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 Linear stability investigations of three-dimensional disturbances in the boundary layer over anisotropic compliant walls(2023) Zengl, Marcus; Rist, Ulrich (apl. Prof. Dr.-Ing.)In dieser Arbeit werden dreidimensionale Störungen in der Grenzschicht über anisotropen nachgiebigen Wänden mit linearer Stabilitätstheorie untersucht. Ein oberflächenbasiertes Modell wird verwendet, um die nachgiebige Wand abzubilden. Hierbei wird das anisotrope Wandmodell von Carpenter erweitert, um einen zusätzlichen Schiebewinkel der Wand bezüglich der Strömungsrichtung einzubringen. Basierend auf diesem Wandmodell wird eine Randbedingung für die Lineare Stabilitätstheorie hergeleitet. Aufgrund der Tatsache, dass diese Randbedingung die Orr-Sommerfeld- und Squire-Gleichung koppelt, wurden zwei neuartige Lösungsverfahren, ein Schießverfahren und ein Matrixlöser, für diesen besonderen Umstand entwickelt. Der Schießlöser transformiert das zugrunde gelegte Eigenwertproblem in ein Randwertproblem und verwendet ein klassisches Schießverfahren zur Lösung des Problems. Um das numerisch steife Problem mit seinem parasitärem Fehlerwachstum zu berücksichtigen beinhaltet das Lösungsverfahren eine Gram-Schmid Orthonormierungsroutine. Durch eine neuartige Skalierung der Phase des zu minimierenden Residuums wird das zeitliche und räumliche Modell robust und performant für gegebene Eigenmoden gelöst. Das durch die gekoppelte Orr-Sommerfeld- and Squire-Gleichung entstehende Eigenwertproblem wird auch mit einer Matrix-basierenden Methode gelöst. Das durch die nachgiebige Wand entstehende zeitliche quadratische Eigenwertproblem wird dabei berücksichtigt. Hierbei wird eine pseudospektrale Diskretisierung mit Chebyshev-Kollokation verwendet. Besonders betrachtet wird die Formulierung des diskretisierten Problems auf seine numerischen Fehler. Die numerische Genauigkeit der Lösungsverfahren wird genau überprüft, um die Gitterunabhängigkeit der Ergebnisse sicherzustellen. Um das Potenzial der nachgiebigen Wände zur Verzögerung des laminar-turbulenten Umschlags zu untersuchen, wurde die Vorgehensweise von Carpenter [15] übernommen. Carpenter optimierte die Parameter der nachgiebigen Wand so, dass Tollmien-Schlichting (TS) Moden so weit wie möglich abgeschwächt werden, während Fluid-Struktur (FISI) Moden grenzwertig stabil bleiben. Dieses Vorgehen wurde ausgewählt, weil Fluid-Struktur Moden absolut instabil sein können, was zu sofortigem Strömungsumschlag führen kann. Stabilitätsrechnungen wurden ausgeführt für zwei Sätze von Wandparametern, die Carpenter mit seinem zweidimensionalen Rahmenwerk optimiert hat. Hierbei wurden nicht nur dreidimensionale Störungen betrachtet, sondern es wurde auch der Einfluss des neu eingebrachten Schiebewinkels der nachgiebigen Wand untersucht. Die Ergebnisse wurden bezüglich der zeitlichen Anfachung der TS- und FISI-Moden, und bezüglich des mit N-Faktoren vorhergesagten Umschlagspunkts beurteilt. Es wird gezeigt, dass dreidimensionale Störungen bestimmte N-Faktoren vor ihren zweidimensionalen Pendants erreichen. Die vorhergesagte laminare Länge ist etwas kürzer als mit zweidimensionalen Verfahren vorhergesagt. Es scheint als ob der eingebrachte Schiebewinkel für die untersuchten Parametersätze keinen Vorteil bezüglich Laminarhaltung bringt. Schließlich wurden optimale Störungen berechnet, um das Transiente Energiewachstum für die anisotrope nachgiebige Wand zu untersuchen. Hierbei wurden die Anfangsverteilungen von Eigenmoden so optimiert, dass deren Überlagerung ein maximales Energiewachstum für eine vorgegebene Zeit erfährt. Die Einhüllende dieser optimalen Störungen wird dann für variierende Wellenzahlen in Strömungs- und Spannweitenrichtung, und variierende Wachstumszeit berechnet. Die Ergebnisse zeigen kein durch die nachgiebige Wand hervorgerufenes relevantes transientes Wachstum. Es wird gezeigt, dass der klassische Mechanismus für transientes Wachstum, der bei der steifen Wand dominiert, nicht verändert wird.