Backward Sweep Research Articles

FBSGraph: Accelerating Asynchronous Graph Processing via Forward and Backward Sweeping

Graph algorithm is pervasive in many applications ranging from targeted advertising to natural language processing. Recently, Asynchronous Graph Processing (AGP) is becoming a promising model to support graph algorithm on large-scale distributed computing platforms because it enables faster convergence speed and lower synchronization cost than the synchronous model for no barrier between iterations. However, existing AGP methods still suffer from poor performance for inefficient vertex state propagation. In this paper, we propose an effective and low-cost forward and backward sweeping execution method to accelerate state propagation for AGP, based on a key observation that states in AGP can be propagated between vertices much faster when the vertices are processed sequentially along the graph path within each round. Through dividing graph into paths and asynchronously processing vertices on each path in an alternative forward and backward way according to their order on this path, vertex states in our approach can be quickly propagated to other vertices and converge in a faster way with only little additional overhead. In order to efficiently support it over distributed platforms, we also propose a scheme to reduce the communication overhead along with a static priority ordering scheme to further improve the convergence speed. Experimental results on a cluster with 1,024 cores show that our approach achieves excellent scalability for large-scale graph algorithms and the overall execution time is reduced by at least 39.8 percent, in comparison with the most cutting-edge methods.

Compensation in Complex Variables for Microgrid Power Flow

The solution of the distribution network power flow in practical applications is based either on the forward–backward sweep method for radial networks, or the current injection method for meshed networks. While the power flow in microgrids that operate in grid-connected mode could be resolved using the above-mentioned approaches, their operation in island mode would require simulating local generation droop controllers for sharing the complex power load and network loss among the generators. This letter proposes a complex power compensation approach, which is based on Wirtinger calculus, for extending the applicability of practical distribution power flow methods to microgrids operating in island mode. Supporting numerical results are reported on microgrids with up to 3139 nodes.

Transport in Short-Period GaAs/AlAs Superlattices with Electric Domains

Electronic transport in short-period GaAs/AlAs superlattices with resonant cavities was studied at room temperature. The evolution of tunneling current at forward and backward bias sweep was investigated. The step-like decrease in current at some threshold voltage was referred to moving domain formation. The current hysteresis observed in current-voltage characteristics was explained by changes in electrical domain regimes. The series of maxima in the current-voltage characteristics was attributed to resonant tunneling of electrons through several barriers inside the domain. The change of threshold voltage for the domain formation at the change of the cavity parameters explained by the excitation of high-amplitude oscillations in the cavity which demonstrated the possibility to excite oscillations in the THz cavity by dynamical negative resistance of SLs with domains.

Assessment of low-fidelity fluid–structure interaction model for flexible propeller blades

Low-fidelity fluid–structure interaction model of flexible propeller blades is assessed by means of comparison with high-fidelity aeroelastic results. The low-fidelity model is based on a coupled extended blade-element momentum model and non-linear beam theory which were both implemented in Matlab. High-fidelity fluid–structure interaction analysis is based on coupling commercial computational fluid dynamics and computational structural dynamics codes. For this purpose, Ansys CFX® and Ansys Mechanical® were used. Three different flexible propeller blade geometries are considered in this study: straight, backward swept, and forward swept. The specific backward and forward swept blades are chosen due to their aeroelastic response and its influence on the propulsive performance of the blade while a straight blade was selected in order to serve as a reference. First, the high-fidelity method is validated against experimental data available for the selected blade geometries. Then the high- and low-fidelity methods are compared in terms of integral thrust and breaking power as well as their respective distributions along the blades are compared for different advancing ratios. In a structural sense, the comparison is performed by analyzing the blade bending and torsional deformation. Based on the obtained results, given the simplicity of the low-fidelity method, it can be concluded that the agreement between the two methods is reasonably good. Moreover, an important result of the comparison study is an observation that the advance ratio is no longer a valid measure of similarity in the case of flexible propeller blades and the behavior of such blades can change significantly with changing operating conditions while keeping the advance ratio constant. This observation is supported by both high- and low-fidelity methods.

