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Path Length (path + length)

Distribution by Scientific Domains

Life Sciences	27%
Chemistry	20%
Medical Sciences	17%
Earth and Environmental Science	7%
Polymers and Materials Science	5%
Information Science and Computing	5%
Engineering	5%
3 Other Domains	14%

Distribution within Life Sciences

Plant Science	44%
Neuroscience	18%

Kinds of Path Length

optical path length

Selected Abstracts

Improvement of the Accuracy in the Optical Hematocrit Measurement by Optimizing Mean Optical Path Length

ARTIFICIAL ORGANS, Issue 9 2009
Shiori Oshima
Abstract Optical techniques have been developed to acquire blood information (e.g., hematocrit [Hct], saturation of oxygen, thrombus) noninvasively and continuously in an artificial heart. For the practical use of an optical Hct measurement, Twersky's theory has been shown to be useful and have a good agreement in forward-scattered measurements. However, it was not applied to backward-scattered measurements, which can provide the measurement with a less demanding spatial requirement. Additionally, optimal measurement for accuracy is not well examined. Therefore, we developed an accurate Hct measurement in an artificial heart using current optical devices. To this end, we focused on optimizing an emitter,detector distance to provide a maximum optical path length. We attached optical emitter and detector fibers on Tygon tubing at various distances to measure forward- and backward-scattered light. Fresh bovine blood (Hct: 30,50%) was circulated in the tubing by a nonpulsatile artificial heart. We calculated the optical path length at various emitter,detector distances by fitting the measured optical outputs and the reference Hcts to Twersky's theory. Then, we performed Hct measurements. As a result, Twersky's theory is applicable not only to forward- but also to backward-scattered measurements in the physiogical Hct range. In both forward- and backward-scattered measurements, calculated optical path lengths become maximum at the same emitter,detector distance. The accuracy of Hct measurement is improved two to three times with the emitter,detector distance compared with other distances. The mean error is less than 1 Hct%. This result shows that an accurate Hct measurement is realized by selecting the optimal emitter,detector distance, which provides maximum optical path length defined by Twersky's theory. Our study provides a framework for the practical and less restrictive application of the optical Hct measurement to patients with an artificial heart. [source]

Actin-dependent motility of melanosomes from fish retinal pigment epithelial (RPE) cells investigated using in vitro motility assays

CYTOSKELETON, Issue 2 2004
E. L. McNeil
Melanosomes (pigment granules) within retinal pigment epithelial (RPE) cells of fish and amphibians undergo massive migrations in response to light conditions to control light flux to the retina. Previous research has shown that melanosome motility within apical projections of dissociated fish RPE cells requires an intact actin cytoskeleton, but the mechanisms and motors involved in melanosome transport in RPE have not been identified. Two in vitro motility assays, the Nitella assay and the sliding filament assay, were used to characterize actin-dependent motor activity of RPE melanosomes. Melanosomes applied to dissected filets of the Characean alga, Nitella, moved along actin cables at a mean rate of 2 ,m/min, similar to the rate of melanosome motility in dissociated, cultured RPE cells. Path lengths of motile melanosomes ranged from 9 to 37 ,m. Melanosome motility in the sliding filament assay was much more variable, ranging from 0.4,33 ,m/min; 70% of velocities ranged from 1,15 ,m/min. Latex beads coated with skeletal muscle myosin II and added to Nitella filets moved in the same direction as RPE melanosomes, indicating that the motility is barbed-end directed. Immunoblotting using antibodies against myosin VIIa and rab27a revealed that both proteins are enriched on melanosome membranes, suggesting that they could play a role in melanosome transport within apical projections of fish RPE. Cell Motil. Cytoskeleton 58:71,82, 2004. © 2004 Wiley-Liss, Inc. [source]

Priority-Driven Acoustic Modeling for Virtual Environments

COMPUTER GRAPHICS FORUM, Issue 3 2000
Patrick Min
Geometric acoustic modeling systems spatialize sounds according to reverberation paths from a sound source to a receiver to give an auditory impression of a virtual 3D environment. These systems are useful for concert hall design, teleconferencing, training and simulation, and interactive virtual environments. In many cases, such as in an interactive walkthrough program, the reverberation paths must be updated within strict timing constraints - e.g., as the sound receiver moves under interactive control by a user. In this paper, we describe a geometric acoustic modeling algorithm that uses a priority queue to trace polyhedral beams representing reverberation paths in best-first order up to some termination criteria (e.g., expired time-slice). The advantage of this algorithm is that it is more likely to find the highest priority reverberation paths within a fixed time-slice, avoiding many geometric computations for lower-priority beams. Yet, there is overhead in computing priorities and managing the priority queue. The focus of this paper is to study the trade-offs of the priority-driven beam tracing algorithm with different priority functions. During experiments computing reverberation paths between a source and a receiver in a 3D building environment, we find that priority functions incorporating more accurate estimates of source-to-receiver path length are more likely to find early reverberation paths useful for spatialization, especially in situations where the source and receiver cannot reach each other through trivial reverberation paths. However, when receivers are added to the environment such that it becomes more densely and evenly populated, this advantage diminishes. [source]

