bluuebunny/crossref_metadata_2025 huggingface dataset:
Photon Beam Diffusion: A Hybrid Monte Carlo Method for Subsurface Scattering
Authors: Habel Ralf , Christensen Per H. , Jarosz Wojciech
Published: July 2013

We present photon beam diffusion, an efficient numerical method for accurately rendering translucent materials. Our approach interprets incident light as a continuous beam of photons inside the material. Numerically integrating diffusion from such extended sources has long been assumed computationally prohibitive, leading to the ubiquitous single‐depth dipole approximation and the recent analytic sum‐of‐Gaussians approach employed by Quantized Diffusion. In this paper, we show that numerical integration of the extended beam is not only feasible, but provides increased speed, flexibility, numerical stability, and ease of implementation, while retaining the benefits of previous approaches. We leverage the improved diffusion model, but propose an efficient and numerically stable Monte Carlo integration scheme that gives equivalent results using only 3–5 samples instead of 20–60 Gaussians as in previous work. Our method can account for finite and multi‐layer materials, and additionally supports directional incident effects at surfaces. We also propose a novel diffuse exact single‐scattering term which can be integrated in tandem with the multi‐scattering approximation. Our numerical approach furthermore allows us to easily correct inaccuracies of the diffusion model and even combine it with more general Monte Carlo rendering algorithms. We provide practical details necessary for efficient implementation, and demonstrate the versatility of our technique by incorporating it on top of several rendering algorithms in both research and production rendering systems.
Probabilistic Visibility Evaluation for Direct Illumination
Authors: Billen Niels , Engelen Björn , Lagae Ares , Dutré Philip
Published: July 2013

The efficient evaluation of visibility in a three‐dimensional scene is a longstanding problem in computer graphics. Visibility evaluations come in many different forms: figuring out what object is visible in a pixel; determining whether a point is visible to a light source; or evaluating the mutual visibility between 2 surface points. This paper provides a new, experimental view on visibility, based on a probabilistic evaluation of the visibility function. Instead of checking the visibility against all possible intervening geometry the visibility between 2 points is now evaluated by testing only a random subset of objects. The result is not a Boolean value that is either 0 or 1, but a numerical value that can even be negative. Because we use the visibility evaluation as part of the integrand in illumination computations, the probabilistic evaluation of visibility becomes part of the Monte Carlo procedure of estimating the illumination integral, and results in an unbiased computation of illumination values in the scene. Moreover, the number of intersections tests for any given ray is decreased, since only a random selection of geometric primitives is tested. Although probabilistic visibility is an experimental and new idea, we present a practical algorithm for direct illumination that uses the probabilistic nature of visibility evaluations.
An Area‐Preserving Parametrization for Spherical Rectangles
Authors: Ureña Carlos , Fajardo Marcos , King Alan
Published: July 2013

We present an area‐preserving parametrization for spherical rectangles which is an analytical function with domain in the unit rectangle [0, 1]2 and range in a region included in the unit‐radius sphere. The parametrization preserves areas up to a constant factor and is thus very useful in the context of rendering as it allows to map random sample point sets in [0, 1]2 onto the spherical rectangle. This allows for easily incorporating stratified, quasi‐Monte Carlo or other sampling strategies in algorithms that compute scattering from planar rectangular emitters.
A Topological Approach to Voxelization
Authors: Laine Samuli
Published: July 2013

We present a novel approach to voxelization, based on intersecting the input primitives against intersection targets in the voxel grid. Instead of relying on geometric proximity measures, our approach is topological in nature, i.e., it builds on the connectivity and separability properties of the input and the intersection targets. We discuss voxelization of curves and surfaces in both 2D and 3D, and derive intersection targets that produce voxelizations with various connectivity, separability and thinness properties. The simplicity of our method allows for easy proofs of these properties. Our approach is directly applicable to curved primitives, and it is independent of input tessellation.
Line‐Sweep Ambient Obscurance
Authors: Timonen Ville
Published: July 2013

Screen‐space ambient occlusion and obscurance have become established methods for rendering global illumination effects in real‐time applications. While they have seen a steady line of refinements, their computational complexity has remained largely unchanged and either undersampling artefacts or too high render times limit their scalability. In this paper we show how the fundamentally quadratic per‐pixel complexity of previous work can be reduced to a linear complexity. We solve obscurance in discrete azimuthal directions by performing line sweeps across the depth buffer in each direction. Our method builds upon the insight that scene points along each line can be incrementally inserted into a data structure such that querying for the largest occluder among the visited samples along the line can be achieved at an amortized constant cost. The obscurance radius therefore has no impact on the execution time and our method produces accurate results with smooth occlusion gradients in a few milliseconds per frame on commodity hardware.
‘Super‐eruptions’ and silicic volcanism from the Yellowstone volcanic field
Authors: Ellis Ben S. , Mark Darren F.
Published: July 2013

