06 Fakultät Luft- und Raumfahrttechnik und Geodäsie

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/7

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Open Access
A time-accurate inflow coupling for zonal LES
(2023) Blind, Marcel P.; Kleinert, Johannes; Lutz, Thorsten; Beck, Andrea
Generating turbulent inflow data is a challenging task in zonal large eddy simulation (zLES) and often relies on predefined DNS data to generate synthetic turbulence with the correct statistics. The more accurate, but more involved alternative is to use instantaneous data from a precursor simulation. Using instantaneous data as an inflow condition allows to conduct high fidelity simulations of subdomains of, e.g. an aircraft including all non-stationary or rare events. In this paper, we introduce a toolchain that is capable of interchanging highly resolved spatial and temporal data between flow solvers with different discretization schemes. To accomplish this, we use interpolation algorithms suitable for scattered data in order to interpolate spatially. In time, we use one-dimensional interpolation schemes for each degree of freedom. The results show that we can get stable simulations that map all flow features from the source data into a new target domain. Thus, the coupling is capable of mapping arbitrary data distributions and formats into a new domain while also recovering and conserving turbulent structures and scales. The necessary time and space resolution requirements can be defined knowing the resolution requirements of the used numerical scheme in the target domain.
Open Access
About the suitability of different numerical methods to reproduce model wind turbine measurements in a wind tunnel with a high blockage ratio
(2018) Klein, Annette Claudia; Bartholomay, Sirko; Marten, David; Lutz, Thorsten; Pechlivanoglou, George; Nayeri, Christian Navid; Paschereit, Christian Oliver; Krämer, Ewald
The paper describes the experimental and numerical investigation of a model wind turbine with a diameter of 3.0 m in a narrow wind tunnel. The objectives of the study are the provision of validation data, the comparison and evaluation of methods of different fidelity and the assessment of the influence of the wind tunnel walls. It turned out, that the accordance between the experimental and numerical results is good, but the wind tunnel walls have to be taken into account for the present setup.
Open Access
Accuracy of the Gamma Re-Theta transition model for simulating the DU-91-W2-250 airfoil at high Reynolds numbers
(2021) Michna, Jan; Rogowski, Krzysztof; Bangga, Galih; Hansen, Martin O. L.
Accurate computation of the performance of a horizontal-axis wind turbine (HAWT) using Blade Element Momentum (BEM) based codes requires good quality aerodynamic characteristics of airfoils. This paper shows a numerical investigation of transitional flow over the DU 91-W2-250 airfoil with chord-based Reynolds number ranging from 3 × 106 to 6 × 106. The primary goal of the present paper is to validate the unsteady Reynolds averaged Navier-Stokes (URANS) approach together with the four-equation transition SST turbulence model with experimental data from a wind tunnel. The main computational fluid dynamics (CFD) code used in this work was ANSYS Fluent. For comparison, two more CFD codes with the Transition SST model were used: FLOWer and STAR-CCM +. The obtained airfoil characteristics were also compared with the results of fully turbulent models published in other works. The XFOIL approach was also used in this work for comparison. The aerodynamic force coefficients obtained with the Transition SST model implemented in different CFD codes do not differ significantly from each other despite the different mesh distributions used. The drag coefficients obtained with fully turbulent models are too high. With the lowest Reynolds numbers analyzed in this work, the error in estimating the location of the transition was significant. This error decreases as the Reynolds number increases. The applicability of the uncalibrated transition SST approach for a two-dimensional thick airfoil is up to the critical angle of attack.
Open Access
Aeroacoustic analysis of the SOFIA telescope cavity by means of flight test data
(2011) Seidenberg, Artem
The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a joint project of the National Aeronautics and Space Administration (NASA) and the German Aerospace Center (DLR) in order to study the universe in the infrared spectrum. Being the largest airborne observatory in the world it consists of a modified Boeing 747SP operating at and above an altitude of 12 km. In particular, a 2.7 m reflecting telescope is integrated in a closable cavity in the rear part of the aircraft body. Once the cavity door is opened for skywatching during operational flights, the telescope is exposed to the free atmosphere and thus to flow-induced vibrations and acoustic fluctuations, which can impact its pointing stability and image quality. The goal of this study thesis was to analyze the aeroacoustic behavior of the SOFIA telescope cavity by means of available flight test data. Therefore, an optimized postprocessing procedure with powerful graphical capabilities has been implemented in order to interpret the test results qualitatively and quantitatively as well as to compare them with previous theoretical findings and numerical simulations. In this way, this analysis provides further important insights as well as a deeper understanding of critical aeroacoustic behavior of the SOFIA configuration and thus contributes to the interdisciplinary telescope pointing optimization.
