Shi Long Lan, Mao Sun
Aerodynamic force and flow structures of two airfoils in tandem configuration performing flapping motions are studied, using the method of solving the Navier-Stokes equations in moving overset grids. Three typical phase differences between the fore- and aft-airfoil flapping cycles are considered.
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Enhancing Thrust in Flapping Airfoils Through Wake Interactions with Oscillating Cylinder
该研究探究振荡圆柱尾流对扑翼翼型推力性能的影响，发现置于该尾流中的翼型推力更高，且翼型斯特劳哈尔数与俯仰振幅直接相关，为优化扑翼翼型效率提供了新思路。
The unsteady aerodynamic thrust and aeroelastic response of a two-dimensional membrane airfoil under prescribed harmonic motion are investigated computationally with a high-order Navier-Stokes solver coupled to a nonlinear membrane structural model. The effects of membrane prestress and elasticity are examined parametrically for selected plunge and pitch-plunge motions at a chord-based Reynolds number of 2500. The importance of inertial membrane loads resulting from the prescribed flapping is also assessed for pure plunging motions. This study compares the period-averaged aerodynamic loads of …
Saving energy and enhancing performance are secular preoccupations shared by both nature and human beings. In animal locomotion, flapping flyers or swimmers rely on the flexibility of their wings or body to passively increase their efficiency using an appropriate cycle of storing and releasing elastic energy. Despite the convergence of many observations pointing out this feature, the underlying mechanisms explaining how the elastic nature of the wings is related to propulsive efficiency remain unclear. Here we use an experiment with a self-propelled simplified insect model allowing to show how…
The source of the hummingbirds distinctive hum is not well understood, but there are clues to its origin in the acoustic nearfield and farfield. To unravel this mystery, we recorded the acoustic nearfield generated by six freely hovering Annas hummingbirds using a 2176 microphone array. We also directly measured the 3D aerodynamic forces generated by the hummingbird in vivo using a new aerodynamic force platform. To determine the degree to which the aerodynamic forces cause the hum, we developed a simple first-principles model to predict the acoustic field radiated by the 3D oscillating forces…
A Robust Data-Driven Model for Flapping Aerodynamics under different hovering kinematics
本研究针对扑翼微型飞行器瞬时气动性能预测难题，提出结合岭回归与高斯过程回归的两阶段数据驱动准稳态降阶模型，在多运动学场景下实现了高精度气动预测并可估计模型不确定性。
How do flapping avian wings exhibit superior aerodynamic performance?
本研究采用自主求解器VorteXdyn，以隼启发的翼模型探究不同雷诺、斯特劳哈尔数下，翼几何保真度对拍动翼空气动力学性能与涡演化的影响，为仿生扑翼飞行器设计提供参考。
The large active wing deformation is a significant way to generate high aerodynamic forces required in bat flapping flight. Besides the twisting, the elementary morphing models of a bat wing are proposed, such as wing-bending in the spanwise direction, wing-cambering in the chordwise direction, and wing area-changing. A plate of aspect ratio 3 is used to model a bat wing and a three dimensional unsteady panel method is applied to predict the aerodynamic forces. It is found that the cambering model has a great positive influence on the lift, followed by area-changing model and then the bending …
Flying animals resort to fast, large-degree-of-freedom motion of flapping wings, a key feature that distinguishes them from rotary or fixed-winged robotic fliers with limited motion of aerodynamic surfaces. However, flapping-wing aerodynamics are characterised by highly unsteady and three-dimensional flows difficult to model or control, and accurate aerodynamic force predictions often rely on expensive computational or experimental methods. Here, we developed a computationally efficient and data-driven state-space model to dynamically map wing kinematics to aerodynamic forces/moments. This mod…
Data-Driven Discovery and Formulation Refines the Quasi-Steady Model of Flapping-Wing Aerodynamics
本研究通过数据驱动方法从5000个候选函数中识别并公式化三个被忽视的机制，显著提升了准定常扑翼气动模型在鹰蛾和果蝇飞行中的预测精度，为昆虫飞行理解和仿生机器人开发提供快速分析工具。
