bluuebunny/crossref_metadata_2025_split huggingface dataset:
Flutter Susceptibility Approximation via Curve Fitting and MAC-Analysis in an Automated Optimization Design Process
Authors: Blocher Michael , Aulich Anne-Laure 
Published: November 2013

At present, several institutes of the German Aerospace Center (DLR) jointly work on the design of a counter rotating fan made of carbon fiber reinforced material. To generate blades with superior aerodynamic properties, a multidimensional optimization tool developed by the Institute of Propulsion Technology is used. Since aeroelastic problems often arise during late design stages and are inevitably very expensive to solve, the goal is to include aeroelastic estimations into this optimization tool which is used in the early design phase.
Assuming that rotors generally show a relatively sinusoidal behavior of the damping-over-IBPA-diagram at operating points sufficiently far away from the surge line, not all IBPAs need to be investigated in order to determine the lowest aerodynamic damping.
An algorithm was implemented that predicts the IBPA with the lowest damping value from three already calculated IBPAs through which a sine curve is fitted. It was established that this algorithm is valid for damping-vs-IBPA-curves that possess a roughly sinusoidal shape.
A further investigation of the possibilities of time reduction in the automated design process revealed that flow solutions and structural properties do not change significantly during one optimization run. Consequently, a method was found to measure the similarity of two different cases. For very similar cases it can be assumed that the aeroelastic properties do not change significantly. Therefore, just the IBPA with the predictably lowest aerodynamic damping value needs to be calculated.
An introduction to the theory of these approaches is presented and examples arising during the design of the counter rotating fan are given.
Aeroelastic Investigation of a Transonic Research Compressor
Authors: Leichtfuß S. , Holzinger F. , Brandstetter C. , Wartzek F. , Schiffer H. P. 
Published: November 2013

The trend in modern compressor design is towards higher stage loading and less structural damping, resulting in increased flutter risk. The understanding of the underlying aeroelastic effects, especially at highly loaded BLISK rotors, is small. This paper reports on the analysis of flutter phenomena in a modern transonic compressor.
The geometry examined here is the one-and-a-half stage transonic research compressor operated by Technische Universität Darmstadt. High blade deflections recorded during throttling measurements point to an aerodynamic excitation. Therefore, numerical investigations are carried out using the CFD-Code TRACE developed at the German Aerospace Center (DLR). Simulations are compared to measured compressor speed lines to validate the steady state results. The open source Finite Element code CalculiX is used to simulate the rotor blade eigenmodes and -frequencies. The results are then used in time-linearized calculations to determine the onset of flutter. These calculations confirm that there is an aerodynamic excitation of the first torsional eigenmode and blade flutter is at risk.
A sensitivity study is carried out to further investigate the aerodynamic conditions under which structural vibrations become unstable and to identify influencing factors.
A Review of the AFRL Turbine Research Facility
Authors: Anthony Richard J. , Clark John P. 
Published: November 2013

The Air Force Research Laboratory Turbine Research Facility is a transient blowdown facility that allows simultaneous measurement of unsteady heat transfer and aerodynamics on full scale engine hardware. It is unique for its size and consequent blowdown duration. Compared to engine validation testing, full scale, short duration turbine-rig testing is able to provide very large amounts of rotating turbine flowfield information using far less energy, orders of magnitude lower cost, and greater instrumentation selection.
This paper provides an updated review of the facility’s history, operation, and enhancements since its initial construction over two decades ago. Historical connections to pioneering work in short-duration turbine heat transfer testing are highlighted, and an overview of past developments, features, and capabilities is given. More recent experimental and computational integration is described using a suite of in-house developed CFD design and analysis tools. Example test programs include a non-proprietary 1+ 1/2 stage research turbine rig, which is the most heavily instrumented high pressure turbine tested in the facility to date.. Recent data illustrates the character of unsteady airfoil shock interactions that may lead to large levels of resonant stress or turbine high cycle fatigue. The paper ends with a brief discussion of future work.
Performance Estimation of a Turbine Under Partial-Admission and Flow Pulsation Conditions at Inlet
Authors: Ni Ron Ho , Humber William , Ni Michael , Sondergaard Rolf , Ooten Mike 
Published: November 2013

