bluuebunny/crossref_metadata_2025 huggingface dataset:
Linear temporal and spatio-temporal stability analysis of a binary liquid film flowing down an inclined uniformly heated plate
Authors: HU JUN , HADID HAMDA BEN , HENRY DANIEL , MOJTABI ABDELKADER
Published: March 2008

Temporal and spatio-temporal instabilities of binary liquid films flowing down an inclined uniformly heated plate with Soret effect are investigated by using the Chebyshev collocation method to solve the full system of linear stability equations. Seven dimensionless parameters, i.e. the Kapitza, Galileo, Prandtl, Lewis, Soret, Marangoni, and Biot numbers (Ka,G,Pr,L, χ,M,B), as well as the inclination angle (β) are used to control the flow system. In the case of pure spanwise perturbations, thermocapillary S- and P-modes are obtained. It is found that the most dangerous modes are stationary for positive Soret numbers (χ≥0), and oscillatory for χ<0. Moreover, the P-mode which is short-wave unstable for χ=0 remains so for χ<0, but becomes long-wave unstable for χ>0 and even merges with the long-wave S-mode. In the case of streamwise perturbations, a long-wave surface mode (H-mode) is also obtained. From the neutral curves, it is found that larger Soret numbers make the film flow more unstable as do larger Marangoni numbers. The increase of these parameters leads to the merging of the long-wave H- and S-modes, making the situation long-wave unstable for any Galileo number. It also strongly influences the short-wave P-mode which becomes the most critical for large enough Galileo numbers. Furthermore, from the boundary curves between absolute and convective instabilities (AI/CI) calculated for both the long-wave instability (S- and H-modes) and the short-wave instability (P-mode), it is shown that for small Galileo numbers the AI/CI boundary curves are determined by the long-wave instability, while for large Galileo numbers they are determined by the short-wave instability.
Three-dimensional instabilities in compressible flow over open cavities
Authors: BRÈS GUILLAUME A. , COLONIUS TIM
Published: March 2008

Direct numerical simulations are performed to investigate the three-dimensional stability of compressible flow over open cavities. A linear stability analysis is conducted to search for three-dimensional global instabilities of the two-dimensional mean flow for cavities that are homogeneous in the spanwise direction. The presence of such instabilities is reported for a range of flow conditions and cavity aspect ratios. For cavities of aspect ratio (length to depth) of 2 and 4, the three-dimensional mode has a spanwise wavelength of approximately one cavity depth and oscillates with a frequency about one order of magnitude lower than two-dimensional Rossiter (flow/acoustics) instabilities. A steady mode of smaller spanwise wavelength is also identified for square cavities. The linear results indicate that the instability is hydrodynamic (rather than acoustic) in nature and arises from a generic centrifugal instability mechanism associated with the mean recirculating vortical flow in the downstream part of the cavity. These three-dimensional instabilities are related to centrifugal instabilities previously reported in flows over backward-facing steps, lid-driven cavity flows and Couette flows. Results from three-dimensional simulations of the nonlinear compressible Navier–Stokes equations are also reported. The formation of oscillating (and, in some cases, steady) spanwise structures is observed inside the cavity. The spanwise wavelength and oscillation frequency of these structures agree with the linear analysis predictions. When present, the shear-layer (Rossiter) oscillations experience a low-frequency modulation that arises from nonlinear interactions with the three-dimensional mode. The results are consistent with observations of low-frequency modulations and spanwise structures in previous experimental and numerical studies on open cavity flows.
Gaseous detonation propagation in a bifurcated tube
Authors: WANG C. J. , XU S. L. , GUO C. M.
Published: March 2008

Gaseous detonation propagation in a bifurcated tube was experimentally and numerically studied for stoichiometric hydrogen and oxygen mixtures diluted with argon. Pressure detection, smoked foil recording and schlieren visualization were used in the experiments. Numerical simulation was carried out at low initial pressure (8.00kPa), based on the reactive Navier–Stokes equations in conjunction with a detailed chemical reaction model. The results show that the detonation wave is strongly disturbed by the wall geometry of the bifurcated tube and undergoes a successive process of attenuation, failure, re-initiation and the transition from regular reflection to Mach reflection. Detonation failure is attributed to the rarefaction waves from the left-hand corner by decoupling leading shock and reaction zones. Re-initiation is induced by the inert leading shock reflection on the right-hand wall in the vertical branch. The branched wall geometry has only a local effect on the detonation propagation. In the horizontal branch, the disturbed detonation wave recovers to a self-sustaining one earlier than that in the vertical branch. A critical case was found in the experiments where the disturbed detonation wave can be recovered to be self-sustaining downstream of the horizontal branch, but fails in the vertical branch, as the initial pressure drops to 2.00kPa. Numerical simulation also shows that complex vortex structures can be observed during detonation diffraction. The reflected shock breaks the vortices into pieces and its interaction with the unreacted recirculation region induces an embedded jet. In the vertical branch, owing to the strength difference at any point and the effect of chemical reactions, the Mach stem cannot be approximated as an arc. This is different from the case in non-reactive steady flow. Generally, numerical simulation qualitatively reproduces detonation attenuation, failure, re-initiation and the transition from regular reflection to Mach reflection observed in experiments.
Analysis of sheet-like thermal plumes in turbulent Rayleigh–Bénard convection
Authors: SHISHKINA OLGA , WAGNER CLAUS
Published: March 2008

Sheet-like thermal plumes are investigated using time-dependent and three-dimensional flow fields obtained from direct numerical simulations and well-resolved large-eddy simulations of turbulent Rayleigh–Bénard convection in water (Prandtl number Pr=5.4) in a cylindrical container with the aspect ratio Γ=1 and for the Rayleigh numbers Ra=2×109 and 2×1010.

