Now showing 1 - 10 of 17
  • Publication
    Metadata only
    The Paddle: A Novel Procedure for Artificial Gust Generation in a Wind Tunnel
    (Deutsche Gesellschaft für Laser-Anemometrie GALA e.V., 2023) ; ;
    Kähler, C.J.
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    Fuchs, T.
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    Hain, R.
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    Scharnowski, S.
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    Ruck, B.
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    Leder, A.
  • Publication
    Metadata only
    A novel approach for artificially generating horizontal wind gusts based on a movable plate: The Paddle
    (Elsevier, 2022-09) ; ;
    Neumann, Torben
    The paper is concerned with a novel approach to generate horizontal wind gusts in classical wind tunnels. Assuming an open test section of an Eiffel- or Göttingen-type wind tunnel, the new wind gust generator denoted ‘‘The Paddle’’ can be easily retrofitted to such experimental setups at low costs. The device is constructed based on commercially available components including a programmable software tool that allows to adjust all relevant kinematic parameters. Five predefined motion patterns of the paddle are investigated which differ according to the achieved blocking ratio of the wind tunnel nozzle as well as the velocity and acceleration of the downward and upward motion of the paddle. It is shown that the shape and intensity of the generated gusts can be fully controlled by these parameters. That guarantees a strong individual adjustability and customization of the induced gusts. Furthermore, a synchronization of the wind gust generator with measurement devices such as laser-Doppler anemometer or particle-image velocimetry is easy to implement. An extensive measurement campaign has shown that the generated horizontal gusts are highly reproducible for the presently investigated laminar boundary layer and vary in the core area of the gust measurements solely according to free-stream turbulence of the wind tunnel.
  • Publication
    Metadata only
    Experimental investigations on the dynamic behavior of a 2-DOF airfoil in the transitional Re number regime based on digital-image correlation measurements
    © 2020 The Authors The present paper investigates the fluid–structure interaction (FSI) of a wing with two degrees of freedom (DOF), i.e., pitch and heave, in the transitional Reynolds number regime. This 2-DOF setup marks a classic configuration in aeroelasticity to demonstrate flutter stability of wings. In the past, mainly analytic approaches have been developed to investigate this challenging problem under simplifying assumptions such as potential flow. Although the classical theory offers satisfying results for certain cases, modern numerical simulations based on fully coupled approaches, which are more generally applicable and powerful, are still rarely found. Thus, the aim of this paper is to provide appropriate experimental reference data for well-defined configurations under clear operating conditions. In a follow-up contribution these will be used to demonstrate the capability of modern simulation techniques to capture instantaneous physical phenomena such as flutter. The measurements in a wind tunnel are carried out based on digital-image correlation (DIC). The investigated setup consists of a straight wing using a symmetric NACA 0012 airfoil. For the experiments the model is mounted into a frame by means of bending and torsional springs imitating the elastic behavior of the wing. Three different configurations of the wing possessing a fixed elastic axis are considered. For this purpose, the center of gravity is shifted along the chord line of the airfoil influencing the flutter stability of the setup. Still air free-oscillation tests are used to determine characteristic properties of the unloaded system (e.g. mass moment of inertia and damping ratios) for one (pitch or heave) and two degrees (pitch and heave) of freedom. The investigations on the coupled 2-DOF system in the wind tunnel are performed in an overall chord Reynolds number range of 9.66×103≤Re≤8.77×104. The effect of the fluid-load induced damping is studied for the three configurations. Furthermore, the cases of limit-cycle oscillation (LCO) as well as diverging flutter motion of the wing are characterized in detail. In addition to the DIC measurements, hot-film measurements of the wake flow for the rigid and the oscillating airfoil are presented in order to distinguish effects originating from the flow and the structure.
