Littérature scientifique sur le sujet « D-separation criterion »
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Articles de revues sur le sujet "D-separation criterion"
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Băncioiu, Camil, et Remus Brad. « Analyzing Markov Boundary Discovery Algorithms in Ideal Conditions Using the d-Separation Criterion ». Algorithms 15, no 4 (23 mars 2022) : 105. http://dx.doi.org/10.3390/a15040105.
Texte intégralRésumé :
This article proposes the usage of the d-separation criterion in Markov Boundary Discovery algorithms, instead of or alongside the statistical tests of conditional independence these algorithms usually rely on. This is a methodological improvement applicable when designing, studying or improving such algorithms, but it is not applicable for productive use, because computing the d-separation criterion requires complete knowledge of a Bayesian network. Yet Bayesian networks can be made available to the algorithms when studied in controlled conditions. This approach has the effect of removing sources of suboptimal behavior, allowing the algorithms to perform at their theoretical best and providing insights about their properties. The article also discusses an extension of this approach, namely to use d-separation as a complement to the usual statistical tests performed on synthetic datasets in order to ascertain the overall accuracy of the tests chosen by the algorithms, for further insights into their behavior. To exemplify these two approaches, two Markov Boundary Discovery algorithms were used, namely the Incremental Association Markov Blanket algorithm and the Iterative Parent–Child-Based Search of Markov Blanket algorithm. Firstly, these algorithms were configured to use d-separation alone as their conditional independence test, computed on known Bayesian networks. Subsequently, the algorithms were configured to use the statistical G-test complemented by d-separation to evaluate their behavior on synthetic data.
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Habib, M. A., H. M. Badr, S. A. M. Said, I. Hussaini et J. J. Al-Bagawi. « On the Development of Deadleg Criterion ». Journal of Fluids Engineering 127, no 1 (1 janvier 2005) : 124–35. http://dx.doi.org/10.1115/1.1852481.
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Corrosion in deadlegs occurs as a result of water separation due to the very low flow velocity. This work aims to investigate the effect of geometry and orientation on flow field and oil/water separation in deadlegs in an attempt for the development of a deadleg criterion. The investigation is based on the solution of the mass and momentum conservation equations of an oil/water mixture together with the volume fraction equation for the secondary phase. Results are obtained for two main deadleg orientations and for different lengths of the deadleg in each orientation. The considered fluid mixture contains 90% oil and 10% water (by volume). The deadleg length to diameter ratio (L/D) ranges from 1 to 9. The results show that the size of the stagnant fluid region increases with the increase of L/D. For the case of a vertical deadleg, it is found that the region of the deadleg close to the header is characterized by circulating vortical motions for a length l≈3 D while the remaining part of the deadleg occupied by a stagnant fluid. In the case of a horizontal deadleg, the region of circulating flow extends to 3–5 D. The results also indicated that the water volumetric concentration increases with the increase of L/D and is influenced by the deadleg orientation. The streamline patterns for a number of cases were obtained from flow visualization experiments (using 200 mW Argon laser) with the objective of validating the computational model.
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Neal, R. M. « On Deducing Conditional Independence from d-Separation in Causal Graphs with Feedback (Research Note) ». Journal of Artificial Intelligence Research 12 (1 mars 2000) : 87–91. http://dx.doi.org/10.1613/jair.689.
Texte intégralRésumé :
Pearl and Dechter (1996) claimed that the d-separation criterion for conditional independence in acyclic causal networks also applies to networks of discrete variables that have feedback cycles, provided that the variables of the system are uniquely determined by the random disturbances. I show by example that this is not true in general. Some condition stronger than uniqueness is needed, such as the existence of a causal dynamics guaranteed to lead to the unique solution.
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Richards, K. J., D. A. Smeed, E. J. Hopfinger et G. Chabert D'Hières. « Boundary-layer separation of rotating flows past surface-mounted obstacles ». Journal of Fluid Mechanics 237 (avril 1992) : 343–71. http://dx.doi.org/10.1017/s0022112092003446.
