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 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 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 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.Item Open Access A reinforcement learning based slope limiter for second‐order finite volume schemes(2023) Schwarz, Anna; Keim, Jens; Chiocchetti, Simone; Beck, AndreaHyperbolic equations admit discontinuities in the solution and thus adequate and physically sound numerical schemes are necessary for their discretization. Second‐order finite volume schemes are a popular choice for the discretization of hyperbolic problems due to their simplicity. Despite the numerous advantages of higher‐order schemes in smooth regions, they fail at strong discontinuities. Crucial for the accurate and stable simulation of flow problems with discontinuities is the adequate and reliable limiting of the reconstructed slopes. Numerous limiters have been developed to handle this task. However, they are too dissipative in smooth regions or require empirical parameters which are globally defined and test case specific. Therefore, this paper aims to develop a new slope limiter based on deep learning and reinforcement learning techniques. For this, the proposed limiter is based on several admissibility constraints: positivity of the solution and a relaxed discrete maximum principle. This approach enables a slope limiter which is independent of a manually specified global parameter while providing an optimal slope with respect to the defined admissibility constraints. The new limiter is applied to several well‐known shock tube problems, which illustrates its broad applicability and the potential of reinforcement learning in numerics.Item 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, WernerAbstractWind 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 AbstractItem Open Access Evaluate the performance of a camber controlled cycloidal rotor(2022) Huang, DoudouThe curvature of the airfoil has a significant effect on the performance of the cycloidal rotor system. This paper aims to improve the aerodynamic performance of the cycloidal rotor system by utilizing dynamical morphing blades in a CFD model. Particularly, three different camber morphing concepts, including leading edge deflection, trailing edge deformation, and cambered NACA profile, are employed to a baseline 2-bladed system with rotating and pitching NACA0015 aerofoils. Based on these three camber concepts, a series of URANS 2-D numerical simulations in OpenFOAM are conducted for blades with different morphing degrees and positions. The simulation results verified that the flow field condition could be optimized and thus significant improvement in thrust and efficiency could be achieved by properly tuning the morphing control.Item Open Access Investigation of a realistic flap modeling using a combination of Chimera method and grid deformation on a wing fuselage configuration(2023) Hillebrand, Marco; Müller, Jens; Ullah, Junaid; Lutz, ThorstenFlap deflections of an aircraft wing for active load alleviation within CFD simulations are realized using pure grid deformation due to time saving and low modeling complexity. In this case, spanwise gaps are neglected, which are present in reality during a flap deflection. Another possibility to realize the deflections is the combination of pure grid deformation and Chimera method, which allows the modeling of the gap between flap and wing or consecutive flaps. The overall aim of this work is the analysis of the aerodynamic effects caused by the different modeling approaches realizing leading and trailing edge flap deflections. The comparison of the modeling methods is investigated on the DLR LEISA configuration, which is a generic wing‐fuselage configuration. For active gust load alleviation, the leading edge flaps are deflected downward and the trailing edge flaps are deflected upward. Due to the downward deflection of the leading edge flaps, vortices are formed using the combined Chimera method as a result of the gap consideration. These vortices lead to a local drag increase resulting in a difference between both modeling methods in the spanwise as well as global drag coefficient. With the pure grid deformation these vortices do not occur. Due to the upward trailing edge deflection, the combined Chimera method leads to a pressure compensation via the effective gap enlargement, which is not present in the pure grid deformation. Overall, the combined Chimera method offers a good possibility to model the induced drag as well as the pressure compensation at a large flap deflection.Item Open Access Aerodynamic and acoustic simulations of thick flatback airfoils employing high order DES methods(2022) Bangga, Galih; Seel, Ferdinand; Lutz, Thorsten; Kühn, TimoThe 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.