Computational Economics Research Articles

Assessment of a film-cooling flow structure by large-eddy simulation

Large-eddy simulation results using the approximate-deconvolution subgrid-scale model (ADM) are reported for a jet-in-crossflow configuration related to the film-cooling of turbine blades. Here, relatively cool air is ejected from a large isobaric plenum through an oblique round nozzle into a hotter turbulent boundary layer. The parameters are chosen in accordance with typical film-cooling conditions of turbines. For computational economy, a steady mean-turbulent inflow profile is used for the crossflow. The adequacy of this simplification is evaluated. The properties of the mean flow field are investigated and the mean film-cooling efficiency is compared to results from the literature. Special attention is paid to the flow structure inside the jet nozzle, which is considered to have a key influence on the mixing process. The physical understanding of film cooling is deepened with an illustration of the instantaneous flow field and its connection to the findings for the mean flow.

Large‐Eddy simulation of film cooling: treatment of inlet boundary conditions for the isobaric plenum and instantaneous vortex structures

AbstractWe present large‐eddy simulation (LES) results of a jet‐in‐crossflow configuration that was previously investigated in the experiments of Sinha et al. (J. Turbomach., Vol. 113, 1991). In this film‐cooling setup, relatively cool air is ejected from a large isobaric plenum through an oblique round nozzle into a hotter turbulent boundary layer. In accordance with typical film‐cooling conditions, the jet‐to‐crossflow velocity and temperature ratios were chosen as 0.25 and 0.5, respectively (defined with bulk quantities for the jet and freestream quantities for the boundary layer). The numerical simulations are performed with the compressible finite‐volume flow solver for structured multi‐block grids NSMB. The approximate‐deconvolution subgrid‐scale model (ADM) accounts for the non‐resolved scales. The problem of the unknown plenum pressure is handled by a combination of a sponge region with a non‐reflecting boundary condition at the plenum inlet. For computational economy, the turbulent boundary layer of the experiment is replaced by a steady mean turbulent inflow profile. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

An enthalpy-based hybrid lattice-Boltzmann method for modelling solid–liquid phase transition in the presence of convective transport

An extended lattice Boltzmann model is developed for simulating the convection–diffusion phenomena associated with solid–liquid phase transition processes. Macroscopic hydrodynamic variables are obtained through the solution of an evolution equation of a single-particle density distribution function, whereas, the macroscopic temperature field is obtained by solving auxiliary scalar transport equations. The novelty of the present methodology lies in the formulation of an enthalpy-based approach for phase-change modelling within a lattice-Boltzmann framework, in a thermodynamically consistent manner. Thermofluidic aspects of phase transition are handled by means of a modified enthalpy–porosity formulation, in conjunction with an appropriate enthalpy-updating closure scheme. Lattice-Boltzmann simulations of melting of pure gallium in a rectangular enclosure, Rayleigh–Bénard convection in the presence of directional solidification in a top-cooled cavity, and crystal growth during solidification of an undercooled melt agree well with the numerical and experimental results available in the literature, and provide substantial evidence regarding the upscaled computational economy provided by the present methodology.

Pricing for Utility-Driven Resource Management and Allocation in Clusters

Users perceive varying levels of utility for each different job completed by the cluster. Therefore, there is a need for existing cluster resource management systems (RMS) to provide a means for the user to express its perceived utility during job submission. The cluster RMS can then obtain and consider these user-centric needs such as Quality-of-Service requirements in order to achieve utility-driven resource management and allocation. We advocate the use of computational economy for this purpose. In this paper, we describe an architectural framework for a utility-driven cluster RMS. We present a user-level job submission specification for soliciting user-centric information that is used by the cluster RMS for making better resource allocation decisions. In addition, we propose a dynamic pricing function that the cluster owner can use to determine the level of sharing within a cluster. Finally, we define two user-centric performance evaluation metrics: Job QoS Satisfaction and Cluster Profitability for measuring the effectiveness of the proposed pricing function in realizing utility-driven resource management and allocation.

A New Surface Model for β-Plane Turbulence

Abstract This paper introduces a new numerical model for studying wave–turbulence interactions in a continuously stratified rotating flow, having a uniform potential vorticity and a rigid boundary. The meridional variation in the Coriolis parameter (β effect), a channel geometry, and the first-order nonlinear terms in a small Rossby number expansion are included into the surface quasigeostrophic dynamics. The model contains important dynamical characteristics of three-dimensional flows such as advection by ageostrophic winds, and stretching and tilting of relative vorticity. Nevertheless, it has the computational economy of two-dimensional flows. Long-term direct numerical simulations are performed for decaying turbulence arising from random initial conditions. In addition to the formation of steady zonal jets, frequently reported as a possible end state under the β effect, this model exhibits several other realistic physical effects that were lacking in the previously studied β-plane models for wave–turbulence interactions. There is a significant contrast in the spatial and time scales of the formed eddies, resulting in high meridional asymmetry of the flow. Mean surface cooling and persistent large-scale blocking eddies are observed as well. The surface potential temperature variance spectrum exhibits a well-resolved k−5/3 inertial range.

