The Experts below are selected from a list of 41433 Experts worldwide ranked by ideXlab platform
Quanquan C. Liu - One of the best experts on this subject based on the ideXlab platform.
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Inapproximability of the Standard Pebble Game and Hard to Pebble Graphs
arXiv: Computational Complexity, 2017Co-Authors: Erik D. Demaine, Quanquan C. LiuAbstract:Pebble games are single-player games on DAGs involving placing and moving Pebbles on nodes of the graph according to a certain set of rules. The goal is to Pebble a set of target nodes using a minimum number of Pebbles. In this paper, we present a possibly simpler proof of the result in [CLNV15] and strengthen the result to show that it is PSPACE-hard to determine the minimum number of Pebbles to an additive $n^{1/3-\epsilon}$ term for all $\epsilon > 0$, which improves upon the currently known additive constant hardness of approximation [CLNV15] in the standard Pebble game. We also introduce a family of explicit, constant indegree graphs with $n$ nodes where there exists a graph in the family such that using constant $k$ Pebbles requires $\Omega(n^k)$ moves to Pebble in both the standard and black-white Pebble games. This independently answers an open question summarized in [Nor15] of whether a family of DAGs exists that meets the upper bound of $O(n^k)$ moves using constant $k$ Pebbles with a different construction than that presented in [AdRNV17].
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WADS - Inapproximability of the Standard Pebble Game and Hard to Pebble Graphs
Lecture Notes in Computer Science, 2017Co-Authors: Erik D. Demaine, Quanquan C. LiuAbstract:Pebble games are single-player games on DAGs involving placing and moving Pebbles on nodes of the graph according to a certain set of rules. The goal is to Pebble a set of target nodes using a minimum number of Pebbles. In this paper, we present a possibly simpler proof of the result in [4] and strengthen the result to show that it is PSPACE-hard to determine the minimum number of Pebbles to an additive \(n^{1/3-\epsilon }\) term for all \(\epsilon > 0\), which improves upon the currently known additive constant hardness of approximation [4] in the standard Pebble game. We also introduce a family of explicit, constant indegree graphs with n nodes where there exists a graph in the family such that using \(0< k < \sqrt{n}\) Pebbles requires \(\varOmega ((n/k)^k)\) moves to Pebble in both the standard and black-white Pebble games. This independently answers an open question summarized in [14] of whether a family of DAGs exists that meets the upper bound of \(O(n^k)\) moves using constant k Pebbles with a different construction than that presented in [1].
Shengyao Jiang - One of the best experts on this subject based on the ideXlab platform.
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Numerical Study on Discharging Characteristics of Pebble Cluster Flow in Pebble Bed
Volume 3: Student Paper Competition; Thermal-Hydraulics; Verification and Validation, 2020Co-Authors: Nan Gui, Xingtuan Yang, Shengyao JiangAbstract:Abstract To better understand the flow features of Pebble cluster in Pebble bed, discharging of the Pebble cluster were simulated by DEM. The Pebble entangled cluster was composed of eight particles connected by rigid bonds and the simulated cluster models are divided into two types: axisymmetric u-particle and distorted z-particle. The simulation starts with the closed discharge outlet and the bonded clusters with different ID are randomly added from the entrance section. The Pebbles fall freely and accumulate freely in the Pebble bed. The discharge hole opens after all the Pebbles being stationary for a period. Then the Pebbles are discharged from the Pebble bed under gravity. The discharging process is time-dependent bulk-movement behavior. There is not much mixing between layers on the boundary. The vertical end makes the packing loose, but also intensifies the interaction between particles due to entanglement. Consequently, the discharge features of Pebble clusters of different included angles were quantified. The results show that the Pebble discharging speeds depend on entanglement angle (α of u-particle and η of z-particle) and discharging outlet diameter. A large included angle may play the role of retarding or inhibiting the discharging flowrate. Therefore, the entanglement of particles component also always plays the key role of retarding the discharge.
