Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session ZC04: Aerodynamics: Fluid-Structure Interactions, Membranes and Flags |
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Chair: Sonya Tiomkin, University of South Florida Room: 101 |
Tuesday, November 21, 2023 12:50PM - 1:03PM |
ZC04.00001: Deformation, drag, and vortex shedding from a porous membrane disk Alexander Gehrke, Sakthi Swarrup, Kenneth Breuer Compliant and porous materials like cloth are applied to increase the drag and stability in the design of parachutes. The porosity can stabilize the wake and lead to increased drag. However, the effect of porosity on elastically deforming membranes has not been studied to great extent. As the membrane stretches under pressure the pores expand and increase the flow through the membrane. This gives rise to rich dynamics between fluid and structure. In this study, we present a theoretical model based on experimental data on the interplay between membrane loading, deformation, and leakage of hyper-elastic, porous membrane discs. We perform aerodynamic force measurement of porous membrane discs in wind tunnel experiments to determine the drag variation of membranes with different porosity and material properties. Additional flow field and deformation measurements are conducted to relate the membrane dynamics to the unsteady flow over and through the porous membranes. The added porosity stabilizes oscillations, observed previously in non-porous, compliant membranes. The reduced vortex shedding leads to an overall reduction in unsteady force variation acting on the bulged membranes. |
Tuesday, November 21, 2023 1:03PM - 1:16PM Author not Attending |
ZC04.00002: Abstract Withdrawn
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Tuesday, November 21, 2023 1:16PM - 1:29PM |
ZC04.00003: The hysteresis in flutter amplitude of almost ideally flat flexible membranes Holger Mettelsiefen, Vrishank Raghav Fluid flow along a flexible membrane which is clamped at its leading edge can lead to self-excited oscillations, also referred to as flutter. Interestingly, the critical fluid velocity that is required to initiate flutter is typically higher than the velocity below which a fluttering membrane stops moving. Systematic investigations of this hysteresis have been impeded in past studies because experimentally measured critical flow velocities can be highly sensitive to flatness defects in the membranes. The present study investigates the hysteresis in flutter amplitude of specimens whose flatness defects are negligible with respect to their thickness, as quantified by means of stereophotogrammetry. We present how the two dimensionless parameters mass ratio and aspect ratio determine the critical velocities of onset and ceasing of flutter. Remarkably, significant hysteresis is observed also in slender flags. Based on the observations, a hypothesis for the origin of the hysteresis in flutter amplitude is derived, which is valid for cases where flatness defects are not dominant. This hypothesis is further supported qualitatively by low-order modeling with ordinary differential equations. |
Tuesday, November 21, 2023 1:29PM - 1:42PM |
ZC04.00004: Flow-induced deformations of Kagome kirigami patterns Agathe Schmider, Sophie Ramananarivo Kirigami, a technique inspired by the Japanese art of paper cutting, is emerging as an efficient method to produce highly deformable sheets with programmable mechanical properties that are determined by the cutting pattern. Upon loading, the sheet extends bi-axially through the opening of its slits, effectively acting as a poroelastic membrane in fluid flow. In this experimental study, we explore the influence of the cutting parameters on the three-dimensional deformation of a sheet with triangular rotating facets (Kagome) immersed in a flow. The cutting pattern dictates both the elastic resistance to deformation of the membrane and the fluid loading, which is dependent on the pore geometry and local degree of opening. Through this study on uniform kirigami patterns, we aim to identify the design rules to then produce targeted deformations in flow by spatially varying the cut motif. |
Tuesday, November 21, 2023 1:42PM - 1:55PM |