Computation of Thermal Condition in an Induction Motor during Star-Delta Starting

The problem of two-dimensional heat flow in the stator of induction motor is solved by use of a finite element formulation and arch-shaped elements in the r-θ plane of the cylindrical co-ordinate system. The shape functions and exact solution matrices are derived algebraically for utmost economy in computation. The temperature distribution has been determined considering convection from the stator frame and cylindrical air-gap surface. Finally, the temperatures obtained by this two-dimensional approximation have been compared for different stator currents considering the time required for each stator current during the transient in star delta starting.

A new model for effects of impersistent joint sets on rock slope stability

Joints often have profound effects on elastic properties and strength of rock masses and therefore on rock slope stability. In surface mine slopes, joints are usually too numerous to be taken into account individually, so an equivalent properties approach is necessary. Previous work that treats a jointed rock mass as an equivalent composite material of joints and intact rock between resulted in excellent agreement between estimated and true rock mass elastic moduli in case of fully persistent joints, for example, [Pariseau WG. An equivalent plasticity theory for jointed rock masses. Int J Rock Mech Min Sci 1999;36(7):907–18]. Rock mass failure mechanisms on joints and through intact rock were retained using influence functions obviating the need for an equivalent rock mass strength. Impersistent joints, which are not continuous on a joint plane, can be accommodated [Pariseau WG. Effects of joint persistence on jointed rock masses. Trans Soc Min Metal Explor 2003;314:121–8]. However, slope stability analysis of a large, deep copper mine indicated a need for computational efficiency even in two-dimensional analysis of vertical sections through pit walls [Puri S. Role of joint persistence in rock slope stability. MS thesis, Department of Mining Engineering, University of Utah, 2006]. The analyses by the popular finite element method used small elements of bench size near the pit slopes and much larger elements away from the pit walls. The reasons for the graded meshes were numerical accuracy (small elements) and computational economy (large elements). Small elements contained only a few joints, while the larger elements contained hundreds of joints and led to impractical computation run times, on the order of days. A new modeling procedure that recognizes sufficiently large elements as representative volume elements (RVEs) assists in overcoming this obstacle. A companion improvement embeds a sufficient number of joints in RVE-size elements that then allows for recovery of joint failure mechanisms that are lost with the RVE designation. These innovations reduced PC (Pentium-4) run times by two orders of magnitude to a few hours.

A case for cooperative and incentive-based federation of distributed clusters

Research interest in Grid computing has grown significantly over the past five years. Management of distributed resources is one of the key issues in Grid computing. Central to management of resources is the effectiveness of resource allocation as it determines the overall utility of the system. The current approaches to brokering in a Grid environment are non-coordinated since application-level schedulers or brokers make scheduling decisions independently of the others in the system. Clearly, this can exacerbate the load sharing and utilization problems of distributed resources due to sub-optimal schedules that are likely to occur. To overcome these limitations, we propose a mechanism for coordinated sharing of distributed clusters based on computational economy. The resulting environment, called Grid-Federation, allows the transparent use of resources from the federation when local resources are insufficient to meet its users’ requirements. The use of computational economy methodology in coordinating resource allocation not only facilitates the Quality of Service (QoS)-based scheduling, but also enhances utility delivered by resources. We show by simulation, while some users that are local to popular resources can experience higher cost and/or longer delays, the overall users’ QoS demands across the federation are better met. Also, the federation’s average case message-passing complexity is seen to be scalable, though some jobs in the system may lead to large numbers of messages before being scheduled.

Finite-Element Simulation of Ductile Fracture in Reduced Section Pull-Plates Using Micromechanics-Based Fracture Models

Micromechanics-based models that capture interactions of stress and strain provide accurate criteria to predict ductile fracture in finite-element simulations of structural steel components. Two such models—the void growth model and the stress modified critical strain model are applied to a series of twelve pull-plate tests that represent reduced (or net) section conditions in bolted and reduced beam section connections. Two steel varieties, A572 Grade 50 and a high-performance Grade 70 bridge steel are investigated. The models are observed to predict fracture much more accurately than basic longitudinal strain criteria, by capturing stress–strain interactions that lead to fracture. The flat stress and strain gradients in these pull plates allow the use of relatively coarse finite-element meshes providing economy of computation while capturing fundamental material behavior and offering insights into localized ductile fracture effects.

