Browsing by Author "Lutz, Thorsten"

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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
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 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
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
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, Rolf
The 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.
Open Access
An improved second-order dynamic stall model for wind turbine airfoils
(2020) Bangga, Galih; Lutz, Thorsten; Arnold, Matthias
Robust and accurate dynamic stall modeling remains one of the most difficult tasks in wind turbine load calculations despite its long research effort in the past. In the present paper, a new second-order dynamic stall model is developed with the main aim to model the higher harmonics of the vortex shedding while retaining its robustness for various flow conditions and airfoils. Comprehensive investigations and tests are performed at various flow conditions. The occurring physical characteristics for each case are discussed and evaluated in the present studies. The improved model is also tested on four different airfoils with different relative thicknesses.The validation against measurement data demonstrates that the improved model is able to reproduce the dynamic polar accurately without airfoil-specific parameter calibration for each investigated flow condition and airfoil.This can deliver further benefits to industrial applications where experimental/reference data for calibrating the model are not always available.
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, Thorsten
Flap 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.
Open Access
Mach and Reynolds number effects on transonic buffet on the XRF-1 transport aircraft wing at flight Reynolds number
(2023) Waldmann, Andreas; Ehrle, Maximilian C.; Kleinert, Johannes; Yorita, Daisuke; Lutz, Thorsten
This work provides an overview of aerodynamic data acquired in the European Transonic Windtunnel using an XRF-1 transport aircraft configuration both at cruise conditions and at the edges of the flight envelope. The goals and design of the wind tunnel test were described, highlighting the use of the cryogenic wind tunnel’s capability to isolate the effects of M∞, Re∞and the dynamic pressure q / E . The resulting dataset includes an aerodynamic baseline characterization of the full span model with vertical and horizontal tailplanes and without engine nacelles. The effects of different inflow conditions were studied using data from continuous polars, evaluating the changes in aeroelastic deformation which are proportional to q / E and the influence of M∞and Re∞on the shock position. Off-design data was analyzed at the lowest and highest measured Mach numbers of 0.84 and 0.90, respectively. Wing lower surface flow and underside shock motion was analyzed at negative angles of attack using cpdistribution and unsteady pressure transducer fluctuation data, identifying significant upstream displacement of the shock close to the leading edge. Wing upper-side flow and the shock motion near buffet onset and beyond was analyzed using unsteady pressure data from point transducers and unsteady pressure-sensitive paint (PSP) measurements. Buffet occurs at lower angles of attack at high Mach number, and without clearly defined lift break. Spectral contents at the acquired data points in the buffet range suggest broadband fluctuations at Strouhal numbers between 0.2 and 0.6, which is consistent with recent literature. The spanwise shock propagation velocities were determined independently via analysis of unsteady PSP and pressure transducers to be in the range between us/u∞=0.24and 0.32, which is similarly in line with published datasets using other swept wing aircraft models.
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, Ewald
This 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.
Open Access
Numerical analyses and optimizations on the flow in the nacelle region of a wind turbine
(2018) Weihing, Pascal; Wegmann, Tim; Lutz, Thorsten; Krämer, Ewald; Kühn, Timo; Altmikus, Andree
The present study investigates flow dynamics in the hub region of a wind turbine focusing on the influence of nacelle geometry on the root aerodynamics by means of Reynolds averaged Navier–Stokes simulations with the code FLOWer. The turbine considered is a generic version of the Enercon E44 converter incorporating blades with flat-back-profiled root sections. First, a comparison is drawn between an isolated rotor assumption and a setup including the baseline nacelle geometry in order to elaborate the basic flow features of the blade root. It was found that the nacelle reduces the trailed circulation of the root vortices and improves aerodynamic efficiency for the inner portion of the rotor; on the other hand, it induces a complex vortex system at the juncture to the blade that causes flow separation. The origin of these effects is analyzed in detail. In a second step, the effects of basic geometric parameters describing the nacelle have been analyzed with the purpose of increasing the aerodynamic efficiency in the root region. Therefore, three modification categories have been addressed: the first alters the nacelle diameter, the second varies the blade position relative to the nacelle and the third comprises modifications in the vicinity of the blade-nacelle junction. The impact of the geometrical modifications on the local flow physics are discussed and assessed with respect to aerodynamic performance in the blade root region. It was found that increasing the nacelle diameter deteriorates the root aerodynamics, since the flow separation becomes more pronounced. Possible solutions identified to reduce the flow separation are a shift of the blade in the direction of the rotation or the installation of a fairing fillet in the junction between the blade and the nacelle.