Yellowstone is perhaps the world's most famous ‘super‐volcano’ with an explosive history stretching back more than 2 million years and abundant contemporary evidence of a voluminous magmatic system. The first explosive eruption at Yellowstone was the largest, the Huckleberry Ridge Tuff with a volume estimated at 2500 km3 of deposit. However, recent work using high‐precision geochronology has suggested that this ‘poster‐child’ of super‐eruptions is actually two distinct events separated by at least six thousand years. The geochronological data indicating differences between the constituent parts of the Huckleberry Ridge Tuff is supported by textural, geochemical and isotopic evidence from numerous studies. Advances in both technology and approaches in the sphere of geochronology are allowing for ever more closely spaced events to be temporally resolved; allowing re‐investigation of deposits considered to represent ‘super‐eruptions’ at much higher resolution. The appreciation that the Huckleberry Ridge Tuff represents two distinct events illustrates that these large, regionally catastrophic events from Yellowstone occurred more frequently than previously thought. Moreover, by being able to better constrain the intervals between super‐eruptions we can investigate the timescales of magma generation during quiescent periods.
Workplace representatives: strategic actors of union renewal?
Authors: Murray Gregor , Lévesque Christian , Dufour Christian , Hege Adelheid
Published: July 2013

Workplace representatives (shop stewards) provide insight into union transformations. This article explores the renewed research interest in terms of the representativeness of unionism and of workplace representatives, the complexity of the sites of representation and employer strategies, the search for new references and the centrality of workplace representatives in union renewal strategies.
A synthesis of research on training of union delegates
Authors: Peetz David , Alexander Michael
Published: July 2013

Through several surveys, we find that union training and education is important to boost delegates' confidence and activism. Quality, breadth and coverage all matter. Union training should be conceptualised broadly and include what happens outside and after the classroom, both formally and informally. The results raise important practical challenges including structural questions for unions.
Empowerment and union workplace delegates: a gendered analysis
Authors: Le Capitaine Catherine , Murray Gregor , Lévesque Christian
Published: July 2013

This article examines the empowerment of female and male workplace union delegates in the education sector in Quebec (Canada). Key power resources and strategic capabilities are significant variables for both sexes, highlighting commonalities in delegate empowerment. Other differences point to the gendered construction of delegate empowerment, with important consequences for union renewal strategies.
‘A mysterious malady of the times’
Authors: De Cauwer, Stijn
Published: July 2013

Eric Santner has described a peculiar agitation that increasingly haunted people in the wake of the shift in political theology after the French Revolution. The ‘substance’ that previously gave the sovereign his or her splendour was turned into an immanent pressure: an irritation of the flesh. Early in his life, Robert Musil visited a psychiatric hospital in Rome. He was struck by the physical agitation of the patients and sensed a similarity between their spasms and the general state of society. He repeatedly compared his times to a madhouse. This agitation was increased by the prevailing moral rigidity. Moreover, both Ulrich, the protagonist of Der Mann ohne Eigenschaften, and Musil himself display a remarkable incapacity to adopt a social position in society. Der Mann ohne Eigenschaften is a novel that takes up the task to find a way to cope with the agitation of the flesh, as Santner described it. Musil called the problems of his time ‘symptoms’ and with his novel he wanted to find a way to work through these symptoms. Musil's work is thus a rich exploration of the phenomena Santner has described.
Did Jesus Discover Forgiveness?
Authors: Bash Anthony
Published: July 2013

This essay explores Hannah Arendt's claim that Jesus was the “discoverer” of forgiveness. It assesses Charles Griswold's view that person‐to‐person forgiveness is in evidence in Greek culture and practice before Jesus. The essay refines Griswold's view and suggests that person‐to‐person forgiveness is a cultural universal. The essay makes observations about the significance of the different words that denote person‐to‐person forgiveness; it also explores the implications of reading the New Testament writings on person‐to‐person forgiveness in the chronological order in which they were written. From a close reading of the early New Testament documents, the essay makes two suggestions about the Western tradition of forgiveness. First, it suggests that Paul the apostle is the first to identify person‐to‐person forgiveness as a moral virtue. Second, it suggests that in the Synoptic tradition, Jesus is the first to identify person‐to‐person forgiveness as a discrete category of behavior distinct, for example, from pardoning, excusing, waiving, or ignoring the wrongs of others.
The Justice of Forgiveness
Authors: Philpott Daniel
Published: July 2013

Over the past generation, forgiveness has entered the political sphere in countries all over the globe that are addressing the past injustices of war, dictatorship, genocide, and the maltreatment of native peoples. Among the international community, however, the practice is controversial, criticized as unjust for burdening victims and foregoing deserved punishment. This essay argues that forgiveness is not contrary to justice but rather reflective of it if justice means restoration of right relationship, a concept embedded in the scriptures and traditions of Judaism, Christianity, and Islam. Conceived of in this way, forgiveness can avoid the potential injustices with which it is charged and contribute positively to the reconstruction of political orders.
Forgiveness in The Arab and Islamic Contexts
Authors: Abu‐Nimer Mohammed , Nasser Ilham
Published: July 2013