To analyse quantitatively the physical properties of the sheet-like thermal plumes and the turbulent background and to obtain the temperature threshold which separates these two different flow regions, the temperature dependences of the conditionally averaged local heat flux, thermal dissipation rate and selected components of the velocity and vorticity fields are studied. It is shown that the sheet-like plumes are characterized by high values of the local heat flux and relatively large absolute values of the vertical components of the vorticity and velocity fields. The borders of these plumes are indicated by large values of the thermal dissipation rate and large absolute values of the horizontal vorticity components. In contrast to the sheet-like thermal plumes, the turbulent background is characterized by low values of the thermal dissipation rate, local heat flux and vertical vorticity component. The highest values of the local heat flux and the highest absolute values of the vertical vorticity component are found in the regions where the sheet-like plumes strike against each other. Fluid swirling at these places forms the stems of the mushroom-like thermal plumes which develop in the bulk of the Rayleigh–Bénard cell.

Further, formulae to calculate the curvature, thickness and length of the plumes are introduced. Geometrical properties such as plume area, diameter, curvature, thickness and aspect ratio together with the physical properties of the sheet-like plumes such as temperature, heat flux, thermal dissipation rate, velocity and vorticity are investigated.
The turbulent equilibration of an unstable baroclinic jet
Authors: ESLER J. G.
Published: March 2008

The evolution of an unstable baroclinic jet, subject to a small perturbation, is examined numerically in a quasi-geostrophic two-layer β-channel model. After a period of initial wave growth, wave breaking leads to turbulence within each layer, and to the eventual equilibration of the flow. The equilibrated flow must satisfy certain dynamical constraints: total momentum is conserved, the total energy is bounded and the flow must be realizable via some area-preserving (diffusive) rearrangement of the initial potential vorticity field in each layer. A theory is introduced that predicts the equilibrated flow in terms of the initial flow parameters. The idea is that the equilibrated state minimizes available potential energy, subject to the constraints on total momentum and total energy, and the further ‘kinematic’ constraint that the potential vorticity changes through a process of complete homogenization within well-delineated regions in each layer. Within a large region of parameter space, the theory accurately predicts the cross-channel structure and strength of the equilibrated jet, the regions where potential vorticity mixing takes place, and total eddy mass (temperature) fluxes. Results are compared with predictions from a maximum-entropy theory that allows for more general rearrangements of the initial potential vorticity field, subject to the known dynamical constraints. The maximum-entropy theory predicts that significantly more available potential energy is released than is observed in the simulations, and that an unphysical ‘exchange’ of bands of fluid will occur across the channel in the lower layer. The kinematic constraint of piecewise potential vorticity homogenization is therefore important in limiting the ‘efficiency’ of release of available potential energy in unstable baroclinic flows. For a typical initial flow, it is demonstrated that if the dynamical constraints alone are considered, then over twice as much potential energy is available for release compared to that actually released in the simulations.
Parametrically forced gravity waves in a circular cylinder and finite-time singularity
Authors: DAS S. P. , HOPFINGER E. J.
Published: March 2008

In this paper we present results on parametrically forced gravity waves in a circular cylinder in the limit of large fluid-depth approximation. The phase diagram that shows the stability-forcing-amplitude threshold and the wave-breaking threshold has been determined in the frequency range of existence of the lowest axisymmetric wave mode. The instability is shown to be supercritical for forcing frequencies at and above the natural frequency and subcritical below in a frequency range where the instability and breaking thresholds do not coincide. Above the instability threshold, the growth in wave amplitude is exponential, but with an initial time delay. The wave-amplitude response curve of stationary wave motions exhibits steady-state wave motion, amplitude modulations and bifurcations to other wave modes at frequencies where the parametric instability boundary of the axisymmetric mode overlaps with the neighbouring modes. The amplitude modulations are either on a slow time scale or exhibit period tripling and intermittent period tripling, without wave breaking. In the wave-breaking regime, a finite-time singularity may occur with intense jet formation, a phenomenon demonstrated by others in fluids of high viscosity and large surface tension. Here, this singular behaviour with jet formation is demonstrated for a low viscosity and low kinematic surface tension liquid. The results indicate that the jet is driven by inertial collapse of the cavity created at the wave trough. Therefore, the jet velocity is determined by the wave fluid velocity but depends, in addition, on kinematic surface tension and viscosity as these affect the last stable wave crest shape and the cavity size.
Admissibility region for rarefaction shock waves in dense gases
Authors: ZAMFIRESCU CALIN , GUARDONE ALBERTO , COLONNA PIERO
Published: March 2008