Similar and functionally typical kinematic reaching parameters in 7- and 15-month-old in utero cocaine-exposed and unexposed infants

DEVELOPMENTAL PSYCHOBIOLOGY, Issue 3 2004
E. Z. Tronick
Abstract This study examined the effects of intrauterine cocaine exposure on the reaches of 19 exposed and 15 unexposed infants at 7 and 15 months using kinematic measures. Infants sat at a table and reached for a rattle, a toy doll, and a chair. Videotaped reaches were digitized using the Peak Performance system. Kinematic movement variables were extracted (e.g., reach duration, peak velocity, movement units, path length) and ratios computed (e.g., path length divided by number of movement units). Regardless of exposure status, reaches of older infants were faster, more direct, had fewer movement units, and covered more distance with the first movement unit. Exposed infants covered more distance per movement unit than unexposed infants, but there were no other significant differences. Reaches of exposed and unexposed infants were essentially similar. Importantly, reach parameters for these high-risk infants were similar to reach parameters for infants at lower social and biological risk. © 2004 Wiley Periodicals, Inc. Dev Psychobiol 44: 168,175, 2004. [source]

Use of terrain variables for mapping gully erosion susceptibility in Lebanon

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 12 2007
Rania Bou Kheir
Abstract This paper predicts the geographic distribution and size of gullies across central Lebanon using a geographic information system (GIS) and terrain analysis. Eleven primary (elevation; upslope contributing area; aspect; slope; plan, profile and tangential curvature; flow direction; flow width; flow path length; rate of change of specific catchment area along the direction of flow) and three secondary (steady-state; quasi-dynamic topographic wetness; sediment transport capacity) topographic variables were generated and used along with digital data collected from other sources (soil, geology) to statistically explain gully erosion field measurements. Three tree-based regression models were developed using (1) all variables, (2) primary topographic variables only and (3) different pairs of variables. The best regression tree model combined the steady-state topographic wetness and sediment transport capacity indices and explained 80% of the variability in field gully measurements. This model proved to be simple, quick, realistic and practical, and it can be applied to other areas of the Mediterranean region with similar environmental conditions, thereby providing a tool to help with the implementation of plans for soil conservation and sustainable management. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Particle path length distributions in meandering gravel-bed streams: results from physical models

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2003
Richard S. Pyrce
Abstract In gravel-bed rivers with well-de,ned pool,bar morphology, the path length of transported bed particles must be, at least during ,channel-forming' ,ows, equal to the length scale of the morphology. This is the basis for some methods for estimating bed material transport rates. However, previous data, especially from ,eld tests, are often strongly positively skewed with mean much shorter than the pool,bar spacing. One possible explanation is that positively skewed distributions occur only in channels lacking distinct pool,bar topography or only at lower discharges in pool,bar channels. A series of ,ume experiments using ,uorescent tracers was used to measure path length distributions in low-sinuosity meandering channels to assess the relation with channel morphology and ,ow conditions. At channel-forming ,ows, 55 to 75 per cent of the tracer grains were deposited on the ,rst point bar downstream of the point of tracer input, with 15 per cent passing beyond the ,rst bar. Path length distributions are symmetrical with mean equal to the pool,bar spacing and can be described with a Cauchy distribution. In some cases there was a secondary mode close to the point of tracer introduction; this bimodal distribution ,ts a combined gamma,Cauchy distribution. Only when discharge was reduced below the channel-forming ,ow were frequency distributions unimodal and positively skewed with no relation to the pool,bar spacing. Thus, path length distributions become more symmetrical, and mean path length increases to coincide with pool,bar spacing, as ,ow approaches channel-forming conditions. This is a substantial modi,cation of existing models of particle transfer in gravel-bed rivers. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Electrochemical Detection for Capillary Electrophoresis Microchips: A Review

ELECTROANALYSIS, Issue 13 2005
Joseph Wang
Abstract Electrochemistry detection offers considerable promise for capillary-electrophoresis (CE) microchips, with features that include remarkable sensitivity, portability, independence of optical path length or sample turbidity, low cost and power requirements, and high compatibility with modern micromachining technologies. This article highlights key strategies in controlled-potential electrochemical detectors for CE microchip systems, along with recent advances and directions. Subjects covered include the design of the electrochemical detection system, its requirements and operational principles, common electrode materials, isolation from the separation voltage, derivatization reactions, typical applications, and future prospects. It is expected that electrochemical detection will become a powerful tool for CE microchip systems and will lead to the creation of truly portable (and possibly disposable) devices. [source]

