Showing papers by "Bell Labs published in 1990"
TL;DR: In this paper, the authors demonstrate in vivo images of brain microvasculature with image contrast reflecting the blood oxygen level, which can be used to provide in vivo real-time maps of blood oxygenation in the brain under normal physiological conditions.
Abstract: Paramagnetic deoxyhemoglobin in venous blood is a naturally occurring contrast agent for magnetic resonance imaging (MRI). By accentuating the effects of this agent through the use of gradient-echo techniques in high fields, we demonstrate in vivo images of brain microvasculature with image contrast reflecting the blood oxygen level. This blood oxygenation level-dependent (BOLD) contrast follows blood oxygen changes induced by anesthetics, by insulin-induced hypoglycemia, and by inhaled gas mixtures that alter metabolic demand or blood flow. The results suggest that BOLD contrast can be used to provide in vivo real-time maps of blood oxygenation in the brain under normal physiological conditions. BOLD contrast adds an additional feature to magnetic resonance imaging and complements other techniques that are attempting to provide positron emission tomography-like measurements related to regional neural activity.
5,812 citations
TL;DR: The proposed measure, the association ratio, estimates word association norms directly from computer readable corpora, making it possible to estimate norms for tens of thousands of words.
Abstract: The term word association is used in a very particular sense in the psycholinguistic literature (Generally speaking, subjects respond quicker than normal to the word nurse if it follows a highly associated word such as doctor ) We will extend the term to provide the basis for a statistical description of a variety of interesting linguistic phenomena, ranging from semantic relations of the doctor/nurse type (content word/content word) to lexico-syntactic co-occurrence constraints between verbs and prepositions (content word/function word) This paper will propose an objective measure based on the information theoretic notion of mutual information, for estimating word association norms from computer readable corpora (The standard method of obtaining word association norms, testing a few thousand subjects on a few hundred words, is both costly and unreliable) The proposed measure, the association ratio, estimates word association norms directly from computer readable corpora, making it possible to estimate norms for tens of thousands of words
4,272 citations
TL;DR: At high magnetic fields (7 and 8.4 T), water proton magnetic resonance images of brains of live mice and rats under pentobarbital anesthetization have been measured by a gradient echo pulse sequence with a spatial resolution of 65 × 65‐ μm pixel size and 700‐μm slice thickness.
Abstract: At high magnetic fields (7 and 8.4 T), water proton magnetic resonance images of brains of live mice and rats under pentobarbital anesthetization have been measured by a gradient echo pulse sequence with a spatial resolution of 65 x 65-microns pixel size and 700-microns slice thickness. The contrast in these images depicts anatomical details of the brain by numerous dark lines of various sizes. These lines are absent in the image taken by the usual spin echo sequence. They represent the blood vessels in the image slice and appear when the deoxyhemoglobin content in the red cells increases. This contrast is most pronounced in an anoxy brain but not present in a brain with diamagnetic oxy or carbon monoxide hemoglobin. The local field induced by the magnetic susceptibility change in the blood due to the paramagnetic deoxyhemoglobin causes the intra voxel dephasing of the water signals of the blood and the surrounding tissue. This oxygenation-dependent contrast is appreciable in high field images with high spatial resolution.
2,246 citations
TL;DR: It is shown that the islands formed in Stranski-Krastanow (SK) growth of Ge on Si(100) are initially dislocation free, and the limiting critical thickness of coherent SK islands is shown to be higher than that for 2D growth.
Abstract: We show that the islands formed in Stranski-Krastanow (SK) growth of Ge on Si(100) are initially dislocation free. Island formation in true SK growth should be driven by strain relaxation in large, dislocated islands. Coherent SK growth is explained in terms of elastic deformation around the islands, which partially accommodates mismatch. The limiting critical thickness, ${\mathit{h}}_{\mathit{c}}$, of coherent SK islands is shown to be higher than that for 2D growth. We demonstrate growth of dislocation-free Ge islands on Si to a thickness of \ensuremath{\approxeq}500 \AA{}, 50\ifmmode\times\else\texttimes\fi{}higher than ${\mathit{h}}_{\mathit{c}}$ for 2D Ge/Si epitaxy.
