06 Fakultät Luft- und Raumfahrttechnik und Geodäsie
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/7
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Item Open Access Aeroacoustic simulation of turbulent boundary layer induced automotive gap noise(2021) Erbig, Lars; Munz, Claus-Dieter (Prof. Dr. rer. nat.)Item Open Access High order large eddy simulation for the analysis of tonal noise generation via aeroacoustic feedback effects at a side mirror(2017) Frank, Hannes; Munz, Claus-Dieter (Prof. Dr.)In this work, the flow around a side mirror and the resulting tonal noise generation are investigated using highly accurate compressible large eddy simulations. Avoiding tonal noise, which can be perceived as disturbing whistling sound, is a crucial target in automotive aeroacoustics. However, the underlying mechanisms are not completely understood and can typically not be captured with state of the art computational aeroacoustics solvers used in industry. Acoustic feedback effects known from tonal airfoil self-noise are a possible cause at smooth mirror housings that exhibit laminar separation upstream of the trailing edge. Since this application demands high accuracy, a simulation code based on the high order discontinuous Galerkin spectral element method is employed. To enhance geometrical flexibility, it is augmented with an extension to non-conforming curved elements in three dimensions. In the first part of the investigation, the simulation framework is used to analyze an early development stage side mirror exhibiting tonal noise generation. Adopting the corresponding experimental configuration, the study considers an isolated side mirror mounted on the wind tunnel floor. The computational flow field is shown to agree remarkably well with the experimental one based on comparisons with static wall pressure, hotwire and PIV measurements. Discrete peaks are obtained in the computational acoustic spectrum, originating at the trailing edge of the mirror downstream of laminar separation. The identified tonal noise source regions match the experimental ones and quantitative agreement is achieved for one of the tonal peak frequencies. Perturbation simulations reveal global acoustic feedback instabilities selecting the same discrete frequencies observed in the developed flow. The feedback loop comprises convective disturbance growth in the separated shear layer, scattering at the trailing edge and reinforcement through receptivity to the emitted sound in the upstream boundary layer. In a second step, this mechanism is studied in more detail based on a specifically designed simplified two-dimensional model. A subdomain approach is introduced to exploit the two-dimensional shape and circumvent the computational cost associated with the bluff body wake of the model. Simulations of a range of free-stream velocities exhibit tonal frequencies varying similarly to the experimentally observed so-called 'ladder structure'. The tone frequencies are shown to evolve according to a theoretical feedback model based on linear stability theory. Finally, the efficacy of various modifications to the mirror contour to eliminate tonal noise generation is evaluated. The present work contributes to the understanding of tonal noise generation mechanisms and can guide future designs. Moreover, it corroborates the capacity of the present discontinuous Galerkin framework to accurately capture relevant but delicate aeroacoustic effects at complex geometries.Item Open Access A domain decomposition method for the efficient direct simulation of aeroacoustic problems(2008) Utzmann, Jens; Munz, Claus-Dieter (Prof. Dr.)A novel domain decomposition approach is developed in this thesis, which significantly accelerates the direct simulation of aeroacoustic problems. All relevant scales must be resolved with high accuracy, from the small, noise generating flow features (e.g., vortices) to the sound with small pressure amplitudes and large wavelengths. Furthermore, the acoustic waves must be propagated over great distances and without dissipation and dispersion errors. In order to keep the computational effort within reasonable and feasible limits, the calculation domain is divided into subregions with respect to the local physical requirements. In these domains, the numerical method which is most suitable and optimized for the considered subproblem is employed. The proposed