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 Numerical study on the aerodynamic characteristics of the NACA 0018 airfoil at low Reynolds number for Darrieus wind turbines using the Transition SST model(2021) Rogowski, Krzysztof; Królak, Grzegorz; Bangga, GalihA symmetrical NACA 0018 airfoil is often used in such applications as small-to-medium scale vertical-axis wind turbines and aerial vehicles. A review of the literature indicates a large gap in experimental studies of this airfoil at low and moderate Reynolds numbers in the previous century. This gap has limited the potential development of classical turbulence models, which in this range of Reynolds numbers predict the lift coefficients with insufficiently accurate results in comparison to contemporary experimental studies. Therefore, this paper validates the aerodynamic performance of the NACA 0018 airfoil and the characteristics of the laminar separation bubble formed on its suction side using the standard uncalibrated four-equation Transition SST turbulence model and the unsteady Reynolds-averaged Navier-Stokes (URANS) equations. A numerical study was conducted for the chord Reynolds number of 160,000, angles of attack between 0 and 11 degrees, as well as for the free-stream turbulence intensity of 0.05%. The calculated lift and drag coefficients, aerodynamic derivatives, as well as the location and length of the laminar bubble quite well agree with the results of experimental measurements taken from the literature for validation. A sensitivity study of the numerical model was performed in this paper to examine the effects of the time-step size, geometrical parameters and mesh distribution around the airfoil on the simulation results. The airfoil data sets obtained in this work using the Transition SST and the k-ω SST turbulence models were used in the improved double multiple streamtube (IDMS) to calculate aerodynamic blade loads of a vertical-axis wind turbine. The characteristics of the normal component of the aerodynamic blade load obtained by the Transition SST approach are much better suited to the experimental data compared to the k-ω SST turbulence model.Item Open Access The effects of airfoil thickness on dynamic stall characteristics of high‐solidity vertical axis wind turbines(2021) Bangga, Galih; Hutani, Surya; Heramarwan, HenidyaThe flow physics of high solidity vertical axis wind turbines (VAWTs) is influenced by the dynamic stall effects. The present study is aimed at investigating the effects of airfoil thickness on the unsteady characteristics of high solidity VAWTs. Seven different national advisory committee for aeronautics (NACA) airfoils (0008, 0012, 0018, 0021, 0025, 0030, 0040) are investigated. A high fidelity computational fluid dynamics (CFD) approach is used to examine the load and flow characteristics in detail. Before the study is undertaken, the CFD simulation is validated with experimental data as well as large eddy simulation results with sound agreement. The investigation demonstrates that increasing the airfoil thickness is actually beneficial not only for suppressing the dynamic stall effects but also to improve the performance of high solidity turbines. Interestingly this is accompanied by a slight reduction in thrust component. The strength and radius of the dynamic stall vortex decrease with increasing airfoil thickness. The airfoil thickness strongly influences the pressure distributions during dynamic stall process, which is driven by the suction peak near the leading edge. The knowledge gained might be used by blade engineers for designing future turbines and for improving the accuracy of engineering models.Item Open Access A time-accurate inflow coupling for zonal LES(2023) Blind, Marcel P.; Kleinert, Johannes; Lutz, Thorsten; Beck, AndreaGenerating 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.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 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 High-fidelity simulation of dynamic stall on helicopter rotors(2021) Letzgus, Johannes; Krämer, Ewald (Prof. Dr.-Ing.)High-fidelity CFD simulations of helicopter rotors are carried out to investigate the dynamic stall flow phenomenon. The simulations are based on two experimental test cases, namely a model rotor with high cyclic pitch control operated at DLR Göttingen, and a highly-loaded, high-speed turn flight of the Bluecopter demonstrator. URANS and DDES simulations are carried out using the flow solver FLOWer coupled with CAMRAD II. A validation of the numerical methods is conducted based on the experimental model-rotor case, which shows that the onset of dynamic stall and the associated load overshoots agree well in overall. An unprecedented comparison of instantaneous PIV and CFD results reveals that after