Institute of Aerodynamics and Gas Dynamics

Research

in the working group Aircraft Aerodynamics

INAFLOWT

INnovative Actuation Concepts for Engine/Pylon/Wing Separation FLOw Control, Design, Build and Wind Tunnel Test

more on project INAFLOWT

Influence of the missing acceleration of the flow due to the absence of the leading edge slat in the area of the engine nacelle on the flow separation at an angle of attack of 27°.

Current Projects

 

FOR 2895

Unsteady flow and interaction phenomena at High Speed Stall conditions

A research initiative funded by DFG (FOR 2895), HGF and DLR.

FOR 2895

Characterization of the vortex structures in the slat cut-out area of the large model behind the UHBR engine at an angle of attack of 27°.


INAFLOWT


IN
novative Actuation Concepts for Engine/Pylon/Wing Separation FLOw Control, Design, Build and Wind Tunnel Test

Contact person: M.Sc. Junaid Ullah, Dr.-Ing. Thorsten Lutz

Background and scope

The strong increase in air traffic together with the ambitious goal of the EU to reduce COx and NOx levels by up to 90% by 2050 compared to 2000 poses major challenges for aircraft manufacturers. Optimizing the engines of passenger aircraft is one way of getting a little closer to the prescribed goal. Special attention is paid to dual-flow engines with a very large bypass ratio, so-called UHBR (Ultra High Bypass Ratio) engines. In terms of fuel consumption, these engines have significantly better characteristics than those in which a large part of the flow is directed through the core engine. The large nacelle diameter of the UHBR engines and the maintenance of a certain ground clearance to the runway result in a close coupling between engine and wing when integrated into the wing. In order to avoid a collision of the high-lift systems with the engine during landing, no leading edge slats are used in the area of the UHBR engine. The turbulent flow from the engine components and the pylon, as well as the missing acceleration effect of the leading edge flap, lead to flow separation in the wake of the UHBR engine. The consequences are a stall occurring approx. 2° earlier and a degradation of the maximum lift by approx. 10%. By active flow control, especially by suction and blowing on the flow surface, the separation areas can be reduced and the stall can be shifted to higher angles of attack again. However, most flow actuators that have been part of numerical and experimental investigations have the disadvantage that they cannot be used for applications in civil aviation due to their low robustness, high weight and high mass flow requirement. Therefore, within the Clean Sky 2 project INAFLOWT, the EU has set itself the goal to develop flow actuators which can be used on the wing of a passenger aircraft.

Wind tunnel tests with a configuration representative for a high-lift system-engine combination of an airliner under realistic inflow conditions should show whether the developed actuators bring the hoped-for advantage. Within the AFloNext (Active Flow- Loads & Noise control on Next Generation Wing) project, actuators operating according to the synthetic jet and pulsed jet principles have already been tested on the wind tunnel model mentioned above. Especially the results with the pulsed jet actuator were very promising. However, the pulsed jet actuators have the disadvantage of an excessively large mass flow requirement, which cannot be met by bleed air from the engine alone. The SaOB (Suction and Oscillatory Blowing) actuator developed by Tel-Aviv University can close this gap by additionally blowing out the extracted air. At the same time, the boundary layer suction has a positive effect on the tendency of the flow to become detached. Therefore, the SaOB actuator, whose advantages over the other state of the art actuators have so far been demonstrated almost exclusively under laboratory conditions, will be investigated in detail under realistic conditions within the framework of the INAFLOWT project. A benchmark study at the end of the project will compare the actuator technologies to highlight the improvements achieved with the SaOB actuator.

 

Project partners and work at the IAG

Five research institutions and universities from four different countries are actively involved in the INAFLOWT project. The Tel-Aviv University (TAU), Israel Aerospace Industries (IAI), the Aerospace Research and Test Institute in the Czech Republic (VZLU), the Central Aerohydrodynamic Institute in Russia and the IAG. While TAU and TsAGI deal with the experimental work, the other project partners are active in the numerical field. TAU mainly deals with the experimental investigation of the internal actuator flow and the wind tunnel tests of a generic, downscaled wind tunnel model. The generic wind tunnel model is used to test different actuator concepts in order to select the most promising concept and transfer it to the more cost-intensive wind tunnel tests of the large model. TsAGI has the experimental means with the large subsonic wind tunnel T-101 to test the configuration under flight-relevant inflow conditions. IAI's main task is to design the generic wind tunnel model to ensure that, despite the differences in the wind tunnel configurations and Reynolds numbers, the same flow conditions prevail as on the large model. VZLU performs actuator internal simulations to optimize the actuator geometry with respect to pressure losses and to provide actuator boundary conditions for the external flow simulations on the high-lift configuration.

