bluuebunny/crossref_metadata_2025 huggingface dataset:
The Evidentiary Value of Challenge Trials for Three Pre‐harvest Food Safety Topics: A Systematic Assessment
Authors: Wisener L. V. , Sargeant J. M. , O'Connor A. M. , Faires M. C. , Glass‐Kaastra S. K.
Published: November 2013

Reducing zoonotic pathogens in food animals prior to harvest will reduce the pathogen burden that enters the food chain and the environment. Consequently, the burden of enteric illness in humans may be reduced. Evaluating interventions to reduce a pathogen in animals often begins with challenge trials, in which animals are deliberately exposed to the pathogen under controlled conditions. Challenge trials are subsequently followed by field trials, also known as randomized controlled trials, in which the animals are naturally exposed to the pathogen. Challenge trials can most effectively inform field trials only if they precede field trials, are robust, internally valid and transparently reported. Using systematic review and meta‐analysis methodology, we examined the pre‐harvest food safety literature for three intervention—pathogen—species combinations: probiotics/competitive exclusion products in ruminants to reduceEscherichia coliO157 shedding, vaccines in ruminants to reduceE. coliO157 shedding and vaccines in swine to reduceSalmonellashedding. We examined two outcomes, prevalence of faecal shedding at the end of the trial and prevalence of faecal shedding throughout the trial period, to compare challenge trials and field trials. We found that challenge trials occurred concurrently with field trials, challenge trials suffered from reporting deficiencies of methodological features, challenge trials tended to report a more favourable outcome than field trials, and there was some evidence of publication bias among all three intervention–pathogen–species combinations. Challenge trials would better serve to inform field trials if they precede field trials, are methodologically sound, include transparent reporting and are published regardless of their results. In addition, due to our findings of greater efficacy reported among challenge trials compared with field trials, risk models predicting the public health benefits of pre‐harvest interventions to reduce zoonotic pathogens in livestock might be best served by field trial results alone.
Longitudinal and Transverse Phonon Transport in Dielectric Crystals
Authors: Tzou D. Y.
Published: November 2013

Longitudinal and transverse modes of thermal disturbances in isotopically pure alkali-halide crystals are derived from phonon hydrodynamics. Guyer-Krumhansl (GK) model of phonon scattering is first recovered by relating the first and second viscosity to the relaxation times. Helmholtz potentials are then introduced to split the two modes from the heat flux vector. It has been found that the scalar potential coincides with the heat equation in the dual-phase-lag model, while the vector potential describes dispersive transverse phonons and introduces a new type of equation in microscale heat transfer.
Stagnation-Point Flow Toward a Stretching/Shrinking Sheet in a Nanofluid Containing Both Nanoparticles and Gyrotactic Microorganisms
Authors: Zaimi Khairy , Ishak Anuar , Pop Ioan
Published: November 2013

The stagnation-point flow and heat transfer toward a stretching/shrinking sheet in a nanofluid containing gyrotactic microorganisms with suction are investigated. Using a similarity transformation, the nonlinear system of partial differential equations is converted into nonlinear ordinary differential equations. These resulting equations are solved numerically using a shooting method. The skin friction coefficient, local Nusselt number, local Sherwood number, and the local density of the motile microorganisms as well as the velocity, temperature, nanoparticle volume fraction and the density of motile microorganisms profiles are analyzed subject to several parameters of interest, namely suction parameter, thermophoresis parameter, Brownian motion parameter, Lewis number, Schmidt number, bioconvection Péclet number, and the stretching/shrinking parameter. It is found that dual solutions exist for a certain range of the stretching/shrinking parameter for both shrinking and stretching cases. The results indicate that the skin friction coefficient, local Nusselt number, local Sherwood number, and the local density of the motile microorganisms increase with suction effect. It is also observed that suction widens the range of the stretching/shrinking parameter for which the solution exists.
Effect of Turbulence and Flow Distortion on the Performance of Conical Diffusers Operating on Supercritical Carbon Dioxide
Authors: López A. , Monje B. , Sánchez D. , Chacartegui R. , Sánchez T.
Published: November 2013

A rapidly growing interest in the supercritical carbon dioxide power cycle has been observed in the last years due to the superb performance of this system in concentrated solar and nuclear applications; a sample of this interest is the number of technical publications submitted to Turbo Expo in the last couple of years. As active members of the supercritical carbon dioxide (SCO2) community, the authors of this work have lately studied the fundamentals of SCO2 flows. The approach followed has nevertheless been different to that of most researchers since it has concentrated on simple devices rather than on an entire turbomachinery. Thus, recent contributions by the authors have shown that major differences are to be expected when air and SCO2 diffuse through simple conical divergent ducts at subsonic speeds, most of which derive from the very different characteristics and performance of the boundary layer when adverse pressure gradients are faced. In particular, the effects of geometry (i.e. divergence angle) and aerodynamic blockage on the static pressure rise coefficient of such a conical diffuser have been reported by the authors in recent technical works. This work presents the effects of other aerodynamic features of the inlet flow to a conical diffuser on the capacity to convert kinetic energy into static pressure. Two flow features are studied: (i) the distortion of the inlet velocity distribution and (ii) the turbulence intensity of the inlet flow. A parallel analysis is developed for air and SCO2 showing that the effects of both distortion and turbulence on diffuser performance are sensitive to the working fluid of choice.
CFD Thermohydrodynamic Analysis of 3-D Sector-Pad Thrust Bearings With Rectangular Dimples
Authors: Papadopoulos C. I. , Kaiktsis L. , Fillon M.
Published: November 2013