In the vapour phase and close to the liquid–vapour saturation curve, fluids made of complex molecules are expected to exhibit a thermodynamic region in which the fundamental derivative of gasdynamic Γ is negative. In this region, non-classical gasdynamic phenomena such as rarefaction shock waves are physically admissible, namely they obey the second law of thermodynamics and fulfil the speed-orienting condition for mechanical stability. Previous studies have demonstrated that the thermodynamic states for which rarefaction shock waves are admissible are however not limited to the Γ<0 region. In this paper, the conditions for admissibility of rarefaction shocks are investigated. This results in the definition of a new thermodynamic region – the rarefaction shocks region – which embeds the Γ<0 region. The rarefaction shocks region is bounded by the saturation curve and by the locus of the states connecting double-sonic rarefaction shocks, i.e. shock waves in which both the pre-shock and post-shock states are sonic. Only one double-sonic shock is shown to be admissible along a given isentrope, therefore the double-sonic states can be connected by a single curve in the volume–pressure plane. This curve is named the double sonic locus. The influence of molecular complexity on the shape and size of the rarefaction shocks region is also illustrated by using the van der Waals model; these results are confirmed by very accurate multi-parameter thermodynamic models applied to siloxane fluids and are therefore of practical importance in experiments aimed at proving the existence of rarefaction shock waves in the single-phase vapour region as well as in future industrial applications operating in the non-classical regime.
Quenching of vortex breakdown oscillations via harmonic modulation
Authors: LOPEZ J. M. , CUI Y. D. , MARQUES F. , LIM T. T.
Published: March 2008

Vortex breakdown is a phenomenon inherent to many practical problems, such as leading-edge vortices on aircraft, atmospheric tornadoes, and flame-holders in combustion devices. The breakdown of these vortices is associated with the stagnation of the axial velocity on the vortex axis and the development of a near-axis recirculation zone. For large enough Reynolds number, the breakdown can be time-dependent. The unsteadiness can have serious consequences in some applications, such as tail-buffeting in aircraft flying at high angles of attack. There has been much interest in controlling the vortex breakdown phenomenon, but most efforts have focused on either shifting the threshold for the onset of steady breakdown or altering the spatial location of the recirculation zone. There has been much less attention paid to the problem of controlling unsteady vortex breakdown. Here we present results from a combined experimental and numerical investigation of vortex breakdown in an enclosed cylinder in which low-amplitude modulations of the rotating endwall that sets up the vortex are used as an open-loop control. As expected, for very low amplitudes of the modulation, variation of the modulation frequency reveals typical resonance tongues and frequency locking, so that the open-loop control allows us to drive the unsteady vortex breakdown to a prescribed periodicity within the resonance regions. For modulation amplitudes above a critical level that depends on the modulation frequency (but still very low), the result is a periodic state synchronous with the forcing frequency over an extensive range of forcing frequencies. Of particular interest is the spatial form of this forced periodic state: for modulation frequencies less than about twice the natural frequency of the unsteady breakdown, the oscillations of the near-axis recirculation zone are amplified, whereas for modulation frequencies larger than about twice the natural frequency the oscillations of the recirculation zone are quenched, and the near-axis flow is driven to the steady axisymmetric state. Movies are available with the online version of the paper.
Précis ofThe Rational Imagination: How People Create Alternatives to Reality
Authors: Byrne Ruth M. J.
Published: March 2008

The human imagination remains one of the last uncharted terrains of the mind. People often imagine how events might have turned out “if only” something had been different. The “fault lines” of reality, those aspects more readily changed, indicate that counterfactual thoughts are guided by the same principles as rational thoughts. In the past, rationality and imagination have been viewed as opposites. But research has shown that rational thought is more imaginative than cognitive scientists had supposed. InThe Rational Imagination,I argue that imaginative thought is more rational than scientists have imagined. People exhibit remarkable similarities in the sorts of things they change in their mental representation of reality when they imagine how the facts could have turned out differently. For example, they tend to imagine alternatives to actions rather than inactions, events within their control rather than those beyond their control, and socially unacceptable events rather than acceptable ones. Their thoughts about how an event might have turned out differently lead them to judge that a strong causal relation exists between an antecedent event and the outcome, and their thoughts about how an event might have turned out the same lead them to judge that a weaker causal relation exists. In a simple temporal sequence, people tend to imagine alternatives to the most recent event. The central claim in the book is that counterfactual thoughts are organised along the same principles as rational thought. The idea that the counterfactual imagination is rational depends on three steps: (1) humans are capable of rational thought; (2) they make inferences by thinking about possibilities; and (3) their counterfactual thoughts rely on thinking about possibilities, just as rational thoughts do. The sorts of possibilities that people envisage explain the mutability of certain aspects of mental representations and the immutability of other aspects.
Counterfactuals in science and engineering
Authors: Chandrasekharan Sanjay , Nersessian Nancy J.
Published: March 2008