Modelling and allocation of open‐UPQC‐integrated PV generation system to improve the energy efficiency and power quality of radial distribution networks

The study presents the modelling and allocation strategy for open unified power quality conditioner (UPQC-O) integrated photovoltaic (PV) generation system in radial distribution networks to improve the energy efficiency and PQ. An UPQC is a custom power device, which consists of series and shunt inverters. In UPQC-O, these inverters are placed in different locations in a network. There is a communication channel to share the information among these inverters to select the respective set point. Two models proposed are: (i) UPQC-O with battery and PV array (UPQC-O-WB) and (ii) UPQC-O with only PV array (UPQC-O-WOB). In UPQC-O-WB, the energy generated by PV array is stored during its operation hour to utilise it during peak hour. However, in UPQC-O-WOB, the energy generated by PV array is directly injected to the network. The proposed models are incorporated in the forward–backward sweep load flow to determine the operational parameters such as bus voltage. An optimisation problem is formulated to determine the optimal placement of UPQC-O with PV array in distribution networks. The objective function includes the investment and operational costs of inverters, battery and PV array, and the cost of energy loss. The particle swarm optimisation is used as the solution strategy.

Aerodynamic Instabilities of Swept Airfoil Design in Transonic Axial-Flow Compressors

This paper investigates the effect of a swept airfoil design on the aerodynamic instabilities of transonic axial-flow compressors based on global stability analysis. An approach aimed at predicting the stall inception point is developed by integrating an improved blade force model and a new mathematical treatment method for steady flow. This approach takes both the concrete blade geometry and complicated base flow into consideration, which promises to provide a highly efficient assessment of the stall margin of a compressor in the design phase. Specifically, the relevant validation is conducted through a typical transonic axial-flow compressor, which shows that the predicted stall inception point agrees well with the experimental result. Furthermore, the compressor is also adopted as a prototype, from which a series of forward- and backward-swept rotors are modeled. Aerodynamic instabilities in the form of eigenvalues for each swept rotor, along with the throttling process, are predicted by solving the developed stall inception model. The comparison of the predicted results for these rotors shows that the backward sweep deteriorates the stall margin; the larger the backward-swept degree, the poorer the compressor stability becomes. However, the forward sweep does not exhibit remarkable and regular influence on the stall margin of the compressor in the present research. A discussion of the predicted results is made from the perspectives of blade loading and passage shock location.

A Stochastic Dual Dynamic Programming Framework for Multiscale MPC

We derive and interpret stochastic dual dynamic programming (SDDP) from the perspective of MPC to argue that SDDP provides a scalable approach to handle complex MPC applications with uncertainties evolving over long time horizons and with fine time resolutions. Scalability is enabled by the use of a nested cutting-plane scheme, which uses forward and backward sweeps along the time horizon to adaptively construct and refine cost-to-go functions. SDDP can also select random paths on-the-fly, thus avoiding the exponential complexity associated with the construction of scenario trees and enabling the construction of statistical performance bounds. Our work seeks to establish connections between SDDP and MPC that allow us to obtain performance bounds for deterministic and stochastic MPC and to propose new MPC schemes that can deal with multiple timescales. We demonstrate the developments using a stationary battery system that participates in multiscale energy markets.

Transport in Short-Period GaAs/AlAs Superlattices with Electric Domains

AbstractElectronic transport in short-period GaAs/AlAs superlattices with resonant cavities was studied at room temperature. The evolution of tunneling current at forward and backward bias sweep was investigated. The step-like decrease in current at some threshold voltage was referred to moving domain formation. The current hysteresis observed in current-voltage characteristics was explained by changes in electrical domain regimes. The series of maxima in the current-voltage characteristics was attributed to resonant tunneling of electrons through several barriers inside the domain. The change of threshold voltage for the domain formation at the change of the cavity parameters explained by the excitation of high-amplitude oscillations in the cavity which demonstrated the possibility to excite oscillations in the THz cavity by dynamical negative resistance of SLs with domains.

Siting and sizing of distributed generators based on improved simulated annealing particle swarm optimization.

