Space-time Characteristics and Experimental Analysis of Broadening First-order Sea Clutter in HF Hybrid Sky-surface Wave Radar

In high frequency (HF) hybrid sky-surface wave radar, the first-order sea clutter broadening is very complex and serious under the influence of ionosphere and bistatic angle, which affects the detection of ship target. This paper analyzes the space-time characteristics based on the HF sky-surface wave experimental system. We first introduce the basic structure, working principle and position principle based on our experimental system. Also analyzed is the influence of ionosphere and bistatic angle on the space-time coupling characteristics of broadening first-order sea clutter and the performance of space-time adaptive processing (STAP). Finally, the results of theoretic analysis are examined with the experimental data. Simulation results show that the results of experiment consist with that of theoretic analysis

Evolution of Anisotropy in Granular Materials: Effect of Particle Rolling and Particle Crushing

The effect of particle rolling and crushing on the evolutions of the two types of anisotropy, i.e., anisotropy of particle packing (microstructure) and anisotropy of force chains, is investigated numerically using the discrete element method. To this end, the classical fabric tensor is adopted to describe the anisotropy of microstructure, while two similar orientation tensors defined by the directions of contact forces are used to characterize the anisotropy of force chains. Numerical results show that the evolutions of anisotropy follows the same tendency as the stress–strain curve, and the anisotropy of force chains is more intense than that of the microstructure. In addition, particle rolling exerts different effect on anisotropy before and after the peak stress state, and particle crushing decreases the anisotropy of granular materials.Представлено чисельне дослідження за допомогою методу дискретних елементів впливу скочування і дроблення частинок на еволюцію анізотропій скочування частинок (мікроструктура) і силового ланцюжка. Для опису анізотропії мікроструктури використовується структурний класичний тензор, а два аналогічних тензора орієнтації, що характеризуються напрямком контактних зусиль, – для визначення анізотропії силового ланцюжка. Результати чисельного дослідження показали, що еволюція анізотропій має той же характер, що і залежність деформації від напруження, однак анізотропія силового ланцюжка є більш інтенсивною порівняно з анізотропією мікроструктури. Більш того, скочування частинок по-різному впливає на анізотропію до і після досягнення максимального значення напруження, в той час як дроблення частинок зменшує анізотропію гранульованих матеріалів.Представлено численное исследование с помощью метода дискретных элементов влияния скатывания и дробления частиц на эволюцию анизотропий скатывания частиц (микроструктура) и силовой цепочки. Для описания анизотропии микроструктуры используется структурный классический тензор, а два аналогичных тензора ориентации, характеризующихся направлением контактных усилий, – для определения анизотропии силовой цепочки. Результаты численного исследования показали, что эволюция анизотропий имеет тот же характер, что и зависимость деформации от напряжения, однако анизотропия силовой цепочки является более интенсивной по сравнению с анизотропией микроструктуры. Более того, скатывание частиц по-разному влияет на анизотропию до и после достижения максимального значения напряжения, тогда как дробление частиц уменьшает анизотропию гранулированных материалов

Approximating the minimum directed tree cover

Given a directed graph $G$ with non negative cost on the arcs, a directed tree cover of $G$ is a rooted directed tree such that either head or tail (or both of them) of every arc in $G$ is touched by $T$. The minimum directed tree cover problem (DTCP) is to find a directed tree cover of minimum cost. The problem is known to be $NP$-hard. In this paper, we show that the weighted Set Cover Problem (SCP) is a special case of DTCP. Hence, one can expect at best to approximate DTCP with the same ratio as for SCP. We show that this expectation can be satisfied in some way by designing a purely combinatorial approximation algorithm for the DTCP and proving that the approximation ratio of the algorithm is $\max\{2, \ln(D^+)\}$ with $D^+$ is the maximum outgoing degree of the nodes in $G$.Comment: 13 page

Hydraulic characteristics of smart reactor for a nominal condition

SMART (System-integrated Modular Advanced ReacTor) is an integral-type reactor being developed, which has major components including core, pumps, steam generators and a pressurizer inside the reactor vessel. In order to analyze the various safety features of the reactor, the quantification for the flow and pressure distributions are very important. A test facility, named “SCOP”, was designed based on the conservation of Euler number which is a ratio of pressure drop to dynamic pressure with a sufficiently high Reynolds number. In order to preserve the flow distribution characteristics, the SCOP is linearly reduced with a scaling ratio of 1/5. For the present work, a total of 9 tests were performed for a nominal SMART flow condition. By using the test results, a statistical final flow distribution for the SMART reactor were presented. The current data could be applied for the validation of a CFD analysis method as well as reactor safety and system performance analysesPaper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012