The notion of mutation is applicable to the generation of novel designs and solutions in engineering and science. This suggests that engineers and scientists have to work against the biases identified in counterfactual thinking. Therefore, imagination appears a lot less rational than claimed in the target article.
Three steps to rational imagining?
Authors: Church Jennifer
Published: March 2008

Ruth Byrne presents a three-step argument to the conclusion that counterfactual imagining is rational. Insofar as this argument is valid, the conclusion is weaker than it seems. More importantly, it does not represent the central contributions of this book – contributions that, if anything, point instead to what is irrational about counterfactual imagining.
Counterfactuals need not be comparative: The case of “As if”
Authors: Markman Keith D. , McMullen Matthew N.
Published: March 2008

Byrne (2005) assumes that counterfactual thinking requires a comparison of facts with an imagined alternative. In our view, however, this assumption is unnecessarily restrictive. We argue that individuals do not necessarily engage in counterfactual simulations exclusively to evaluate factual reality. Instead, comparative evaluation is often suspended in favor of experiencing the counterfactual simulation as if it were real.
Imagination as a source of rationality in development
Authors: Markovits Henry
Published: March 2008

Byrne's book makes a strong case for the important role of imagination as a creator of possibilities that are used to understand complex relations, while remaining rational. I suggest that imagination also serves a critical developmental role by creating possibilities that are not rational, and that act to modify the nature of the cognitive processes that are used to define rationality.
Imagination is only as rational as the purpose to which it is put
Authors: Shtulman Andrew
Published: March 2008

Byrne's criteria for considering imagination rational do not accord with standard notions of rationality. A different criterion – that is, the correspondence between an inference strategy and its domain of application – is offered and illustrated with recent work on possibility judgment. This analysis suggests that, although imagination can be put to rational purposes, imagination itself should not be considered rational.
How rational is the imagination?
Authors: Sternberg Robert J.
Published: March 2008

Byrne has written a terrific book that is, nevertheless, based on a mistaken assumption – that imagination is largely rational. I argue in this commentary that her book follows very well, if one accepts her assumption of rationality, but that the bulk of the evidence available to us contradicts this assumption.
The development of the counterfactual imagination
Authors: Van Reet Jennifer , Pinkham Ashley M. , Lillard Angeline S.
Published: March 2008

How the rational imagination develops remains an open question. The ability to imagine emerges early in childhood, well before the ability to reason counterfactually, and this suggests that imaginative thought may facilitate later counterfactual ability. In addition, developmental data indicate that inhibitory control may also play a role in the ability to reason counterfactually.
Emotional consequences of alternatives to reality: Feeling is for doing
Authors: Zeelenberg Marcel
Published: March 2008

When creating alternatives to reality, people often feel emotions in response to these imaginary worlds. I argue that these emotions serve an important purpose. They signal how the world could have been better and prioritize actions to bring this better world about.
Neuroborreliosis in the South West of England
Authors: LOVETT J. K. , EVANS P. H. , O'CONNELL S. , GUTOWSKI N. J.
Published: March 2008

Although Lyme borreliosis is increasingly diagnosed in the United Kingdom, few systematic studies have been performed there. UK data suggest that the commonest complications are neurological, but inadequate information exists about their nature and the incidence of late neuroborreliosis. Local data are necessary because clinical presentations may show geographical variation. This study aimed to provide data on clinical manifestations in an area of South West England and to estimate treatment delay. We reviewed clinical records of 88 patients in the Royal Devon and Exeter Hospital catchment area who had positive Borrelia antibody tests during a 5-year period. Fifty-six (64%) reported tick bites. The commonest presentations were erythema migrans (65%) and arthralgia/myalgia (27%). However, 22 patients (25%) had neurological symptoms other than headache alone. Fourteen had facial palsy, eight had confusion/drowsiness, four had meningism, five had radiculopathy, two had sixth nerve palsies, and two had peripheral neuropathies. No late, progressive or atypical neurological syndromes were found. Neurological manifestations were generally predictable and usually included either (or all) of meningoencephalitis, facial palsy or radiculopathy.
A foodborne outbreak of enterotoxigenicE. coliandSalmonellaAnatum infection after a high-school dinner in Denmark, November 2006
Authors: PAKALNISKIENE J. , FALKENHORST G. , LISBY M. , MADSEN S. B. , OLSEN K. E. P. , NIELSEN E. M. , MYGH A. , BOEL J. , MØLBAK K.
Published: March 2008

