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This book reports on the latest numerical and experimental findings in the field of high-lift technologies. It covers interdisciplinary research subjects relating to scientific computing, aerodynamics, aeroacoustics, material sciences, aircraft structures, and flight mechanics. The respective chapters are based on papers presented at the Final Symposium of the Collaborative Research Center (CRC) 880, which was held on December 17-18, 2019 in Braunschweig, Germany. The conference and the research presented here were partly supported by the CRC 880 on “Fundamentals of High Lift for Future Civil Aircraft,” funded by the DFG (German Research Foundation). The papers offer timely insights into high-lift technologies for short take-off and landing aircraft, with a special focus on aeroacoustics, efficient high-lift, flight dynamics, and aircraft design.

### Unconventional Applications and New Approaches for Flow Control

This article consists of a review of unconventional applications of flow control interspersed with new techniques for actuation. The flow control studies are aimed at solving a broad set of aerodynamic and propulsion flow problems targeting commercial and military applications. The applications range from aerodynamic performance improvements, up to solutions to airplane operational issues. The flow control techniques are used for reducing drag, controlling flow separation, as well as manipulating vortical flow structures for achieving a desired objective. An important driver for the development of the new techniques is a result of particular focus on issues of practical integration, where actuation input is within available resources onboard. A systematic approach based on computational simulations is used to provide insight into the flow problem, facilitate root cause analyses, and develop flow control approaches. Both fluidic actuation and morphing structures are considered. Actuator concepts and aspects of system integration are also introduced.

Arvin Shmilovich

### A Brief Overview on Past Research on Coandă Assisted STOL Transport Aircraft

The Coordinated Reserch Center SFB 880 aimed at developing technologies that enable aircraft designs for use on short airfields in the vicinity of populated urban areas. Capabilities for short take-off and landing have been in the focus of research already at the early stages of jet driven air transport in the second half of the previous century. However, only few aircraft applying such technologies went into serial production. This contribution aims to give a brief overview on the most remarkable aircraft projects without any claim on completeness. It focuses on large aircraft and thus omits—only for briefness—all technology demonstrations made on small aircraft and most military fighter aircraft. It intends to provide an overview on invented technologies in order to provide a basis to evaluate the significant achievements made on the SFB 880.

Jochen Wild

### Actuation System for Effective Flow Control of an Internally Blown Coanda Flap

The focus of the work is on the evaluation, development and integration of a robust actuator system for three-dimensional flow control of a blown Coanda flap to improve the high lift system of commercial aircraft. As part of the research work presented, the system is integrated into a wind tunnel model in order to demonstrate its functionality under aerodynamic loads. The system is based on individual bending transducers that can vary the height of the blowing slot dynamically. There are 33 actuated segments to implement static and dynamic actuation along the wing span (3D-actuation). All segments can be controlled independently and thus offer great optimization potential for an effective flow control. The first aerodynamic results show an increase in lift of up to ∆Cl = 0.57. These aerodynamic gains are achieved at amplitudes that do not require the lip segments to completely close or open the blowing slot, which shows the advantage of the current lip design that enables activation with independently controlled stationary and unsteady components.

Peter Wierach, Jan Petersen, Christian Behr, Michael Sinapius

### Micro Sensor System for Active Flow Control

In the course of this research project, the distribution of wall shear stresses and pressures on high-lift flap models in water and air tunnel experiments needs to be acquired in order to serve an active blowing control system. An important requirement for the acquisition of pressure and wall-shear stress data is not to affect the flow by their measurement. To fulfill this specification, a novel sensor system has been designed, which can be integrated flush into the surface of experimental models. In the course of creating an electrically self-contained and fluidically impact-less sensor system, novel custom-engineered MEMS pressure sensors and electro-thermal wall shear-stress sensors have been researched, designed, evaluated individually and finally joint into structure integrable sensor modules. The individual sensor designs and their microtechnical fabrication processes including the combination into a sensor system are presented and evaluated.