Open Access
Numerical simulation of wake interactions on a tandem wing configuration in high-speed stall conditions
(2023) Kleinert, Johannes; Stober, Jonathan; Lutz, Thorsten
In 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.
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, Thorsten
Helicopters 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.
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, Ewald
RANS 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.
Open Access
Simulation of transonic buffet with an automated zonal DES approach
(2020) Ehrle, Maximilian; Waldmann, Andreas; Lutz, Thorsten; Krämer, Ewald
A 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.
Open Access
Utilizing high fidelity data into engineering model calculations for accurate wind turbine performance and load assessments under design load cases
(2022) Bangga, Galih; Parkinson, Steven; Lutz, Thorsten
Wind turbines often have lower performance and experience higher loading in real operation compared to the original design performance. The reasons stem from the influences of complex atmospheric turbulence, blade contamination, surface imperfection and airfoil-shape changes. Engineering models used for designing wind turbines are limited to information derived from blade sectional datasets, while details on the three-dimensional blade characteristics are not captured. In these studies, a dedicated strategy to improve the prediction accuracy of engineering model calculations will be presented. The main aim is to present an elaborated effort to obtain a better estimate of the turbine loads in realistic operating conditions. The present studies are carried out by carefully utilizing data from high fidelity Computational Fluid Dynamics (CFD) computations into Blade Element Momentum (BEM) and Vortexline methods. The results are in a good agreement with detailed field measurement data of a 2.3 MW turbine. The studies are further extended to a large turbine having a rated power of 10 MW to provide an overview of its suitability for large turbines. Finally, calculations of the wind turbine under a realistic IEC design load case are demonstrated. The studies highlight important considerations for engineering modeling using BEM and Vortexline methods.
Open Access
Wake tail plane interactions for a tandem wing configuration in high-speed stall conditions
(2023) Kleinert, Johannes; Ehrle, Maximilian; Waldmann, Andreas; Lutz, Thorsten
In this work, wake-tail plane interactions are investigated for a tandem wing configuration in buffet conditions, consisting of two untapered and unswept wing segments, using hybrid Reynolds-Averaged Navier–Stokes / Large Eddy Simulations (RANS/LES) with the Automated Zonal Detached Eddy Simulation (AZDES) method. The buffet on the front wing and the development of its turbulent wake are characterized, including a spectral analysis of the fluctuations in the wake and a modal analysis of the flow. The impact of the wake on the aerodynamics and loads of the rear wing is then studied, with a spectral analysis of its lift and surface pressure oscillations. Finally, the influence of the position and the incidence angle of the rear wing is investigated. For the considered flow conditions, 2D buffet is present on the front wing. During the downstream movement of the shock, the amount of separation reaches its minimum and small vortices are present in the wake. During the upstream movement of the shock, the amount of separation is at its maximum and large turbulent structures are present accompanied by high fluctuation levels. A distinct peak in the corresponding spectra can be associated with vortex shedding behind the wing. The impingement of the wake leads to a strong variation of the loading of the rear wing. A low-frequent oscillation of the lift, attributed to the change of the intensity of the downwash generated by the front segment, can be distinguished from high-frequent fluctuations that are caused by the impingement of the wake’s turbulent structures.