In industrialized countries enterotoxigenicEscherichia coli(ETEC) is mainly diagnosed as a cause of travellers' diarrhoea, but it is also known to cause foodborne outbreaks. We report an outbreak of acute gastroenteritis caused by ETEC serotypes O92:H−and O153:H2 as well asSalmonellaAnatum, which affected around 200 students and teachers after a high-school dinner in Greater Copenhagen, Denmark, November 2006. A retrospective cohort study showed that consumption of pasta salad with pesto was associated with an increased risk of illness (attack rate 59·4%; risk ratio 2·6, 95% confidence interval 1·2–5·7). Imported fresh basil used for preparation of the pesto was the most likely source of contamination. Although ETEC is associated with travellers' diarrhoea in Denmark, this outbreak suggests that a proportion of sporadic ETEC infections might be caused by contaminated imported foodstuffs. To improve food safety further, it is important to target this poorly regulated and researched area.
Amplitude of the transient visual evoked potential (tVEP) as a function of achromatic and chromatic contrast: Contribution of different visual pathways
Authors: SOUZA GIVAGO S. , GOMES BRUNO D. , LACERDA ELIZA MARIA C.B. , SAITO CÉZAR A. , FILHO MANOEL DA SILVA , SILVEIRA LUIZ CARLOS L.
Published: March 2008

We investigated how the stimulation mode influences transient visual evoked potentials (tVEP) amplitude as a function of contrast of achromatic and isoluminant chromatic gratings. The chromatic stimulation probed only responses to the red-green axis. Visual stimuli were monocularly presented in a 5° diameter circle, achromatic and chromatic horizontal gratings, 1 Hz pattern reversal stimulation, and achromatic and chromatic gratings, 300 ms onset per 700 ms offset stimulation. For the achromatic pattern reversal stimulation, a double slope function describes how the P100 amplitude varied as a function of log contrast which had a limb at low-to-medium contrasts and another limb at high contrasts. For the achromatic onset/offset stimulation, C2 amplitude saturated at the highest contrast tested and a single straight line described how it changed along most of the contrast range. Both presentation modes for chromatic gratings resulted in amplitude versus log contrast relations which were well described by single straight lines along most of the contrast range. The results may be interpreted as if at 2 cpd, achromatic pattern reversal stimulation evoked the activity of at least two visual pathways with high and low contrast sensitivity, respectively, while achromatic onset/offset stimulation favored the activity of a pathway with high contrast sensitivity. The neural activity in the M pathway is the best candidate to be the high contrast mechanism detected with pattern reversal and pattern onset/offset VEPs. The activity of color opponent pathways such as the P and K pathways either combined or in isolation seems to be responsible for VEPs obtained with isoluminant chromatic gratings at both presentation modes. When the amplitudes of chromatic VEPs were plotted in the same contrast scale as used for achromatic VEPs, chromatic contrast thresholds had similar values to those of the achromatic mechanism with high contrast sensitivity.
Resetting the Nominal Mapping Parameter in L2 English: Definite article use and the count–mass distinction
Authors: SNAPE NEAL
Published: March 2008

This paper presents two experiments in the acquisition of the nominal domain in English by Japanese and Spanish second language (L2) learners. The first experiment tests the L2 learners' ability to distinguish between count and mass nouns using a grammaticality judgement task and the second experiment tests learners on different types of definite (based on J. Hawkins's 1978 taxonomy) in count and mass contexts by means of a forced-choice elicitation task. The claim by Chierchia (1998a, b) is that there is a Nominal Mapping Parameter and the three languages discussed in this paper each have a different parametric value. The aim of the paper is to test Japanese and Spanish L2 learners of English to see whether they can reset the parameter to the English setting.
Assessing the presence of lexical competition across languages: Evidence from the Stroop task
Authors: COSTA ALBERT , ALBAREDA BÁRBARA , SANTESTEBAN MIKEL
Published: March 2008

Do the lexical representations of the non-response language enter into lexical competition during speech production? This issue has been studied by means of the picture–word interference paradigm in which two paradoxical effects have been observed. The so-called CROSS-LANGUAGE IDENTITY EFFECT (Costa, Miozzo and Caramazza, 1999) has been taken as evidence against cross-linguistic lexical competition. In contrast, the so-called PHONO-TRANSLATION EFFECT (Hermans, Bongaerts, De Bot and Schreuder, 1998) has been interpreted as revealing lexical competition across languages. In this article, we assess the reliability of these two effects by testing Spanish–Catalan highly-proficient bilinguals performing a Stroop task. The results of the experiment are clear: while the cross-language identity facilitation effect is reliably replicated, the phono-translation interference effect is absent from the Stroop task. From these results, we conclude that we should be cautious when drawing strong conclusions about the presence of competition across languages based on the phono-translation effect observed in the picture–word interference paradigm.
Molecular mechanisms and treatment of bone metastasis
Authors: Clines Gregory A. , Guise Theresa A.
Published: March 2008

The metastasis of cancer cells to bone alters bone architecture and mineral homeostasis. As described by the ‘seed and soil’ hypothesis, bone represents a fertile ground for cancer cells to flourish. A ‘vicious cycle’ of reciprocal bone–cancer cellular signals occurs with osteolytic (bone-resorbing) metastases, and a similar mechanism likely modulates osteoblastic (bone-forming) metastatic lesions as well. The development of targeted therapies either to block initial cancer cell chemotaxis, invasion and adhesion or to break the ‘vicious cycle’ is dependent on a more complete understanding of bone metastases. Although bisphosphonates delay progression of skeletal metastases, it is clear that more-effective therapies are needed. Cancer-associated bone morbidity remains a major public health problem, and to improve therapy and prevention it is important to understand the pathophysiology of the effects of cancer on bone. This review details scientific advances in this area.
The galactic cycle of extinction
Authors: Gillman Michael , Erenler Hilary
Published: March 2008

