Rainer Schnell

Investigation of the Tonal Acoustic Field of a Transonic Fanstage by Time-Domain CFD-Calculations With Arbitrary Blade Counts

In order to investigate the tonal noise generated by viscous blade/vane interaction in a typical transonic bypass engine fan stage, time domain calculations were carried out with the non-linear Navier-Stokes solver TRACE. The propagating acoustic field was obtained by a modal decomposition of near field flow data. Comparisons with available experimental data in the bypass duct casing are presented. Different OGV designs of the investigated fanstage were evaluated with respect to the emitted tonal noise of the stage. To ensure the correct modal structure of the acoustic field it is vital to employ the correct blade count ratio in numerical simulations. To accomplish this efficiently, phase-lagged boundary conditions were incorporated into the employed flow solver. The method has been validated using a counter-rotating propfan configuration and was then applied for the three dimensional, spatially highly resolved fanstage calculation.Copyright

Investigation of Blade Tip Interaction With Casing Treatment in a Transonic Compressor—Part I: Particle Image Velocimetry

A single-stage transonic axial compressor was equipped with a casing treatment (CT), consisting of 3.5 axial slots per rotor pitch in order to investigate the predicted extension of the stall margin characteristics both numerically and experimentally. Contrary to most other studies the CT was designed especially accounting for an optimized optical access in the immediate vicinity of the CT, rather than giving maximum benefit in terms of stall margin extension. Part 1 of this two-part contribution describes the experi¬mental investigation of the blade tip interaction with casing treatment using Particle image velocimetry (PIV). The nearly rectangular geometry of the CT cavities allowed a portion of it to be made of quartz glass with curvatures matching the casing. Thus the flow phenomena could be observed with essentially no disturbance caused by the optical access. Two periscope light sheet probes were specifically designed for this application to allow for precise alignment of the laser light sheet at three different radial positions in the rotor passage (87.5%, 95% and 99%). For the outermost radial position the light sheet probe was placed behind the rotor and aligned to pass the light sheet through the blade tip clearance. It was demonstrated that the PIV technique is capable of providing velocity information of high quality even in the tip clearance region of the rotor blades. The chosen type of smoke-based seeding with very small particles (about 0.5 µm in diameter) supported data evaluation with high spatial resolution, resulting in a final grid size of 0.5 x 0.5 mm. The PIV data base established in this project forms the basis for further detailed evaluations of the flow phenomena present in the transonic compressor stage with CT and allows validation of accompanying CFD calculations using the TRACE code. Based on the combined results of PIV measurements and CFD calculations of the same compressor and CT geometry a better understanding of the complex flow characteristics can be achieved, as detailed in Part 2 of this paper.

Compressor Leading Edge Sensitivities and Analysis with an Adjoint Flow Solver

Based on the results of a prior study about fan blade degradation, which state a noticeable influence of small geometric changes on the fan performance, an adjoint computational fluid dynamics method is applied to systematically analyze the sensitivities of fan blade performance to changes of the leading edge geometry.As early as during manufacture, blade geometries vary due to fabrication tolerances. Later, when in service, engine operation results in blade degradation which can be reduced but not perfectly fixed by maintenance, repair and overhaul processes. The geometric irregularities involve that it is difficult to predict the blade’s aerodynamic performance. Therefore, the aim of this study is to present a systematic approach for analyzing geometric sensitivities for a fan blade.To demonstrate the potential, two-dimensional optimizations of three airfoil sections at different heights of a transonic fan blade are presented. Although the optimization procedure is limited to the small area of the leading edge, the resulting airfoil sections can be combined to a three-dimensional fan blade with an increased isentropic efficiency compared to the initial blade.Afterwards, an adjoint flow solver is applied to quasi-three-dimensional configurations of an airfoil section in subsonic flow with geometric leading edge variations in orders representative for realistic geometry changes. Validations with non-linear simulation results demonstrate the high quality of the adjoint results for small geometric changes and indicate physical effects in the leading edge region that influence the prediction quality.© 2013 ASME

Analyzing and optimizing geometrically degraded transonic fan blades by means of 2D and 3D simulations and cascade measurements