method differs from established approaches, e.g. the grid coupling is not limited to Chimera techniques but presents a consistent way for the space-time coupling of high order methods. Various domain decomposition options are examined and implemented in a common code framework. In the subdomains, the Navier-Stokes, Euler and linearized Euler equations are solved, for which methods from the discontinuous Galerkin (DG), finite volume (FV) and finite difference (FD) class are available with their respective special properties. For example, DG methods are very suitable for highly accurate solutions on unstructured grids due to their locality, while FD methods are very efficient on Cartesian grids for the simulation of linear wave propagation. In turn, FV methods are very robust in the presence of strong gradients, e.g. shocks. All implemented methods have in common, that they are explicit one-step time integration schemes and thus are especially applicable for unsteady calculations. Furthermore, their order of accuracy in space and time may be chosen arbitrarily. A newly developed numerical solver, the STE-FV method on Cartesian grids, closes the gaps in the repertoire of numerical schemes in the coupling framework. It forms a fast high order method that features great robustness also at nonlinearities by employing a WENO algorithm. For validation purposes, convergence studies and benchmark tests, e.g. the popular double Mach reflection in 2D and an explosion in 3D, are performed for the STE-FV method with orders in space and time up to six and beyond. The coupling of different grids is based on high order interpolations and the data exchange over the ghost elements of the calculation domains. The Gauss integration points in the cells are used here in order to find a source domain for the interpolation and for providing high order boundary conditions afterwards. The grids are not required to be matching or overlapping. Furthermore, arbitrary constellations of structured and unstructured grids are possible. The optimal time steps, which can be different of each other, are allowed in the subregions. This is made possible by employing the Cauchy-Kovalevskaja procedure, which delivers a Taylor series that provides boundary information for the intermediate points of time for domains with a smaller time step. The implementation structure inside the code framework is largely modular. The fluid and acoustics solvers can be used as stand-alone codes, and also new ones can be easily added. Furthermore, external programs, which may run on separate computer systems, can be linked to the framework. The distribution to different system architectures is also possible for the internal solvers. Hence, the respective properties of the numerical methods regarding vectorization and parallelization can be exploited in an optimal way. It is shown on the basis of convergence studies for different constellations of grids, equations and methods, that the domain decomposition approach is capable of maintaining high order of accuracy globally. An examination regarding high-frequency perturbations reveals a natural filtering process if perturbations cannot be resolved on a coarse mesh anymore. Hence, a spatial filtering operator is not a necessity. Another study shows, that the magnitude of reflections occurring at the domain boundaries are in good accordance with theoretical estimations. Besides the change from nonlinear to linear equations, also the jump in resolution matters in this context. However, the reflections are negligible in general. The accuracy and efficiency of the proposed domain decomposition method is illustrated for benchmark examples like the acoustic scattering at a sphere or at multiple cylinders and for the Von Karman vortex street. Here, especially the method's potential for efficient far field calculations becomes clear, but also the advantages in the presence of complex geometries are emphasized. Finally, the simulation of a nozzle flow with a supersonic free jet and the associated noise underlines the practical applicability of the domain decomposition approach.Item Open Access Numerical simulation of wake interactions on a tandem wing configuration in high-speed stall conditions(2023) Kleinert, Johannes; Stober, Jonathan; Lutz, ThorstenIn this work, the interaction of the separated wake of the front wing with the rear wing of a tandem configuration