The paper presents a detailed computational study of flow patterns and performance indices in a dimpled parallel thrust bearing. The bearing consists of eight pads; the stator surface of each pad is partially textured with rectangular dimples, aiming at maximizing the load carrying capacity. The bearing tribological performance is characterized by means of Computational Fluid Dynamics (CFD) simulations, based on the numerical solution of the Navier-Stokes and energy equations for incompressible flow. Realistic boundary conditions are implemented. The effects of operating conditions and texture design are studied for the case of isothermal flow. First, for a reference texture pattern, the effects of varying operating conditions, in particular minimum film thickness (thrust load), rotational speed and feeding oil pressure are investigated. Next, the effects of varying texture geometry characteristics, in particular texture zone circumferential/radial extent, dimple depth and texture density on the bearing performance indices (load carrying capacity, friction torque and friction coefficient) are studied, for a representative operating point. For the reference texture design, the effects of varying operating conditions are further investigated, by also taking into account thermal effects. In particular, adiabatic conditions and conjugate heat transfer at the bearing pad are considered. The results of the present study indicate that parallel thrust bearings textured by proper rectangular dimples are characterized by substantial load carrying capacity levels. Thermal effects may significantly reduce load capacity, especially in the range of high speeds and high loads. Based on the present results, favorable texture designs can be assessed.
Effects of Engine Wash Frequency on GT Degradation in Natural Gas Compressor Stations
Authors: Botros K. K. , Golshan H. , Rogers D.
Published: November 2013

Gas Turbine (GT), like other prime movers, undergoes wear and tear over time which results in performance drop as far as available power and efficiency is concerned. In addition to routine wear and tear, the engine also undergoes corrosion, fouling etc. due to the impurities it breathes in. It is standard procedure to ‘wash’ the engine from time to time to revive it. However, it is important to establish a correct schedule for the wash to ensure optimal maintenance procedure. This calls for accurate prediction of the performance degradation of the engine over time. In this paper, a methodology is presented to predict the performance degradation in a GE LM2500+ Gas Turbine engine used at one of TransCanada’s pipeline system, Canada. Evaluation of various components of the GT gas path, in particular the air compressor side of the engine since it is most prone to fouling and degradation is presented and correlated to the frequency and span of offline engine washes. Other components, such as the high pressure turbine and the power turbine are also evaluated. Although the results presented are for a specific engine type, the general framework of the model could be used for other engines as well to quantify degradation over time of other components within the GT engine. This model combines Gas Path Analysis (GPA) to evaluate the thermodynamic parameters over the engine cycle followed by parameter estimation based on Error-in-Variable Model (EVM) to filter the data of possible noise due to instrumentation errors. The model helps quantify the degradation in the engine performance over time and also indicates the effectiveness of each engine wash. The analysis will lead to better scheduling of the engine wash thereby optimizing operational costs as well as engine overhaul time.
Numerical Characterization of Swirl Brakes for High Pressure Centrifugal Compressors
Authors: Da Soghe Riccardo , Micio Mirko , Andreini Antonio , Facchini Bruno , Innocenti Luca , Ceccherini Alberto
Published: November 2013

High pressure centrifugal compressors continue to experience vibrations due to rotordynamic stability. The main cause for aero-induced exciting forces that affects the stability, is the tangential velocity component of the gas entering the many labyrinth seals throughout the machine. In order to control or limit these swirling flows, swirl brakes are generally implemented both at the impeller eye seals and at the balance piston or division wall seal of a centrifugal compressor. This paper deals with the aerodynamic characterization, by means of CFD, of such kind of devices. Several design parameters, such as teeth lean, angle of attack and pitch-to-chord ratio have been considered and also the operating conditions (pressure level and swirl at the swirl brake inlet) are accounted for. This paper aims to improve the physical understanding of the fluid flow of centrifugal compressors swirl brakes allowing an optimization of such systems.
A Compact Diffuser and Scroll for a Centrifugal Compressor
Authors: Rodgers Colin , Brown Daniel , Thompson Robert
Published: November 2013

This paper describes the test development of a compact diverging sidewall spiral diffuser for a small gas turbine auxiliary power units (APU’s) of conventional load compressor type wherein the gas generator or core module directly drives a separate centrifugal load compressor to supply a combination of both compressor air for main engine starting and air conditioning, together with shaft power to drive an engine mounted electric generator.