Texte intégralRésumé :
This paper describes laboratory experiments on the flow over a three-dimensional hill in a rotating fluid. The experiments were carried out in towing tanks, placed on rotating tables. Rotation is found to have a strong influence on the separation behind the hill. The topology of the separation is found to be the same for all the flows examined. The Rossby number R in the experiments is of order 1, the maximum value being 6. The separated flow is dominated by a single trailing vortex. In the majority of cases the surface stress field has a single separation line and there are no singular points. In a few experiments at the highest Rossby numbers the observations suggest more complex stress fields but the results are inconclusive.A criterion for flow separation is sought. For values of D/L > 1, where D is the depth of the flow and L the lengthscale of the hill, separation is found to be primarily dependent on R. At sufficiently small values of R separation is suppressed and the flow remains fully attached.Linear theory is found to give a good estimate for the critical value of R for flow separation. For hills with a moderate slope (slope ≤ 1) this critical value is around 1, decreasing with increasing slope. It is postulated that the existence of a single dominant trailing vortex is due to the uplifting and subsequent turning of transverse vorticity generated by surface pressure forces upstream of the separation line.
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Framstad, Nils Chr. « Portfolio Theory forα-Symmetric and Pseudoisotropic Distributions:k-Fund Separation and the CAPM ». Journal of Probability and Statistics 2015 (2015) : 1–11. http://dx.doi.org/10.1155/2015/235452.
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The shifted pseudoisotropic multivariate distributions are shown to satisfy Ross’ stochastic dominance criterion for two-fund monetary separation in the case with risk-free investment opportunity and furthermore to admit the Capital Asset Pricing Model under an embedding inLαcondition if1<α≤2, with the betas given in an explicit form. For theα-symmetric subclass, the market without risk-free investment opportunity admits2d-fund separation ifα=1+1/(2d-1),d∈N, generalizing the classical elliptical cased=1, and we also give the precise number of funds needed, from which it follows that we cannot, except degenerate cases, have a CAPM without risk-free opportunity. For the symmetric stable subclass, the index of stability is only of secondary interest, and several common restrictions in terms of that index can be weakened by replacing it by the (no smaller) indices of symmetry/of embedding. Finally, dynamic models with intermediate consumption inherit the separation properties of the static models.
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Zhang, Jiawei, Yong Gu, Aihua Chen et Yuguo Yu. « Unveiling Dynamic System Strategies for Multisensory Processing : From Neuronal Fixed-Criterion Integration to Population Bayesian Inference ». Research 2022 (20 août 2022) : 1–20. http://dx.doi.org/10.34133/2022/9787040.
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Multisensory processing is of vital importance for survival in the external world. Brain circuits can both integrate and separate visual and vestibular senses to infer self-motion and the motion of other objects. However, it is largely debated how multisensory brain regions process such multisensory information and whether they follow the Bayesian strategy in this process. Here, we combined macaque physiological recordings in the dorsal medial superior temporal area (MST-d) with modeling of synaptically coupled multilayer continuous attractor neural networks (CANNs) to study the underlying neuronal circuit mechanisms. In contrast to previous theoretical studies that focused on unisensory direction preference, our analysis showed that synaptic coupling induced cooperation and competition in the multisensory circuit and caused single MST-d neurons to switch between sensory integration or separation modes based on the fixed-criterion causal strategy, which is determined by the synaptic coupling strength. Furthermore, the prior of sensory reliability was represented by pooling diversified criteria at the MST-d population level, and the Bayesian strategy was achieved in downstream neurons whose causal inference flexibly changed with the prior. The CANN model also showed that synaptic input balance is the dynamic origin of neuronal direction preference formation and further explained the misalignment between direction preference and inference observed in previous studies. This work provides a computational framework for a new brain-inspired algorithm underlying multisensory computation.
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Aghsaee, Payam, Leon Boegman, Peter J. Diamessis et Kevin G. Lamb. « Boundary-layer-separation-driven vortex shedding beneath internal solitary waves of depression ». Journal of Fluid Mechanics 690 (24 novembre 2011) : 321–44. http://dx.doi.org/10.1017/jfm.2011.432.
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AbstractWe investigate global instability and vortex shedding in the separated laminar boundary layer beneath internal solitary waves (ISWs) of depression in a two-layer stratified fluid by performing high-resolution two-dimensional direct numerical simulations. The simulations were conducted with waves propagating over a flat bottom and shoaling over relatively mild $(S= 0. 05)$ and steep $(S= 0. 1)$ slopes. Over a flat bottom, the potential for vortex shedding is shown to be directly dependent on wave amplitude, for a particular stratification, owing to increase of the adverse pressure gradient ($\mathrm{d} P/ \mathrm{d} x\gt 0$ for leftward propagating waves) beneath the trailing edge of larger amplitude waves. The generated eddies can ascend from the bottom boundary to as high as 33 % of the total depth in two-dimensional simulations. Over sloping boundaries, global instability occurs beneath all waves as they steepen. For the slopes considered, vortex shedding begins before wave breaking and the vortices, shed from the bottom boundary, can reach the pycnocline, modifying the wave breaking mechanism. Combining the results over flat and sloping boundaries, a unified criterion for vortex shedding in arbitrary two-layer continuous stratifications is proposed, which depends on the momentum-thickness Reynolds number and the non-dimensionalized ISW-induced pressure gradient at the point of separation. The criterion is generalized to a form that may be readily computed from field data and compared to published laboratory experiments and field observations. During vortex shedding events, the bed shear stress, vertical velocity and near-bed Reynolds stress were elevated, in agreement with laboratory observations during re-suspension events, indicating that boundary layer instability is an important mechanism leading to sediment re-suspension.