Automated ultrasound-assisted method for the determination of the oxidative stability of virgin olive oil

EUROPEAN JOURNAL OF LIPID SCIENCE AND TECHNOLOGY, Issue 2 2007
José Platero-López
Abstract A fast and automated method is proposed for determining the oxidative stability of virgin olive oil by using ultrasound. The ultrasound microprobe (3,mm in diameter) was directly immersed into the olive oil sample contained in a test tube. The most influential variables in the oxidation process, namely pulse amplitude, duty cycle, irradiation time, and sample amount, were optimized. The oil absorbance at 270,nm was continuously monitored by oil recirculation through a 0.1-mm path length flow cell connected to a fiber optic microspectrometer. This short path length allowed the direct monitoring of absorbance without needing any sample dilution. The ultrasound energy was applied during 35,min, and the resulting increase in absorbance was continuously monitored. The difference between the final and the initial absorbance at 270,nm of a set of virgin olive oil samples was closely correlated with their oxidative stability calculated by the Rancimat method (R2,=,0.9915). The resulting equation enabled the prediction of the oxidative stability of virgin olive oil in a short period of time (35,min), by using a simple, inexpensive, automatic and easy-to-use system. [source]

Vents and seals in non-steady-state chambers used for measuring gas exchange between soil and the atmosphere

EUROPEAN JOURNAL OF SOIL SCIENCE, Issue 4 2001
G. L. Hutchinson
Summary Despite decades of research to define optimal chamber design and deployment protocol for measuring gas exchange between the Earth's surface and the atmosphere, controversy still surrounds the procedures for applying this method. Using a numerical simulation model we demonstrated that (i) all non-steady-state chambers should include a properly sized and properly located vent tube; (ii) even seemingly trivial leakiness of the seals between elements of a multiple-component chamber results in significant risk of measurement error; (iii) a leaking seal is a poor substitute for a properly designed vent tube, because the shorter path length through the seal supports much greater diffusive gas loss per unit of conductance to mass flow; (iv) the depth to which chamber walls must be inserted to minimize gas loss by lateral diffusion is smaller than is customary in fine-textured, wet or compact soil, but much larger than is customary in highly porous soils, and (v) repetitive sampling at the same location is not a major source of error when using non-steady-state chambers. Finally, we discuss problems associated with computing the flux of a gas from the non-linear increase in its concentration in the headspace of a non-steady-state chamber. [source]

Subtle myelin defects in PLP-null mice ,

GLIA, Issue 3 2006
Jack Rosenbluth
Abstract This study explores subtle defects in the myelin of proteolipid protein (PLP)-null mice that could potentially underlie the functional losses and axon damage known to occur in this mutant and in myelin diseases including multiple sclerosis. We have compared PLP-null central nervous system (CNS) myelin with normal myelin using ultrastructural methods designed to emphasize fine differences. In the PLP-null CNS, axons large enough to be myelinated often lack myelin entirely or are surrounded by abnormally thin sheaths. Short stretches of cytoplasm persist in many myelin lamellae. Most strikingly, compaction is incomplete in this mutant as shown by the widespread presence of patent interlamellar spaces of variable width that can be labeled with ferricyanide, acting as an aqueous extracellular tracer. In thinly myelinated fibers, interlamellar spaces are filled across the full width of the sheaths. In thick myelin sheaths, they appear filled irregularly but diffusely. These patent spaces constitute a spiral pathway through which ions and other extracellular agents may penetrate gradually, possibly contributing to the axon damage known to occur in this mutant, especially in thinly myelinated fibers, where the spiral path length is shortest and most consistently labeled. We show also that the "radial component" of myelin is distorted in the mutant ("diagonal component"), extending across the sheaths at 45° instead of 90°. These observations indicate a direct or indirect role for PLP in maintaining myelin compaction along the external surfaces of the lamellae and to a limited extent, along the cytoplasmic surfaces as well and also in maintaining the normal alignment of the radial component. © 2006 Wiley-Liss, Inc. [source]

Intrahippocampal administration of BDNF in adult rats affects short-term behavioral plasticity in the Morris water maze and performance in the elevated plus-maze