Effects of a gurney flap were numerically investigated on the supercritical NASA airfoil by solving the two-dimensional Reynolds-averaged Navier-Stokes equations for a range of transonic Mach numbers and angles of attack, using turbulence compressible KW SST model. The height of the gurney flap was selected to be 1.65 percent chord length. A high-resolution mesh was applied to accurately predict the flow field specifically in the vicinity of the airfoil. Below the drag divergence Mach number, the gurney flap has a remarkable influence on the aerodynamic coefficients especially at -1 and 0 degr…
While dragonflies are highly agile flyers, some key aerodynamic mechanisms responsible for their flight performance remain inadequately understood. Based on forward flight conditions, we investigate dragonfliess spanwise aerodynamic behaviors associated with flapping wings phasing relationship. Overall, the leading edge vortex (LEV) on the forewing forms without the influence of the hindwing. For hindwing, the wing root region prominently displays a trailing edge vortex (TEV). In the inner span region, the vortical flow structures around the hindwing is influenced by the forewings LEV when bot…
On the unsteady aerodynamics of flapping wings under dynamic hovering kinematics
本文采用重叠网格与LES模拟Re~O(10^3)下刚性半椭圆翼的激进悬停扑动，揭示附加质量、尾流作用与LEV在不同阶段产生升力峰值的机制，并用扩展POD分离主导流动结构。
Airfoil2Vec: Spectral Geometry-Conditioned Neural Surrogate Models for Airfoil Aerodynamics and a Downforce-Generating CFD Dataset
本文提出Airfoil2Vec，一种结合谱几何编码的神经代理模型，用于快速预测翼型流场，并在新数据集上验证了其高效性与泛化能力。
The unsteady characteristics of the flow over thick flatback airfoils have been investigated by means of CFD calculations. Sandia airfoils which have 35% maximum thickness with three different trailing edge thicknesses were selected. The calculations provided good results compared with available experimental data with regard to the lift curve and the impact of trailing edge thickness. Unsteady CFD simulations revealed that the Strouhal number is found to be independent of the lift coefficient before stall and increases with the trailing edge. The present work shows the dependency of the Strouh…
Aerodynamic performance and flow mechanism of 3D flapping wing using discrete vortex method
本文采用离散涡方法模拟三维矩形扑翼，比较刚性、弯曲、扭转及弯扭耦合机翼的升推力、推进效率与涡动机理，并分析运动学参数影响。
Morphing wings comprised of fixed leading sections with piezocomposite trailing control surfaces have emerged as a novel active control technique for unmanned aerial vehicles. However, the wake dynamics and aerodynamic performance of such hybrid airfoil configuration has not been thoroughly investigated. In this paper, direct numerical simulations of two-dimensional flows over hybrid airfoils comprised of NACA 0012 leading sections with piezocomposite trailing control surfaces are performed at a fixed Reynolds number of 1000. The effects of length and camber of the trailing control surface on …
Efficient aerodynamic coefficients prediction with a long sequence neural network
本文提出AirfoilNet神经网络，通过融合数学计算与神经网络实现端到端空气动力学系数预测，结合GRUNet和ResMLP预测头支持迭代优化，在精度、泛化性和推理速度上表现优异。
Unsteady airfoil aerodynamics in attached flow: From unsteady thin airfoil theory to wind turbine application
该研究针对风力机附着流非定常空气动力学，将其转化为BEM、LL、AL方法的气动力系数，经案例验证可提升计算精度，明确了关键建模选择与误差来源。
Wing flexibility plays an essential role in the aerodynamic performance of insects due to the considerable deformation of their wings during flight under the impact of inertial and aerodynamic forces. These forces come from the complex wing kinematics of insects. In this study, both wing structural dynamics and flapping wing motion are taken into account to investigate the effect of wing deformation on the aerodynamic efficiency of a bumblebee in tethered flight. A fluid-structure interaction solver, coupling a mass-spring model for the flexible wing with a pseudo-spectral code solving the inc…
Biological flapping wing fliers operate efficiently and robustly in a wide range of flight conditions and are a great source of inspiration to engineers. The unsteady aerodynamics of flapping-wings are dominated by large-scale vortical structures that augment the aerodynamic performance but are sensitive to minor changes in the wing actuation. We experimentally optimise the pitch angle kinematics of a flapping wing system in hover to maximise the stroke average lift and hovering efficiency using a evolutionary algorithm and in-situ force and torque measurements at the wing root. Additional flo…
