Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session L30: Flapping Wings and General Aerodynamics |
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Chair: Jack Elliott, Minnesota State University, Mankato Room: 255 B |
Monday, November 25, 2024 8:00AM - 8:13AM |
L30.00001: Drag-Thrust Transition of the Oscillating NACA0012 Airfoil at Varying Pitch Biases James Luo, CHK Williamson The experiments conducted in this research aim to characterize the flow physics governing the harmonically heaving and pitching NACA0012 airfoil at varying pitch biases. By introducing a pitch bias, the NACA0012 airfoil experiences a nonzero mean angle of attack over each cycle, in contrast to the classical case with zero pitch bias. Consequently, the NACA0012 airfoil produces a mean lift force over the course of each cycle in tandem with either a drag or thrust force. The experiments performed comprise of parametric sweeps across reduced frequency, amplitude, Reynolds numbers, and varying pitch biases. The Cornell Cyber-Physics Fluid Dynamics (CPFD) Facility conducts the sweep through this multi-dimensional parameter space by automated batch experimentation. Collectively, the experiments highlight the transition between thrust-producing and drag-producing regimes as well as characterize the kinematics and flow physics that enable efficient production of lift alongside thrust. Particle Image Velocimetry (PIV) experiments are also performed in the CPFD facility to identify modes of vortex shedding and the significance of the Leading Edge Vortex (LEV) in this family of flows. |
Monday, November 25, 2024 8:13AM - 8:26AM |
L30.00002: A Comparative Study of Asymmetrically Flapping Single and Tandem Finite Wings with 2D Airfoils Adhiraj Mukhopadhyay, Sunil Manohar Dash, Sophie F Armanini The study compares the aerodynamic performance of a 2D NACA0012 airfoil with that of a finite rectangular single wing with an aspect ratio (AR) of 1.85 through numerical simulations. This comparison is done at a flow Reynolds number (Re) of 5000, with the upstroke accounting for 70% of the total asymmetric flapping cycle time of 1.14s. Differences in aerodynamic performance observed between the wings are linked to changes in vortex dynamics. In addition, 3D tandem wing simulations are performed on a previously identified optimal arrangement of asymmetrically flapping finite wings with identical dimensions. Here, the rear wing motion leads the front wing by a phase difference of 10⁰. Comparisons between this optimal configuration and a pair of identical rectangular wings with the same motion show that the former performs better with a total lift coefficient of 1.59 compared to the latter's 1.30. Efforts are made to minimise adverse 3D aerodynamic effects such as spanwise flow and leading-edge vortex disruption. This involves designing new finite wings with a higher aspect ratio (AR of 3.7) for both single and tandem wing cases which enhances the aerodynamic performance, bringing results closer to the 2D cases. Details on the flow physics will be covered in the presentation. |
Monday, November 25, 2024 8:26AM - 8:39AM |
L30.00003: Aerodynamic forces on a flat plate undergoing asynchronous sinusoidal heaving and pitching motion Ching Chang, Wen-Chun Chen Unsteady aerodynamic has drawn a lot of attentions in recent years given its implication in biological fluid dynamics, such as the flight of insects and birds. Its fundamental physic is a dynamical system that the flow dynamic and the unsteady motion of solid objects are coupled. We numerically investigated a flat plate immersed in a steady freestream undergoing flapping motions. Computational fluid dynamics (CFD) with finite volume method is implemented to solve two-dimensional Navier–Stokes equations for Reynolds number 600~900. Both cross-stream translation and pitching motions are set to be sinusoidal and a phase difference is introduced. We explored a range of motion frequency and phase angle; and the transition for drag becoming thrust is identified. Additionally, we modeled the same problem using a discrete vortex method. Conventional CFD using a Eulerian mesh for these unsteady problems are generally expansive and time-consuming. Discrete vortex method serves as a reduced-order model in Lagrangian sense, and is right for vortex-dominated flows such as massive separation flows or vortex shedding. Despite the inviscid nature, vortex methods capture the essential interplay between vortices shed from both the leading and trailing edges. The forces computed using vortex method are compared and validated with the CFD result. |
Monday, November 25, 2024 8:39AM - 8:52AM |