This essay explores the current and historical meaning of forgiveness in Arab and Islamic cultural and religious contexts. It also hopes to encourage further empirical research on this understudied topic in both religious and peacebuilding studies. In addition to the perceived meaning of forgiveness in an Arab Islamic context, this essay examines the links between forgiveness and reconciliation. Relying on religious sources including the Qur'an and Hadith, as well as certain events in Islamic history, the essay identifies various ways to conceptualize and explain the meaning of forgiveness. This theoretical and conceptual segment is followed by a section which explores current perceptions of forgiveness among Arab Muslim teachers in five different communities. The empirical data for this analysis is based on a larger comparative regional study that has been completed through surveys and structured interviews with educators from Lebanon, Jordan, Egypt, and Palestinians from the West Bank and Israel. Our study concluded that the teachers' perceptions of forgiveness were mainly derived from religious sources and identities and that Islamic religious discourse provided a solid foundation for framing the meaning of forgiveness.
Moral Landscapes and Ethical Vistas
Authors: Zehr Nahed Artoul
Published: July 2013

This book discussion demonstrates how the War on Terror—now spanning two American administrations—has affected the role and authority of historical American traditions on the justice of war. It highlights the continuing importance of four principles rooted in American moral and ethical traditions: legitimate authority; guidelines on preemption; the protection of civilians; and the importance of democratic discourse in the deliberation of American conduct.
Do Term Limits Restrain State Fiscal Policy? Approaches for Causal Inference in Assessing the Effects of Legislative Institutions
Authors: Keele Luke , Malhotra Neil , McCubbins Colin H.
Published: July 2013

Scholars of state politics are often interested in the causal effects of legislative institutions on policy outcomes. For example, during the 1990s a number of states adopted term limits for state legislators. Advocates of term limits argued that this institutional reform would alter state policy in a number of ways, including limiting state expenditures. We highlight a number of research design issues that complicate attempts to estimate the effect of institutions on state outcomes by addressing the question of term limits and spending. In particular, we focus on (1) treatment effect heterogeneity and (2) the suitability of nonterm‐limit states as good counterfactuals for term‐limit states. We compare two different identification strategies to deal with these issues: differences‐in‐differences (DID) estimation and conditioning on prior outcomes with an emphasis on synthetic case control. Using more rigorous methods of causal inference, we find little evidence that term limits affect state spending. Our analysis and results are informative for researchers seeking to assess the causal effects of state‐level institutions.
Limiting Party Representation: Evidence from a Small Parliamentary Chamber
Authors: Sharman Campbell
Published: July 2013

Parties are seen as vital for the maintenance of parliamentary government and as necessary intermediaries between voters and legislators; an elected parliamentary chamber not controlled by parties is highly anomalous. This study contrasts the party‐controlled Tasmanian lower house with its Independent‐dominated elected upper house and finds that the major source of constraints on party representation is not a clientelistic style of politics but the persistence of a distinctive institutional design and electoral rules based on fixed terms and annual staggered elections. The consequences of these rules are explored for their effects on voter choice and legislative behaviour.
Voting from the Pew: The Effect of Senators' Religious Identities on Partisan Polarization in the U.S. Senate
Authors: McTague John , Pearson‐Merkowitz Shanna
Published: July 2013

Partisan polarization in the Senate is in part a product of the increased sorting of evangelical Christians into the Republican caucus. The relationship between senators' religious identities, party affiliation, and ideology has changed since the 1970s. Whereas congressional party caucuses in the past were more diverse in their religious composition, evangelical Christian senators have sorted themselves into the party that most closely resembles the values of their religious identities, leading to greater overall polarization.
L'Artiste comme ready‐made
Authors: Pedersen Christian Bank
Published: July 2013

Cet article traite de « l'Œuvre de Beck », « Das Beckwerk », et de ses incursions artistiques et politiques dans la littérature danoise de ces dernières années. En prenant son point de départ dans la réflexion sur l'identité proposée par « Das Beckwerk », l'article étudie plus spécifiquement la question politique et esthétique que pose L'Action suicidaire. Le récit de la tentative d'introduire la Démocratie en Irak, l'an 2004 : quel est l'état de la démocratie aujourd'hui ? Cette question est discutée en relation avec l'histoire de l'art et de la littérature aux XIXe et XXe siècles.
Phytogenic biosynthesis and emission of methyl acetate
Authors: JARDINE KOLBY , WEGENER FREDERIK , ABRELL LEIF , VAN HAREN JOOST , WERNER CHRISTIANE
Published: July 2013

