bluuebunny/crossref_metadata_2025_split huggingface dataset:
On Inflow Distortion Decay in the Blade Cascades
Authors: Vassiliev Vladimir , Kappis Wolfgang , Granovsky Andrey , Shchaulov Vladimir 
Published: November 2013

Flow distortions in front of turbomachines can lead to additional mechanical and thermal loading of the blades, and may influence the operational stability of the engine and the lifetime of its parts. In current design practice the impact of inflow distortions is accounted by safety margins, which are often conservative. Therefore the knowledge of real limits helps to increase operational flexibility and recommended service intervals.
The distortions inside turbomachines have a wide spectrum of sizes and frequencies depending on their origin. Some of them were extensively studied in the recent years, including wakes interaction with blades, hot streaks propagation in turbines, and some other phenomenon. However, there are areas of practical importance, which are not yet covered by available studies. One of this distortion types is investigated in the present work.
These are distortions of a size, comparable with the inlet guide vane pitch of a compressor or the nozzle guide vane pitch of a turbine, both steady in the absolute frame of reference. Such distortions in front of a compressor can be caused, for example, by fouling of the intake, or by some structures purposely installed inside the intake. The distortions in front of the expansion turbine can be caused by structures.
Main purpose of the study is to provide an understanding of the decay mechanism of distortions considered. For this purpose the numerical and theoretical analysis of inflow distortion propagation inside the set of stationary and rotating blades cascades is performed. These cascades represent the mid-section of a typical multistage compressor and turbine.
The analysis showed that the decay mechanism is different for temperature distortions and total pressure distortions, and that the latter one is stronger affected by the blading configuration.
Prediction of Flow in an Exhaust Gas Turbine Diffuser With a Scale-Adaptive Simulation Model
Authors: Volkmer Silke , Schatz Markus , Casey Michael , Montgomery Matthew 
Published: November 2013

The prediction of the flow in a gas turbine exhaust diffuser of a combined cycle power plant is particularly difficult as maximum performance is obtained with highly loaded diffusers, which operate close to boundary layer separation. CFD (computational fluid dynamics) simulations then need to cope with complex phenomena such as smooth wall separation, recirculation, reattachment, blockage and free shear layer mixing. Recent studies based on the RANS (Reynolds-Averaged-Navier-Stokes) approach demonstrate the challenge for two-equation turbulence models to predict separation and mixing of the flow correctly in such highly loaded diffusers and identify that more accurate methods are needed. Hence, the application of a hybrid Scale-Adaptive Simulation (SAS) is investigated and the CFD results are compared with experimental results from an in-house test rig.
In the present study the flow in a model exhaust diffuser (for heavy-duty gas turbine diffuser applications typical Reynolds number 1.5×106 and inlet Mach number 0.6) is examined with unsteady RANS (URANS) simulations with the SST (Shear Stress Transport) model as well as a hybrid Scale-Adaptive Simulation (SAS) model. The SAS model switches from URANS to a mode similar to a Large Eddy Simulation (LES) in unsteady flow regions to resolve various scales of detached eddies. The current study shows that with the SST model similar results are obtained with RANS and URANS simulations, whereas the more complex SAS model leads to a much better resolution of the unsteady fluctuations. However, the time-averaged results of the SAS calculations overpredict the blockage of the separation and hub wake. This results in an underprediction of the pressure recovery and the mixing of the flow compared to the simpler two-equation models and also compared to experimental results.
Aerodynamic Loss Measurements in a Compressible Flow Vane Cascade Showing the Influence of Reynolds Number Lapse
Authors: Mihelish M. P. , Ames F. E. 
Published: November 2013

Turbofan engines are designed to operate over a range of conditions depending on the mission of the aircraft. Many unmanned aerial vehicles (UAVs) have been designed to use turbofans and these aircraft can operate at service ceilings up to 65,000 ft. At these altitudes the pressure drops to about 6% of ambient conditions and Reynolds numbers throughout the turbine drop precipitously. Understanding and designing for the influence of these large changes in engine Reynolds numbers over the aircraft’s operating range is key to the success of the aircraft and its mission. The present paper documents the influence of chord exit Reynolds number (90,000 to 720,000), inlet turbulence (0.8% and 9%), and exit Mach number (0.7, 0.8, and 0.9) on the exit losses of a first vane design consistent with turbofan engines designed to power high altitude UAVs.
These aerodynamic loss measurements were acquired in a new closed loop compressible flow facility. This closed loop wind tunnel was designed to test turbine cascades over a wide range of Reynolds numbers by controlling the pressure in the tunnel. The current measurements were acquired using a five-hole cone probe controlled with a sealed two-axis traversing system. This first stage vane test section was configured in a three full passage four-vane linear cascade arrangement with upper and lower bleed flows. The low turbulence condition was developed using a flow conditioning section consisting of two perforated plates and two fine-mesh screens and a 4.7 to 1 area ratio nozzle. The high turbulence condition was developed using a simulated combustor. The present results show a significant increase in exit losses with decreasing Reynolds number. This increase in losses is largely due to the thickening of the wake.
Correlation of Parameters to Instability and Chaos of a Horizontal Flexible Rotor Ball Bearing System
Authors: Gupta T. C. , Gupta K. 
Published: November 2013