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Malamataris, N. A., I. E. Sarris, D. P. Pazis et A. Liakos. « A comprehensive study of the onset of boundary layer separation in the unbounded flow around a circular cylinder ». Physics of Fluids 34, no 10 (octobre 2022) : 103607. http://dx.doi.org/10.1063/5.0101413.
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The onset of boundary layer separation in the unbounded flow around a circular cylinder is determined directly with a numerical experiment with accuracy to the second decimal point of the Reynolds number, Re. The governing equations are the steady state, two dimensional Navier–Stokes equations, which are solved with Galerkin finite elements. A new criterion, the entrance ratio [Formula: see text] with D the diameter of the cylinder and L the entrance length of the domain, is introduced in addition to the traditional blockage ratio; the aim is to establish conditions for unbounded flow in both flow directions. A hypothesis is formulated and verified for the onset of separation leading to the conclusion that this flow undergoes a supercritical pitchfork bifurcation with respect to fixed points on the boundary. Results are presented for various blockage ratios, B, in the range [Formula: see text] and constant entrance ratio [Formula: see text] until asymptotic behavior is observed for the onset of separation in the Reynolds number, in the angle and the length of the smallest eddy. Onset of separation occurs in the region [Formula: see text]. The bifurcation diagram of the flow is calculated. Streamlines of the flow domain before and after the bifurcation point are shown, including images of the smallest possible eddies. In addition, coefficients of base suction and total drag are computed and shown in the regime [Formula: see text]. The results are compared with available laboratory and numerical measurements.
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Cheng, W., D. I. Pullin et R. Samtaney. « Large-eddy simulation of flow over a grooved cylinder up to transcritical Reynolds numbers ». Journal of Fluid Mechanics 835 (27 novembre 2017) : 327–62. http://dx.doi.org/10.1017/jfm.2017.767.
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We report wall-resolved large-eddy simulation (LES) of flow over a grooved cylinder up to the transcritical regime. The stretched-vortex subgrid-scale model is embedded in a general fourth-order finite-difference code discretization on a curvilinear mesh. In the present study $32$ grooves are equally distributed around the circumference of the cylinder, each of sinusoidal shape with height $\unicode[STIX]{x1D716}$, invariant in the spanwise direction. Based on the two parameters, $\unicode[STIX]{x1D716}/D$ and the Reynolds number $Re_{D}=U_{\infty }D/\unicode[STIX]{x1D708}$ where $U_{\infty }$ is the free-stream velocity, $D$ the diameter of the cylinder and $\unicode[STIX]{x1D708}$ the kinematic viscosity, two main sets of simulations are described. The first set varies $\unicode[STIX]{x1D716}/D$ from $0$ to $1/32$ while fixing $Re_{D}=3.9\times 10^{3}$. We study the flow deviation from the smooth-cylinder case, with emphasis on several important statistics such as the length of the mean-flow recirculation bubble $L_{B}$, the pressure coefficient $C_{p}$, the skin-friction coefficient $C_{f\unicode[STIX]{x1D703}}$ and the non-dimensional pressure gradient parameter $\unicode[STIX]{x1D6FD}$. It is found that, with increasing $\unicode[STIX]{x1D716}/D$ at fixed $Re_{D}$, some properties of the mean flow behave somewhat similarly to changes in the smooth-cylinder flow when $Re_{D}$ is increased. This includes shrinking $L_{B}$ and nearly constant minimum pressure coefficient. In contrast, while the non-dimensional pressure gradient parameter $\unicode[STIX]{x1D6FD}$ remains nearly constant for the front part of the smooth cylinder flow, $\unicode[STIX]{x1D6FD}$ shows an oscillatory variation for the grooved-cylinder case. The second main set of LES varies $Re_{D}$ from $3.9\times 10^{3}$ to $6\times 10^{4}$ with fixed $\unicode[STIX]{x1D716}/D=1/32$. It is found that this $Re_{D}$ range spans the subcritical and supercritical regimes and reaches the beginning of the transcritical flow regime. Mean-flow properties are diagnosed and compared with available experimental data including $C_{p}$ and the drag coefficient $C_{D}$. The timewise variation of the lift and drag coefficients are also studied to elucidate the transition among three regimes. Instantaneous images of the surface, skin-friction vector field and also of the three-dimensional Q-criterion field are utilized to further understand the dynamics of the near-surface flow structures and vortex shedding. Comparison of the grooved-cylinder flow with the equivalent flow over a smooth-wall cylinder shows structural similarities but significant differences. Both flows exhibit a clear common signature, which is the formation of mean-flow secondary separation bubbles that transform to other local flow features upstream of the main separation region (prior separation bubbles) as $Re_{D}$ is increased through the respective drag crises. Based on these similarities it is hypothesized that the drag crises known to occur for flow past a cylinder with different surface topographies is the result of a change in the global flow state generated by an interaction of primary flow separation with secondary flow recirculating motions that manifest as a mean-flow secondary bubble. For the smooth-wall flow this is accompanied by local boundary-layer flow transition to turbulence and a strong drag crisis, while for the grooved-cylinder case the flow remains laminar but unsteady through its drag crisis and into the early transcritical flow range.