Acetylation of plant metabolites fundamentally changes their volatility, solubility and activity as semiochemicals. Here we present a new technique termed dynamic 13C‐pulse chasing to track the fate of C1–3 carbon atoms of pyruvate into the biosynthesis and emission of methyl acetate (MA) and CO2. 13C‐labelling of MA and CO2 branch emissions respond within minutes to changes in 13C‐positionally labelled pyruvate solutions fed through the transpiration stream. Strong 13C‐labelling of MA emissions occurred only under pyruvate‐2‐13C and pyruvate‐2,3‐13C feeding, but not pyruvate‐1‐13C feeding. In contrast, strong 13CO2 emissions were only observed under pyruvate‐1‐13C feeding. These results demonstrate that MA (and other volatile and non‐volatile metabolites) derive from the C2,3 atoms of pyruvate while the C1 atom undergoes decarboxylation. The latter is a non‐mitochondrial source of CO2 in the light generally not considered in studies of CO2 sources and sinks. Within a tropical rainforest mesocosm, we also observed atmospheric concentrations of MA up to 0.6 ppbv that tracked light and temperature conditions. Moreover, signals partially attributed to MA were observed in ambient air within and above a tropical rainforest in the Amazon. Our study highlights the potential importance of acetyl coenzyme A (CoA) biosynthesis as a source of acetate esters and CO2 to the atmosphere.
Variation in the carbon and oxygen isotope composition of plant biomass and its relationship to water‐use efficiency at the leaf‐ and ecosystem‐scales in a northern Great Plains grassland
Authors: FLANAGAN LAWRENCE B. , FARQUHAR GRAHAM D.
Published: July 2013

Measurements of the carbon (δ13Cm) and oxygen (δ18Om) isotope composition of C3 plant tissue provide important insights into controls on water‐use efficiency. We investigated the causes of seasonal and inter‐annual variability in water‐use efficiency in a grassland near Lethbridge, Canada using stable isotope (leaf‐scale) and eddy covariance measurements (ecosystem‐scale). The positive relationship between δ13Cm and δ18Om values for samples collected during 1998–2001 indicated that variation in stomatal conductance and water stress‐induced changes in the degree of stomatal limitation of net photosynthesis were the major controls on variation in δ13Cm and biomass production during this time. By comparison, the lack of a significant relationship between δ13Cm and δ18Om values during 2002, 2003 and 2006 demonstrated that water stress was not a significant limitation on photosynthesis and biomass production in these years. Water‐use efficiency was higher in 2000 than 1999, consistent with expectations because of greater stomatal limitation of photosynthesis and lower leaf ci/ca during the drier conditions of 2000. Calculated values of leaf‐scale water‐use efficiency were 2–3 times higher than ecosystem‐scale water‐use efficiency, a difference that was likely due to carbon lost in root respiration and water lost during soil evaporation that was not accounted for by the stable isotope measurements.
Efficiencies for Partially Wetted Spine Fins: Uniform Cross Section, Conical, Concave Parabolic, and Convex Parabolic Spines
Authors: Pirompugd Worachest , Wongwises Somchai
Published: July 2013

In this study, efficiencies for partially wetted fins for the uniform cross section spine, conical spine, concave parabolic spine, and convex parabolic spine are presented using an analytical method. Depending on the set of boundary conditions, there are two methods for deriving the efficiencies of partially wet fins for each spine. The eight equations for fin efficiencies were investigated. Fin efficiency is a function of the length of the dry portion. Thus, the equations for calculating the length of the dry portion are also presented. The findings indicate that a larger cross-sectional fin results in a higher conduction heat transfer rate. Contrarily, the fin efficiency is lower. This is different from the longitudinal fin, for which the trend lines of heat transfer rate and fin efficiency are the same. This converse relationship is due to the effect of the ratio of the cross-sectional area to the surface area. Moreover, partially wet fin efficiencies decrease with increased relative humidity. For convenience, the approximate equation for efficiencies for partially wet fins, which is derived from the equations for fully wet and fully dry fin efficiencies, is also presented.
Turbine Inflow Characterization at the National Wind Technology
Center
Authors: Clifton Andrew , Schreck Scott , Scott George , Kelley Neil , Lundquist Julie K.
Published: July 2013

Utility-scale wind turbines operate in dynamic flows that can vary significantly over time scales from less than a second to several years. To better understand the inflow to utility-scale turbines on time scales from seconds to minutes, the National Renewable Energy Laboratory installed and commissioned two inflow measurement towers at the National Wind Technology Center near Boulder, Colorado, in 2011. These towers are 135 m tall and instrumented with sonic anemometers, cup anemometers, wind vanes, and temperature measurements to characterize the inflow wind speed and direction, turbulence, stability and thermal stratification for two utility-scale turbines. In this paper, we present variations in mean and turbulent wind parameters with height, atmospheric stability, and as a function of wind direction that could be important for turbine operation, and for the persistence of turbine wakes. Wind speed, turbulence intensity, and dissipation are all factors that affect turbine performance. Our results show that these all vary with height across the rotor disk, demonstrating the importance of measuring atmospheric conditions that influence wind turbine performance at multiple heights in the rotor disk, rather than relying on extrapolation from conditions measured at lower levels.
Workspace Analysis of a Novel Six-Degrees-of-Freedom Parallel Manipulator With Coaxial Actuated Arms
Authors: Isaksson Mats , Watson Matthew
Published: July 2013

