7,057 research outputs found
CFD applications in chemical propulsion engines
The present research is aimed at developing analytical procedures for predicting the performance and stability characteristics of chemical propulsion engines. Specific emphasis is being placed on understanding the physical and chemical processes in the small engines that are used for applications such as spacecraft attitude control and drag make-up. The small thrust sizes of these engines lead to low nozzle Reynolds numbers with thick boundary layers which may even meet at the nozzle centerline. For this reason, the classical high Reynolds number procedures that are commonly used in the industry are inaccurate and of questionable utility for design. A complete analysis capability for the combined viscous and inviscid regions as well as for the subsonic, transonic, and supersonic portions of the flowfield is necessary to estimate performance levels and to enable tradeoff studies during design procedures
Derivative Computations and Robust Standard Errors for Linear Mixed Effects Models in lme4
While robust standard errors and related facilities are available in R for
many types of statistical models, the facilities are notably lacking for models
estimated via lme4. This is because the necessary statistical output, including
the Hessian and casewise gradient of random effect parameters, is not
immediately available from lme4 and is not trivial to obtain. In this article,
we supply and describe two new functions to obtain this output from Gaussian
mixed models: estfun.lmerMod() and vcov.full.lmerMod(). We discuss the
theoretical results implemented in the code, focusing on calculation of robust
standard errors via package sandwich. We also use the Sleepstudy data to
illustrate the code and compare it to a benchmark from package lavaan.Comment: Accepted at Journal of Statistical Softwar
An overview of the Penn State Propulsion Engineering Research Center
An overview of the Penn State Propulsion Engineering Research Center is presented. The following subject areas are covered: research objectives and long term perspective of the Center; current status and operational philosophy; and brief description of Center projects (combustion, fluid mechanics and heat transfer, materials compatibility, turbomachinery, and advanced propulsion concepts)
A Generic Framework for Engineering Graph Canonization Algorithms
The state-of-the-art tools for practical graph canonization are all based on
the individualization-refinement paradigm, and their difference is primarily in
the choice of heuristics they include and in the actual tool implementation. It
is thus not possible to make a direct comparison of how individual algorithmic
ideas affect the performance on different graph classes.
We present an algorithmic software framework that facilitates implementation
of heuristics as independent extensions to a common core algorithm. It
therefore becomes easy to perform a detailed comparison of the performance and
behaviour of different algorithmic ideas. Implementations are provided of a
range of algorithms for tree traversal, target cell selection, and node
invariant, including choices from the literature and new variations. The
framework readily supports extraction and visualization of detailed data from
separate algorithm executions for subsequent analysis and development of new
heuristics.
Using collections of different graph classes we investigate the effect of
varying the selections of heuristics, often revealing exactly which individual
algorithmic choice is responsible for particularly good or bad performance. On
several benchmark collections, including a newly proposed class of difficult
instances, we additionally find that our implementation performs better than
the current state-of-the-art tools
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