Cluster Formation Process Research Articles

CB‐MAC: a novel cluster‐based MAC protocol for VANETs

Due to relative mobility, topology changes rapidly with frequent link breakage in vehicular ad hoc networks (VANETs). Clustering VANETs into small groups limits channel contention and controls the network topology efficiently. In this study, a novel cluster-based medium access control (CB-MAC) protocol is proposed for VANETs. The cluster formation process is defined. Moreover, cluster head election and cluster merging processes are described for efficient communication in the cluster as well as out of the cluster. The mechanism defined in IEEE 802.11 standard is specially designed for only direct communications and is not suitable for cluster-based communications. Therefore, new control packets are introduced and the existing control packet format is modified to support cluster-based communications. For effective MAC protocol design, the request to send (RTS)/clear to send (CTS) mechanism is not used for safety messages which are of broadcast nature. On the other hand, the RTS/CTS mechanism is used for non-safety data delivery to eliminate hidden node problem. Markov chain model-based analytical model is presented to explore the performance of the proposed CB-MAC protocol. The proposed protocol is validated by numerical studies. The numerical results exhibit that the proposed CB-MAC protocol improves system performance and satisfies the delay constraint of 100 ms for safety messages.

Size-dependent variability in the formation and trade-offs of facultative aggregations in golden orb-web spiders (Nephila clavipes)

The vast majority (> 99.99%) of spiders are solitary, but in some species, individuals form stable groups or, alternatively, looser aggregations that fluctuate over space and time. While previous work has documented trade-offs associated with being part of intraspecific groups, rather than being solitary, we know less about why grouping behavior would be facultative. Residing in a group can reduce predation risk, but also lead to competition for food, and we hypothesized that variation in these trade-offs could provide insights into the optional nature of clustering. We used a Costa Rican population of golden orb-web spiders, Nephila clavipes, to ask two questions. First, we tested whether a spider’s size influenced the trade-offs of clustering, and second, whether inter-annual variation in predator and prey abundance could help explain clustering patterns. We also examined the process of cluster formation and dissolution in relation to spider size. Data collected on over 1500 spiders over 4 years indicated that in each year, small spiders experienced the greatest reduction in predation if clustered and also were most likely to initiate aggregations. Overall rates of predation varied significantly among years, and small spiders were most likely to cluster in the year with highest predation. We also detected a cost of clustering in the form of reduced prey capture, but only in years with relatively high overall prey acquisition. Together, these findings suggest that trade-offs that vary individually and over time could influence a spider’s decision to cluster and, hence, explain the facultative nature of aggregating in this population. Animals within a population can show variability in a range of behaviors, such as foraging and predator avoidance, and at times, the ideal way of behaving depends on individual or environmental conditions. In this study, we asked why some female golden orb-web spiders (Nephila clavipes) form groups, while in the same population, others are solitary. This is an interesting question given how rare aggregations are among spiders. In our study, small spiders benefited most from clustering in terms of higher survival when aggregated compared to when solitary. At the same time, spiders of all sizes captured less prey when aggregated, although this cost was only detected in years with relatively abundant prey. Together, these findings suggest that trade-offs that vary individually and over time could influence a spider’s decision to cluster and, hence, explain the optional nature of aggregating in this population.

Low-mass star formation and subclustering in the H II regions RCW 32, 33, and 27 of the Vela Molecular Ridge