Open Access
Aeroacoustic simulation of turbulent boundary layer induced automotive gap noise
(2021) Erbig, Lars; Munz, Claus-Dieter (Prof. Dr. rer. nat.)
Open Access
Aerodynamic and acoustic simulations of thick flatback airfoils employing high order DES methods
(2022) Bangga, Galih; Seel, Ferdinand; Lutz, Thorsten; Kühn, Timo
The results of high fidelity aerodynamic and acoustic computations of thick flatback airfoils are reported in the present paper. The studies are conducted on a flatback airfoil having a relative thickness of 30% with the blunt trailing edge thickness of 10% relative to chord. Delayed Detached-Eddy Simulation (DDES) approaches in combination with high order (5th) flux discretization WENO (Weighted Essentially Non-Oscillatory) and Riemann solver are employed. Two variants of the DES length scale calculation methods are compared. The results are validated against experimental data with good accuracy. The studies provide guideline on the mesh and turbulence modeling selection for flatback airfoil simulations. The results indicate that the wake breakdown is strongly influenced by the spanwise resolution of the mesh, which directly contributes to the prediction accuracy especially for drag force and noise emission. The Reynolds normal stress and the Reynolds stress component have the largest contributions on the mixing process, while the contribution of the component is minimal. Proper orthogonal decomposition is further performed to gain deeper insights into the wake characteristics.
Open Access
Aerodynamic characteristics of airfoil and vertical axis wind turbine employed with gurney flaps
(2021) Chakroun, Yosra; Bangga, Galih
In the present studies, the effects of Gurney flaps on aerodynamic characteristics of a static airfoil and a rotating vertical axis wind turbine are investigated by means of numerical approaches. First, mesh and time step studies are conducted and the results are validated with experimental data in good agreement. The numerical solutions demonstrate that the usage of Gurney flap increases the airfoil lift coefficient CL with a slight increase in drag coefficient CD. Furthermore, mounting a Gurney flap at the trailing edge of the blade increases the power production of the turbine considerably. Increasing the Gurney flap height further increases the power production. The best performance found is obtained for the maximum height used in this study at 6% relative to the chord. This is in contrast to the static airfoil case, which shows no further improvement for a flap height greater than 0.5%c. Increasing the angle of the flap decreases the power production of the turbine slightly but the load fluctuations could be reduced for the small value of the flap height. The present paper demonstrates that the Gurney flap height for high solidity turbines is allowed to be larger than the classical limit of around 2% for lower solidity turbines.
Open Access
Aerodynamic interactions between distributed propellers and the wing of an electric commuter aircraft at cruise conditions
(2024) Schollenberger, Michael; Kirsch, Bastian; Lutz, Thorsten; Krämer, Ewald; Friedrichs, Jens
Beneficial interactions that occur between propellers and the wing can be used to increase the overall efficiency of an aircraft in cruise flight. Different concepts with such interacting propellers are distributed propulsion (DP) and wingtip mounted propellers (WTP). For DP, a full distribution over the entire span can be distinguished from a partial distribution, concentrating the propellers at the wing tip area. The paper focuses on the energy efficiency in cruise flight as a result of the interactions and provides a general comparison of the concepts (WTP, full and partial DP) with a Beechcraft 1900D commuter aircraft as a reference. Parametric CFD studies varying the number and the position of the propellers are performed with a half-wing model. The simulations are performed with the second-order finite-volume flow solver TAU, developed by the German Aerospace Center (DLR), employing Reynolds-averaged Navier-Stokes (RANS) equations. The propellers are modeled using an Actuator Disk (ACD). An algorithm is used to reach cruise condition by iteratively adjusting the propeller rotational speed and the wing angle of attack. The CFD results are analyzed and evaluated with respect to the overall efficiency including the aerodynamic efficiency of the wing as well as the propulsive efficiency of the propellers. The parameter study shows that in cruise flight partial DP is more efficient than a full DP. The pure WTP configuration was found as the optimum of the propeller distribution along the wing, resulting in a saving of required power of 5.6%, relative to the reference configuration.
Open Access
Aerodynamic interactions on Airbus Helicopters' compound helicopter RACER
(2023) Frey, Felix; Krämer, Ewald (Prof. Dr.-Ing.)