Parallel manipulators possess several advantages compared to serial robots, including the possibilities for high acceleration and high accuracy positioning of the manipulated platform. However, the majority of all proposed parallel mechanisms suffer from the combined drawbacks of a small positional workspace in relation to the manipulator footprint and a limited range of rotations of the manipulated platform. This paper analyses a recently proposed six-degrees-of-freedom parallel mechanism that aims to address both these issues while maintaining the traditional advantages of a parallel mechanism. The investigated manipulator consists of six actuated coaxial upper arms that are allowed to rotate indefinitely around a central cylindrical base column and a manipulated platform where five of the six joint positions are collinear. The axis-symmetric arm system leads to an extensive positional workspace while the proposed link arrangement increases the range of achievable platform rotations. The manipulator workspace is analyzed in detail and two methods to further increase the rotational workspace are presented. It is shown that the proposed manipulator has the possibility of a nonsingular transition of assembly modes, which extends the usable workspace. Furthermore, it is demonstrated how an additional kinematic chain can be utilized to achieve infinite platform rotation around one platform axis. By introducing additional mobility in the manipulated platform, a redundantly actuated mechanism is avoided.
The Use of Generalized Laguerre Polynomials in Spectral Methods for Solving Fractional Delay Differential Equations
Authors: Khader M. M.
Published: July 2013

In this paper, an efficient numerical method for solving the fractional delay differential equations (FDDEs) is considered. The fractional derivative is described in the Caputo sense. The proposed method is based on the derived approximate formula of the Laguerre polynomials. The properties of Laguerre polynomials are utilized to reduce FDDEs to a linear or nonlinear system of algebraic equations. Special attention is given to study the error and the convergence analysis of the proposed method. Several numerical examples are provided to confirm that the proposed method is in excellent agreement with the exact solution.
Development of Computational Models for Evaluation of Mechanical and Hemodynamic Behavior of an Intravascular Stent
Authors: Lee Kuang-Huei , Hsiao Hao-Ming , Liao Ying-Chih , Chiu Yi-Hsiang , Tee Yu-Seng
Published: July 2013

Coronary artery stenting has emerged as the gold standard for the treatment of cardiovascular diseases. With this revolutionary technology, stents today have extended their indications from coronary artery diseases to many other applications including carotid, renal, and femoral arteries. A stent is a small, coiled wire-mesh tube that can be deployed into a narrowed artery and expanded by a catheter during angioplasty. Inserting such a foreign body into human is likely to induce adverse biological reactions such as restenosis. It is believed that the low wall shear stress (WSS) region on the stented arterial wall is related to restenosis [1–2].
Blood Cell Adhesion on a Polymeric Heart Valve Leaflet Processed Using Magnetic Abrasive Finishing
Authors: Boggs Taylor , Yamaguchi Hitomi , Tran-Son-Tay Roger , DeGroff Curt , Al-Mousily Faris
Published: July 2013

Heart valve prosthetics replace damaged, malfunctioning valves to improve a patient’s quality of life. Current mechanical valves are durable but suffer from thrombogenicity and flow separation, and can cause blood damage leading to coagulation. While bioprosthetic valves have better haemodynamic function than mechanical valves, the valves suffer from tears due to inflammation and collagen degradation. The absence of living tissue leaves them unable to repair themselves and their antigenicity must be masked. Complications due to thrombosis occur between 1.5% and 3% per year for current mechanical and bioprosthetic valves [1]. Polymeric valves have the potential to exhibit improved haemodynamic performance over mechanical valves without the complications associated with bioprosthetic valves; current issues associated with polymeric valves include calcification, hydrolysis, and durability [2].
Portable Plasma Medical Device for Infection Treatment and Wound Healing
Authors: Thiyagarajan Magesh
Published: July 2013

The purpose of this study was to determine the effects of plasma treatment on bacteria in liquid phases. We predict that the plasma gas can penetrate the liquid culture media and plasma treatment will efficiently kill the bacteria at unique time and distance parameters. It is also hypothesized that less stringent plasma treatment will negatively affect the growth rate of some species of bacteria and possibly their pathogenicity. The bacteria were exposed to hot and cold plasma at various time lengths and distance parameters. Our results indicated that 2 minutes of hot plasma treatment with the plasma torch 5 cm away from the liquid culture is effective in killing/sterilizing cultures of S. aureus, S. pyogenes, Salmonella spp, N. meningitidis, and E. coli. Five minutes of cold plasma with the probe immersed 1–2 cm inside the liquid culture were needed to kill the bacteria. The portable nonthermal plasma system can be used for infection treatment and wound healing applications affected by the microbes studied in this research [1–4].
Speed Line Computation of a Transonic Compressor Stage With Unsteady CFD Methods
Authors: Biesinger Thomas , Cornelius Christian , Nürnberger Dirk , Rube Christoph
Published: July 2013

The flow through a transonic compressor stage is dominated by unsteady effects such as shock propagation and wake shedding. An accurate prediction of the performance of a compressor, i.e. operating range and efficiency, may require the modeling of unsteady effects. Steady CFD methods cease to converge too early when the stall limit is approached. Efficient unsteady CFD methods such as the transient time-inclining (TI) method and the perturbation based non-linear harmonic (NLH) method perform better and are becoming increasingly popular in the industry. Both methods consider the actual blade count ratio for each passage while using a single passage model.