Context. Most stars are born in clusters, and recent results suggest that star formation (SF) preferentially occurs in subclusters. Studying the morphology and SF history of young clusters is crucial for understanding early cluster formation processes. Aims. We aim to identify the embedded population of young stellar objects (YSOs) down to the low-mass stars in the M-type regime in the three H II regions RCW 33, RCW 32, and RCW 27, which are located in the northwestern region of the Vela Molecular Ridge. Our aim is to characterize their properties, such as morphology and extent of the clusters in the three H II regions, derive stellar ages, and determine the connection of the SF history with the environment. Methods. Through public photometric surveys such as Gaia, VPHAS+, 2MASS, and Spitzer/GLIMPSE, we identify YSOs with classical techniques aimed at detecting IR, Hα, and UV excesses as signatures of circumstellar disks and accretion. In addition, we implement a method for distinguishing main-sequence (MS) stars and giants in the M-type regime by comparing the reddening derived in several optical/IR color-color diagrams, assuming suitable theoretical models. Since this diagnostic is sensitive to stellar gravity, the procedure allows us to also identify pre-MS (PMS) stars. Results. Using the classical membership criteria, we find that a large population of YSOs shows signatures of circumstellar disks with or without accretion. In addition, with the new technique of M-type star selection, we find a rich population of young M-type stars whose spatial distribution strongly correlates with the more massive population. We find evidence of three young clusters, with different morphology, for which we estimate the individual distances using TGAS Gaia data of the brighter subsample. In addition, we identify field stars falling in the same region by securely classifying them as giants and foreground MS stars. Conclusions. We identify the embedded population of YSOs down to about 0.1 M⊙ that is associated with the three H II regions RCW 33, RCW 32, and RCW 27 and the three clusters Vela T2, Cr 197, and Vela T1, respectively. All the three clusters are located at a similar distance, but they have very different morphologies. Our results suggest a decreasing SF rate in Vela T2 and triggered SF in Cr 197 and Vela T1.

Теоретичні особливості та складники розвитку регіонального ринку праці

Мета роботи – визначити теоретичні особливості та обґрунтувати складники розвитку регіонального ринку праці.
 Дизайн/Метод/Підхід дослідження. Застосовано системний підхід при обґрунтуванні концептуальної схеми розвитку регіонального ринку праці та метод логічного узагальнення – при обґрунтуванні чинників розвитку регіонального ринку праці відповідно до очікувань його суб’єктів. Інформаційна основа дослідження – монографічні роботи та наукові публікації за його тематикою.
 Результати дослідження. Обґрунтовано доцільність запровадження заходів щодо розвитку ринку праці в Україні на її сучасному етапі соціально-економічного та суспільно-політичного розвитку. Визначено основних суб’єктів ринку праці, до яких було віднесено найманих працівників, роботодавців та органи влади. Узагальнено чинники розвитку ринку праці на основі диференціації цілей суб’єктів ринку праці. Обґрунтовано характерні риси розвитку ринку праці. Створено концептуальну схему розвитку регіонального ринку праці, що враховує очікування та вимоги суб’єктів, бар’єри та проблеми на шляху розвитку, напрями задоволення очікувань і подолання бар’єрів й індикатори розвитку ринку праці.
 Практичне значення дослідження. Результати дослідження можуть стати основою для формування політики активізації процесів кластероутворення з метою досягнення тенденцій розвитку регіонального ринку праці та прискореного задоволення очікувань його суб’єктів, що забезпечить синергетичний ефект.
 Оригінальність/Цінність/Наукова новизна дослідження. Запропоновано визначення сутності поняття «розвиток регіонального ринку праці», у якому, на відміну від існуючих трактувань, зосереджено увагу на обґрунтуванні напрямів задоволення очікувань та вимог суб’єктів ринку праці шляхом подолання існуючих й прогнозованих бар’єрів і проблем задля досягнення індикаторів розвитку ринку праці.
 Обмеження дослідження/Перспективи подальших досліджень. Результати дослідження мають бути покладеними в основу діагностики регіональних ринків праці України за основними чинниками їх розвитку.
 Тип статті – теоретична.

Spectrum aware clustering in cognitive radio ad hoc networks

Cognitive radio (CR) is an emerging technology developed for efficient utilization of the radio spectrum. CRN utilizes CR technology and enables the unlicensed users also referred as secondary users (SUs) to access free portions of the licensed spectrum in an opportunistic manner. To support scalability and stability in distributed CRNs also referred as cognitive radio ad hoc networks (CRAHNs), SUs are often organized into smaller groups known as clusters. Spectrum aware clustering is considered as the key technique to overcome numerous is-sues associated with the dynamic nature of CRAHNs. This article focuses on clustering in CRAHNs and presents a comprehensive review of various spectrum aware clustering algorithms presented in the literature. The article highlights notable clustering metrics and includes the description of cluster formation and maintenance process. The article also renders potential research gaps in existing research works and discusses open challenges and issues that need to be addressed for efficient clustering in CRAHNs.