The APU capability to efficiently supply both pneumatic and electrical power demands a load compressor with a wide flow range at rated speed and low resisting torque during start which is achieved with modulated variable inlet guide vanes (IGV’s).

The compressor vaned diffuser design was unlike most conventional channel diffusers in that the diffusing passage downstream of the throat had diverging sidewalls to diffuse the flow radial component prior to discharge into the delivery scroll.

Test evaluation of this diffuser was conducted on a specially designed turbodrive compressor rig at pressure ratios up to 5:1 and rotational speeds up to 65000 rpm. Test results showed that the design performance requirements were satisfied.

The uniqueness of the design later prompted a CFD diagnosis of the compressor as a code verification case the computational results of which are presented in comparison to the experimental data.
A Study on Optimal Design of Filament Winding Composite Tower for 2 MW Class Horizontal Axis Wind Turbine Systems
Authors: Kong Changduk , Lim Sungjin , Park Hyunbum
Published: November 2013

Although the tower of the horizontal axis wind turbine system is a simple structure relatively to the wind turbine rotor system, its production cost tower is about 20–25% of the whole wind turbine system cost. If a composite materials tower is used instead of the existing steel tower, the production cost can be reduced due to use of low cost composite materials, simple manufacturing process, easy transportation and easy assembly. However studies on the composite materials towers are very few.

Therefore, in this study a specific structural design procedure for 2 MW class glass/polyester face sheets-sand/ polyester core sandwich composite wind turbine system towers is newly proposed through load case study, trade-off study, optimal structural design and structural analysis. Optimal tower design can minimize both weight and cost. In the structural design of the tower, three kinds of loads such as wind load, blades, nacelle and tower weight and blade aerodynamic drag load should be considered. Initial structural design is carried out using the netting rule and the rule of mixture. Then the structural safety and stability are confirmed using a commercial finite element code, MSC NASTRAN/PATRAN. It is confirmed that the finally proposed tower meets the tower design requirements.
A Study on Aerodynamic and Structural Design of High Efficiency Composite Blade of 1 MW Class HAWTS Considering Fatigue Life
Authors: Kong Changduk , Kim Minwoong , Park Gilsu
Published: November 2013

In this work, 1 MW class horizontal axis wind turbine blade configuration is properly sized and analyzed using the newly proposed aerodynamic design procedure and the in-house code developed by authors, and its design results are verified through comparison with experimental results of the previously developed wind turbine blade. The wind turbine structural design is carried out using the Glass/Epoxy composite materials and the simplified deign methods by the netting rule and the rule of mixture. The structural safety of the designed blade structure is investigated through the various load case studies, and stress, deformation, buckling and vibration analyses using a commercial FEM code, MSC.NASTRAN. Finally the required 20 years fatigue life is confirmed using the modified Spera’s empirical formulae.
Three-Dimensional Model for Manufacturing and Inspection
Authors: Pippenger Brian S.
Published: November 2013

Three-dimensional Model-Based Engineering (MBE) along with Quality Information Framework (QIF) is an approach to product design, manufacturing, and support where a digital three-dimensional representation of the product serves as the normative source for information communicated throughout the product’s lifecycle and supply chain. MBE simplifies data management and provides a more powerful communication medium than 2D-based environments. This is not just using a model for reference or a visual aid. The model will be the definition for the parts being manufactured, inspected, and built into full engines. The use of two-dimensional prints will be outdated and a culture change will be needed to embrace this change. Many organizations are implementing MBE and are using this technology to produce aerospace products. Manufacturing and inspection functions are dependent on the models from cradle to grave of the product lifecycle. These smart models will have all of the necessary dimensional metrology interoperability, GD&T encoded and product manufacturing information (PMI) data all in a standardized format associated within the model. This will allow for a model that has less errors, more functionality with manufacturing and inspection systems. The information included in the Model Based Definition (MBD) will be part of the QIF.

The QIF fully defines quality measurement plans, measurement results, measurement rules, measurement resources, and results analysis. This combined with the PMI for manufacturing will provide a comprehensive MBD that can be used for all of the manufacturing process in the lifecycle of the product.

The culture will need to progress from two-dimensional paper prints to smart three-dimensional models that are rich with data. These models will drive the process and be the final word for part acceptance; paper prints will not be needed or produced. These types of models will drive almost all of the military’s new designs and if the organization is not prepared for this change, it will lose many opportunities to be competitive.
Static Destabilizing Behavior for Gas Annular Seals at High Eccentricity Ratios
Authors: Childs Dara W. , Arthur Stephen P.
Published: November 2013

The centering force of a plain, annular, liquid seal in a centered position has a pronounced effect on the rotordynamic behavior of pumps due to the Lomakin Effect. In centrifugal injection compressors, the centering force for a hole-pattern-stator/smooth-rotor for a balance-piston seal in a straight-through design or a division-wall seal in a back-to-back design has an impact on the rotordynamic behavior of high-pressure injection compressors. Annular seals for pumps and compressors are designed to operate in a nominally centered position. Previous test results show that the direct (centering) stiffness for liquid seals and hole-pattern-stator seals remain relatively constant out to eccentricity ratios of 0.5. For liquid seals, tests have shown positive centering capability at eccentricity ratios on the order of 0.7.