In this thesis, the mutual effects of main rotor, wings, and lateral rotors of Airbus Helicopters' compound helicopter RACER are addressed under different flight conditions. In order to isolate the interactional phenomena, trimmed high-fidelity computational fluid dynamics (CFD) simulations are not only conducted on a detailed representation of the full compound helicopter but also on reduced configurations which are created by removing the respective other components of interest. By comparing the aerodynamic performance of main rotor, wings, and lateral rotors for the different computations, mutual influences can consequently be determined, which are further divided into first- and second-order effects. With the help of a loose coupling between the comprehensive analysis (CA) tool HOST and the CFD solver FLOWer, realistically trimmed free-flight conditions are determined for the relevant flight states. The first of these is RACER's cruise flight at 220kts, which represents the operating condition the compound helicopter was specifically designed for. Due to the elevated advance ratio of over 0.5 and the resulting azimuthal variation of main rotor inflow, a strong asymmetry is witnessed in unsteady Reynolds-averaged Navier-Stokes (URANS) simulations, where most of the interactional phenomena originating from or affecting the advancing side of the rotor disk are significantly stronger than on the retreating side. In order to account for the large regions of separated flow below the wings, RACER's hover as the second flight condition of interest is analysed by means of delayed detached-eddy simulations (DDES). It is not only that second-order effects generally play a more important role for this flight state, but that asymmetries in aerodynamic interactions are not linked to the main rotor and its thrust distribution but rather to the different operating conditions of the lateral rotors where the right-hand rotor generates reverse thrust to provide anti-torque.
Open Access
Aeroelastic analysis of wind turbines under turbulent inflow conditions
(2021) Guma, Giorgia; Bangga, Galih; Lutz, Thorsten; Krämer, Ewald
The aeroelastic response of a 2 MW NM80 turbine with a rotor diameter of 80 m and interaction phenomena are investigated by the use of a high-fidelity model. A time-accurate unsteady fluid–structure interaction (FSI) coupling is used between a computational fluid dynamics (CFD) code for the aerodynamic response and a multi-body simulation (MBS) code for the structural response. Different CFD models of the same turbine with increasing complexity and technical details are coupled to the same MBS model in order to identify the impact of the different modeling approaches. The influence of the blade and tower flexibility and of the inflow turbulence is analyzed starting from a specific case of the DANAERO experiment, where a comparison with experimental data is given. A wider range of uniform inflow velocities are investigated by the use of a blade element momentum (BEM) aerodynamic model. Lastly a fatigue analysis is performed from load signals in order to identify the most damaging load cycles and the fatigue ratio between the different models, showing that a highly turbulent inflow has a larger impact than flexibility, when low inflow velocities are considered. The results without the injection of turbulence are also discussed and compared to the ones provided by the BEM code AeroDyn.
Open Access
Assessment of low‐frequency aeroacoustic emissions of a wind turbine under rapidly changing wind conditions based on an aero‐servo‐elastic CFD simulation
(2023) Wenz, Florian; Maas, Oliver; Arnold, Matthias; Lutz, Thorsten; Krämer, Ewald
A meteorologically challenging situation that represents a demanding control task (rotational speed, pitch and yaw) for a wind turbine is presented and its implementation in a simulation is described. A high-fidelity numerical process chain, consisting of the computational fluid dynamics (CFD) solver FLOWer, the multi-body system (MBS) software SIMPACK and the Ffowcs Williams-Hawkings code ACCO, is used. With it, the aerodynamic, servoelastic and aeroacoustic (<20 Hz) behaviour of a generic wind turbine during a meteorological event with strong and rapid changes in wind speed and direction is investigated. A precursor simulation with the meteorological model system PALM is deployed to generate realistic inflow data. The simulated strong controller response of the wind turbine and the resulting aeroelastic behaviour are analysed. Finally, the low-frequency sound emissions are evaluated and the influence of the different operating and flow parameters during the variable inflow is assessed. It is observed that the wind speed and, linked to it, the rotational speed as well as the turbulence intensity are the main influencing factors for the emitted low-frequency sound power of the wind turbine. Yawed inflow, on the other hand, has little effect unless it changes the operational mode to load reduction, resulting in a swap of the main emitter from the blades to the tower.
Open Access
Auslegung und Charakterisierung eines dreidimensionalen Scramjet-Einlaufs mit hohem Verdichtungsverhältnis und variabler Innenkontraktion
(2014) Hohn, Oliver; Krämer, Ewald (Prof. Dr.-Ing.)