The influence that the kinematics of pitching and heaving 2D airfoils have on the aerodynamic forces is investigated using Direct Numerical Simulations and a force decomposition algorithm. Large amplitude motions are considered (of the order of one chord), with moderate Reynolds numbers and reduced frequencies of order 1, varying the mean pitch angle and the phase shift between the pitching and heaving motions. Our results show that the surface vorticity contribution (viscous effects) to the aerodynamic force is negligible compared to the contributions from the body motion (fluid inertia) and …
Comparing design and off-design aerodynamic performance of a natural laminar airfoil
本文通过数值模拟对比SHM1自然层流翼型在亚声速与跨声速设计及非设计工况下的气动性能，分析激波、分离和载荷分布，为分离与激波控制优化提供基准。
Unsteady inviscid flow models of wings and airfoils have been developed to study the aerodynamics of natural and man-made flyers. Vortex methods have been extensively applied to reduce the dimensionality of these aerodynamic models, based on the proper estimation of the strength and distribution of the vortices in the wake. In such modeling approaches, one of the most fundamental questions is how the vortex sheets are generated and released from sharp edges. To determine the formation of the trailing-edge vortex sheet, the classical Kutta condition can be extended to unsteady situations by rea…
For present study, setting Strouhal Number (St) as control parameter, numerical simulations for flow past oscillating NACA-0012 airfoil at 1,000 Reynolds Numbers (Re) are performed. Temporal profiles of unsteady forces; lift and thrust, and their spectral analysis clearly indicate the solution to be a period-1 attractor for low Strouhal numbers. This study reveals that aerodynamic forces produced by plunging airfoil are independent of initial kinematic conditions of airfoil that proves the existence of limit cycle. Frequencies present in the oscillating lift force are composed of fundamental (…
In nature, phase transitions prevail amongst inherently different systems, while frequently showing a universal behavior at their critical point. As a fundamental phenomenon of fluid mechanics, recent studies suggested laminar-turbulent transition belonging to the universality class of directed percolation. Beyond, no indication was yet found that directed percolation is encountered in technical relevant fluid mechanics. Here, we present first evidence that the onset of a laminar separation bubble on an airfoil can be well characterized employing the directed percolation model on high fidelity…
High-fidelity wall-resolved large-eddy simulations (LES) are utilized to investigate the flow-physics of small-amplitude pitch oscillations of an airfoil at Re = 100,000. The investigation of the unsteady phenomenon is done in the context of natural laminar flow airfoils, which can display sensitive dependence of the aerodynamic forces on the angle of attack in certain "off-design" conditions. The dynamic range of the pitch oscillations is chosen to be in this sensitive region. Large variations of the transition point on the suction-side of the airfoil are observed throughout the pitch cycle r…
Flow control for turbulent skin-friction drag reduction is applied to a transonic airfoil to improve its aerodynamic performance. The study is based on direct numerical simulations (with up to 1.8 billions cells) of the compressible turbulent flow around a supercritical airfoil, at Reynolds and Mach numbers of $Re_\infty= 3 \times 10^5$ and $M_\infty =0.7$. Control via spanwise forcing is applied over a fraction of the suction side of the airfoil. Besides locally reducing friction, the control modifies the shock wave and significantly improves the aerodynamic efficiency of the airfoil by incre…
Quasi-steady aerodynamics predicts the dynamics of flapping locomotion
本文通过准稳气动力学模型揭示振翅运动中升力与阻力的变化规律，展示了其在不同雷诺数和攻角下的应用范围和物理意义。
The aerial environment in the operating domain of small-scale natural and artificial flapping wing fliers is highly complex, unsteady and generally turbulent. Considering flapping flight in an unsteady wind environment with a periodically varying lateral velocity component, we show that body rotations experienced by flapping wing fliers result in the reorientation of the aerodynamic force vector that can render a substantial cumulative deficit in the vertical force. We derive quantitative estimates of the body roll amplitude and the related energetic requirements to maintain the weight support…
