Cluster Lifetime Research Articles

A Mobility-Aware General-Purpose Vehicular Ad-Hoc Network Clustering Scheme

Clustering is an effective method of topology control for wireless ad-hoc networks. Clusters introduce a structure in a flat network and can guarantee some basic levels of performance in presence of high mobility and large number of nodes. Therefore clusters can effectively increase the network capacity by the spatial reuse of network resources. Robust clustering schemes designed specifically for VANETs must address cluster stability and the high-mobility exhibited by the nodes, while reducing communication and computational costs. This paper presents a distributed multi-hop clustering scheme designed for VANETs, which utilizes mobility information to generate a stable cluster structure, maintained through the propagation of periodic DSRC safety messages. Our scheme takes the directionality of VANET nodes into consideration, where clusters are formed only between vehicles exhibiting a similar mobility pattern. Simulation results confirm that the execution of our scheme, due to its features, increases the average cluster lifetime and stability, and reduces the amount of communication and processing needed to form stable cluster structures in VANETs.

Adaptive coarse-grained Monte Carlo simulation of reaction and diffusion dynamics in heterogeneous plasma membranes

BackgroundAn adaptive coarse-grained (kinetic) Monte Carlo (ACGMC) simulation framework is applied to reaction and diffusion dynamics in inhomogeneous domains. The presented model is relevant to the diffusion and dimerization dynamics of epidermal growth factor receptor (EGFR) in the presence of plasma membrane heterogeneity and specifically receptor clustering. We perform simulations representing EGFR cluster dissipation in heterogeneous plasma membranes consisting of higher density clusters of receptors surrounded by low population areas using the ACGMC method. We further investigate the effect of key parameters on the cluster lifetime.ResultsCoarse-graining of dimerization, rather than of diffusion, may lead to computational error. It is shown that the ACGMC method is an effective technique to minimize error in diffusion-reaction processes and is superior to the microscopic kinetic Monte Carlo simulation in terms of computational cost while retaining accuracy. The low computational cost enables sensitivity analysis calculations. Sensitivity analysis indicates that it may be possible to retain clusters of receptors over the time scale of minutes under suitable conditions and the cluster lifetime may depend on both receptor density and cluster size.ConclusionsThe ACGMC method is an ideal platform to resolve large length and time scales in heterogeneous biological systems well beyond the plasma membrane and the EGFR system studied here. Our results demonstrate that cluster size must be considered in conjunction with receptor density, as they synergistically affect EGFR cluster lifetime. Further, the cluster lifetime being of the order of several seconds suggests that any mechanisms responsible for EGFR aggregation must operate on shorter timescales (at most a fraction of a second), to overcome dissipation and produce stable clusters observed experimentally.

Microscopic origin of slow dynamics at the good glass forming composition range in Zr1−xCux metallic liquids

We have studied the structure and dynamics of a series of Zr1−xCux metallic liquids at temperatures of 1500 and 1300 K. We found that as the Cu composition increases, the Zr–Zr order undergoes considerable transition, which can be attributed to the different size of Zr and Cu atoms. The diffusivities of both Cu and Zr atoms become lower at the composition range corresponding to good glass forming region, i.e., 0.50≤x≤0.70. We found that the lower diffusivities for the intermediate composition range are correlated with the concentration of some specific clusters, including icosahedra, which can be screened out automatically and unbiasedly by using direct atomic dynamics analysis. Our analysis for the dynamic properties of high temperature liquids of ZrCu metallic alloys shows that the icosahedral clusters together with some other pentagon-rich Voronoi clusters are responsible for slowing dynamics. Cluster lifetime analysis indicates that the slow clusters mobility could be originated in their energy.