Diese Arbeit als Bestandteil des Graduiertenkollegs GRK-1095 „Aero-thermodynamische Auslegung eines Scramjet-Antriebssystems für zukünftige Raumtransportsysteme“ der Deutschen Forschungsgemeinschaft (DFG) befasst sich mit Untersuchungen an Scramjet-Einläufen mit hohen Verdichtungsverhältnissen, wobei der Schwerpunkt darin lag, den Übergang von vormals vorherrschenden zweidimensionalen zu dreidimensionalen Geometrien zu erreichen. Dazu wurde zunächst der bestehende zweidimensionale Doppelrampen-Einlauf GK-01 der ersten Leitkonfiguration des Graduiertenkollegs modifiziert, um mit diesem Erkenntnisse über Aspekte zu erlangen, auf die bei der Auslegung einer neuen, vollständig drei-dimensionalen Einlaufgeometrie besonderes Augenmerk gelegt werden muss. Dies betraf insbesondere die aerodynamischen und aerothermodynamischen Auswirkungen durch zusätzliche Seitenwandkompression sowie Veränderungen beim Innenkontraktionsverhältnis des Einlaufs. Basierend auf diesen Erkenntnissen wurde mittels einer CFD-Parameterstudie die Einlaufgeometrie der neuen dreidimensionalen Gesamtkonfiguration des Graduiertenkollegs festgelegt. Das Betriebsverhalten des auf Basis dieser Parameterstudie entwickelten und gefertigten Einlaufmodells GK-3D wurde anschließend im Hyperschallwindkanal H2K der Abteilung Überschall- und Hyperschalltechnologien des DLR in Köln eingehend an unterschiedlichen Betriebspunkten und bei verschiedenen Bedingungen experimentell untersucht. Die Einlaufströmung wurde in den Windkanalversuchen mit Strömungsvisualisierung durch Schattenaufnahmen und Wand- und Pitotdruckmessungen erfasst. Für das neu ausgelegte GK-3D-Modell wurde ein Druckmessrechen entwickelt, der neben Pitotröhrchen auch über statische Drucksonden verfügt. Zudem wurde der Massenstrom mit einer Drossel gemessen, mit der auch der Brennkammergegendruck variiert wurde, um die Grenzen des Betriebsbereichs des Einlaufs zu ermitteln. Die Wärmelasten auf den externen Verdichtungsrampen wurden mittels Infrarot-Thermografie bestimmt. Die zusätzliche Seitenwandkompression im modifizierten 2D-Einlauf verursachte starke Änderungen der externen Strömung, die das Startverhalten des Einlaufs negativ beeinflussten. Dadurch war eine optimale Anpassung der Lippenposition an die veränderte Strömungsstruktur nicht möglich, so dass mit dieser Art der Seitenwandkompression, im Gegensatz zu zusätzlichen seitlichen Kompressionskeilen im internen Strömungskanal, aufgrund des größeren Spillage-Massenstroms keine bedeutend höhere Verdichtung erzielt werden konnte. Die Eckenwirbel waren bei zusätzlicher Seitenwandkompression deutlich stärker und wurden durch die Interaktion mit dem zweiten Rampenstoß nochmals enorm verstärkt, wodurch die Grenzschichten in weiten Bereichen der externen Rampen ablösten. Im 2D-Fall und bei interner Seitenwandkompression war es möglich, die Innenkontraktion deutlich zu erhöhen und durch den zusätzlich eingefangenen Massenstrom die Effizienz zu steigern. Zur Untersuchung des Betriebsverhaltens des neuen 3D-Einlaufs wurden das Startverhalten, der Einfluss der Innenkontraktion, der Reynoldszahl und die Änderung der Flugbahnwinkel betrachtet. Das Startverhalten stimmte dabei gut mit Erfahrungswerten von anderen 3D-Einläufen überein. Die Innenkontraktion hatte keine entscheidenden Auswirkungen auf das Strömungsfeld und das Leistungsvermögen des Einlaufs. Die Grenzen des Betriebsbereichs verschoben sich mit steigender Innenkontraktion jedoch zu höheren Druckverhältnissen. Die Stoß-Grenzschicht-Interaktion des Rampenstoßes mit der Haubenoberfläche, die bei Fällen mit hoher Innenverdichtung auftritt, erwies sich nicht als problematisch. Dies war auch bereits bei den Voruntersuchungen mit dem modifizierten 2D-Einlauf der Fall. Größere Auswirkungen ergaben sich durch Flugbahnwinkel, welche die effektiven Kompressionswinkel der Rampe bzw. der Seitenwände und den Fangquerschnitt verändern und damit auch die Druck- und Massenstromverhältnisse. Hinsichtlich eines sicheren Betriebs sind vor allem hohe positive Anstellwinkel als kritisch einzustufen, da die Druck- und Massenstromverhältnisse derart stark absinken, dass die Zündung und Stabilität der Verbrennung eventuell