At the IAG, the flow around both high-lift configurations is numerically investigated. Simulations are performed both with and without actuators. The investigation of different actuator concepts on the generic configuration by means of CFD should limit the number of actuator configurations to be investigated in the wind tunnel in addition to the definition of the actuator positions. The simulations of the large model with the final actuator configuration will finally be used for a benchmark study in which comparisons to the actuators used in the AFloNext project will be made.

For the CFD simulations at IAG the flow solver DRL TAU-Code is used. For the URANS calculations of the external flow around the high-lift configurations, the actuators are modelled only through the suction holes and blow-off nozzles. A complete modelling of the actuators would be associated with an unrealistic computing time, despite the use of the computing clusters of the HLRS. The transient boundary conditions at the outlet of the suction hole and at the inlet of the blow-out hole are taken from the internal actuator simulations of VZLU.

The validation of the simulation results by means of experimental data also provides the possibility to evaluate the limits of the URANS application using the flow solver DLR TAU code for simulations with flow actuators and thus to draw conclusions for future numerical investigations of actuator influences on the external bypass flow.

Results

Characterization of the vortex structures in the slat cut-out area of the large model behind the UHBR engine at an angle of attack of 27°.

Publications / Links

Contact person: M.Sc. Jens Müller, Dr.-Ing. Thorsten Lutz

Background and motivation

An essential component of future aircraft development is the ability to describe an aircraft and its characteristics as accurately and comprehensively as possible in the computer with the associated simulation models in the sense of a product as a virtual aircraft model. In the context of the joint project VitAM, the idea of the Virtual Aircraft Model is to be implemented and demonstrated in industrially relevant applications.

The IAG is investigating the effects of realistic atmospheric turbulence on the loads and aerodynamics of transport aircraft. In addition to fundamental investigations on the propagation of turbulence in CFD simulations, two approaches to modeling atmospheric disturbances in CFD simulation are compared. These are the Resolved Atmosphere Approach (RAA), which records all interactions between aircraft and atmospheric turbulence, and the Disturbance Velocity Approach (DVA), which records only the influence of the disturbances on the aircraft, but not the effect of aerodynamics on atmospheric turbulence. While the RAA requires a very high computational effort, the DVA offers the advantage of using a coarser network resolution and thus a lower computation time.

Objectives in the IAG work packages

  • Creation of an interface for turbulence injection in CFD simulations.
  • Performing tests to propagate turbulence through a simple flow field.
  • Determination of the necessary network resolution and numerical parameters.
  • Investigation of the interactions between inflow turbulence and flow around a profile / airfoil using DVA and RAA. Characterization of the correlation between the turbulence fed in and the load fluctuations at the profile/wing.
  • Comparison of the simulation approaches RAA and DVA.
  • Simulation of the flight of a complete aircraft configuration (ATRA) by realistic atmospheric turbulence using DVA and RAA.
Electric wing tip mounted propellers for the development of energy-efficient and noise reduced airplanes.

Contact Person: M.Sc. Michael Schollenberger, Dr.-Ing. Thorsten Lutz

Projekt Description
The aim of the project ELFLEAN as part of the Federal Aviation Research Programme of Germany (LuFo) V-3 is to investigate aircraft configurations with wingtip mounted propellers. An advantageous use of the aerodynamic interactions between the propellers and the wing vortex system, should lead to an increase in the energy efficiency of the configuration and therefore to a reduction of the power requirement. The reduction of the power requirement is intended to compensate for the central disadvantage of electric aircraft, the power supply. In addition a reduction in pollution and noise emissions should be enabled. By reducing the power requirements and the resulting savings in direct operating cost (DOC), the concept has the potential of a commercial advantage over conventional configurations. In the longer term, the concept should contribute to emission-neutral aviation.

ELFLEAN
is a multidisciplinary project of three institutes of the University of Stuttgart, the Institute of Aircraft Design (IFB), the Institute of Flight Mechanics and Flight Control (iFR) and the Institute of Aerodynamics and Gas Dynamics (IAG). The interdisciplinary character is intended to optimize the efficiency of the entire system before maximizing a partial aspect and thereby increase the chances of success of the concept. At the IFB flight tests should be carried out with a scaled unmanned demonstrator platform with wingtip mounted propellers based on the electric aircraft e-Genius. The aim is to quantify the impact of wingtip propellers on flight performance and assign them to the various effects. The flight mechanical characteristics of the configuration are investigated at the iFR. At the IAG, in order to expand the theoretical understanding of the aerodynamic interactions, detailed investigations are carried out by (U)/RANS-based numerical models.