Algebraic correction methods for computational assessment of clone overlaps in DNA fingerprint mapping

BackgroundThe Sulston score is a well-established, though approximate metric for probabilistically evaluating postulated clone overlaps in DNA fingerprint mapping. It is known to systematically over-predict match probabilities by various orders of magnitude, depending upon project-specific parameters. Although the exact probability distribution is also available for the comparison problem, it is rather difficult to compute and cannot be used directly in most cases. A methodology providing both improved accuracy and computational economy is required.ResultsWe propose a straightforward algebraic correction procedure, which takes the Sulston score as a provisional value and applies a power-law equation to obtain an improved result. Numerical comparisons indicate dramatically increased accuracy over the range of parameters typical of traditional agarose fingerprint mapping. Issues with extrapolating the method into parameter ranges characteristic of newer capillary electrophoresis-based projects are also discussed.ConclusionAlthough only marginally more expensive to compute than the raw Sulston score, the correction provides a vastly improved probabilistic description of hypothesized clone overlaps. This will clearly be important in overlap assessment and perhaps for other tasks as well, for example in using the ranking of overlap probabilities to assist in clone ordering.

Foreword to the Special Issue on Computational Heat Transfer

Foreword to the Special Issue on Computational Heat Transfer

Effective elastic properties for unpoled barium titanate

Piezoelectric devices with complex electrode geometries may contain regions of ferroelectric material that remain unpoled. It is desirable to account for these non-piezoelectric regions for device optimization, since the unpoled and poled material properties differ. The lack of published elastic properties for unpoled ferroelectrics, specifically the numerous commercial PZT compositions, reflects the difficulty of experimental measurement. In this work, a method was developed to predict unpoled properties from more commonly available poled material data. Barium titanate was chosen for study as a representative ferroelectric, with both single crystal and polycrystalline (ceramic) properties available. Finite element micro-mechanical models were created with a focus on computational economy. This allowed larger ensembles of results to be computed, providing accurate effective compliances when averaged. The modelling methodology predicted the elastic properties for unpoled barium titanate to within ∼10% of measured experimental values.

Barriers for Students’ Learning of Holistic Systems Development

Introduction In the 1980s it was put forward that integration of the application and problem domain, i.e. the users' area and IT system's tasks respectively, requires that the systems developer possesses particular competences. Until that point in time, systems development had been firmly positioned within the functionalist paradigm, but now a number of new schools and methods, advocated by among others Winston W. Royce, Pelle Ehn, Enid Mumford, Peter Checkland, David Avison, and A.T. Wood-Harper, proposed that the systems developer should have interdisciplinary skills and be able to move between different paradigms. In other words, the system developer should be able to use, in addition to the functionalist paradigm, the social relativist, the radical structuralist, and the neohumanist paradigms (Hirschheim & Klein, 1989). Thus, with regard to understanding, communicating, and designing, the 'good' and 'reflective' systems developer is someone who can facilitate between users and programmers and navigate easily within and between the application and the problem domain (Curtis, Krasner, & Iscoe, 1988; Mathiassen & Purao, 2002). Already in the mid 1980s the Copenhagen Business School designed a bachelor programme in computer science and economy, called HA/DOK. The purpose of HA/DOK was, and continuous to be, to educate students with competences in holistic systems development, i.e. with competences in facilitating between and integrating the application and problem domain. In Denmark, similar education programmes were later developed at Roskilde University, Arhus School of Business, Arhus University, and the IT University. This paper investigates the Information Processing (IP) course, which is located in the fifth semester of HA/DOK. More specifically, the point of departure is the summative evaluation that concluded the course in January 2007 and that puzzled the authors, as well as the students. During a period of four days we held oral exams with 47 students, and therefore 12 groups. While preparing and having the exams we noticed that: * All students experienced the same problems and barriers irrespective of how they performed at the exam. * The students' different level of competence influenced if and how they dealt with the problems and barriers. * The students had a strong tendency to act in accordance with the functionalist paradigm (Hirschheim & Klein, 1989) when other approaches would have been the natural choice. We asked ourselves if the observed pattern was a result of: the exam, the IP course in isolation, or of the bachelor programme as a whole. Recently greater attention has been paid to formulation and assessment of learning objectives and outcomes in higher education, i.e. to outcome based academic programmes (Abraham, 2006), but a literature search in relevant databases revealed that so far little research has addressed the relationship between ISD educations' intention, course objectives, and the actual learning outcome. (However, see Cope (2003) and Abraham (2006) for interesting, related studies of learning outcomes in undergraduate ISD programmes). To understand the local problem at the IP course and to contribute to the ISD literature in general, we decided to start an action research collaboration that investigates which barriers students experience when learning holistic systems development and why. We consider the observation of a gap between programme intention, course objectives, and learning outcome as well as our analysis here of to be relevant for ISD education planers, teachers, and organizations that employ students with integrative and facilitator competences. The paper is structured as follows. First HA/DOK and the IP course are described. Then the research approach is presented. In the next three sections we investigate at what level the gap between intention and learning arises, i.e. at exam, course, and/or study programme level, what the problem is, and what causes it. …