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Effects of restitution coefficient on Pebble motion in a thin Pebble bed – Lagrangian analysis
Annals of Nuclear Energy, 2020Co-Authors: Nan Gui, Xingtuan Yang, Shengyao JiangAbstract:Abstract The core of Pebble bed type reactor (HTGR) is a packed bed composed of spherical Pebbles (fuel element and graphite moderator). The restitution coefficient is an important parameter which is directly related to the flow of the core Pebbles and affects the motion trajectory and stacking state of the fuel Pebbles. Herein, Discrete Element Method (DEM) is used to simulate Pebble flows within a thin Pebble bed. The packing peaks, apex angles, trajectory and velocity deviations, residence time and residence ratios are analyzed in details. The influence of restitution coefficient on the motion characteristics of Pebble flow are studied based on the trajectory of fuel Pebble. A new evaluation criterion for the uniformity of Pebble flows is put forward, and its influencing mechanisms are explored. The relationship between the restitution coefficient and Pebble motion is proposed, which can help understand the flow uniformity of fuel Pebbles in nuclear reactor core.
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Effects of 3D contraction on Pebble flow uniformity and stagnation in Pebble beds
Nuclear Engineering and Technology, 2020Co-Authors: Nan Gui, Xingtuan Yang, Shengyao JiangAbstract:ABSTRACT Pebble flow characteristics can be significantly affected by the configuration of Pebble bed, especially for HTGR Pebble beds. How to achieve a desired uniform flow pattern without stagnation is the top priority for reactor design. Pebbles flows inside some specially designed Pebble bed with arc-shaped contraction configurations at the bottom, including both concave-inward and convex-outward shapes are explored based on discrete element method. Flow characteristics including Pebble retention, residence-time frequency density, flow uniformity as well as axial velocity are investigated. The results show that the traditionally designed Pebble bed with cone-shape bottom is not the most preferred structure with respect to flow pattern for reactor design. By improving the contraction configuration, the flow performance can be significantly enhanced. The flow in the convex-shape configuration featured by uniformity, consistency and less stagnation, is much more desirable for Pebble bed design. In contrast, when the shape is from convex-forward to concave-inward, the flow shows more nonuniformity and stagnation in the corner although the average cross-section axial velocity is the largest due to the dominant middle Pebbles.
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a review of Pebble flow study for Pebble bed high temperature gas cooled reactor
Experimental and Computational Multiphase Flow, 2019Co-Authors: Shengyao Jiang, Xingtuan Yang, Nan GuiAbstract:The Pebble bed high temperature gas-cooled reactor is a promising generation-IV reactor, which uses large fuel Pebbles and helium gas as coolant. The Pebble bed flow is a fundamental issue for both academic investigation and engineering application, e.g., reactor core design and safety analysis. This work performed a review of recent progress on Pebble flow study, focusing on the important issues like Pebble flow, gas phase hydrodynamics, and inter-phase heat transfer (thermal hydraulics). Our group’s researches on Pebble flow have also been reviewed through the aspects of phenomenological observation and measurement, voidage distribution, geometric and parameter optimization, Pebble flow mechanisms, flow regime categorization, and fundamentals of modelings of Pebble flow and radiation. Finally, the major problems or possible directions of research are concluded which would be some of our focuses on the Pebble bed flow study.
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Effect of bed configuration on Pebble flow uniformity and stagnation in the Pebble bed reactor
Nuclear Engineering and Design, 2014Co-Authors: Nan Gui, Xingtuan Yang, Shengyao JiangAbstract:Pebble flow uniformity and stagnation characteristics are very important for the design of Pebble bed high temperature gas-cooled reactor. Pebble flows inside some specifically designed contraction configurations of Pebble bed are studied by discrete element method. The results show the characteristics of stagnation rates, recycling rates, radial distribution of Pebble velocity and residence time. It is demonstrated clearly that the bed with a brachistochrone-shaped configuration achieves optimum levels of flow uniformity and recycling rate concentration, and almost no Pebbles are stagnated in the bed. Moreover, the optimum choice among the arc-shaped bed configurations is demonstrated too. Detailed information shows the quantified characteristics of bed configuration effects on flow uniformity. In addition, a good design of the Pebble bed configuration is suggested.
Erik D. Demaine - One of the best experts on this subject based on the ideXlab platform.