nicht mehr gewährleistet sind. Die Variation der Reynoldszahl lieferte nur kleine Einflüsse auf das Leistungsvermögen und die Effizienz des Einlaufs im ungedrosselten Betriebsfall. Die Betriebsgrenzen lagen bei hoher Reynoldszahl jedoch deutlich niedriger, das Blockieren der Einlaufströmung passierte viel schneller (d.h. bei niedrigeren Brennkammergegendrücken). Mit IR-Thermografie konnten Erkenntnisse über die Höhe der maximalen auftretenden Wärmelasten und die Orte, an denen sie auftreten, erlangt werden. Diese und besonders auch deren Position änderten sich stark bei Variation der Flugbedingung, vor allem durch Flugbahnwinkel. Es konnten daraus jedoch keine genaueren Einblicke in die Strömungsstruktur auf den externen Rampen, vor allem das Transitionsverhalten, gewonnen werden. Zudem wurden alternative, analytische Auslegeverfahren basierend auf Streamline-Tracing betrachtet, um die hier angewandte und mit hohem Arbeitsaufwand verbundene Auslegungsstrategie einer Parameterstudie effizienter zu gestalten. Dazu wurden ein REST-Einlauf und ein Einlauf auf Basis einer Busemannströmung entwickelt, denen jeweils die gleichen Flächenverhältnisse wie beim GK-3D-Einlauf zugrunde liegen. Das Auslegungsverfahren für REST-Einläufe erwies sich dabei als nur sehr eingeschränkt tauglich für die vorliegenden hohen Verdichtungsverhältnisse, da die so erlangten Einlaufgeometrien extrem lang werden und somit ungeeignet für das hypothetische Flugexperiment des GRK-Teilprojekts C1 wären. Basierend auf Busemann-Strömungen war es möglich, Einlaufgeometrien zu erlangen, die zwar immer noch länger als der GK-3D-Einlauf waren, aber deutlich kompakter gestaltet werden konnten als die REST-Einläufe. Mit diesen Einläufen konnten zudem bedeutend höhere Wirkungsgrade erzielt werden als mit den anderen Konfigurationen. Jedoch lag deren Leistungsvermögen in Form der Verdichtungs- und Temperaturverhältnisse geringfügig niedriger als das des 3D-Einlaufs.
Open Access
Behandlung von Strömungsproblemen in Raketendüsen bei Überexpansion
(2001) Frey, Manuel; Wagner, Siegfried (Prof. Dr.-Ing.)
Die vorliegende Arbeit untersucht die strömungsmechanischen Ursachen der bei Strömungsablösung in Raketendüsen beobachteten Seitenkräfte und gibt Hinweise zu ihrer Quantifizierung. Zunächst werden die im Abgasstrahl von überexpandierten Düsenströmungen auftretenden Stoßmuster analysiert. In gekürzten idealen Düsen tritt wie erwartet nur die reguläre oder Machreflexion des Überexpansionsstoßes an der Düsenlängsachse auf. Im Gegensatz dazu kann sich im Abgasstrahl einer schuboptimierten Düse ein drittes, bisher unbekanntes und nun als 'Kappenmuster' bezeichnetes Stoßmuster einstellen, welches sich als inverse Machreflexion des in schuboptimierten Düsen erzeugten inneren Stoßes deuten läßt. Für den Fall der Freistrahlablösung wird ein Modell zur Bestimmung des Ablösepunktes vorgeschlagen. Es wird gezeigt, daß die weniger erforschte Ablösung mit Wiederanlegen anders als bisher angenommen durch die Konturgebung der Düse bedingt ist. Sie tritt nur in schuboptimierten Düsen bei Existenz eines Kappenmusters auf, weil dieses die abgelöste Strömung in Richtung Düsenwand umlenkt und so zum Wiederanlegen führt. Videoanalysen und numerische Simulationen weisen die Ablösung mit Wiederanlegen erstmals auch in Großtriebwerken nach. Beim Anfahren und Abschalten von schuboptimierten Düsen kann es zum Umschlag zwischen den beiden Ablösetypen kommen. Eigens durchgeführte Modellversuche zeigen, daß die mit Abstand größten Seitenkräfte im diesem Augenblick auftreten, weil der Umschlag asymmetrisch erfolgt. Seitenkraftmessungen aus Triebwerksversuchen belegen dies auch für Großtriebwerke. Folglich können Seitenkräfte durch die Verwendung einer gekürzten idealen anstatt einer schuboptimierten Kontur entscheidend verringert werden, weil dadurch das Wiederanlegen der abgelösten Strömung vermieden wird. In den Modellversuchen erzeugt dementsprechend eine schuboptimierte Düse fast dreimal so hohe Seitenkräfte wie eine vergleichbare gekürzte ideale Düse.
Open Access
Berechnung und Verminderung von Strömungsgeräuschen an Profilen
(2004) Guidati, Gianfranco; Wagner, Siegfried (Prof. Dr.-Ing.)