Background
Wingtip mounted propellers use the aerodynamic interaction that occurs between the propeller and the wing vortex system. On the one hand, the tangential velocity of the propeller slipstream can be used to counteract the induced downwash of the wing tip vortex and thus to reduce the induced drag of the wing. On the other hand, the wing tip vortices can increase the inflow velocity of the propeller blades and thus the propeller thrust. Both effects result in a reduced power requirement of the overall configuration.
The interactions are dependent on flow conditions as well as geometric and operational parameters, in particular the relative propeller position upstream (tractor configuration) and downstream (pusher configuration) to the wing. Due to the large number of occurring effects and design parameters, wingtip mounted propellers are a complex technology.

Work at the IAG
IIn the field of aerodynamics, the questions should be clarified, which aspects contain the aerodynamic interactions in detail, how these can be numerically determined, verified by flight tests and taken into account for the design of wingtip mounted propeller configuration. Various numerical methods are used for the propeller simulation, approximating models such as Actuator Disk and Actual Line as well as a fully blade resolved calculation by Chimera rotation.

The project steps are:

  1. Verificate the numerical methods for the simulation of aerodynamic interactions between wingtip mounted propellers and the wing vortex system.
  2. Understand the aerodynamic interactions in more detail.
  3. Generate a detailed database of the design parameters.
  4. Investigate the aerodynamic design of wingtip mounted propellers.

 

Supported by Federal Ministry for Economic Affairs and Energie on the basis of a descision by the German Bundestag

Completed projects:

Contact: Dipl.-Ing. Rouven Mayer, Dr.-Ing. Thorsten Lutz

Background and motivation

A wing in laminar flow leads to a significant reduction of the aerodynamic resistance compared to a wing in turbulent flow. Airbus has been working with partners on this technology for many years. However, the realisation of a laminar wing for operational use still requires a great deal of effort, such as answering questions on the economic production and durability of the very smooth surfaces required.

The Low Drag Aircraft (LDA) technology program, which has been running since 2008, is designed to demonstrate the technical maturity of an NLF (Natural Laminar Flow) wing up to technology maturity level 6. Within the framework of the LDAinOP subproject, key technologies for this type of transonic wing are to be investigated. In addition to improved laminar retention in climb flight through the intelligent use of a flap, this flap is also to be used in cruise flight to adjust the wing curvature and thus to position the shock wave occurring in the transonic speed range. This is of interest in so far as the natural laminar flow requires less wing sweep. As a consequence, the flight Mach number above which wave resistance occurs is considerably reduced. On the basis of shock positioning, various measures for wave resistance reduction will therefore be investigated. In the past, the so called Shock Control Bump (SCB) has proved to be an efficient control measure. However, this only works when the position is correct with regard to the compression impact and tends to have a detrimental effect when the flight Mach number is lower, so that adaptive structural concepts are to be investigated within the framework of the project.

Works at the IAG

Within the framework of the LDAinOP project at the IAG, an SCB design for wave resistance reduction at high transonic flight Mach numbers for a modern laminar wing using numerical methods is being developed. A CFD-based numerical process chain is used, which allows the numerical optimization of SCBs. Based on the optimization results, industrially applicable design guidelines are developed. In addition, the IAG, with its many years of experience, supports the project partners in the structural implementation of the SCB design. Consequently, one goal of the project is to demonstrate the structural feasibility of such adaptive structures.

Contact: Dipl.-Ing. Rouven Mayer, Dipl.-Ing. Steffen Bogdanski, Dr.-Ing. Thorsten Lutz

Background and motivation

With regard to the development of future transport aircraft, resistance reduction and the associated lower fuel consumption play a central role. Laminar wings show great potential here. The aim of the BUTERFLI project, carried out in European-Russian cooperation, is an in-depth analysis of the underlying aerodynamics in order to establish laminar wing technology in industry in the medium term and thus improve the flight performance of future commercial aircraft. For this purpose, the BUTERFLI project investigates different phenomena with experimental and numerical methods, which are related to a laminar wing operating in the transonic speed range. In addition to the buffet, i.e. the repeated detachment and reapplication of the boundary layer combined with an impact and lift oscillation, on a laminar profile and a turbulently flowed around supercritical profile, cross-flow instabilities on a swept wing are also investigated. In addition, investigations are carried out using various flow influencing techniques. In addition to contour bumps (Shock Control Bumps (SCB)), active measures such as plasma actuators are also used.

Works at the IAG

Within the framework of the BUTERFLI project, 3D-SCBs are being investigated and designed at the IAG, which will both reduce the characteristic impedance occurring in the transonic velocity range and increase the flight range limit specified by the occurrence of Buffet. For a laminar profile, numerical optimizations of the SCBs are carried out at the IAG. An existing, CFD-based, numerical process chain is used, which includes RANS simulations in addition to automated, high-quality networking of the profile geometry with a structured network. URANS simulations will then be performed for selected SCBs to provide in-depth insights into buffet behaviour, buffet boundaries and buffet amplitudes.