stall onset, only the DDES captures the chaotic nature of separated flow and exhibits small-scale vortical structures that correlate nicely with the measurement. However, the DDES suffers from the numerical artifact of modeled-stress depletion leading to grid-induced separation. Therefore, several improvements to the so-called boundary-layer shielding are investigated for both dynamic stall cases and found to eliminate the issue. Also, a shear-layer-adaptive filter width is successfully applied to the LES mode of the DDES that promotes a more realistic development of flow instabilities in separated shear layers. Concerning the turn flight simulation of the Bluecopter, the computed main rotor control angles agree very well with the flight-test measurements. A comparison of the pitch-link loads shows a good correlation regarding the overall trends and a significant improvement over a lower-order analysis. However, the pitch-link-load amplitudes are still underpredicted. Furthermore, the flow field is found to be highly unsteady and complex throughout a large portion of the azimuth, exhibiting strong separation and multiple dynamic stall events that are partly triggered by blade-vortex interaction.Item Open Access DNS of a turbulent boundary layer using inflow conditions derived from 4D-PTV data(2021) Appelbaum, Jason; Ohno, Duncan; Rist, Ulrich; Wenzel, ChristophUnsteady, 3D particle tracking velocimetry (PTV) data are applied as an inlet boundary condition in a direct numerical simulation (DNS). The considered flow case is a zero pressure gradient (ZPG) turbulent boundary layer (TBL) flow over a flat plate. The study investigates the agreement between the experimentally measured flow field and its simulated counterpart with a hybrid 3D inlet region. The DNS field inherits a diminishing contribution from the experimental field within the 3D inlet region, after which it is free to spatially evolve. Since the measurement does not necessarily provide a spectrally complete description of the turbulent field, the spectral recovery of the flow field is analyzed as the TBL evolves. The study summarizes the pre-processing methodology used to bring the experimental data into a form usable by the DNS as well as the numerical method used for simulation. Spectral and mean flow analysis of the DNS results show that turbulent structures with a characteristic length on the order of one average tracer particle nearest neighbor radius r¯NN or greater are well reproduced and stay correlated to the experimental field downstream of the hybrid inlet. For turbulent scales smaller than r¯NN, where experimental data are sparse, a relatively quick redevelopment of previously unresolved turbulent energy is seen. The results of the study indicate applicability of the approach to future DNS studies in which specific upstream or far field boundary conditions (BCs) are required and may provide the utility of decreasing high initialization costs associated with conventional inlet BCs.Item Open Access Gust alleviation by spanwise load control applied on a forward and backward swept wing(2023) Klug, Lorenz; Ullah, Junaid; Lutz, Thorsten; Streit, Thomas; Heinrich, Ralf; Radespiel, RolfThe present paper investigates the feasibility of gust load alleviation at transonic speeds on a backward swept and a forward swept transport aircraft configuration. Spanwise-distributed control surfaces at the leading and trailing edges are employed to control gust-induced wing bending as well as wing torsion moments. The deflection amplitude and temporal flap actuation are determined by a novel scheme that builds on the aerodynamic strip theory. The aerodynamic effectiveness of the actuators is taken from a data base, computed from either 2D infinite swept wing simulations, or from yawed computations that take the effects of boundary-layer cross flow and the local sweep angle of the control surface into account. The present numerical flow simulations reveal that careful application of control laws at the trailing edge alleviates wing bending moments caused by strong vertical gusts by 85-90%, for both aircraft configurations. The application of leading-edge flaps introduces significant nonlinear aerodynamic interactions, that make the control of torsional moments comparably challenging. Here, the present results indicate that about 60% of wing torsion loads due to strong gusts can be removed.