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Inapproximability of the Standard Pebble Game and Hard to Pebble Graphs
arXiv: Computational Complexity, 2017Co-Authors: Erik D. Demaine, Quanquan C. LiuAbstract:Pebble games are single-player games on DAGs involving placing and moving Pebbles on nodes of the graph according to a certain set of rules. The goal is to Pebble a set of target nodes using a minimum number of Pebbles. In this paper, we present a possibly simpler proof of the result in [CLNV15] and strengthen the result to show that it is PSPACE-hard to determine the minimum number of Pebbles to an additive $n^{1/3-\epsilon}$ term for all $\epsilon > 0$, which improves upon the currently known additive constant hardness of approximation [CLNV15] in the standard Pebble game. We also introduce a family of explicit, constant indegree graphs with $n$ nodes where there exists a graph in the family such that using constant $k$ Pebbles requires $\Omega(n^k)$ moves to Pebble in both the standard and black-white Pebble games. This independently answers an open question summarized in [Nor15] of whether a family of DAGs exists that meets the upper bound of $O(n^k)$ moves using constant $k$ Pebbles with a different construction than that presented in [AdRNV17].
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WADS - Inapproximability of the Standard Pebble Game and Hard to Pebble Graphs
Lecture Notes in Computer Science, 2017Co-Authors: Erik D. Demaine, Quanquan C. LiuAbstract:Pebble games are single-player games on DAGs involving placing and moving Pebbles on nodes of the graph according to a certain set of rules. The goal is to Pebble a set of target nodes using a minimum number of Pebbles. In this paper, we present a possibly simpler proof of the result in [4] and strengthen the result to show that it is PSPACE-hard to determine the minimum number of Pebbles to an additive \(n^{1/3-\epsilon }\) term for all \(\epsilon > 0\), which improves upon the currently known additive constant hardness of approximation [4] in the standard Pebble game. We also introduce a family of explicit, constant indegree graphs with n nodes where there exists a graph in the family such that using \(0< k < \sqrt{n}\) Pebbles requires \(\varOmega ((n/k)^k)\) moves to Pebble in both the standard and black-white Pebble games. This independently answers an open question summarized in [14] of whether a family of DAGs exists that meets the upper bound of \(O(n^k)\) moves using constant k Pebbles with a different construction than that presented in [1].
Mamdouh M. Abdeen - One of the best experts on this subject based on the ideXlab platform.
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Reappraisal of strain estimations and measurement methods in the Hammamat Group sediments: Comparison of primary and secondary grain fabrics with new data from Wadi Zeidun and Wadi Arak basins, CED, Egypt
Precambrian Research, 2014Co-Authors: A. Fowler, Mamdouh M. AbdeenAbstract:Abstract In this study, Pebble and sand grain shape and orientation data from undeformed and deformed conglomerates and sandstones in the Egyptian Central Eastern Desert (CED) Wadi Zeidun and Wadi Arak basins were collected, with the aim of providing more accurate tectonic strain estimations for deformed Hammamat sediments by comparing their primary and secondary sedimentary fabrics. Common assumptions about primary Pebble fabrics and Pebble deformation behaviour in previous strain studies of the region are shown to be invalid. This raises doubts about the accuracy of earlier reported strain estimates for the Hammamat that were based on such assumptions. For low strains in the Hammamat sediments, the best strain measurement method is normalized Fry (NFry) analysis of sand grain centre-to-centre distance data. An alternative is conventional Fry analysis of Pebble ellipse centre-to-centre distance data, provided that large numbers of Pebbles (∼300) are used, and the Pebbles form closely packed aggregates with high degrees of anticlustering. This study also provides systematic guidelines for Pebble axial length and orientation measurements in the field. The progressive changes in the mechanical behaviour of Pebbles with increasing bulk strain are discussed.
Joshua J. Cogliati - One of the best experts on this subject based on the ideXlab platform.