1,751 citations
Book•
01 Jan 1990TL;DR: This chapter discusses the evolution of C++ and ANSI C, a standards-based programming language, and its applications in the rapidly changing environment, as well as some of the techniques used to design and implement these systems.
Abstract: Preface. 1. Introduction. Overview. Syntax Notation. Evolution of C++. Acknowledgements. 2. Lexical Conventions. Tokens. Comments. Identifiers. Keywords. Literals. Implementation Dependencies. 3. Basic Concepts. Declarations and Definitions. Scopes. Program and Linkage. Start and Termination. Storage Classes. Types. Lvalues. Name Spaces. Numerical Limits. 4. Standard Conversions. Integral Promotions. Integral Conversions. Float and Double. Floating and Integral. Arithmetic Conversions. Pointer Conversions. Reference Conversions. Pointers to Members. Arithmetic Conversions. 5. Expressions. Primary Expressions. Postfix Expressions. Unary Operators. Explicit Type Conversion. Pointer-to-Member Operators. Multiplicative Operators. Additive Operators. Shift Operators. Relational Operators. Equality Operators. Bitwise AND Operator. Bitwise Exclusive OR Operator. Bitwise Inclusive OR Operator. Logical AND Operator. Logical OR Operator. Conditional Operator. Assignment Operators. Comma Operator. Constant Expressions. 6. Statements. Labeled Statement. Expression Statement. Compound Statement, or Block. Selection Statements. Iteration Statements. Jump Statements. Declaration Statement. Ambiguity Resolution. 7. Declarations. Specifiers. Enumeration Declarations. Asm Declarations. Linkage Specifications. Linkage Specifications. Type-safe Linkage. Limitations. 8. Declarators. Type Names Meaning of Declarators. Function Definitions. Initializers. Pointers to Members. 9. Classes. Class Names. Class Members. Member Functions. Static Members. Unions. Bit-Fields. Nested Class Declarations. Local Class Declarations. Local Type Names. Interfaces. 10. Derived Classes. Multiple Base Classes. Virtual Functions. Abstract Classes. Summary of Scope Rules. Single Inheritance. Multiple Inheritance. Multiple Inheritance and Casting. Multiple Inheritance and Implicit Conversion. Virtual Base Classes. Virtual Base Classes and Casting. Single Inheritance and Virtual Functions. Multiple Inheritance and Virtual Functions. Instantiation of Virtual Functions. Virtual Base Classes with Virtual Functions. Renaming. 11. Member Access Control. Access Specifiers. Access Specifiers for Base Classes. Access Declarations. Friends. Protected Member Access. Access to Virtual Functions. Multiple Access. General Ideas. Per Class Protection. Access Control. 12. Special Member Functions. Constructors. Temporary Objects. Conversions. Destructors. Free Store. Initialization. Constructors and Destructors. Copying Class Objects. Temporary Elimination. Access Control and Special Functions. Summary of Member, Friend, and Special Functions. 13. Overloading. Declaration Matching. Argument Matching. Address of Overload Function. Overloaded Operators. 14. Templates. Templates. Class Templates. Type Equivalence. Function Templates. Declarations and Definitions. Member Function Templates. Friends. Static Members and Variables. 15. Exception Handling. Exception Handling. Throwing an Exception. Constructors and Destructors. Handling and Exception. Exception Specifications. Special Functions. Exceptions and Access. 16. Preprocessing. Phases of Preprocessing. Trigraph Sequences. Macro Definition and Expansion. File Inclusion. Conditional Compilation. Line Control. Error Directive. Pragmas. Null Directive. Predefined Names. C++ Constructs versus #define. Compatibility. Classic C Preprocessing. 17. Grammar Summary. Keywords. Expressions. Declarations. Declarators. Class. Statements. Preprocessor. Templates. Exception. 18. Compatibility. Extensions. C++ and ANSI C. Anachronisms. ANSI/ISO Resolutions. Index. 0201514591T04062001
886 citations
TL;DR: In this paper, the power limitations of light-wave systems were analyzed as a function of a number of wavelength-multiplexed channels and methods for scaling these results with changes in system parameters were presented.