New test results are presented for annular gas seals with smooth and hole-pattern stators, moving from centered to fully eccentric (contact) positions. The test fluid is air. The tests are for non-rotating conditions with a supply pressure up to 70 bars and pressure rations (PRs) of 0.4, 0.5, and 0.6. The exit Mach numbers for the tests are on the order of 0.33, so compressibility effects are not pronounced.

Test results are presented for the net reaction force for a smooth seal and a hole-pattern-stator gas annular seal in moving from centered to fully eccentric positions. Comparisons are made between measurements and predictions from a bulk-flow code.

The smooth seal creates significantly larger de-centering forces than the HP seal. Its measured de-centering force increases steadily as the stator approaches the wall. The negative slope of the reaction force also increases, indicating a steadily increasing negative stiffness. The de-centering force increased with decreasing PR, increasing ΔP, and is much larger than predicted.

The HP seal de-centering reaction force is bi-linear with an approximate constant slope out to ε0 ≅ 0.5, and then a much steeper slope out to wall contact. The model does not predict the bilinear behavior but does a reasonable job of predicting the force magnitude at contact.

The de-centering forces measured here have not previously been reported and emphasize the importance of concentric assembly of annular seals in pumps and compressors. Unanticipated eccentricities that create negative stiffness will lead to over prediction of critical speeds and overly optimistic stability calculations.
Effects of Blade Counts and Clocking on the Unsteady Profile Pressure Distribution in an Axial-Radial Combined Compressor
Authors: Zhao Ben , Yang Ce , Hu Liangjun , Lao Dazhong
Published: November 2013

A new hypothesis is presented for the superimposed effects of the blade pressure distribution in a multistage compressor. The effects of the unsteady pressure fluctuations on the blade surface are separated into three groups. The influences of the upstream or downstream rotors can be obtained by numerical simulation for the R/S or S/R configuration; the data produced by all the influences can be obtained from the R/S/R configuration. The effects of the blade counts and clocking on the superimposed effects, acting on the profile pressure distribution, are studied using a special data analysis method that had been previously developed by the authors. The results indicate that the blade counts of the upstream and downstream rotors determine the periods of the unsteady pressure fluctuations on the stator surface. The clocking moving blade rows modulate the relative superimposed phases and interactions between two rotors such that the unsteady pressure fluctuates with different amplitudes on the surface of the stator blade.
Multidisciplinary Automated Optimization Strategy on a Counter Rotating Fan
Authors: Aulich Anne-Laure , Goerke Daniel , Blocher Michael , Nicke Eberhard , Kocian Frank
Published: November 2013

Multidisciplinary automated optimization processes are nowadays essential to obtain optimal turbomachinery components. However, an extremely large number of free variables, constraints and objectives results in a complex task. This paper presents an optimization strategy developed to handle with different constraints, design goals concerning aerodynamic, mechanic and aeroelastic, and finally manufacturing aspects. This strategy has been applied to a counter rotating integrated shrouded propfan, which is developed within a DLR-project. Both rotors have been already aerodynamic optimized in a first design phase coupled with a mechanical analysis of the CF/PEEK blades with titanium clevises. Detailed analysis showed high displacements and unreliable Campbell-Diagrams. To reach a rig-ready design a new optimization strategy has been developed.

The optimizations feature more than hundred free variables, two objective functions, as well as a high number of aerodynamic and mechanical constraints. The mechanical behavior of the blades has been improved step by step in four successive aeromechanical optimizations. To secure the improvement obtained in one optimization, their objective functions become constraints in the next step. In the first optimization, the efforts have been focused on reducing the maximal absolute displacements in several operation points. In the second one, the scattering of the maximal absolute displacements between several operation points have been reduced. In the third optimization, the Campbell-Diagrams have been additionally optimized. Although the aerodynamic performance remained on a good level, it decreased a little bit in this design phase. For this reason, an additional fourth optimization was performed with the objective to increase the fan efficiency by keeping the good mechanical behavior reached before.

The presented optimization strategy has been successfully completed and the best members obtained show an almost satisfactory mechanical feasibility in view of the planned rig test.
Approximate Analytical Model for Oil Film Force of Finite Length Squeeze Film Damper Under Low Reynolds Number
Authors: Wang Yongliang , Gao Yu , Zhong Jingjun , Yang Ling , Lu Huawei
Published: November 2013

Squeeze film dampers (SFDs) are widely used in aero-engines and other high speed rotating machines as damping elements, owing to their remarkable damping effect. The oil-film force model of SFDs is the key to investigate the dynamic characteristics of the rotor-bearing systems involving SFDs. In this paper, the analytical solution of the oil film pressure of a finite length SFD is obtained by employing the separation of variables method to solve the Reynolds equation (at low Reynolds number) based upon the dynamic π boundary condition. The analytical expression of the oil film force is then derived by applying the integral method. The oil film force from the analytical model is compared with the results from other well-known methods, i.e. the long bearing approximation, the short bearing approximation and the finite difference method. The results clearly show that within a wider length-diameter ratio range, the newly proposed model can accurately predict the oil film characteristics of the SFDs at low Reynolds numbers.
Measurements Versus Predictions for the Static and Dynamic Characteristics of a Four-Pad, Rocker-Pivot, Tilting-Pad Journal Bearing
Authors: Tschoepe David P. , Childs Dara W.
Published: November 2013