is investigated for high-speed stall conditions by means of hybrid RANS/LES simulations, using the zonal AZDES method. After a characterization of the transonic buffet on the front wing, the development of the separated turbulent wake behind the wing is investigated. The interaction of the separated wake with the rear wing is then analyzed in detail. The results reveal that there is a strong variation in the wake characteristics over the buffet cycle, caused by the varying amount of separation on the front wing. During the upstream movement of the shock, the flow is largely separated, resulting in a thick wake with strong, high-frequent fluctuations that can be attributed to large turbulent vortices. On the contrary, when the shock travels downstream, there is only a small amount of separation present, resulting in a thin wake with comparatively low fluctuations that are caused by corresponding smaller turbulent vortices. The impact of the wake of the front wing causes a strong variation in the rear wing loading. An oscillation with a comparatively low frequency can be distinguished from high-frequent fluctuations. The low-frequent oscillation is caused by the variation in the downwash behind the front wing as its lift changes during the buffet cycle. The high-frequent fluctuations are due to the impingement of the turbulent structures onto the rear wing. Because both size and frequency of those vortices vary significantly within the buffet cycle, the amplitude and frequency of the lift and surface pressure fluctuations also change accordingly.Item Open Access Untersuchungen zum externen und internen Strömungsfeld eines Scramjet Triebwerkseinlaufs bei unterschiedlichen Betriebspunkten(2009) Häberle, Jürgen; Krämer, Ewald (Prof. Dr.-Ing.)Die vorliegende Arbeit entstand im Rahmen des Graduiertenkollegs 1095/1 „Aerothermodynamische Auslegung eines Scramjet Antriebssystems für zukünftige Raumtransportsysteme“ und befasst sich mit der experimentellen Untersuchung des Strömungsfelds eines Scramjet-Einlaufsystems unter aerodynamischen und aerothermodynamischen Gesichtspunkten. Wesentliche Ziele der Arbeit bestanden darin, die Stoß-Grenzschicht-Interaktion im Halsbereich zu untersuchen und ein umfassendes Verständnis der aerodynamischen und aerothermodynamischen Vorgänge im externen und insbesondere im internen Strömungsfeld eines Scramjet-Einlaufs zu erlangen. Hierzu wurden zwei unterschiedliche Einläufe bei Ma = 6 und Ma = 7 untersucht. Um die auftretenden Strömungstopologien untersuchen zu können, wurden diverse Messtechniken eingesetzt. Hierzu zählte die Messung des statischen Druckes an der Ober- und Unterseite entlang der Mittellinie der Einläufe und die Messung des Pitotdruckes in der Brennkammereintrittsebene. Zusätzlich wurde mit Hilfe einer Drossel der vom Einlauf gefangene Massenstrom bestimmt sowie der simulierte Brennkammergegendruck variiert. Die Innenströmung wurde optisch mit Hilfe von Schattenaufnahmen visualisiert. Mit Hilfe der Infrarot-Thermographie wurde die zeitliche Entwicklung der Oberflächentemperatur der Isolatorseitenwand und der externen Rampen gemessen, im Rahmen der Datenauswertung wurde hieraus die Wärmestromdichte berechnet und anschließend die dimensionslose Stantonzahl dargestellt. Im Rahmen dieser Arbeit wurde erstmals am H2K die IR-Thermographie auf interne Strömungsfelder angewendet und ausgewertet. Die Veränderung der Strömungstopologie mit einhergehender Variation der aerothermodynamischen Belastung konnte für verschiedene Betriebszustände und Konfigurationen anhand der 2D-Verteilung der Stantonzahl entlang der Isolatorseitenwand dargestellt und diskutiert werden. Zur Untersuchung der Stoß-Grenzschicht-Interaktion im Halsbereich waren beide Modelle mit einer optionalen passiven Absaugung im Halsbereich ausgestattet. Hierdurch war es möglich, die Veränderung der Strömungstopologie bei starker Stoß-Grenzschicht-Interaktion mit dem Fall geringer (keiner) Stoß-Grenzschicht-Interaktion zu vergleichen. Um das Betriebsverhalten der Einlaufsysteme zu untersuchen und insbesondere die sich verändernden Strömungsbedingungen am Brennkammereintritt zu quantifizieren, wurde mit Hilfe der Drossel das Gegendruckverhältnis variiert. Die Erhöhung des statischen Druckes am Ende des Isolators führte zu einem Druckanpassungsgebiet im Isolator. Im Bereich dieses Druckanpassungsgebietes kam es zur Aufdickung bzw. im Extremfall zur