Computer
Authors: Crowell Benjamin
Published: October 2003

This note describes an open-source computer program that allows students to check their answers to symbolic homework problems. Traditional introductory physics courses emphasize solving a problem to obtain a numerical result, but this skill is only weakly correlated with conceptual understanding.1 We state the laws of physics in the language of algebra, but many students find it difficult to connect a purely symbolic equation with its interpretation; to them, an equation is nothing more than an algorithm for numerical computation.
An Historico-Critical Account of Potential Energy: Is PE Really Real?
Authors: Hecht Eugene
Published: October 2003

This paper critically explores the familiar concept of potential energy (PE), with the intent of addressing the issue of whether it is “real” or not. We begin with an historical account of the development of the idea of energy, examining the original motivations for the introduction of the notion of PE. This is followed by a sample of the arguments existing in the literature (from the 1880s through the 20th century) against the legitimacy of PE; that is, arguments maintaining that potential energy is not a real observable physical quantity. Today potential energy is so widely and unquestioningly accepted that it seems almost unthinkable that anyone ever seriously challenged its veracity. Using relativistic considerations it will be shown that PE is as real as mass is real. Nonetheless it will be argued that the concept of potential energy, however real, is actually superfluous.
Contribution of the Patagonia Icefields of South America to Sea Level Rise
Authors: Rignot Eric , Rivera Andrés , Casassa Gino
Published: October 2003

Digital elevation models of the Northern and Southern Patagonia Icefields of South America generated from the 2000 Shuttle Radar Topography Mission were compared with earlier cartography to estimate the volume change of the largest 63 glaciers. During the period 1968/1975–2000, these glaciers lost ice at a rate equivalent to a sea level rise of 0.042 ± 0.002 millimeters per year. In the more recent years 1995–2000, average ice thinning rates have more than doubled to an equivalent sea level rise of 0.105 ± 0.011 millimeters per year. The glaciers are thinning more quickly than can be explained by warmer air temperatures and decreased precipitation, and their contribution to sea level per unit area is larger than that of Alaska glaciers.
Always Good Turing: Asymptotically Optimal Probability Estimation
Authors: Orlitsky Alon , Santhanam Narayana P. , Zhang Junan
Published: October 2003

While deciphering the Enigma code, Good and Turing derived an unintuitive, yet effective, formula for estimating a probability distribution from a sample of data. We define the attenuation of a probability estimator as the largest possible ratio between the per-symbol probability assigned to an arbitrarily long sequence by any distribution, and the corresponding probability assigned by the estimator. We show that some common estimators have infinite attenuation and that the attenuation of the Good-Turing estimator is low, yet greater than 1. We then derive an estimator whose attenuation is 1; that is, asymptotically it does not underestimate the probability of any sequence.
Hematopoietic Cell Regulation by Rac1 and Rac2 Guanosine Triphosphatases
Authors: Gu Yi , Filippi Marie-Dominique , Cancelas Jose A. , Siefring Jamie E. , Williams Emily P. , Jasti Aparna C. , Harris Chad E. , Lee Andrew W. , Prabhakar Rethinasamy , Atkinson Simon J. , Kwiatkowski David J. , Williams David A.
Published: October 2003

The Rho guanosine triphosphatases (GTPases) Rac1 and Rac2 are critical signaling regulators in mammalian cells. The deletion of both Rac1 and Rac2 murine alleles leads to a massive egress of hematopoietic stem/progenitor cells (HSC/Ps) into the blood from the marrow, whereas
Rac1
–/–
but not
Rac2
–/–
HSC/Ps fail to engraft in the bone marrow of irradiated recipient mice. In contrast, Rac2, but not Rac1, regulates superoxide production and directed migration in neutrophils, and in each cell type, the two GTPases play distinct roles in actin organization, cell survival, and proliferation. Thus, Rac1 and Rac2 regulate unique aspects of hematopoietic development and function.
Diffusion Dynamics of Glycine Receptors Revealed by Single-Quantum Dot Tracking
Authors: Dahan Maxime , Lévi Sabine , Luccardini Camilla , Rostaing Philippe , Riveau Béatrice , Triller Antoine
Published: October 2003