The aerodynamic performance of a transonic airfoil with spanwise forcing
本文通过直接数值模拟研究展向行波力对跨声速翼型激波与边界层相互作用的控制效果，发现合适的控制参数可推迟激波、形成与摩擦减阻相关的分离区，从而提高气动效率。
A quantitative analysis of leading edge vortices in flapping wing aerodynamics
本研究提出一种定量描述扑翼前缘涡的方法，结合Q准则、细化算法等识别涡结构与核心并计算流动量，应用于前飞扑翼数据，揭示LEV发展分裂规律及与截面升力的关联。
NeuralFoil: An Airfoil Aerodynamics Analysis Tool Using Physics-Informed Machine Learning
本研究提出开源工具NeuralFoil，融合物理信息机器学习与解析模型，在保证精度的前提下实现远快于XFoil的翼型空气动力学分析，支持梯度优化与不确定性量化，可快速生成接近专家设计水平的翼型。
Figuring out the right airfoil is a crucial step in the preliminary stage of any aerial vehicle design, as its shape directly affects the overall aerodynamic characteristics of the aircraft or rotorcraft. Besides being a measure of performance, the aerodynamic coefficients are used to design additional subsystems such as a flight control system, or predict complex dynamic phenomena such as aeroelastic instability. The coefficients in question can either be obtained experimentally through wind tunnel testing or, depending upon the accuracy requirements, by numerically simulating the underlying …
The generation of uniform, periodic gust disturbances in an experimental context is demonstrated using a single oscillating airfoil. A pitching and heaving symmetric airfoil is suggested as a simpler alternative to existing gust-generation methods. The Theodorsen theory of unsteady aerodynamics is used as an analytical tool to dictate the kinematics necessary to produce well-defined sinusoidal gusts downstream of the airfoil. These analytic predictions improve the symmetry of fluctuations in the vertical velocity induced by the airfoil, as well as minimize the influence of vorticity shed by th…
A comprehensive and detailed overview of the flow topology over a cambered NACA 65(1)-412 airfoil at Re = 20,000 is presented for angles of attack ranging from 0° to 10° using high-order direct numerical simulations. It is shown that instabilities bifurcate the flow and cause it to change at a critical angle of attack from laminar separation without reattachment over a laminar separation bubble at the trailing edge to a bubble at the leading edge. The transition of the flow regimes is governed by the Karman vortex shedding of the pressure side boundary layer at the trailing edge, Kelvin-Helmho…
A convolutional autoencoder is trained using a database of airfoil aerodynamic simulations and assessed in terms of overall accuracy and interpretability. The goal is to predict the stall and to investigate the ability of the autoencoder to distinguish between the linear and non-linear response of the airfoil pressure distribution to changes in the angle of attack. After a sensitivity analysis on the learning infrastructure, we investigate the latent space identified by the autoencoder targeting extreme compression rates, i.e. very low-dimensional reconstructions. We also propose a strategy to…
Rotor blades of wind turbines in the atmospheric boundary layer regularly experience the aerodynamic phenomenon of dynamic stall consisting of a temporary overshoot of lift and detrimental fatigue loads. Particularly the formation of dynamic stall under three-dimensional inflow conditions raises open questions. Aerodynamic behavior of a DU 91-W2-250 wind profile undergoing light dynamic stall is thus analyzed in a wind tunnel. Effects of a gust with streamwise and spanwise periodic variation are investigated by comparing total and local lift generation with flow formation above the airfoil. Th…
Spanwise Control Authority of Synthetic Jets on a Stalled Airfoil
研究低频和高频合成喷嘴控制策略对NACA 0025翼型气动效应的影响，分析流场稳定性及展向控制能力变化。
Scaling laws for the sound generation of bio-inspired flapping wings
针对受生物启发扑翼的发声标度律研究空白，基于势流理论与Ffowcs Williams-Hawkings声学类比提出悬停扑翼发声标度律，经数值模拟验证吻合良好，可助力微型飞行器的气动与声学协同设计。