Abstract: Optical nonlinearities in the context of lightwave systems limitations are described. The nature and severity of system degradation due to stimulated Raman scattering, carrier-induced phase noise, stimulated Brillouin scattering, and four-photon mixing are discussed. The system power limitations are plotted as a function of a number of wavelength-multiplexed channels. Methods for scaling these results with changes in system parameters are presented. >
787 citations
TL;DR: Evidence for a flux-periodic persistent current in each ring of average value 3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$e${\ensurmath{
u}}_{\mathit{F}}$/L.
Abstract: We have measured the low-temperature magnetization response of ${10}^{7}$ isolated mesoscopic copper rings to a slowly varying magnetic flux. At sufficiently low temperature, the total magnetization response oscillates as a function of the enclosed magnetic flux on the scale of half of a flux quantum. The amplitude of the oscillatory moment is \ensuremath{\approxeq}1.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{Am}}^{2}$ and decreases exponentially with increasing temperature on the scale of the correlation energy ${\mathit{E}}_{\mathit{c}}$=hD/(2L${)}^{2}$\ensuremath{\approxeq}80 mK. This is evidence for a flux-periodic persistent current in each ring of average value 3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$e${\ensuremath{
u}}_{\mathit{F}}$/L.
701 citations
TL;DR: In this article, a concurrent construction of random packings of 2000 rigid disks in the plane, subject to periodic boundary conditions on a square primitive cell, has been proposed to generate the histories of colliding and jamming disks.
Abstract: Random packings ofN⩽2000 rigid disks in the plane, subject to periodic boundary conditions on a square primitive cell, have been generated by a concurrent construction which treats all disks on an equal footing, as opposed to previously investigated sequential constructions. The particles start with random positions and velocities, and as they move about they grow uniformly in size, from points to jammed disks. The collection of packings displays several striking geometric features. These include (for largeN) typically polycrystalline textures with irregular grain boundaries and linear shear fractures. The packings occasionally contain monovacancies and trapped but unjammed “rattler” disks. The latter appear to be confined to the grain boundaries. The linear shear fractures preserve bond orientational order, but disrupt translational order, within the crystalline grains. A new efficient event-driven simulation algorithm is employed to generate the histories of colliding and jamming disks. On a computer which can process one million floating-point instructions per second the algorithm processes more than one million pairwise collisions per hour.
700 citations
TL;DR: The question of phase detection in photonic communications systems that use linear optical amplifiers is considered, owing to the nonlinear Kerr effect in the transmission fiber, which limits the capacity and range of such systems to a range of a few thousand kilometers.
Abstract: Spontaneous emission noise limits the capacity and range of photonic communications systems that use linear optical amplifiers. We consider here the question of phase detection in such systems. Amplitude-to-phase-noise conversion occurs owing to the nonlinear Kerr effect in the transmission fiber, resulting in optimal phase noise performance when the nonlinear phase shift of the system is approximately 1 rad. Error-free state-of-the-art systems that use phase detection at multigigabit rates are thereby limited to a range of a few thousand kilometers.
679 citations
TL;DR: A phenomenological model of a system of antiferromagnetically correlated spins is shown to give a good quantitative description of NMR, nuclear-quadrupole-resonance, and Knight-shift measurements on yttrium, planar copper, and planar oxygen sites in YBa{sub 2}Cu{sub 3}O{sub 7}.
Abstract: A phenomenological model of a system of antiferromagnetically correlated spins is shown to give a good quantitative description of NMR, nuclear-quadrupole-resonance, and Knight-shift measurements on yttrium, planar copper, and planar oxygen sites in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. The antiferromagnetic correlation length is estimated to be \ensuremath{\sim}2.5 lattice constants at T=100 K. The temperature dependence of the correlation length ceases at ${\mathit{T}}_{\mathit{x}}$\ensuremath{\simeq}100 K. The enhancement of the observed relaxation rates over what is expected for weakly interacting electrons is calculated and shown to be large. Extension of the calculation to other cuprate superconductors is discussed.
662 citations
01 Sep 1990
TL;DR: Haptic display as an augmentation to visual display can improve perception and understanding both of force fields and of world models populated with impenetrable objects and the present tool promises to yield new chemistry research results.