This paper describes a procedure for assessing the efficiency of a turbine operating under unsteady periodic flow at the inlet. Time-resolved flow simulations of the modified Garrett turbine case A3K7 are conducted with time-varying inlet conditions upstream of the turbine. A mass-weighted moving average is applied to the instantaneous data over a period corresponding to the pulsing frequency of the time-varying inlet conditions. The resulting time averaged data is then used to assess the convergence of the numerical solution and evaluate the turbine performance.
Three methods of computing the time-averaged turbine efficiency are presented. The first method, referred to as the TP method, is based upon the use of mass-weighted average of the total temperature (Tt) and total pressure (Pt). The second method, the TS method, uses the mass-weighted average of total temperature (Tt) and entropy (S). The third method, the WS method, employs the moving average of specific work output (Ws) and work lost due to entropy (S) increase.
A comparison of time-averaged specific work output, stage efficiency, and rotor efficiency is made for three turbine operating configurations: partial admission, synchronized pulsation, and sequential pulsing flow conditions of the sector inlets. Results show that for the tested rotor speed, the efficiency increases as the inlet pulsing frequency is reduced, and that when inlet sectors are not opened simultaneously, a large drop in efficiency occurs due to spillage of high energy fluid between the open and closed sectors. Additionally, it is shown that due to the non-convective nature of total pressure, the TP method for efficiency is not reliable while the TS and WS methods are able to produce consistent results.
Investigation of Efficient CFD Methods for the Prediction of Blade Damping
Authors: Elder Robin , Woods Ian , Patil Sunil , Holmes William , Steed Robin , Hutchinson Brad 
Published: November 2013

Accurate and efficient prediction of blade damping is one essential element in the engineering of durable and reliable compressors and turbines. Over the years, a variety of empirical and linearized methods have been developed and used, and have served well. Recently, the development of efficient unsteady CFD methods combined with an expansion in available and affordable computing power has enabled CFD analysis of blade damping. This paper looks at the prediction of aerodynamic blade damping using some recently developed CFD methods.
Unsteady CFD methods are used to predict the fluid flow in a transonic fan rotor, with tip Mach number of about 1.4. Deformation of the blade is determined from a mechanical pre-stressed modal analysis. In this investigation, blade motion for the first bending moments is prescribed in the CFD code, for a range of nodal diameters. After periodic unsteady solutions are obtained, damping coefficients are calculated based on the predicted blade forces and the specified blade motion.
Traditional unsteady CFD methods require the simulation of many blades in a given row, depending on the nodal diameter. For instance, for a nodal diameter of four, a wheel with 22 blades would require simulation of eleven blades. Computational methods have been developed which now enable simulation of only a few (1 or 2) blades per row yet yield the full sector solution, thus providing considerable savings in computing time and machine resources. The properties of the available methods vary, but one method, the Fourier Transformation method, has the property that it is frequency preserving, and hence suitable for the present task.
Fourier Transformation predictions, for a variety of nodal diameters, are compared with full sector predictions. Positive damping was predicted for this range of nodal diameters at design speed near peak efficiency operating condition indicating a stable system. The Fourier Transformation predictions for blade aerodynamic damping match very closely the reference full sector solutions. The Fourier transformation methods also provide solutions 3.5 times faster than average periodic reference cases.
Applicability of Quasi-3D Blade Design Methods to Profile Shape Optimizaton of Turbine Blades
Authors: Gezork Tobias , Völker Stefan 
Published: November 2013

The performance of gas turbine airfoils is continually improved by creating advanced aerodynamic and thermal designs. Optimization methods are used to handle the increasing complexity of such a design. However, optimization is expensive when performed based on 3D CFD calculations. Therefore, an optimization strategy based on simpler, less expensive analysis methods is desirable.
Oftentimes, a so-called quasi-3D (Q3D) approach is used, where 2D calculations are carried out on multiple, radially stacked meridional blade sections. This paper investigates the applicability of such an approach for optimization with regard to blade profile loss.
Obviously, certain physical effects are neglected using this approach, leading to errors in the predicted blade performance. Still, optimization based on Q3D calculations might be possible if the error is consistent, i.e. not random.
For this purpose, a design of experiment (DOE) was carried out to compare and correlate loss predictions from Q3D calculations and high-fidelity 3D CFD calculations for gas turbine blades. It is shown that the total pressure loss coefficients found with both the Q3D and 3D calculations correlate well (75–90%) to warrant the use of a Q3D method for profile shape optimization. Subsequently, an optimization is performed to demonstrate the applicability of the method.
Small Wind Turbines in the Built Environment: Influence of Flow Inclination on the Potential Energy Yield
Authors: Bianchi Serena , Bianchini Alessandro , Ferrara Giovanni , Ferrari Lorenzo 
Published: November 2013