Die Arbeit behandelt die Berechnung und Verminderung von Strömungsgeräuschen an Tragflügelprofilen. Dabei werden zwei Mechanismen der Lärmentstehung betrachtet. Vorderkantenlärm entsteht aus der Wechselwirkung von Turbulenz in der Zuströmung mit der Vorderkante eines Profils. Hinterkantenlärm entsteht aus der Wechselwirkung von Grenzschichtturbulenz mit der Profilhinterkante. Für beide Mechanismen werden Vorhersageverfahren vorgestellt. Berechnungen und Messungen zeigen, dass Vorderkantenlärm stark von der Form der Profilnase abhängt. Dabei erzeugen stumpfe Profile bis zu 10 dB weniger Lärm als solche mit einer spitzen Vorderkante. Für den Hinterkantenlärm wurde ein Vorhersageverfahren mit einem Profilentwurfsprogramm und einem automatischen Optimierungsalgorithmus gekoppelt. Berechnungen zeigen, dass Lärmreduktionen bis zu 4 dB durch einen gekoppelten aerodynamischen / aeroakustischen Profilentwurf erreicht werden können.
Open Access
CFD simulation of Jet and Vortex Actuators (JaVA) with and without cross flow boundary layer
(2010) Rashad, Muhammad Aqeel; Rist, Ulrich (Apl. Prof. Dr.-Ing.)
In the present study an active flow control actuator is studied numerically. This type of actuator was first studied experimentally by Lachowicz et al. and called “Jet and Vortex Actuator” (JaVA). This kind of active flow control actuator produces different flow fields depending upon the frequency and the scaled amplitude of the imposed oscillation. Thus, it can be used to produce different net reactions, like vertical jets, wall jets, or a vortex flow. The actuator under consideration consists of a cavity and a rigid plate which serves as the actuation surface. The actuator plate acts like a piston pumping air out of the cavity on the down stroke and sucking air into the cavity on the upstroke. Different cases are selected for validation of the simulations. Our simulations yield the unsteady flow field, whereas only time-averaged data are available from literature. Thus, our simulations provide extra details of the flows through the gaps intended for a better understanding of the actuator flow. Qualitative and quantitative comparisons of the time-averaged data with the experiments are very encouraging. Especially, the different flow regimes appear for the same parameters as in the experiments. Different parameters like cavity depth and plate position relative to the cavity upper wall and compressibility effects are also studied. In the present work we also presented a modified design in order to improve and understand flow mechanisms. In this new design the horizontal plate is stationary and a vertical plate inside the cavity moves left and right to push fluid through the narrow and wide gap alternatively. Interestingly, in this new design like in the original, once again we get different flow modes like vertical jet, angled jet, vortex and wall jet but from the narrow gap. Different parameters like cavity depth and gap widths and plate position relative to the cavity upper wall are also studied. Relations between different nondimensional parameters like Reynolds number, Stokes number, Strouhal number and scaled amplitude and their effect upon the flow field are also presented in detail. In the present work 3D results are also presented for the modified design. It was found that strong 3D end effects are present. The flow field is completely different for 2D and 3D simulations. For the same set of parameters in 2D we get a very nice vortex as where in 3D we get a vertical jet. We also presented results with a cross-flow boundary layer for 2D cases of original and modified design with different orientations of gaps with respect to the cross-flow. It was found that configuration-2 in the original design in which the narrow gap comes first to the cross flow and the wide gap later, is more effective with respect to increasing the flow momentum close to the wall and in controlling the boundary layer. For the 3D modified design with the cross flow boundary layer two different gap orientations with respect to the oncoming flow are used. In the first configuration the gaps are oriented across the flow with the wide gap coming first and the narrow later. In the second configuration the gaps are oriented along the boundary layer. In the 3D case this second configuration turns out to be the most effective with respect to increasing the flow momentum close to the wall, such that this kind of actuator can be used for boundary layer (separation) control. For the present investigations the commercial CFD-software FLUENT is used for flow calculation and visualization. The accompanying grid-generation software GAMBIT is used for geometry specification and grid generation.