Mechanics of Molecular Bond Clusters between Elastic Media: Stochastic-Elastic Coupling in Cell-Matrix Adhesion

Focal adhesions are clusters of specific receptor-ligand bonds that link an animal cell to an extracellular matrix. A capability to control focal adhesions, for which a quantitative description of the collective behavior of multiple molecular bonds is a critical step, is essential for tissue and cellular engineering. While the behavior of single molecular bonds is governed by statistical mechanics at small scale, continuum mechanics should be valid at large scale. How can this transition be modeled and can this tell us something about the mechanics of cell adhesion? Here we develop a stochastic-elasticity model of a periodic array of adhesion clusters between two dissimilar elastic media subjected to an inclined tensile stress, in which stochastic descriptions of molecular bonds and elastic descriptions of interfacial traction are unified in a single modeling framework. A fundamental scaling law of interfacial traction distribution is established to govern the transition between uniform and cracklike singular distributions of the interfacial traction within molecular bonds. Guided by this scaling law, we perform Monte Carlo simulations to investigate the effects of cluster size, cell/matrix modulus and loading direction on lifetime and strength of the adhesion clusters. The results show that intermediate adhesion size, stiff substrate, cytoskeleton stiffening, and low-angle pulling are factors that contribute to the stability of focal adhesions. The predictions of our model provide feasible explanations for a wide range of experimental observations and suggest possible mechanisms by which cells can modulate adhesion and deadhesion via cytoskeletal contractile machinery.

Deducing the Milky Way's Massive Cluster Population

AbstractRecent near-infrared surveys of the galactic plane have been used to identify new massive cluster candidates. Follow up study indicates about half are not true, gravitationally-bound clusters. These false positives are created by high density fields of unassociated stars, often due to a sight-line of reduced extinction. What is not so easy to estimate is the number of false negatives, clusters which exist but are not currently being detected by our surveys. In order to derive critical characteristics of the Milky Way's massive cluster population, such as cluster mass function and cluster lifetimes, one must be able to estimate the characteristics of these false negatives. Our group has taken on the daunting task of attempting such an estimate by first creating the stellar cluster imaging simulation program, MASSCLEAN. I will present our preliminary models and methods for deriving the biases of current searches.

Lifetime and Strength of Periodic Bond Clusters between Elastic Media under Inclined Loading

Focal adhesions are clusters of specific receptor-ligand bonds that link an animal cell to an extracellular matrix. To understand the mechanical responses of focal adhesions, here we develop a stochastic-elasticity model of a periodic array of adhesion clusters between two dissimilar elastic media subjected to an inclined tensile stress, in which stochastic descriptions of molecular bonds and elastic descriptions of interfacial traction are unified in a single modeling framework. We first establish a fundamental scaling law of interfacial traction distribution and derive a stress concentration index that governs the transition between uniform and cracklike singular distributions of the interfacial traction within molecular bonds. Guided by this scaling law, we then perform Monte Carlo simulations to investigate the effects of cluster size, cell/extracellular matrix modulus, and loading direction on lifetime and strength of the adhesion clusters. The results show that intermediate adhesion size, stiff substrate, cytoskeleton stiffening, and low-angle pulling are factors that contribute to the stability of focal adhesions. The predictions of our model provide feasible explanations for a wide range of experimental observations and suggest possible mechanisms by which cells can modulate adhesion and deadhesion via cytoskeletal contractile machinery and sense mechanical properties of their surroundings.

Star cluster disruption

AbstractStar clusters are often used as tracers of major star-formation events in external galaxies as they can be studied out to much greater distances than individual stars. It is vital to understand their evolution if they are used to derive, for example, the star-formation history of their host galaxy. More specifically, we want to know how cluster lifetimes depend on their environment and on structural properties such as mass and radius. This review presents a theoretical overview of the early evolution of star clusters and the consequent long-term survival chances. It is suggested that clusters forming with initial densities of ≳104 M⊙ pc−3 survive the gas expulsion, or ‘infant mortality,’ phase. At ~10Myr, they are bound and have densities of ~103±1 M⊙ pc−3. After this time, they are stable against expansion through stellar evolution, encounters with giant molecular clouds and will most likely survive for another Hubble time if they are located in a moderate tidal field. Clusters with lower initial densities (≲100 M⊙ pc−3) will disperse into the field within a few 10s of Myrs. Some discussion is given on how extragalactic star cluster populations, and especially their age distributions, can be used to gain insight into disruption.