Measured and predicted static and dynamic characteristics are provided for a four-pad, rocker-pivot, tilting-pad journal bearing in the load-on-pad and load-between-pad orientations. The bearing has the following characteristics: 4 pads, .57 pad pivot offset, 0.6 L/D ratio, 60.33 mm (2.375in) pad axial length, 0.08255 mm (0.00325 in) radial clearance in the load-on-pad orientation, and 0.1189 mm (0.00468 in) radial clearance in the load-between-pad orientation. Tests were conducted on a floating test bearing design with unit loads ranging from 0 to 2903 kPa (421.1 psi) and speeds from 6.8 to 13.2 krpm.

For all rotor speeds, hot-clearance measurements were taken to show the reduction in bearing clearance due to thermal expansion of the shaft and pads during testing. As the testing conditions get hotter, the rotor, pads, and bearing expand, decreasing radial bearing clearance. Hot-clearance measurements showed a 16–25% decrease in clearance compared to a clearance measurement at room temperature. To look at the radial thermal gradient in the loaded pad, embedded thermocouples were inserted inside the bearing pad near the bearing housing. Results showed a 5–25°C decrease in temperature from the rotor side of the pad to the temperatures near the bearing housing. This radial temperature gradient caused an uneven thermal deflection in the pad, changing the pads’ radii of curvature.

For all test conditions, dynamic tests were performed over a range of excitation frequencies to obtain complex dynamic stiffness coefficients as a function of frequency. The direct real dynamic stiffness coefficients were then fitted with a quadratic function with respect to frequency. From the curve fit, the frequency dependence was captured by including a virtual-mass matrix [M] to produce a frequency independent [K][C][M] model.

The direct dynamic stiffness coefficients for the load-on-pad orientation showed significant orthotropy, while the load-between-pad did not. The load-between-pad showed slight orthotropy as load increased. Experimental cross-coupled stiffness coefficients were measured in both load orientations, but were of the same sign and significantly less than direct stiffness coefficients.

In both orientations the imaginary part of the measured dynamic stiffness increased linearly with increasing frequency, allowing for frequency-independent direct damping coefficients.

Rotordynamic coefficients presented were compared to predictions from two Reynolds-based models. The models showed the importance of taking into account pivot contact flexibility and different pad geometries (due to the reduction in bearing clearance during testing) in predicting rotordynamic coefficients. If either of these two inputs were incorrect, then predictions for the bearings impedance coefficients were very inaccurate.
Investigation of Flow and Mixing Phenomena in an Oil Supply Pocket of a Heavy Duty Gas Turbine Journal Bearing
Authors: Uhkoetter Stephan , aus der Wiesche Stefan , Joos Franz
Published: November 2013

In recent year, bearing designers have focused on reduced-temperature designs; whereby better results have been achieved by applying the cool inlet oil more effectively and reducing the hot oil carryover. The key factor in this improvement is the hot-cool oil mixing process in the oil supply pockets. However, relatively little is know about the details of the underlying flow and mixing phenomena, and the classical one dimensional lubrication theory fails to provide an explanation. A detail experimental investigation of the flow and mixing phenomena in a typical oil supply pocket of a heavy duty gas turbine was performed by employing a large-scale model of the geometry. This model has a similarity in respect to Reynolds number, and, by using laser Doppler anemometry (LDA), it permitted optical flow visualization and measurements. In addition to a thin shear layer close to the moving boundary, flow patterns including recirculation zones were detected in the larger part of the flow domain. Fully three-dimensional computational fluid dynamics (CFD) simulations were performed, too. With regard to the observed flow and mixing phenomena, large eddy simulation (LES) methods offer great potential because this approach can directly resolve the large eddies, and it is the interaction between the shear layer and these large eddies that mainly govern the mixing phenomena. The comparison between experimental data and LES results showed a reasonable agreement. Furthermore, the potential of more conventional turbulence models such as the classical k-ε-model were also assessed.
Surrogate-Based Recurrence Framework Approach to Unsteady Aerodynamic Modeling of Wind Turbine Airfoils
Authors: Liu Pengyin , Zhu Xiaocheng , Yu Guohua , Du Zhaohui
Published: November 2013