Jan Niklas Haus, Martin Schwerter, Marcel Gäding, Anke Vierheller, Monika Leester-Schädel, Andreas Dietzel

### Active Flow Control Experiments on a High-Lift Configuration

The present study investigates the lift gains generated by the superposition of a periodic actuation component onto a steady component on an airfoil with a highly deflected Coanda flap. The presented results are drawn from two experiments conduced in the water and in the wind tunnel. For the water tunnel experiment, periodic actuation is provided by two synchronized specially modified valves that deliver actuation frequency up to 30 Hz. For the wind tunnel experiment, the unsteady actuation is generated by 33 specially-designed individually-controlled lip segments that deliver actuation frequencies up to 300 Hz. The results demonstrate the benefits of superimposing a periodic component onto the steady actuation component for a separated or partially-attached flow, where a lift increase of up to $$\varDelta C_l =0.6$$ is achieved. Among spanwise-varied actuation, the traveling wave yield the highest lift gains with a relatively high wave frequency and long wavelength.

Richard Semaan, Yosef El Sayed, Stefan Loges, Bernd R. Noack, Rolf Radespiel

### Friction Drag Reduction by Transversal Spanwise Traveling Waves of Ribbed Surfaces

Friction drag reduction in zero-pressure gradient turbulent boundary layer flow is investigated by a combined passive and active method. The passive method is riblet structured surfaces, which have been proven to effectively reduce turbulent friction drag. Additionally, the surface undergoes a spanwise traveling transversal wave motion which constitutes the active method. This hybrid, i.e., passive and active, method is tested in a turbulent boundary layer flow at a Reynolds number of $$Re_\theta = 1,000$$ . The results are compared against those of a stationary smooth flat plate, a stationary ribbed flat plate, and a moving smooth flat plate. For the transversal wave motion two amplitudes are considered while the wave length and the frequency are not varied. The results show a slightly higher drag reduction for the hybrid method compared to the single drag reduction technique. Furthermore, the hybrid method is less susceptible to variations of the wave motion. The largest relative drag decrease of the hybrid method is achieved for the smallest amplitude, i.e., the lowest additional energy input. The mean velocity gradient near the wall is reduced for the hybrid method compared to the non-actuated cases at the tips and in the valleys of the riblets. The Reynolds stresses show a significant decrease of the turbulent intensities in the near-wall region especially for the large amplitude case. The location of the peak of the streamwise stresses and the wall-normal vorticity fluctuations is shifted off the wall.

Pascal S. Meysonnat, Marian Albers, Benedikt Roidl, Wolfgang Schröder

### Circulation Control Experiments on a Vertical Tail

Steady tangential blowing on a vertical tail was experimentally investigated. Two different actuation configurations, a continuous slot and discrete slots, were implemented close to the rudder hinge line. The main objective of this investigation is to increase the total side force of the vertical tail. To attain this, flow separation on the rudder, which occurs at large deflection angles, must be delayed. A secondary objective is to determine the efficiency of such configurations. The continuous slot configuration showed the overall largest increase of the side force, but at low values for the efficiency. In contrast, the discrete slot configuration demanded 34% less of the momentum coefficient compared at a similar side force. The most efficient configuration was found for discrete jets blowing locally at mid-span. Moreover, the tangential wall jets of the discrete slot configuration generate on a swept wing longitudinal vortices.

Vickram M. Singh, Peter Scholz

### Benefits of Applying Pulsed Jet Actuation on a Vertical Stabilizer

With active flow control (AFC) it is possible to increase the control authority of a Vertical Tail Plane (VTP), which could lead to smaller stabilizers on future airplanes. While this hypothesis is the main reason for research on active flow control on stabilizers, with higher control authority of the VTP, the required sideslip or rudder deflection angles for certain flight scenarios could be reduced. In the current experiment, Pulsed Jet Actuation (PJA) was applied on a VTP wind-tunnel model and a wide range of sideslip, rudder deflection angles, and momentum coefficients was investigated. The measured data indicates that pulsed blowing with $$C_{\mu }=1.5$$ % at the rudder is capable of reducing the required sideslip angle by $$\varDelta \beta \approx 55$$ % when the rudder deflection angle is constant. If the sideslip angle is kept constant the required rudder deflection angle can be reduced by $$\varDelta \delta _r\approx 40$$ %.

