Large Void Regions Research Articles

Joint Effects of Void Region Size and Sink Architecture on Underwater WSNs Lifetime

Underwater wireless sensor networks (UWSNs) typically suffer from the communication void region problem. A common method to handle the void region problem is to re-route the packets around the void regions. As the size of the void regions increases, packets require more hops to circumvent the large void regions, resulting in a short network lifetime. On the other hand, the void region problem is more destructive in UWSNs utilizing a single-sink architecture than a multi-sink architecture since nodes consume excessive energy for bypassing the void regions to reach the sink node, which can be positioned in a hard-to-reach area in single-sink UWSNs. In this work, an integer linear programming (ILP) model is developed for maximizing UWSNs lifetime while bypassing the void regions. Solving the ILP model to optimality, the joint impact of the void region size and the sink architecture type on UWSNs lifetime is investigated. The results show that the performance of UWSNs significantly drops as the size of the void region grows such that UWSNs lifetimes shorten by up to 61% as the total void region size is one-quarter of the network size. Moreover, multi-sink UWSNs yield better performance than single-sink UWSNs in the void region problem.

Impact of Dispersion Force Schemes on Liquid Systems: Comparing Efficiency and Drawbacks for Well-Targeted Test Cases.

First-principles molecular dynamics (FPMD) calculations were performed on liquid GeSe4 with the aim of inferring the impact of dispersion (van der Waals, vdW) forces on the structural properties. Different expressions for the dispersion forces were employed, allowing us to draw conclusions on their performances in a comparative fashion. These results supersede previous FPMD calculations obtained in smaller systems and shorter time trajectories by providing data of unprecedented accuracy. We obtained a substantial agreement with experiments for the structure factor regardless of the vdW scheme employed. This objective was achieved by using (in addition to FPMD with no dispersion forces) a selection of vdW schemes available within density functional theory. The first two are due to Grimme, D2 and D3, and the third one is devised within the so-called maximally localized Wannier functions approach (MLWF). D3 results feature a sizeable disagreement in real space with D2 and MLWF in terms of the partial and total pair correlation functions as well as the coordination numbers. More strikingly, total and partial structure factors calculated with D3 exhibit an unexpected sharp increase at low k. This peculiarity goes along with large void regions within the network, standing for a phase separation of indecipherable physical meaning. In view of these findings, further evidence of unconventional structural properties found by employing D3 is presented by relying on results obtained for a complex ionic liquid supported on a solid surface. The novelty of our study is multifold: new, reliable FPMD data for a prototypical disordered network system, convincing agreement with experimental data and assessment of the impact of dispersion forces, with emphasis on the intriguing behavior of one specific recipe and the discovery of common structural features shared by drastically dissimilar physical systems when the D3 vdW scheme is employed.

Effect of thermal oxidation on helium implanted 316L stainless steel

The effect of thermal oxide layer on He implanted 316L stainless steel was studied to evaluate experimentally how thermal oxidation affects the diffusion and distribution of He in the material. In the case of thermal oxidation of a He implanted sample, with an increase in oxidation time, the max swelling height increases logarithmically as a function of time and finally saturates for all samples except for the lowest dose of implanted He. Concerning TEM results, two void regions are identified. Similar to the calculation, the total irradiated depth was around 250 nm and the large void region was formed around 100–150 nm depth. On the other hand, the small void region was observed immediately under oxide layer from the thermal oxidation. In contrast, there were no voids in the altered zone near the metal/oxide interface in the non-thermal oxidized/He implanted sample. This description of the phenomena was justified using the Kirkendall effect and the Point Defect Model.

Hybrid approaches for accelerated convergence of block-Jacobi iterative methods for solution of the neutron transport equation

