Eberhard Nicke

Numerical Investigation of Casing Treatment Mechanisms With a Conservative Mixed-Cell Approach

A conservative mixed-cell approach of second-order accuracy is presented and applied to investigate the mechanisms of a self-recirculating casing treatment coupled with a transonic compressor rotor. The mixed cell is a computational cell that may show up at the zonal interface boundary, the face of which is partially solid and partially fluid, if the azimuthal open area of casing treatment does not fully contact with the whole annulus of blade passage. The mixed-cell approach is essentially an extension of the conservative zonal approach by incorporating special mixed-cell handling at the zonal interface and it allows a great flexibility to the grid generation for the patched zones with the best grid topology. The mixed-cell approach is extremely useful for solving the unsteady interaction problems within turbomachinery and its application for simulating the coupled flow through the rotor and the casing treatment is reported. The calculated results and analysis reveal an effective stall margin extension of the casing treatment herein by weakening or even destroying the tip leakage vortex, and expose the different tip flow topologies between the cases with the casing treatment and with the untreated smooth wall. It is found that the casing treatment only slightly decreases the overall efficiency at the design point, but it is beneficial to the overall efficiency at the off-design operating conditions and it can improve the inflow conditions to the downstream stator blade row as well.© 2003 ASME

Automated Multidisciplinary Optimization of a Transonic Axial Compressor

The current paper describes DLR’s optimizer AutoOpti, the implementation of the metamodel “Kriging” as accelerating technique, and the process chain in the automated, multidisciplinary optimization of fans and compressors on basis of a recent full stage optimization of a highly loaded, transonic axial compressor. Methods and strategies for an aerodynamic performance map optimization coupled with a finite element analysis on the structural side are presented. The high number of 231 free design parameters, a very limited number of CFD simulations, and conflicting demands both within the aerodynamic requirements and between the disciplines are a challenging optimization task. To navigate such a multi-dimensional search space, metamodels have successfully been used as accelerating technique. Using four aerodynamic operating points at two rotational speeds allows adjusting a required stability margin and optimizing the working line performance under this constraint. The investigated compressor concept is a highly loaded transonic stage with a single row rotor and a tandem stator, designed for a very high total pressure ratio. A. Introduction ompressors for aircraft engines are constantly developed towards higher aerodynamic loading to reduce the installation length, weight, and number of parts with no degradation in efficiency. This leads to more complex geometries and consequently to more complex flow structures. An automated optimization approach is to be preferred in order to take advantage of new design freedoms, while reducing or at least maintaining development time. Automated optimization is also suggested by recent progress in simulation technologies in several fields such as steady and unsteady computational fluid dynamics (CFD), structural and thermal finite element analysis (FEM). Moreover, processors have become increasingly powerful, and parallel computing on huge clusters can be considered state of the art technology for CFD and FEM applications. Thus, it has become possible to employ optimization methods in the design of various parts of heavy duty gas turbines and aircraft engines, even when calculations require large computational resources.

Advanced Nonaxisymmetric Endwall Contouring for Axial Compressors by Generating an Aerodynamic Separator—Part I: Principal Cascade Design and Compressor Application

Modern methods for axial compressor design are capable of shaping the blade surfaces in a three-dimensional way. Linking these methods with automated optimization techniques provides a major benefit to the design process. The application of nonaxisymmetric contoured endwalls is considered to be very successful in turbine rotors and vanes. Concerning axial compressors, nonaxisymmetric endwalls are still a field of research. This two-part paper presents the recent development of a novel endwall design. An aerodynamic separator, generated by a nonaxisymmetric endwall groove, interacts with the passage vortex. This major impact on the secondary flow results in a significant loss reduction because of load redistribution, reduction in recirculation areas, and suppressed corner separation. The first paper deals with the development of the initial endwall design using a linear compressor cascade application. A brief introduction of the design methods is provided, including the automated optimization and the 3D process chain with a focus on the endwall contouring tool. Hereafter, the resulting flow phenomena and physics due to the modified endwall surface are described and analyzed in detail. Additionally, the endwall design principal is transferred to an axial compressor stage. The endwall groove is applied to the hub and casing endwalls of the stator, and the initial numerical investigation is presented. For highly loaded operating points, the flow behavior at the hub region can be improved in accord with the cascade results. Obviously, the casing region is dominated by the incoming tip vortex generated by the rotor and still remains an area for further investigations concerning nonaxisymmetric endwall contouring.

Automated multiobjective optimisation in axial compressor blade design

This paper presents an automated multiobjective design methodology for the aerodynamic optimisation of turbomachinery blades. In this approach several operating-points of the compressor are considered and the flow-characteristics of the different flow-solutions are combined to one or more objective functions. The optimisation strategy is based on multiobjective asynchronous evolutionary algorithms (MOEA’S) which are accelerated using additive local neural networks and kriging procedures. Common operators: Mutation, Crossover and Differential-Evolution are used to create a new population. In addition to these common operators the optimisation runs temporarily on the response-surface created by the neural networks and/or kriging-processes respectively. Only the Pareto-optimal solutions obtained from this metamodel are evaluated using the numerical expensive flow-solver. Therefore, the response-surface is just a new operator that creates auspicious members. One of the main differences between the presented code to usual and traditional MOEA’S is the selection of parents. While traditional codes choose potential parents of a new population from the previous population, the current method selects parents from the database of all evaluated members. This approach allows the user to run the optimisation asynchronously and with a smaller size of population, treducing numerical costs, without influencing the diversity of the optimal solutions over the whole Pareto-front. This aspect is very important when evaluating very complex and/or discontinuous fronts.