Background and motivation

With high flight Mach numbers and lift coefficients, highly complex, transient impact-interface interactions occur at the wing, which can be associated with local or complete impact-induced detachments. The detached transient flow may strike the tail and cause critical pressure and load fluctuations. This interaction between the detached vane flow and the tailplane flow leads to phenomena which are not yet clarified in detail. Within the framework of the IAG contribution, state-of-the-art numerical calculation models, which will be further developed within C2A2S2E and ComFliTe, will be used and evaluated for the study of these phenomena and qualified in the sense of the preparation of best practice guidelines. This should contribute to the extension of the use of CFD methods for predicting aircraft characteristics to the limits of the flight regime. The methodological developments and detailed validations carried out by the partners within ComFliTe as well as the preparatory work of the IAG will be used in a targeted manner.

Goals of the working package of the IAG
  • Testing and validation of newly developed turbulence models as well as hybrid RANS-/LES methods for prediction of complex shock-interface interactions including shock oscillations (buffet)
  • Studies for low-dissipation calculation of the propagation of transient, detached flow structures
  • Study of the influence of shock induced, massive flow separation at the wing on the excitation of the tailplane
  • Application of currently developed numerical methods (e.g. higher order methods) for more complex test cases
  • Evaluation of coupling approaches for the calculation of the trimmed, flexible aircraft
  • Identification of the limits of URANS methods with regard to the aerodynamic task under consideration and evaluation of the ability of URANS and DES methods to reproduce inherent transient flow phenomena with temporally invariant inflow.
Results

Fluid structure coupling between the flow solver TAU and a Nastran FEM model Propagation of the detached wake on a profile with subsonic incident flow Numerical simulation with hybrid RANS-LES methods in the area of the shock buffet
AP75 AP75 AP72
For a larger view please click on the picture!

Publications

Dipl.-Ing. Sebastian Illi, Th. Lutz and E. Krämer:
Transonic Tail Buffet Simulations on the ATRA Research Aircraft
Notes on Numerical Fluid Mechanics and Multidisciplinary Design, Volume 123, Springer, Germany, 2013.

Dipl.-Ing. Sebastian Illi, P. Gansel, Th. Lutz and E. Krämer:
Hybrid RANS-LES Wake Studies of an Airfoil in Stall
CEAS Aeronautical Journal, Volume 4, Issue 2, pp 139-150, Springer, Germany, June 2013.

P. Gansel,  Dipl.-Ing. Sebastian Illi, Dr. Th. Lutz and Prof. Dr.-Ing. Ewald Krämer:
Numerical Simulation of Low Speed Stall and Analysis of Turbulent Wake Spectra
The 15th International Conference on Fluid Flow Technologies, Budapest, Hungary, September 4-7, 2012.

P. Gansel, S. A. Illi, A. Kalimullina and Th. Lutz:
The Spectral Analysis of Unsteady Pressure Coefficient at the Wing Trailing Edge
16th International Conference on the Methods of Aerophysical Research, Kazan, Russia, August 20 - 26, 2012.

Dipl.-Ing. Sebastian Illi, Th. Lutz and E. Krämer:
On the capability of unsteady RANS to predict transonic buffet
Third Symposium "Simulation of Wing and Nacelle Stall", Braunschweig, Germany, June 22 - 23, 2012.

A. Klein, Dipl.-Ing. Sebastian Illi, Th. Lutz and E. Krämer:
Wall Effects and Corner Separations for Subsonic and Transonic Flow Regimes
High Performance Computing in Science and Engineering '11, Springer, Germany, 2012.

Dipl.-Ing. Sebastian Illi, Th. Lutz and E. Krämer:
Improved physical understanding and modeling – DES/LES & PIV
CFD and Experiment – Integration of Simulation, Göttingen, Germany, April 5 - 6, 2011.

Dipl.-Ing. Sebastian Illi, Dr. Th. Lutz and Prof. Dr.-Ing. Ewald Krämer:
Simulation of pressure and shock induced separation using DES implementations in the DLR-TAU Code
Second Symposium "Simulation of Wing and Nacelle Stall", Braunschweig, Germany, June 22 - 23, 2010.

Links

Official ComFliTe website of the DLR
Federal Ministry of Education and Research

Contact: Dipl.-Ing. Katharina Wawrzinek, Dr.-Ing. Thorsten Lutz

Background and motivation

Turbulences in the inflow, as they occur in the atmosphere, cause load fluctuations and can strongly influence the aerodynamics of an airfoil, especially in the stable area. A major goal of the research group is the study of these complex interactions and the improvement of numerical calculation models to predict the occurring effects. Within the framework of this new subproject, simulations on two reference configurations using a Reynolds Stress Model of turbulence and the new hybrid method of Detached-Eddy Simulation will be carried out complementary to the work of the partners. The subproject contributes to the process development with a model for the detection of anisotropies in synthetic turbulence generation. The aim of the work is the validation of these methods and the analysis of the interactions by parametric investigations. The influence of artificial two- and three-dimensional disturbances on a high-lift configuration measured in a wind tunnel as well as the interaction of an atmospheric inflow with a high-lift wing are considered. In addition to the effects of turbulent inflows on pressure distribution, loads and stall properties of wings, this subproject will also specifically investigate the influence on the development of wing wake, which is relevant for the inflow of a tail unit.