HIPPOCAMPUS, Issue 7 2004
Francesca Cirulli
Abstract The present study evaluated the effects of a single intrahippocampal administration of brain-derived neurotrophic factor (BDNF) on memory retention in a water maze. Adult rats were trained in a water maze (acquisition phase, day 1). Immediately after the last training trial subjects were injected in the right hippocampus with either BDNF (24 ,g) or phosphate-buffered saline (1 ,l). On day 2, all subjects were tested for memory retention in a probe trial and were subsequently tested for reversal learning. While no differences emerged in the probe trial, BDNF-treated subjects showed a shorter latency and a shorter path length to reach the platform during the reversal phase. A significant difference in their "turn angle" and in their swim paths suggests that they might have used a different search strategy compared with controls. Moreover, all subjects also underwent an elevated-plus maze test. BDNF-treated-animals showed a clear tendency to spend a greater amount of time in the open arms and a significantly higher frequency of grooming behavior and of the stretched-attend posture in this maze area, but no differences in locomotion. Overall, these results indicate that administration of BDNF improves performance in a spatial memory task and has enduring effects on emotional behavior. © 2004 Wiley-Liss, Inc. [source]

Davydov's solitons in a homogeneous nucleotide chain

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2010
Victor D. Lakhno
Abstract Charge transfer in homogeneous nucleotide chains is modeled on the basis of Holstein Hamiltonian. The path length of Davydov solitons in these chains is being studied. It is shown that in a dispersionless case, when the soliton velocity V is small, the path length grows exponentially as V decreases. In this case, the state of a moving soliton is quasisteady. In the presence of dispersion determined by the dependence ,2 = , + V,2, the path length in the region 0 < V < V0 is equal to infinity. In this case, the phonon environment follows the charge motion. In the region V > V0, the soliton motion is accompanied by emission of phonons which leads to a finite path length of a soliton. The latter tends to infinity as V , V0 + 0 and V , ,. The presence of dissipation leads to a finite soliton path length. An equilibrium velocity of soliton in an external electric field is calculated. It is shown that there is a maximum intensity of an electric field at which a steady motion of a soliton is possible. The soliton mobility is calculated for the stable or ohmic brunch. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source]

Tree search algorithm for assigning cooperating UAVs to multiple tasks

INTERNATIONAL JOURNAL OF ROBUST AND NONLINEAR CONTROL, Issue 2 2008
Steven J. Rasmussen
Abstract This paper describes a tree search algorithm for assigning cooperating homogeneous uninhabited aerial vehicles to multiple tasks. The combinatorial optimization problem is posed in the form of a decision tree, the structure of which enforces the required group coordination and precedence for cooperatively performing the multiple tasks. For path planning, a Dubin's car model is used so that the vehicles' constraint, of minimum turning radius, is taken into account. Due to the prohibitive computational complexity of the problem, exhaustive enumeration of all the assignments encoded in the tree is not feasible. The proposed optimization algorithm is initialized by a best-first search and candidate optimal solutions serve as a monotonically decreasing upper bound for the assignment cost. Euclidean distances are used for estimating the path length encoded in branches of the tree that have not yet been evaluated by the computationally intensive Dubin's optimization subroutine. This provides a lower bound for the cost of unevaluated assignments. We apply these upper and lower bounding procedures iteratively on active subsets within the feasible set, enabling efficient pruning of the solution tree. Using Monte Carlo simulations, the performance of the search algorithm is analyzed for two different cost functions and different limits on the vehicles' minimum turn radius. It is shown that the selection of the cost function and the limit have a considerable effect on the level of cooperation between the vehicles. The proposed deterministic search method can be applied on line to different sized problems. For small-sized problems, it provides the optimal solution. For large-sized problems, it provides an immediate feasible solution that improves over the algorithm's run time. When the proposed method is applied off line, it can be used to obtain the optimal solution, which can be used to evaluate the performance of other sub-optimal search methods. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Modified methodology for computing interference in LEO satellite environments

INTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS AND NETWORKING, Issue 6 2003
Raúl Chávez Santiago
Abstract Computing interference is very important in satellite networks design in order to assure the electromagnetic compatibility (EMC) with other radiocommunication systems. There are different methods to compute interference in geostationary (GEO) satellite systems including conventional methods using link budget equations and alternate methods such as increase in noise temperature. However, computing interference in low earth orbit (LEO) systems represents a different problem. Due to the special characteristics of this kind of orbits, the elevation angle at any site changes continuously over time, meaning a time dependent change of the propagation path length between an interfering transmitter and an interfered-with receiver, and of the discrimination provided by the transmitting and/or the receiving antenna. Thus, conventional interference prediction methods developed for fixed links must be adapted to the case of LEO systems. To overcome this problem a mathematical model that characterizes the path length variations by an average value obtained from the probability density function of the varying distance between an interfering transmitter and an interfered- with receiver is proposed in this paper. This average path length enables the use of conventional link budget methods to reduce the computation time for the evaluation of interference in LEO satellite environments. Two practical examples show the possible applications of the proposed model. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Role of Wavelength Adaptation in the Initiation, Maintenance, and Pharmacologic Suppression of Reentry

JOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY, Issue 6 2001
STEVEN D. GIROUARD Ph.D.
Wavelength Adaptation and Reentry.Introduction: The stability of reentry is thought to depend on a critical balance between the spatial extent of refractory tissue in a reentrant wave (i.e., wavelength ,) and the reentrant path length. Because considerable evidence suggests that , changes continuously in space and time during abrupt rate changes associated with the onset of tachycardia, we hypothesized that beat-by-beat adaptation of , to the dimensions of the reentrant path plays a central role in the mechanism of initiation of reentry. Methods and Results: To investigate the dynamic relationship between , and path length during initiation of reentry, optical mapping with voltage-sensitive dyes was used in a guinea pig model of reentrant ventricular tachycardia (VT). In this model, a computer-guided laser obstacle precisely controlled the position and dimensions of the reentrant path. Under control perfusion and after addition of 15 , M d-sotalol, , was monitored during steady-state pacing, premature stimulation, and the initiating beats leading to nonsustained and sustained VT. During control perfusion, reentrant VT was reproducibly induced in 8 of 8 hearts, whereas in the presence of d-sotalol, reentry could only be initiated in 1 of 8 hearts due primarily to the failure of , to adapt to the reentrant path length. During successful initiation of VT, a consistent sequence was observed. The sequence was characterized by antidromic and orthodromic propagation around both sides of the anatomic obstacle, followed by unidirectional block of the antidromic impulse and persistence of reentry only if the , of the orthodromic impulse adapted to the reentrant path (, < path length). d-Sotalol prevented initiation of VT by altering , adaptation of the orthodromic wave; however, it failed to terminate ongoing VT because reverse use-dependence developed after several beats of tachycardia. Conclusion: In an experimental model where ,, path length, and cellular action potentials were monitored during initiation of reentry, we found that, in contrast to termination, the initiation of reentry and the transition from nonsustained to sustained VT is strongly dependent on beat-to-beat adaptation of , to the dimensions of the reentrant path. [source]

Assessing normal pulse wave velocity in the proximal pulmonary arteries using transit time: A feasibility, repeatability, and observer reproducibility study by cardiovascular magnetic resonance

JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 5 2007
MRCP, William M. Bradlow BM
Abstract Purpose To calculate pulse wave velocity (PWV) in the proximal pulmonary arteries (PAs) by cardiovascular magnetic resonance (CMR) using the transit-time method, and address respiratory variation, repeatability, and observer reproducibility. Materials and Methods A 1.9-msec interleaved phase velocity sequence was repeated three times consecutively in 10 normal subjects. Pulse wave (PW) arrival times (ATs) were determined for the main and branch PAs. The PWV was calculated by dividing the path length traveled by the difference in ATs. Respiratory variation was considered by comparing acquisitions with and without respiratory gating. Results For navigated data the mean PWVs for the left PA (LPA) and right PA (RPA) were 2.09 ± 0.64 m/second and 2.33 ± 0.44 m/second, respectively. For non-navigated data the mean PWVs for the LPA and RPA were 2.14 ± 0.41 m/second and 2.31 ± 0.49 m/second, respectively. No statistically significant difference was found between respiratory non-navigated data and navigated data. Repeated on-table measurements were consistent (LPA non-navigated P = 0.95, RPA non-navigated P = 0.91, LPA navigated P = 0.96, RPA navigated P = 0.51). The coefficients of variation (CVs) were 12.2% and 12.5% for intra- and interobserver assessments, respectively. Conclusion One can measure PWV in the proximal PAs using transit-time in a reproducible manner without respiratory gating. J. Magn. Reson. Imaging 2007;25:974,981. © 2007 Wiley-Liss, Inc. [source]

Infrared stealth and anticorrosion performances of organically modified silicate-NiZn ferrite/polyaniline hybrid coatings

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 3 2008
K. H. Wu
Abstract Hybrid coatings based on organically modified silicate-Ni0.5Zn0.5Fe2O4/polyaniline were synthesized through a sol,gel technique with different NiZn ferrite/polyaniline weight ratio (1/1, 1/2, 1/5). These hybrid films were deposited via spin coating onto an aluminum alloy to improve the corrosion protection and to act as infrared stealth coatings. The effects induced by the NiZn ferrite/polyaniline hybrids on the chain dynamic, ferromagnetic behavior, infrared stealth, and anticorrosion performances of the coated samples were investigated. The rotating-frame spin-lattice relaxation times and scale of the spin-diffusion path length indicated that the configuration of the hybrid films was highly cross-linked and dense. The thermal extinction of the hybrid coatings increased with the increase in the polyaniline content. Potentio-dynamic and salt-spray analysis revealed that the hybrid films provided an exceptional barrier and corrosion protection in comparison with untreated aluminum alloy substrates. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 926,935, 2008 [source]

