11,991 research outputs found
Two-dimensional numerical simulation of a Stirling engine heat exchanger
The first phase of an effort to develop multidimensional models of Stirling engine components is described; the ultimate goal is to model an entire engine working space. More specifically, parallel plate and tubular heat exchanger models with emphasis on the central part of the channel (i.e., ignoring hydrodynamic and thermal end effects) are described. The model assumes: laminar, incompressible flow with constant thermophysical properties. In addition, a constant axial temperature gradient is imposed. The governing equations, describing the model, were solved using Crank-Nicloson finite-difference scheme. Model predictions were compared with analytical solutions for oscillating/reversing flow and heat transfer in order to check numerical accuracy. Excellent agreement was obtained for the model predictions with analytical solutions available for both flow in circular tubes and between parallel plates. Also the heat transfer computational results are in good agreement with the heat transfer analytical results for parallel plates
Heat transfer in oscillating flows with sudden change in cross section
Oscillating fluid flow (zero mean) with heat transfer, between two parallel plates with a sudden change in cross section, was examined computationally. The flow was assumed to be laminar and incompressible with inflow velocity uniform over the channel cross section but varying sinusoidally with time. Over 30 different cases were examined; these cases cover wide ranges of Re sub max (187.5 to 30000), Va (1 to 350), expansion ratio (1:2, 1:4, 1:8, and 1:12) and A sub r (0.68 to 4). Three different geometric cases were considered (asymmetric expansion and/or contraction, symmetric expansion/contraction, and symmetric blunt body). The heat transfer cases were based on constant wall temperature at higher (heating) or lower (cooling) value than the inflow fluid temperature. As a result of the oscillating flow, the fluid undergoes sudden expansion in one half of the cycle and sudden contraction in the other half. One heating case is examined in detail, and conclusions are drawn from all the cases (documented in detail elsewhere). Instantaneous friction factors and heat transfer coefficients, for some ranges of Re sub max and Va, deviated substantially from those predicted with steady state correlations
Large evolution of the bilinear Higgs coupling parameter in SUSY models and reduction of phase sensitivity
The phases in a generic low-energy supersymmetric model are severely
constrained by the experimental upper bounds on the electric dipole moments of
the electron and the neutron. Coupled with the requirement of radiative
electroweak symmetry breaking, this results in a large degree of fine tuning of
the phase parameters at the unification scale. In supergravity type models,
this corresponds to very highly tuned values for the phases of the bilinear
Higgs coupling parameter and the universal trilinear coupling . We
identify a cancellation/enhancement mechanism associated with the
renormalization group evolution of , which, in turn, reduces such
fine-tuning quite appreciably without taking recourse to very large masses for
the supersymmetric partners. We find a significant amount of reduction of this
fine-tuning in nonuniversal gaugino mass models that do not introduce any new
phases.Comment: Version to appear in Phys.Rev.D. Insignificant changes like a few
typos corrected. 26 pages, 7 figures, LaTe
CP-odd Phase Correlations and Electric Dipole Moments
We revisit the constraints imposed by electric dipole moments (EDMs) of
nucleons and heavy atoms on new CP-violating sources within supersymmetric
theories. We point out that certain two-loop renormalization group corrections
induce significant mixing between the basis-invariant CP-odd phases. In the
framework of the constrained minimal supersymmetric standard model (CMSSM), the
CP-odd invariant related to the soft trilinear A-phase at the GUT scale,
theta_A, induces non-trivial and distinct CP-odd phases for the three gaugino
masses at the weak scale. The latter give one-loop contributions to EDMs
enhanced by tan beta, and can provide the dominant contribution to the electron
EDM induced by theta_A. We perform a detailed analysis of the EDM constraints
within the CMSSM, exhibiting the reach, in terms of sparticle spectra, which
may be obtained assuming generic phases, as well as the limits on the CP-odd
phases for some specific parameter points where detailed phenomenological
studies are available. We also illustrate how this reach will expand with
results from the next generation of experiments which are currently in
development.Comment: 31 pages, 21 eps figures; v2: additional remarks on 2-loop threshold
corrections and references added; v3: typos corrected, to appear in Phys.
Rev.