The present study deals with the influence of geometrically degraded transonic engine fan blades on the fan’s aerodynamic behavior. The study is composed of three phases; the first consists of 3D simulations to point out changes in the performance parameters caused by blade degradations. In the second phase, 2D optimizations are carried out to determine the potential of redesigning the blade and in the third phase, measurements on a transonic cascade are used to experimentally verify the numeric results.During engine operation as well as maintenance processes, geometric variations of the fan blades, and especially of the blades’ leading edges, are observed. They mainly originate from the ambient conditions under which the engine is operated. Though the deformations of the blade differ widely, several typical degradation types can be identified. In advance of the study, these degradation types have been systematized and simplified models representing different degrees of degradation have been built.In the first phase, the models are aerodynamically analyzed by means of 3D simulations. A high influence on the performance parameters is found for a fan blade exposed to long-term erosion. The model’s characteristics are a blunt leading edge and a reduced chord length. In contrast, the performance parameters of a model representing a re-contoured blade (reduced chord length but reshaped leading edge) are shown to be similar to those of a new fan blade. This leads to the conclusion that an eroded blade may offer almost the initial performance parameters as long as the leading edge is well reshaped.Since the model of the long-term eroded blade shows great changes in the fan’s performance and the best optimization potential, this has been chosen for the further analysis in the following phases.In the second phase, 2D optimizations are applied to three airfoil sections at different heights of the blade. The parameterization used is limited to a small area of the leading edge; the shape of the rest of the blade is kept constant. The optimizations lead to loss reduction and demonstrate the potential of the optimization process.The third phase is carried out in the Transonic Cascade Wind Tunnel of the Institute of Propulsion Technology in Cologne. As the transonic part of the fan blade is the most sensitive to geometric changes, a transonic airfoil with long-term erosion has been chosen. During the tests, the following measurement techniques are applied: Static pressure probes to determine the Mach number distribution, a 3-hole probe to detect exit angle and loss distribution, Schlieren photographs and PIV-measurements to locate the shock system, the L2F method to measure the cascade inflow angle and to resolve the boundary layer distribution and Liquid crystal measurements to observe transition activities. The full analysis of the measurements with PIV, L2F and Liquid Crystals are still in progress, but the evaluation of the loss polar and the Schlieren photographs show increased losses for the degraded blade and a good match with the numeric results.Copyright

Investigation of Blade Tip Interaction With Casing Treatment in a Transonic Compressor—Part II: Numerical Results

A single stage transonic axial compressor was equipped with a casing treatment consisting of 3.5 axial slots per rotor pitch in order to investigate its influence on stall margin characteristics, as well as on the rotor near tip flow field, both numerically and experimentally. Contrary to most other studies, a generic casing treatment (CT) was designed to provide optimal optical access in the immediate vicinity of the CT, rather than for maximum benefit in terms of stall margin extension. The second part of this two-part paper deals with the numerical developments and their validation, carried out in order to efficiently perform time-accurate casing treatment simulations. The numerical developments focus on the extension of an existing coupling algorithm in order to carry out unsteady calculations with any exterior geometry coupled to the main flow passage (in this case a single slot), having an arbitrary pitch. This extension is done by incorporating frequency domain, phase-lagged boundary conditions into this coupling procedure. Whereas the phase lag approach itself is well established and validated for standard rotor-stator calculations, its application to casing treatment simulations is new Its capabilities and validation will be demonstrated on the given compressor configuration, making extensive use of the detailed particle image velocimetry flow field measurements near the rotor tip. Instantaneous data at all measurement planes will be compared for different rotor positions with respect to the stationary slots in order to evaluate the time-dependent interaction between the rotor and the casing treatment.