This paper proposes a method for predicting unsteady aerodynamics of wind turbine airfoils using surrogate-based recurrence framework (SBRF) method. Using specified simulation results generated by the CFD method in some conditions, the unsteady aerodynamic model could be established by the Kriging surrogate model. Then, time-domain predictions of unsteady lift, moment, and drag in different conditions can be gained by the SBRF method with minimal computational expense. Some parameters have been set according to the operational condition of wind turbines so as to describe the unsteady aerodynamic modeling problem. The unsteady aerodynamic performance of the wind turbine airfoils in some training conditions is carried out by the commercial CFD simulator CFX, the results of which could be utilized to build the SBRF. Then the predicted time-varying aerodynamic characteristics of wind turbine airfoils in the validated condition could be obtained by the SBRF method and the CFD simulation, respectively. It is revealed from the results that the time-varying aerodynamic characteristics of wind turbine airfoils in most dynamic stall cases can accurately approximate by the SBRF method. In addition, the SBRF method has relatively less computational cost compared with the CFD method. Therefore, it can be used as the foundation of aero-elastic analysis and design optimization studies of wind turbines.
Efficient Techniques for the Computation of the Nonlinear Dynamics of a Foil-Air Bearing Rotor System
Authors: Pham Hai , Bonello Philip
Published: November 2013

The foil-air bearing (FAB) plays a key role in the development of high speed, economical and environmentally friendly oil-free turbomachinery. However, FABs are known to be capable of introducing undesirable nonlinear effects into the dynamic response of a rotor-bearing system. This necessitates a means for calculating the nonlinear response of rotor systems with FABs. Up to now, the computational burden introduced by the interaction of the dynamics of the rotor, air film and foil structure has been overcome by uncoupling these three subsystems, introducing the potential for significant error. This paper performs the time domain solution of a simple rotordynamic system without uncoupling the state variables. This solution is then used as a reference for the verification of two proposed novel methods for reducing the computational burden: (a) use of harmonic balance; (b) use of Galerkin transformation. The applicability and accuracy of these two methods is illustrated on a simple symmetric rotor-FAB system.
Investigation of Wake Induced Transition in Low-Pressure Turbines Using Large Eddy Simulation
Authors: Nagabhushana Rao V. , Tucker P. G. , Jefferson-Loveday R. J. , Coull J. D.
Published: November 2013

Modern ‘high-lift’ blade designs incorporated into the low pressure turbine (LPT) of aero-engines typically exhibit a separation bubble on the suction surface of the airfoil. The size of the bubble and the loss it generates is governed by the transition process in the separated shear layer. However, the wakes shed by the upstream blade rows, the turbulent fluctuations in the free-stream and the roughness over the blade complicates the transition process.

The current paper numerically investigates the transition of a separated shear layer over a flat plate with an elliptic leading edge using large eddy simulations (LES). The upper wall of the test section is inviscid and specifically contoured to impose a streamwise pressure distribution over the flat plate to simulate the suction surface of a LPT blade. The influences of free-stream turbulence (FST), periodic wake passing and streamwise pressure distribution (blade loading) are considered.

The simulations were carried out at a Reynolds number of 83,000 based on the length of the flat plate (S0 = 0.5m) and the velocity at the nominal trailing edge (UTE ∼ 2.55 m/s). A high turbulence intensity of 4% and a dimensionless wake passing frequency (fr = fwakeS0/UTE, where fwake is the dimensional wake frequency) of 0.84 is chosen for the study. Two different distributions representative of a ‘high-lift’ and an ‘ultra-high-lift’ turbine blade are examined. An in-house, high order, flow solver is used for the Large Eddy Simulations (LES). The Variational Multi-scale approach is used to account for the sub-grid scale stresses.

Results obtained from the current LES compare favorably with the extensive experimental data previously obtained for the test cases considered. The LES results are then used to further explore the flow physics involved in the transition process, in particular the role of Klebanoff streaks and their influence on performance. The additional effect of surface roughness of the blade has also been studied for one of the blade loadings. The benefit that roughness can offer for highly loaded turbine blades is demonstrated.
Investigation of Rotor-Casing Interactions in the Centrifugal Compressor of a Helicopter Engine
Authors: Meingast Markus B. , Batailly Alain , Legrand Mathias , Ousty Jean-Philippe
Published: November 2013

The phenomenology of rotor-casing setups experiencing contact interactions is still poorly understood, particularly when complex geometries such as centrifugal compressors are involved. Although interaction phenomena have been witnessed and recorded during industrial experiments, the physical understanding of what occurs during these interactions is limited. The usual design approach is to consider possible modal interaction points in a linear framework and move these outside of normal operating conditions by means of minor geometric changes. Based on this linear approach, no information on the severity of these interactions is available to the designer. Besides, a possible interaction point appearing in the linear framework may not produce any harmful interactions, thereby increasing design restrictions.

Based on an in-house numerical strategy previously presented, contact interactions for a flexible centrifugal compressor from a helicopter engine and rigid casing setup are investigated. By imposing a small deformation on the casing geometry, blade/casing contact is initiated and subsequent interactions feature complex phenomena that are analyzed. In comparison to previous interaction simulations involving axial compressors, a higher degree of complexity of the numerical simulations stems from a strong curvature of the blade and very significant blade-disk coupling. This coupling presents itself towards the trailing edge where compressor mode shapes indicate a significant component normal to the casing surface. Accordingly, these modes may lead to large amplitude contact forces.