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Nichol, Christine P., et Basil D. Roufogalis. « Influence of associated lipid on the properties of purified bovine erythrocyte acetylcholinesterase ». Biochemistry and Cell Biology 69, no 2-3 (1 février 1991) : 154–62. http://dx.doi.org/10.1139/o91-023.
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Acetylcholinesterase has been isolated from bovine erythrocyte membranes by affinity chromatography using a m-trimethylammonium ligand. The purified enzyme had hydrophobic properties by the criterion of phase partitioning into Triton X-114. The activity of the hydrophobic enzyme was seen as a slow-moving band in nondenaturing polyacrylamide gels. After treatment with phosphatidylinositol-specific phospholipase C, another form of active enzyme was produced that migrated more rapidly toward the anode in these gels. This form of the enzyme partitioned into the aqueous phase in Triton X-114 phase separation experiments and was therefore hydrophilic. The hydrophobic form bound to concanavalin A in the absence of Triton X-100. As this binding was partially prevented by detergent, but not by α-methyl mannoside, D-glucose, or myo-inositol, it is in part hydrophobic. Erythrocyte cell membranes showed acetylcholinesterase activity present as a major form, which was hydrophobic by Triton X-114 phase separation and in nondenaturing gel electrophoresis moved at the same rate as the purified enzyme. In the membrane the enzyme was more thermostable than when purified in detergent. The hydrophobic enzyme isolated, therefore, represents a native form of the acetylcholinesterase present in the bovine erythrocyte cell membrane, but in isolation its stability becomes dependent on amphiphile concentration. Its hydrophobic properties and lectin binding are attributable to the association with the protein of a lipid with the characteristics of a phosphatidylinositol.Key words: acetylcholinesterase, bovine erythrocytes, phosphatidylinositol-specific phospholipase C, phase separation, affinity chromatography.
Thèses sur le sujet "D-separation criterion"
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CALIGARIS, SILVIA. « A Causal Graphs - based approach for assessing gender disparities : an application to child health & ; nutrition in China ». Doctoral thesis, Università degli Studi di Milano-Bicocca, 2014. http://hdl.handle.net/10281/83241.
Texte intégralRésumé :
Most of gender statistical measures proposed in the last decades are in fact composite indicators,
i.e. weighted linear combinations of basic statistics such as ratios, percentages etc. Composite indicators then
involves several arbitrary choices -for instance the weighting/aggregating system, variables selection,
standardization affecting both indexes transparency and interpretation. Furthermore gender inequality is a
complex latent phenomenon, a collection of disparate and inter-linked issues that can be hardly caught in a
single indicator. The development of statistical tools and ad hoc models is then required.
The aim of this work is to explore the potential of graphical models as a language able to clearly represent the
complex relationships among variables involved in the statistical measuring the gender disparities. In particular
we will focus on causal graphs allowing to deep and interpret the causal mechanism that may originate gender
gaps as well as to explore the effects of gender tailored policies. Causal models indeed provide transparent
mathematical tools to formulate the assumptions underlying all causal inference, to translate them in term of
joint distribution and to read off the conditional independences using the d-separation criterion (Pearl 2000). It
is thus possible deriving causal effects in non-experimental studies, representing policies’ effects and
interventions through the do operator, controlling confounders and interpreting counterfactuals.