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The effects of temperatures on the Pebble flow in a Pebble bed high temperature reactor
2012Co-Authors: R. S. Sen, Joshua J. Cogliati, H. D. GougarAbstract:The core of a Pebble bed high temperature reactor (PBHTR) moves during operation, a feature which leads to better fuel economy (online refueling with no burnable poisons) and lower fuel stress. The Pebbles are loaded at the top and trickle to the bottom of the core after which the burnup of each is measured. The Pebbles that are not fully burned are recirculated through the core until the target burnup is achieved. The flow pattern of the Pebbles through the core is of importance for core simulations because it couples the burnup distribution to the core temperature and power profiles, especially in cores with two or more radial burnup 'zones '. The Pebble velocity profile is a strong function of the core geometry and the friction between the Pebbles and the surrounding structures (other Pebbles or graphite reflector blocks). The friction coefficient for graphite in a helium environment is inversely related to the temperature. The Thorium High Temperature Reactor (THTR) operated in Germany between 1983 and 1989. It featured a two-zone core, an inner core (IC) and outer core (OC), with different fuel mixtures loaded in each zone. The rate at which the IC was refueled relative to the OCmore » in THTR was designed to be 0.56. During its operation, however, this ratio was measured to be 0.76, suggesting the Pebbles in the inner core traveled faster than expected. It has been postulated that the positive feedback effect between inner core temperature, burnup, and Pebble flow was underestimated in THTR. Because of the power shape, the center of the core in a typical cylindrical PBHTR operates at a higher temperature than the region next to the side reflector. The friction between Pebbles in the IC is lower than that in the OC, perhaps causing a higher relative flow rate and lower average burnup, which in turn yield a higher local power density. Furthermore, the Pebbles in the center region have higher velocities than the Pebbles next to the side reflector due to the interaction between the Pebbles and the immobile graphite reflector as well as the geometry of the discharge conus near the bottom of the core. In this paper, the coupling between the temperature profile and the Pebble flow dynamics was analyzed by using PEBBED/THERMIX and PebbleS codes by modeling the HTR-10 reactor in China. Two extreme and opposing velocity profiles are used as a starting point for the iterations. The PEBBED/THERMIX code is used to calculate the burnup, power and temperature profiles with one of the velocity profiles as input. The resulting temperature profile is then passed to PebbleS code to calculate the updated Pebble velocity profile taking the new temperature profile into account. If the aforementioned hypothesis is correct, the strong temperature effect upon the friction coefficients would cause the two cases to converge to different final velocity and temperature profiles. The results of this analysis indicates that a single zone Pebble bed core is self-stabilizing in terms of the Pebble velocity profile and the effect of the temperature profile on the Pebble flow is insignificant. (authors)« less
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Pebble bed Pebble motion: Simulation and Application
2011Co-Authors: Joshua J. CogliatiAbstract:Pebble bed reactors (PBR) have moving graphite fuel Pebbles. This unique feature provides advantages, but also means that simulation of the reactor requires understanding the typical motion and location of the granular flow of Pebbles. This report presents a method for simulation of motion of the Pebbles in a PBR. A new mechanical motion simulator, PebbleS, efficiently simulates the key elements of motion of the Pebbles in a PBR. This model simulates gravitational force and contact forces including kinetic and true static friction. It's used for a variety of tasks including simulation of the effect of earthquakes on a PBR, calculation of packing fractions, Dancoff factors, Pebble wear and the Pebble force on the walls. The simulator includes a new differential static friction model for the varied geometries of PBRs. A new static friction benchmark was devised via analytically solving the mechanics equations to determine the minimum Pebble-to-Pebble friction and Pebble-to-surface friction for a five Pebble pyramid. This pyramid check as well as a comparison to the Janssen formula was used to test the new static friction equations. Because larger Pebble bed simulations involve hundreds of thousands of Pebbles and long periods of time, the PebbleS code has been parallelized.more » PebbleS runs on shared memory architectures and distributed memory architectures. For the shared memory architecture, the code uses a new O(n) lock-less parallel collision detection algorithm to determine which Pebbles are likely to be in contact. The new collision detection algorithm improves on the traditional non-parallel O(n log(n)) collision detection algorithm. These features combine to form a fast parallel Pebble motion simulation. The PebbleS code provides new capabilities for understanding and optimizing PBRs. The PebbleS code has provided the Pebble motion data required to calculate the motion of Pebbles during a simulated earthquake. The PebbleS code provides the ability to determine the contact forces and the lengths of motion in contact. This information combined with the proper wear coefficients can be used to determine the dust production from mechanical wear. These new capabilities enhance the understanding of PBRs, and the capabilities of the code will allow future improvements in understanding.« less
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Pebble bed Pebble motion: Simulation and Applications
arXiv: Computational Physics, 2011Co-Authors: Joshua J. CogliatiAbstract:This dissertation presents a method for simulation of motion of the Pebbles in a PBR. A new mechanical motion simulator, PebbleS, efficiently simulates the key elements of motion of the Pebbles in a PBR. This model simulates gravitational force and contact forces including kinetic and true static friction. It's used for a variety of tasks including simulation of the effect of earthquakes on a PBR, calculation of packing fractions, Dancoff factors, Pebble wear and the Pebble force on the walls. The simulator includes a new differential static friction model for the varied geometries of PBRs. A new static friction benchmark was devised via analytically solving the mechanics equations to determine the minimum Pebble-to-Pebble friction and Pebble-to-surface friction for a five Pebble pyramid. This pyramid check as well as a comparison to the Janssen formula was used to test the new static friction equations. Because larger Pebble bed simulations involve hundreds of thousands of Pebbles and long periods of time, PebbleS runs on shared memory architectures and distributed memory architectures. For the shared memory architecture, the code uses a new O(n) lock-less parallel collision detection algorithm to determine which Pebbles are likely to be in contact. The PebbleS code provides new capabilities for understanding and optimizing PBRs. The PebbleS code has provided the Pebble motion data required to calculate the motion of Pebbles during a simulated earthquake. The PebbleS code provides the ability to determine the contact forces and the lengths of motion in contact. This information combined with the proper wear coefficients can be used to determine the dust production from mechanical wear. These new capabilities enhance the understanding of PBRs, and the capabilities of the code will allow future improvements in understanding.