Separation of benzene and deuterated benzenes by reversed-phase and recycle liquid chromatography using monolithic capillary columns

JOURNAL OF SEPARATION SCIENCE, JSS, Issue 15-16 2004
Lee Wah Lim
Abstract An alternate pumping-recycle system utilizing a commercially available low dead-volume switching valve was developed for microcolumn LC. The recycle system had two separation columns, and the dead volume of the recycling lines was kept to a minimum by avoiding passage of the sample through the pump chamber, sample injector, and the normal path length of a conventional UV detector. The drawback of the high total back pressure caused by the second column that is placed after the detector was overcome by on-column detection, and this eliminated the need for a high pressure flow cell. The system was used for the separation of an authentic mixture of benzene, benzene-1,3,5-d3, and benzene-d6. Baseline separation was accomplished after six cycles and the calculated theoretical plate number for benzene was 230,000. It was observed that the theoretical plate number (N) increased linearly with increasing number of cycles, and the N per unit time increased with increasing inlet pressure. The separation conditions were optimized and the separation of benzene and benzene-d6 was accomplished within 75 min at 2.5 MPa inlet pressure. [source]

USING NETWORK ANALYSIS TO CHARACTERIZE FOREST STRUCTURE

NATURAL RESOURCE MODELING, Issue 2 2008
MICHAEL M. FULLER
Abstract Network analysis quantifies different structural properties of systems of interrelated parts using a single analytical framework. Many ecological phenomena have network-like properties, such as the trophic relationships of food webs, geographic structure of metapopulations, and species interactions in communities. Therefore, our ability to understand and manage such systems may benefit from the use of network-analysis techniques. But network analysis has not been applied extensively to ecological problems, and its suitability for ecological studies is uncertain. Here, we investigate the ability of network analysis to detect spatial patterns of species association in a tropical forest. We use three common graph-theoretic measures of network structure to quantify the effect of understory tree size on the spatial association of understory species with trees in the canopy: the node degree distribution (NDD), characteristic path length (CPL), and clustering coefficient (CC). We compute the NDD, CPL, and CC for each of seven size classes of understory trees. For significance testing, we compare the observed values to frequency distributions of each statistic computed from randomized data. We find that the ability of network analysis to distinguish observed patterns from those representing randomized data strongly depends on which aspects of structure are investigated. Analysis of NDD finds no significant difference between random and observed networks. However, analysis of CPL and CC detected nonrandom patterns in three and one of the seven size classes, respectively. Network analysis is a very flexible approach that holds promise for ecological studies, but more research is needed to better understand its advantages and limitations. [source]

The fractional congestion bound for efficient edge disjoint routing

NETWORKS: AN INTERNATIONAL JOURNAL, Issue 3 2008
Alok Baveja
Abstract This article investigates the following problem: Given the fractional relaxation of the edge disjoint routing problem, how small a fractional congestion is sufficient to guarantee efficient edge disjoint routing? That is, what is the largest possible value v such that a fractional flow with congestion at most v, can be efficiently converted into an edge disjoint routing? Leighton, Lu, Rao, and Srinivasan (SIAM J Comput 2001) have established that fractional congestion of at most the order of O(1/(d log k)) is sufficient, where d is the maximum path length in the fractional relaxation, and k is the number of pairs to be routed. It is also known that ,(1/d) is the correct bound, if we are only interested in an existence result (Leighton, Rao, and Srinivasan, Hawaii International Conference on System Sciences, 1998). Motivated by the fact that d is small for many types of routing problems, specifically, polylogarithmic for expander graphs, this article improves upon the former result by showing O(1/(d log d)) fractional congestion to suffice. © 2007 Wiley Periodicals, Inc. NETWORKS, 2008 [source]

Algorithms to calculate the distribution of the longest path length of a stochastic activity network with continuous activity durations

NETWORKS: AN INTERNATIONAL JOURNAL, Issue 3 2006
Lawrence M. Leemis
Abstract We develop algorithms to calculate the probability distribution of the longest path of an arbitrary stochastic activity network with continuous activity durations by three techniques: recursive Monte Carlo simulation, series-parallel reduction, and conditioning. Examples illustrate the use of the three techniques. © 2006 Wiley Periodicals, Inc. NETWORKS, Vol. 48(3), 143,165 2006 [source]

Shortest-path network interdiction,

NETWORKS: AN INTERNATIONAL JOURNAL, Issue 2 2002
Eitan Israeli
Abstract We study the problem of interdicting the arcs in a network in order to maximize the shortest s,t path length. "Interdiction" is an attack on an arc that destroys the arc or increases its effective length; there is a limited interdiction budget. We formulate this bilevel, max,min problem as a mixed-integer program (MIP), which can be solved directly, but we develop more efficient decomposition algorithms. One algorithm enhances Benders decomposition by adding generalized integer cutting planes, called "supervalid inequalities" (SVIs), to the master problem. A second algorithm exploits a unique set-covering master problem. Computational results demonstrate orders-of-magnitude improvements of the decomposition algorithms over direct solution of the MIP and show that SVIs also help solve the original MIP faster. Published 2002 Wiley Periodicals, Inc. [source]

