Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session G03: Aerodynamics: Fluid-Structure Interactions, Membranes, Flutter I |
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Chair: Emmanuel Branlard, NREL Room: 130 |
Sunday, November 20, 2022 3:00PM - 3:13PM |
G03.00001: Gust Response of Free-Falling Porous Disks Ignazio Maria Viola, Chandan Bose Studies on the dispersal of plant seeds have recently revealed new fluid mechanics features that aid diaspores to remain airborne and travel long distances. Diaspores are the biological compounds comprising the seed and other tissues dispersed with it. Understanding the aerodynamics underpinning their dispersal can inspire the design of novel unpowered flying devices with high endurance and range. Dandelion diaspores, for example, employ a bundle of bristles called pappus, which gives way to the formation of a steady Separated Vortex Ring (SVR) in their wake. Because the formation of the SVR is due to the low pressure in the near wake of the pappus, it has been associated with drag enhancement. Here, we model the flight of dandelion diaspores as a free-falling porous disk and study how the topology of the SVR changes during flight for different governing parameters. Furthermore, the changes in the flight behaviour under the influence of wind gusts of varying intensity are studied to reveal the underlying mechanisms behind the high endurance of dandelion seeds. High-fidelity simulations are performed by solving the incompressible Navier-Stokes equation at a Reynolds number of the order of 100. We solve the Darcy-Brinkman-Forchheimer equation in the porous disk, and the incompressible Navier-Stokes equations in the clear fluid region around the disk. The fluid governing equations are weakly coupled with the Newton-Euler equations of motion using a partitioned approach. The results of this work reveal that the morphology of the dandelion diaspore is optimal not only for a steady fall in quiescent flow, but also in gusty flow conditions. This research will directly benefit the design of potential control strategies for insect-scale drones, having multi-fold applications including environmental monitoring. |
Sunday, November 20, 2022 3:13PM - 3:26PM |
G03.00002: Membrane flutter in three-dimensional inviscid flow Christiana Mavroyiakoumou, Silas D Alben Many previous works have studied fluid-structure interactions induced by thin flexible bodies. In most of these studies the body is nearly inextensible, with a moderate bending modulus. Here we consider softer materials — extensible membranes — that have zero bending modulus, and undergo significant stretching in a fluid flow that can lead to flutter. Examples include rubber, textile fabric, and the skin of swimming and flying animals. We develop a mathematical model and numerical method to study the large-amplitude flutter of rectangular membranes that shed a trailing vortex-sheet wake in a 3D inviscid flow. This extends our previous work on membrane dynamics in a 2D flow, where the membrane is a 1D curvilinear segment that undergoes small and large deflections. Here we consider 12 distinct boundary conditions at the membrane edges and compute the stability thresholds and the subsequent large-amplitude dynamics across the three-parameter space of membrane mass ratio, pretension, and stretching rigidity. We find that 3D dynamics in the 12 cases naturally form four groups based on the conditions at the leading and trailing edges. The conditions at the side edges are generally less important, but may have qualitative effects on the membrane dynamics — e.g. steady versus unsteady, periodic versus chaotic, or the variety of spanwise curvature distributions — depending on the group and the physical parameter values. |
Sunday, November 20, 2022 3:26PM - 3:39PM |
G03.00003: Non Quasi-Steady Effects of a Heaving Rectangular Cylinder at Low Reynolds Number Ahmed M Naguib, Alireza Safaripour, Mark Feero, Manoochehr M Koochesfahani This study is motivated by understanding the galloping instability of rectangular cylinders in the Reynolds number range 1,000 < Re <10,000 (based on the cylinder thickness). The particular focus of the investigation is to examine the presence of non quasi-steady behavior over a range of reduced velocity, oscillation amplitude and Reynolds number. To this end, experiments are conducted in a water tunnel to characterize the unsteady force acting on, and the boundary layer around a rectangular cylinder undergoing forced harmonic oscillation. The results show that non quasi-steady effects are present even at reduced velocities as high as 30 and oscillation amplitudes as small as 20% of the cylinder thickness. An analysis is conducted to understand the role of the non quasi-steady component of the force in galloping instability of the cylinder, and how this role is affected by key parameters. |
Sunday, November 20, 2022 3:39PM - 3:52PM |
G03.00004: Rigid and flexible wings subject to large intensity turbulence: a two-dimensional particle image velocimetry study Craig Thompson, Hulya Biler, Sean P Symon, Bharathram Ganapathisubramani A rigid and flexible wing were subjected to turbulent conditions generated by a Makita style active grid at Re=200,000. Both the suction and pressure side of the wing were illuminated in the chordwise plane, and two cameras were mounted such that full coverage of the surrounding flow field was obtained. Concurrently, a 6-axis load cell was used to determine the resulting forces and moments generated by the wings. Preliminary analysis suggests that there is an interaction between the freestream turbulence and the response of the wing. The study found that what can be perceived as random flow perturbations in the form of free stream turbulence can still cause excitations in the structural modes of a wing dependant on the length scale of those random perturbations. The study found a torsional mode was excited when the integral length scale was half of that of the airfoils chord, and a bending mode was excited when the integral length scale was equal to the airfoils chord. |
Sunday, November 20, 2022 3:52PM - 4:05PM |
G03.00005: Investigating the interplay between vortex dynamics and compressibility in transonic airfoil flutter Jacob M Turner, Jung-Hee Seo, Rajat Mittal Airfoil flutter characteristics and limit cycle oscillation (LCO) behavior are profoundly influenced by compressibility effects. Non-linear mechanisms, such as shock-induced separation have been shown to significantly reduce the critical speed for flutter resulting in the so-called "transonic dip". Despite extensive research into this domain, many of the key mechanisms are still not fully explained. This is partly due to the large parameter space involved and the limited use of high-fidelity simulations. In this work, we focus on the aerodynamic instabilities arising from pitching oscillations of a NACA0012 airfoil at a Reynolds number of 10,000. Direct numerical simulations based on a high-order compressible flow immersed boundary method (ViCComp3D) are used for this purpose. The influence of Mach number and frequency on the flutter boundary is categorized using an energy map approach based on forced pitching oscillations. Through this approach, we identify LCOs which are highly dependent on the free-stream Mach number. New insights into the transonic dip mechanisms are provided through detailed analysis of the unsteady aerodynamic forces and flow visualization. |
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