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 J03: Aerodynamics: Flapping and Oscillating Wings |
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Chair: Anya Jones, U Maryland Room: 130 |
Sunday, November 20, 2022 4:35PM - 4:48PM |
J03.00001: On the aerodynamic performance of the leading-edge tubercle elliptic flapping airfoil in forward flight condition Gangadhar V R Pinapatruni, Sunil Manohar M Dash, Jit Sinha, Kim Boon Lua In this study, the aerodynamic performance of the flapping elliptic airfoil with and without the leading-edge sinusoidal tubercles was investigated using experiments and numerical simulations in the presence of an incoming flow having a Reynolds number of 5000. Here, the amplitude and wavelength of the tubercles were fixed at 0.125 and 0.5 times the airfoil chord, respectively. The effect of the flapping frequency on the airfoil aerodynamic performance was also accessed by varying the Strouhal number (St) between 0.2 to 0.6 while keeping the maximum effective angle of attack as 15o. It is noticed that the time-averaged thrust coefficient (Ct) over a flapping cycle increases with St for both types of airfoils up to a critical St (=0.5) and degrades after that. In addition, the Ct values of tubercle airfoil are seen to be inferior compared to the smooth-edge airfoil at considered St. On the other hand, the tubercle airfoil's aerodynamic power requirement is less than smooth-edged airfoil, yielding a better aerodynamic efficiency at higher St. Moreover, the near wake structures of the tubercle airfoil with the counter-rotating vortex pairs between the crest and trough region are distinctive from that of the smooth-edged airfoil. Those details will be discussed in the presentation. |
Sunday, November 20, 2022 4:48PM - 5:01PM |
J03.00002: Influence of wing acceleration on the growth and trajectory of the leading-edge vortex Abbishek Gururaj, Sarah E Morris, Mahyar Moaven, Brian Thurow, Vrishank Raghav The presence of a stable leading-edge vortex (LEV) over an insect wing is responsible for the generation of high lift. The LEV stability is mediated by the rotational accelerations and their influence in the quasi-steady phase has been studied. However, understanding the LEV dynamics during the transient or accelerating phase of the wing motion is equally essential and has not received much attention. This study aims to understand the role of wing acceleration on the forces and flowfield evolving over a rotating wing. The wing acceleration will be experimentally varied while keeping the Reynolds number (Re = 1500), Rossby number (Ro =4.5), and aspect ratio (AR = 5) constant. The evolution of the circulatory lift force will be studied and correlated to the LEV circulation for different acceleration conditions. Furthermore, the trajectory of the LEV will be studied for different conditions. Preliminary flow visualization experiments have shown that increasing the wing acceleration increases vorticity production and leads to earlier separation. The flowfield data will be used to understand the correlation between the wing acceleration, LEV circulation, trajectory, and separation |
Sunday, November 20, 2022 5:01PM - 5:14PM |
J03.00003: Development of surrogate models for unsteady flow fields using a deep neural network Hamid R Karbasian, Wim M van Rees In this study, we show the application of deep learning in developing a surrogate model for unsteady flow problems, with a focus on flows past single or multiple moving bodies in 2D. Our proposed surrogate model is developed in two stages, based on data from direct numerical simulations of the Navier-Stokes equations. First, we use Proper Orthogonal Decomposition (POD) as an invertible operator to transfer information from the high-dimensional physical space (i.e. the simulation data) to a lower-dimensional latent space and vice versa. Then, we use the Long-Short Term Memory (LSTM) recurrent neural network to extract non-linear dynamics in the temporal solutions in the latent space. We show that this surrogate model is capable of dynamic reconstruction and prediction of unsteady flow fields involving the flow past flapping ellipses, achieving a speedup of at least two orders of magnitude compared with the numerical simulations. We will discuss the possibilities offered by this surrogate model approach for design, optimization, and control of challenging unsteady flow problems. |
Sunday, November 20, 2022 5:14PM - 5:27PM |
J03.00004: Vortex Shedding Modes of the Self-Propelled Airfoil James Luo, CHK Williamson In the present research, we study the vortex formation and vortex shedding of an oscillating NACA0012 airfoil that is free to "self-propel" in the streamwise direction. Experiments are conducted by imposing heaving and pitching motions of varying amplitude onto the airfoil, resulting in a "Heave-Pitch Diagram". We consider two cases for the self-propelled airfoil. In the first case, the airfoil's oscillatory motion is symmetric, resulting in zero mean angle of attack and no net lift production over a cycle. In the second case, the airfoil's motion becomes asymmetric by introducing an offset angle into the pitching motion trajectory. Consequently, the airfoil experiences a nonzero mean angle of attack and generates a net lift force over a cycle. Through PIV flow visualization throughout our Heave-Pitch diagram, we characterize the wake structure and vortex dynamics for both the airfoil undergoing symmetric oscillations and the airfoil undergoing asymmetric oscillations. While the symmetric configuration yields the inverse von Kármán street, the vortex dynamics of the asymmetric configuration leads to the formation of a series of vortex pairs in the wake. As a consequence of vortex-vortex interactions, these pairs can deflect at angles relative to the flow's center plane. |