The higher order effects from ball bearing nonlinearities cause complex vibration characteristics in rotor ball bearing systems. The sources of nonlinearities are internal radial clearance, Hertzian contact forces between balls and races and varying compliance effect. The same authors in their earlier work have identified the sets of parameters corresponding to instability and chaos for a horizontal flexible rotor supported on deep groove ball bearing. To the best of author’s knowledge, there is not much work reported in the literature on the dynamic analysis for instability and chaos, which is based on energy functions and bearing loads. Extending the preceding research work in the present paper by using a typical set of parameters and specifications of rotor ball bearing system, a correlation of parameters to instability and chaos is attempted using different energy functions associated with the dynamical system.
A generalized Timoshenko beam finite element formulation is used to model the flexible rotor shaft. To achieve the convergence of solution with smaller number of elements, shape functions are derived from the exact solutions of governing differential equations of Timoshenko beam element. The sources of excitation are rotating unbalance and parametric excitation due to varying compliance of ball bearing during motion. For the bearing used in the present paper, the ratio of these excitation frequencies comes out to be an irrational number. Therefore, the dynamic response would be quasi-periodic with time period equal to infinity. To extend the use of non-autonomous shooting method to derive quasi-periodic solution, the fixed point algorithm (FPA) proposed in the literature is used to deduce the time period for non-autonomous shooting algorithm. The shooting method otherwise is used only to derive periodic solutions. Thus the non-autonomous shooting method coupled with fixed point algorithm (FPA) is used to compute the quasi-periodic solution, which also gives the monodromy matrix. The eigenvalues of the monodromy matrix, called Floqoet multipliers, give information about instability. The chaotic nature of the dynamic response is established by the maximum value of Lyapunov exponent.
Once the instability and chaos is confirmed based on computed values of Floquet multipliers and Lyapunov exponents, the nature of the work done (positive or negative) by different conservative and non-conservative forces and moments during motion are analyzed and the fundamental causes, which make the system response unstable and / or chaotic, are established.
Flexible Seal Strip Design for Advanced Labyrinth Seals in Turbines
Authors: Herrmann N. , Dullenkopf K. , Bauer H.-J. 
Published: November 2013

The paper discusses the potential benefit of flexible seal strips in labyrinth seals for turbines. By reducing the radial stiffness compared to a standard straight and stiff knife, seal clearance could be reduced without significantly reducing the seal durability and long-term performance. As contact between the seal strips and the rotor can occur especially during transient operating phases, a more flexible design of the seal strips can prevent damage and wear, keeping the discharge rates constantly low. However, the pressure difference across the fin will cause a deflection of the seal strip due to the increased flexibility and thus creating an additional possible risk for an unwanted contact. Pressure balanced designs and supports on the low pressure side are used on the investigated seal designs to eliminate that risk.
To give evidence of possible performance gain a standard labyrinth seal configuration is compared to two configurations with segmented and curved seal strips. In a first step, the discharge coefficient and the leakage rates for the nominal seal design are calculated using two-dimensional CFD. In order to investigate the impact of a worn seal tip on the leakage flow, the geometry change due to a rubbing event is simulated with FEA tools. Therefore, a specific high-speed wear model is implemented and calibrated by experimental data, enabling the correct cooling effects and plastic deformation. The discharge coefficient and the leakage mass flow rates of the worn geometry are then again modeled with CFD for the various seal configurations and compared to the unworn state.
The study shows that a wise combination of the advantages of flexible curved seal strips can be used to reduce the leakage rates significantly, improving the life time of seal elements at the same time.
A New Partial Admission Method for Turbocharger Turbine Control at Off-Design
Authors: Challand Sebastian , Dirschauer Eckart , Ilievski Mirko , Casey Michael , Schatz Markus 
Published: November 2013