Stephan Löffler, Julien Weiss

### Manipulation of Leading-Edge Vortex Flow

At very high angles of attack, the leading-edge vortex of a semi-slender delta wing becomes unsteady and collapses, endangering the flight stability. Active flow control by pulsed blowing stabilizes the vortex system, enlarging the flight envelope for such wing configurations. The reattachment of the separated shear layer during post-stall causes a lift increase of more than 50% and offers a great perspective for active flow control in aeronautical applications. However, the interactions of shear layer vortices with the jet-induced vortices are complex. This paper attempts to shed a light on the flow field response to pulsed blowing at the leading edge of a generic half delta wing model with a $$65^\circ$$ sweep angle based on detached eddy simulations.

Andrei Buzica, Christian Breitsamter

### Large-Displacement Morphing Wing Leading Edge Droop Nose: Optimization, Manufacture and Instrumentation

This paper provides an overview of research into a morphing wing leading edge allowing smooth and continuous shape changes. Together with high-lift technologies investigated in the Collaborative Research Center 880 over the past nine years, the morphing droop nose has the potential to generate very high lift coefficients with low noise. Multiple design strategies were explored over the course of research and summarized here. Design features include novel, multifunctional, hybrid tailored composites comprising fiberglass and elastomeric materials, integral stringers to facilitate the flow of internal and external forces, and the use of optimization chains to synthesize the numerous design variables. The results of experimental tests show in general the suitability of the design methods, as well as a number of lessons learned, including the need for tighter aerodynamic-structural coupling, that are useful for future work.

Srinivas Vasista, Johannes Riemenschneider, Hans Peter Monner, Ralf Keimer, Felix Nolte, Peter Horst

### Large-Displacement Morphing Wing Leading Edge Droop Nose: Structural Concept, Testing and Systems Integration

The design of a large displacement morphing wing leading edge from the structural point of view is presented in this article. The overall structural concept of the design and the novel approach of an hybrid rubber/composite skin are discussed. Key features such as the integral stringers, industrialization aspects concerning anti-/deicing strategies and the integration of Lightning Strike Protection are outlined. Furthermore, an energy based composite crack onset criterion is derived as it was used for the skin design. Finally, a test rig for a morphing leading edge is presented.

Felix Nolte, André Hannig, Peter Horst, Srinivas Vasista, Hans Peter Monner

### Semi-Analytical Approach to the Determination of Free-Edge Stress Fields in Cylindrically Curved Composite Laminated Shells

This paper aims at contributing to the field of highly efficient analysis methods by presenting a semi-analytical approach which enables an adequate prediction of the three-dimensional stress fields in cylindrically curved orthotropic laminated composite shells subjected to bending load. The approach incorporates a layerwise plane-strain analysis in the innermost regions of the shell wherein the stresses are specified in terms of Airy’s stress function. In order to realize an accurate determination of the expected three-dimensional stress field in the vicinity of the free edge, the plane-strain formulation is upgraded by a displacement-based approach wherein each laminate layer is discretized into a number of mathematical layers with respect to the thickness direction. The underlying differential equations and boundary conditions that govern the free-edge effect in cylindrically curved composite shells are derived by virtue of the principle of minimum elastic potential. The results of the developed analysis method are verified by comparison with detailed finite element computations, and it is found that the semi-analytical approach works with comparable accuracy, however, only at a fraction of the required computational effort.

Andreas Kappel, Christian Mittelstedt

### Design of Active High Lift Wing Configurations Via Fluid-Structure Interaction Simulation

The wing of an active high lift aircraft configuration with an UHBR engine is structurally sized. The FEA details droop nose and flaps and uses loads from 3D CFD RANS simulations for the fully stressed design. The sizing yields a comparable skin thickness distribution compared to a similar wing configuration with a Turboprop engine. The differences in mass result mainly from the relatively high UHBR engine weight and the higher sweep angle. Furthermore the steady aeroelastic equilibrium is computed with a partitioned approach for the landing configuration at optimal circulation control. Flow separation is initiated at the end of the unprotected leading edge, propagating to the outboard main wing and aileron. The effect of the wing elasticity onto the aerodynamics is negligible due to the high stiffness of the UHBR configuration.