Distributed power grids generally contain multiple diverse types of distributed generators (DGs). Traditional particle swarm optimization (PSO) and simulated annealing PSO (SA-PSO) algorithms have some deficiencies in site selection and capacity determination of DGs, such as slow convergence speed and easily falling into local trap. In this paper, an improved SA-PSO (ISA-PSO) algorithm is proposed by introducing crossover and mutation operators of genetic algorithm (GA) into SA-PSO, so that the capabilities of the algorithm are well embodied in global searching and local exploration. In addition, diverse types of DGs are made equivalent to four types of nodes in flow calculation by the backward or forward sweep method, and reactive power sharing principles and allocation theory are applied to determine initial reactive power value and execute subsequent correction, thus providing the algorithm a better start to speed up the convergence. Finally, a mathematical model of the minimum economic cost is established for the siting and sizing of DGs under the location and capacity uncertainties of each single DG. Its objective function considers investment and operation cost of DGs, grid loss cost, annual purchase electricity cost, and environmental pollution cost, and the constraints include power flow, bus voltage, conductor current, and DG capacity. Through applications in an IEEE33-node distributed system, it is found that the proposed method can achieve desirable economic efficiency and safer voltage level relative to traditional PSO and SA-PSO algorithms, and is a more effective planning method for the siting and sizing of DGs in distributed power grids.

Sensitivity analysis of convergence characteristics in power flow methods for distribution systems

This paper presents an evaluation of the main convergence characteristics of multiphase power flow methods in unbalanced distribution systems. Two widely used methods for solving unbalanced power flows are analyzed: methods based on Newton–Raphson (NR) and methods based on backward forward sweep (BFS). The limits and robustness of the BFS method and NR method are tested in the following aspects: (i) variation of the X/R ratio, (ii) load’s increase up to the convergence limit, (iii) load model impacts (ZIP model), and (iv) voltage regulators modeling impacts. Some models presented in the literature were implemented and tested. Tests are performed on the IEEE 34 and IEEE 123 systems. Analytical explanations are also presented.

Interferometric near-infrared spectroscopy (iNIRS): performance tradeoffs and optimization

Interferometric near-infrared spectroscopy (iNIRS) is a time-of-flight- (TOF-) resolved sensing modality for determining optical and dynamical properties of a turbid medium. iNIRS achieves this by measuring the interference spectrum of light traversing the medium with a rapidly tunable, or frequency-swept, light source. Thus, iNIRS system performance critically depends on the source and detection apparatus. Using a current-tuned 855 nm distributed feedback laser as the source, we experimentally characterize iNIRS system parameters, including speed, sensitivity, dynamic range, TOF resolution, and TOF range. We also employ a novel Mach-Zehnder interferometer variant with a multi-pass loop to monitor the laser instantaneous linewidth and TOF range at high tuning speeds. We identify and investigate tradeoffs between parameters, with the goal of optimizing performance. We also demonstrate a technique to combine forward and backward sweeps to double the effective speed. Combining these advances, we present in vivo TPSFs and autocorrelations from the mouse brain with TOF resolutions of 22-60 ps, 36-47 dB peak-sidelobe dynamic range, 4-10 μs autocorrelation lag time resolution, a TOF range of nanoseconds or more, and nearly shot noise limited sensitivity.

An implicit simplified sphere function-based gas kinetic scheme for simulation of 3D incompressible isothermal flows

In this work, an implicit simplified sphere function-based gas kinetic scheme (SGKS) is presented for simulation of 3D incompressible isothermal flows. At first, the numerical fluxes of governing equations are reconstructed by the local solution of Boltzmann equation with sphere function distribution. Due to incompressible limit, the sphere at cell interface can be approximately considered to be symmetric as shown in the work. Besides that, the energy equation is usually not needed for simulation of incompressible isothermal flows. With all these simplifications, the formulations of the simplified SGKS can be expressed concisely and explicitly. Secondly, the commonly-used implicit Lower-Upper Symmetric Gauss-Seidel (LU-SGS) method is adopted to further improve the computational efficiency and numerical stability of present scheme. In LU-SGS method, only a forward and a backward sweep are needed for marching the conservative variables in time. As a result, the simplified SGKS with the LU-SGS method can be implemented easily. Numerical experiments, including the 3D lid-driven cavity flow and flow over a backward-facing step, showed that the incompressible isothermal flows can be well simulated by the developed scheme and its computational efficiency is significantly higher than that of the original SGKS and the lattice Boltzmann flux solver (LBFS). In addition, it was found that the present scheme with the LU-SGS method is more efficient than that with the explicit Euler method, and the speedup ratio is about 2 to 5.