Increasing interest is being paid by architects, project developers and local governments to understand where small wind turbines can effectively be exploited to provide delocalized power in the built environment.
The wind conditions in the rooftop area of buildings in urban locations are, however, very complex and the real adaptability of wind turbines to these environments is not yet tested both in terms of real producibility and of structural compatibility with the building themselves. In these installations, in particular, the flow which incomes on the rotors is often inclined with respect to the horizontal direction due to the interaction with the building façade and the roof.
A correct estimation of the impact of an inclined flow on the performance of horizontal-axis wind turbines therefore becomes a very relevant issue to correctly predict the potential energy yield of a machine.
To this purpose, a simulation code based on a Blade Element Momentum (BEM) approach was developed and validated by means of experimental data found in the literature. The code was then used to evaluate the energetic suitability of a small-size wind turbine installation in the rooftop of a building in a conventional European city.
A numerical CFD analysis was carried out to characterize the flow field in the rooftop area of different buildings. The flow velocity modulus and direction were calculated for several oncoming wind profiles: the results were projected into an available wind power curve in the rooftop of the building.
The effective energy-yield capabilities were then corrected using the model for the flow inclination as a function of the specific flow conditions in the rooftop area.
The results were finally exploited to analyze the energy-oriented feasibility of an installation in a similar context.
Numerical Investigation of the Unsteady Interaction Within a Close-Coupled Centrifugal Compressor Used in an Aero Engine
Authors: Wilkosz Benjamin , Zimmermann Markus , Schwarz Philipp , Jeschke Peter , Smythe Caitlin 
Published: November 2013

The present work forms part of a research project of the Institute of Jet Propulsion and Turbomachinery at the RWTH Aachen University in collaboration with GE Aviation. The subject is the detailed numerical analysis of the unsteady flow field focusing on the interaction between the impeller and the passage diffuser of a close-coupled transsonic centrifugal compressor used in an aero engine.
The centrifugal compressor investigated is characterized by a close-coupled impeller and passage diffuser with a radial gap of only 3.6%. The close coupling tends to provide a high aerodynamic efficiency but simultaneously cause a high unsteady interaction between the impeller and the diffuser. These unsteady effects can have a significant impact on the performance of both components [1,2] and present a challenge to state-of-the-art numerical methods. With increasing compressor efficiency, the more important it is to have a understanding of the detailed unsteady flow physics. Experimental data was obtained from a state-of-the art centrifugal compressor test rig located at the Institute of Jet Propulsion [3]. Steady and unsteady pressure measurements within the impeller and diffuser are used to gain detailed information on the temporal, time-averaged and spectral pressure distributions within the stage to validate the CFD.
The work presented shows the unsteady phenomena caused by the interaction as well as the location and propagation of these phenomena within the centrifugal stage. Within the impeller, the exducer is in first order excited by the BPF of the diffuser, whereas in the diffuser both the BPF, as well as the PPF, are present up until the end of the pipe-diffuser. Significant effects on the integral component performance could only be identified for the impeller. Special focus is paid to evaluate the diffuser upstream pressure field, since this is the major source of unsteadiness within the impeller. The performance of the rotor decreases due to the unsteady interaction. This effect is traced back to the unsteady tip-clearance flow, in which the time-averaged mass transport decreases, whereas the specific entropy production increases in a nonlinear way. Within the diffuser, local effects counteracting with respect to the integral performance are found. In front of the throat, there is less decay in total pressure, as a result of tangentially expanding pressure waves. Within the passage an decrease in flow uniformity in the unsteady flow is identified as the reason for the lower diffusion up until the throat and higher losses within the downstream diffuser passage.
Forced Response Analysis of a Steam Turbine Shrouded Blade: Numerical Analysis and Experiments
Authors: Piraccini Francesco , Lorusso Salvatore , Maceli Nicola , Ryman John 
Published: November 2013

Faced with the present transformation of the world economy, steam turbine manufacturers are seeking ways to remain competitive in their respective markets.
Having longer Low Pressure (LP) blades and seeking for higher rotating speeds have always been two effective methods to improve the Steam Turbine efficiency, therefore to reduce steam consumption and related plant costs.
Both trends have increased the risk of failure for forced response due to the occurrence of resonance or to the decrease of alternating stress margins.
Because of large uncertainties in the estimation of friction damping and aerodynamic excitation, the prediction of dynamic response of the long blades in the LP section is still a challenge for the analytical tools; therefore, expensive activities for experimental validation are usually required.
In order to reduce design costs and time, a set of tools has been developed and validated using the test data collected during a full-scale test vehicle campaign in steam (Low Pressure Development Turbine - LPDT). In this study, the validation activity related to the blade response due to nozzle stimulus is reported.
As a first step, a steady state CFD analysis was performed at the operating conditions where significant response was observed, caused by the resonance with the Nozzle Passing Frequency (NPF). Then, an unsteady CFD analysis of the bucket blade was conducted considering the perturbation due to the nozzles. Subsequently, the computed unsteady pressure distribution on the blade airfoil was mapped onto a finite element model and forced response analyses were performed to estimate the bucket dynamic response. The predicted response was compared against measured test data and good correlation was found.
Excited Blade Vibration for Aeroelastic Investigations of a Rotating Blisk Using Piezo-Electric Macro Fiber Composites
Authors: Belz Joachim , May Markus , Siemann Jan , Seume Joerg R. , Voigt Christian , Böhmer Heiko , Grüber Björn 
Published: November 2013