Fabian Runge, Kay Sommerwerk, Michael Rohdenburg, Matthias C. Haupt

### Aeroacoustic Analysis of a Circulation Control Airfoil with Solid and Porous Trailing Edge

In aerodynamic research circulation control airfoils are shown to yield performance benefits compared to conventional multi-element high-lift systems. Their application on transport aircraft allows to consider new take-off and landing procedures with reduced flight speed and steep climbs and descends. These procedures can help to reduce the noise impact on the ground close to airports. Furthermore, there are hints that the noise emission from such high-lift systems is lower than from a conventional wing with slotted slat and flap. The present paper shows the results of numerical and experimental investigations to understand the noise sources of a circulation control airfoil. Additionally the application of porous materials to the flap trailing edge is considered as a means for further noise reduction.

Lennart Rossian, Karl-Stéphane Rossignol, Roland Ewert, Michaela Herr, Jan W. Delfs

An important design objective of future aircraft configurations is a low noise transmission into the passenger cabin. Mechanical models and its numerical solution allow analyses of the noise transmission from the source to the passenger ear. Within this contribution, numerical methods are applied within a multidisciplinary simulation chain resulting in a prediction of cabin noise due to fluid noise sources. Noise by the jet and the turbulent boundary layer are expected as two major noise sources which are considered in this contribution. Jet noise is computed for two different engine configurations which are compared to the turbulent boundary layer noise in the passenger cabin. The contribution of the engine fan is not accounted for within this study. In the multidisciplinary simulation chain, the hybrid Computational Aeroacoustics solver PIANO combined with the Fast Random Particle Mesh method is applied to compute the pressure fluctuations due to jet noise on the outer skin of the fuselage. For the turbulent boundary layer, analytic calculations of auto-spectra and wavenumber-spectra are transferred to pressure fluctuations on the outer skin as well. The loads are applied to a wave-resolving finite element model of the research aircraft by the Collaborative Research Center 880. The model is derived from the available design data and solved with the in-house research code elPaSo. First, the results show a much lower sound pressure level in the cabin induced by a novel ultra–high–bypass ratio engine in comparison with a conventional engine. Second, it is shown that the turbulent boundary layer becomes the dominating noise source for cabin noise in future aircraft driven by a third generation engine.

Christopher Blech, Christina K. Appel, Roland Ewert, Jan W. Delfs, Sabine C. Langer

### Aircraft Cabin Noise Prediction Under Uncertainty

The predictive capability of numerical aircraft passenger cabin noise simulations critically depends on the exact knowledge of input data. We perform a forward uncertainty quantification study to analyse the influence of uncertain parameters in a finite element model of an aircraft’s fuselage. In particular, uncertainties associated with material parameters are considered. The generalised polynomial chaos and the multi element generalised polynomial chaos methods are employed to perform the uncertainty quantification study. The multi element approach automatically detects strong variations in the parameter domain through local adaptivity. However, it still requires roughly the same total number of deterministic model solutions as standard polynomial chaos, since additional effort is needed to steer the refinement. Both methods yield accurate surrogate models which can be used to efficiently carry out a sensitivity and uncertainty analysis of the frequency response function.

Shreyas M. Guruprasad, Prem R. Mohan Ram, Christopher Blech, Ulrich Römer, Sabine C. Langer

### Adjoint-Based Identification of Sound Sources for Sound Reinforcement and Source Localization

The identification of sound sources is a common problem in acoustics. Different parameters are sought, among these are signal and position of the sources. We present an adjoint-based approach for sound source identification, which employs computational aeroacoustic techniques. Two different applications are presented as a proof-of-concept: optimization of a sound reinforcement setup and the localization of (moving) sound sources.

Mathias Lemke, Lewin Stein

### Air Curtain Flow Control for Aerodynamic Noise Reduction

In this paper, dual-jet planar air curtains are demonstrated to be able to successfully remove aerodynamic noise radiated from tandem rods in a crossflow. Single-jet air curtains have shown significant promise as a low noise technology but can introduce additional noise sources such as lip and mixing noise. In this work, dual-jet air curtains are shown to address these obstacles, achieving the same shielding height with a significantly lower overall system noise. Particle image velocimetry and flow visualisation methods allow the flow fields to be examined, and a numerical analysis with computational fluid dynamics to be validated. It is demonstrated that a judicious choice of flow parameters can result in the complete removal of the aerodynamic noise with almost no acoustic penalty. Further development of air curtain flow control is required to establish whether this might be a viable innovative solution as a low noise technology for landing gear noise reduction.