The main objective of this paper is to explain these methods and benchmark their performance with respect to reliable near stall predictions. Computed compressor characteristics and blade row interaction effects of the Purdue Transonic Research Compressor are compared to measurement data. The stator row is found to be limited at the casing in all of the unsteady simulation results. This effect is also qualitatively predicted by steady results calculated at a lower back pressure level. The NLH method is significantly faster than the other transient methods and the TI method resolves more flow detail on identical meshes.
Study on the Impact of Fouling on Axial Compressor Stage
Authors: Chen Shaowen , Zhang Chen , Shi Hui , Wang Songtao , Wang Zhongqi
Published: July 2013

Mechanistic research on the fouling of the compressor is necessary to delay the deterioration caused by fouling during long-term operation, and to explore methods that will lower compressor component deterioration, thereby improving the overall performance.

The effects of fouling on the performance of an axial compressor stage were investigated numerically. As a representative of the realistic compressor stages, the NASA Stage 35 was considered to perform a numerical investigation by means of a commercial computational fluid dynamic code. The numerical model was validated by comparing with the experimental data available from literatures. The computed performance maps and exit parameter distributions showed a good agreement with experimental data. The model was then used to simulate the effect of fouling on compressor stage by various fouling configurations including added thickness and surface roughness levels. The mechanism of the compressor deterioration due to fouling was discussed in detail. As a result, despite the contribution of added thickness on the work capacity, it substantially narrowed the table operating ranges substantially, causing a greater effect on the overall compressor performance. The influence of roughness applied in the rotor is similar to that in the whole stage, including the drop in mass flow rate at choked and near stall point, pressure ratio, and efficiency, whereas, compressor performance slightly decreases in the stator. When the surface roughness is equal to 50 μm, the drop in mass flow rate under a low Reynolds number is less than that under normal conditions, with little influence on the stable operating range.
Numerical Investigation of the Effect of Jet-Flaps in a Transonic Turbine Cascade With Subsonic Operating Conditions
Authors: Chen Ping-Ping , Qiao Wei-Yang , Wang Pu-Wei
Published: July 2013

The objective of this article is to analyze numerically the effect of jet-flap on transonic high-pressure turbine cascades, which is one of usable flow control methods for a variable-geometry turbine (VGT). This paper is mainly on the discussion of the effect of different jet-flap design parameters with subsonic operating conditions: jet mass flow rate, jet slot location, jet blowing direction and jet slot width. With two-dimensional steady simulations, the jet-flap changes the cascade turning, the passage throat velocity distribution and throat area effectively, which will then contribute to a different turbine performance. Using a Cm = 2% pressure side (PS) jet-flap with 1.025mm slot width and counter-axial (-x) jet blowing direction, the incoming mass flow rate is diminished by 16.7%, the turning angle augmented by 3.7 degrees, but with an extended total-pressure loss coefficient from 0.05 (no jets) to 0.09.
Implementation and Assessment of an Advanced Plenum Boundary Condition for Turbine Design Calculations
Authors: Jefferson-Loveday Richard , Tucker Paul , Northall John
Published: July 2013

In an effort to provide predictions for hot annulus ingestion flows across turbine rim-seals and shroud cavity interfaces, this paper describes a new plenum boundary condition based on characteristic boundary methods that has been formulated and implemented into the Rolls-Royce in-house CFD solver HYDRA. It is assessed for a rim-seal on a 1.5 stage research turbine at two different operating points and seal mass flow rates. For shroud cavities it is tested on a recent engine geometry design at realistic operating conditions. Downstream total pressure loss profiles show a significant improvement for the rim seal and encouraging improvement for the shroud cavity geometry when compared with the original HYDRA boundary conditions. The improvement is assessed through examination of flow fields at the cavity/mainstream interface. The observed flows show improved agreement with the fully meshed cavity test-cases.
The Design Space Boundaries for High Flow Capacity Centrifugal Compressors
Authors: Rusch Daniel , Casey Michael
Published: July 2013

A methodology has been derived allowing a fast preliminary assessment of the design of centrifugal compressors specified for high specific swallowing capacity. The method is based on one-dimensional (1D) design point values using classical turbomachinery analysis to determine the inlet geometry for the maximum mass flow function. The key results are then expressed in a series of diagrams which draw out the nature of the conflicting boundary conditions of the design. In particular it is shown how the inlet casing relative Mach number causes the design flow coefficient to decrease with the total pressure ratio and determines the inlet eye diameter. Physically-based boundaries of operation are added to the diagrams giving guidelines for the proper choice of specification values to the designer. In addition, links are given to some well-known impeller efficiency correlations, so that a preliminary estimate of the performance can be made. Comparisons are made with a range of compressor data which supports the approach. The derived methodology allows any given specifications to be checked rapidly for feasibility and development risk or can be used to define a challenging specification for the design of a new product.
Performance Uncertainty Quantification for Centrifugal Compressors: Part 2—Flange to Flange Variability
Authors: Sankaran Sivasubramaniyan , Sassanelli Giuseppe , Iurisci Giuseppe , Panizza Andrea
Published: July 2013