We show the potential of such models through an application to real data from China Health and Nutrition
Survey 2011 ; in particular we explore the eventual existence of gender discrimination in children’ nutrition and
health as possible indicator of preference for sons. The analysis takes in exam socio-demographic, economical
as well as biological variables. Resorting to the PC algorithm and the IDA algorithm, we aim to learn the
underlying causal structure and to estimate causal effect of siblings on children’ nutrition from observational
data.
Chapitres de livres sur le sujet "D-separation criterion"
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Hofer-Szabó, Gábor. « Bell’s Local Causality is a d-Separation Criterion ». Dans Springer Proceedings in Mathematics & ; Statistics, 67–82. Singapore : Springer Singapore, 2018. http://dx.doi.org/10.1007/978-981-13-2487-1_2.
Texte intégralActes de conférences sur le sujet "D-separation criterion"
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Rienstra, Tjitze, Matthias Thimm, Kristian Kersting et Xiaoting Shao. « Independence and D-separation in Abstract Argumentation ». Dans 17th International Conference on Principles of Knowledge Representation and Reasoning {KR-2020}. California : International Joint Conferences on Artificial Intelligence Organization, 2020. http://dx.doi.org/10.24963/kr.2020/73.
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We investigate the notion of independence in abstract argumentation, i.e., the question of whether the evaluation of one set of arguments is independent of the evaluation of another set of arguments, given that we already know the status of a third set of arguments. We provide a semantic definition of this notion and develop a method to discover independencies based on transforming an argumentation framework into a DAG on which we then apply the well-known d-separation criterion. We also introduce the SCC Markov property for argumentation semantics, which generalises the Markov property from the classical acyclic case and guarantees the soundness of our approach.
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Kim, Y., Y. J. Chao et P. S. Lam. « Numerical Simulation of Cup-Cone Fracture in a Round Tensile Bar ». Dans ASME 2008 Pressure Vessels and Piping Conference. ASMEDC, 2008. http://dx.doi.org/10.1115/pvp2008-61186.
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Cup and cone fracture for a round tensile bar under tension is analyzed numerically using finite element method. Finite strain analysis with general solid elements is performed to simulate the development of necking, flat cracking at the center of the bar, propagation of the flat crack towards the free surface, and then slant 45° cracking leading to the final separation of the tensile bar. The key fracture stages and parameters in the simulation of this entire process include (a) a fracture criterion for the flat cracking at the center of the bar, (b) flat crack propagation using a constant crack-tip-opening-angle and/or displacement (CTOA/CTOD), (c) a criterion for determining the transition from flat cracking to slant cracking, (d) slant crack propagation using a constant shear CTOA/CTOD. Details of the numerical process are provided.
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Zhu, Haiwen, Jianjun Zhu, Jiecheng Zhang et Hong-Quan Zhang. « Efficiency and Critical Velocity Analysis of Gravitational Separator Through CFD Simulation ». Dans ASME 2017 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2017. http://dx.doi.org/10.1115/imece2017-71230.
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Oil and gas producing rates have increased rapidly with the development of shale oil and fracturing technology. Besides, advances in horizontal wells have increased the slugging issue, especially in complex geometry wells. Therefore, Artificial lift systems, especially rod-pumps and electrical submersible pumps, always suffer from associated gas and require an economical way to avoid problems like gas lock, gas pound, gas interference and slugging. Among all kinds of the downhole separator, and the gravitational separator are the most economical devices, which can handle severe slugging problems. The rule of thumb liquid maximum downward velocity for the gravitational separator is 0.6 in/s [13, 14]. However, the criterion needs to be improved by considering pressure, temperature and fluid properties. This article first uses CFD simulations to validate the critical liquid velocity and then obtains pressure field, velocity profile, gas distribution and sensitivity factors under complicated field situations. The results could be used to develop an empirical or even a mechanistic efficiency prediction model in the future. In this paper, 2-D simulation is first utilized to study the critical separation velocity and effective parameters. Comparing with Stokes’ law, the simulation shows density, and surface tension have a strong effect of critical separation velocity, while viscosity has lower influence. Then extended 2-D simulations are conducted on different inner tube to annulus connection geometry, which shows a strong effect on separation efficiency. Later on, 3-D CFD simulations are generated based on a newly designed separator by TUALP and an existing design from the Don-Nan separator. Simulations are used to validate 2-D conclusion and illustrate the improvement of the new design.