Gareth J. Bennett, Kun Zhao

### Aerodynamics and Loss Accounting of a Low Pressure Ratio Fan for an Over-Wing Mounted Engine

In recent research, several Boundary Layer Ingesting propulsors concepts have been investigated with varying degrees of airframe integration. The contribution compares the well-known approach of a $$60^\circ$$ circumferential inlet distortion with a configuration of a partly wing-embedded and a fully embedded engine integration. The second type is characterized by a local distortion while the whole flow field of the third shows flow variations. Conventional distortion metrics are getting close to their limits for an adequate assessment. The focus of this contribution lies in highlighting differences in loss generation through the fan stage and at dedicated stations. The stator blade row has a changing role in terms of entropy production, depending on the distortion type. Concepts for a stator refitting are presented and successfully applied. Almost one-third of efficiency drop is recovered for the final design change.

Daniel Giesecke, Jens Friedrichs

### Aerodynamic Optimization of the OWN Configurations REF3 and REF4 in Cruise Flight

Detailed investigations of interference effects between over-the-wing mounted nacelles (OWN) and airframe were accomplished with the aid of high-fidelity CFD tools on two aircraft configurations. The configurations differ in a variation of the wing planform from a backward (REF3) to a forward swept layout (REF4). The impact of the OWN was investigated by using an automated process chain varying engine position, wing twist distribution, and airfoil section geometry to achieve an aerodynamically efficient aircraft design in the presence of the OWN. Ensuing from the surrogate-based optimization, the data set was used to evaluate the interference between OWN and wing.

Fabian Lange, Ralf Rudnik

### Intake Design and Airframe Integration Sensitivities for an Embedded Over-The-Wing Mounted Ultra-High Bypass Ratio Turbofan

Within the Collaborative Research Centre 880, future technologies are to be developed for the application on a cruise-efficient transport aircraft. Part of this concept is to mount the aircraft engines on the rear part of the wing. This installation position might hold the potential to accommodate aircraft engines with growing engine diameters. Moreover, a synergy between wing and nacelle might lead to a short nacelle design which becomes crucial for increasing bypass ratios. To enhance the coupling between airframe and nacelle, the engine will be partly embedded into the wing. A preliminary design of an embedded ultra-high bypass ratio engine was investigated at high speed conditions. The axial position of the engine was varied at constant lift showing that aircraft performance improves by shifting the nacelle axially downstream. In addition, studies on intake parameter variations for a generic embedded configuration are discussed.

Constance Heykena, Nana Obayashi, Jens Friedrichs

### Multibody Modelling of a UHBR Engine and Its Influence on the Dynamics of an Aircraft Wing

A multibody modelling in SIMPACK of an ultra high bypass ratio (UHBR) engine is presented in this article. In this context, a hybrid approach is used: all rotor components of the engine are created as finite element models in ANSYS Mechanical and are then transferred as model order reduced bodies into the multibody environment. It is thereby possible to account for flexibility and consequently for phenomena such as gyroscopic moments. Multibody simulations are conducted with the UHBR engine model attached to a flexible wing model. The angular momentum of the engine is varied in order to achieve a change of the gyroscopic moments and to assess their influence on the wing dynamics. In that regard, the results revealed a strong gyroscopic coupling between wing and UHBR engine. As a consequence, it can be stated that gyroscopics should not be neglected in the design process of an aircraft.

Martin Schmalz, Holger Hennings

### Active Flow Separation Control on a Generic UHBR Engine High-Lift Configuration by Means of Suction and Oscillatory Blowing

The effect of suction and oscillatory blowing (SaOB) on local flow separation in the slat cutout region downstream from a UHBR engine is investigated by means of URANS simulations. The simulation environment mirrors the wind tunnel test of an UHBR high-lift configuration conducted at the small scale low speed wind tunnel at Tel-Aviv University. The validated CFD results are employed to assess the beneficial effect of SaOB compared to individual actuation by means of suction and oscillatory blowing. Both investigated flow control mechanisms are effective in significantly attenuating flow separation and a combined actuation allows to reduce external mass flow requirements by additional 15%. The excitation of the flow in the slat cutout region affects the flow field on large parts of the upper wing. A decrease in lift augmentation by flow control through SaOB compared to oscillatory blowing is most likely attributable to the sensitivities of the longitudinal vortices and corner flow separation.