Energy-Efficient Dynamic Spatial Resolution Control for Wireless Sensor Clusters

In very large-scale dense sensor network applications, more sensor nodes may be deployed than are required to provide the initial desired spatial resolution. Such over-deployment can extend network life, improve robustness, and accommodate network dynamic. To enable large deployments, tiered and clustered network structures may be adopted for scalability and manageability. This article presents a highly scalable, distributed control method to manage the activity of sensors in each cluster so that dynamic application-specific spatial resolution requirement can be achieved with minimum energy cost and with uniform participation of sensors. The method, Look-Ahead Resolution Control (LARC), utilizes a look-ahead prediction of activities of sensors in the cluster and provides feedback to nodes as part of acknowledgments to transmissions. LARC is shown to be highly responsive to system dynamics such as changes in the resolution requested, node failures or replenishment. Existing control methods not only fall short in network life and scalability, but also do not provide such responsiveness and uniform participation to ensure a diverse representation of sensed data. LARC is extended to tradeoff between energy and delay in recovery from failures of large number of nodes, by multiplexing sleeping, listening, and transmissions probabilistically in such a way that the control overhead is minimized while the delay is bounded. The article presents the control strategy for LARC along with performance studies showing near-uniform participation and near-optimal cluster lifetime.

Multiphoton dissociation of CF3I clusters by IR laser radiation

The results of investigation of dissociation of (CF3I) n clusters upon resonant excitation of vibrations of CF3I molecules by pulsed CO2 laser radiation are reported. The kinetics of dissociation of such clusters is studied and the dissociation yield is measured. It is shown that its value is completely controlled by fluence ΦIR of transmitted IR radiation energy and its time dependence is exponential. The results of measurements show that the dissociation lifetime of clusters is shorter than 10−8 s. The velocity and translational temperature of free CF3I molecules formed during dissociation of clusters are measured, as well as the dependence of these parameters on ΦIR. It is concluded that, under these conditions of IR excitations, the dissociation of (CF3I) n clusters can be treated as consecutive evaporation of molecules.

Topology Stability Analysis and Its Application in Hierarchical Mobile Ad Hoc Networks

The hierarchical architecture has been proven effective for solving the scalability problems in large-scale ad hoc networks. The stability of the hierarchical architecture is a key factor in determining the network performance. Although many solutions have been proposed to construct stable clusters, the maximum stability achievable in mobile environments is still unknown. In this paper, we define three metrics for measuring network stability: (1) the cluster lifetime;(2) the intercluster link lifetime; and (3) the end-to-end path lifetime. We model and analyze the maximum of these lifetimes under the constraint of random node mobility. Analytical results provide the fundamental understanding of the bounds on network stability. Inspired by this understanding, we propose a clustering algorithm and a hierarchical routing protocol that work together to achieve the maximum network stability. The analytical results are verified by simulations.

Biradicalic excited states of zwitterionic phenol-ammonia clusters

Phenol-ammonia clusters with more than five ammonia molecules are proton transferred species in the ground state. In the present work, the excited states of these zwitterionic clusters have been studied experimentally with two-color pump probe methods on the nanosecond time scale and by ab initio electronic-structure calculations. The experiments reveal the existence of a long-lived excited electronic state with a lifetime in the 50-100 ns range, much longer than the excited state lifetime of bare phenol and small clusters of phenol with ammonia. The ab initio calculations indicate that this long-lived excited state corresponds to a biradicalic system, consisting of a phenoxy radical that is hydrogen bonded to a hydrogenated ammonia cluster. The biradical is formed from the locally excited state of the phenolate anion via an electron transfer process, which neutralizes the charge separation of the ground state zwitterion.

Kinetic properties of small He–vacancy clusters in iron

The paper deals with the lattice kinetic Monte-Carlo simulation of the mobility of small vacancy and vacancy–helium clusters in iron in the temperature range of 200–500 C. The input parameters characterizing vacancy and helium binding and mobility were taken from recently published ab initio calculations. The temperature dependence of diffusion coefficients and lifetimes of small clusters have been numerically estimated.

Abundance Profiles in Cooling‐Core Clusters: a Fossil Record of Past AGN‐driven Convection?