Time simulation results are confronted with experimental observations, and the consistency of the behavior of the numerical model with respect to industrial observations is underlined. A frequency domain post-processing of the results reveals specific engine order interactions and frequency spectra are plotted in order to interpret the phenomenon of interest. Such methodology will enable designers to more efficiently discriminate potential critical interaction speeds as compared to the classical linear frequency approach.
Forcing of Separation Bubbles by Main Flow Unsteadiness or Pulsed Vortex Generating Jets: A Comparison
Authors: Lyko Christoph , Dähnert Jerrit , Peitsch Dieter
Published: November 2013

Low pressure turbines typically operate in the low Reynolds number regime. Depending on the loading of the blade, they may exhibit detached flow with associated re-attachment in the rear part of the suction surface. Additionally the flow is highly time-dependent due to the sequence of rotating and stationary blade rows. The work presented in this paper covers experimental efforts taken to investigate this type of flow in detail. Typical low pressure turbine flow conditions have been chosen as baseline for the experimental work. A pressure distribution has been created on a flat plate by means of a contoured upper wall in a low speed wind tunnel. The distribution matches the one of the Pak-B airfoil. Unsteadiness is then super-imposed in two ways: A specific unsteadiness was created by using a Rotating Flap (RF) downstream of the test section. This results in almost sinusoidal periodic unsteady flow across the plate, simulating the interaction between stator and rotor of a turbine stage. Furthermore pulsed blowing by Vortex Generating Jets (VGJ) upstream of the suction peak was used to influence the transition process and development of the separation bubble. Measurements have been performed with hot-wire anemometry. Experimental results are presented to compare both forcing mechanisms. In sinusoidal unsteady main flow the transition occurs naturally by the breakdown of the shear layer instability, which is affected by periodic changes in the overall Reynolds number and thus pressure gradient. In opposition, Active Flow Control (AFC) by VGJ triggers the transition process by impuls and vorticity injection into the boundary layer, while maintaining a constant Reynolds number. The flow fields are compared using phase averaged data of velocity und turbulence intensity as well as boundary layer parameters, namely shape factor and momentum thickness Reynolds number. Finally a model to describe the time mean intermittency distribution is refined to fit the data.
Steady State CFD Investigation of a Radial Compressor Operating With Supercritical CO2
Authors: Rinaldi Enrico , Pecnik Rene , Colonna Pierot
Published: November 2013

The supercritical closed Brayton cycle concept is widely recognized as an attractive new option for energy conversion because of the very high-efficiency, reachable at moderate turbine inlet temperature, and the very compact general assembly. Carbon dioxide is chosen as the working fluid because it allows for its compression to occur close to the critical point at suitable temperatures, and high pressure. Compression work is thus small, if compared for instance to air compression. The concept was first studied in the sixties, and recent interest spreading in the scientific and technical community led to the realization of a small-scale proof-of-concept prototype operating at Sandia’s National Laboratories. Moreover, the CSP SunShot project was recently funded by the U.S. National Renewable Energy Laboratory, and it is aimed at the realization of a multi-megawatt concentrating solar power plant, whereby the power block will be a supercritical CO2 Brayton cycle turbine. Other pre-commercial activities are ongoing. This paper focuses on the study of the fluid dynamics of turbomachinery operating with fluids characterized by a complex thermodynamic behavior. The goal is to develop a complete methodology to help the aerodynamic design of scaled-up turbomachinery for supercritical CO2 gas turbine power plants. Starting from a previous analysis of the impeller of the radial compressor of the Sandia proof-of-concept test bench, the new detailed computational domain includes the tip clearance and the vaned diffuser, and has been obtained using an in-house meshing tool suited for turbomachinery geometries. The steady state interface between the impeller and the diffuser is treated with a mixing-plane. In order to correctly calculate the thermophysical properties of the fluid, affected by strong variations close to the critical point, the solver is coupled with an extensive library for the computation of properties of pure fluids and mixtures. An accurate multiparameter equation of state is selected and a look-up table approach is used to speed up the fluid properties evaluation. The results are finally compared with experimental data, and demonstrate the potential of the tool.
Experimental Validation of Numerically Optimized Short Annular Diffusers
Authors: Cerantola D. J. , Birk A. M.
Published: November 2013

Short annular diffuser systems consisting of a conical expansion section with negative wall angles and a solid diffusing section were tested experimentally and numerically. Three centre bodies with different wall angles and three outer walls with area ratios with respect to the annular diffuser inlet of 1.65, 1.91, and 2.74 were manufactured. The designs were selected from a set of ideal solutions determined by a numerical multiobjective optimization study completed by Cerantola and Birk (2012). Results presented in this paper are Reynolds number independent and are based on tests completed with an inlet Reynolds number of Ret ≈ 1.4 × 105 and Mach number Mt ≈ 0.16. Through considering the various centre body and outer wall configurations, an initial flow angle of 14° provides the best performance and the larger diameter outer wall generates more static pressure recovery at the expense of a reduction in outlet velocity uniformity and greater total pressure loss. The three selected designs were found to be in agreement with their optimum design qualities. When compared to computational solutions, the realizable k-ε turbulence model on coarse grids delivered the best relative comparison whereas the SST turbulence model predicted the best absolute agreement.
Investigation on Loading Pulsation of LP Long Blade Stage in Steam Turbine
Authors: Qi Mingxu , Yang Jiandao , Yang Rui , Yang Hong
Published: November 2013