Semiconductor quantum dots (QDs) are nanometer-sized fluorescent probes suitable for advanced biological imaging. We used QDs to track individual glycine receptors (GlyRs) and analyze their lateral dynamics in the neuronal membrane of living cells for periods ranging from milliseconds to minutes. We characterized multiple diffusion domains in relation to the synaptic, perisynaptic, or extrasynaptic GlyR localization. The entry of GlyRs into the synapse by diffusion was observed and further confirmed by electron microscopy imaging of QD-tagged receptors.
LMO2
-Associated Clonal T Cell Proliferation in Two Patients after Gene Therapy for SCID-X1
Authors: Hacein-Bey-Abina S. , Von Kalle C. , Schmidt M. , McCormack M. P. , Wulffraat N. , Leboulch P. , Lim A. , Osborne C. S. , Pawliuk R. , Morillon E. , Sorensen R. , Forster A. , Fraser P. , Cohen J. I. , de Saint Basile G. , Alexander I. , Wintergerst U. , Frebourg T. , Aurias A. , Stoppa-Lyonnet D. , Romana S. , Radford-Weiss I. , Gross F. , Valensi F. , Delabesse E. , Macintyre E. , Sigaux F. , Soulier J. , Leiva L. E. , Wissler M. , Prinz C. , Rabbitts T. H. , Le Deist F. , Fischer A. , Cavazzana-Calvo M.
Published: October 2003

We have previously shown correction of X-linked severe combined immunodeficiency [SCID-X1, also known as Î³ chain (Î³c) deficiency] in 9 out of 10 patients by retrovirus-mediated Î³c gene transfer into autologous CD34 bone marrow cells. However, almost 3 years after gene therapy, uncontrolled exponential clonal proliferation of mature T cells (with Î³Î´+ or Î±Î²+ T cell receptors) has occurred in the two youngest patients. Both patients' clones showed retrovirus vector integration in proximity to the
LMO2
proto-oncogene promoter, leading to aberrant transcription and expression of
LMO2
. Thus, retrovirus vector insertion can trigger deregulated premalignant cell proliferation with unexpected frequency, most likely driven by retrovirus enhancer activity on the
LMO2
gene promoter.
Critical Roles for Rac1 and Rac2 GTPases in B Cell Development and Signaling
Authors: Walmsley Marita J. , Ooi Steen K. T. , Reynolds Lucinda F. , Smith Susan Harless , Ruf Sandra , Mathiot Anne , Vanes Lesley , Williams David A. , Cancro Michael P. , Tybulewicz Victor L. J.
Published: October 2003

The Rac1 guanosine triphosphatase (GTPase) has been implicated in multiple cellular functions, including actin dynamics, proliferation, apoptosis, adhesion, and migration resulting from signaling by multiple receptors, including the B cell antigen receptor (BCR). We used conditional gene targeting to generate mice with specific Rac1 deficiency in the B cell lineage. In the absence of both Rac1 and the highly related Rac2, B cell development was almost completely blocked. Both GTPases were required to transduce BCR signals leading to proliferation, survival and up-regulation of BAFF-R, a receptor for BAFF, a key survival molecule required for B cell development and maintenance.
Solvent-Free Electrolytes with Aqueous Solution-Like Conductivities
Authors: Xu Wu , Angell C. Austen
Published: October 2003

Aqueous solutions are generally assumed to be superior electrolytic conductors because of the unique dielectric and fluid properties of water. We show that their conductivities can be matched by liquid electrolytes that contain no solvent. These are proton transfer salts that are liquid at ambient temperature. The high conductivities are due to the high fluidity and ionicity rather than some sort of Grotthus mechanism, although in certain cases a mobile proton population may make a non-negligible contribution. The highest conductivities have been obtained when both cations and anions contain protons. At 25°C, values of >150 millisiemens per centimeter (mS cm
–1
) appear possible; at 100°C, 470 mS cm
–1
has been measured. Because of the combination of high ionicity and proton exchange kinetics with low vapor pressure, the systems we describe also make excellent fuel cell electrolytes.
Clinical Potential of the Acyclic Nucleoside Phosphonates Cidofovir, Adefovir, and Tenofovir in Treatment of DNA Virus and Retrovirus Infections
Authors: De Clercq Erik
Published: October 2003