Central peaks in the iron abundance of intracluster plasma are a common feature of cooling-core galaxy clusters. Although they are centrally localized, these abundance peaks have a broader profile than the stars of the brightest cluster galaxy (BCG) that produce the excess iron, indicating that metal-enriched plasma is transported out of the BCG by some process such as turbulent diffusion. The AGN-driven convection model of Chandran & Rasera predicts the turbulent velocity profile in a steady state cluster in which radiative cooling is balanced by heating from AGN-driven convection and thermal conduction. We use the velocity profiles from this model as input into an advection/diffusion model for the transport of metals in the intracluster medium. We compare the results of our model to XMM-Newton and Chandra observations of eight clusters. Assuming a constant turbulence level over a cluster's lifetime equal to the present value, the turbulent velocities in the model can explain the observed abundance profiles in only five of the eight clusters. However, we go on to develop an analytic fit of the turbulent velocity profile as a function of the AGN power. We then deduce for each cluster the average AGN power (during the past ~10 Gyr) required to match the abundance profiles. The required average values are between 1043 and 2 × 1044 erg s−1, while the present AGN powers span a much larger range. Our results suggest that AGN-driven convection can account for the observed abundance profiles if the AGN power varies in time with average values in the above-quoted range.

The Calculation of Positron Lifetime in Ni<sub>3</sub>Al Alloy

Superposed-neutral-atom model and the finite-difference method (SNA-FD) are used to calculate the positron lifetime of different sizes and types of vacancy clusters in Ni3Al alloy. The calculated positron bulk lifetime in Ni3Al agrees well with the experimental results mentioned in literatures. The positron lifetimes of vacancy clusters with the same amount of vacancy but different components are also calculated. The results show that the positron lifetime is different when its components are different. The positron lifetimes of vacancy clusters with a single element but different sizes are also calculated. The result shows that the positron lifetime is almost a constant when its component is only Al vacancy, but the positron lifetime will increase at first and trend to be a constant when its component is only Ni vacancy.

A distributed group mobility adaptive clustering algorithm for mobile ad hoc networks

This paper proposes a distributed group mobility adaptive (DGMA) clustering algorithm for mobile ad hoc networks (MANETs) on the basis of a revised group mobility metric, linear distance based spatial dependency ( LDSD), which is derived from the linear distance of a node’s movement instead of its instantaneous speed and direction. In particular, it is suitable for group mobility pattern where group partitions and mergence are prevalent behaviors of mobile groups. The proposed clustering scheme aims to form more stable clusters by prolonging cluster lifetime and reducing the clustering iterations even in highly dynamic environment. Simulation results show that the performance of the proposed framework is superior to two widely referenced clustering approaches, the Lowest-ID clustering scheme and the mobility based clustering algorithm MOBIC, in terms of average clusterhead lifetime, average resident time, average number of clusterhead changes, and average number of cluster reaffiliations.

Shaping the Globular Cluster Mass Function by Stellar‐Dynamical Evaporation

We show that the globular cluster mass function (GCMF) in the Milky Way depends on cluster half-mass density (rho_h) in the sense that the turnover mass M_TO increases with rho_h while the width of the GCMF decreases. We argue that this is the expected signature of the slow erosion of a mass function that initially rose towards low masses, predominantly through cluster evaporation driven by internal two-body relaxation. We find excellent agreement between the observed GCMF -- including its dependence on internal density rho_h, central concentration c, and Galactocentric distance r_gc -- and a simple model in which the relaxation-driven mass-loss rates of clusters are approximated by -dM/dt = mu_ev ~ rho_h^{1/2}. In particular, we recover the well-known insensitivity of M_TO to r_gc. This feature does not derive from a literal ``universality'' of the GCMF turnover mass, but rather from a significant variation of M_TO with rho_h -- the expected outcome of relaxation-driven cluster disruption -- plus significant scatter in rho_h as a function of r_gc. Our conclusions are the same if the evaporation rates are assumed to depend instead on the mean volume or surface densities of clusters inside their tidal radii, as mu_ev ~ rho_t^{1/2} or mu_ev ~ Sigma_t^{3/4} -- alternative prescriptions that are physically motivated but involve cluster properties (rho_t and Sigma_t) that are not as well defined or as readily observable as rho_h. In all cases, the normalization of mu_ev required to fit the GCMF implies cluster lifetimes that are within the range of standard values (although falling towards the low end of this range). Our analysis does not depend on any assumptions or information about velocity anisotropy in the globular cluster system.