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Development Status of the PebbleS Code for Pebble Mechanics: Improved Physical Models and Speed-up
2009Co-Authors: Joshua J. Cogliati, Abderrafi M. OugouagAbstract:PebbleS is a code for simulating the motion of all the Pebbles in a Pebble bed reactor. Since Pebble bed reactors are packed randomly and not precisely placed, the location of the fuel elements in the reactor is not deterministically known. Instead, when determining operating parameters the motion of the Pebbles can be simulated and stochastic locations can be found. The PebbleS code can output information relevant for other simulations of the Pebble bed reactors such as the positions of the Pebbles in the reactor, packing fraction change in an earthquake, and velocity profiles created by recirculation. The goal for this level three milestone was to speedup the PebbleS code through implementation on massively parallel computer. Work on this goal has resulted in speeding up both the single processor version and creation of a new parallel version of PebbleS. Both the single processor version and the parallel running capability of the PebbleS code have improved since the fiscal year start. The hybrid MPI/OpenMP PebbleS version was created this year to run on the increasingly common cluster hardware profile that combines nodes with multiple processors that share memory and a cluster of nodes that are networked together. The OpenMP portions usemore » the Open Multi-Processing shared memory parallel processing model to split the task across processors in a single node that shares memory. The Message Passing Interface (MPI) portion uses messages to communicate between different nodes over a network. The following are wall clock speed up for simulating an NGNP-600 sized reactor. The single processor version runs 1.5 times faster compared to the single processor version at the beginning of the fiscal year. This speedup is primarily due to the improved static friction model described in the report. When running on 64 processors, the new MPI/OpenMP hybrid version has a wall clock speed up of 22 times compared to the current single processor version. When using 88 processors, a speed up of 23 times is achieved. This speedup and other improvements of PebbleS combine to make PebbleS more capable and more useful for simulation of a Pebble bed reactor. This report details the implementation and effects of the speedup work done over the course of the fiscal year.« less
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Pebble bed reactor dust production model
Fourth International Topical Meeting on High Temperature Reactor Technology Volume 1, 2008Co-Authors: Joshua J. Cogliati, Abderrafi M. OugouagAbstract:The operation of Pebble bed reactors, including fuel circulation, can generate graphite dust, which in turn could be a concern for internal components; and to the near field in the remote event of a break in the coolant circuits. The design of the reactor system must, therefore, take the dust into account and the operation must include contingencies for dust removal and for mitigation of potential releases. Such planning requires a proper assessment of the dust inventory. This paper presents a predictive model of dust generation in an operating Pebble bed with recirculating fuel. In this preliminary work the production model is based on the use of the assumption of proportionality between the dust production and the normal force and distance traveled. The model developed in this work uses the slip distances and the inter-Pebble forces computed by the authors’ PebbleS. The code, based on the discrete element method, simulates the relevant static and kinetic friction interactions between the Pebbles as well as the recirculation of the Pebbles through the reactor vessel. The interaction between Pebbles and walls of the reactor vat is treated using the same approach. The amount of dust produced is proportional to the wear coefficient for adhesive wear (taken from literature) and to the slip volume, the product of the contact area and the slip distance. The paper will compare the predicted volume with the measured production rates. The simulation tallies the dust production based on the location of creation. Two peak production zones from intra Pebble forces are predicted within the bed. The first zone is located near the Pebble inlet chute due to the speed of the dropping Pebbles. The second peak zone occurs lower in the reactor with increased Pebble contact force due to the weight of supported Pebbles. This paper presents the first use of a Discrete Element Method simulation of Pebble bed dust production.