Open Access
Characterization of low levels of turbulence generated by grids in the settling chamber of a laminar wind tunnel
(2022) Romblad, Jonas; Greiner, Michael; Guissart, Amandine; Würz, Werner
AbstractWind tunnel investigations of how Natural Laminar Flow (NLF) airfoils respond to atmospheric turbulence require the generation of turbulence, whose relevant characteristics resemble those in the atmosphere. The lower, convective part of the atmospheric boundary layer is characterized by low to medium levels of turbulence. The current study focuses on the small scales of this turbulence. Detailed hot-wire measurements have been performed to characterize the properties of the turbulence generated by grids mounted in the settling chamber of the Laminar Wind Tunnel (LWT). In the test section, the very low base turbulence level of Tuu ≅ 0.02% (10 ≤ f ≤ 5000 Hz) is incrementally increased by the grids up to Tuu ≅ 0.5%. The turbulence spectrum in the u-direction shows the typical suppression of larger scales due to the contraction between grids and test section. Still, the generated turbulence provides a good mapping of the spectrum measured in flight for most of the frequency range 500 ≤ f ≤ 3000 Hz, where Tollmien-Schlichting (TS)-amplification occurs for typical NLF airfoils. The spectra in v and w-direction exhibit distinct inertial subranges with slopes being less steep compared to the - 5/3 slope of the Kolmogorov spectrum. The normalized spectra in u-direction collapse together well for all grids, whereas in v- and w-directions the inertial- and dissipative subranges are more clearly distinguished for the coarser grids. It is demonstrated that the dissipation rate ε is a suitable parameter for comparing the wind tunnel turbulence with the atmospheric turbulence in the frequency range of interest. By employing the grids, turbulence in the range 4.4 × 10-7 ≤ ε ≤ 0.40 m2/s3 at free-stream velocity U∞ = 40 m/s can be generated in the LWT, which covers representative dissipation rates of free flight NLF applications. In the x-direction, the spectra of the v and w-components develop progressively more pronounced inertial- and dissipative subranges, and the energy below f ≈ 400 Hz decreases. In contrast, the spectral energy of the u-component increases across the whole frequency range, when moving downstream. This behavior can be explained by the combination of energy transport along the Kolmogorov cascade and the incipient return to an isotropic state.Graphic Abstract
Open Access
Compressible multi-phase simulation at extreme conditions using a discontinuous Galerkin scheme
(2015) Fechter, Stefan; Munz, Claus-Dieter (Prof. Dr.)
This work provides a contribution to the approximation of compressible multi-phase flows using a high-order discontinuous Galerkin spectral element method. Compressibility effects have to be considered for operating conditions close to the critical point. Important examples for such extreme ambient conditions include fuel injection systems of aeronautical, automotive and rocket engines. The simulation of compressible multi-phase flows at these ambient conditions imposes high demands on the numerical treatment as well as the numerical method. On the one hand, due to the compressible treatment of both fluid phases and their corresponding numerical methods, especially regarding the numerical resolution of the phase interface. On the other hand, the evaluation of equation of states, that are valid in the vicinity of the critical point, is expensive. As additional challenge are hydrodynamics and thermodynamics coupled closely by the compressible flow equations. This implies that a thermodynamically consistent numerical method has to be chosen. The building blocks of the described numerical method for compressible multi-phase flows include a compressible flow solver for the bulk phases, a level-set based interface tracking method, a comprehensive description of the equation of state and a model for the interface approximation. The interaction of these parts within the solution algorithm is described and validated in the thesis.
Open Access
Computational aeroacoustics with a high order discontinuous Galerkin scheme
(2012) Birkefeld, Andreas; Munz, Claus-Dieter (Prof. Dr. rer. nat. habil.)
The high order discontinuous Galerkin solver NoisSol for the linearized acoustic equations and its application to airfoil noise simulation are presented. Aiming at the fast simulation of the noise generation and propagation in domains with complex geometries, the discretization based on unstructured grids is seen as the favorable strategy. Further important requirements for an aeroacoustic solver are low dissipation and dispersion errors to enable the propagation of waves over a long distance to the far field. Discontinuous Galerkin schemes outrange finite volume schemes in these properties and are consequently the optimum choice for computational aeroacoustics (CAA) on unstructured grids. They furthermore convince with their low demands in grid quality, which avoids mesh postprocessing and optimization and eases the tool chain. For the simulation of aeroacoustics in flows with a low Mach number, the separation of flow and acoustic simulation is favorable, since both have to deal with different space and energy scales. For the acoustic calculation linearized equations can be used. They have transient source terms, which describe the excitation of the sound by flow phenomena. These sources as well as the linearization state of the equations depend on the local flow state. The linearization is done around the time averaged (’mean’) flow field. The transfer of source and mean flow data needs a coupling between both the flow solver grid and the acoustic grid. Therefor for each node or interpolation point in one grid the corresponding element in the other grid has to be known. Since a brute force approach for this search is infeasible for large scale applications, a new search algorithm has been developed. In the presented airfoil noise simulation it has been applied to the search of the corresponding CFD cells for the mean flow values, where it showed impressive results. It is applicable for any grid in 2D and 3D with elements of a standard type, such as triangles, quadrilaterals, tetrahedrons or hexahedrons. A hybrid grid coupling has been developed and implemented with the DLR code PIANO to combine the advantages of the presented DG solver with the undoubted advantages of a finite difference (FD) solver in the obstacle free far field, which include a straightforward mesh generation for rectangular or cuboidal domains and a low memory demand. This work is based on ideas of Schwartzkopff and Utzmann and transfers them to the field of hybrid aeroacoustics. Furthermore, it has focused on industrial applications. Hence the number of schemes, codes and equations involved has been kept very low and the initialization process has been automated as far as possible. This reduces the number of parameters that have to be adjusted by the user and the necessary knowledge of the solvers’ interior. Both the pure DG scheme and the coupled schemes have proven their operability and their ability to maintain the design order of convergence for a planar wave test case. A slat noise propagation simulation has been chosen to show the capabilities of the new framework. It deals with a NASA 30P30N three part airfoil with extended high lift devices in a low Mach flow. The underlying flow and source calculations have been done at the German Aerospace Center. The acoustic simulations with NoisSol and with the coupled framework have shown a very good agreement, both in the pressure field as well as in the frequency spectrum. For the pure DG calculation with its longer simulation time the spectrum has also been compared to CAA results of Dierke et al., which are based on the same source data. The spectra show a very good agreement. The examination of the simulation times reveal a first idea of the performance behavior of the coupled codes. Some further tests will allow a general conclusion in terms of a best practice guide.