Objectives in the IAG work packages

  • Analysis of the transient aerodynamics of aerofoils with inhomogeneous inflow
  • Improvement of synthetic turbulence generation by a physically consistent anisotropy model
  • Validation of the simulation methodology for 2D and 3D disturbances in the wind tunnel
  • Gain of knowledge for flying at border areas near maximum lift in disturbed atmosphere:
    • Influence of atmospheric inflow on loads and stall properties of a wing with a slit flap
    • Influence on the wing wake and the inflow of a tail unit


Links

Official FOR1066 website of the TU Braunschweig
German Research Foundation

Contact: Dipl.-Ing. Philipp Gansel, Dr.-Ing. Thorsten Lutz

Background and motivation

The main objective of the DLR-led joint project HINVA (High-Lift In-Flight Validation) is the development and improvement of prediction methods for the maximum lift of commercial aircraft. For this purpose, detailed flight tests, cryogenic wind tunnel tests and extensive numerical simulations will be carried out. The Airbus A320 ATRA (Advanced Technology Research Aircraft) of DLR serves as the test vehicle. The flow conditions during the stall manoeuvre will be examined for subsonic aircraft with landing and cruise flight configuration. By using modern test and measurement technology, a large database will be obtained during the extensive flight tests, which will serve to validate and evaluate the numerical and experimental methods and contribute to their improvement. In addition to the project partners DLR, Airbus and ETW, the TU Berlin, TU Braunschweig and the University of Stuttgart are represented in the project.

Work at the IAG

At the IAG, numerical investigations are carried out on the interference behaviour of wing and tail unit flow as well as on the trim of a transport aircraft in the low-speed stall area. On the basis of the cruise flight configuration of the ATRA, the local spatial discretization of the wing wake, the fuselage boundary layer and the tailplane itself is investigated with regard to its influence on the inflow of the tailplane. The linear Ca-range is calculated with RANS- and with detached wing boundary layer with URANS-methods. The flow instationarities at the trailing edge of the wing and their propagation to the tailplane are evaluated with regard to the contained spectra and length scales of the turbulent flow, in order to finally gain a statement about their effect on transient elevator loads. In addition, the effects of a numerically determined trim angle of the tail unit compared to a trim angle determined in the flight test are analyzed. On a configuration with simplified wing geometry an exemplary Detached-Eddy-Simulation shall allow the comparison of this simulation method with temporally and spatially resolved turbulent structures to the modelled approach of the RANS equations. The influence of geometric simplifications by neglecting engine and flap track fairings is estimated with RANS methods.


Publications

SOFIA open door test flight courtesy of NASA
SOFIA open door test flight courtesy of NASA

Sofia 

Project description

The Institute of Aerodynamics and Gas Dynamics at the University of Stuttgart is involved in the SOFIA project, a joint project between NASA, DLR, the Institute of Space Systems at the University of Stuttgart and other German and American institutions and companies. The 20-year project focuses on a flying observatory, a Boeing 747 SP with an integrated infrared mirror telescope for astronomical remote sensing.

Infrared radiation provides information about the formation of stars and solar systems, and allows a glimpse into the past. Water vapour absorbs the IR radiation, in the lower part of the earth's atmosphere only a small part of the radiation arrives. Above the tropopause (about 12-15 km altitude), good observation conditions already prevail. Compared to a satellite base, the airborne observatory offers the advantage of considerably lower costs and almost unlimited positioning possibilities. The instruments can be optimized during the flights and permanently adapted to the state of the art.

The mirror telescope (diameter 2.7 m) is located in the rear part of the fuselage, a hole cut into the fuselage structure ensures an unobstructed view to the outside. During take-off, this hole is closed by a door, which is opened for observation during flight.

Background and motivatin

The overflow of the cavity leads to strong turbulences and self-excited pressure fluctuations in the area of the mirror, whereby under flight conditions without active or passive damping mechanisms sound levels of up to 160 dB are achieved in the cavity, which in turn lead to body vibrations. In order to optimize the positioning accuracy of the telescope in order to achieve the highest possible observation quality, pressure fluctuations must be suppressed. Several active and passive methods are available to influence the boundary layer upstream of the cavity or the free shear layer.