Ablösung der Grenzschicht und damit zur Bildung komplexer Stoßstrukturen und der Ausbildung eines inhomogenen Strömungsfeldes in der Brennkammereintrittsebene. Das Verhalten des Scramjet-Einlaufs bei unterschiedlichen Anstellwinkeln wurde speziell unter dem Gesichtspunkt der Variation des gefangenen Massenstroms und der Veränderung der Druckverteilung untersucht. Zusätzlich zu den 2D-Einlaufuntersuchungen wurden für den neu ausgelegten Einlauf Untersuchungen bei erhöhter Innenkompression durchgeführt. Hierbei wurde die Innenkompression in zwei Schritten bis über das Startkriterium nach Kantrowitz hinaus erhöht. Die Erhöhung der Innenkompression erfolgte durch eine Verringerung der Breite des internen Strömungskanals. Hierbei wurde ein ausgeprägter dreidimensionaler Charakter der Strömung beobachtet. Die Veränderung des Strömungsfeldes im Vergleich zum 2D-Einlauf wurde anhand der statischen und Pitotdruckmessungen aufgezeigt. Abschließend wurde in dieser Arbeit das Auswerteverfahren zur Bestimmung der Wärmestromdichte und der daraus berechneten dimensionslosen Stantonzahl erweitert. Zu diesem Zweck wurde exemplarisch für die Basiskonfiguration des GK-Einlaufs die gemessene Temperaturverteilung mit Hilfe eines FEM ANSYS Modells ausgewertet. Dies ermöglichte die dreidimensionale Auswertung der gemessenen zeitabhängigen Oberflächentemperatur. Die Unterschiede zum bisherigen 1D-Auswerteverfahren konnten dargestellt und ausführlich diskutiert werden.Item Open Access Laminar-to-turbulent transition in airfoil boundary layer flows at oscillating inflow conditions(2023) Ohno, Duncan; Rist, Ulrich (apl. Prof. Dr.-Ing.)This thesis numerically investigates the laminar-to-turbulent transition in boundary-layer flows on natural laminar flow airfoils under oscillating inflow conditions. Large-scale fluctuations in the form of periodic vertical gusts generate an oscillating pressure gradient, resulting in a complex transient behavior of the boundary layer. Under these conditions, two scenarios are investigated: an attached flow with natural Tollmien-Schlichting (TS) wave transition and a boundary-layer flow featuring a laminar separation bubble (LSB). The study aims to provide a deeper understanding of the transient mechanisms involved as well as the basis for new transition prediction methods for unsteady conditions. Direct numerical simulations (DNS) are performed where the gust disturbance is imposed on the fully-resolved transitional boundary layers via unsteady boundary conditions. In this novel approach, transient base flows are generated in advance with unsteady Reynolds-averaged Navier-Stokes (URANS) simulations of entire unsteady airfoil flows in conjunction with the disturbance velocity approach (DVA) to introduce sinusoidal gusts. The spatio-temporal evolution of the modal disturbances is analyzed using the continuous wavelet transform (CWT), which is then compared with linear stability theory (LST) by employing a trajectory-following method for transient flows. Several physical effects responsible for the transient characteristics of the studied flows are identified and observations from previous experimental studies are classified. The results demonstrate that the quasi-steady linear theory adequately predicts the transient behavior of the convective modal disturbances for a wide range of gust perturbations. The so-called convective-transition mode with a subsequent calmed region is found for cases with a high degree of unsteadiness. This study is the first to provide a physical explanation for the occurrence of this mechanism for natural transition.Item Open Access Piloted simulation of the rotorcraft wind turbine wake interaction during hover and transit flights(2022) Štrbac, Alexander; Greiwe, Daniel Heinrich; Hoffmann, Frauke; Cormier, Marion; Lutz, ThorstenHelicopters are used for offshore wind farms for maintenance and support flights. The number of helicopter operations is increasing with the expansion of offshore wind energy, which stresses the point that the current German regulations have not yet been validated through scientific analysis. A collaborative research project between DLR, the Technical University of Munich, the University of Stuttgart and the University of Tübingen has been conducted to examine the sizes of the flight corridors on offshore wind farms and the lateral safety clearance for helicopter hoist operations at offshore