Global extinction and geological events have previously been linked with galactic events such as spiral arm crossings and galactic plane oscillation. The expectation that these are repeating predictable events has led to studies of periodicity in a wide set of biological, geological and climatic phenomena. Using data on carbon isotope excursions, large igneous provinces and impact craters, we identify three time zones of high geological activity which relate to the timings of the passage of the Solar System through the spiral arms. These zones are shown to include a significantly large proportion of high extinction periods. The mass extinction events at the ends of the Ordovician, Permian and Cretaceous occur in the first zone, which contains the predicted midpoints of the spiral arms. The start of the Cambrian, end of the Devonian and end of the Triassic occur in the second zone. The pattern of extinction timing in relation to spiral arm structure is supported by the positions of the superchrons and the predicted speed of the spiral arms. The passage times through an arm are simple multiples of published results on impact and fossil record periodicity and galactic plane half-periods. The total estimated passage time through four arms is 703.8 Myr. The repetition of extinction events at the same points in different spiral arm crossings suggests a common underlying galactic cause of mass extinctions, mediated through galactic effects on geological, solar and extra-solar processes. The two largest impact craters (Sudbury and Vredefort), predicted to have occurred during the early part of the first zone, extend the possible pattern to more than 2000 million years ago.
Net-Based Knowledge Communication in Groups
Authors: Buder Jürgen
Published: March 2008

This paper provides a selective review of the emerging research field of net-based knowledge communication in groups. The review is conceptualized around three problems that designers of net-based knowledge communication will face (cluttering of content, lack of interpersonal information, and lack of sequential structure), and provides examples of how the corresponding barriers can be overcome. It is discussed how scenarios of net-based knowledge communication can even surpass the levels of face-to-face communication if the design relies on strengths of computer technologies (permanent storage, representational flexibility, and computational power), and if the communication among participants is complemented and augmented by meaningful interactions with computer tools.
Supporting and Hindering Knowledge Communication in a Collaborative Picture-Sorting Task
Authors: Hansen Miriam , Spada Hans , Schneider Arno
Published: March 2008

Effective knowledge communication presupposes common ground ( Clark & Brennan, 1991 ) that needs to be established and maintained. This is particularly difficult in remote communication as well as in noninteractive settings, because the speaker cannot use gestures or mimic and has to tailor his utterances to the addressee without receiving feedback. In these situations, the speaker may achieve mutual understanding, for example, by adopting the addressee’s perspective. We present a study conducted to test the impact of instructions that support and hinder individual problem-solving and knowledge communication. We used a picture-sorting task requiring individual cognitive processes of feature search ( Treisman & Gelade, 1980 ) in addition to referential communication. As our study focused on the design of utterances, all participants assumed the role of speaker. Participants were told that their descriptions would be recorded and then listened to later by a participant in the role of addressee. Eight sets of pictures were used, which varied on two dimensions: the individual cognitive demands of detecting the relevant features (varied as between-subject factor) and the communicative demands (varied as within-subject factor). A further between-subject factor was the type of instructions: The participants received either a collaboration script as supporting instructions, or time pressure was applied to induce stress, or else they were given no additional instructions (control group). We used the speakers’ verbal utterances to examine the quality of the speakers’ descriptions. For both dimensions of difficulty, we found the expected effects. In the conditions with a collaboration script, there were fewer irrelevant features mentioned and fewer features were described with delay. In the conditions with time pressure, there were fewer irrelevant features described, but the number of correctly described pictures was impaired through the fact that relevant features were also neglected. Under time pressure, speakers tended to provide ambiguous descriptions regarding the frame of reference.
Supporting Experts’ Written Knowledge Communication Through Reflective Prompts on the Use of Specialist Concepts
Authors: Jucks Regina , Schulte-Löbbert Petra , Bromme Rainer
Published: March 2008

Communicating expert knowledge to a lay addressee in writing is a demanding task that requires a great deal of mental effort. This article reports on a study in which experts were prompted to reflect either on a text they had produced (content focus condition) or on its comprehensibility to a layperson (recipient focus condition). A software tool highlighted the specialist terms or concepts used by the expert writers and guided the reflection process. Subsequent to this reflection phase, writers had the opportunity to revise their texts. Experts in the recipient focus condition significantly expanded their texts and made more meaningful revisions. For example, they were more likely than experts in the content focus condition to explain central concepts in their revision. Results are discussed from the perspective of writing theories and in terms of their practical implications for written knowledge communication.
Behavior Speaks Louder than Traits
Authors: Geschke Daniel , Sassenberg Kai , Ruhrmann Georg , Sommer Denise
Published: March 2008