Objectives in the IAG work packages

  • Investigation of SOFIA cavity aeroacoustics with CFD (Computational Fluid Dynamics) and CAA (Computational Aero Acoustics)
  • Studies on passive and active flow influencing
  • Analysis of the influence of flow phenomena on the positioning accuracy of the telescope
  • Testing of hybrid RANS-/LES methods for the simulation of caviy flows
  • Evaluation of aero-optical properties of free shear layers using CFD
  • Detailed analysis of the SOFIA cavity flow using pressure sensors, thermocouples and optical cameras during flight tests.

Results

 

Influencing the shear layer over the cavity with a 3D fence
vortices
Impact of passive flow influence on the telescope
SPL TA Surface
Comparison of CFD and CAA in the prediction of cavity acoustic modes
48Hz Mode
All images are property of NASA and DLR  (Please click on the picture for a larger view!)

Publications

S. Schmid, Th. Lutz and E. Krämer:
Simulation of the Unsteady Cavity Flow of the Stratospheric Observatory For Infrard Astronomy
IUTAM Symposium on "Unsteady Flows and their Control", Vol.14, Springer Berlin Heidelberg New York 2009

S. Schmid, Th. Lutz and E. Krämer:
Passive Control of the Flow Around the Stratospheric Observatory For Infrard Astronomy
AIAA Paper 2008-6717, 2008

Links

German SOFIA Institute

German Center for Aerospace Engineering

National Aeronautics and Space Agency

Contact: Dipl.-Ing. Dina-Marie Zimmermann, Dr.-Ing. Thorsten Lutz

Background and motivation

In boundary areas of the flight envelope, strongly transient inflows at the tail can be caused by detached and thus temporally fluctuating boundary layer conditions at the wing. The transient aerodynamic forces excite dynamic modes of the structure, which must be taken into account in the load calculation of the tailplane. In addition, detachments at the wing affect the downwind and thus change the inflow conditions of the tail, which can negatively influence its effectiveness. In the LuFo IV project ATLAS investigations are therefore carried out on the transient inflow of the tail in the wake of a detached wing flow. Based on a characterisation of the transient inflow at the tail from numerical simulations of detached wing flows, a model is developed to describe the fluctuations in the tail area.

Objectives:
  • Establishment of an efficient chain providing suitable input data for the method used by project partners to calculate transient tailplane loads.

  • Findings on the influence of inflow parameters on the transient fluctuations in the tailplane inflow.

  • Methodology for the efficient determination of transient inflow data at the tail unit based on the flow state at the wing from RANS simulations.

Figure 1: The current concept
Figure 1: The current concept

In times of constantly increasing mobile data traffic with ever faster data transmission rates and required bandwidths, it is becoming increasingly necessary to look for alternatives to the existing ground-based and satellite-based solutions that are able to cope with the data volume of the future and guarantee area-covering and cost-effective use. At the Institute for Statics and Dynamics of Aerospace Structures (ISD) the test vehicle of a multi-segment airship platform ("airworm") has been tested for several years. Equipped with telecommunication devices, the platform is to be positioned at a height of 20 km in the stratosphere as stationary as possible above a fixed point on Earth due to the favourable wind conditions prevailing there (Figure 1). With a corresponding position outside the commercially used airspace of up to about 15 km, the lowest wind speeds in the upper stratosphere occur in this altitude range. On average, these are around 50 km/h in summer and 100 km/h in winter, although considerably higher values may occur in the short term. As is the case with most aircraft, the minimization of structural mass is of decisive importance in the design of high-altitude platforms. In a non-rigid airship, this consists of the mass of the hull and the tail units as well as the payload. The hull mass results from the maximum internal overpressure that the hull must withstand. This maximum internal overpressure is made up of the operating overpressure and the maximum differential pressure of the carrier gas at high pitch.

AirChain_3

Figure 2:
Fluid Mechanical Phenomena

The operating pressure ensures that the airship can withstand under all flight conditions the static bending stresses resulting from an uneven distribution of lift, payload and design mass (in particular the tailplane) as well as the dynamic bending stresses caused by the tailplane and ammunition moments during stabilisation and control of the airship. Both bending stresses and thus the hull mass increase with increasing length-diameter ratio of the airship, whereby the proportional mass of the tail units decreases. In the case of a sphere, both bending stresses disappear, so that five quasi-spherical segments were arranged one behind the other in order to achieve the lowest possible hull mass and at the same time a high aerodynamic quality. The loose coupling of the segments results in an additional form instability. However, the multi-segment "airworm" can be stabilized, as demonstrated in numerous flights of the prototype (Figure 2), by appropriate control using differential thrust of the propellers attached laterally to the first three segments.

AirChain_4
Figure 3:
Prototype

The IRON BIRD system test bench is being developed for the simulation of all systems of this airship-supported elevation platform within the framework of the AirChain project financed by the Landesstiftung Baden-Württemberg foundation. In addition to the dimensioning of the components on board and the complex control system, the IRON BIRD is also used for extensive testing of the overall system. Within the scope of this project, work at the IAG is concerned with aerodynamic problems. This includes the calculation of the propeller maps for the existing controllable pitch propellers.