Sunday, November 20, 2022 5:27PM - 5:40PM |
J03.00005: Effects of Gust Structures on the Vorticity Field of a Pitching Wing Morgan Lusch, Austin Evans, Kundan Panta, Bo Cheng, Azar Eslam-Panah Animals in nature, such as birds and insects, have unique designs and techniques for flying and adapting within unsteady environments. Human-made structures, such as drones and airplanes, have fixed wings that easily malfunction when faced with inclement weather. The intention of this research is to study how birds and insects fly during gusty conditions and how the data can be applied to further improve the design and development of manufactured aircraft. Particle Image Velocimetry (PIV) measurements were used to study a pitching wing in an unsteady environment through varying flow velocities, reduced frequencies, and pitching angles. As the pitch angle increased, it created a larger disturbance in the wake of the wing causing the boundary layer to break off closer to the trailing edge of the wing. The vortices created by the gust generator caused vortex shedding along the wing if the vortex and the boundary layer had opposing direction. If the gust generator vortex and the boundary layer had similar sign, the boundary layer's vorticity would increase in magnitude. This increase of magnitude would move along the surface of the wing and eventually shed a vortex off the trailing edge of the wing. |
Sunday, November 20, 2022 5:40PM - 5:53PM |
J03.00006: Numerical Study of Owls' Leading-Edge Serrations Asif Shahriar Nafi, Nikolaos Beratlis, Roi Gurka, Elias Balaras Nocturnal owls’ species fly almost silently. Their stealth flight is commonly attributed to a combination of their wings’ special microfeatures and kinematics. These features are distributed over the wingspan in both the leading and trailing edge regions as well as over the planform. One of these microfeatures is known as leading-edge serrations. They are rigid, comb-like structures located at the primary feathers. Their length is about 1% of the wing chord and they are aligned at an angle in respect to the incoming flow, positioned in a ‘hooks-like’ formation over the leading-edge surface. In this study, we investigate the flow characteristics over a Barn owl (Tyto alba) wing section with and without leading-edge serrations geometry at an intermediate chord-based Reynolds number (Rec ~ 40,000) using DNS (Direct Numerical Simulation) approach. The simulations are performed at a high angle of attack in gliding mode replicating the owl’s wing position during the approach phase towards the prey. This comparative study enables us to shed light on the impact these serrations have on the flow dynamics developed over the wing. Disparities in the flow turbulence characteristics, as well as boundary layer parameters, are evaluated to understand how the owls’ leading-edge serrations impact the incoming flow. We will demonstrate that the presence of serrations alters the small scales of the flow in the near wake. In addition, detailed analysis of their impact on the aerodynamic performance will be presented. |
Sunday, November 20, 2022 5:53PM - 6:06PM |
J03.00007: Effects of gust structures on aerodynamics of flapping wings Kundan Panta, Morgan Lusch, Austin Evans, Bo Cheng, Azar Eslam-Panah Birds and insects can stabilize themselves in gusty environments with their flapping wings, surpassing what fixed-wing aerial vehicles can do in similar conditions. Here, we test the hypothesis that the plunging movements of a wing can indeed suppress the effects of incoming gust structures by studying both the fluid flow surrounding the wing and the wing's aerodynamic loading. The wing plunged at frequencies determined by Strouhal numbers (St) from 0 (fixed wing) to 0.5, covering the range commonly found in nature. Inside a closed-circuit water channel, a gust generator with sinusoidally pitching vanes created vortex gusts that interacted with the wing. Particle Image Velocimetry (PIV) was used to observe the flow around the wing, and a force/torque transducer was used to measure the loads acting on it. Flow quantities such as circulation, turbulence intensity, and forces were calculated. Because the interactions between the wing and gusty flow was unpredicted, statistics were used instead of the time-series data to quantify the gusts' effects. With increasing St, the circulation and the forces increased in magnitude, but the increase in the spread of both quantities, due to the gusts, lowered. This suggests better predictability of the flow and forces for plunging wings at higher St, which could be exploited for better stabilization against gusts. |
Sunday, November 20, 2022 6:06PM - 6:19PM |
J03.00008: Effects of Camber on Dynamic Stall at Low Reynolds Numbers Colin Stutz, John T Hrynuk, Douglas G Bohl The effects of camber on pitch-and-hold dynamic stall was investigated at a Reynolds number of 12,000 using high speed PIV. Three airfoil shapes (NACA 0012, Eppler 387, and SD 5060) and two dimensionless pitch rates (Ω* = 0.05 and 0.1) were studied undergoing pitching motions from 0 to 55 degrees angle of attack. Many previous observations, such as the effect of pitch rate on the formation location and formation angle of the Dynamic Stall Vortex (DSV), were also observed in this data set. Airfoil thickness was known to have an effect the formation of Shear Layer Vortices (SLVs). This work will demonstrate that SLV formation is instead determined by the amount of curvature on the upper surface as opposed to overall thickness. However, it remains true that the relative importance of the effects of airfoil shape compared to motion parameters is difficult to determine at these low pitch rates. This study will also show that the relationship between SLVs and Trailing Edge Vortices may be more important to the dynamic stall process than previous thought. |