The Parallel Block Jacobi (PBJ) [1,2] scheme is an iterative method with application in neutron transport solution which requires asynchronous sub-domain interfaces, where asynchronous here means flux continuity is not imposed across sub-domain interfaces. In this work, we present the development, analysis, and testing of hybrid methods which utilize both PBJ-type methods and synchronous sweep-based methods in a single iterative step. These methods are developed to reduce the penalty to iterative performance of the former incurred in problems containing optically thin cells, while maintaining the PBJ method's applicability to massively parallel solution on unstructured grids. As a prelude to this ultimate goal, this study is conducted in serial operation on 2-D Cartesian grids to facilitate implementation of the various tested iterative methods, to enable the reported spectral analysis, and for shorter computational execution times allowing the large amounts of performance data to be collected. The analysis consists of two Fourier analyses, demonstrating the sequential execution of a PBJ method and SI to be iteratively robust with respect to optical thickness. The results of these analyses and preliminary computational experimentation demonstrate the effectiveness of methods that execute PBJ and SI only in optically thick and thin regions of a problem, respectively. Further experimentation, including a parametric study, then demonstrates this approach to be effective when the thin region is solved using Inexact Parallel Block Jacobi (IPBJ), with sub-domains of a size such that the applicability to massively parallel solution on unstructured grids is not diminished. Finally, testing on realistic test problems containing large void regions confirms the viability of this approach. Performance comparisons include the traditional iterative acceleration methods Diffusion Synthetic Acceleration, Partial-current Nonlinear Diffusion Acceleration, and Adjacent-cell Preconditioner. The results show that while the hybrid PBJ methods generally consume more iterations than these traditional methods, our hybrid approach maintains an iteration count on the same order of magnitude as these acceleration methods, which are not fully developed for massively parallel solution on unstructured grids.

Quasi-Diffusion Method with 3-D Cross Sections for TREAT Core Analysis

The conventional two-step neutronics method used to perform full-core reactor neutronics simulation has been used successfully for light water reactor steady-state and transient analysis. The first step in the method is to generate assembly homogenized few-group cross sections from a lattice transport calculation at the anticipated range of core conditions. The resulting cross sections are then used in the second step to calculate the whole-core flux distribution using nodal diffusion methods. However, when applying this method to small reactors or some experimental reactors such as the Transient Reactor Test (TREAT) Facility, the bias from approximations used in the conventional two-step method can become significant. A large source of error can be the cross sections that are generated from the assembly calculations with reflective boundary conditions since interassembly neutron leakages in small reactors can be significant. Another source of error can be the presence of large void regions such as in the TREAT core. In the work here, the shortcomings of the two-step method were addressed by using the quasi-diffusion method with cross sections obtained from a whole-core three-dimensional Monte Carlo simulation. For the nonvoid region, the group-averaged cross sections were obtained directly from Monte Carlo simulation results, and the directional diffusion coefficients were generated from flux-weighted transport cross sections and the Edington factors directly from the angular flux distribution from the Monte Carlo results. Discontinuity factors were also used in the nodal solution to preserve the neutron currents between nodes based on the Monte Carlo results. For the void region, the directional diffusion coefficients were optimized to minimize the magnitude of the discontinuity factors and thereby mitigate potential numerical problems in the quasi-diffusion method for full-core simulations. The numerical results from the TREAT core steady-state and transient analysis show that the quasi-diffusion method can reproduce the Monte Carlo whole-core results in steady state and that the transient results are in good agreement with experimental measurements.

Narrow-band topology optimization on a sparsely populated grid

A variety of structures in nature exhibit sparse, thin, and intricate features. It is challenging to investigate these structural characteristics using conventional numerical approaches since such features require highly refined spatial resolution to capture and therefore they incur a prohibitively high computational cost. We present a novel computational framework for high-resolution topology optimization that delivers leaps in simulation capabilities, by two orders of magnitude, from the state-of-the-art approaches. Our technique accommodates computational domains with over one billion grid voxels on a single shared-memory multiprocessor platform, allowing automated emergence of structures with both rich geometric features and exceptional mechanical performance. To achieve this, we track the evolution of thin structures and simulate its elastic deformation in a dynamic narrow-band region around high-density sites to avoid wasted computational effort on large void regions. We have also designed a mixed-precision multigrid-preconditioned iterative solver that keeps the memory footprint of the simulation to a compact size while maintaining double-precision accuracy. We have demonstrated the efficacy of the algorithm through optimizing a variety of complex structures from both natural and engineering systems.

Improved discrete nodal transport method for treating void regions

To solve neutron transport problems with void regions accurately and efficiently, the legacy discrete nodal transport method (DNTM) was improved in three-dimensional Cartesian geometry. Firstly, the efficient nodal-equivalent finite difference (NEFD) algorithm was modified, named M-NEFD, to directly treat the zero total cross section in the denominator for void nodes. Furthermore, an angle projection discrete-ordinates (APSN) method was proposed to combine with the NEFD algorithm, named APSN-NEFD, to treat the void regions without sweeping the void nodes one by one. Thirdly, the angular sweeps within one octant of both M-NEFD and APSN-NEFD were parallelized using OpenMP. Based on these improvements, a discrete-ordinates nodal transport code named NECP-HONESTY has been developed in three-dimensional Cartesian geometry. To test the performance of the method and the code, a fission-source problem with two small void regions and a fixed-source problem with one large void region were presented in this paper by comparing with the Monte Carlo method. It has been demonstrated by the numerical results that the improved DNTM can provide accurate eigenvalue and scalar flux, no matter directly sweeping the void nodes with M-NEFD or skipping the void regions with APSN. In addition, compared with M-NEFD, APSN-NEFD can efficiently reduce both computing time and storage requirement for problem with large void regions.