The impact of message abstractness on recipients is controversial. Research on person perception suggests that abstract messages are more influential, whereas research on communication and its outcomes suggests that concrete, vivid communication has greater impact. It was predicted that the latter also applies to the domain of person perception if the message contains unexpected (i.e., stereotype inconsistent) information. This prediction was tested experimentally. It was found that a message about a target person with unexpected content exerts more influence when it is concrete than when it is abstract. This effect generalized across different sources of communication (mass media and interpersonal communication). In line with earlier findings, messages from the media had a somewhat stronger effect than interpersonal communication. Implications for optimal message composition for the communication of knowledge about people are discussed.
Response of gross ecosystem productivity, light use efficiency, and water use efficiency of Mongolian steppe to seasonal variations in soil moisture
Authors: Li Sheng‐Gong , Eugster Werner , Asanuma Jun , Kotani Ayumi , Davaa Gombo , Oyunbaatar Dambaravjaa , Sugita Michiaki
Published: March 2008

The examination of vegetation productivity and use of light and water resources is important for understanding the carbon and water cycles in semiarid and arid environments. We made continuous measurements of carbon dioxide and water vapor fluxes over an arid steppe ecosystem in Mongolia by using the eddy covariance (EC) technique. These measurements allow an examination of EC‐estimated gross ecosystem productivity (GEP), light use efficiency (LUE), and water use efficiency (WUE) of the steppe. Daily variations of GEP, LUE, and WUE were associated with daily variations of incident photosynthetically active radiation (PAR), ambient temperature (Ta), and vapor pressure deficit (VPD). The magnitudes of these variations were also dependent on canopy development. On the daily basis, GEP linearly correlated with evapotranspiration rate and PAR. LUE correlated positively with leaf area index, Ta, and soil moisture availability but negatively with the surface reflectivity for short‐wave solar radiation. Throughout the growing season, both GEP and LUE responded strongly to precipitation‐fed soil moisture in the top 20 cm of the soil. An examination of the responses of LUE and WUE to PAR under different soil moisture conditions shows that when soil water availability exceeded VPD, the steppe was most efficient in light use, whereas it was less efficient in water use. The multivariate analysis of variance also suggests that soil moisture availability, especially water status in the upper 20‐cm soil layer with dense distribution of grass roots, is the most significant factor that governs GEP, WUE, and LUE. This study provides a preliminary assessment of the use of available water and light by the Mongolian arid steppe ecosystems under seasonally varying soil moisture conditions. A better understanding of these functional responses is required to predict how climate change may affect arid steppe ecosystems.
Evidence from high‐Ni olivines for a hybridized peridotite/pyroxenite source for orogenic andesites from the central Mexican Volcanic Belt
Authors: Straub Susanne M. , LaGatta Alexandra B. , Martin‐Del Pozzo Ana Lillian , Langmuir Charles H.
Published: March 2008

Subduction zone magmatism produces calc‐alkaline andesite melts that combine the high SiO2, Na2O, and K2O abundances of the differentiated continental crust with low FeO, FeO/MgO, and TiO2typical of melts from depleted mantle. Ni‐rich olivines in basaltic andesites and andesites of the central Mexican Volcanic Belt suggest that this dichotomy reflects a particular mechanism of mantle processing in the subduction environment. Hydrous slab components rich in Si, Na, and fluid mobile large‐ion lithophile elements (LILE) transform mantle olivine to “reaction orthopyroxene.” Along the ascent paths, and embedded into surrounding peridotite, secondary pyroxenite lithologies are created that are composed of “reaction orthopyroxene” next to mantle clinopyroxene and orthopyroxene. Partial melts from peridotite and pyroxenite then mix to produce primary calc‐alkaline basaltic andesites and andesites that are rich in Na and LILE. The steady slab flux maintains high levels of Na and LILE in the mantle source but also induces repetitive melting that steadily depletes the subarc mantle in FeO, TiO2, and other high field strength elements. If mantle processing thus creates primary basaltic andesite and andesite melts with the fractionated major element signature of the continental crust, the high magnesium number (Mg # (=Mg/Mg + Fe2+)) ∼60–70 of these melts still requires additional differentiation to arrive at the lower Mg # ∼55 of average continental crust.
Composition and structural control of crustal domains in the central Andes
Authors: Mamani Mirian , Tassara Andrés , Wörner Gerhard
Published: March 2008

Present‐day ratios of Pb isotopes (324 published samples, 435 new) and Nd‐Sr isotopes (150 published, 180 new) on Proterozoic to Holocene igneous, metamorphic, and sedimentary rocks define (at high spatial resolution) distinct isotopic domains of the crust in the central Andes. These domains correlate with the internal compositional structure of the crust as revealed by a three‐dimensional density model. Pb‐Nd isotopic boundaries thus correspond to variations in crustal compositional structure and reflect Proterozoic mafic‐dominated and Paleozoic felsic‐dominated crustal lithologies. Age and composition (mafic versus felsic) of these domains have controlled the rheology of the Andean crust, have influenced crustal deformation patterns, and correlate with the central Andean plateau segmentation.
Microwave dielectric constant of liquid hydrocarbons: Application to the depth estimation of Titan's lakes
Authors: Paillou Philippe , Mitchell Karl , Wall Stephen , Ruffié Gilles , Wood Charles , Lorenz Ralph , Stofan Ellen , Lunine Jonathan , Lopes Rosaly , Encrenaz Pierre
Published: March 2008