Abstract: We began in 1967 a project to develop a haptic+display for 6-D force fields of interacting protein molecules. We approached it in four stages: a 2-D system, a 3-D system tested with a simple task, a 6-D system tested with a simple task, and a full 6-D molecular docking system, our initial goal. This paper summarizes the entire project---the four systems, the evaluation experiments, the results, and our observations. The molecular docking system results are new.Our principal conclusions are:• Haptic display as an augmentation to visual display can improve perception and understanding both of force fields and of world models populated with impenetrable objects.• Whereas man-machine systems can outperform computer-only systems by orders of magnitude on some problems, haptic-augmented interactive systems seem to give about a two-fold performance improvement over purely graphical interactive systems. Better technology may give somewhat more, but a ten-fold improvement does not seem to be in the cards.• Chemists using GROPE-III can readily reproduce the true docking positions for drugs whose docking is known (but not to them) and can find very good docks for drugs whose true docks are unknown. The present tool promises to yield new chemistry research results; it is being actively used by research chemists.• The most valuable result from using GROPE-III for drug docking is probably the radically improved situation awareness that serious users report. Chemists say they have a new understanding of the details of the receptor site and its force fields, and of why a particular drug docks well or poorly.• We see various scientific/education applications for haptic displays but believe entertainment, not scientific visualization, will drive and pace the technology.• The hardware-software system technology we have used is barely adequate, and our experience sets priorities for future development.• Some unexpected perceptual phenomena were observed. All of these worked for us, not against us.
06 Jun 1990
TL;DR: The resulting quasi-semantic classification of nouns demonstrates the plausibility of the distributional hypothesis, and has potential application to a variety of tasks, including automatic indexing, resolving nominal compounds, and determining the scope of modification.
Abstract: A method of determining the similarity of nouns on the basis of a metric derived from the distribution of subject, verb and object in a large text corpus is described. The resulting quasi-semantic classification of nouns demonstrates the plausibility of the distributional hypothesis, and has potential application to a variety of tasks, including automatic indexing, resolving nominal compounds, and determining the scope of modification.
TL;DR: This description of the contrast enhancement has been confirmed by a series of in vitro blood sample experiments and image simulations and a predicted contrast change has been demonstrated in brain images of a mouse placed at two different orientations in the magnet.
Abstract: Unusually high image contrast in vivo magnetic resonance imaging of the brain becomes observable at high magnetic fields when the blood oxygenation level is lowered. The cause of the contrast has been attributed to a magnetic susceptibility effect induced by paramagnetic deoxyhemoglobin in red cells. When the cylinder axis of a blood vessel is not parallel to the main magnetic field, the susceptibility difference produces varying local fields around the blood vessel. In gradient-echo images, not in spin-echo images, these local fields cause intravoxel dephasing of the water signal of the surrounding tissue. This description of the contrast enhancement has been confirmed by a series of in vitro blood sample experiments and image simulations. A predicted contrast change has been demonstrated in brain images of a mouse placed at two different orientations in the magnet. From the simulated images, the dependence of the contrast on the field strength has been estimated.
18 Jun 1990
TL;DR: Algorithms are presented that solve the emptiness problem without explicitly constructing the strongly connected components of the graph representing the product automaton by allowing the algorithms to err with some probability.
Abstract: This article addresses the problem of designing memory-efficient algorithms for the verification of temporal properties of finite-state programs. Both the programs and their desired temporal properties are modeled as automata on infinite words (Buchi automata). Verification is then reduced to checking the emptiness of the automaton resulting from the product of the program and the property. This problem is usually solved by computing the strongly connected components of the graph representing the product automaton. Here, we present algorithms that solve the emptiness problem without explicitly constructing the strongly connected components of the product graph. By allowing the algorithms to err with some probability, we can implement them with a randomly accessed memory of size O(n) bits, where n is the number of states of the graph, instead of O(n log n) bits that the presently known algorithms require.
TL;DR: The dissipation below {ital T}{sub {ital c}} has been studied for representatives of all classes of cuprate high-temperature superconductors, including Ba{sub 2}YCu{sub 3}O{sub 7{minus}{delta}}, and Bi and Tl compounds.
Abstract: The dissipation below ${\mathit{T}}_{\mathit{c}}$ has been studied for representatives of all classes of cuprate high-temperature superconductors, including ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, and Bi and Tl compounds. The results are parametrized in the framework of flux creep, with the largest activation energies found in ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7}$. It is argued that the magnitude of dissipative flux motion is more related to the electronic anisotropy of the material than the actual defect structure. The thermally activated flux creep model, whose parameters are extracted from dc measurements, consistently describes also dynamic measurements, including the irreversibility line and the melting transition. Finally, the similarities in dissipative behavior are emphasized between high-${\mathit{T}}_{\mathit{c}}$ materials, very thin films, and layered low-${\mathit{T}}_{\mathit{c}}$ superconductors.