Load Flow Analysis for AC/DC Distribution Systems with Distributed Generations

Hybrid AC/DC distribution systems are rapidly growing due to the recent developments in power electronic technologies and increasing deployment of DC output type distributed generations (DGs) in distribution voltage levels. The presence of DGs in DC distribution systems imposes new challenges in load flow calculations. In response, this paper is dealing with the load flow calculation problem in AC/DC distribution systems in the presence of different kinds of DGs. For this purpose, mathematical models of different kinds of DGs are presented, and then, the models are incorporated into the sensitivity matrix method for using in load flow calculations. Since the large number of load buses and the high R/X ratios in the AC part of the system can lead to the divergence of Jacobian matrix based methods, the suggested method is based on backward forward sweep (BFS) method. The proposed method is simulated on a test system equipped with different kinds of DGs and converters and the results are presented.

Optimization of Energy Loss Cost of Distribution Networks with the Optimal Placement and Sizing of DSTATCOM Using Differential Evolution Algorithm

This paper presents an optimization of energy loss cost (ELC) of distribution networks (DN) with the optimal allocation and sizing of DSTATCOM to maximizing the total net profit (TNP)/cost savings by using the differential evolution algorithm per annum and planning horizon of DSTATCOM installation scheme. In this approach, the optimal reactive power compensation is the main vital role in solving the objective function. The optimal reactive power compensation with the optimal placement and sizing of DSTATCOM and the improvement in voltage profile of the DN are obtained based on certain objectives such as best reduction in network power loss and the total ELC and the maximization of TNP. The modeling on the size of DSTATCOM is used and was incorporated in DN through the forward–backward sweep load flow technique to evaluate the parameters of load flows in DN. Present worth factor is instigated to evaluate the TNP of the DSTATCOM installation scheme. The proposed method is validated on the 30, 33, and 69-bus DN.

Improved Algorithm for Load Flow Analysis of Radial Distribution System

Objectives: In this paper an improved algorithm for load flow analysis of Radial Distribution Systems (RDS) is presented. The algorithm incorporates probabilistic variations of line and load data along with composite loads. Methodology: The algorithm uses Simple Backward and Forward Sweep based method for solving RDS and has further been modified to solve distribution systems with Distributed Generation (DG). Findings: This Improved algorithm for load flow analysis is tested on standard RDS and considerable promising results have been delivered for the system having Distributed Generator for improving the voltage profile and reduce the losses. Novelty/Improvement: The results obtained are exemplary that confirms usefulness of the algorithm and can have significant impact on future planning and operation purposes of unbalanced RDS.

Agent‐based decentralised load flow computation for smart management of distribution system

Centralised operation of distribution system suffer from inherent difficulties like increased complexities with increased system size, consequent frequent upgradation, delayed decision making and so on. Paradigm shift towards decentralised operations overcomes the aforesaid problems and assist in automated grid operations. Load flow is an indispensable tool, where the system operator does centralised calculation of state variables. This study proposes a novel approach towards decentralised load flow computation for weakly meshed distribution system. Agent technology is employed to design the framework which is based on agent relaying concept. The agents are located on system buses, and have limited access to information pertaining to system topology and nodal injections. By coordinating with neighbouring agents, they aim towards the fulfilment of their own goal, i.e. bus voltage calculation. To achieve this, several behaviours such as forward sweep, backward sweep, break point calculations and scenario generation are incorporated in the proposed system design. The solution thus obtained is decentralised in nature. The agent designing is done under JAVA environment on JADE platform. Two case studies considering 15 bus and 69 bus radial distribution systems are simulated with the proposed concept to show the validity and applicability of the proposed framework for smart grid management systems.

A high‐order backward forward sweep interpolating algorithm for semi‐Lagrangian method

AbstractConventional semi‐Lagrangian methods often suffer from poor accuracy and imbalance problems of advected properties because of low‐order interpolation schemes used and/or inability to reduce both dissipation and dispersion errors even with high‐order schemes. In the current work, we propose a fourth‐order semi‐Lagrangian method to solve the advection terms at a computing cost of third‐order interpolation scheme by applying backward and forward interpolations in an alternating sweep manner. The method was demonstrated for solving 1‐D and 2‐D advection problems, and 2‐D and 3‐D lid‐driven cavity flows with a multi‐level V‐cycle multigrid solver. It shows that the proposed method can reduce both dissipation and dispersion errors in all regions, especially near sharp gradients, at a same accuracy as but less computing cost than the typical fourth‐order interpolation because of fewer grids used. The proposed method is also shown able to achieve more accurate results on coarser grids than conventional linear and other high‐order interpolation schemes in the literature. Copyright © 2017 John Wiley & Sons, Ltd.