Airfoils pitching in the stalled regime have been of keen interest in recent years due to their desirable aerodynamic force characteristics. In this study, we are numerically investigating the unsteady flow past a NACA 0012 airfoil under sinusoidally pitching motion, using a finite volume based sharp interface immersed boundary solver. The flow is investigated in the low Reynolds number regime (Re=3000) for reduced frequencies of 0.1 and 0.5, at three different pivot locations (c/3, c/2 and 2c/3 from the leading edge). The airfoil is subjected to sinusoidal oscillations with its incidence angl…
In this paper, the problem of compressible flow over a thin airfoil located near the ground is studied. A singular integral equation, also known as Possio equation, that relates the pressure jump along the airfoil to its downwash is derived. The derivation of the equation utilizes Laplace transform, Fourier transform, method of images, and theory of Mikhlin multipliers. The existence and uniqueness of solution to the Possio equation is verified for the steady state case and an approximate solution is obtained. The aerodynamic loads are then calculated based on the approximate solution. Moreove…
Numerical Simulation of Wavy-Flap Airfoil Performance at Low Reynolds Number: Insights from Lift and Drag Coefficient Analysis
本研究通过数值模拟分析了低雷诺数下波浪翼型的气动性能，发现其在较大尾部攻角和低攻角下能显著提升升力系数和升阻比，从而提高航空与海洋应用中的气动和燃油效率。
Large-Scale Horizontal Axis Wind Turbine Aerodynamic Efficiency Optimization using Active Flow Control and Synthetic Jets
本研究通过CFD分析DTU-10MW叶片翼型的边界层分离特性，并采用遗传算法优化主动流动控制（AFC）与合成射流参数实现边界层重新附着，使各翼型获得23至36KW的净功率增益，验证了该技术提升大型风力涡轮机性能的有效性。
The effect of air viscosity on the flow around an insect wing increases as insect size decreases. For the smallest insects (wing length R below 1 mm), the viscous effect is so large that lift-generation mechanisms used by their larger counterparts become ineffective. How the weight-supporting vertical force is generated is unknown. To elucidate the aerodynamic mechanisms responsible, we measure the wing kinematics of the tiny wasp Encarsia formosa (0.6 mm R) in hovering or very slow ascending flight and compute and analyze the aerodynamic forces. We find that the insects perform two unusual wi…
The complex wake modifications produced by a Gurney flap on symmetric NACA airfoils at low Reynolds number are investigated. Two-dimensional incompressible flows over NACA 0000 (flat plate), 0006, 0012 and 0018 airfoils at a Reynolds number of $Re = 1000$ are analyzed numerically to examine the flow modifications generated by the flaps for achieving lift enhancement. While high lift can be attained by the Gurney flap on airfoils at high angles of attack, highly unsteady nature of the aerodynamic forces are also observed. Analysis of the wake structures along with the lift spectra reveals four …
Flapping wings enable flying animals and biomimetic robots to generate elevated aerodynamic forces. Measurements that demonstrate this capability are based on tethered experiments with robots and animals, and indirect force calculations based on measured kinematics or airflow during free flight. Remarkably, there exists no method to measure these forces directly during free flight. Such in vivo recordings in freely behaving animals are essential to better understand the precise aerodynamic function of their flapping wings, in particular during the downstroke versus upstroke. Here we demonstrat…
Rapid aerodynamic prediction of swept wings via physics-embedded transfer learning
提出一种物理嵌入的迁移学习框架，利用二维翼型流场预测三维机翼流场，结合后掠理论显著降低误差并减少训练样本需求。
The formation and accretion of ice on the leading edge of an airfoil can be detrimental to aerodynamic performance. Furthermore, the geometric shape of leading edge ice profiles can vary significantly depending on a wide range of physical parameters, which can translate into a wide variability in aerodynamic performance. The purpose of this work is to explore the variability in airfoil aerodynamic performance that results from variability in leading edge ice shape profile. First, we demonstrate how to identify a low-dimensional set of parameters that governs ice shape from a database of ice sh…
Spatio-Temporal Prediction of Unsteady Airfoil Aerodynamics Using Augmented Graph Neural Ordinary Differential Equations with Exogenous Controls
研究非定常翼型空气动力学的时空预测问题，核心方法是将图神经网络与增强神经常微分方程结合形成GNODE，主要贡献是该方法能稳定准确地预测表面力，适合建模含外部输入的非线性时空系统。