A new method for turbocharger control in automotive applications is presented. It is called MEDUSA (Multiple Exhaust DUct with Source Adjustment, European patent application number: 21326 - EP) and is a partial admission system consisting of several separate flow channels that connect the exhaust duct of the engine and individual nozzle segments of the turbine. By opening or closing the individual flow channels using external valves, the turbine flow can be adjusted, hence allowing the whole turbocharger to be controlled. Due to the use of external valves, the system is considerably more robust than other variable geometry systems based on variable inlet guide vanes and thus becomes suitable for application to spark-ignition motors at high temperature.
The paper presents a theoretical assessment of this innovative control system, based on one dimensional considerations and CFD simulations. The CFD-calculations of the MEDUSA-system are compared to those of a turbocharger turbine controlled with a variable inlet nozzle. The results indicate that the performance and operating range of the new system is comparable, or even better, than the currently used variable nozzle systems, especially at low load conditions. This indicates that further experimental work is justified as it could become considerably more effective than the typical waste gate systems used in spark ignition engines and provides a new solution for the turbocharger control in these applications. So far, only radial turbines have been considered for application of this method but it could also be used for mixed-flow or axial turbines.
Extended Turbomachinery Aeromechanical Model
Authors: Romanov Artyom , Di Mare Luca 
Published: November 2013

An extended version of the previously presented linearized one-dimensional turbomachinery performance model is described. The current version of the model is capable of performing forced response and flutter simulations on several stationary and/or rotating bladerows. The amplitude of the perturbation is assumed small thus impact of the perturbations of several sources may be superimposed. The distortion propagation analysis may be performed in the early stages of the design process, or whenever a quick solution is desirable, having only minimal information about the studied geometry.
The approach has a block structure, where each block represents a bladed passage or the empty space between. The blocks contain linearized gas relations that relate the gas state to known changes of enthalpy, entropy and momentum. The blade blocks are represented using the extended semi-actuator disk theory, where the flow inside the passage assumed to be one-dimensional [9]. The model considers frequency scattering for the rotating bladerows and is also using a complete package of linearized loss- and deviation correlations, providing more realistic results. The approach extends the previously presented methods by Amiet [2], [3] and Kaji&Okazaki [4], [5], being capable to handle harmonic distortions of various wavelength-to-chord ratios. Minimal assumptions are made about the studied geometry and nature of the gas, allowing to perform unsteady flow analysis not only on the idealized cases, but on more complex, realistic geometry.
Appropriate non-reflecting boundary conditions are applied at the boundaries of each block, using the hyperbolic characteristic theory, thus facilitating multi-blaredow domains setup and allowing running more complex cases, involving both forced response and flutter.
A number of idealized cases presented by Amiet and Kaji & Okazaki are reproduced to validate the model against the reference data, where a good comparison is achieved. The approach is also tested on the HP compressor of modern design for forced response simulations. Several multi-bladerow cases are also studied.
New Simulation Process for the Life Management of the Gas Turbine Rotors
Authors: Di Sisto Paolo , Parodi Mauro 
Published: November 2013

The Gas Turbine manufacturers are continuously engaged in providing to their customers machines with higher performances, longer lives, better reliability and availability. Since the late 70’s, the gas turbines are designed by using computational tools able to simulate through physic based models the thermal and mechanical behavior of the engines by predicting with high accuracy gas turbine internal pressures, temperatures, stresses etc., across the full flange to flange architecture; but it is only in the last one or two decades that computational predictions have done a huge step forward thanks to the great progress of the information technology. What was before designed with a worst case deterministic approach, it is now designed through optimization, by assessing many different configurations and it is made robust through sensitiveness and statistical assessments.
One further step forward has been made possible by new software tools able to manage simulation process flows made of a variety of applications, including commercial CAD/CAE software, in house software and Excel spreadsheets. The simulation process described in this paper has been developed for rotor life assessments and the proprietary company is currently using the process to manage part of its heavy duty gas turbine fleet engaged in the oil and gas application for the estimation of risk to extend rotor life beyond inspection interval [1].
Individually optimized uniform contrast enhancement in CT angiography for the diagnosis of pulmonary thromboembolic disease—A simulation study
Authors: Xue Ming , Zhang Hao , Kligerman Seth , Klahr Paul , D’Souza Warren , Lu Wei 
Published: November 2013

