Now showing 1 - 10 of 228
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  • Publication
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    Evaluation of an efficient data-driven ANN model to predict agglomerate collisions within Euler–Lagrange simulations
    (Elsevier Science B.V., 2024) ;
    In this study, a recently developed data-driven model for the collision-induced agglomerate breakup (CHERD 195, 2023) is evaluated. It is especially intended for Euler–Lagrange simulations of flows with high mass loadings, where coupled CFD–DEM predictions are too expensive. Therefore, a surrogate model relying on the hard-sphere approach in which agglomerates are represented by effective spheres was developed. Based on a variety of DEM simulations, artificial neural networks were trained to predict the post-collision number of arising fragments, their size distribution and their velocities. In the present contribution, the agglomerate collision model is assessed using the particle-laden flow through a T-junction. Since two fluid streams with agglomerates are injected at both opposite ends, the setup is particularly suitable for investigating breakage caused by collisions. Two flow configurations (laminar flow at Re = 130 and turbulent flow at Re = 8000) and two different powders (primary particle diameter of 0.97 and 5.08 micrometers) are taken into account. The latter allows to study the influence of the strength of the agglomerates on the collision-induced breakage. The laminar case offers the possibility to evaluate the effect of the collision angle in detail. The collision-induced breakage proves to be the most dominant deagglomeration mechanism in both the laminar and turbulent flow scenario. Nevertheless, the role of the fluid stresses and especially the drag stress becomes more prominent in the turbulent case, while in the laminar flow their effects are negligible.
  • Publication
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    A numerical method to mimic an experimental wind gust generator: The immersed boundary gust generator
    (AIP Publishing, 2024) ;
    To generate horizontal wind gusts in a classical wind tunnel, Wood, Breuer, and Neumann [A novel approach for artificially generating horizontal wind gusts based on a movable plate: The paddle,” J. Wind Eng. Ind. Aerodyn. 230, 105170 (2022)] developed a new wind gust generator denoted the “paddle.” The working principle relies on the partial blocking of the outlet of the wind tunnel nozzle by a plate that vertically moves into the free-stream. Based on laser-Doppler anemometer measurements of the velocity at only a few locations, the basic functionality of the device was proven. The objective of the present contribution is to numerically mimic the gust generator and the flow field induced by the paddle in the test section. Contrary to the single-point measurements, the three-dimensional time-resolved simulation delivers the entire flow field and thus allows to investigate all details of the generated gust. To describe the paddle motion, the immersed boundary method with a continuous and direct forcing approach is implemented into a finite-volume flow solver for large-eddy simulations. A uniform and a non-uniform distribution of the Lagrangian markers are investigated where the latter ensures that an excessive increase in the computational resources required can be avoided. The predictions allow to characterize the resulting flow features induced by the paddle in great detail. Furthermore, a comparison of the numerical and experimental results is carried out based on the time histories of the streamwise and vertical velocity components at certain positions showing a close agreement. Finally, the forces acting on the fluid by the moving paddle are evaluated.
  • Publication
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    Numerical study of the hydrodynamic stability of a wind-turbine airfoil with a laminar separation bubble under free-stream turbulence
    (AIP Publishing, 2023-08)
    Fava, Thales C. L.
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    Lobo, Brandon A.
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    Nogueira, P.A.S
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    Schaffarczyk, Alois P.
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    Henningson, Dan S.
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    Hanifi, Ardeshir
    The interaction of several instabilities and the influence of free-stream turbulence on laminar-turbulent transition on a 20% thick wind-turbine blade section with a laminar separation bubble (LSB) are investigated with wall-resolved large-eddy simulations (LES). Turbulence intensities (TI) of 0%, 2.2%, 4.5%, 8.6%, and 15.6% at chord Reynolds number 105 are considered. Linear receptivity occurs for the most energetic disturbances; high-frequency perturbations are excited via non-linear mechanisms for TI >= 8:6%. Unstable Tollmien–Schlichting (TS) waves appear in the inflectional flow region for TI <= 4:5%, shifting to inviscid Kelvin–Helmholtz (KH) modes upon separation and forming spanwise rolls. Sub-harmonic secondary instability occurs for TI = 0%, with rolls intertwining before transition. Streaks spanwise modulate the rolls and increase their growth rates with TI for TI <= 4:5%, reducing separation and shifting transition upstream. The TI = 4:5% case presents the highest perturbations, leading to the smallest LSB and most upstream transition. Earlier inception of TS/KH modes occurs on low-speed streaks, inducing premature transition. However, for TI = 8:6%, the effect of the streaks is to stabilize the attached mean flow and front part of the LSB. This occurs due to the near-wall momentum deficit alleviation, leading to the transition delay and larger LSB than TI = 4:5%. This also suppresses separation and completely stabilizes TS/KH modes for TI = 15:6%. Linear stability theory predicts well the modal evolution for TI <= 8:6%. Optimal perturbation analysis accurately computes the streak development upstream of the inflectional flow region but indicates higher amplification than LES downstream due to the capture of low-frequency, oblique modal instabilities from the LSB. Only low-amplitude [O(1%)] streaks displayed exponential growth in the LES since non-linearity precludes the appearance of these modes.