Formation of Small Clusters in the Free Expanding Water Vapor Plume

The results of calculations of water vapor outflow from a source, in which the pressure and temperature are maintained constant, into a vacuum through an orifice in the infinitely thin wall are presented. The calculations are carried out using the direct simulation Monte Carlo method when the processes of water cluster formation/decay in the flow field are taken into account. The vapor flow regimes transient with respect to the Knudsen number are considered. The effect of the cluster formation processes on the parameters of flow which becomes significant for the mole cluster fraction above 4%is analyzed. The effect of “freezing” the mole cluster concentrations with increase in the distance from the source is demonstrated. The influence of the probabilities of monomer and dimer association during pair collisions, used in the model, on themole dimer fraction is investigated. The results obtained are compared with the available experimental data.

Energy Conservation Using RR Algorithm in Dynamic Cluster Based WSN

The energy consumption during the cluster head selection phase in a hierarchal and dynamic cluster based wireless sensor network had been considered insignificant in the previous research works. However, we have shown in our previous works that around 25% network energy is exhausted only in repeated cluster formation process of the network. We have proposed a round rotation (RR) protocol, which can be used in any dynamic cluster based network that can substantially conserve the overhead energy (the energy consumed in the random cluster head setup phase) of the network. It was shown that 20% of the total network energy consumed in data transmission can be conserved by minimizing the transmission of unnecessary control messages in the cluster setup phase. This paper also compares the overhead energy consumed in our proposed RR mechanism with three different mechanisms, (1) generic dynamic cluster based protocols, (2) centrally controlled dynamic cluster based protocols and (3) (re-clustering avoidance RCA) protocols. Results show that network lifetime can be improved by 25% using our proposed RR technique. The optimal value of cluster heads has also been calculated considering the network energy consumed in the different protocols.

Structural and Dielectric Studies on Na2O-PbO-SiO2 Glasses

Structural studies on x Na2O.(50-x)PbO.50SiO2 glasses have been carried out by different spectroscopic techniques. FTIR, EDX, NMR and dielectric spectroscopies are applied to follow the change in glass structure in terms of bridging oxygen (BO), non-bridging oxygen (NBO), and cluster species upon adding of Na2O at expense of PbO. NBOs are formed in the silicate network upon Na2O addition, since Na2O plays the role of glass modifier. Changes in relative area of Q2 (obtained from IR analysis) and NMR chemical shift of silicon nuclei with increasing Na2O are indicative for formation of the less shielded silicate units. While in sodium rich silicate glasses, the structural role of Na2O is changed due to changing Na coordination. FTIR analysis could be used to elucidate the changes related to changing the role of Na2O. The data obtained are correlated with that obtained from EDX spectroscopy. Moreover, the role of Na2O on the process of cluster formation when it substitutes PbO is determined. NBOs only are formed in the silicate network upon Na2O addition up to 30 mol%. On the other hand, in sodium rich silicate glasses an additional type of oxygen (free oxygen O2−) is present. The free oxygen is required for sodium to form aggregated cluster, specially at high concentration of Na2O (50 mol%). An increase in Na2O concentration in Na2O-rich silicate network results in increasing Na coordination instead of breaking more silicon–oxygen bonds. Scanning electron micrographs (SEM) and EDS spectroscopy in correlation with FTIR results confirm the presence of Na cluster species. The effect of clusters formation on AC conductivity was discussed according to jump relaxation model. Some parameters related to AC conductivity are found to be affected by the presence of cluster species in the glass network.

A tale of two clusters: knowledge and emergence

Cluster emergence is an important topic but weakly conceptualized in the literature. Focusing on the interaction of the local knowledge pool and firm growth, the paper develops a comprehensive framework to understand cluster emergence. In the framework, the cluster formation process starts with the collision of local and external knowledge which generates an innovation and stimulates the creation of local pioneering firms in a new field. To support the growth of follow-up entrants in the new industry, the local knowledge pool needs to be expanded and deepened through local knowledge sharing and external knowledge inflows. The enlarged local knowledge pool enables local firms to grow and explore other fields further. To promote cluster emergence, public policies need to facilitate the interaction of the local knowledge pool and firm growth. The paper illustrates the interactive framework with two aluminum extrusion clusters in China that emerged in different ways over different time periods.