This paper investigates stability analysis of flapping flight. Due to time-varying aerodynamic forces, such systems do not display fixed points of equilibrium. The problem is therefore approached via a limit cycle analysis based on Floquet theory. Stability is assessed from the eigenvalues of the Jacobian matrix associated to the limit cycle, also known as the Floquet multipliers. We developed this framework to analyze the flapping flight equations of motion of a bird in the longitudinal plane. Such a system is known to be not only non-linear and time-dependent, but also driven by state-depend…
We report direct numerical simulations of a pair of wings in horizontal tandem configuration, to analyze the effect of their aspect ratio on the flow and the aerodynamic performance of the system. The wings are immersed in a uniform free-stream at Reynolds number Re = 1000, and they undergo heaving and pitching oscillation with Strouhal number St = 0.7. The aspect ratios of forewing and hindwing vary between 2 and 4. The aerodynamic performance of the system is dictated by the interaction between the trailing edge vortex (TEV) shed by the forewing and the induced leading edge vortex formed on …
A minimal wake-vortex model explains formation flight of flapping birds
本文提出一种简化模型，解释了扑翼鸟类编队飞行中涡流相互作用的空气动力学机制，揭示了编队飞行能减少11%的机械功率。
The present paper numerically investigates flow control over NACA0021 (National Advisory Committee for Aeronautics) airfoil by applying a leading-edge tubercle as a passive flow control device at transitional Reynolds number (Re) regime. The study includes simulations over the unmodified airfoil and modified airfoil operating at Re=120000 using kT-kL-ω based Delayed Detached Eddy Simulation (DDES) model. The calculations include both the pre-stall and post-stall regions, while the performance of the mean flow aerodynamic properties like pressure, lift, skin-friction, and drag coefficients are …
Nature-inspired three-dimensional surface serration topologies enable silent flight by suppressing airfoil-turbulence interaction noise
融合猫头鹰羽毛与蝉翼结构提出三维表面锯齿拓扑，通过增强涡量、抑制偶极子和四极子声源，实现翼型降噪与推进效率同步提升。
Wake transition and aerodynamics of a dragonfly-inspired airfoil
本研究在二维$α-Re$参数空间中，结合线性稳定性分析与三维非线性直接数值模拟，揭示仿蜻蜓翼型的流场转捩机制、稳定性特性及空气动力学性能与参数的关联。
An approximation model based on convolutional neural networks (CNNs) is proposed for flow field predictions. The CNN is used to predict the velocity and pressure field in unseen flow conditions and geometries given the pixelated shape of the object. In particular, we consider Reynolds Averaged Navier-Stokes (RANS) flow solutions over airfoil shapes. The CNN can automatically detect essential features with minimal human supervision and shown to effectively estimate the velocity and pressure field orders of magnitude faster than the RANS solver, making it possible to study the impact of the airf…
A Method for Evaluation of Aerodynamic Lift and Drag Based on Statistical Mechanics
本文提出基于统计力学的“体积元”方法，用于数值评估空气动力升力与阻力，适用于凸形翼型，结合蒙特卡洛模拟可处理凹形翼型，并拓展至量子气体应用。
Aerodynamic ground effect in flapping-wing insect flight is of importance to comparative morphologies and of interest to the micro-air-vehicle (MAV) community. Recent studies, however, show apparently contradictory results of either some significant extra lift or power savings, or zero ground effect. Here we present a numerical study of fruitfly sized insect takeoff with a specific focus on the significance of leg thrust and wing kinematics. Flapping-wing takeoff is studied using numerical modelling and high performance computing. The aerodynamic forces are calculated using a three-dimensional…
The Harmonic Pitching NACA 0018 Airfoil in Low Reynolds Number Flow
本研究探究低雷诺数下对称NACA 0018翼型谐波俯仰运动的气动性能，通过Transition SST等湍流模型模拟不同频率、振幅及稳态工况，发现该模型在特定场景的优势，为厚翼型气动行为及湍流建模提供见解。
A vortex-lattice method for wing aerodynamics that uses nonlinear airfoil data is presented. Two applications of this procedure are presented: Direct Design of a Flying Wing and Inverse Identification from wind tunnel measurements with low-aspect ratio wings. A Newton method is employed, which not only allows very fast solutions to the nonlinear equations but enables the calculation of static and dynamic stability and control derivatives without further cost.
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