ZC04.00005: Propulsive characteristics of heaving membrane wings with passive pitching Sonya Tiomkin, Justin W Jaworski Membrane wings possess several aerodynamic advantages over rigid wings due to their passive adaptation to the surrounding flow. The dynamic and lift responses of heaving membrane wings depend strongly on the reduced frequency of the prescribed motion, and are strongly enhanced near resonance. However, the unsteady fluid load on the airfoil also produces an aerodynamic pitch moment that incites passive pitch oscillations. In pursuit of a model to evaluate the benefits of passively pitching, heaving membrane wings, a theoretical framework is developed for the propulsive characteristics of two-dimensional flapping membrane wings in an inviscid flow. An extensible membrane of small camber is assumed with a constant tension along its length. The membrane is attached to a torsional spring at the leading edge, where the heaving motion is prescribed. The aerodynamic load on the airfoil is obtained using unsteady thin airfoil theory, which is coupled with the membrane dynamic motion and the passive pitch oscillations of the airfoil. Closed-form expressions are derived for the thrust of passively pitching heaving membrane wings and are compared to the results available for rigid thin airfoils to assess the effect of membrane flexibility on the propulsion potential of flapping wings. |
Tuesday, November 21, 2023 1:55PM - 2:08PM |
ZC04.00006: Strongly-coupled fluid-structure interaction of lightweight membranes and shells Marin Lauber, Gabriel D Weymouth, Georges Limbert Numerical fluid-structure interaction simulations of lightweight, flexible structures, such as membranes and shells, are notoriously tricky due to the large deformations and the significant added-mass effect. These effects generate a very strongly coupled problem that requires implicit coupling algorithms to solve them or, in extreme cases, prevent reaching a stable solution altogether. |
Tuesday, November 21, 2023 2:08PM - 2:21PM |
ZC04.00007: Experimental Investigation for a flapping flag using coupled POD for FSI Rodrigo Padilla, Vibhav Durgesh Proper Orthogonal Decomposition has been extensively used to study complex fluid dynamic flows. However, for experimental Fluid-Structure Interaction (FSI) studies, the application of POD has been limited to fluid flow only. Goza and Colonius [JF&S:2018] developed the coupled modal analysis formulation encompassing fluid and structure together for the computational FSI studies. For this study, we used the approach provided by Goza and Colonius and developed a framework for coupled POD applied to the experimental FSI study. For this study, 2D PIV experiments were performed for flapping flags with dimensionless rigidity 4.3 x10-3, and 2.6 x 10-3. The flag's mass and aspect ratios were 1.47 and 0.25, respectively. The Reynolds number for this study is 6.7 x 104 and 8.7 x 104. For coupled POD analysis, the illumination of the membrane from the raw PIV images was used to estimate the structure energy for the coupled kernel. The coupled POD formulation for the FSI study was validated using synthetic data. The results demonstrated a coupling between the modes that captured the limit-cycle oscillations and observed the change in the modal energy distribution with a change in the flag oscillations. |
Tuesday, November 21, 2023 2:21PM - 2:34PM |
ZC04.00008: Flow-induced oscillations of a rigid inverted flag and their application to energy harvesting Winthrop Townsend, Cecilia Huertas-Cerdeira The interaction between a flexible cantilever plate and a uniform flow that impinges on its free end is known to produce large amplitude limit cycle oscillations (LCOs) over a finite range of flow speeds. The above plate configuration is commonly referred to as an inverted flag or a cantilever plate in reverse axial flow and has potential applications in energy harvesting and vortex generation. |
Tuesday, November 21, 2023 2:34PM - 2:47PM |
ZC04.00009: Shaping it up: how aspect ratio affects the dynamics of flapping flags Gaétan Raynaud, Karen Mulleners Flexible objects like flags remain steady in a low-velocity flow but undergo high-magnitude flapping beyond a critical velocity. Prior studies on the fluid-structure interaction of flags have mainly focused on the instability mechanisms. The influence of the flag's morphology on its deflection and interaction with the unsteady wake flow has received less attention. We specifically focus on the effect of aspect ratio on the flapping dynamics of rectangular flags. We install paper flags of different shapes in an open-section wind tunnel. The experimental set-up is fully automated, which enables to explore a large range of geometries at varying wind speeds. A load cell and an event-based camera provide time-resolved force and deformation measurements. Our results suggest that the aspect ratio does not affect the onset of flapping but affects the dynamic response in the flapping regime. Variations of Strouhal number and other metrics will reveal the role of three-dimensional effects in the dynamics of flapping flags. |
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