Sunday, November 20, 2022 6:19PM - 6:32PM |
J03.00009: Different routes to optimal flapping in unsteady conditions Rodrigo Vilumbrales Garcia, Gabriel D Weymouth, Bharathram Ganapathisubramani Fish can significantly improve their swimming performance if their kinematics and paths are adequately adapted to the incoming flow. When the optimum path is unknown, the first step is to accurately predict the performance that could be obtained for several route candidates. Next, we can select the trajectory that increases the thrust or efficiency. In this study, we develop a sparse database of cases where a flapping foil executes transition motions inside an unsteady incoming flow. The delay component with respect to the upstream wake governs the course of the foil. Next, we use a Koopman operator-based approach to obtain several predictive models to estimate the performance of the foil for a given candidate path. We found that the predictive error is reduced to less than 20% for all test cases only after incorporating a wake model that calculates the relative location between the foil and the incoming upstream vortices. The wake model-based predictive tool will be used to develop a new transition approach to position the foil on the optimum route to maximise its performance. |
Sunday, November 20, 2022 6:32PM - 6:45PM |
J03.00010: Fluid-structure interaction of bat-inspired membrane wings Sushrut Kumar, Jung-Hee Seo, Dimitri A Skandalis, Cynthia F Moss, Rajat Mittal Bat flight involves some of the most complex wing kinematics in nature. These kinematics enable the aerial agility needed to navigate through tight spaces and complex environments. The bat wing is formed from a membrane reinforced by elastic and inelastic elements and deforms under muscular, inertial, and aerodynamic loads, which has substantial implications for the development of aerodynamic forces during the flapping cycle. In this work, we study the aerodynamics of these flexible wings by using a fluid-structure interaction method, with kinematics based on experimental measurements. To fully understand the lift generating mechanism of these complex flapping wings, it is imperative to understand the role of vortices, viscous effects, and kinematics of bat wings. This is accomplished using the force partitioning method (FPM) which allows us to decompose the total lift into the previously defined mechanisms. |
Sunday, November 20, 2022 6:45PM - 6:58PM |
J03.00011: Thermal convection analysis of the low Reynolds number flow over an airfoil with simultaneous pitching and plunging motions. José G Montiel Galindo, Rubén Ávila Rodríguez In the literature, several studies have been carried out considering only the hydrodynamic characteristics of pitching and plunging motions. However, the influence of the thermal convection on the aerodynamic performance has not been widely explored. In this study the laminar flow over a NACA0012 airfoil with simultaneous pitching and plunging motions with thermal convection is presented. The non-dimensional, non-steady, two dimensional, Navier-Stokes equations coupled with the energy equation, considering the Boussinesq approximation, are numerically solved by using the spectral element method. The Reynolds number varies from 100 to 1000, while the Rayleigh number is in the range of 1000 to 5x106. For the pitching motion, the amplitude is between 2° and 10°, and the non dimensional reduced frequency varies from 2 to 9. For the plunging motion the non dimensional amplitude varies from 0.0125 to 0.045, and the reduced frequency range is the same as for the pitching motion. It is found that an increase of the Rayleigh number has an adverse effect on the aerodynamic performance. A three dimensional map involving the Reynolds and Rayleigh numbers and the reduced frequency shows the behaviour of the aerodynamic performance and the heat transfer rate. |
Sunday, November 20, 2022 6:58PM - 7:11PM |
J03.00012: Mitigation of Thrust Deterioration at High Flapping Frequencies of a Two-dimensional Elliptic Flapping Airfoil Using Asymmetric Flapping Strokes in the Forward Flight Jit Sinha, Sohan Roy, Sunil Manohar Dash In this study, the deteriorated time-averaged thrust performance of a two-dimensional flapping elliptic airfoil as seen in the forward flight at high flapping frequencies (or beyond critical Strouhal number, Stcr) is mitigated by adopting asymmetric St for the downstroke and upstroke of a flapping cycle. Here, three different pivot locations, three effective angle of attack amplitudes, and St ≥ Stcr are considered for investigation, keeping the Reynolds number fixed as 5000. Furthermore, we have selected the St for the downstroke with a value more than Stcr, but the upstroke St = Stcr. Note that the observations are periodic and reciprocate suitably when the stroke St selection is reversed in the flapping cycle. It is found that the asymmetric St configurations of the flapping strokes yield more time-averaged thrust and non-zero lift than the symmetric one at St ≥ Stcr. In addition, we noticed that the pivot point location near the airfoil leading edge produces a higher lift than the cases where the pivot point is at the center or near the trailing edge. We have further analyzed the transient thrust, lift force profiles, and near wake flow structures to understand better this improved aerodynamic performance with asymmetric St, which will be discussed in the presentation. |
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