  • Publication
    Metadata only
  • Publication
    Open Access
    Experimentelle Untersuchungen zur Fluid-Struktur-Interaktion einer deformierbaren Membran-Halbkugel in turbulenter Strömung
    (Universitätsbibliothek der HSU / UniBwH, 2019) ; ;
    Helmut-Schmidt-Universität / Universität der Bundeswehr Hamburg
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    Manhart, Michael
    Membrane structures are of increasing interest for modern civil engineering due to their adaptable application. The safe assembly and operation of membranous buildings in urban regions is a challenging task due to permanently changing environmental conditions. A critical issue is the dynamic response of the flexible membrane to wind loads which has to be addressed as a primary design criterion for this type of structure in the future. The influence of wind loads on the deformable structure form a multi-physical problem since fluid and structure mechanics have to be considered simultaneously to encompass the whole problem. This leads to the motivation of this thesis in which the fluid-structure interaction (FSI) of a thin-walled and air-inflated membranous structure immersed in a turbulent boundary layer is investigated. The highly flexible structure has the shape of a hemisphere. In order to maintain its hemispherical form and to attain a resistance against wind loads, the flexible structure is pressurized by a slight gauge pressure pre-stressing the membrane. In this configuration, the membranous hemisphere is considered as an air-inflated building often seen as roofing for temporary facilities. The fluid-structure interaction of the flexible hemisphere immersed in a turbulent flow is experimentally investigated in a wind tunnel. For this purpose, an appropriate flexible model is manufactured using a casting procedure, where the structure of the hemisphere is based on a silicone material. A second fully rigid hemispherical model is manufactured out of aluminum serving as a reference for the flow field studies. The first investigation focuses on the turbulent flow around the rigid hemisphere at Re = 50,000. Large effort is put into the generation of the required thick turbulent boundary layer at the inlet of the test section of the wind tunnel, since it is an essential boundary condition of the experimental case. The flow around the solid bluff body is measured by laser-Doppler (LDA) and constant-temperature anemometry (CTA). Two characteristic vortex shedding processes are observed in the wake of the hemisphere: An asymmetric von Karman and an arch-type symmetric type. Both vortex patterns alternate in an irregular manner in time, where only one is present in the wake during a certain period of time. The time-averaged data reveal the characteristic phenomena forming around the hemisphere such as the horseshoe vortex system, the free shear layer and the recirculation region. All data are furthermore used for the successful validation of a large-eddy simulation which is taken from the literature. After this initial flow field study, the coupled problem is observed. For this purpose, the flexible hemisphere is once more exposed to a turbulent boundary layer at three Reynolds numbers (50,000, 75,000 and 100,000). This setup is used to examine the interaction between the flow and the pressurized membrane at different free stream velocities. Furthermore, the flow field around the rigid hemisphere is measured again in order to maintain comparability between the used measurement equipment and the extended Reynolds number range. The experiments are carried out by combining particle-image-velocimetry (PIV) for the flow field and high-speed digital-image correlation (DIC) measurements for the deformation of the oscillating membranous structure. Moreover, a constant-temperature anemometer is utilized in order to evaluate the velocity spectra at locations close to the wall. This is necessary to connect the non-synchronized fluid and structure measurements. Afterwards the spectra of the velocity fluctuations (CTA) and the structure oscillations (DIC) are compared. This procedure leads to the characterization of the underlying FSI mechanisms. As before, the two main vortex shedding types (von Karman and symmetric arch-type) are observed at all Reynolds numbers. These are also identified in the unsteady structure excitations. With increasing Re number the time-averaged deformations of the structure as well as the observed amplitudes of the oscillation increase. The displacements of the structure strongly influence the time-averaged flow field revealing a significant difference in the wake. A thorough analysis of the comprehensive data sets for the fluid flow and the displacements of the structure leads to the characterization of the behavior of the flexible structure under changing flow conditions. Again, the experimental results are supported by complementary numerical investigations based on large eddy simulations for the fluid and a finite-element solver for the structure taken from the literature.
  • Publication
    Metadata only
    Particle-Image Velocimetry and the Assignment Problem
    (Springer, 2018)
    Butz, Franz-Friedrich
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    Fügenschuh, Armin
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  • Publication
    Metadata only
    Numerical studies on the instantaneous fluid–structure interaction of an air-inflated flexible membrane in turbulent flow
    (2018) ;
    Apostolatos, Andreas
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    Bletzinger, Kai Uwe
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    Wüchner, Roland
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    The present paper is the numerical counterpart of a recently published experimental investigation by Wood et al. (2018). Both studies aim at the investigation of instantaneous fluid–structure interaction (FSI) phenomena observed for an air-inflated flexible membrane exposed to a turbulent boundary layer, but looking at the coupled system based on different methodologies. The objective of the numerical studies is to supplement the experimental investigations by additional insights, which were impossible to achieve in the experiments. Relying on the large-eddy simulation technique for the predictions of the turbulent flow, non-linear membrane elements for the structure and a partitioned algorithm for the FSI coupling, three cases with different Reynolds numbers (Re=50,000, 75,000 and 100,000) are simulated. The time-averaged first and second-order moments of the flow are presented as well as the time-averaged deformations and standard deviations. The predictions are compared with the experimental references data solely available for 2D planes. In order to better comprehend the three-dimensionality of the problem, the data analysis of the predictions is extended to 3D time-averaged flow and structure data. Despite minor discrepancies an overall satisfying agreement concerning the time-averaged data is reached between experimental data in the symmetry plane and the simulations. Thus for an in-depth analysis, the numerical results are used to characterize the transient FSI phenomena of the present cases either related to the flow or to the structure. Particular attention is paid to depict the different vortex shedding types occurring at the top, on the side and in the wake of the flexible hemispherical membrane. Since the fluid flow plays a significant role in the FSI phenomena, but at the same the flexible membrane with its eigenmodes also impacts the deformations, the analysis is based on the frequencies and Strouhal numbers found allowing to categorize the different observations accordingly.