wind turbines. This paper details the results of piloted helicopter simulations in a realistic offshore wind farm scenario. The far-wake of rotating wind turbines and the near-wake of non-rotating wind turbines have been simulated with high-fidelity computational fluid dynamics under realistic turbulent inflow conditions. The resulting flow fields have been processed by superposition during piloted simulations in the research flight simulator AVES to examine the flight corridors in transit flights and the lateral safety clearance in hovering flights. The results suggest a sufficient size for the flight corridor and sufficient lateral safety clearance at the offshore wind turbines in the considered scenarios.Item Open Access Stability of a laminar streaky boundary-layer behind a roughness element(2015) Shin, Yong-su; Rist, Ulrich (apl. Prof. Dr.-Ing.)Analysis of flow instability is of importance to understand laminar-turbulent transition which is a crucial factor for aerodynamic performance. The present study deals with influences of a roughness element on the flow instability of a laminar boundary-layer. Roughness elements in laminar boundary-layers generate localized disturbances. They grow transiently and formulate streamwise elongated streaky structures downstream. Spanwise periodicity of these streaky structures disturbs the streamwise development of two-dimensional Tollmien-Schlichting waves in a laminar boundary-layer. In this way, a delay of the laminar-turbulent transition is achieved (flow stabilization). On the other side, physically unavoidable velocity reduction behind the roughness elements brings on high shear layers in wall-normal direction at the same time. Also, separations or strong vortical structures occur occasionally depending on both shape of the roughness elements and flow conditions. They cause a flow destabilization and sometimes trigger a bypass transition. Because these two opposite phenomena happen concurrently and interact with each other, it is difficult to precisely understand the instability mechanisms provoked by the roughness elements. Therefore, the goal of the present work is to study the stability of a laminar streaky layer induced by a roughness element. This work consists of two parts: Bi-global linear stability analysis and experimental measurements. A complex instability procedure of the three-dimensional streaky layers arranged parallel in streamwise and periodical in spanwise direction can be analysed by a bi-global approach. Corroborating experiments were conducted in the laminar water channel at the University of Stuttgart. Simultaneous operation of two hot-film probes and signal processing enabled to find the theoretically calculated unstable eigenmodes in practical flow. In addition, observations of both velocity distribution in a complex flow field and nonlinear vortex structures were carried out by a flow visualization using a hydrogen-bubble method and Particle-Image-Velocimetry measurements. As a result, a streaky layer which includes streamwise elongated high- and low-speed streaks and a separation behind the roughness element was found by a CFD computation using a laminar solver and confirmed by time-averaged experimental velocity components. The bi-global LST identified two highly unstable eigenmodes. These eigenmodes oscillate symmetrically and asymmetrically with respect to the spanwise coordinates and were accordingly termed varicose and sinuous mode, respectively. Their streamwise evolution depends mainly on a streamwise development of the streaks. Experimental results confirmed the presence of these two unstable modal modes. The varicose mode dominants flow instability, and the sinuous mode has a smaller signal-to-noise ratio. Additionally, an external forcing was tried to increase the initial amplitude of the smaller sinuous mode with respect to the varicose one. Despite some deficiencies of the experimental setup, a separate artificial amplification of a specific eigenmode, i.e. the sinuous mode, was possible. In the latter part of the present study, the nonlinear behaviour of the streaky layer further downstream and the breakdown under an over-critical condition were explored. Because the linear theory cannot calculate nonlinear instability, complex three-dimensional flows and vortical structures were investigated by experimental flow visualization methods, and an evolution from nonlinear streaks to hairpin vortices was detected.Item Open Access Methodik zur näherungsweisen Vorhersage des laminar-turbulenten Umschlags an Hubschrauberrotoren(2016) Heister, Christoph; Krämer, Ewald (Prof. Dr.