Cassini RADAR reveals the surface of Titan since flyby Ta acquired on October 2004. The RADAR instrument discovered volcanic structures, craters, dunes, channels, lakes and seas. In particular, flyby T16 realized in July 2006 imaged tens of radar‐dark features close to Titan's north pole. They are interpreted as lakes filled with liquid hydrocarbons – mainly methane, a key material in the geologic and climatic history of Titan. In order to perform quantitative analysis and modeling of the radar response of Titan's lakes, the dielectric constant of liquid hydrocarbons is a crucial parameter, in particular to estimate the radar wave attenuation. We present here first measurements of the dielectric constant of LNG (Liquefied Natural Gas), mainly composed of methane, at Ku‐band (10–13 GHz): we obtained a value ɛ = 1.75 – 0.002j. This value is used to model the radar backscattering of lakes observed during T16 flyby. Using a two‐layer scattering model, we derive a relationship that is used to estimate a minimum depth for Titan's lakes. The proposed relationship is also coherent with the observation that the larger and then the deeper lakes are also the darker in radar images.
A kinematic model for the East African Rift
Authors: Stamps D. Sarah , Calais Eric , Saria Elifuraha , Hartnady Chris , Nocquet Jean‐Mathieu , Ebinger Cynthia J. , Fernandes Rui M.
Published: March 2008

The kinematics of the East African Rift (EAR) is the least well‐known of all major plate boundaries. Here, we show that present‐day data (a GPS+DORIS geodetic solution and earthquake slip vectors) are consistent with 3.2 Myr‐average spreading rates and transform‐fault azimuths along the Southwest Indian Ridge and support a kinematic model that includes three subplates (Victoria, Rovuma, and Lwandle) between Nubia and Somalia. Continental rifting in the EAR appears to involve localized strain along narrow rift structures that isolate large lithospheric blocks.
Spectral description of oceanic near‐surface variability
Authors: Elipot Shane , Lumpkin Rick
Published: March 2008

This paper provides a spectral description of near‐surface oceanic velocity variability on a global scale. Rotary spectra of drifter velocities are estimated in zonal bands of the Indian, Pacific and Atlantic Oceans. The partition of energy between cyclonic and anticyclonic frequencies clearly identifies inertial oscillations at all latitudes. The meridional distribution of near‐inertial energy is described in detail. The polarization of super‐inertial energy is consistent with internal waves dynamics. Thus, the distribution of super‐inertial energy may be a manifestation of freely propagating near‐inertial waves on the β‐plane.
Azimuthal seismic anisotropy constrains net rotation of the lithosphere
Authors: Becker T. W.
Published: March 2008

Net rotation (NR) of the lithosphere is found in hot spot reference frames, important for tectonics and plume models, but difficult to constrain. Using mantle flow and crystallographic texture modeling, I show that NRs lead to mantle shearing which is recorded in azimuthal seismic anisotropy. The NR amplitude in some hot spot models is so large that it degrades the model fit to anisotropy significantly. Smaller NRs, such as predicted by flow models with stiff continental keels, are consistent with seismology, however.
Electronic structure of oxide, peroxide, and superoxide clusters of the 3d elements: A comparative density functional study
Authors: Uzunova Ellie L. , Mikosch Hans , Nikolov Georgi St.
Published: March 2008

The 3d-element transition metal dioxide MO2, peroxide M(O2), and superoxide MOO clusters (M=Sc–Zn), are studied by density functional theory with the B1LYP functional. The reliability of the methods and basis sets employed was tested by a reinvestigation of the monoxides, for which a database of experimental data is available. The global minima on the M+O2 potential energy surfaces correspond to dioxide structure, the only exception being CuOO, with a superoxide structure. All Zn dioxygen clusters are thermodynamically unstable—their ground states lie higher than the dissociation limit to Zn+O2. Our calculations are in favor of the high-spin configurations for the FeO2, CoO2, and NiO2 ground states, which are still a subject of extensive theoretical and experimental studies. These assignments are confirmed by the coupled-cluster method, CCSD(T), except for NiO2. Based on the existence of a stable NiO2 monoanion in a B14 state, however, it can be concluded that NiO2 in its A15 state should also be stable. The vibrational frequencies are calculated for clusters entrapped in the cubic cell of solid Ar matrix and compared with those obtained for gas-phase clusters. The matrix has no influence on the vibrations of the monoxides and most of the dioxides; however, Co and Ni—dioxoclusters interact strongly with the atoms from the noble gas matrix. The most intense frequencies in the IR spectra are shifted to lower energies and the ordering of the low-lying electronic states by stability is also reversed. According to the electrostatic potential maps, the oxygen atoms in the peroxides are more nucleophilic than those in the dioxides and superoxides. The terminal oxygen atom in superoxides is more nucleophilic than its M-bonded oxygen atom, though charge distribution analysis predicts a smaller negative charge on the terminal oxygen. TiO2 is the only dioxide in which nucleophilic character in the vicinity of the metal cation is induced.