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Cao, Teng, Nagabhushana Rao Vadlamani, Paul G. Tucker, Angus R. Smith, Michal Slaby et Christopher T. J. Sheaf. « Fan-Intake Interaction Under High Incidence ». Dans ASME Turbo Expo 2016 : Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2016. http://dx.doi.org/10.1115/gt2016-56561.
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In this paper, we present an extensive numerical study on the interaction between the downstream fan and the flow separating over an intake under high incidence. The objectives of this investigation are twofold: (a) to gain qualitative insight into the mechanism of fan-intake interaction and (b) to quantitatively examine the sensitivity of the flow distortion (in terms of distortion coefficient DC60), to the key design parameters of the intake (Length, L / Diameter, D). Both steady and unsteady Reynolds Averaged Numerical Simulations (RANS) were carried out. For the steady calculations, a low order fan model has been used while a full 3D geometry has been used for the unsteady RANS. The numerical methodology is also thoroughly validated against the measurements for the intake-only and fan-only configurations on a high bypass ratio turbofan intake and fan respectively. To systematically study the effect of fan on the intake separation and explore the design criteria, a simplified intake-fan configuration has been considered. In this fan-intake model, the ratio of the intake length to diameter (L/D) can be conveniently altered without affecting other parameters. The key results indicate that, depending on L/D, the fan has either suppressed the level of the post separation distortion or increased the separation-free operating range. At the lowest L/D (∼ 0.17), around a 5° increase in the separation-free angle of incidence was achieved. This delay in the separation-free angle of incidence decreased with increasing L/D. At the largest L/D (∼ 0.44), the fan was effective in suppressing the post-separation distortion rather than entirely eliminating the separation. Isentropic Mach number distributions over the intake lip for different L/D’s revealed that the fan accelerates the flow upstream of the fan face, thereby decreasing the distortion level in the immediate vicinity. However, this acceleration effect decayed rapidly with increasing upstream distance from the fan-face.
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Perl, M., Q. Ma et C. Levy. « The Influence of a Non-Aligned Semi-Elliptical Surface Crack on a Quarter-Circle Corner Crack in an Infinitely Large Plate Under Uniaxial Tension ». Dans ASME 2016 Pressure Vessels and Piping Conference. American Society of Mechanical Engineers, 2016. http://dx.doi.org/10.1115/pvp2016-63081.
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The evaluation of non-aligned multiple cracks is required in various fitness-for-service codes. For non-aligned parallel cracks, on-site inspection needs to decide whether the cracks should be treated as coalesced or separate multiple cracks for Fitness-for-Service. In the existing literature, criteria and standards for the adjustment of multiple nonaligned cracks are very source dependent, and those criteria and standards are often derived from on-site service experience without rigorous and systematic verification. Based on this observation, the authors previously reported on the effect between an edge and an embedded parallel crack in 2-D scenarios and, more recently, in 3-D scenarios of circular cracks. Since realistic crack configurations detected using non-destructive methods are generally 3-D in nature, the study of 3-D effect of non-aligned cracks with different shapes is deemed necessary in order to obtain more practical guidance in the usage of rules speculated in Fitness-for-Service codes. In this study, a quarter-circle corner crack is considered to interact with a semi-elliptic non-aligned surface flaw in an infinitely large plate under uniaxial tension. While keeping the geometry of the quarter-circle corner crack unchanged, a pair of horizontal (H) and vertical (S) separation distances between the two cracks is chosen followed by a detailed analysis of the effect of crack shape on the 3D SIFs for various embedded semi-elliptical cracks. The analysis is repeated for various combinations of separation distances S and H. The results from this study are collectively significant to the understanding of the correlation between the criteria and standards in Fitness-for-Service community and the consequence of their usage in engineering practice.
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Levy, C., M. Perl et Q. Ma. « The Effect of a Quarter-Circle Corner Crack on the Distribution of the SIF Along the Front of a Non-Aligned Semi-Elliptical Surface Crack in an Infinitely Large Plate Under Uniaxial Tension ». Dans ASME 2017 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2017. http://dx.doi.org/10.1115/imece2017-72468.