This paper presents a methodology to control flange to flange performance prediction of centrifugal compressors using a probabilistic approach. In order to have reliable prediction for the performance of centrifugal compressors, a thorough knowledge of critical parameters contributing to the deviation and an efficient way to control the variation of these parameters becomes necessary. This paper discusses about a robust methodology for identifying and controlling the variation of these parameters and hence the predicted performance. This probabilistic technique involves a Design of Experiments (DoE) study to handle large number of input parameters, sensitivity study to identify critical ones and a Monte-Carlo based approach to identify the uncertainty in flange to flange performance. This approach takes into consideration the compressor stage performance variability driven by impeller manufacturing tolerances, statoric component losses variability and leakages variability in order to compute overall performance variation in a compressor. An in-house developed probabilistic optimization code (PEZ) is interfaced with a well-validated & calibrated thermodynamic tool to analyse large sets of possible combinations and to provide best possible solution for a given design space. This concept is successfully applied for different compressor configurations by varying the stage numbers and process conditions. The results give an insight on the main sources and magnitude of variations on compressor performance, thus enabling to control the predictions in an efficient way.

This methodology will provide a novel and an efficient way to generate robust compressor performance, where it will be possible to take into account design and manufacturing uncertainty. The use of this methodology can thus drastically improve the performance predictability and risk associated with each compressor selection.
Numerical Study of Reynolds Number Effects of Cavity Volumes and Seal Gaps on LP Turbine Performance
Authors: Reichstein Georg A. , Rose Martin G. , Staudacher Stephan , Engel Karl
Published: July 2013

A two-stage turbine is tested in cooperation between the Institute of Aircraft Propulsion Systems (ILA) and MTU Aero Engines GmbH (MTU). The experimental results taken in the Altitude Test Facility (ATF) are used to assess the quality of the numerical simulation with regard to cavity size and seal gap height. The analysis focuses on a range of small Reynolds numbers, from as low as 35,000 up to 88,000. Circumferentially averaged five-hole-probe area traverse data is compared to steady multistage CFD predictions. Previous analysis showed the simulation with cavities to be superior to the approach without cavities. For most of the Reynolds lapse numerically changing the cavity volume is of no significance for the prediction of the main flow. Only at the smallest Reynolds number these trends diverge. Numerically changing the seal gap height forces the prediction closer to the experimental data on global values. At the smallest Reynolds number the improvements from changing the gap height cease to exist.
Blade Loading Effects on Axial Turbine Tip Leakage Vortex Dynamics and Loss
Authors: Huang A. C. , Greitzer E. M. , Tan C. S. , Clemens E. F. , Gegg S. G. , Turner E. R.
Published: July 2013

Numerical simulations have been carried out to define the loss generation mechanisms associated with tip leakage in un-shrouded axial turbines. Tip clearance vortex dynamics are a dominant feature of two mechanisms important in determining this loss: (i) decreased swirl velocity due to vortex line contraction in regions of decreasing axial velocity, i.e., adverse pressure gradient and (ii) vortex breakdown and reverse flow in the vortex core. The mixing losses behave differently from the conventional view of flow exiting a turbine tip clearance. More specifically, it is shown, through both control volume arguments and computations, that as a swirling leakage flow passes through a pressure rise, such as in the aft portion of the suction side of a turbine blade, the mixed-out loss can either decrease or increase. For turbines the latter typically occurs if the deceleration is large enough to initiate vortex breakdown, and it is demonstrated that this is the case in modern turbines. The effect of blade pressure distribution on clearance losses is illustrated through computational examination of two turbine blades, one with forward loading at the tip and one with aft loading. A 15% difference in leakage loss is found between the two, due to lower clearance vortex deceleration (lower core static pressure rise) with forward loading, and hence lower vortex breakdown loss. Additional computational experiments, carried out to define the effects of blade loading, incidence, and solidity, are found to be consistent with the proposed ideas linking blade pressure distribution, vortex breakdown and turbine tip leakage loss.
Experimental Study of Surge Precursors in a High-Speed Multistage Compressor
Authors: Courtiade Nicolas , Ottavy Xavier
Published: July 2013

Pressure measurements using high frequency response sensors have been carried out on the 3rd rotor of the 3 1/2-stage high speed compressor CREATE (rotation speed: 11543 RPM, Rotor 1 tip speed: 313 m/s), over the complete characteristic line and during the surge transient. It seems that precursors to the instabilities occurring near surge are observable at stable operating points. Just before the surge, these precursors characterized as rotating disturbances transition smoothly to part span rotating stall cells, and bring the machine to surge as they grow and reach a critical size: this paper presents a detailed analysis of the phenomena of rotating stall and surge transient and shows that it is possible to develop an anti-surge active control system based on the early detection of the precursors.
Effects of Blade Damage on the Performance of a Transonic Axial Compressor Rotor
Authors: Li Yanling , Sayma Abdulnaser
Published: July 2013

Gas turbine axial compressor blades may encounter damage during service for various reasons. Debris from casing or foreign objects may impact blades causing damage near the rotor’s tip. This may result in deterioration of performance and reduction in the surge margin. Ability to assess the effect of damaged blades on the compressor performance and stability is important at both the design stage and in service. The damage to compressor blades breaks the cyclic symmetry of the compressor assembly. Thus computations have to be performed using the whole annulus. Moreover, if rotating stall or surge occurs, the downstream boundary conditions are not known and simulations become difficult.