The acyclic nucleoside phosphonates HPMPC (cidofovir), PMEA (adefovir), and PMPA (tenofovir) have proved to be effective in vitro (cell culture systems) and in vivo (animal models and clinical studies) against a wide variety of DNA virus and retrovirus infections: cidofovir against herpesvirus (herpes simplex virus types 1 and 2 varicella-zoster virus, cytomegalovirus [CMV], Epstein-Barr virus, and human herpesviruses 6, 7, and 8), polyomavirus, papillomavirus, adenovirus, and poxvirus (variola virus, cowpox virus, vaccinia virus, molluscum contagiosum virus, and orf virus) infections; adefovir against herpesvirus, hepadnavirus (human hepatitis B virus), and retrovirus (human immunodeficiency virus types 1 [HIV-1] and 2 [HIV-2], simian immunodeficiency virus, and feline immunodeficiency virus) infections; and tenofovir against both hepadnavirus and retrovirus infections. Cidofovir (Vistide) has been officially approved for the treatment of CMV retinitis in AIDS patients, tenofovir disoproxil fumarate (Viread) has been approved for the treatment of HIV infections (i.e., AIDS), and adefovir dipivoxil (Hepsera) has been approved for the treatment of chronic hepatitis B. Nephrotoxicity is the dose-limiting side effect for cidofovir (Vistide) when used intravenously (5 mg/kg); no toxic side effects have been described for adefovir dipivoxil and tenofovir disoproxil fumarate, at the approved doses (Hepsera at 10 mg orally daily and Viread at 300 mg orally daily).
Infections in Patients with Inherited Defects in Phagocytic Function
Authors: Andrews Timothy , Sullivan Kathleen E.
Published: October 2003

Patients with defects in phagocytic function are predisposed to intracellular microorganisms and typically have early dissemination of the infection. Recognition of the underlying disorder and aggressive antimicrobial therapy has been beneficial for the patients. Improved understanding of the pathophysiology has also affected patient management by allowing specific, targeted immunomodulatory intervention. The disorders described in this review are not common but have had a significant impact on our understanding of the role of phagocytic cells in host defense. Conversely, understanding the role of the neutrophil and macrophage in infection has benefited not just the patients described in this review but also other patients with similar disease processes.
Babesia divergens
, a Bovine Blood Parasite of Veterinary and Zoonotic Importance
Authors: Zintl Annetta , Mulcahy Grace , Skerrett Helen E. , Taylor Stuart M. , Gray Jeremy S.
Published: October 2003

Babesia divergens
is an intraerythrocytic protozoan parasite, transmitted by the tick
Ixodes ricinus
, and is the main agent of bovine babesiosis in Europe. It is not only a cause of significant loss to the cattle industry; it can also infect immunocompromised humans, causing medical emergencies characterized by rapid fulmination and parasitemias that may exceed 70%. The current emphasis in Europe on sustainable agriculture and extensification is likely to lead to an increase in vector tick populations with increased risk of infection. Despite the veterinary and zoonotic importance of this parasite, relatively little research has been carried out on
B. divergens
, and many questions regarding the parasite's epidemiology and the host's response remain unanswered. A better understanding of the species' biology and host-parasite interactions may lead to improved control mechanisms and new trends in vaccine and antibabesial drug development. This review provides the first comprehensive summary of
B. divergens
biology, including its morphology, life cycle, and host specificity, and the current state of knowledge of both human and bovine infections.
Role of Toll-Like Receptors in Pathogen Recognition
Authors: Janssens S. , Beyaert R.
Published: October 2003

The innate immune system relies on a vast array of non-clonally expressed pattern recognition receptors for the detection of pathogens. Pattern recognition receptors bind conserved molecular structures shared by large groups of pathogens, termed pathogen-associated molecular patterns. The Toll-like receptors (TLRs) are a recently discovered family of pattern recognition receptors which show homology with theDrosophilaToll protein and the human interleukin-1 receptor family. Engagement of different TLRs can induce overlapping yet distinct patterns of gene expression that contribute to an inflammatory response. The TLR family is characterized by the presence of leucine-rich repeats and a Toll/interleukin-1 receptor-like domain, which mediate ligand binding and interaction with intracellular signaling proteins, respectively. Most TLR ligands identified so far are conserved microbial products which signal the presence of an infection, but evidence for some endogenous ligands that might signal other danger conditions has also been obtained. Molecular mechanisms for pathogen-associated molecular pattern recognition still remain elusive but seem to be more complicated than initially anticipated. In most cases, direct binding of microbial ligands to TLRs still has to be demonstrated. Moreover,DrosophilaTLRs bind endogenous ligands, generated through a proteolytic cascade in response to an infection. In the case of endotoxin, recognition involves a complex of TLR4 and a number of other proteins. Moreover, TLR heterodimerization further extends the spectrum of ligands and modulates the response towards specific ligands. The fact that TLR expression is regulated in both a cell type- and stimulus-dependent fashion further contributes to the complexity.
Prevention of Cytomegalovirus Disease in Recipients of Allogeneic Stem Cell Transplants
Authors: Meijer Ellen , Boland Greet J. , Verdonck Leo F.
Published: October 2003