L30.00004: Investigating the Dynamics of Airfoil Propulsion to Study the Collective Motion in Aquatic and Avian Animals Durga Charan, Adhiraj Mukhopadhyay, Sunil Manohar Dash, Sunita Mishra The present study investigates the self-propulsion fluid dynamics of oscillating airfoils to understand the unique collective motions observed in aquatic and avian animals. Here, the computational fluid dynamics simulations are performed on heaving NACA0017 airfoil in both single and tandem arrangements. The heaving airfoils' ability to generate propulsive forces is evaluated at different heaving frequencies. In addition, in-house experiments are conducted on the airfoils with aspect ratios of 3 and 4.5 to validate the numerical observations. The simulation results of tandem airfoils indicate the formation of the stable equilibrium gap between the airfoils at the induced velocity wavelength downstream of the trailing airfoil. Furthermore, this equilibrium gap is closely linked to the frequency and amplitude of heaving kinematics. Our results indicate that the use of multiple airfoils can effectively enhance velocity while simultaneously reducing energy consumption. These findings provide insight into the basic principles that control coordinated group motions in nature and open up opportunities for additional investigation into self-organization in these complex biological systems. The flow structure and aerodynamic forces will be thoroughly discussed in the presentation. |
Monday, November 25, 2024 8:52AM - 9:05AM |
L30.00005: Effects of Flapping Frequency on the Thrust Performance of Elliptical Airfoils in Transverse Offset Tandem Configurations Rahul Ranjan, Sunil Manohar Dash, Kim Boon Lua In this study, the effects of non-dimensional flapping frequency (Strouhal number, St) and transverse offset on the aerodynamic thrust performance of a two-dimensional tandem elliptical flapping airfoil in forward flight condition is investigated at a Reynolds number of 5000. The front and rear airfoils are identical, with an aspect ratio of 8. They are positioned at 1 chord length (c) apart along the longitudenal direction, with a transverse offset of 0.5c. Here, St is varied from 0.2 to 0.7 to examine thrust performance at different effective angles of attack amplitudes (α0) of 10°, 15°, and 20°. It is observed that as the St value initially increases, the time-averaged thrust coefficient for both the front and rear airfoils also increases until reaching a critical St value (Stcr). Beyond this Stcr, the thrust declines for all α0 values. The rear airfoil's thrust performance surpasses the front airfoil for St < Stcr but experiences quick reductions in thrust at higher St (St > Stcr). Additionally, both inline and transverse offset tandem systems show a similar trend in time-averaged thrust. The improved thrust performance of the rear airfoil is due to the strong Leading-Edge Vortex (LEV) and the constructive interaction of the shear layer. In the presentation, a detailed flow physics will be discussed. |
Monday, November 25, 2024 9:05AM - 9:18AM |
L30.00006: Dynamic Response of a Free-flying Airfoil to Prescribed Vortex Gusts Eric Edward Handy-Cardenas, Bingfei Yan, Jennifer A. Franck, Kenneth S Breuer The unfavorable interaction of modern aircraft with vortex gusts is an increasingly important issue as aircraft become smaller, more lightweight, flexible, and agile. In this work, an airfoil's dynamic response to a disturbance is studied experimentally in a water flume using a two-foil system, and results are compared with numerical simulations. The upstream foil acts as a disturbance generator that produces a vortex gust of desired strength, sign, and location by following a prescribed heaving and pitching trajectory. The downstream airfoil is controlled experimentally by a force-feedback cyber-physical system (CPS) that enables it to “freely fly” – to move in response to flow disturbances and realistic flight dynamics. Using Particle Image Velocimetry (PIV) we first validate the vortex generator to ensure a single coherent vortex with desired characteristics is produced. Secondly, we characterize the forces experienced by a constrained airfoil as a vortex impinges on it and we compare this with classic unsteady aerodynamics models. Finally, the dynamic response of a two-degree-of-freedom CPS free-flying airfoil in which the wing can pitch and heave in response to a gust is characterized. |
Monday, November 25, 2024 9:18AM - 9:31AM |
L30.00007: Effect of end-walls on the separated flow over a compressor blade at Re=20,000 Sergio B Castiblanco-Ballesteros, Bjoern F Klose, Geoffrey R Spedding, Gustaaf B Jacobs The effects of end-walls on the separated flow over a compressor blade for a chord based Reynolds number of Re=20,000 is investigated. The span of the blade is three times the chord length. The blade is bounded by end-walls on both ends. The blade is not twisted and is defined entirely by a NACA 65(1)-412 airfoil cross section. Direct Numerical Simulation is conducted with a discontinuous Galerkin based Navier-Stokes solver. The airfoil is placed under an angle of attack of four degrees with respect to the free-stream condition. Flow structures are visualized with the finite-time Lyapunov (FTLE) and Q-criterion. The FTLE visualizations show a pair of horse-shoe vortices next to the end-wall. A major vortical structure originates at the intersection of the airfoil and the end-wall and interacts with the nominally two-dimensional vortex street in the midspan region. The vortex street arcs and breaks periodically which leads to a low-frequency oscillation of lift and drag. The arcing further induces a flow from the end-wall towards mid-span inside the separation region. |
Monday, November 25, 2024 9:31AM - 9:44AM |