This paper presents an unsteady CFD analysis of compressor performance with tip curl damage. Tip curl damage typically occurs when rotor blades hit a loose casing liner. The computations were performed up to the stall boundary, predicting rotating stall patterns. The aim is to assess the effect of blade damage on stall margin and provide better understanding of the flow behaviour during rotating stall. Computations for the undamaged rotor are also performed for comparison. A transonic axial compressor rotor is used for the time-accurate numerical unsteady flow simulations, with a variable choked nozzle downstream simulating an experimental throttle. One damaged blade was introduced in the rotor assembly and computations were performed at 60% of the design rotational speed. It was found that there is no significant effect on the compressor stall margin due to one damaged blade despite the differences in rotating stall patterns between the undamaged and damaged assemblies.
The Potential of Rotor and Stator Clocking in a 2.5-Stage Low-Speed Axial Compressor
Authors: Städing J. , Friedrichs J. , Waitz T. , Dobriloff C. , Becker B. , Gümmer V.
Published: July 2013

Detailed experimental investigations have been conducted to gain profound knowledge of airfoil clocking mechanisms in axial compressors. Clocking, the circumferential indexing of adjacent rotor or stator rows with equal blade counts, is known as a potential means to modify the flow field in multistage turbo-machinery and increase overall efficiencies of both turbines and compressors. These beneficial effects on turbomachine performance are due to wake-airfoil interactions and primarily depend on the alignment of a downstream blade or vane row with upstream wake trajectories that are generated in the same frame of reference. The present survey describes and discusses the experimental research on Rotor and Stator Clocking effects in a low-speed 2.5-stage axial flow compressor. For both Rotor and Stator Clocking, variations of Stage 2 performance have been found that are sinusoidal in trend over the clocking angle and originate from a significant change in static pressure rise across the clocked blade rows. Time-averaged measurements basically suggest the highest pressure gain, if the upstream wakes pass through mid-passage of the downstream blade row. In case of Rotor Clocking, this may even lead to a variation in compressor operating range. The fundamental aerodynamic mechanism responsible for the clocking effect can be attributed to a shift of the suction-sided boundary layer transition over the clocking angle. Regarding overall Stage 2 performance, the investigations show that Full Clocking, i.e. the combination of Rotor and Stator Clocking, nearly doubles the potential of single row indexing.
The Influence of Metamodeling Techniques on the Multidisciplinary Design Optimization of a Radial Compressor Impeller
Authors: Chahine Christopher , Seume Joerg R. , Verstraete Tom
Published: July 2013

Aerodynamic turbomachinery component design is a very complex task. Although modern CFD solvers allow for a detailed investigation of the flow, the interaction of design changes and the three dimensional flow field are highly complex and difficult to understand. Thus, very often a trial and error approach is applied and a design heavily relies on the experience of the designer and empirical correlations. Moreover, the simultaneous satisfaction of aerodynamic and mechanical requirements leads very often to tedious iterations between the different disciplines. Modern optimization algorithms can support the designer in finding high performing designs. However, many optimization methods require performance evaluations of a large number of different geometries. In the context of turbomachinery design, this often involves computationally expensive Computational Fluid Dynamics and Computational Structural Mechanics calculations. Thus, in order to reduce the total computational time, optimization algorithms are often coupled with approximation techniques often referred to as metamodels in the literature. Metamodels approximate the performance of a design at a very low computational cost and thus allow a time efficient automatic optimization. However, from the experiences gained in past optimizations it can be deduced that metamodel predictions are often not reliable and can even result in designs which are violating the imposed constraints. In the present work, the impact of the inaccuracy of a metamodel on the design optimization of a radial compressor impeller is investigated and it is shown if an optimization without the usage of a metamodel delivers better results. A multidisciplinary, multiobjective optimization system based on a Differential Evolution algorithm is applied which was developed at the von Karman Institute for Fluid Dynamics. The results show that the metamodel can be used efficiently to explore the design space at a low computational cost and to guide the search towards a global optimum. However, better performing designs can be found when excluding the metamodel from the optimization. Though, completely avoiding the metamodel results in a very high computational cost. Based on the obtained results in present work, a method is proposed which combines the advantages of both approaches, by first using the metamodel as a rapid exploration tool and then switching to the accurate optimization without metamodel for further exploitation of the design space.