Texte intégralRésumé :
The evaluation of the mutual effect of non-aligned multiple cracks is a prerequisite in applying fitness-for-service codes. For non-aligned parallel cracks, during on-site inspection, one needs to decide whether the cracks should be treated as coalesced or separate multiple cracks for Fitness-for-Service. In the existing literature, criteria and standards for the adjustment of multiple nonaligned cracks are very source dependent, and those criteria and standards are often derived from on-site service experience without rigorous and systematic verification. Based on this observation, the authors previously reported on the influence of an embedded crack on an edge crack in 2-D scenarios and, more recently, in 3-D scenarios of the influence of a surface crack on a quarter-circle corner crack. However, realistic crack configurations detected using non-destructive methods are generally 3-D in nature and their influences are mutual. Thus the SIF distribution characteristics along the surface crack is equally important as the SIF distribution of the corner crack when Fitness-for-Service rules are to be applied. Therefore, non-aligned flaws with different configurations and shapes and the SIFs along their crack fronts are deemed necessary in order to obtain more practical guidance in the usage of rules speculated in Fitness-for-Service codes. In this study, the characteristics of the SIF distribution along a semi-elliptic non-aligned surface crack is examined under the influence of a quarter-circle corner crack of various geometries in an infinitely large plate. For any given geometry of a quarter-circle corner crack, a pair of horizontal (H) and vertical (S) separation distances between the two cracks is chosen followed by a detailed analysis of the effect of the quarter-circle corner crack on the 3D SIFs of the surface crack at different ellipticities. The analysis is repeated for various combinations of separation distances S and H. The results from this study are collectively significant to the understanding of the correlation between the criteria and standards in Fitness-for-Service community and the consequence of their usage in engineering practice.
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Gallus, H. E., et H. Hoenen. « Experimental Investigations of Airfoil- and Endwall Boundary Layers in a Subsonic Compressor Stage ». Dans ASME 1986 International Gas Turbine Conference and Exhibit. American Society of Mechanical Engineers, 1986. http://dx.doi.org/10.1115/86-gt-143.
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Criteria for the maximum diffusion allowable in a blade row without reaching stall play an important part in the design of highly loaded axial-flow compressors. Most of these criteria for maximum blade loading were derived from wind tunnel measurements of 2-d-steady cascade flow. As the flow field in turbomachines is extremely unsteady and of three-dimensional nature the boundary layers are influenced by these effects. The paper deals with the results of boundary layer measurements in a stator blade channel of a subsonic axial-flow compressor stage at various operating points between unthrottled and highly throttled flow (near stall). In front of the stator, the time-averaged velocity profiles as well as the fluctuations due to the unsteady flow field downstream of the rotor were measured. The growing of the separation zones inside the channel with increased blade load is studied in detail. Photos of flow visualization in the boundary layers by dye-injection and flow patterns derived from hot-wire measurements illustrate the physics of boundary layer behavior and separation due to increasing stator blade load. The investigations include measurements of the turbulence energy and a frequency analysis of the velocity fluctuations in the boundary layers.
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Hustad, Carl-W., Inge Trondstad, Roger E. Anderson, Keith L. Pronske et Fermin Viteri. « Optimization of Thermodynamically Efficient Nominal 40 MW Zero Emission Pilot and Demonstration Power Plant in Norway ». Dans ASME Turbo Expo 2005 : Power for Land, Sea, and Air. ASMEDC, 2005. http://dx.doi.org/10.1115/gt2005-68640.
Texte intégralRésumé :
In Aug 2004 the Zero Emission Norwegian Gas (ZENG) project team completed Phase-1: Concept and Feasibility Study for a 40 MW Pilot & Demonstration (P&D) Plant, that is proposed will be located at the Energy Park, Risavika, near Stavanger in South Norway during 2008. The power plant cycle is based upon implementation of the natural gas (NG) and oxygen fueled Gas Generator (GG) (1500°F/1500 psi) successfully demonstrated by Clean Energy Systems (CES) Inc. The GG operations was originally tested in Feb 2003 and is currently (Feb 2005) undergoing extensive commissioning at the CES 5MW Kimberlina Test Plant, near Bakersfield, California. The ZENG P&D Plant will be an important next step in an accelerating path towards demonstrating large-scale (+200 MW) commercial implementation of zero-emission power plants before the end of this decade. However, development work also entails having a detailed commercial understanding of the techno-economic potential for such power plant cycles: specifically in an environment where the future penalty for carbon dioxide (CO2) emissions remains uncertain. Work done in dialogue with suppliers during ZENG Project Phase-1 has cost-estimated all major plant components to a level commensurate with engineering pre-screening. The study has also identified several features of the proposed power plant that has enabled improvements in thermodynamic efficiency from 37% up to present level of 44–46% without compromising the criteria of implementation using “near-term” available technology. The work has investigated: i. Integration between the cryogenic air separation unit (ASU) and the power plant. ii. Use of gas turbine technology for the intermediate pressure (IP) steam turbine. iii. Optimal use of turbo-expanders and heat-exchangers to mitigate the power consumption incurred for oxygen production. iv. Improved condenser design for more efficient CO2 separation and removal. v. Sensitivity of process design criteria to “small” variations in modeling of the physical properties for CO2/steam working fluid near saturation. vi. Use of a second “conventional” pure steam Rankine bottoming cycle. In future analysis, not all these improvements need necessarily be seen to be cost-effective when taking into account total P&D program objectives; thermodynamic efficiency, power plant investment, operations and maintenance cost. However, they do represent important considerations towards “total” optimization when designing the P&D Plant. We observe that Project Phase-2: Pre-Engineering & Qualification should focus on optimization of plant size with respect to total capital investment (CAPEX); and identification of further opportunities for extended technology migration from gas turbine environment that could also permit raised turbine inlet temperatures (TIT).