Junaid Ullah, Shay Monat, Avraham Seifert, Thorsten Lutz, Ewald Krämer

### Use of MDO Techniques for the Design of Morphing Aircraft

This paper describes the application of multidisciplinary optimization techniques to the design of adaptive morphing wings based on compliant structures. The investigated morphing concept is the active camber variation limited to the leading and the trailing edges of the wing, keeping the central wingbox undeformed. The developed optimization framework consists of different steps. Starting from the mission requirements, typically the aircraft efficiency improvement in one or more flight conditions, the first step aims at the design of the optimal morphing aerodynamic shape of the wing enabling the desired performances, also taking into account structural and material limitations for the skin. Then, dedicated tools are applied, at increasing detail level, to design the internal structure that realizes the requested morphing shape change. The proposed procedure has been applied over the years in different research projects. Among the others, the results for the NOVEMOR project and the Clean Sky 2 AIRGREEN project are reported, showing the suitability of the developed framework for the design of optimum solutions in the morphing field.

Alessandro De Gaspari, Vittorio Cavalieri, Sergio Ricci

### CRC 880 Vehicle Concepts and Comparative Noise Assessment

The Coordinated Research Centre 880 (CRC 880) focuses on the development of a new active high-lift system, which enables short takeoff and landing, low fuel consumption, and low noise immission. In order to assess the active high-lift system on an aircraft level, several vehicle variants, that are equipped with this active high-lift system, have been developed within the 9-year funding period of the CRC 880. This article describes the CRC 880 vehicle concepts and evaluates their advantages and disadvantages in detail. Furthermore, the noise prediction methodology is described and the noise immission of the vehicle concepts is evaluated on a comparative basis. Results indicate that a propeller-driven short takeoff and landing (STOL) aircraft is cost-efficient but has increased noise immission compared to a conventional turbofan aircraft. On the other side, STOL aircraft with turbofan engines located above and aft of the main wing for fan sound shielding are more expensive but offer significant noise reductions.

Jason Blinstrub, Lothar Bertsch, Wolfgang Heinze

### Aerodynamic Analysis of a STOL Propeller Aircraft with Active High-Lift and Control Surface Systems

The aerodynamic properties of a state-of-the-art short take-off and landing aircraft design with circulation control and slipstream deflection in landing configuration have been investigated via RANS simulations. The present paper summarizes the findings concerning one engine inoperative conditions (OEI), active control surface design, and ground effect. It is shown that the unfavorable aircraft’s aerodynamic characteristics under OEI conditions, including a degraded maximum lift coefficient and extraordinarily high yawing moments, can be improved via passive measures, such as nacelle strakes and tail fences. The resulting requirements for lateral control forces can be fulfilled by the use of circulation controlled rudder and ailerons. Simulations of the 3D configuration in ground proximity yield a beneficial effect on the lift coefficient despite contrary indications from 2D computations. Furthermore, the simulations show a significant pitch down tendency due to ground proximity.

Dennis Keller, Ralf Rudnik

### Understanding the Flight Mechanics of Active High-Lift Aircraft Configurations: Achievements of the CRC 880 Project

Flight mechanics of short take-off and landing aircraft with active high-lift system and propeller engines exhibit excellent low airspeed flight performance on the one hand but also flight mechanical challenges on the other. These are results of previous work of the 60s as well as of the Collaborative Research Centre CRC 880. In the CRC 880 project, new knowledge about unsatisfactory handling qualities was gained by flight dynamics modeling and simulations based on a high-fidelity computational fluid dynamics database of the CRC reference aircraft. This paper summarizes the results of the flight dynamics that confirm excellent flight performance, minor challenges in the longitudinal motion and major challenges in the lateral-directional motion. Passive and active countermeasures are discussed to improve the handling qualities. Moreover, it is shown that only partial failures of the active high-lift system can be compensated. Finally, the paper gives an outlook on future work.

Yannic Beyer, Jobst H. Diekmann, Nicolas Fezans, Meiko Steen

### A Multifidelity Approach for Uncertainty Propagation in Flight Dynamics

This paper is concerned with the estimation of failure probabilities and statistical moments in a flight dynamics context with multifidelity Monte Carlo techniques. A propulsion-based emergency autoland scenario for the ATTAS research aircraft is considered, where stochastic wind disturbances complicate the landing. All statistical quantities of interest are inferred from samples of the wind shear and the corresponding aircraft touchdown parameters. The underlying Monte Carlo methodology ensures un-biased and reliable estimation of statistical quantities of interest. The advantages and limitations of using black-box surrogates as low-fidelity models are revealed through a series of numerical examples. In particular, signifficant computational gains can be achieved in some cases, however, advanced surrogate modeling may be required for some solution parameters.