Robust clustering for ad hoc cognitive radio network

AbstractCluster structure in cognitive radio (CR) networks facilitates cooperative spectrum sensing, routing, and other functionalities. Unlicensed channels, which are temporally available for a group of CR users in one area, consolidate the group into a cluster. More available unlicensed channels in a cluster make the cluster more likely to uphold against the licensed users' influence, making clusters more robust. This paper analyzes the problem of how to form robust clusters in a CR network such that CR systems benefit from collaboration within clusters despite intense primary user activity. We give a formal description of the robust clustering problem, prove it to be NP‐hard, and propose both centralized and distributed solutions. The congestion game model is adopted to analyze the process of cluster formation, which not only contributes to the design of the distributed clustering scheme but also provides a guarantee on the convergence to a Nash equilibrium and the convergence speed. The proposed distributed clustering scheme outperforms state‐of‐the‐art in terms of cluster robustness, convergence speed, and overhead. Extensive simulations are presented supporting the theoretical claims.

Methods to Identify Star Clusters in The Large Magellanic Cloud (LMC)

The Large Magellanic Cloud (LMC) hosts a few thousand star clusters which are ideal tools to study stellar evolution, star formation history, cluster formation and dissolution processes in the galaxy. Although many surveys (like IRSF, OGLE II etc) of the LMC have been carried out, a large fraction of clusters, mainly poor ones are yet to be identified. Also, the parameters of already identified clusters are not well studied. In this context, we have tried to explore that among the available and upcoming surveys which survey (optical/NIR) can be used to efficiently detect and study the clusters in the LMC. We have found that the available OGLE-III optical data is ideal for this purpose, but only for young clusters, whereas Deeper optical data from DECAM survey, OGLE-IV and skymapper are ideal to study poor and old clusters. We have also found that one can combine the ongoing VISTA data with upcoming optical data (OGLE IV) and estimate the cluster parameters more accurately.

Social-Energy-Aware User Clustering for Content Sharing Based on D2D Multicast Communications

With the ever-increasing demands for content sharing, device-to-device (D2D) multicast content sharing is becoming a promising technology to improve the quality of local area services. In order to guarantee the implementation of D2D multicast content sharing, the main concern is the user clustering, i.e., cluster head (CH) selection and cluster formation. Most of the existing works fail to design a distributed scheme with a concern of the incentive to stimulate cooperation. In this paper, we model a novel user clustering problem with the target of maximizing the energy efficiency of the D2D multicast network, where both the social tie information and the pricing scheme are adopted to stimulate cooperation. Due to its NP-hard property, it is decomposed into two subproblems. Specifically, a CH selection algorithm based on social maximum weight is first proposed to discriminate the proper CHs from multitudinous candidates and restrict the upper bound of the number of the selected CHs. After that, we model the cluster formation process as a non-transferable utility coalition formation game, and a distributed coalition formation algorithm for cluster formation is proposed based on preference relationship and switch operations. Importantly, the final coalition structure is proved to be Nash stable. Numerical results show that our proposed scheme outperforms other three baseline schemes.

Further insights into the metal ion binding abilities and the metalation pathway of a plant metallothionein from Musa acuminata.

The superfamily of metallothioneins (MTs) combines a diverse group of metalloproteins, sharing the characteristics of rather low molecular weight and high cysteine content. The latter provides MTs with the capability to coordinate thiophilic metal ions, in particular those with a d 10 electron configuration. The sub-family of plant MT3 proteins is only poorly characterized and there is a complete lack of three-dimensional structure information. Building upon our previous results on the Musa acuminata MT3 (musMT3) protein, the focus of the present work is to understand the metal cluster formation process, the role of the single histidine residue present in musMT3, and the metal ion binding affinity. We concentrate our efforts on the coordination of ZnII and CdII ions, using CoII as a spectroscopic probe for ZnII binding. The overall protein-fold is analysed with a combination of limited proteolytic digestion, mass spectrometry, and dynamic light scattering. Histidine coordination of metal ions is probed with extended X-ray absorption fine structure spectroscopy and CoII titration experiments. Initial experiments with isothermal titration calorimetry provide insights into the thermodynamics of metal ion binding.