Patent•
26 Dec 1990TL;DR: In this paper, the information needs of an incoming caller with the particular expertise of a telemarketing agent who handles such calls are matched by making a search for a match between the first caller skill number and an available agent possessing that skill.
Abstract: Automatic Call Distribution (ACD) equipment is improved by matching the information needs of an incoming caller with the particular expertise of a telemarketing agent who handles such calls. Each incoming call is assigned up to three prioritized skill numbers that estimate skill requirements of the calling party. Each agent possesses up to four skill numbers that represent various particular abilities of that agent. Numbers 1 through 9 are used. Upon arrival of the incoming call, a search is made for a match between the first caller skill number and an available agent possessing that skill. If no match is found, a similar search is made with respect to the second caller skill number. After a predetermined delay, a search is made with respect to the third caller skill number. When a match is found, the incoming call is connected to the agent possessing that skill. The longer that an incoming call remains unanswered, the larger the pool of agents becomes that will be allowed to handle the call. Although the specific expertise sought may not be matched, the present technique optimizes the matching process when a time constraint is imposed.
TL;DR: A method of verifying the identity of a user based on a stream of latency periods between keystrokes, and results from trial usage of the system are reported.
Abstract: The variables that help make a handwritten signature a unique human identifier also provide a unique digital signature in the form of a stream of latency periods between keystrokes. This article describes a method of verifying the identity of a user based on such a digital signature, and reports results from trial usage of the system.
TL;DR: In this article, Bovey and Jelinsky proposed a two-dimensional nuclear magnetic resonance spectroscopy (2D NMS) method, which is based on the Coupling of Nuclear Spins (CNS).
Abstract: Fundamental Principles. L.W. Jelinsky, Experimental Methods. The Chemical Shift. Coupling of Nuclear Spins. Nuclear Relaxation and Chemical Rate Processes. P.A. Mirau and F.A. Bovey, Two-Dimensional Nuclear Magnetic Resonance Spectroscopy. Macromolecules. NMR of Solids. F.A. Bovey, L.W. Jelinsky, P.A. Mirau, Special Topics. Appendixes. Author Index. Subject Index.
TL;DR: The feasibility of using controlled manipulation of actively translocating organelles to measure direct force is demonstrated and the force driving a single organelle along microtubules is determined, allowing the force generated by a single motor to be 2.6 × 10-7 dynes.
Abstract: Organelle transport along microtubules is believed to be mediated by organelle-associated force-generating molecules. Two classes of microtubule-based organelle motors have been identified: kinesin and cytoplasmic dynein. To correlate the mechanochemical basis of force generation with the in vivo behaviour of organelles, it is important to quantify the force needed to propel an organelle along microtubules and to determine the force generated by a single motor molecule. Measurements of force generation are possible under selected conditions in vitro, but are much more difficult using intact or reactivated cells. Here we combine a useful model system for the study of organelle transport, the giant amoeba Reticulomyxa, with a novel technique for the non-invasive manipulation of and force application to subcellular components, which is based on a gradient-force optical trap, also referred to as 'optical tweezers'. We demonstrate the feasibility of using controlled manipulation of actively translocating organelles to measure direct force. We have determined the force driving a single organelle along microtubules, allowing us to estimate the force generated by a single motor to be 2.6 x 10(-7) dynes.
TL;DR: Le rapport d'intensite du dichroisme circulaire magnetique entre les limites L 2 and L 3 differe nettement de celui predit pour un simple modele decomposition/echange de the bande de valence.
Abstract: Magnetic circular dichroism (MCD) has been observed at the ${\mathit{L}}_{2,3}$ absorption edges of ferromagnetic nickel by use of circularly-polarized soft-x-ray synchrotron radiation. The MCD intensity ratio between the ${\mathit{L}}_{2}$ and ${\mathit{L}}_{3}$ edges is found to differ appreciably from that predicted by a simple exchange-split-valence-band model. Fine MCD features, imperceptible in the absorption spectra, are also observed, and a tentative interpretation is given. This work, demonstrating the feasibility of MCD measurements in the soft-x-ray region, provides a new approach to study 3d and 4f ferromagnetic systems with their respective dipole-permitted 2p\ensuremath{\rightarrow}3d and 3d\ensuremath{\rightarrow}4f transitions.