The main risk factors for cytomegalovirus (CMV) disease in recipients of allogeneic stem cell transplants (SCT) are recipient CMV seropositivity and acute graft-versus-host disease. Currently, two antiviral strategies, prophylactic or preemptive antiviral treatment, are used for prevention of CMV disease. Preemptive treatment is most favorable when short-term (14-day) treatment is applied. Several methods are available for monitoring of CMV reactivation. PCR-based CMV DNA detection assays are the most sensitive methods; however, the clinical benefit of this high sensitivity is unclear. Even more, there is lack of clarity whether PCR tests can better be performed with plasma, whole blood, or peripheral blood leukocyte samples. Recovery of a CMV-specific CD8
+
cytotoxic-T-lymphocyte (CTL) response is necessary for preventing CMV reactivation and disease. Reconstitution of absolute CMV-specific CTL counts to values above 10 × 10
6
to 20 × 10
6
CTLs/liter is associated with protection from CMV disease. In the near future, preemptive therapy might be withheld in patients with CMV reactivation who are shown to have adequate CMV-specific cytotoxic T-cell levels. Antiviral therapy with (val)acyclovir has been studied only as prophylactic treatment for prevention of CMV infection. High-dose oral valacyclovir is more effective than acyclovir when used in addition to preemptive treatment of CMV reactivation with ganciclovir or foscarnet. Three antiviral drugs have been tested for preemptive therapy of CMV reactivation and/or treatment of CMV disease. Although intravenous ganciclovir is considered the drug of choice, foscarnet has similar efficacy and less toxicity, especially hematologic toxicity. Cidofovir has not been tested extensively, but so far the results are disappointing. Oral valganciclovir for preemptive treatment of SCT recipients is currently being studied. In addition to antiviral therapy, adoptive immunotherapy with CMV-specific cytotoxic T cells as prophylactic or preemptive therapy is a very elegant strategy; however, generation of these cells is expensive and time-consuming, and therefore the therapy is not available at every transplantation center. Magnetic selection of CMV-specific CD8
+
T cells from peripheral blood by using HLA class I-peptide tetramers may be very promising, making this strategy more accessible.
Potential Uses of Probiotics in Clinical Practice
Authors: Reid Gregor , Jass Jana , Sebulsky M. Tom , McCormick John K.
Published: October 2003

Probiotics are defined as live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. There is now mounting evidence that selected probiotic strains can provide health benefits to their human hosts. Numerous clinical trials show that certain strains can improve the outcome of intestinal infections by reducing the duration of diarrhea. Further investigations have shown benefits in reducing the recurrence of urogenital infections in women, while promising studies in cancer and allergies require research into the mechanisms of activity for particular strains and better-designed trials. At present, only a small percentage of physicians either know of probiotics or understand their potential applicability to patient care. Thus, probiotics are not yet part of the clinical arsenal for prevention and treatment of disease or maintenance of health. The establishment of accepted standards and guidelines, proposed by the Food and Agriculture Organization of the United Nations and the World Health Organization, represents a key step in ensuring that reliable products with suitable, informative health claims become available. Based upon the evidence to date, future advances with single- and multiple-strain therapies are on the horizon for the management of a number of debilitating and even fatal conditions.
Bacterial Wall as Target for Attack
Authors: Koch Arthur L.
Published: October 2003

When Bacteria, Archaea, and Eucarya separated from each other, a great deal of evolution had taken place. Only then did extensive diversity arise. The bacteria split off with the new property that they had a sacculus that protected them from their own turgor pressure. The saccular wall of murein (or peptidoglycan) was an effective solution to the osmotic pressure problem, but it then was a target for other life-forms, which created lysoymes and β-lactams. The β-lactams, with their four-member strained rings, are effective agents in nature and became the first antibiotic in human medicine. But that is by no means the end of the story. Over evolutionary time, bacteria challenged by β-lactams evolved countermeasures such as β-lactamases, and the producing organisms evolved variant β-lactams. The biology of both classes became evident as the pharmaceutical industry isolated, modified, and produced new chemotherapeutic agents and as the properties of β-lactams and β-lactamases were examined by molecular techniques. This review attempts to fit the wall biology of current microbes and their clinical context into the way organisms developed on this planet as well as the changes arising since the work done by Fleming. It also outlines the scientific advances in our understanding of this broad area of biology.