Simulation of defect formation, amorphization and cluster formation processes in nc-TiN/a-Si3N4 nanocomposite under Xe irradiation

The research of defect formation and clusterization processes by means of a molecular dynamics method both in nc-TiN nanocrystals and amorphous a-Si3N4 matrix, as the constituents of nc-TiN/a-Si3N4 nanocomposite, under exposure to Xe implantation was the aim of the present study. Dependences of the clustered Xe atoms fraction on their concentration and temperature of post-irradiation annealing were analyzed. At defect formation process in nc-TiN nanocrystals, there is a size effect consisting in intensification of the radiation point defects formation with the reduction of nc-TiN nanocrystals size and concurrent predominant formation of the dangling Si- and N-bonds in a-Si3N4 matrix. Accumulation of these defects at the irradiation leads to amorphization of nc-TiN nanocrystals with the size less than 8 nm and to formation of the nanopores in a-Si3N4 matrix. The important role of the radiation defects subsystem in transport processes of implanted Xe both in TiN close-packed lattice as well as in a-Si3N4 amorphous matrix is shown. There is a much higher extent of intensity of xenon atoms clusterization processes in the amorphous matrix. The results of the simulation are compared to existing experimental data.

Three candidate double clusters in the LMC: truth or dare?

The Large Magellanic Cloud (LMC) hosts a large number of candidate stellar cluster pairs. Binary stellar clusters provide important clues about cluster formation processes and the evolutionary history of the host galaxy. However, to properly extract and interpret this information, it is crucial to fully constrain the fraction of real binary systems and their physical properties. Here we present a detailed photometric analysis based on ESO-FORS2 images of three candidate cluster multiplets in the LMC, namely SL349-SL353, SL385-SL387-NGC1922 and NGC1836-BRHT4b-NGC1839. For each cluster we derived ages, structural parameters and morphological properties. We have also estimated the degree of filling of their Roche lobe, as an approximate tool to measure the strength of the tidal perturbations induced by the LMC. We find that the members of the possible pairs SL349-SL353 and BRHT4b-NGC1839 have a similar age ($t = 1.00 \pm 0.12$ Gyr and $t = 140 \pm 15$ Myr, respectively), thus possibly hinting to a common origin of their member systems We also find that all candidate pairs in our sample show evidence of intra-cluster overdensities that can be a possible indication of real binarity. Particularly interesting is the case of SL349-SL353. In fact, SL353 is relatively close to the condition of critical filling, thus suggesting that these systems might actually constitute an energetically bound pair. It is therefore key to pursue a detailed kinematic screening of such clusters, without which, at present, we do not dare making a conclusive statement about the true nature of this putative pair.

A new clustering approach in wireless sensor networks using fuzzy system

In recent years, wireless sensor networks (WSNs) have attracted many researchers due to their widely usage in a wide range of applications. One of the most important problems in these networks is energy consumption that has a direct effect on network lifetime. Clustering is one of the most important solutions in order to overcome the problem. Energy resource limitation is a fundamental problem in WSNs and clustering protocols provide suitable procedures in order to enhance network lifetime. However, they impose high energy consumption on cluster heads (CH), and therefore, in each round, the protocol should reform clusters and change CH in order to enhance network lifetime. Although these protocols are proper for clustering, do not guarantee suitable CH selection. In this paper, a novel energy-efficient method is proposed using fuzzy logic and three parameters including the amount of energy in CH, distance from CH to base station, and the number of connections in CH. In fact, we focus on the cluster formation process. The proposed model is compared to the well-known low-energy adaptive clustering hierarchy protocol. Simulation results demonstrate that the proposed protocol improves network lifetime.

