Against the background of the high development status of modern axial compressors, a further performance enhancement is linked with the extension of the design space in the development process and the concentration on the essential loss mechanisms in the compressor. The performance of a compressor cascade is considerably influenced by secondary flow effects in the near endwall region, since up to 50 percent (for low aspect ratio) of the losses in the bladed channel of a turbomachinery are linked to the endwalls. In this context the application of non-axisymmetric profiled endwalls provides a potential for compressor improvement. The paper presents the detailed experimental and numerical investigation of controlling the endwall cross flow in a compressor cascade. The general approach is based on a boundary layer fence arrangement, whose application on the compressor endwall works as a non-axisymmetric endwall contour. This non-axisymmetric endwall modification constrains the interaction of the endwall cross flow with the suction side boundary layer, thus the onset of the corner separation is delayed and a significant loss reduction of 8 percent is achieved. The experiments were carried out in a linear compressor cascade at the high-speed cascade wind tunnel of the DLR in Berlin at peak efficiency (design point) and off-design of the cascade at Mach number M = 0.67. Furthermore, high fidelity 3D-RANS flow simulations were performed in order to analyze the complex blade and endwall boundary layer interaction. The combined consideration of experimental and numerical flow pattern allows a detailed interpretation and description of the resulting flow phenomena.

The present study demonstrates the aerodynamic and acoustic optimization potential of a counter rotating open rotor. The objective was to maximize the propeller efficiency at top-of-climb conditions and to minimize the noise emission at takeoff while fulfilling the given thrust specifications at two operating conditions (Takeoff and Top of Climb) considered. Both objectives were successfully met by applying an efficient multi-objective optimization procedure in combination with a 3D RANS method. The acoustic evaluation was carried out with a coupled U-RANS and an analytic far field prediction method based on an integral Ffowcs-Williams Hawkings approach. This first part of the paper deals with the application of DLR’s CFD method TRACE to counter rotating open rotors. This study features the choice and placement of boundary conditions, resolution requirements and a corresponding meshing strategy. The aerodynamic performance in terms of thrust, torque and efficiency was evaluated based on steady state calculations with a mixing plane placed in between both rotors, which allowed for an efficient and reliable evaluation of the performance, in particular within the automatic optimization. The aerodynamic optimization was carried by the application of AutoOpti, a multi-objective optimization procedure based on an evolutionary algorithm which also was developed at the Institute of propulsion technology at DLR. The optimization presented in this paper features more than 1600 converged 3D steady-state CFD simulations at two operating conditions, Takeoff and Top of Climb respectively. In order to accelerate the optimization process a surrogate model based on a Kriging interpolation on the response surfaces was introduced. The main constrains and regions of interest during the optimization where a given power split between the rotors at takeoff, retaining an axial outflow at the aft rotor exit at top of climb and fulfilling the given thrust specifications at both operating conditions. Two objectives were defined: One was to maximize the (propeller) efficiency at top of climb conditions. The other objective was an acoustic criteria aiming at decreasing the rotor/rotor interaction noise at takeoff by smoothening the front rotor wakes. Approximately 100 geometric parameters were set free during the optimization to allow for a flexible definition of the 3D blade geometry in terms of rotor sweep, aft rotor clipping, hub contour as well as a flexible definition of different 2D profiles at different radial locations. The acoustic evaluation was carried out based on unsteady 3D-RANS computations with the same CFD method (TRACE) involving an efficient single-passage phase-lag approach. These unsteady results were coupled with the integral Ffowcs-Williams Hawkings method APSIM via a permeable control surface covering both rotors. The far field directivities and spectra for a linear microphone array were evaluated, here mainly at the takeoff certification point. This (still time consuming) acoustic evaluation was carried out after the automatic optimization for a few of the most promising individuals only and results will be presented in comparison with the baseline configuration. This detailed acoustic evaluation also allowed for an assessment of the effectiveness of the acoustic cost function as introduced within the automatic optimization.

The following paper deals with the development of an optimized fillet and an endwall contour for reducing the total pressure loss and for homogenizing the outflow of a highly loaded cascade with a low aspect ratio. The NACA-65 K48 cascade profile without a fillet and without endwall contouring is used as a basis. Optimizations are performed using the DLR in-house tool AutoOpti and the RANS-solver TRACE. Three operating points at an inflow Mach number of 0.67 with different inflow angles are used to secure a wide operating range of the optimized design. At first only a fillet is optimized. The optimized fillet is small at the leading edge and rather high, wide and thick towards the trailing edge. It reduces the total pressure loss and homogenizes the outflow up to a blade height of 20 %. Following this a combined optimization of the endwall and the fillet is performed. The optimized contour leads to the development of a vortex, which changes the secondary flow in such a way, that the corner separation is reduced, which in turn significantly reduces the total pressure loss up to 16 % in the design operating point. The contour in the outflow region leads to a significant homogenization of the outflow in the near wall region.

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.

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. Introductionompressors 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. B. Optimizer AutoOpti Multiobjective Optimization Strategies in Turbomachinery DesignThe simulation-speedup and the emergence of improved optimization algorithms nowadays enable the development and use of automatic optimization methodologies to perform complex multi-disciplinary and multiobjective optimization processes in turbomachinery design. Such automated, computer assisted design-concepts have the potential to:• Create new design-ideas for turbomachinery components and support the engineer.• Reduce the number of design iterations within and between different disciplines like aerodynamic, structural and thermal analysis.• Generate design compromises between the disciplines.• Improve gas turbine performance and stability.