Inferring gene regulatory networks from gene expression data by a dynamic Bayesian network-based model
Enabled by recent advances in bioinformatics, the inference of gene regulatory networks (GRNs) from gene expression data has garnered much interest from researchers. This is due to the need of researchers to understand the dynamic behavior and uncover the vast information lay hidden within the networks. In this regard, dynamic Bayesian network (DBN) is extensively used to infer GRNs due to its ability to handle time-series microarray data and modeling feedback loops. However, the efficiency of DBN in inferring GRNs is often hampered by missing values in expression data, and excessive computation time due to the large search space whereby DBN treats all genes as potential regulators for a target gene. In this paper, we proposed a DBN-based model with missing values imputation to improve inference efficiency, and potential regulators detection which aims to lessen computation time by limiting potential regulators based on expression changes. The performance of the proposed model is assessed by using time-series expression data of yeast cell cycle. The experimental results showed reduced computation time and improved efficiency in detecting gene-gene relationships
Inferring gene regulatory networks from gene expression data by a dynamic Bayesian network-based model
Enabled by recent advances in bioinformatics, the inference of gene regulatory networks (GRNs) from gene expression data has garnered much interest from researchers. This is due to the need of researchers to understand the dynamic behavior and uncover the vast information lay hidden within the networks. In this regard, dynamic Bayesian network (DBN) is extensively used to infer GRNs due to its ability to handle time-series microarray data and modeling feedback loops. However, the efficiency of DBN in inferring GRNs is often hampered by missing values in expression data, and excessive computation time due to the large search space whereby DBN treats all genes as potential regulators for a target gene. In this paper, we proposed a DBN-based model with missing values imputation to improve inference efficiency, and potential regulators detection which aims to lessen computation time by limiting potential regulators based on expression changes. The performance of the proposed model is assessed by using time-series expression data of yeast cell cycle. The experimental results showed reduced computation time and improved efficiency in detecting gene-gene relationships
A new Fe substituted arsenotungstate [Fe 2(AsWO)(AsOH)]: Synthesis, structure, characterization and magnetic properties
The iron(III)-containing arsenotungstate [Fe 2(AsWO)(AsOH)] (1) was prepared via a simple, one-pot reaction in aqueous basic medium. The compound was isolated as its sodium salt, and structurally-characterized by Single Crystal X-ray Diffraction (SCXRD), Powder X-ray Diffraction (PXRD), Fourier-Transform Infrared (FT-IR) spectroscopy, Thermogravimetric Analysis (TGA) and elemental analysis. Its magnetic properties are reported; the antiferromagnetic coupling between the two FeIII centers is unusually weak as a result of the bridging geometry imposed by the rigid arsenotungstate metalloligands
The Electric Dipole Moment and CP Violation in in SUGRA Models with Nonuniversal Gaugino Masses
The constraints of electric dipole moments (EDMs) of electron and neutron on
the parameter space in supergravity (SUGRA) models with nonuniversal gaugino
masses are analyzed. It is shown that with a light sparticle spectrum, the
sufficient cancellations in the calculation of EDMs can happen for all phases
being order of one in the small tan case and all phases but
() order of one in the large tan case. This is
in contrast to the case of mSUGRA in which in the parameter space where
cancellations among various SUSY contributions to EDMs happen
must be less than for small and for
large . Direct CP asymmetries and the T-odd polarization of lepton in
are investigated in the models. In the large tan case,
and for l= () can be enhanced by about a factor of
ten (ten) and ten (three) respectively compared to those of mSUGRA.Comment: 12 pages, latex, 4 figures, a few change
Is simulation the only keystone of surgical training?