Investigation of Blade Tip Interaction With Casing Treatment in a Transonic Compressor: Part 2—Numerical Results

A single-stage transonic axial compressor was equipped with a casing treatment, consisting of 3.5 axial slots per rotor pitch in order to investigate its influence on stall margin characteristics as well as on the rotor near tip flowfield both numerically and experimentally. Contrary to most other studies a generic Casing Treatment was designed to provide optimal optical access in the immediate vicinity of the CT, rather than for maximum benefit in terms of stall margin extension. The second part of this two-part paper deals with the numerical developments, and their validation, carried out in order to efficiently perform time-accurate casing-treatment simulations. The numerical developments focus on the extension of an existing coupling algorithm in order to carry out unsteady calculations with any exterior geometry coupled to the main flow passage (in this case a single slot) having an arbitrary pitch. This extension is done by incorporating frequency domain, phase-lagged boundary conditions into this coupling procedure. Whereas the phaselag approach itself is well established and validated for standard rotor-stator calculations, its application to casing treatment simulations is new. Its capabilities and validation will be demonstrated on the given compressor configuration, making extensive use of the detailed PIV flowfield measurements near the rotor tip. Instantaneous data at all measurement planes will be compared for different rotor positions with respect to the stationary slots in order to evaluate the time-dependent interaction between the rotor and the casing treatment.

On the Impact of Geometric Variability on Fan Aerodynamic Performance, Unsteady Blade Row Interaction and its Mechanical Characteristics

The focus of the present study is to assess and quantify the uncertainty in predicting the steady and unsteady aerodynamic performance as well as the major mechanical characteristics of a contrarotating turbofan, primarily due to geometric variations stemming from the manufacturing process. The basis of this study is the optically scanned blisk of the first rotor, for which geometric variations from blade to blade are considered. In a first step, selected profile sections of the first rotor were evaluated aerodynamically by applying the 2D coupled Euler/boundary-layer solver mises. Statistical properties of the relevant flow quantities were calculated firstly based on the results of the nine manufactured blades. In a second step, the geometric variations were decomposed into their corresponding eigenforms by means of principal component analysis (PCA). These modes were the basis for carrying out Monte Carlo (MC) simulations in order to analyze in detail the blades aerodynamic response to the prescribed geometric variations. By means of 3D-computational fluid dynamics (CFD) simulations of the entire fan stage for all the nine scanned rotor 1 blade geometries, the variation of the overall stage performance parameters will be quantified. The impact of the instrumentation will be discussed, here partly doubling the standard deviation of the major performance indicators for the instrumented blades and also triggering a premature laminar/turbulent transition of the boundary layer. In terms of the unsteady blade row interaction, the standard deviation of the resulting blade pressure amplitude shall be discussed based on unsteady simulations, taking advantage of a novel harmonic balance approach. It will be shown that the major uncertainty in terms of the predicted blade pressure amplitude is in the aft part of the front rotor and results from upstream shock/blade interaction. Apart from the aerodynamic performance, an analysis of the mechanical properties in terms of Campbell characteristics and eigenfrequencies was carried out for each of the scanned blades of rotor 1, reflecting the frequency scattering of each eigenmode due to geometric variability.

Aerodynamic Performance Characteristics of the Installe V2527 Fan at Ground Operation

This paper describes the aerodynamic assessment of the V2500 fan stage during ground operation at maximum fan rotational speed. In order to analyze the ground effect and resulting vortex ingestion into the fan, full annulus uRANS computations of the fan including the nacelle and the ground were performed. The numerical results were used prior to the tests at DLRs research aircraft ATRA to support planning the experimental setup in terms of measurement plane location and seeding introduction. In return, the obtained PIV measurement results were used to verify the numerical data. In particular, the formation of the ground vortex, although its location being highly unstable in the experiments, was observed in both, numerical and experimental results in a very similar fashion and its characteristics could be studied in detail. The numerical results furthermore allowed for a detailed assessment of the interaction between this incoming vortical distortion and the fan blades. Apart from high-fidelity uRANS computations, the fan performance over the entire speed range or fight regime respectively was evaluated by single passage and steady state RANS simulations. Appropriate boundary conditions were derived from a thermodynamic cycle model of the entire engine which was validated with available data from engine acceptance tests. The CFD results in terms of performance characteristics were then introduced again into the cycle model to update and further improve the cycle model.