TL;DR: The band-gap luminescence is not from the exciton state, but from a surface trapped state and results in long-lived bleach and induced-absorption features in pump-probe experiments.
Abstract: We use transient optical hole burning and photoluminescence to investigate the static and dynamic electronic properties of 32-\AA{} CdSe quantum dots. We observe a number of discrete electronic transitions, resolve LO-phonon progressions, and obtain homogeneous linewidths and electron--LO-phonon couplings. We find that the band-gap luminescence is not from the exciton state, but from a surface trapped state. Rapid (\ensuremath{\sim}160 fs) trapping into these surface states results in long-lived (\ensuremath{\sim}10--100 ns) bleach and induced-absorption features in pump-probe experiments.
01 Jul 1990
TL;DR: Contents: Guidance (Kinematics, Control, and Trajectory Generation), Sensors, Navigation, Maps Representation, Sensing Strategies, Motion Planning, Systems.
Abstract: Contents: Guidance (Kinematics, Control, and Trajectory Generation.- Sensors.- Navigation (Position and Copurse Estimation).- Map Representation.- Sensing Strategies.- Motion Planning.- Systems.
TL;DR: Using a model to detect clocking hazards, two linear programs are investigated: minimizing the clock period, while avoiding clock hazards, and for a given period, maximizing the minimum safety margin against clock hazard.
Abstract: Improving the performance of a synchronous digital system by adjusting the path delays of the clock signal from the central clock source to individual flip-flops is investigated. Using a model to detect clocking hazards, two linear programs are investigated: (1) minimizing the clock period, while avoiding clock hazards, and (2) for a given period, maximizing the minimum safety margin against clock hazard. These programs are solved for a simple example, and circuit simulation is used to contrast the operation of a resulting circuit with the conventionally clocked version. The method is extended to account for clock skew caused by relative variations in the drive capabilities of N-channel versus P-channel transistors in CMOS. >
TL;DR: The authors discuss and document a parameter estimation algorithm for data sequence modeling involving hidden Markov models that uses the state-optimized joint likelihood for the observation data and the underlying Markovian state sequence as the objective function for estimation.
Abstract: The authors discuss and document a parameter estimation algorithm for data sequence modeling involving hidden Markov models. The algorithm, called the segmental K-means method, uses the state-optimized joint likelihood for the observation data and the underlying Markovian state sequence as the objective function for estimation. The authors prove the convergence of the algorithm and compare it with the traditional Baum-Welch reestimation method. They also print out the increased flexibility this algorithm offers in the general speech modeling framework. >
TL;DR: The modifications made to a connected word speech recognition algorithm based on hidden Markov models which allow it to recognize words from a predefined vocabulary list spoken in an unconstrained fashion are described.
Abstract: The modifications made to a connected word speech recognition algorithm based on hidden Markov models (HMMs) which allow it to recognize words from a predefined vocabulary list spoken in an unconstrained fashion are described. The novelty of this approach is that statistical models of both the actual vocabulary word and the extraneous speech and background are created. An HMM-based connected word recognition system is then used to find the best sequence of background, extraneous speech, and vocabulary word models for matching the actual input. Word recognition accuracy of 99.3% on purely isolated speech (i.e., only vocabulary items and background noise were present), and 95.1% when the vocabulary word was embedded in unconstrained extraneous speech, were obtained for the five word vocabulary using the proposed recognition algorithm. >
TL;DR: In this paper, a non-inertial (zero Taylor number) viscoelastic instability was discovered for Taylor-Couette flow of dilute polymer solutions, where the critical De was observed to increase as e decreases, in agreement with the theory.