A novel analytical approach for sizing and placement of DG in radial distribution system

Rapid depletion of fossil fuel reserves and its increasing demand necessitates the search for other alternatives. Severe environmental impacts caused by the fossil fuel-based power plants and the escalating fuel costs are the major challenges faced by the electricity supply industry. The integration of distributed generators (DG) especially, wind and solar systems to the grid can offer viable solutions to address these challenges. In radial distribution system, backward forward sweep algorithm (BFSA) is a simple, efficient and accurate algorithm used for load flow analysis to find the voltage profile and electrical power losses. This paper applies a new algorithm, bus voltage sensitivity index (BVSI) on balanced distribution networks for the proper placement of DG. The bus having least BVSI gives the highest priority for active power. The results show that the proposed method converges to better solutions much faster than the genetic algorithm (GA)-based BFSA.

Optimization of Distribution Generating Unit using Voltage Stability Index and Analytical Approach

Background/Objectives: Voltage stability has become an important issue of power system stability. In this paper Optimization of Distribution Generating Unit using Voltage Stability Index (VSI) and Analytical Approach with Forward, Backward Sweep method (FBS). Methods/Statistical analysis: In this work a novel approach has been implemented to reduce the losses in the radial distribution system. This work has been done in two phases, identification of weak nodes with voltage stability index, which are most sensitive toward voltage collapse using the Branch Injection and Branch Current (BIBC) and Branch Current and Branch Voltage (BCBV) matrix have been analyzed from the first phase of work. From second phase, optimal allocation of Distribution Generation (DG) with objective as minimizing the distribution line losses with cost attributes. It is vitally essential feature to characterize the size and location of DG to be placed with the cost of that DG installed allowing for determine cost of loss. Application/Improvements: By virtue of some intrinsically characteristics of distribution systems, as the structure is radial in nature, immensely colossal number of nodes, with an extensive range of R/X ratios, the conservative techniques diverges for the distribution system on the determination of optimum size and location of distributed generations. The projected analysis is implemented on IEEE 15, IEEE 33, and IEEE 69 bus standard test systems.

Traces of Floating Archosaurs: An Interpretation of the Enigmatic Trace Fossils from the Triassic of the Tabular Cover of Southern Spain

abstractTwenty-six enigmatic footprints composed of three elongated parallel depressions (two lateral ones and a shorter central one located forward with respect to the others) are described from Middle Triassic red beds of the Tabular Cover in southern Spain. The purpose of this work is the interpretation of these enigmatic tracks. The footprints range from 28 to 48 cm in length and 23 to 44 cm in width. Limestones on top of the footprint-bearing sandstone contain invertebrate traces such as Planolites, Thalassinoides, and Rhizocorallium, and bivalves (Pleuromya, Trigonodus, and Unionites), typical of marine and brackish environments. They indicate that these footprints developed in a coastal or nearly coastal environment. Whereas the enigmatic trace fossils correspond to tridactyl footprints, the digital marks are parallel and do not show any divarication as in typical walking footprints of archosauria. These traces were left when the trackmaker was swimming in a waterbody deep enough for floating only occasionally touching the ground with the digit tips. The tracks formed as a result of the backward sweeping of the tips when the digits came in contact with the bottom while the animal was buoyed by water. They differ from typical swim traces of other archosaurs (such as theropods) that show three longitudinal scratches because in studied tracks the axial digit (III) of the bipedal tridactyl archosaur sinks in the sediment and gives a short and deep scratch. The other digit scratches (from digits II and IV) would correspond to the upper surface of the toes. The studied tracks correspond to Characichnos ichnofacies (swimming tracks composed by parallel scratch marks) and they were made by a bipedal tridactil archosaur or some functionally tridactyl, chirothere tracemaker.