Multidimensional modelling of diesel combustion by a detailed kinetic scheme and comparison with in-cylinder optical measurements

The present work aims at a comprehensive study of diesel combustion based on detailed kinetic modelling and experimental investigations performed on an optically accessible engine. A diesel fuel surrogate model of combustion is included in a three-dimensional CFD code properly designed for application to internal combustion engines. The fuel spray is treated by discrete droplet models tuned on the ground of measurements of penetration lengths effected in a vessel at controlled conditions. The detailed chemistry of the turbulent reacting flow is computed by means of the partially stirred reactor approximation, which leads to an appreciable computational economy. Test-bench experimental data allow the code validation and the assessment of its predictability as operating conditions are modified. Optical measurements clarify major physical and chemical aspects related to fuel autoignition and combustion evolution. The proposed approach is believed to be valuable in optimising performances of diesel engines equipped with Common Rail (CR) injection systems.

Motorcycle Steering Control by Road Preview

The main objectives of the work described are to devise an effective path-based motorcycle simulation capability and to add to understanding of how riders control motorcycles. Optimal linear preview control theory was previously applied to the tracking of a roadway by a car, using a simple car model operating in fixed control. Similar theory is applied to path control of motorcycles. The simple car previously employed is replaced by a much more elaborate motorcycle. The steering angle control used previously is changed into steering torque control. Rider upper body lean torque is also allowed as a control input. The machine speed is considered constant but is a parameter of the motion. The objective of the optimal control is to minimize a weighted sum of tracking errors, rider lean angle and control power. The time-invariant optimal control corresponding to a white noise disturbance and to an infinite optimization horizon is found for many situations, involving variations in machine speed and performance priorities. Tight controls, corresponding to high weightings on performance, and loose controls, corresponding to high weightings on control power, are identified. Results show the expected pattern for preview control, that information well into the future is of limited value in determining the present control inputs. Full system performance is achievable with only finite preview. The extent of the preview necessary for full performance is determined as a function of machine speed and performance priorities. This necessary preview is found to be in accord with conventional wisdom of motorcycle riding and rider training. Optimal path tracking preview controls are shown to represent the inverse dynamics of the motorcycle. New light is shed on the relative effectiveness of steering torque and body lean torque controls. Simulations of an optimally controlled motorcycle and rider combination are conducted. A typical lane change path and an S-shaped path from the literature are used. For a chosen speed, optimal controls are installed on the machine for which they were derived and simulation results showing tracking performance, control inputs, and other responses are included. Transformation of the problem from a global description, in which the optimal control is found, to a local description corresponding to the rider’s view, is described. It is concluded that a motorcycle rider model representing a useful combination of steering control capability and computational economy has been established. The model yields new insights into rider and motorcycle behavior.

3D Local Earthquake Hypocenter Determination with an Irregular Shortest-Path Method

A simple but robust method for joint (multiple) earthquake location in arbitrary 3D velocity media is presented in this article. It exploits the advantages of the irregular shortest-path method (irregular spm) for the raytracing, and uses a damped minimum norm, constrained least-squares solution for the source parameter (hypocenter and origin time) update. Special features of the scheme, made possible by the irregular spm, are that the raytracing needs to be carried out only once and the elements of the Jacobian matrix are calculated directly. Six supplementary sources are added around a trial (to be updated) source and trilinear interpolation used to form the travel times and the derivatives. Comparison tests against the regular spm and the finite-difference method show that the newly developed procedure is able to achieve comparable accuracy in both the travel-time calculation and in the hypocentral location solution, but at considerable computational economy. Further numerical tests for a complex 3D model indicate that the method is accurate, with a location error in depth comparable to that in the horizontal plane, at least for the station distribution used. In a practical sense, there is no requirement to have a close initial guess of the source coordinates and origin time before the location process is undertaken if the sources are not too far from the seismic network. The results show that this approach is fairly insensitive to modest levels of random noise, but like all hypocenter determination algorithms, it strongly depends on having the correct velocity model.