Purpose:To improve the diagnostic quality of CT pulmonary angiography (CTPA) by individually optimizing a biphasic contrast injection function to achieve targeted uniform contrast enhancement. To compare the results against a previously reported discrete Fourier transform (DFT) approach.Methods:This simulation study used the CTPA datasets of 27 consecutive patients with pulmonary thromboembolic disease (PE). An optimization approach was developed consisting of (1) computation of the impulse enhancement function (IEF) based on a test bolus scan, and (2) optimization of a biphasic contrast injection function using the IEF in order to achieve targeted uniform enhancement. The injection rates and durations of a biphasic contrast injection function are optimized by minimizing the difference between the resulting contrast enhancement curve and the targeted uniform enhancement curve, while conforming to the clinical constraints of injection rate and total contrast volume. The total contrast volume was limited first to the clinical standard of 65 ml, and then to the same amount used in the DFT approach for comparison. The optimization approach and the DFT approach were compared in terms of the root mean square error (RMSE) and total contrast volume used.Results:When the total contrast volume was limited to 65 ml, the optimization approach produced significantly better contrast enhancement (closer to the targeted uniform contrast enhancement) than the DFT approach (RMSE 17 HU vs 56 HU,p < 0.00001). On average, the optimization approach used 63 ml contrast, while the DFT approach used 50 ml with four patients exceeding 65 ml. When equivalent total contrast volume was used for individual patient, the optimization approach still generated significantly better contrast enhancement (RMSE 44 HU vs 56 HU, p < 0.01). Constraints for the injection function could be easily accommodated into the optimization process when searching for the optimal biphasic injection function.Conclusions:The optimization approach generated individually optimized biphasic injection functions yielding significantly better contrast enhancement compared to the DFT approach. This new approach has the potential to improve the diagnostic quality of CTPA for PE.
Cost-Effectiveness of Oral Anticoagulants for Treatment of Atrial Fibrillation
Authors: Canestaro William J. , Patrick Amanda R. , Avorn Jerry , Ito Kouta , Matlin Olga S. , Brennan Troyen A. , Shrank William H. , Choudhry Niteesh K. 
Published: November 2013

Background—
New anticoagulants may improve health outcomes in patients with atrial fibrillation, but it is unclear whether their use is cost-effective.

Methods and Results—
A Markov state transition was created to compare 4 therapies: dabigatran 150 mg BID, apixaban 5 mg BID, rivaroxaban 20 mg QD, and warfarin therapy. The population included those with newly diagnosed atrial fibrillation who were eligible for treatment with warfarin. Compared with warfarin, apixaban, rivaroxaban, and dabigatran, costs were $93 063, $111 465, and $140 557 per additional quality-adjusted life year gained, respectively. At a threshold of $100 000 per quality-adjusted life year, apixaban provided the greatest absolute benefit while still being cost-effective, although warfarin would be superior if apixaban was 2% less effective than expected. Although apixaban was the optimal strategy in our base case, in probabilistic sensitivity analysis, warfarin was optimal in an equal number of iterations at a cost-effectiveness threshold of $100 000 per quality-adjusted life year.

Conclusions—
While at a standard cost-effectiveness threshold of $100 000 per quality-adjusted life year, apixaban seems to be the optimal anticoagulation strategy; this finding is sensitive to assumptions about its efficacy and cost. In sensitivity analysis, warfarin seems to be the optimal choice in an equal number of simulations. As a result, although all the novel oral anticoagulants produce greater quality-adjusted life expectancy than warfarin, they may not represent good value for money.
Poleward Shift and Change of Frontal Activity in the Southern Hemisphere over the Last 40 Years
Authors: Solman Silvina A. , Orlanski Isidoro 
Published: November 2013

Abstract
Several studies have documented the poleward shift of the midlatitude westerly jet of the Southern Hemisphere during the last decades of the twentieth century, mainly during the warm season. In this work the consistency between this change and the seasonal changes in frontal activity and precipitation are explored. The authors also attempt to identify the correlation between frontal activity and precipitation changes.
Frontal activity is defined using the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) dataset for the period 1962–2001 as the temperature gradient times the relative vorticity at 850 hPa. Considering cyclonic systems only, an enhancement of the frontal activity at high latitudes in the last two decades is apparent. However, the pattern of frontal activity change is not zonally symmetric, with the zonal asymmetries consistent with the climate change signal of the zonal anomaly of the 300-hPa geopotential height.
The pattern of precipitation change, showing midlatitude drying and high-latitude moistening, is consistent with the pattern of the frontal activity change, explaining to a large extent both the zonal mean and asymmetric rainfall changes. This consistency is also found in terms of the year-to-year variability of the zonal mean at both mid- and high latitudes. However, the frontal activity has a complex relationship with rainfall (not every frontal system is associated with rainfall events), and this consistency is unclear over some specific regions.
Results presented here highlight the robust link between the change in the asymmetric component of the upper-level circulation, the frontal activity, and rainfall over the mid- to high latitudes of the Southern Hemisphere.