Ulrich Römer, Shanza Ali Zafar, Nicolas Fezans

### Aeroelastic Instabilities of Wings with Active High-Lift Devices—A Reduced-Order Model

A reduced-order model is developed to study the parameter-dependent aeroelastic behaviour of two wing configurations with high-lift devices. One is the wing of a conventional turboprop aircraft, the other a wing with over-the-wing mounted ultra high bypass ratio engine. Characteristic aerodynamic loads are investigated with steady and unsteady flow simulations of a 2D profile section. 3D effects are taken into account using an adapted lifting line theory according to Prandtl. Structure and aerodynamic loads are coupled in modal space to predict aeroelastic instabilities. Bending and bending-torsion instabilities due to the high-lift systems become visible.

Nora Neuert, Ian Krukow, Dieter Dinkler

### Production and Characterization of Porous Materials with Customized Acoustic and Mechanical Properties

An overriding objective of the CRC 880 was to reduce noise of future civil aircraft. It was shown that the usage of porous materials can effectively reduce aircraft noise. They are used at the trailing edge replacing a section of the solid wing surface. In order to improve the acoustic properties of these materials, a rolling process was established which allows grading of the material. Initially, it was unclear how well the technique could be applied to porous materials and what effects it would have onto their structure and properties. A porous aluminum as well as a sintered fiber felt was investigated, which showed good results in the acoustic wind tunnel (AWB at DLR Braunschweig) in the unrolled state. In this article it is shown that the rolling process is very well suited to change porosity and thus properties of porous materials in a targeted manner without negatively affecting the mechanical stability. Both, the influence of cold rolling on the porous structure and the mechanical properties were investigated.

Jörn Tychsen, Joachim Rösler

### Massively Parallel Lattice Boltzmann Simulations of Turbulent Flow over and Inside Porous Media

Porous trailing edges have been proposed as a means to reduce acoustic emissions from aircraft wings. However, the influence of the porous material on the aerodynamic performance of the wing has to be investigated. In this work we report DNS/LES simulations of turbulent flow over a DLR-F16 wing profile at a Reynolds number of $$10^6$$ using a cumulant lattice Boltzmann method. The cumulant LBM is implemented in the object-oriented framework VirtualFluids. Due to the requirement of resolving the boundary layer, the resulting simulation setups consist of more than two billion grid nodes and $$72.9\times 10^{9}$$ degrees of freedom distributed on a locally refined three-dimensional grid requiring massively parallel simulations. We discuss modeling and scaling aspects of our approach and present computational results including experimental validation.

Konstantin Kutscher, Martin Geier, Manfred Krafczyk

### Scale-Resolving Simulation of Aeroacoustic Sound from Coanda Flaps

This manuscript summarizes the work performed by the authors (project A8) as part of the Collaborative Research Centre CRC 880 Fundamentals of High Lift for Future Civil Aircraft. Our work aimed to further enhance the understanding of the source mechanisms of solid and porous materials inside turbulent flows by the use of scale-resolving simulations. On the basis of a novel Large Eddy Simulations approach, called Overset-LES, the source mechanism and sound reduction of porous materials were investigated. The governing equations are based on the compressible Navier-Stokes equations in perturbation form. The presence of porous material was modelled by a volume-averaged approach. This volume-averaged model, including nonlinear terms, serves to clarify the sound generation while fully accounting for the nonlinear interaction between fluid and acoustics. The developed hybrid zonal simulation tool with volumetric inflow forcing was applied to a NACA0012 airfoil’s trailing-edge, with or without a porous insert, at representative Mach and Reynolds number of 0.1118 and $$1.0\times 10^6$$ , respectively. Three dimensional simulations of the NACA0012 trailing edge (with a solid and porous inset) showed good agreement with aerodynamic and aeroacoustic validation experiments. Applying porous material to the trailing-edge’s noise confirms the results reported in literature and underlines the validity of the porous model as well as it illustrates possible further applications. The same computational approach was applied to the situation of a Coanda flap, showing that such computations are feasible with practical computational resources.