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Perl, M., C. Levy et Q. Ma. « The Reciprocal Effect Among a Quarter-Circle Corner Crack and a Non-Aligned Surface Crack of Comparable Size in an Infinitely Large Plate Under Uniaxial Tension ». Dans ASME 2018 Pressure Vessels and Piping Conference. American Society of Mechanical Engineers, 2018. http://dx.doi.org/10.1115/pvp2018-84035.
Texte intégralRésumé :
The reciprocal effect between a quarter-circle corner crack and a non-aligned surface crack of comparable size is addressed in the present study. The significance of understanding the reciprocal effect between the non-aligned parallel cracks is to assist in the evaluation of non-aligned multiple cracks as required in various fitness-for-service codes. For non-aligned parallel cracks, on-site inspection needs to decide whether the cracks should be treated as coalesced or separate multiple cracks. In the existing literature, criteria and standards for the adjustment of multiple non-aligned cracks are very source dependent, and those criteria and standards are often derived from on-site service experience without rigorous and systematic verification. Based on this observation, the authors previously reported on the effect of an embedded parallel crack on an edge crack in 2-D scenarios and, more recently, in 3-D scenarios of a circular corner crack influenced by a parallel surface crack. It may be just as important to evaluate the mutual effect of a quarter-circle corner crack on a non-aligned surface crack as reflected in their stress intensity factors (SIFs). In the present study, the quarter-circle corner crack and the non-aligned surface crack are assumed to be of the same length a2 = a1 = 15mm. While keeping throughout the entire analysis the geometry of the quarter-circle corner crack unchanged, the relative depth of the semi-elliptical surface crack is varied so that b1/a1 = 0.2–1.0. For each particular case a pair of horizontal (H) and vertical (S) separation distances between the two cracks is chosen (H/a2 = 0.4–2 and S/a2 = −0.5–2) and the SIFs along the 3D crack fronts are extracted for both the corner and the surface crack. The reciprocal effect on the SIFs for both cracks are discussed. It is found that the mutual influence between the corner crack and the surface crack are equally important, and each may dominate the decision making based on present criteria and standards in Fitness-for-Service.
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Langtry, R. B., F. R. Menter, S. R. Likki, Y. B. Suzen, P. G. Huang et S. Vo¨lker. « A Correlation-Based Transition Model Using Local Variables : Part II — Test Cases and Industrial Applications ». Dans ASME Turbo Expo 2004 : Power for Land, Sea, and Air. ASMEDC, 2004. http://dx.doi.org/10.1115/gt2004-53454.
Texte intégralRésumé :
A new correlation-based transition model has been developed, which is built strictly on local variables. As a result, the transition model is compatible with modern CFD methods using unstructured grids and massive parallel execution. The model is based on two transport equations, one for the intermittency and one for the transition onset criteria in terms of momentum thickness Reynolds number. The proposed transport equations do not attempt to model the physics of the transition process (unlike e.g. turbulence models), but form a framework for the implementation of correlation-based models into general-purpose CFD methods. Part I of this paper gives a detailed description of the mathematical formulation of the model and some of the basic test cases used for model validation. Part II (this part) of the paper details a significant number of test cases that have been used to validate the transition model for turbomachinery and aerodynamic applications, including the drag crisis of a cylinder, separation induced transition on a circular leading edge and natural transition on a wind turbine airfoil. Turbomachinery test cases include a highly loaded compressor cascade, a low-pressure turbine blade, a transonic turbine guide vane, a 3-D annular compressor cascade and unsteady transition due to wake impingement. In addition, predictions are shown for an actual industrial application, namely a GE Low-Pressure turbine vane. In all cases good agreement with the experiments could be achieved and the authors believe that the current model is a significant step forward in engineering transition modeling.