These are attached laterally to the first three segments of the "airworm" and provide the necessary propulsion, whereby the stabilisation and directional control of the multi-segment airship is also achieved by differential thrust. These serve as the basis for the flight-mechanical models of the flight control systems.

AirChain_5
Figure 4:
Wind Tunnel Model

Although today's theoretical approaches and their implementation in numerical simulations are subject to continuous improvement, the validation of the obtained results by means of wind tunnel tests is still an important component in the design process of a flying object. This applies in particular to the very strong three-dimensional airship flow, which is characterized by massive separation. This still poses great challenges for today's computational methods, especially in the case of the transient detachments that frequently occur at large angles of attack or thrust due to the low possible flight speeds under the influence of gusts and flight manoeuvres. For novel configurations such as the "airworm", in which the individual segments can additionally assume different flight angles to each other, very complex flow-mechanical phenomena arise, especially at the bends (Figure 3). For such multiple buckled configurations no aerodynamic data base exists yet, so that experimental investigations are necessary to determine the coefficients and to validate flow solvers. Figure 4 shows the configurations to be investigated within the wind tunnel measurement series.

AirChain_6
Figure 5:
Model in the wind tunnel

For this purpose, a 4m long wind tunnel model was created for the Gust Wind Tunnel of the institute, at which the required segment angles can be adjusted (Figure 5). In addition to the total loads on the configuration, the individual segment loads must also be determined. The former are recorded with the aid of a rope-supported model suspension. The model, sufficiently ballasted, is suspended from six ropes equipped with a force transducer. The aerodynamic loads on the model are determined by means of the rope forces and the rope directions, for which the individual suspension ropes are each provided with three reflecting markers, the positions of which are recorded with the aid of a photogrammetric method (Figure 6). The individual segment loads can be determined by integrating the pressure distribution on the model surface.

Multi Ojective Dynamical Aircraft Synthesis

High-precision modeling of flight dynamics and aerodynamics for a flexible aircraft program aeronautical research 2003-2007 of the BMWT. The aim of the project is to establish a method for the generation of high-precision state space models of a flexible aircraft. For the first time, nonlinear transonic and viscous effects should be considered Figure 1: Process chain MODYAS Figure 2: Deformation of a dynamically stressed wing At IAG, the necessary RANS calculations for the trimmed, aeroelastically deformed aircraft are performed. For this purpose, the RANS solver FLOWer is coupled with a method for the automatic generation of structured multi-block grids and the FEM program Nastran modular. In the weakly coupled CFD-CSD simulation, not only the aeroelastic wing deformation but also the deformations of the fuselage and the tail units are to be taken into account. The results of the aeroelastic simulation serve the DLR Institute of Aeroelasticity as a basis for the calculation of the transient air force matrices using a Transonic Doubled Lattice (TDLM) method. The Institute of Flight Mechanics and Flight Control (IFR) uses these air force matrices as the basis for creating a state space model of the flexible aircraft.

 

 

Figure 1: Adaptive Wing
Figure 1: Adaptive Wing

The project deals with concepts and aerodynamics of adaptive transonic wings. To cope with the problem of flight at transonic Mach numbers adaptive mechanisms are introduced. Aerodynamic efficiency at off-design conditions is improved by the application of a shock control bump (SCB), a concept first introduced in 1992 by Ashill, Fulker and Shires, on a variable camber (VC) airfoil. Since a SCB has to be properly shaped and positioned to generate a favourable effect, relevant geometrical parameters are investigated using direct numerical optimisation. An optimisation environment was developed consisting of a hybrid optimiser, a geometry module and a coupled Euler boundary-layer code. For a specified off-design condition bump shapes are optimised, while the influence of various geometric bump representations is investigated. Shape optimisations for an adaptive bump are carried out for different Mach numbers at a fixed lift coefficient. To overcome the problem of narrow Mach regions of significant drag reduction for one-point designed bumps, multi-point designs are performed.
The physical effect of the SCB is based on the highly nonlinear character of transonic flows. The SCB maps the contour of supercritical wing sections onto a smaller scale thus inducing isentropic compression waves upstream of the shockwave. This leads to a significant decrease of wave drag.
Determination of the exact flight condition in real flight represents a sophisticated task. Thus it is anticipated that SCBs for practical use must yield a reduced sensitivity to small changes of the onset flow. Because of the narrow Mach region of reduced drag coefficients for a one-point designed SCB multi-point designs were introduced. The objective function for the multi-point optimisation was changed to be represented by the sum of two drag coefficients at two different Mach numbers. On the left hand side the drag coefficient is plotted vs. the Mach number for a one-point design and several two-point optimised bumps. Since no weighting factors were involved in the optimisation process, the lower edge of the Mach-region implicitly has a lower priority than the upper edge since it introduces less wave drag that can be reduced. Thus the bump optimised for the most extended region of Mach numbers even shows a higher drag coefficient for the lower design Mach number compared to the clean airfoil while being favourable in the remaining design region. However, it can be stated that at the cost of less maximum drag reduction the region in which the bump is effective is broadened.