Collision frequency and radial distribution function in particle-laden turbulent channel flow

We performed Eulerian–Lagrangian direct numerical simulation of particle-laden channel flow at a frictional Reynolds number of 950. A fully parallelized deterministic particle collision algorithm is applied for elastic collisions between two particles and particles and the walls. A total number of 51 million mono-disperse particles is considered, resulting in a particle volume fraction close to 1×10−4. We studied the results of the simulation after a statistically steady turbulent state and particle concentration were reached. In this state the particles close to the walls are preferentially located in the low-speed streaks, whereas the particle distribution in the center of the channel is rather non-uniform as well, showing large void regions. The presence of the particles results in a decrease of the turbulence dissipation rate of 20% close to the walls. We studied in particular the particle collision frequency as a function of the wall-normal coordinate and compared the simulation results with two theoretical expressions involving the radial distribution function at contact and the mean relative velocity of two colliding particles. It appeared that the radial distribution function diverges if the distance between the particles approaches the particle diameter. This is not only caused by the drift mechanisms but also by repeated collision events, which occur relatively more often in the center of the channel. We proposed a collision criterion that distinguishes repeated collisions from single collisions, and which is easy to apply during a simulation and in the computation of the radial distribution function and the mean relative velocity of two colliding particles. Simulation results and the two theoretical expressions for the collision frequency are in good agreement if this criterion is applied.

Exotic Ground States of Directional Pair Potentials via Collective-Density Variables

Collective-density variables have proved to be a useful tool in the prediction and manipulation of how spatial patterns form in the classical many-body problem. Previous work has employed properties of collective-density variables along with a robust numerical optimization technique to find the classical ground states of many-particle systems subject to radial pair potentials in one, two and three dimensions. That work led to the identification of ordered and disordered classical ground states. In this paper, we extend these collective-coordinate studies by investigating the ground states of directional pair potentials in two dimensions. Our study focuses on directional potentials whose Fourier representations are non-zero on compact sets that are symmetric with respect to the origin and zero everywhere else. We choose to focus on one representative set which has exotic ground-state properties: two circles whose centers are separated by some fixed distance. We obtain ground states for this "two-circle" potential that display large void regions in the disordered regime. As more degrees of freedom are constrained the ground states exhibit a collapse of dimensionality characterized by the emergence of filamentary structures and linear chains. This collapse of dimensionality has not been observed before in related studies.

Development of analysis method for sodium void reactivity of step type FBR cores by using group-wise Monte Carlo code

A new method is proposed to separate the sodium void reactivity of step type FBR cores to components including non-leakage terms and a leakage term by using a newly developed perturbation code MCPERT where fluxes and adjoint fluxes are derived from a group-wise Monte Carlo code. The step type FBR core is a core where the height of the inner core is smaller than that of the outer core and a large sodium plenum region is located above the core so as to decrease the sodium void reactivity. The conventional diffusion perturbation method cannot treat such a large void region due to the diffusion approximation, while the Monte Carlo code can treat it exactly. In this study, a group-wise Monte Carlo code GMVP with a 70-group constant set JFS-3-J3.3 is employed to evaluate the neutron fluxes and adjoint fluxes which are used as inputs to the MCPERT code to evaluate the non-leakage terms. The leakage term is derived from the difference of the total sodium void reactivity evaluated by the direct calculation of GMVP and the summation of the non-leakage terms. It is found that the proposed method can provide the result approximately consistent to the ratio of the reactivity components derived from the conventional method.

Galaxy N-point correlation functions - Theoretical amplitudes for arbitrary N

Scaling arguments have suggested the hierarchical form of galaxy N-point correlation functions, kappa/sub N/ = Q/sub N/xi/sup N/-1, where xi is the two-point function, but such arguments are inherently unable to predict the amplitudes Q/sub N/. Assuming this form, substituting in the BBGKY hierarchy, and using graph theory combinatories, I find amplitudes Q/sub N/ = (4Q/N)/sup N/-2N/(2N-2), where Q is the reduced three-point amplitude. As an application, I compute the probability of large void regions in the hierarchical picture.