34,016 research outputs found
On Validating an Astrophysical Simulation Code
We present a case study of validating an astrophysical simulation code. Our
study focuses on validating FLASH, a parallel, adaptive-mesh hydrodynamics code
for studying the compressible, reactive flows found in many astrophysical
environments. We describe the astrophysics problems of interest and the
challenges associated with simulating these problems. We describe methodology
and discuss solutions to difficulties encountered in verification and
validation. We describe verification tests regularly administered to the code,
present the results of new verification tests, and outline a method for testing
general equations of state. We present the results of two validation tests in
which we compared simulations to experimental data. The first is of a
laser-driven shock propagating through a multi-layer target, a configuration
subject to both Rayleigh-Taylor and Richtmyer-Meshkov instabilities. The second
test is a classic Rayleigh-Taylor instability, where a heavy fluid is supported
against the force of gravity by a light fluid. Our simulations of the
multi-layer target experiments showed good agreement with the experimental
results, but our simulations of the Rayleigh-Taylor instability did not agree
well with the experimental results. We discuss our findings and present results
of additional simulations undertaken to further investigate the Rayleigh-Taylor
instability.Comment: 76 pages, 26 figures (3 color), Accepted for publication in the ApJ
Integrating Symbolic and Neural Processing in a Self-Organizing Architechture for Pattern Recognition and Prediction
British Petroleum (89A-1204); Defense Advanced Research Projects Agency (N00014-92-J-4015); National Science Foundation (IRI-90-00530); Office of Naval Research (N00014-91-J-4100); Air Force Office of Scientific Research (F49620-92-J-0225
The STRESS Method for Boundary-point Performance Analysis of End-to-end Multicast Timer-Suppression Mechanisms
Evaluation of Internet protocols usually uses random scenarios or scenarios
based on designers' intuition. Such approach may be useful for average-case
analysis but does not cover boundary-point (worst or best-case) scenarios. To
synthesize boundary-point scenarios a more systematic approach is needed.In
this paper, we present a method for automatic synthesis of worst and best case
scenarios for protocol boundary-point evaluation.
Our method uses a fault-oriented test generation (FOTG) algorithm for
searching the protocol and system state space to synthesize these scenarios.
The algorithm is based on a global finite state machine (FSM) model. We extend
the algorithm with timing semantics to handle end-to-end delays and address
performance criteria. We introduce the notion of a virtual LAN to represent
delays of the underlying multicast distribution tree. The algorithms used in
our method utilize implicit backward search using branch and bound techniques
and start from given target events. This aims to reduce the search complexity
drastically. As a case study, we use our method to evaluate variants of the
timer suppression mechanism, used in various multicast protocols, with respect
to two performance criteria: overhead of response messages and response time.
Simulation results for reliable multicast protocols show that our method
provides a scalable way for synthesizing worst-case scenarios automatically.
Results obtained using stress scenarios differ dramatically from those obtained
through average-case analyses. We hope for our method to serve as a model for
applying systematic scenario generation to other multicast protocols.Comment: 24 pages, 10 figures, IEEE/ACM Transactions on Networking (ToN) [To
appear
Fully leakage-resilient signatures revisited: Graceful degradation, noisy leakage, and construction in the bounded-retrieval model
We construct new leakage-resilient signature schemes. Our schemes remain unforgeable against an adversary leaking arbitrary (yet bounded) information on the entire state of the signer (sometimes known as fully leakage resilience), including the random coin tosses of the signing algorithm. The main feature of our constructions is that they offer a graceful degradation of security in situations where standard existential unforgeability is impossible
History and development of validation with the ESP-r simulation program
It is well recognised that validation of dynamic building simulation programs is a long-term complex task. There have been many large national and international efforts that have led to a well-established validation methodology comprising analytical, inter-program comparison and empirical validation elements, and a significant number of tests have been developed. As simulation usage increases, driven by such initiatives as the European Energy Performance of Buildings Directive, such tests are starting to be incorporated into national and international standards. Although many program developers have run many of the developed tests, there does not appear to have been a systematic attempt to incorporate such tests into routine operation of the simulation programs. This paper reports work undertaken to address this deficiency. The paper summarizes the tests that have been applied to the simulation program ESP-r. These tests have been developed within International Energy Agency Annexes, within CEN standards, within various large-scale national projects, and by the UK's Chartered Institution of Building Services Engineers. The structure used to encapsulate the tests allows developers to ensure that recent code modifications have not resulted in unforeseen impacts on program predictions, and allows users to check for themselves against benchmarks
Monolithic simulation of convection-coupled phase-change - verification and reproducibility
Phase interfaces in melting and solidification processes are strongly
affected by the presence of convection in the liquid. One way of modeling their
transient evolution is to couple an incompressible flow model to an energy
balance in enthalpy formulation. Two strong nonlinearities arise, which account
for the viscosity variation between phases and the latent heat of fusion at the
phase interface.
The resulting coupled system of PDE's can be solved by a single-domain
semi-phase-field, variable viscosity, finite element method with monolithic
system coupling and global Newton linearization. A robust computational model
for realistic phase-change regimes furthermore requires a flexible
implementation based on sophisticated mesh adaptivity. In this article, we
present first steps towards implementing such a computational model into a
simulation tool which we call Phaseflow.
Phaseflow utilizes the finite element software FEniCS, which includes a
dual-weighted residual method for goal-oriented adaptive mesh refinement.
Phaseflow is an open-source, dimension-independent implementation that, upon an
appropriate parameter choice, reduces to classical benchmark situations
including the lid-driven cavity and the Stefan problem. We present and discuss
numerical results for these, an octadecane PCM convection-coupled melting
benchmark, and a preliminary 3D convection-coupled melting example,
demonstrating the flexible implementation. Though being preliminary, the latter
is, to our knowledge, the first published 3D result for this method. In our
work, we especially emphasize reproducibility and provide an easy-to-use
portable software container using Docker.Comment: 20 pages, 8 figure
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