Experimental investigations of the aerodynamic damping of compressor blades are usually performed by forcing the blades externally to a motion that is similar to a given mode shape and inter-blade phase angle (IBPA) while measuring the motion-induced unsteady pressure distribution. Evaluating this to an aerodynamic work entry from the fluid to the blade, at least a local contribution to the flutter (in-)stability can be determined. Test rigs are usually non-rotating linear or rotating annular cascade wind tunnels. In the latter case, besides measuring in and transmitting data out of the rotating system, the excitation of the blades themselves is still a challenge.
In the present case a blisk rotor with realistic fan blade geometries and flow conditions was investigated aeroelastically. For the excitation of the 1st bending (1F) mode shape of the blading a sector of five blades was chosen. In this sector the natural vibration of the blading, represented by constant vibration amplitudes and a given IBPA should be simulated. Therefore the blades were equipped with Macro Fiber Composites (MFC). These foils of piezoelectric material expand and contract due to the applied high voltage. A control system was developed to adjust the amplitude and the IBPA of the blade vibration. For the transmission of the high voltage, a separate data transmission system on the base of liquid metals was chosen. The blade vibration was measured by strain gauges and additionally monitored by a specific rig system.
The aeroelastic investigations were carried out in the compressor test facility M2VP of the DLR Institute of Propulsion Technology in Cologne. During the measurement, the MFCs were able to excite the blades to a certain extent. The paper will present the technique to excite the blades of a compressor blisk by means of MFCs as well as achieved vibration amplitudes and limitations under engine-like operating conditions.
Numerical Investigation of an Additively Manufactured Compressor Casing: The Effect of Auxetic Structures on the Tip Clearances
Authors: Rockel David , Weihard Stefan , Hachmann Andreas , Hupfer Andreas , Kau Hans-Peter 
Published: November 2013

A new approach for a compressor casing design has been developed using new design possibilities offered by additive manufacturing technologies. Tip clearance behaviour is a serious problem in compressor design leading to increased aerodynamic losses and limiting the operating range of a compressor through a decrease in surge margin. Efforts are necessary to minimise the tip clearance in the load cycle of an engine.
The approach presented in this work uses auxetic structures in a double-walled compressor casing with the aim to force the radial expansion behaviour of the inner casing wall in order to adapt the casing to the rotor. The studies of the casing and the calculations of the transient rotor displacement are carried out by using coupled-temperature-displacement calculations using commercial FEM-tools.
Elements which influence the radial expansion of the casing are identified with the aim of generating a modular system for future compressor casing developments. So the casing elements are adaptable to the radial expansion of a certain rotor. An improved tip clearance behaviour in relevant operating points is achieved. In the latter part of the paper, practical design considerations are made as the auxetic structures are integrated into the CAD model of an existing compressor casing. Design challenges are also briefly discussed.
This paper presents a new potential application of additive manufacturing technologies in future aero engine design. The focus of this paper is on the practical implementation of the new design freedom using the example of a compressor casing. The advantages for the overall engine performance as well as identified problems are also discussed qualitatively.
Aerodynamic Design and Analysis of Centrifugal Turbine Cascades
Authors: Persico G. , Pini M. , Dossena V. , Gaetani P. 
Published: November 2013

The centrifugal turbine architecture represents a promising solution for Organic Rankine Cycle (ORC) Systems, in the small-to-medium power range. The large volumetric expansion ratios occurring in ORC turbines complicate the design of the turbo-expander, making the centrifugal arrangement worth of interest with respect to conventional architectures. A new-concept centrifugal turbine has been recently proposed by the authors, based on the design of Ljungström but implementing a stator-rotor arrangement, which allows for multi-stage assembly without compromising compactness.
To properly evaluate the potential of the centrifugal turbine solution, reliable data on cascade aerodynamic performances are required, but they are still lacking in the open literature. In this paper the aerodynamics of radial-outward turbine cascades is discussed, on the basis of Computational Fluid-Dynamics (CFD) simulations. A weakly transonic operating condition is selected and for that different classes of profiles are tested.
Results show that the intrinsic diverging shape of the radial-outward configuration complicates the blade design; if the flow deflection is not properly controlled along the streamwise direction, the bladed duct can result in a converging-diverging channel, leading to unexpected chocked flows and shocks even in weakly transonic configurations. The indications achieved by the comparison between different blade profiles are gathered to define guidelines for the design of novel elliptic profiles, which allow to control the flow acceleration process, providing high aerodynamic efficiency. Off-design performances of the elliptic profiles are finally addressed by studying the response of the cascade to different expansion ratios and high incidence angles.