Topological Defects and Non-homogeneous Melting of Large 2D Coulomb Clusters

The configurational and melting properties of large two-dimensional clusters of charged classical particles interacting with each other via the Coulomb potential are investigated through the Monte Carlo simulation technique. The particles are confined by a harmonic potential. For a large number of particles in the cluster (N>150) the configuration is determined by two competing effects, namely in the center a hexagonal lattice is formed, which is the groundstate for an infinite 2D system, and the confinement which imposes its circular symmetry on the outer edge. As a result a hexagonal Wigner lattice is formed in the central area while at the border of the cluster the particles are arranged in rings. In the transition region defects appear as dislocations and disclinations at the six corners of the hexagonal-shaped inner domain. Many different arrangements and type of defects are possible as metastable configurations with a slightly higher energy. The particles motion is found to be strongly related to the topological structure. Our results clearly show that the melting of the clusters starts near the geometry induced defects, and that three different melting temperatures can be defined corresponding to the melting of different regions in the cluster.Comment: 7 pages, 11 figures, submitted to Phys. Rev.

Hydraulic characteristics of smart reactor for a nominal condition

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.SMART (System-integrated Modular Advanced ReacTor) is an integral-type reactor being developed, which has major components including core, pumps, steam generators and a pressurizer inside the reactor vessel. In order to analyze the various safety features of the reactor, the quantification for the flow and pressure distributions are very important. A test facility, named “SCOP”, was designed based on the conservation of Euler number which is a ratio of pressure drop to dynamic pressure with a sufficiently high Reynolds number. In order to preserve the flow distribution characteristics, the SCOP is linearly reduced with a scaling ratio of 1/5. For the present work, a total of 9 tests were performed for a nominal SMART flow condition. By using the test results, a statistical final flow distribution for the SMART reactor were presented. The current data could be applied for the validation of a CFD analysis method as well as reactor safety and system performance analyses.dc201

Understorey plant community and light availability in conifer plantations and natural hardwood forests in Taiwan

Questions: What are the effects of replacing mixed species natural forests with Cryptomeria japonica plantations on understorey plant functional and species diversity? What is the role of the understorey light environment in determining understorey diversity and community in the two types of forest? Location: Subtropical northeast Taiwan. Methods: We examined light environments using hemispherical photography, and diversity and composition of understorey plants of a 35‐yr C. japonica plantation and an adjacent natural hardwood forest. Results: Understorey plant species richness was similar in the two forests, but the communities were different; only 18 of the 91 recorded understorey plant species occurred in both forests. Relative abundance of plants among different functional groups differed between the two forests. Relative numbers of shade‐tolerant and shade‐intolerant seedling individuals were also different between the two forest types with only one shade‐intolerant seedling in the plantation compared to 23 seedlings belonging to two species in the natural forest. In the natural forest 11 species of tree seedling were found, while in the plantation only five were found, and the seedling density was only one third of that in the natural forest. Across plots in both forests, understorey plant richness and diversity were negatively correlated with direct sunlight but not indirect sunlight, possibly because direct light plays a more important role in understorey plant growth. Conclusions: We report lower species and functional diversity and higher light availability in a natural hardwood forest than an adjacent 30‐yr C. japonica plantation, possibly due to the increased dominance of shade‐intolerant species associated with higher light availability. To maintain plant diversity, management efforts must be made to prevent localized losses of shade‐adapted understorey plants

An Alternative Method to Deduce Bubble Dynamics in Single Bubble Sonoluminescence Experiments

In this paper we present an experimental approach that allows to deduce the important dynamical parameters of single sonoluminescing bubbles (pressure amplitude, ambient radius, radius-time curve) The technique is based on a few previously confirmed theoretical assumptions and requires the knowledge of quantities such as the amplitude of the electric excitation and the phase of the flashes in the acoustic period. These quantities are easily measurable by a digital oscilloscope, avoiding the cost of expensive lasers, or ultrafast cameras of previous methods. We show the technique on a particular example and compare the results with conventional Mie scattering. We find that within the experimental uncertainties these two techniques provide similar results.Comment: 8 pages, 5 figures, submitted to Phys. Rev.

Cubic Interactions in PP-Wave Light Cone String Field Theory

We use the supergravity modes to clarify the role of the prefactor in the light-cone superstring field theory on PP-wave background. We verify some of the proposals of the recent paper hep-th/0205089 and give further evidence for the correspondence between N=4 SYM gauge theory and string theory on PP-wave. We also consider energy-preserving processes and find that they give vanishing cubic interaction Hamiltonian matrix.Comment: 16+1 pages, no figures, LaTeX (v3: references, minor changes and a clarification about Neumann matrices added; to appear in Physical Review D