Open Access
Computational studies of massively separated wake flows of transport aircraft
(2021) Waldmann, Andreas; Krämer, Ewald (Prof. Dr.)
This work focuses on the investigation of flow phenomena associated with low speed stall using a representative commercial transport aircraft configuration. Subsonic stall at high Reynolds number involves a highly complex turbulent flow field, which is difficult to analyze in ist entirety via experimental methods. Various computational approaches based on URANS and hybrid RANS/LES were evaluated, utilizing validation data from the European Transonic Windtunnel. Scale-resolving computational approaches were leveraged to gain deeper insight into the processes occurring in such a wake. DDES-based methods were found to be able to resolve the flow features occurring at the separation location and in the wake. An extensive study on the impact of solver settings, computational grids, model geometry and inflow Reynolds number was carried out in order to permit a validation of the chosen approach. Using these findings, the massively separated wake flow was studied at three angles of attack in post stall conditions. Three different regimes of formation of the separated wake were identified via the main locations where turbulence kinetic energy is produced. Analysis of anisotropy, turbulence length scales and signal characteristics provided insight into the propagation of the wake and the mixing processes. Modal analysis of the wake dynamics enabled the detection of a near-wing recirculation area and a von Kármán vortex street in the wake. Flow structures associated with both phenomena result in tailplane load fluctuations at their respective characteristic frequencies.
Open Access
Control of the secondary crossflow instability : direct numerical simulation of localized suction in three-dimensional boundary layers
(2013) Friederich, Tillmann; Rist, Ulrich (Prof. Dr.-Ing.)
Transition control by localized “pinpoint” suction in a three-dimensional boundary-layer flow with crossflow is investigated by means of direct numerical simulation. The control of large-amplitude steady crossflow vortices with active secondary instability constitutes hereby an alternative promising possibility to maintain laminar flow on relevant regions of airliner wings (active laminar flow control) resulting in a significant reduction of drag and thus also of greenhouse gas emissions. Up to date, laminar flow control applied to tackle crossflow instability aims at a reduction of the primary crossflow instability, i.e. hindering the development of large-amplitude, secondarily unstable crossflow vortices. The classically applied homogeneous suction focuses on reducing the crossflow in the quasi two-dimensional base flow which results in hindered growth of crossflow vortices, later-induced secondary instability and hence delayed laminarturbulent transition. On the other hand, techniques like the “distributed roughness elements” method or “distributed flow deformation” excite locally stable or weakly unstable crossflow vortex modes, leading to “benign” crossflow vortices that are spaced narrower than the naturally amplified ones while suppressing all other modes, including the most unstable ones. In the current work, the three-dimensional nonlinear disturbance state with large-amplitude steady crossflow vortices including already active secondary instability is controlled by means of pinpoint hole suction. The influence of hole-suction modeling on the effects of pinpoint suction is checked in the first part of the work where results from a numerically extensive compressible simulation are discussed that comprises the channel flow below the suction orifice. Although the actual wall-normal velocity distribution in the suction hole deviates from the modeled prescribed distribution, it is shown that at equal mass flux the effects on the crossflow vortex are virtually identical and secondary instability is equally attenuated. In the second part of the work, localized pinpoint suction through holes is activated below the updraft side of the primary vortices. It is shown that the overall vortical motion is reduced by the imposed negative wall-normal velocity component while simultaneously the growth of unstable high-frequency secondary instability modes is attenuated that are located in a high-shear layer above the suction holes. Thereby, the associated flow field is stabilized and laminar-turbulent transition is significantly delayed or suppressed. A case with homogeneous suction at equal suction rate is shown to be virtually ineffective at this transition stage, while cases with slit suction, where the slits extend in spanwise direction, perform still well; however, the attenuation found in cases with concentrated hole suction is not reached. In all considered setups the suction-induced increased wall shear is by far overcompensated by the much stronger decrease due to the transition delay resulting in an overall effective drag reduction.