Nucleation of NaCl Nanoparticles in Supercritical Water: Molecular Dynamics Simulations

Formation of NaCl nanoparticles in supercritical water is studied using molecular dynamics simulation method. We have simulated particle nucleation and growth in NaCl-H2O fluids, with salt concentration of 5.1 wt %, in the temperature and density range of 673-1073 K and 0.17-0.34 g/cm(3), respectively. The cluster size distributions, the size of critical nuclei and cluster lifetimes are reported. The size distribution of emerging clusters shows a very strong dependence on the system's density, with larger clusters forming at lower densities. Clusters consisting of approximately 14-24 ions appear critical for the thermodynamic states examined. The local structures of critical clusters are found to be amorphous. The lifetime values for clusters containing more than 20 ions are in the range of 10-50 ps. We have calculated the NaCl nucleation rates, which appear to be on the order of 10(28) cm(-3) s(-1).

DEEP ADVANCED CAMERA FOR SURVEYS IMAGING IN THE GLOBULAR CLUSTER NGC 6397: DYNAMICAL MODELS

We present N-body models to complement deep imaging of the metal-poor core-collapsed cluster NGC 6397 obtained with the Hubble Space Telescope. All simulations include stellar and binary evolution in step with the stellar dynamics and account for the tidal field of the Galaxy. We focus on the results of a simulation that began with 100, 000 objects (stars and binaries), 5% primordial binaries, and Population II metallicity. After 16 Gyr of evolution, the model cluster has about 20% of the stars remaining and has reached core collapse. We compare the color-magnitude diagrams of the model at this age for the central region and an outer region corresponding to the observed field of NGC 6397 (about 2-3 half-light radii from the cluster center). This demonstrates that the white dwarf (WD) population in the outer region has suffered little modification from dynamical processes?contamination of the luminosity function by binaries and WDs with non-standard evolution histories is minimal and should not significantly affect measurement of the cluster age. We also show that the binary fraction of main-sequence stars observed in the NGC 6397 field can be taken as representative of the primordial binary fraction of the cluster. For the mass function (MF) of the main-sequence stars, we find that although this has been altered significantly by dynamics over the cluster lifetime, especially in the central and outer regions, the position of the observed field is close to optimal for recovering the initial MF of the cluster stars (below the current turn-off mass). More generally we look at how the MF changes with radius in a dynamically evolved stellar cluster and suggest where the best radial position to observe the initial MF is for clusters of any age. We discuss computational constraints that restrict the N-body method to non-direct models of globular clusters currently, how this affects the interpretation of our results regarding NGC 6397, and future plans for models with increased realism.

Lifetime and Strength of Adhesive Molecular Bond Clusters between Elastic Media

With a long-term objective toward a quantitative understanding of cell adhesion, we consider an idealized theoretical model of a cluster of molecular bonds between two dissimilar elastic media subjected to an applied tensile load. In this model, the distribution of interfacial traction is assumed to obey classical elastic equations whereas the rupture and rebinding of individual molecular bonds are governed by stochastic equations. Monte Carlo simulations that combine the elastic and stochastic equations are conducted to investigate the lifetime of the bond cluster as a function of the applied load. We show that the interfacial traction is generally nonuniform and for a given adhesion size the average cluster lifetime asymptotically approaches infinity as the applied load is reduced to below a critical value, defined as the strength of the cluster. The effects of elastic moduli, adhesion size, and rebinding rate on the cluster lifetime and strength are studied under strongly nonuniform distributions of interfacial traction. Although overly simplified in a number of aspects, our model seems to give predictions that are consistent with relevant experimental observations on focal adhesion dynamics.

Measurement of Large-Scale Cluster Power Characteristics for Geometric Channel Models

Directional urban radio channel measurements are analyzed in this paper. The mean powers, the shadow fading, and the lifetimes of individual propagation clusters extracted from the measured data are studied. The cluster shadow fading is an input parameter of several directional channel models, but measured values for the fading have not been published previously. Also, measured numbers of strong, significant propagation clusters are essential for the channel models.