Fan design method improvement: an efficient use of aero-acoustic predictions in a multi-fidelity optimization framework

This thesis proposes a new methodology for the formulation of a design strategy involving multi-fidelity optimization techniques. The latter offer additional design capabilities in the engineering toolbox by combining accurate predictions from sophisticated models, often time demanding, with models providing cheap and quick predictions, but less accurate. Their proper applications suffer, however, from the little literature and guidelines allowing the design community to refer to best practices or criterion on which the selection and combination of models may be based. To reproduce an aero-acoustic multi-fidelity environment, different strategies in model downgrading were considered. Based on the current state-of-the-art, models and methods of distinct accuracy and dedicated to the predictions of turbofan aero-acoustic performances were identified as candidates for optimization applications. The whole consists of RANS and HB methods to solve computational fluid dynamics (CFD) problems and whose solutions are, respectively, used to perform RANS-informed analytical and radial mode analysis acoustic methods. To extend the range of fidelity levels on which the models are performed, a set of degraded conditions, including mesh density reductions and blade geometry simplifications, are defined. The main part of the thesis consists into finding, by means of a statistical-based approach, the best combinations of aero-acoustic models. For this purpose, aerodynamic and acoustic quantities, predicted at different levels of fidelity, were intensively compared and the strength of their relationships assessed over a statistical population. Modelling parameters playing a role in the robustness of the predictions performed under degraded conditions are pinpointed. Subsequently, the most promising combination of models is implemented in an automated optimization method and its capabilities are demonstrated in the multidisciplinary optimization of a highly-constrained counter-rotating turbofan. To validate the design strategy derived from the statistical-based approach, the prediction quality of the model combination is compared with aerodynamic and acoustic experimental data, as well as with predictions from more accurate models. Finally, the effects of two different turbulence models used for the CFD simulations on the predictions are highlighted

Fan design method improvement

In dieser Arbeit wird eine Methode zur Formulierung einer Designstrategie vorgeschlagen, die multi-fidelity Optimierungsverfahren beinhaltet. Das Fehlen von Richtlinien erschwert jedoch ihre korrekte Anwendung. Auf der Grundlage des aktuellen Stands der Technik wurden Modelle und Methoden mit verschiedener Genauigkeit, die für die Vorhersage der aeroakustischen Leistung von Turbofans geeignet sind, als Kandidaten für Optimierungsanwendungen identifiziert. Mithilfe eines statistischen Ansatzes sollen die besten Kombinationen von aeroakustischen Modellen gefunden werden. Anschließend wird die vielversprechendste Modellkombination in einem multi-fidelity Optimierungsproblem eines gegenläufigen Turbofans verwendet. Um die aus dem statistischen Ansatz abgeleitete Designstrategie zu validieren, wird die Vorhersagequalität der Modellkombination mit aerodynamischen und akustischen experimentellen Daten verglichen

VERIFICATION OF THE THREE DIMENSIONAL SHOCK-STRUCTURES IN AN S-SHAPED TRANSONIC UHBR FAN-ROTOR

The German Aerospace Center (DLR) has designed and tested a 1:3 scaled model S-shaped fan rotor as an example for a medium pressure ratio propulsor with potential application to future UHBR aero engines. In the present study, the attention will focus on the tip region in which the flow field is subject to complex flow phenomena and to the impact of the Sshape feature on the radial shock structure. Steady numerical simulations with DLR in-house solver TRACE as well as measurements were carried out. The casing is instrumented with ten piezoelectric static pressure transducers over the rotor pitch. Particle Image Velocimetry (PIV) is used to catch the flow velocities at three radial blade positions. All experimental data require a phase-locked ensemble averaging procedure. The results include the global performance of the compressor and detailed comparisons between simulations and measurements to validate the shock structures as well as the highly three dimensional-design S-shape fan

Design of a counter rotating fan using a multidisciplinary and multifidelity optimisation under high level of restrictions

A model scale ducted counter-rotating fan was designed by DLR, ONERA, COMOTI and Safran Aircraft Engines within the frame of the EU-project COBRA, (Europe-Russia cooperation within FP7 program coordinated by ONERA for the European side and CIAM for the Russian side). The design was very challenging due to the multidisciplinary aspects of the problem and the high number of very restrictive constraints. This work is based on the previous EU-project VITAL (within FP6 program), objectives of which have been motivated by the ACARE-goals, namely reducing noise by 10 dB, NOx by 60% and the specific fuel consumption by 20%. The COBRA project investigated further different technical solutions to overcome the insufficient noise performance of the VITAL CRTF. An acoustic level reduction by 3 dB for all acoustic operating points has been defined as target compared to the best VITAL CRTF. To achieve these goals, a higher bypass ratio is explored resulting in much lower blade tip speeds and blade count while maintaining good aerodynamic performances. The multidisciplinary approach and the numerical tools used are described. The final CRTF geometry is based on a multiobjective and multifidelity optimisation method and derived from an initial geometry (V0) provided by Safran Aircraft Engines. During the optimisation process, all created members are evaluated with 3D RANS calculations for the CRTF aerodynamics performance, and FEM calculations for the blade structural properties. A simple acoustic evaluation provided by ONERA using steady calculation results allows to estimate the CRTF acoustic level at approach condition. The number of blades chosen fixed during the optimisation is 11 blades for the front fan and 8 for the rear fan. More than 100 free parameters are used to characterize one blade geometry, which is defined by five profiles evenly distributed in the spanwise direction. The final geometry (V4) of the counter-rotating fan achieves an efficiency similar to the previous VITAL CRTF at design point but reduces the noise of -3 dB at approach operating point, while satisfying the mechanical constraint of titanium and maintaining a very strict range of torque ratio as well as a very low residual swirl. Concerning this last point, the multifidelity optimisation method has been very helpful to respond the high level of requirements

Evaluation of the aerodynamic performance of the Counter Rotating Turbo Fan COBRA by means of experimental and numerical data

In the present study, steady numerical simulations performed on the counter rotating turbo fan (CRTF) COBRA are compared with experimental data carried at the CIAM C-3A test-bench in Moscow. For this purpose, a systematic analysis of the measurement uncertainties was performed for the global aerodynamic performances of the CRTF, namely, the massflow, the total pressure ratio, the isentropic efficiency, as well as the torque ratio applied on both fan rows. Several numerical models are investigated to highlight their effects on the aforementioned predicted quantities. Differences in modeling consist in grid resolutions and the use of two turbulence models popular in the turbomachinery community. To match as much as possible the experiment running conditions, the performance map of the CRTF is simulated using the exact measured speed ratio and massflow. The comparisons show good estimations of the numerical simulation over the entire performance map. The main differences between the turbulence models occur at part-speed close to stall conditions. More surprisingly at aerodynamic design point, the importance of the turbulence modeling on the predicted torque ratio has been pointed out