Reduced basis methods for Stokes equations in domains with non-affine parameter dependence

In this paper we deal with reduced basis techniques applied to Stokes equations. We consider domains with different shape, parametrized by affine and non-affine maps with respect to a reference domain. The proposed method is ideally suited for the repeated and rapid evaluations required in the context of parameter estimation, design, optimization, and real-time control. An “empirical”, stable and inexpensive interpolation procedure has permitted to replace non-affine coefficient functions with an expansion which leads to a computational decomposition between the off-line (parameter independent) stage for reduced basis generation and the on-line (parameter dependent) approximation stage based on Galerkin projection, used to find a new solution for a new set of parameters by a combination of previously computed stored solutions. As in the affine case this computational decomposition leads us to preserve reduced basis properties: rapid and accurate convergence and computational economies. The applications and results are based on parametrized geometries describing domains with curved walls, for example a stenosed channel and a bypass configuration. This method is well suited to treat also problems in fixed domain with non-affine parameters dependence expressing varying physical coefficients.

A multi‐term semi‐energy finite strip method for post‐buckling analysis of composite plates

AbstractIn this paper a finite strip approach based on the concept of a rigorous post‐buckling solution for composite plates, namely the semi‐energy approach, is presented. Moreover, in semi‐energy finite strip approach, the out‐of‐plane displacement field of the finite strip is the only displacement which is postulated by a deflected form. The postulated deflected form is substituted into von Kármán's compatibility equation which is solved exactly to obtain the corresponding forms of the mid‐plane stresses and displacements. The solution of von Kármán's compatibility equation and the postulated out‐of‐plane deflected form are then used to evaluate the potential energy of the related finite strip. Finally, by invoking the principle of minimum potential energy, the equilibrium equations of the finite strip are derived. The developed finite strip method (FSM) is applied to analyse the post‐buckling behaviour of laminated composite plates. As far as, the validity and numerical efficiency of the developed semi‐energy FSM is concerned, the results obtained from semi‐energy FSM approach are compared with those obtained from a fairly conventional and well exploited finite strip technique, namely the full‐energy finite strip method. The study of the results revealed that the developed semi‐energy FSM provides a very higher convergence and stress distribution properties in comparison with the full‐energy solution by incorporating considerably less number of degrees of freedom. This means that from the computational economy point of view, the developed semi‐energy FSM is superior to the full energy FSM. Copyright © 2006 John Wiley & Sons, Ltd.

Parallel Simulation Study of a Laser Surface Alloying Process

ABSTRACT Numerical simulation of transient phase-change processes is massively time consuming, and can benefit from parallel processing to reduce the computational costs. In the present work, an implicit finite-volume-based three-dimensional, transient, turbulent model is developed to describe the transport phenomena for a typical laser surface alloying process, and the code is subsequently parallelized for effective computational economy. Parallelization of the code is done following the pipelining method, which uses temporal parallelism in a distributed computing environment and runs on a cluster of workstations (COW) connected with local area networking (LAN). Equal partitioning of the spatial domain into subdomains along the longer geometric dimension is performed, in order to decompose the computational domain. The Message Passing Interface (MPI) utility is used for communications among the subdomains on each processor. To demonstrate the parallel performance, a systematic analysis is carried out, with up to 16 nodes and various grid sizes. It is found that the overall parallel performance improves up to a certain number of processors, and then degrades with an increase in number of processors, due to communication delays in the LAN and pipeline hazards.

Adaptive refinement of all-hexahedral elements for three-dimensional metal forming analysis

In many 3-D forging simulations, the master-grid method or uniform-grid method is adopted as an effect tool for hexahedral element mesh generation, because the method is robust and reliable to construct the intermediate mesh during forging simulation. However, almost equal-sized element mesh is employed regardless of the local characteristics of the deforming region. In order to overcome this drawback, a new refinement technique for hexahedral element mesh is proposed by selecting the element regions to be refined and iteratively inserting the zero-thickness element layers into the necessary interfaces of elements. To generate refined mesh adaptive to the gradient of physical property or to the complexity of geometry, the desired mesh density is obtained from the Z – Z posteriori error analysis. In the course of expanding and smoothing the refined mesh to ensure the desired mesh density, the weighting factor based on mesh density of neighbor entities is introduced to the conventional Laplacian smoothing method. Comparative simulations of the selected forging processes have shown that the proposed refinement technique is effective from the viewpoint of computational economy of computations and accuracy.