The influence of species and growing conditions on the 18-O enrichment of leaf water and its impact on ,effective path length'

NEW PHYTOLOGIST, Issue 3 2009
Ansgar Kahmen
Summary ,,The stable oxygen isotope ratio (,18O) of plant material has been shown to contain essential information on water and carbon fluxes at the plant and ecosystem scales. However, the effective path length (Lm), a parameter introduced to leaf-water models still requires a comprehensive biological characterization to allow interpretation of ,18O values in plant material with confidence. ,,Here, we tested the variability of Lm across and within three species that developed leaves in environments with different relative humidity. We also tested whether the Lm of fully developed leaves is affected by short-term fluctuations in relative humidity. ,,We determined that significant differences in Lm exist among Phaseolus vulgaris, Rizinus communis and Helianthus annuus. Within a given species, however, Lm values did not differ significantly among individuals. ,,These findings indicate that Lm is species specific and a relatively constant parameter and that Lm will not obscure the interpretation of ,18O values in plant material of a given species. We urge caution, however, because values for Lm are derived from fitting leaf-water models to measured values of ,18O, so care must be taken in assigning a ,cause' to values of Lm as they likely capture a combination of different biological leaf properties [source]

Dynamics of water transport and storage in conifers studied with deuterium and heat tracing techniques

PLANT CELL & ENVIRONMENT, Issue 1 2006
F. C. MEINZER
ABSTRACT The volume and complexity of their vascular systems make the dynamics of long-distance water transport in large trees difficult to study. We used heat and deuterated water (D2O) as tracers to characterize whole-tree water transport and storage properties in individual trees belonging to the coniferous species Pseudotsuga menziesii (Mirb.) Franco and Tsuga heterophylla (Raf.) Sarg. The trees used in this study spanned a broad range of height (13.5,58 m) and diameter (0.14,1.43 m). Sap flow was monitored continuously with heat dissipation probes near the base of the trunk prior to, during and following injection of D2O. The transit time for D2O transport from the base of the trunk to the upper crown and the tracer residence time were determined by measuring hydrogen isotope ratios in water extracted from leaves sampled at regular intervals. Transit times for arrival of D2O in the upper crown ranged from 2.5 to 21 d and residence times ranged from 36 to 79 d. Estimates of maximum sap velocity derived from tracer transit times and path length ranged from 2.4 to 5.4 m d,1. Tracer residence time and half-life increased as tree diameter increased, independent of species. Species-independent scaling of tracer velocity with sapwood-specific conductivity was also observed. When data from this study were combined with similar data from an earlier study of four tropical angiosperm trees, species-independent scaling of tracer velocity and residence time with sapwood hydraulic capacitance was observed. Sapwood capacitance is an intrinsic tissue-level property that appears to govern whole-tree water transport in a similar manner among both tracheid- and vessel-bearing species. [source]

Differences in hydraulic architecture account for near-isohydric and anisohydric behaviour of two field-grown Vitis vinifera L. cultivars during drought

PLANT CELL & ENVIRONMENT, Issue 8 2003
H. R. SCHULTZ
ABSTRACT A comparative study on stomatal control under water deficit was conducted on grapevines of the cultivars Grenache, of Mediterranean origin, and Syrah of mesic origin, grown near Montpellier, France and Geisenheim, Germany. Syrah maintained similar maximum stomatal conductance (gmax) and maximum leaf photosynthesis (Amax) values than Grenache at lower predawn leaf water potentials, ,leaf, throughout the season. The ,leaf of Syrah decreased strongly during the day and was lower in stressed than in watered plants, showing anisohydric stomatal behaviour. In contrast, Grenache showed isohydric stomatal behaviour in which ,leaf did not drop significantly below the minimum ,leaf of watered plants. When g was plotted versus leaf specific hydraulic conductance, Kl, incorporating leaf transpiration rate and whole-plant water potential gradients, previous differences between varieties disappeared both on a seasonal and diurnal scale. This suggested that isohydric and anisohydric behaviour could be regulated by hydraulic conductance. Pressure-flow measurements on excised organs from plants not previously stressed revealed that Grenache had a two- to three-fold larger hydraulic conductance per unit path length (Kh) and a four- to six-fold larger leaf area specific conductivity (LSC) in leaf petioles than Syrah. Differences between internodes were only apparent for LSC and were much smaller. Cavitation detected as ultrasound acoustic emissions on air-dried shoots showed higher rates for Grenache than Syrah during the early phases of the dry-down. It is hypothesized that the differences in water-conducting capacity of stems and especially petioles may be at the origin of the near-isohydric and anisohydric behaviour of g. [source]