Advanced Nonaxisymmetric Endwall Contouring for Axial Compressors by Generating an Aerodynamic Separator—Part II: Experimental and Numerical Cascade Investigation

Modern methods for axial compressor design are capable of shaping the blade surfaces in a three-dimensional way. Linking these methods with automated optimization techniques provides a major benefit to the design process. The application of nonaxisymmetric contoured endwalls is considered to be very successful in turbine rotors and vanes. Concerning axial compressors, nonaxisymmetric endwalls are still a field of research. This two-part paper presents the recent development of a novel endwall design. A vortex created by a nonaxisymmetric endwall groove acts as an aerodynamic separator, preventing the passage vortex from interacting with the suction side boundary layer. This major impact on the secondary flow results in a significant loss reduction by means of load redistribution, reduction in recirculation areas, and suppressed corner separation. Part I of this paper deals with the endwall design and its compressor application. The resulting flow phenomena and physics are described and analyzed in detail. The second paper presents the detailed experimental and numerical investigation of the developed endwall groove. The measurements carried out at the transonic cascade wind tunnel of DLR in Cologne, demonstrated a considerable influence on the cascade performance. A loss reduction and redistribution of the cascade loading were achieved at the aerodynamic design point, as well as near the stall condition of the cascade. This behavior is well predicted by the numerical simulation. The combined analysis of experimental and numerical flow patterns allows a detailed interpretation and description of the resulting flow phenomena. In this context, high fidelity 3D-Reynolds-averaged Navier―Stokes flow simulations are required to analyze the complex blade and endwall boundary layer interaction.

Design of a Highly Efficient Low-Noise Fan for Ultra-High Bypass Engines

In order to achieve an environmental-friendly engine i.e. with more efficiency and less noise emission, a geared ultra-high bypass ratio fan test rig has been designed within the EU-funded project SILENCE®. Engine cycle requirements were untypical in terms of mass flow rate, pressure ratio and BPR. In order to reach the desired mass flow rate and simultaneously to avoid a strong interaction of the shocks with the boundary layer an S-shape leading edged rotor with forward sweep close to casing has been designed. Specific blade numbers for rotor and stator has been used to minimize the rotor-stator interaction noise. For the same purpose a backward swept bypass stator has been designed. There are two stators in the core duct in order to bring the flow to zero swirl which is a necessity for test rig measurements. The main design loop includes blade shape and the flow path optimization as well as the computation of stress distribution in all blades and the rotor disc. The fan is being manufactured from titanium because of its specific aeroelastic properties and stators are made of steel. The rig is scheduled to be tested in 2006 for its aerodynamic and aeroacoustic performance.Copyright

Axis-Asymmetric Profiled Endwall Design by Using Multiobjective Optimisation Linked With 3D RANS-Flow-Simulations

Secondary flow loss in modern axial compressors is considered to be the prime reason for the reduction of overall isentropic efficiency in these engine components. This paper presents a new methodology to diminish blade secondary loss and endwall loss by an axis-asymmetric modification of endwalls using an automated multiobjective optimizer in conjunction with 3D-RANS-flow-simulations. In order to obtain a favorable design for a wide operating range, the most important operating-points are considered in the optimization process. The existing multiobjective optimization package is enhanced by implementation of DLR’s in-house 3D-flow-solver TRACE. A straightforward stator optimization was performed for a 3D-process-chain test run. Finally, the novel endwall design technique is introduced and the first optimization results and further studies are discussed.Copyright

Multidisciplinary Automated Optimization strategy on a Counter Rotating Fan

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.Copyright

Design of an economical counter rotating Fan:comparison of the calculated and measured steady and unsteady results

Within the framework of the EU funded Project VITAL, SNECMA (Group Safran), as the work package leader, developed a counter rotating low-speed fan-concept for a high bypass ratio engine. The detailed aerodynamic and mechanical optimization of one blading version (CRTF2.b) was carried out at the German Aerospace Center (DLR), by applying one of the newest design methods featuring a multi-objective automatic optimization method based on an Evolutionary Algorithm [1].The final design goals were high efficiency, a sufficient stall margin and adequate acoustic performances for the given cycle parameters. The fan stage developed was tested in an anechoic test facility at CIAM in Moscow. The test routine included the measurement of the performance map based on total pressure and total temperature measurements at the inlet and the outlet of the test rig and acoustic measurement as well.The unsteady flow field of the low speed Contra-Rotating Turbo Fan has been measured with four hot-wire probes at different axial positions.In the evaluation the measured data are compared with high resolution CFD results. Special emphasis was given to the comparison of the radial distribution of total pressure and total temperature in the bypass channel, the comparison of the measured and the calculated fan maps and to the comparison of the hot-wire measurements with high resolution, unsteady CFD results. The tests and the URANS-results confirmed the design goals.Copyright

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