L30.00008: A variational approach to modeling a cylinder in ground effect Hever J Zelaya Solano, Mark Andrew Stremler For high Reynolds number flow over a cylinder near a moving surface, a potential flow model can be used to represent flow over the leading edge. However, the potential flow solution requires knowledge of the circulation around the cylinder. This circulation value can be found with an auxiliary condition using an energy method. Multiple choices for this variational condition exist. Gol'dshtik & Khanin (1978) postulated an ad-hoc variational approach that looks only at the velocity on the cylinder boundary. Guided by the novel work of Gonzalez & Taha (2022), we consider an extension of Gauss' principle that takes into account the entire velocity field. We will compare the predictions of these two models with experiments by considering the pressure distribution and forces on the cylinder as a function of proximity to the wall. We also discuss improvements to the model predictions through ad-hoc inclusion of flow separation. |
Monday, November 25, 2024 9:44AM - 9:57AM |
L30.00009: Stokes-Dependent Droplet Collection Efficiency on a NACA 0012 Airfoil: Insights from Droplet-Informed Simulations Arash Shad, Hashnayne Ahmed, Nadim Zgheib, S. Balachandar, S. A. Sherif We conducted three-dimensional, Euler-Lagrange simulations of a droplet-laden airflow impinging on a NACA 0012 airfoil under various operating conditions, where a Langevin model was used to include the effect of free-stream turbulence on droplet trajectory. We considered free-stream velocities in the range of 60≤U≤240 m/s, chord lengths in the range of 0.5≤c≤2 m, and droplet diameters in the range 1≤dp≤160 microns. To minimize the computational cost of collecting statistically significant data, we employ statistical overloading of droplets, allowing us to simulate millions of impinging droplets in a short time span. We monitored the droplet collection efficiency〈β〉as well as droplet impingement velocity〈Vimp〉and angle 〈Θ〉. Our findings indicate that droplet collection efficiency, which quantifies the likelihood of droplet impingement on the airfoil surface, increases with droplet size and free-stream velocity but decreases with airfoil size. However, we demonstrate that the Stokes number, which integrates these parameters, primarily dictates droplet impingement behavior. Furthermore, we observe droplet behavior becoming Stokes number independent at very small and very large values of the Stokes number. |
Monday, November 25, 2024 9:57AM - 10:10AM |
L30.00010: Modeling and Computation of Aerodynamic Nosecone Ablation Siva Thangam, Igbal Mehmedagic This presentation involves the modeling and simulation of nosecone ablation due to aerodynamic heating of a projectile in highspeed flight. Generic projectiles with plastic hollow nosecones (for housing the radar system) in supersonic flight are considered for the purpose of developing efficient designs for high-speed projectiles. Computations are performed using an efficient anisotropic turbulence model that has wide applicability for aerodynamic applications. The time-averaged equations of motion and energy are solved using the modeled form of transport equations for the turbulence kinetic energy and specific turbulent dissipation rate with an efficient finite-volume algorithm. The possible melting and phase change during flight conditions are modeled in the context projectile design. The liquidized material flow due to ablation along the solid surface of the projectile in supersonic flow uses a multiphase model of the plastic nosecone melt material under influence of the high velocity gas. In this context, the stratified/free-surface flow in which two immiscible fluids are separated by a clearly defined interface is prescribed by the model. Computational findings show that commonly used plastic nosecone would ablate during high-speed flights. The magnitude of ablation, changes to the shape and resultant aerodynamic penalty for the projectile are quantified and discussed. |
Monday, November 25, 2024 10:10AM - 10:23AM |
L30.00011: Regulated Formation of a Ground Vortex in Suction Flow Over a Surface Bojan Vukasinovic, Derek A Nichols, Ari N Glezer The formation of a columnar ground vortex by the interaction of uniform flow over a plane surface with suction into an adjacent cylindrical conduit and the ingestion of the vortex into conduit's inlet are investigated in wind tunnel experiments using planar/stereo PIV with specific emphasis on the role of the surface vorticity layer in the vortex initiation and sustainment. It is shown that when combinations of the inlet momentum flux, crosswind speed, and inlet elevation exceed critical threshold, wall-normal columnar vortices are spawned within a counter current shear layer that forms over the ground plane within a streamwise domain on the inlet's leeward side by the suction flow into the duct. At low suction speeds, these wall-normal vortices are advected downstream, but their celerity is reversed with increased suction and they are advected towards the inlet, gain circulation stretch and are ingested into the inlet. Reduction of the counter current shear within the wall vorticity layer by deliberate, partial bypass of the inlet face flow through the periphery of the cylindrical conduit can significantly delay the ingestion of the ground vortex to substantially higher formation threshold. |
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