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 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 Simulation of transonic buffet with an automated zonal DES approach(2020) Ehrle, Maximilian; Waldmann, Andreas; Lutz, Thorsten; Krämer, EwaldA study of transonic buffet on the NASA Common Research Model at flight Reynolds numbers is presented. The ability of two different hybrid RANS/LES models as well as the URANS approach for resolving three-dimensional buffet motion was evaluated by means of spectral analysis. Automated Zonal DES and URANS simulations show similar results in terms of buffet frequency and spanwise propagation of buffet cells, whereas the Delayed Detached Eddy Simulation results indicate a strong interaction between flow separation and shock motion. The extracted characteristic frequencies which are associated with transonic buffet are located in a range of Sr = 0.2-0.65 for URANS and AZDES and are therefore in accordance with findings from related recent research. Furthermore, the simulation time series were investigated and a structure of spanwise moving buffet cells with varying convection speed and wavelength could be observed.Item Open Access Reynolds number and wind tunnel wall effects on the flow field around a generic UHBR engine high-lift configuration(2020) Ullah, Junaid; Prachař, Aleš; Šmíd, Miroslav; Seifert, Avraham; Soudakov, Vitaly; Lutz, Thorsten; Krämer, EwaldRANS simulations of a generic ultra-high bypass ratio engine high-lift configuration were conducted in three different environments. The purpose of this study is to assess small scale tests in an atmospheric closed test section wind tunnel regarding transferability to large scale tests in an open-jet wind tunnel. Special emphasis was placed on the flow field in the separation prone region downstream from the extended slat cut-out. Validation with wind tunnel test data shows an adequate agreement with CFD results. The cross-comparison of the three sets of simulations allowed to identify the effects of the Reynolds number and the wind tunnel walls on the flow field separately. The simulations reveal significant blockage effects and corner flow separation induced by the test section walls. By comparison, the Reynolds number effects are negligible. A decrease of the incidence angle for the small scale model allows to successfully reproduce the flow field of the large scale model despite severe wind tunnel wall effects.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.Item Open Access Aeroelastic analysis of wind turbines under turbulent inflow conditions(2021) Guma, Giorgia; Bangga, Galih; Lutz, Thorsten; Krämer, EwaldThe 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.Item Open Access The near-wake development of a wind turbine operating in stalled conditions : part 1: assessment of numerical models(2024) Weihing, Pascal; Cormier, Marion; Lutz, Thorsten; Krämer, EwaldThis study comprehensively investigates the near-wake development of a model wind turbine operating at a low tip-speed ratio in stalled conditions. In the present paper, part 1, different ways of representing the turbine, which include a full geometrical representation and modeling by means of the actuator line method, and different approaches for the modeling of turbulence are assessed. The simulation results are compared with particle image velocimetry (PIV) measurements from the MEXICO and New MEXICO experiments. A highly resolved numerical setup was created and a higher-order numerical scheme was applied to target an optimal resolution of the tip vortex development and the wakes of the blades. Besides the classical unsteady Reynolds-averaged methodology, a recently developed variant of the detached-eddy simulation (DES) was employed, which features robust shielding capabilities of the boundary layers and enhanced transition to a fully developed large-eddy simulation (LES) state. Two actuator line simulations were performed in which the aerodynamic forces were either evaluated by means of tabulated data or imposed from the averaged blade loads of the simulation with full blade geometry. The purpose is to distinguish between the effects of the force projection and the force calculation in the underlying blade-element method on the blade wake development. With the hybrid Reynolds-averaged Navier-Stokes (RANS)-LES approach and the geometrically fully resolved rotor blade, the details of the flow of the detached blade wake could be resolved. The prediction of the wake deficit also agreed very well with the experimental data. Furthermore, the strength and size of the blade tip vortices were correctly predicted. With the linear unsteady Reynolds-averaged Navier-Stokes (URANS) model, the wake deficit could also be described correctly, yet the size of the tip vortices was massively overestimated. The actuator line method, when fed with forces from the fully resolved simulation, provides very similar results in terms of wake deficit and tip vortices to its fully resolved parent simulation. However, using uncorrected two-dimensional polars shows significant deviations in the wake topology of the inner blade region. This shows that the application in such flow conditions requires models for rotational augmentation. In part 2 of the study, to be published in another paper, the development and the dynamics of the early tip vortex formation are detailed.Item 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, EwaldA 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.