  • Publication
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    Effects of Threadlike Roughness on a High-Slenderness Finned Projectile at Supersonic Speeds
    (2023-06)
    Michalski, Sebastian
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    Hruschka, Robert
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    Klatt, Daniel
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    Bastide, Myriam
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    The aerodynamic properties of a high-slenderness finned projectile with and without a threadlike roughness element on its shaft are evaluated by force and moment balance measurements in a supersonic blow-down tunnel. The test conditions cover a Mach number range from 2.5 to 4.1 at diameter-based Reynolds numbers in the order of 8 × 10^5. The forebody axial force is increased by approximately 15 % by the roughness element at zero angle of attack. Further measurements at non-zero angles of attack show that this axial force increase growths over-proportionately with the angle of attack. Whereas a minor influence by the roughness element is observed for the normal force, the pitching moment is either decreased by about 7 % or increased by about 13 % depending on the flow condition and angle of attack. Flow visualization by a high-resolution schlieren setup gives insight into the flow physics responsible for these phenomena. The schlieren setup also enables the detection of the laminar-turbulent boundary layer transition.
  • Publication
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    Data-driven ANN approach for binary agglomerate collisions including breakage and agglomeration
    The present contribution is a follow-up of a recently conducted study to derive a data-driven model for the breakage of agglomerates by wall impacts. This time the collision-induced breakage of agglomerates and concurrently occurring particle agglomeration processes are considered in order to derive a model for Euler--Lagrange methods, in which agglomerates are represented by effective spheres. Although the physical problem is more challenging due to an increased number of influencing parameters, the strategy followed is very similar. In a first step extensive discrete element simulations are carried out to study a variety of binary inter-agglomerate collision scenarios. That includes different collision angles, collision velocities, agglomerate sizes and powders. The resulting extensive database accounts for back-bouncing, agglomeration and breakage events. Subsequently, the collision database is used for training artificial neural networks to predict the post-collision number of arising entities, their size distributions and their velocities. Finally, it is shown how the arising data-driven model can be incorporated into the Euler--Lagrange framework to be used in future studies for efficient computations of flows with high mass loadings.
  • Publication
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    On the laminar–turbulent transition mechanism on megawatt wind turbine blades operating in atmospheric flow
    (Copernicus, 2023-03-06)
    Lobo, Brandon Arthur
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    Özçakmak, Özge Sinem
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    Madsen, Helge Aagaard
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    Schaffarczyk, Alois Peter
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    Sørensen, Niels N.
    Among a few field experiments on wind turbines for analyzing laminar–turbulent boundary layer transition, the results obtained from the DAN-AERO and aerodynamic glove projects provide significant findings. The effect of inflow turbulence on boundary layer transition and the possible transition mechanisms on wind turbine blades are discussed and compared to CFD (computational fluid dynamics) simulations of increasing fidelity (Reynolds-averaged Navier–Stokes, RANS; unsteady Reynolds-averaged Navier–Stokes, URANS; and large-eddy simulations, LESs). From the experiments, it is found that the transition scenario changes even over a single revolution with bypass transition taking place under the influence of enhanced upstream turbulence, for example, such as that from wakes, while natural transition is observed in other instances under relatively low inflow turbulence conditions. This change from bypass to natural transition takes place at azimuthal angles directly outside the influence of the wake indicating a quick boundary layer recovery. The importance of a suitable choice of the amplification factor to be used within the eN method of transition detection is evident from both the RANS and URANS simulations. The URANS simulations which simultaneously check for natural and bypass transition match very well with the experiment. The LES predictions with anisotropic inflow turbulence show the shear-sheltering effect and a good agreement between the power spectral density plots from the experiment and simulation is found in case of bypass transition. A condition to easily distinguish the region of transition to turbulence based on the Reynolds shear stress is also observed. Overall, useful insights into the flow phenomena are obtained and a remarkably consistent set of conclusions can be drawn.
  • Publication
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    Aeroelastic response of an elastically mounted 2-DOF airfoil and its gust-induced oscillations
    The paper is concerned with numerical investigations on the effect of vertical wind gusts on airfoils in a parameter range relevant for Micro-Air Vehicles. Using a simplifiedsubstitute model instead of an elastic wing, a rigid but elastically mounted airfoil with two degrees of freedom (heave and pitch) is considered. The coupled problem is tackled by a partitioned fluid–structure interaction coupling scheme based on the large-eddy simulation (LES) technique and a rigid-body solver. In order to describe the effect of deterministic 1-cosine gusts of different gust lengths and gust strengths, the split velocity method (SVM) is incorporated into the simulation framework relying on the Arbitrary Lagrangian–Eulerian (ALE) formulation on temporally varying control volumes. First the flow fields and the corresponding aerodynamic forces during the direct airfoil–gust interaction are compared for a fixed and an elastically mounted airfoil. The intrinsic study on the elastic case includes nine different gust scenarios in the transitional Reynolds number regime in order to investigate the resulting flow fields and motion patterns and to answer the question whether limit-cycle oscillations (LCO) or even flutter can be induced. The results show that in seven of the studied cases, the airfoil–gust interaction leads to sustained heave and pitch oscillations of bounded amplitudes (i.e., LCO). Further investigations clarify that this can be physically attributed to the laminar separation taking place on the upper and lower surfaces of the airfoil. The two strongest gust cases, however, excite the airfoil to levels above its critical angle of attack and triggered a pitch-induced diverging flutter. An energy analysis of both characteristic scenarios (i.e., LCO and flutter) further elucidates the differences between both cases. The former case is driven by the heave motion, whereas the pitch DOF acts as an energy sink. Contrarily, in the case of flutter the pitching motion is powering the coupled system, whereas the heave motion dissipates energy.