Modelling and Validation of a V2500 Honeycomb-Core Fan Blade

Structural mechanics and also aerodynamics and aero elastics have a need for detailed and exact models of the regarded structures to create reliable results. The Institute of Structures and Design of the German Aerospace Center (DLR) located in Stuttgart has considerable experience in the field of mechanical analysis, design and assessment of aero engine structures as well as in the processing of these structures for further applications in aerodynamics and aero elastics. It is important to ensure that the hereby used methods in modelling and simulation are producing authentic results and the created data is feasible for usage in the linked disciplines. Therefore, the simulated behavior of a fan blade model of the engine of DLR research aircraft A320-ATRA (Advanced Technology Research Aircraft) was validated with measurements of real fan blades during the SAMURAI project. The paper describes a modelling technique for a fan blade with titanium honeycomb core in its context of an IAE V2500 aero engine based on provided CAD-data, x-ray and CT scans and measurements on existing structures. The fan blade model and particularly the influence of the honeycomb core were validated against Eigen frequencies and masses of real blades. With this baseline, simulations for several loading cases respectively several operating points were performed. The gained results were used on the one hand for aerodynamics and engine performance calculations in the context of the project and on the other hand for comparison with fan blade deformations which were determined by IPCT-measurements (Image Pattern Correlation Technique) on the real engine in operation.

Optimised Aerodynamic Design of an OGV With Reduced Blade Count for Low Noise Aircraft Engines

In order to meet the ACARE environmental goals further noise reduction from aircraft operation is required relative to that achieved to date. To support this aim the European Commission Seventh Framework Programme promotes the “Optimisation for low Environmental Noise impact AIRcraft (OPENAIR)” programme. Within the OPENAIR programme Rolls-Royce leads the Integrated Propulsion System Design sub platform containing a task to produce multi disciplinary optimized Fan System outlet guide vane (OGV) designs. As a partner in this task, DLR have undertaken an aerodynamic optimized design for a novel OGV concept as the first part within this task.As a starting point, Rolls-Royce specified a high bypass ratio turbo fan engine style and a novel OGV number and space envelope within the engine. Within the EU FP6 VITAL programme a lower number of OGVs compared to current practice was investigated [1] and Rolls-Royce had previously undertaken rig tests of a range of fan / OGV blade ratios and showed potential noise penalties but also benefits, particularly for the higher tone harmonics and broadband noise for very low numbers of OGVs [2]. The advent of multidisciplinary optimization techniques for noise, aerodynamics and mechanical constraints led Rolls-Royce to specify a very low number of OGVs to investigate the benefits and the mitigation of the penalties using these optimization techniques. The multidisciplinary design optimization was planned to be undertaken in two parts. The aim of the first part, to be reported in this paper, was to achieve a good aerodynamic design for the novel OGV concept. Controlling the aerodynamic secondary losses was expected to be a challenge for the novel concept. The second part of the optimisation will also include noise.As part of the initial design optimization study within OPENAIR, DLR present a multi objective aerodynamic design optimization for the novel very low OGV number and space envelope specified. The blade number of the OGV was reduced from 42 to 14. The first design phase covered the two dimensional airfoil section design for the reduced blade count OGV. In order to achieve an enhanced aerodynamic performance with a reduced blade number an automated process chain was used to couple the DLR in house optimiser AutoOpti [3, 4] with the two dimensional flow solver MISES [5]. Three operating points were considered in the aerodynamic design covering a wide range of the compressor operating map.It has been found that the design of a low blade count OGV is feasible by means of good aerodynamic performance. Additionally the aerodynamic losses could be reduced in all operating points further and small flow separation areas on the suction surface closed to the hub and tip endwalls implied a further improvement potential. Therefore, the second design phase was focused on the three dimensional endwall optimisation including a variable flow duct and fillet geometry. An automated process chain was established for this purpose. As a result the flow separation was prevented in all operating points. In the last step of the DLR work a high fidelity URANS simulation of the configuration with the newly designed OGV was conducted to assess its aero-acoustical behavior at this stage of the aerodynamically optimized OGV design and the emitted sound power level downstream the OGV for the optimised low blade number configuration is presented in this work.Copyright