Abstract: A non-inertial (zero Taylor number) viscoelastic instability is discovered for Taylor–Couette flow of dilute polymer solutions. A linear stability analysis of the inertialess flow of an Oldroyd-B fluid (using both approximate Galerkin analysis and numerical solution of the relevant small-gap eigenvalue problem) show the growth of an overstable (oscillating) mode when the Deborah number exceeds f(S) e−½, where e is the ratio of the gap to the inner cylinder radius, and f(S) is a function of the ratio of solvent to polymer contributions to the solution viscosity. Experiments with a solution of 1000 p.p.m. high-molecular-weight polyisobutylene in a viscous solvent show an onset of secondary toroidal cells when the Deborah number De reaches 20, for e of 0.14, and a Taylor number of 10−6, in excellent agreement with the theoretical value of 21. The critical De was observed to increase as e decreases, in agreement with the theory. At long times after onset of the instability, the cells become small in wavelength compared to those that occur in the inertial instability, again in agreement with our linear analysis. For this fluid, a similar instability occurs in cone-and-plate flow, as reported earlier. The driving force for these instabilities is the interaction between a velocity fluctuation and the first normal stress difference in the base state. Instabilities of the kind that we report here are likely to occur in many rotational shearing flows of viscoelastic fluids.
01 May 1990
TL;DR: A mapping based on Hilbert's space-filling curve is presented, which out-performs previously proposed mappings on average over a variety of different operating conditions.
Abstract: There is often a need to map a multi-dimensional space on to a one-dimensional space. For example, this kind of mapping has been proposed to permit the use of one-dimensional indexing techniques to a multi-dimensional index space such as in a spatial database. This kind of mapping is also of value in assigning physical storage, such as assigning buckets to records that have been indexed on multiple attributes, to minimize the disk access effort.In this paper, we discuss what the desired properties of such a mapping are, and evaluate, through analysis and simulation, several mappings that have been proposed in the past. We present a mapping based on Hilbert's space-filling curve, which out-performs previously proposed mappings on average over a variety of different operating conditions.
TL;DR: A simple and efficient method for evaluating the performance of an algorithm, rendered as a directed acyclic graph, on any parallel computer is presented and its application to several common algorithms shows that it is surprisingly accurate.
Abstract: A simple and efficient method for evaluating the performance of an algorithm, rendered as a directed acyclic graph, on any parallel computer is presented. The crucial ingredient is an efficient approximation algorithm for a particular scheduling problem. The only parameter of the parallel computer needed by our method is the message-to-instruction ratio $\tau$. Although the method used in this paper does not take into account the number of processors available, its application to several common algorithms shows that it is surprisingly accurate.
TL;DR: The ion-dissociation spectra have multiple peaks caused by multiphoton transitions during dissociation of H{sub 2} gas by intense 532-nm, 100-ps laser pulses.
Abstract: We report a new kind of multiphoton dissociation in intense laser fields. ${\mathrm{H}}_{2}^{+}$ molecular ions were formed in above-threshold ionization of ${\mathrm{H}}_{2}$ gas by intense 532-nm, 100-ps laser pulses. Observations suggest that multiphoton couplings soften the molecular bond, resulting in dissociation at laser intensities over 50 TW/${\mathrm{cm}}^{2}$. The ion-dissociation spectra have multiple peaks caused by multiphoton transitions during dissociation of ${\mathrm{H}}_{2}^{+}$.
TL;DR: The heat flux (Nusselt number) as a function of Rayleigh number is deduced from the presence of a mean flow and the nesting of the thermal boundary layer within the viscous one.
Abstract: The heat flux (Nusselt number) as a function of Rayleigh number, ${\mathit{N}}_{\mathrm{Nu}}$\ensuremath{\approxeq}0.3${\mathit{N}}_{\mathrm{Ra}}^{2/7}$, is deduced from the presence of a mean flow and the nesting of the thermal boundary layer within the viscous one. The numerical coefficients are obtained from those known empirically for turbulent boundary layers. The consistency of our assumptions as a function of Prandtl number limits this regime to (${10}^{7}$--${10}^{8}$)${\mathit{N}}_{\mathrm{Pr}}^{5/3}$\ensuremath{\lesssim}${\mathit{N}}_{\mathrm{Ra}}$\ensuremath{\lesssim} (${10}^{13}$--${10}^{15}$)${\mathit{N}}_{\mathrm{Pr}}^{4}$. The Bolgiano-Obukhov ${\mathit{k}}^{\mathrm{\ensuremath{-}}7/5}$ spectrum for the temperature fluctuations is inconsistent with a simple scaling treatment of the equations.