Evaluating different soil and plant hydraulic constraints on tree function using a model and sap flow data from ponderosa pine

PLANT CELL & ENVIRONMENT, Issue 7 2001
M. Williams
Relationships between tree size and physiological processes such as transpiration may have important implications for plant and ecosystem function, but as yet are poorly understood. We used a process-based model of the soil,plant,atmosphere continuum to investigate patterns of whole-tree sap flow in ponderosa pine trees of different size and age (36 m and ,250 years versus 13 m and 10,50 years) over a developing summer drought. We examined three different hypothetical controls on hydraulic resistance, and found that size-related differences in sap flow could be best explained by absolute differences in plant resistance related to path length (hypothesis 1) rather than through different dynamic relationships between plant resistance and leaf water potential (hypothesis 2), or alterations in rates of cumulative inducement and repair of cavitation (hypothesis 3). Reductions in sap flow over time could be best explained by rising soil,root resistance (hypothesis 1), rather than by a combination of rising plant and soil,root resistance (hypothesis 2), or by rising plant resistance alone (hypothesis 3). Comparing hourly predictions with observed sap flow, we found that a direct relationship between plant resistance and leaf water potential (hypothesis 2) led to unrealistic bimodal patterns of sap flow within a day. Explaining seasonal reduction in sap flow purely through rising plant resistance (hypothesis 3) was effective but failed to explain the observed decline in pre-dawn leaf water potential for small trees. Thus, hypothesis 1 was best corroborated. A sensitivity analysis revealed a significant difference in the response to drought-relieving rains; precipitation induced a strong recovery in sap flow in the hypothetical case of limiting soil,root resistance (hypothesis 1), and an insignificant response in the case of limiting plant resistance (hypothesis 3). Longer term monitoring and manipulation experiments are thus likely to resolve the uncertainties in hydraulic constraints on plant function. [source]

Ultrasonic measurement of residual wall thickness during gas assisted injection molding,

POLYMER ENGINEERING & SCIENCE, Issue 11 2007
E.C. Brown
Ultrasonic technology provides a powerful and noninvasive method of in-process measurement during injection molding and extrusion. Changes in the velocity, attenuation and reflection coefficients of high frequency sound waves can be related to the state and conditions of the materials through which they propagate. The velocity of an ultrasonic wave changes with density and elastic moduli; this allows information on solidification and material properties to be collected during the molding cycle. The time of flight of the wave is a function of velocity and path length. This paper shows that it can be correlated with the residual wall thickness of polymer in the mold during gas assisted injection molding. POLYM. ENG. SCI., 47:1730,1739, 2007. © 2007 Society of Plastics Engineers [source]

Influence of day length, ambient temperature, and seasonality on daily travel distance in the Yunnan snub-nosed monkey at Jinsichang, Yunnan, China

AMERICAN JOURNAL OF PRIMATOLOGY, Issue 3 2009
Ren Baoping
Abstract This article examines the effect of ambient temperature, day length, weather conditions, and seasonality on daily path length (DPL) of a free-ranging group of Yunnan snub-nosed monkeys (Rhinopithecus bieti) using an auto-released GPS collar. Data were collected from December 17, 2003 to October 22, 2004 at Laojunshan in northwestern Yunnan province, China. The average DPL of the monkey group was 909±472,m (n=291), with the shortest distance being 180,m and the longest distance 3,626,m. Ambient temperature and day length were found to affect DPL. Both factors were positively correlated with DPL, which means that the monkey group traveled greater distances on longer and warmer days. At the study site, three distinct seasons were identified, and DPL did not vary significantly across these periods. The time of sunrise was not correlated with DPL. Nevertheless, we sometimes observed the group starting its daily trip later on cloudy days than on sunny days. Furthermore, weather conditions (e.g. rainy, cloudy, and sunny) did not influence the average DPL of the study group. Overall we found that the primary factors affecting DPL in R. bieti were day length and ambient temperature, especially daily highest temperature. Am. J. Primatol. 71:233,241, 2009. © 2008 Wiley-Liss, Inc. [source]

Least path criterion (LPC) for unique indexing in a two-dimensional decagonal quasilattice

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 5 2002
N. K. Mukhopadhyay
The least path criterion or least path length in the context of redundant basis vector systems is discussed and a mathematical proof is presented of the uniqueness of indices obtained by applying the least path criterion. Though the method has greater generality, this paper concentrates on the two-dimensional decagonal lattice. The order of redundancy is also discussed; this will help eventually to correlate with other redundant but desirable indexing sets. [source]