Sliding grid and dual-time step method-based unsteady flow simulation on the final two stages of a steam turbine is performed without geometry scaling. The behavior and characteristic of unsteady pressure fluctuations on the blade surfaces of stator and rotor blades are investigated in details. FFT is performed on the blade surface pressure fluctuation to estimate the region covered by the unsteady flow perturbation from upstream and downstream. Results show that for long blade turbine stage investigated, pressure loading fluctuation is significant, with the maximum amplitude being equivalent up to level of 61% pressure loading. The loading fluctuation follows the trend of increasing from hub to shroud in stator while contrary trend is followed by the rotor. Among the unsteady perturbation sources causing the significant pressure fluctuations in long blade turbine stage, trailing edge shock is the dominating one, leading significant pressure distortion along circumferential direction, which induces strong interaction on downstream blade row.
Aerodynamic Analysis of a Boundary-Layer-Ingesting Distortion-Tolerant Fan
Authors: Florea Razvan V. , Voytovych Dmytro , Tillman Gregory , Stucky Mark , Shabbir Aamir , Sharma Om , Arend David J.
Published: November 2013

The paper describes the aerodynamic CFD analysis that was conducted to address the integration of an embedded-engine (EE) inlet with the fan stage. A highly airframe-integrated, distortion-tolerant propulsion preliminary design study was carried out to quantify fuel burn benefits associated with boundary layer ingestion (BLI) for “N+2” blended wing body (BWB) concepts. The study indicated that low-loss inlets and high-performance, distortion-tolerant turbomachines are key technologies required to achieve a 3–5% BLI fuel burn benefit relative to a baseline high-performance, pylon-mounted, propulsion system. A hierarchical, multi-objective, computational fluid dynamics-based aerodynamic design optimization that combined global and local shaping was carried out to design a high-performance embedded-engine inlet and an associated fan stage. The scaled-down design will be manufactured and tested in NASA’s 8′×6′ Transonic Wind Tunnel. Unsteady calculations were performed for the coupled inlet and fan rotor and inlet, fan rotor and exit guide vanes. The calculations show that the BLI distortion propagates through the fan largely un-attenuated. The impact of distortion on the unsteady blade loading, fan rotor and fan stage efficiency and pressure ratio is analyzed. The fan stage pressure ratio is provided as a time-averaged and full-wheel circumferential-averaged value. Computational analyses were performed to validate the system study and design-phase predictions in terms of fan stage performance and operability. For example, fan stage efficiency losses are less than 0.5–1.5% when compared to a fan stage in clean flow. In addition, these calculations will be used to provide pretest predictions and guidance for risk mitigation for the wind tunnel test.
Creep and Fatigue of Single Crystal and Directionally Solidified Nickel-Base Blades via a Unified Approach Based on Hill48 Potential Function: Part 1 — Plasticity and Creep
Authors: Ramaglia Alessandro D. , Villari Paolo
Published: November 2013

A simple phenomenological approach is devised based on the formalism introduced by Hill’s work on plasticity which allows for a unified description of the plastic-creep and fatigue responses of Single Crystal (SX) and Directionally Solidified (DS) nickel-base blades used in heavy duty gas turbines. To this end, the mechanical behavior of real SX and DS materials is approximated by adopting simplified classes of anisotropy able to reproduce the most relevant features of their material response. The hypotheses behind such choice depend from one side on a precise balance of various sources of uncertainties in both design analyses and experimental data and from the other side help in guaranteeing the respect of constraints on robustness, availability and maintainability of the design practice.

In this first part the modeling of plasticity and creep is discussed and accompanied by FEM analyses of real components. The set of tools presented here will be completed in the second part of this work where fatigue will be investigated.
Creep and Fatigue of Single Crystal and Directionally Solidified Nickel-Base Blades via a Unified Approach Based on Hill48 Potential Function: Part 2 — Low Cycle Fatigue
Authors: Ramaglia Alessandro D. , Villari Paolo
Published: November 2013

In the first part of this work the modeling of the anisotropic plastic and creep response of Single Crystal (SX) and Directionally Solidified (DS) nickel-base blades was discussed.

In this second part, the set of tools based on the Hill’s potential function is extended by showing how Hill like operators can be used to describe basic anisotropic features of the low cycle fatigue of SX and DS nickel-base superalloys. To this end a new Hill operator is introduced in order to set empirical life correlations and straightforward calibration procedures were set. The work is accompanied by applications on real heavy duty gas turbine components.