No abstract availabl
Describing Function Inversion: Theory and Computational Techniques
In the last few years the study of nonlinear mechanics has received the attention of numerous investigators, either under the scope of pure mathematics or from the engineering point of view. Many of the recent developments are based on the early works of H. Poincare [1] and A. Liapunov [2] As examples can be cited the perturbation method, harmonic balance, the second method of Liapunov, etc. An approximate technique developed almost simultaneously by C. Goldfarb [3] in the USSR, A. Tustin [4] in England, R. Kochenburger [5] in the USA, W, Oppelt [6] in Germany and J. Dutilh [7] and C. Ecary [8] in France, known as the describing function technique, can be considered as the graphical solution of the first approximation of the method of the harmonic balance. The describing function technique has reached great popularity, principally because of the relative ease of computation involved and the general usefulness of the method in engineering problems. However, in the past, the describing function technique has been useful only in analysis. More exactly, it is a powerful tool for the investigation of the possible existence of limit cycles and their approximate amplitudes and frequencies. Several extensions have been developed from the original describing function technique. Among these can be cited the dual-input describing function, J. C. Douce et al. [9]; the Gaussian-input describing function, R. C, Booton [10]; and the root-mean-square describing function, J. E. Gibson and K. S. Prasanna-Kumar [11]. In a recent work which employs the describing function, C. M„ Shen [12] gives one example of stabilization of a nonlinear system by introducing a saturable feedback. However, Shen’s work cannot be qualified as a synthesis method since he fixes a priori the nonlinearity to be introduced in the feedback loop. A refinement of the same principle used by Shen has been proposed by R. Haussler [13] The goal of this new method of synthesis is to find the describing function of the element being synthesized. Therefore, for Haussler’s method to be useful, a way must be found to reconstruct the nonlinearity from its describing function. This is called the inverse-describing-function-problem and is essentially a synthesis problem. This is not the only ease in which the inverse-describing-function-problem can be useful. Sometimes, in order to find the input-output characteristic of a physical nonlinear element, a harmonic test can be easier to perform rather than a static one (which also may be insufficient). The purpose of this report is to present the results of research on a question which may then be concisely stated as; If the describing function of a nonlinear element is known, what is the nonlinearity? The question may be divided into two parts, the first part being the determination of the restrictions on the nonlinearity (or its describing function) necessary to insure that the question has an answer, and the second part the practical determination of that answer when it exists. Accordingly, the material in this report is presented in two parts. Part I is concerned with determining what types of nonlinearities are (and what types are not) uniquely determined by their conventional (fundamental) describing function. This is done by first showing the non-uniqueness in general of the describing function, and then constructing a class of null functions with respect to the describing function integral, i.e., a class of nonlinearities not identically zero whose describing functions are identically zero. The defining equations of the describing function are transformed in such a manner as to reduce the inverse describing function problem to the problem of solving a Volterra integral equation, an approach similar to that used by Zadeh [18]. The remainder of Part I presents the solution of the integral equations and studies the effect of including higher order harmonics in the description of the output ware shape. The point of interest here is that inclusion of the second harmonic may cause the describing function to become uniquely invertible in some cases. Part II presents practical numerical techniques for effecting the inversion of types of describing functions resulting from various engineering assumptions as to the probable form of the nonlinearities from which said describing functions were determined. The most general method is numerical evaluation of the solution to the Volterra integral equations developed in Part I, A second method, which is perhaps the easiest to apply, requires a least squares curve fit to the given describing function data. Then use is made of the fact that the describing function of a polynomial nonlinearity is itself a polynomial to calculate the coefficients in a polynomial approximation to the nonlinearity. This approach is indicated when one expects that the nonlinearity is a smooth curve, such as a cubic characteristic. The third method presented assumes that the nonlinearity can be approximated by a piecewise linear discontinuous function, and the slopes and y-axis intercepts of each linear segment are computed. This approach is indicated when one expects a nonlinearity with relatively sharp corners. It may toe remarked that the polynomial approximation and the piecewise linear approximation are derived independently of the material in Part I. All three methods presented in Part II are suited for use with experimental data as well as with analytic expressions for the describing functions involved. Indeed, an analytical expression must toe reduced to discrete data for the machine methods to the of use. To the best of the authors® knowledge, research in the area of describing function inversion has been nonexistent with the exception of Zadeh’s paper [18] in 1956. It seems that a larger effort in this area would toe desirable in the light of recent extensions of the describing function itself to signal stabilization of nonlinear control systems by Oldentourger and Sridhar [19] and Boyer [20] and the less restrictive study of dual-input describing functions for nonautonomous systems by Gibson and Sridhar [21]. There presently exist techniques for determining a desired describing function for use in avoiding limit cycle oscillations in an already nonlinear system (Haussler [13]), and the methods presented in this report now allow the exact synthesis of the nonlinear element from the describing function data
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