Paul Bernicke, Varun B. Ananthan, Rinie A. D. Akkermans, Jürgen Dierke, Roland Ewert

### Numerical Simulations and Experiments for Flows over Porous Media

A numerical model for flow through non-uniform porous media is developed. This model is verified with the help of generic test cases for laminar and turbulent flows and numerical results are presented for a non-uniform porous medium at the trailing edge of an airfoil. Aerodynamic analysis of an experimental and numerical investigation for flows through porous trailing edge for high-lift airfoil is presented.

### Towards Faster Design Cycles Through Gradient-Based Optimization

As design tasks are significantly complexifying over time, engineers seek more often assistance by optimization tools in their daily design task. Those cost-effective tools propose innovative solutions which could not be found intuitively by human designers. However, as more and more freedom is given to the design problem, classical optimization algorithms tend to require a significant computational cost. This can be reduced remarkably by using gradient-based optimization algorithms, but these require the computation of the gradient. The present paper discusses the advantages of gradient-based methods and illustrates their capabilities on some turbomachinery design problems compared to gradient-free methods.

Tom Verstraete, Lasse Mueller, Mohamed Aissa, Arnaud Chatel

### A Review of Turbomachinery Noise: From Analytical Models to High-Fidelity Simulations

To address the need for noise predictions of any rotating machine, two different approaches are presented: fast-running analytical models as pre-design tools, and numerical methods to provide detailed analysis and physical insight in the noise sources. For both, a methodology in three steps is proposed, which includes the definition of the excitation, the blade response, and the propagation of the equivalent noise source to the far-field. For all machines, the excitation can be either vortical or acoustic gusts, and the far-field propagation is provided by an acoustic analogy either in free field or in a duct. Only the second step is either an isolated blade response for low speed ventilators or a cascade response for high-speed turbomachines. Overall, analytical models are shown to provide good and fast first sound estimates at pre-design stages, and to easily separate the different noise sources. On the numerical side, for all machines, unsteady Reynolds-Averaged Navier-Stokes simulations are shown to yield accurate tonal noise of the most complex configurations. Wall-modeled Large Eddy Simulations can provide the broadband noise part over most rotating components with good overall sound power level predictions. An accurate and efficient alternative to yield both contributions at once appears to be the hybrid Lattice-Boltzmann/Very Large Eddy Simulations.

Stéphane Moreau

### Experimental Validation of an Optimized Design Process for Transonic Mixed-Flow Compressors

A novel active high-lift system for future commercial aircraft uses electrically driven compressors to provide the required jet momentum for steady blowing over a Coanda flap. This application necessitates high total pressure ratios and high flow rates. A newly developed aeromechanical optimization approach was applied to fulfill these requirements under the given constraints. The optimization resulted in an mixed-flow compressor design featuring a transonic flow regime. A prototype of this compressor stage was designed and manufactured. In this paper, the optimization process is extended to account for the requirements of the electrical components of the compressor system. The compressor performance of the prototype at rotational speeds up to the design speed of 60,000 rpm is measured. A sensitivity study and post-test calculations using Computational Fluid Dynamics are performed. To correct for the influence of leakage flow within the design optimization process, a simple speed-dependent penalty function is implemented. The results confirm the design calculations at points with sufficient surge margin.

Niklas Maroldt, Felix Kauth, Joerg R. Seume

### Optimization of Integrated Compressor Drives for Electrically Powered High-Lift Systems

This chapter presents the holistic optimization process of the electrical components for an advanced high-lift system. The design steps for the reference electrical machine and the power electronics are presented and the results of the holistic optimization routine are shown and discussed.

Gerrit Narjes, Jan-Kaspar Müller, Matthias Kalla, Axel Mertens, Bernd Ponick

### Efficiency of Active Flow Control in an Unsteady Stator Vane Flow Field

Active flow control (AFC) has been proven to be an effective method to counteract flow disturbances and improve the performance of flow systems. Here, we compare the efficiency of open- and closed-loop AFC for a periodically disturbed compressor stator cascade flow. While still being effective in damping disturbances, it turned out that an additional efficiency gain could not be confirmed for this experiment.

Simon J. Steinberg, Rudibert King