-Ing.)In der vorliegenden Arbeit wird eine approximative Methodik zur Vorhersage des laminar-turbulenten Grenzschichtumschlags an Hubschrauberrotoren für reibungsbehaftete Strömungssimulation mittels URANS Verfahren entwickelt. Anhand der Methodik wird der Einfluss von laminarer Strömung auf die benötigte Rotorleistung und den generierten Rotorschub untersucht und mit voll-turbulenten Ergebnissen verglichen. Zur Berechnung der laminaren Grenzschicht wird ein Integralverfahren nach Schlichting/Walz in Kombination mit einem Geschwindigkeitsansatz für rotierende Blätter nach Blaser und Velkoff verwendet. Die Vorhersage des laminar-turbulenten Umschlags erfolgt mittels empirischer Kriterien. Aufgrund der komplexen Rotorströmung wird ein breites Spektrum von Grenzschichtinstabilitäten berücksichtigt, darunter Tollmien-Schlichting-Wellen, laminare Ablösung, Querströmung, Bypass-Mechanismen und Anlegelinieninstabilitäten. Die Umschlagsvorhersage berücksichtigt den Turbulenzgrad des simulierten Blattnachlaufes. Eine Detektion des Grenzschichtrandes aus der URANS Lösung ist nicht erforderlich. Die grundlegende Validierung der approximativen Methode erfolgt anhand der Nachrechnung von experimentellen Testfällen mit laminar-turbulentem Grenzschichtumschlag. Dazu zählen ein Laminarprofil, ein oszillierendes Hubschrauberprofil sowie ein schiebendes Flügelsegment. Als Testfall mit Rotationseinfluss werden der Flugversuch eines Hubschraubers im Schwebeflug und die Rotorströmung eines Mach-skalierten Windkanal-Hubschraubermodells im Vorwärtsflug nachgerechnet. Die berechneten laminar-turbulenten Umschlagslagen an den Rotorblättern zeigen generell eine gute Übereinstimmung zu den experimentellen Messungen. Gegenüber der üblichen Annahme einer voll-turbulenten Rotorströmung bewirkt die Berücksichtigung der laminaren Lauflängen eine Verringerung des Rotorleistungsbedarfs von 3- 5%. Der Rotorschub bleibt von der laminaren Strömung praktisch unbeeinflusst.Item Open Access On the investigation of oblique shock‐wave/turbulent boundary‐layer interactions with a high‐order discontinuous Galerkin method(2022) Gao, Min; Kuhn, Thomas; Munz, Claus‐DieterShock-wave/turbulent boundary-layer interactions are still a challenge for numerical simulation. The shock capturing needs dissipation to avoid spurious oscillations while turbulence will be falsified by introducing dissipation. Especially, an accurate prediction of quantities such as the skin-friction coefficient inside the interaction area of shock wave and turbulent flow is a critical point. In this article, we investigate a wall-resolved large eddy simulation of oblique shock-wave/turbulent boundary-layer interactions by a high-order discontinuous Galerkin scheme. The high-order scheme handles the turbulent flow very well. The shock-capturing is confined to the near shock region by switching locally to a finite volume second-order TVD scheme on subcells. This strategy is completed with the application of a shock indicator to a filtered flow field. A global spanwise filter is applied to avoid switching on the shock-capturing procedure in regions of under-resolved turbulent structures. We validate our numerical results first at shock-wave/laminar boundary-layer interaction. The main simulation under consideration is a Mach 2 turbulent boundary-layer with an inlet momentum-thickness Reynolds number of 1628, interacting with an oblique shock that deflects the incoming flow by 8°. We employ a reformulated synthetic eddy method at the inlet to avoid the influence of recycling-based turbulence generating schemes on the low-frequency unsteadiness. The anisotropic linear forcing technique is adopted to further reduce the turbulence recovery length. Through the spectral analysis of wall pressure probes, a typical Strouhal number of around 0.03 is observed. We attribute the discrepancies between an experimental scaling law and our computation to the three-dimensional sidewall effects in the experiment. With the assistance of numerical results from this article and other authors, a new scaling law for the spanwise-periodic computations is suggested to quantify the difference between experimental and computed data.