Fuzzy Logic Based Clustering Algorithm for Wireless Sensor Networks

WSNs have many applications in modern life. Thus, optimization of the network operation is required to maximize its lifetime. The energy is a major issue in order to increase the lifetime of WSNs. The clustering algorithm is one of the proposed algorithms to enhance the lifetime of WSNs. The operation of the clustering algorithm is divided into cluster heads (CHs) selection and cluster formation. However, most of the previous works have focused on CHs selection, and have not considered the cluster formation process, which is the important issue in clustering algorithm based routing schemes, and it can drastically affect the lifetime of WSNs. In this paper, a Fuzzy Logic based Clustering Algorithm for WSN (CAFL) has been proposed to improve the lifetime of WSNs. This approach uses fuzzy logic for CHs selection and clusters formation processes by using residual energy and closeness to the sink as fuzzy inputs in terms of CH selection, and residual energy of CH and closeness to CHs as fuzzy inputs in terms of clusters formation. Simulation results justify its efficiency.

An optimal clustering mechanism based on Fuzzy-C means for wireless sensor networks

In order to balance the node’s energy consumption and extend the lifetime under energy-constrained wireless sensor networks, an energy-efficient clustering algorithm based on Fuzzy-C means for wireless sensor networks is proposed. Taking into account the uneven distribution of the sensor nodes and the uncertainty of the radio channel, the cluster formation process of nodes is modeled as a fuzzy partition of sample space in this paper. Firstly, the overall energy consumption of the networks is analyzed, and the optimal number of cluster heads is estimated based on node’s density. Secondly, in the design of the objective function, the distance from the node to the cluster head and the weight of the membership values are considered. Then, the improved Fuzzy-C means clustering algorithm is proposed to divide the sensor nodes into a specified number of clusters. Finally, a single hop communication mode is used for intra cluster communication, and inter cluster communication adopts a multi-hop communication mode. The simulation results show that the proposed algorithm can obtain uniform spatial distribution of cluster heads and balance the energy consumption of network effectively.

Stabilization of high-temperature disorder of fluorine sublattice by quenching in calcium fluoride crystals

The superionic state of crystals with the fluorite structure caused by disordering of anion sublattice is accompanied by the formation of short-lived clusters due to the correlation between anti-Frenkel defects at the increase of their concentration. The existence of clusters comprising anion vacancies as well as interstitial and displaced anions at high temperatures was deduced from the analysis of neutron scattering. The quenched CaF2 crystals show intense ultraviolet absorption with a long-wavelength tail that can be ascribed to the clusters thus testifying the opportunity of a high-temperature disorder freezing. It was found that the fluorine vacancies introduced into the crystal during preheating in the reduction atmosphere of calcium vapor trigger and intensify the process of cluster formation. The model that takes into account a strong electron-vibration coupling gives rise to the reconstruction of both the valence and conduction bands in the cluster region thereby clarifying the nature of absorption spectra of quenched crystals.

Effect of Ni and vacancy concentration on initial formation of Cu precipitate in Fe–Cu–Ni ternary alloys by molecular dynamics simulation**Project supported by the National Natural Science Foundation of China (Grant No. 51301102) and the Science and Technology Commission of Shanghai Municipality, China (Grant No. 15ZR1416000).

In the present work, the effects of Ni atoms and vacancy concentrations (0.1%, 0.5%, 1.0%) on the formation process of Cu solute clusters are investigated for Fe–1.24%Cu–0.62%Ni alloys by molecular dynamics (MD) simulations. The presence of Ni is beneficial to the nucleation of Cu precipitates and has little effect on coarsening rate in the later stage of aging. This result is caused by reducing the diffusion coefficient of Cu clusters and the dynamic migration of Ni atoms. Additionally, there are little effects of Ni on Cu precipitates as the vacancy concentration reaches up to 1.0%, thereby explaining the embrittlement for reactor pressure vessel (RPV) steel. As a result, the findings can hopefully provide the important information about the essential mechanism of Cu cluster formation and a better understanding of ageing phenomenon of RPV steel. Furthermore, these original results are analyzed with a simple model of Cu diffusion, which suggests that the same behavior could be observed in Cu-containing alloys.