292 research outputs found
Mitigating Cascading Failures in Interdependent Power Grids and Communication Networks
In this paper, we study the interdependency between the power grid and the
communication network used to control the grid. A communication node depends on
the power grid in order to receive power for operation, and a power node
depends on the communication network in order to receive control signals for
safe operation. We demonstrate that these dependencies can lead to cascading
failures, and it is essential to consider the power flow equations for studying
the behavior of such interdependent networks. We propose a two-phase control
policy to mitigate the cascade of failures. In the first phase, our control
policy finds the non-avoidable failures that occur due to physical
disconnection. In the second phase, our algorithm redistributes the power so
that all the connected communication nodes have enough power for operation and
no power lines overload. We perform a sensitivity analysis to evaluate the
performance of our control policy, and show that our control policy achieves
close to optimal yield for many scenarios. This analysis can help design robust
interdependent grids and associated control policies.Comment: 6 pages, 9 figures, submitte
Advanced Methodology for Simulation of Complex Flows Using Structured Grid Systems
Detailed simulations of viscous flows in complicated geometries pose a significant challenge to current capabilities of Computational Fluid Dynamics (CFD). To enable routine application of CFD to this class of problems, advanced methodologies are required that employ (a) automated grid generation, (b) adaptivity, (c) accurate discretizations and efficient solvers, and (d) advanced software techniques. Each of these ingredients contributes to increased accuracy, efficiency (in terms of human effort and computer time), and/or reliability of CFD software. In the long run, methodologies employing structured grid systems will remain a viable choice for routine simulation of flows in complex geometries only if genuinely automatic grid generation techniques for structured grids can be developed and if adaptivity is employed more routinely. More research in both these areas is urgently needed
First ultraviolet outburst detected from ASASSN-18eh strengthens its interpretation as a cataclysmic variable
As part of the Transient UV Objects project, we have discovered a new
outburst (at the beginning of October 2020) of the candidate cataclysmic
variable (CV) ASASSN-18eh using the UV/Optical Telescope aboard the Neil
Gehrels Swift Observatory. During the outburst its brightness increased by
about 6 mag in UV compared to its brightness in the quiescent state. The
properties of this outburst are consistent with it being a dwarf nova, strongly
supporting the CV nature of ASASSN-18eh
Visualization of three dimensional CFD results
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1989.Includes bibliographical references (leaves 87-89).by David Laurence Modiano.M.S
Adaptive mesh Euler equation computation of vortex breakdown in delta wing flow
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1993.Includes bibliographical references (leaves 139-146).by David Laurence Modiano.Ph.D
Adaptive Mesh Euler Equation Computations of Vortex Breakdown in Delta Wing Flow
A solution method for the three-dimensional Euler equations is formulated and implemented.
The solver uses an unstructured mesh of tetrahedral cells and performs
adaptive refinement by mesh-point embedding to increase mesh resolution in regions
of interesting flow features. The fourth-difference artificial dissipation is increased to
a higher order of accuracy using the method of Holmes and Connell. A new method
of temporal integration is developed to accelerate the explicit computation of unsteady
flows. The solver is applied to the solution of the flow around a sharp edged delta wing,
with emphasis on the behavior of the leading edge vortex above the leeside of the wing at
high angle of attack, under which conditions the vortex suffers from vortex breakdown.
Large deviations in entropy, which indicate vortical regions of the flow, specify the region
in which adaptation is performed. Adaptive flow calculations are performed at ten
different angles of attack, at seven of which vortex breakdown occurs. The aerodynamic
normal force coefficients show excellent agreement with wind tunnel data measured
by Jarrah, which demonstrates the importance of adaptation in obtaining an accurate
solution. The pitching moment coefficient and the location of vortex breakdown are
compared with experimental data measured by Hummel and Srinivasan, with which
fairly good agreement is seen in cases in which the location of breakdown is over the
wing. A series of unsteady calculations involving a pitching delta wing were performed.
The use of the acceleration technique is validated. A hysteresis in the normal force is
observed, as in experiments, and a lag in the breakdown position is demonstrated
Enhancing network robustness via shielding
We consider shielding critical links to guarantee network connectivity under geographical and general failure models. We develop a mixed integer linear program (MILP) to obtain the minimum cost shielding to guarantee the connectivity of a single SD pair under a general failure model, and exploit geometric properties to decompose the shielding problem under a geographical failure model. We extend our MILP formulation to guarantee the connectivity of the entire network, and use Benders decomposition to significantly reduce the running time by exploiting its partial separable structure. We also apply simulated annealing to solve larger network problems to obtain near-optimal solutions in much shorter time. Finally, we extend the algorithms to guarantee partial network connectivity, and observe significant reduction in shielding cost, especially when the failure region is small. For example, when the failure region radius is 60 miles, we observe as much as 75% reduction in shielding cost by relaxing the connectivity requirement to 95% on a major US infrastructure network
Adaptive Mesh Refinement in Curvilinear Body-Fitted Grid Systems
To be truly compatible with structured grids, an AMR algorithm should employ a block structure for the refined grids to allow flow solvers to take advantage of the strengths of unstructured grid systems, such as efficient solution algorithms for implicit discretizations and multigrid schemes. One such algorithm, the AMR algorithm of Berger and Colella, has been applied to and adapted for use with body-fitted structured grid systems. Results are presented for a transonic flow over a NACA0012 airfoil (AGARD-03 test case) and a reflection of a shock over a double wedge
Computations of Unsteady Viscous Compressible Flows Using Adaptive Mesh Refinement in Curvilinear Body-fitted Grid Systems
A methodology for accurate and efficient simulation of unsteady, compressible flows is presented. The cornerstones of the methodology are a special discretization of the Navier-Stokes equations on structured body-fitted grid systems and an efficient solution-adaptive mesh refinement technique for structured grids. The discretization employs an explicit multidimensional upwind scheme for the inviscid fluxes and an implicit treatment of the viscous terms. The mesh refinement technique is based on the AMR algorithm of Berger and Colella. In this approach, cells on each level of refinement are organized into a small number of topologically rectangular blocks, each containing several thousand cells. The small number of blocks leads to small overhead in managing data, while their size and regular topology means that a high degree of optimization can be achieved on computers with vector processors
Detection of Plasmodium falciparum male and female gametocytes and determination of parasite sex ratio in human endemic populations by novel, cheap and robust RTqPCR assays
The presence of Plasmodium falciparum gametocytes in peripheral blood is essential for human to mosquito parasite transmission. The detection of submicroscopic infections with gametocytes and the estimation of the gametocyte sex ratio are crucial to assess the human host potential ability to infect mosquitoes and transmit malaria parasites
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