Figure 2 and 3: Polar Diagrams of Adaptive Wing
SFB_2

Because of its wave drag reducing capability an SCB applied additionally onto a VC airfoil promises a further increase of the aerodynamic efficiency. Direct numerical optimisations for a VC-SCB combination were carried out in order to estimate the additional improvement. On the right hand side the lift to drag ratio is plotted against the lift coefficient for the clean airfoil, the envelope of a VC-airfoil (green line), the envelope of a SCB-airfoil (blue line) as well as the envelope of a VC-SCB combination (orange line). Significant additional gains for the combination are visible. Investigations of the resulting optimised bump geometries of the VC-SCB combination show a noticeably reduced bump height compared to the optimised SCB only geometry. Further investigations regarding this project can be found in the following publications:

A. Sommerer, Th. Lutz and S. Wagner:
Numerical Optimization of Adaptive Transonic Airfoils With Variable Camber
Proceedings 22nd ICAS Congress, Harrogate, United Kingdom, August 27 - September 1, 2000, ICAS Paper ICA2.111

A. Sommerer, Th. Lutz and S. Wagner:
Design of Adaptive Transonic Airfoils By Means of Numerical Optimization
Proceedings ECCOMAS 2000: European Congress on Computational Methods in Applied Sciences and Engineering, September 11-14, 2000, Barcelona, Spain

Panel calculation for the airship LOTTE
Panel calculation for the airship LOTTE

Research Group Airship Technology (FOGL)Aircraft.

The Airship Technology Research Group (FOGL) was established by the German Research Foundation (DFG) at the beginning of 1997 at the University of Stuttgart and was funded for a total of six years. The initiator of the research group was Prof. Dr.-Ing. B. Kröplin from the Institute for Statics and Dynamics of Aerospace Structures (ISD). In addition to the ISD, the Institute of Aerodynamics and Gas Dynamics (IAG), the Institute of Flight Mechanics and Flight Control (IFR) and the Centre for Solar Energy and Hydrogen Research (ZSW) were involved. The objective of the research group was to deepen the understanding of relevant physical relationships and phenomena as a basis for the development of new tools and for the adaptation of existing tools for the design and analysis of modern airships.

Since the peak phase of the airships, hardly any scientific investigations have been carried out on the aerodynamics of "lighter-than-air" configurations. At the beginning of FOGL's work, no current published aerodynamic data was available that would serve as a sound basis for validating modern measurement techniques. This concerned on the one hand studies on the topology of the fluid bed separation from the fuselage and on the development of the associated wake, as well as investigations on interference effects between fuselage and tail fins. The results have deepened the understanding of the relevant fluid mechanics phenomena. In addition, they served as a basis for physically meaningful modelling within the framework of a parallel process development. One focus of the theoretical work was the development and implementation of an efficient coupled 3D panel boundary layer method, in which airship-specific aerodynamic phenomena are specifically captured. Approaches for the higher-quality modelling of free vortex layers, as they can detach from the inclined hull or the side or front edges of the tail units, have been developed. The resulting calculation program UNIPAC enables the stationary analysis of driven LTA configurations as well as the recording of relevant transient effects with a justifiable computing time requirement.
A further focus of the theoretical work was the performance of RANS analyses (Reynolds-Averaged-Navier-Stokes) for the reference configuration LOTTE, which was measured in detail. Extensive comparative calculations document characteristic differences between different turbulence models and knowledge and experience could be gained for the meaningful selection and application of suitable models (as well as for the design of structured grids).

AirChain_3
Comparison calculated (RANS) and measured pressure distribution for the airship LOTTE
AirChain_3
Measurements on LOTTE in the Gust Wind Tunnel of the IAG
AirChain_3
Boundary layer measurements and flow visualization on the Zeppelin NT in flight test


The results achieved within the framework of FOGL's work have been documented in numerous publications and can also be found, for example, in the specialist book "Lighter than Air - Transport and Carrier Systems".

The aim of the ELFLEAN project in the Aviation Research Programme (LuFo) V-3 is the investigation of aircraft configurations with surface propellers.

more about the ELFLEAN project

Dr.-Ing. Thorsten Lutz

This picture showsThorsten Lutz
Dr.-Ing.

Thorsten Lutz

Head of working group Aircraft Aerodynamics / Head of working group Wind Energy

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