Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session S02: Aerodynamics: Fixed, Flapping and Rotating Wings (5:45pm  6:30pm CST)Interactive On Demand

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S02.00001: Aerodynamics of Highly Cambered Circular Arcs With a Sharp Leading Edge at Low Reynolds Numbers JeanBaptiste Souppez, Ignazio Maria Viola The flow around cambered circular arcs with a sharp leading edge is a paradigm that underpins a vast array of cambered thin wings. Yet, some key features of this flow condition, such as the impact of the leadingedge bubble on the boundary layer regime and trailingedge separation, remain to be fully characterized. Here, particle image velocimetry was employed to portray the flow field around such geometries at incidences beyond the ideal angle of attack. The study revealed a combination of a critical Reynolds number and a critical angle of attack to trigger transition and to delay trailing edge separation. However, for the range of Reynolds number tested, from 54k to 220k, the leadingedge bubble was always turbulent. In fact, in the subcritical regime, relaminarisation occurred at the reattachment point. Conversely, in the post critical regime, the reattached boundary layer was turbulent all the way to trailingedge separation. These findings reveal the critical effect of the leadingedge flow on the global flow field and associated forces experienced by thin, highly chambered wings with leadingedge separation. These results may further contribute to applications ranging from downwind yacht sails to micro aerial vehicles. [Preview Abstract] 

S02.00002: The Forward Fin  First Results M G Mungal, Tioga Benner Motivated by the surprising existence of forward facing fins in certain tropical fish, this study reports on a combined numerical and experimental investigation of lowaspectratio forward vs. backward facing fins from low to high angles of attack, at moderate Reynolds numbers. Several cases are investigated using the STARCCM$+$ code with a few select cases investigated in a wind tunnel. The experimental lift and drag measurements and surface flow visualizations support the flow dynamics found in the numerical simulations which show, as expected, a complex flow with positive and negative interacting shed vortices. The forward fin, either in single or tandem configurations, produces a smoother lift curve with angle of attack (``rolling stall'') while the backward configuration exhibits the lift decrease associated with classical stall (``sudden stall''). Straight fins show a ``twotime stall'' behavior previously reported by others. The forward fin shows less drag relative to the backward fin for angles of attack less than 20 degrees while the reverse is true at larger angles up to 60 degrees. The dynamics of a developing stall bubble rationalizes the results which are reported for both pointed and rounded fins. [Preview Abstract] 

S02.00003: Aspect ratio effects on the aerodynamic performance of flapping wings in tandem configuration Rafael Jurado, Gonzalo Arranz, Oscar Flores, Manuel GarcíaVillalba Dragonflies are frequently used as bioinspired models for micro air vehicles due to its great manoeuvrability. However, the complex mechanisms underlying the wingwing interaction are not properly understood yet. In this work, direct numerical simulations have been used to study the aerodynamic performance of a pair of heavingpitching wings in horizontal tandem configuration in forward flight. The kinematics of the wings are chosen from a 2D configuration optimised to maximize the propulsive efficiency. The Reynolds number based on the freestream velocity and chord of the wing is $Re = U c / \nu = 1000$. The influence of the aspect ratio (AR) of the hindwing on the aerodynamic performance of the system is analysed, considering hind wings with $AR = 4, 3$ and $2$, keeping $AR = 4$ for the fore wing. Results show that the propulsive efficiency is similar for all studied cases as the force coefficients are barely affected when the hindwing aspect ratio is changed. Decreasing the hindwing aspect ratio helps to avoid the forewing tip vortex but also increases the finite wing effects. Funding: Spanish Ministry of Economy and Competitiveness (DPI201676151C22R) and Red Espa\~nola de Supercomputaci\'on (IM201930011). [Preview Abstract] 

S02.00004: Analysis of the Clapping Effect in Bioinspired Flapping Wing Robots Dipan Deb, Miquel Balta Manich, Haithem E. Taha The goal of this research is to investigate different designs of the bioinspired flapping wing robot in terms of thrust and the power consumption. Initially two kinds of mechanical birds were investigated, one with two wings and another with four. It has been observed that for same power consumption the four wings bird is generating more thrust. This phenomenon has been observed for a wide range of flapping frequencies (422Hz. To check whether clapping has any effect, two new birds with four wings were designed: one that clapped partially, and another that was made with a separator in between the wings; which prevents the wings from clapping. When these new models were tested, it has been observed that they generate lesser Thrust than the full clap bird. Therefore, the phenomenon of clapping is changing the nature of the flow in such a way that the bird is getting some extra thrust. To understand the flow physics, a smoke flowvisualization setup has been made to investigate the flow field around the flapping wing qualitatively. The flow visualization was done different sections of the wing to capture the 3D nature of the flow field. Presently, load cell data have been acquired to measure the thrust and lift time history over the flapping cycle. [Preview Abstract] 

S02.00005: Effect of Turbulence Modeling on Dynamic Stall Prediction Nabil Khalifa, Amir Rezaei, Haithem Taha A harmonically pitching NACA 0012 airfoil is studied computationally to investigate the three dimensional nature of Dynamic Stall phenomenon and the effect of turbulence modeling on its prediction. In this study, we performed 2D and 3D Computational simulations of NavierStokes equations using two different turbulence modeling techniques: Unsteady Reynolds Averaged Navier Stokes (URANS) and Detached Eddy Simulation (DES) defined as a RANSLarge Eddy Simulation (LES) hybrid model. The study was performed at a Reynolds number of $1.35\times 10^5$ to facilitate comparison with experimental results available in literature. The $k\omega$ Shear Stress Transport ($SST$) model was used for turbulence modeling in both URANS and DES simulations. Results show that all the models agreed with experimental data during upstroke. However, only 3D DES was able to capture the $C_{L}$ peak value. In downstroke, 3D URANS show better agreement than 2D URANS with experimental data, while 3D DES surpasses the URANS models significantly, especially at the beginning of downstroke. This study concludes that 3D computational setups are required for proper simulation of the Dynamic Stall phenomenon, which accentuates its 3D nature. [Preview Abstract] 

S02.00006: A Deep Neural Network (DNN) Model for Predicting the Propulsive Performances of 3D Printed Tandem Flapping Wings with Stroke TimeAsymmetry Guangjian Wang, Zhen Wei Teo, Bing Feng Ng The applications of tandem flapping wings on the design of micro aerial vehicles are inspired by birds and flying insects, which normally operate their wings in timeasymmetric strokes. Previous numerical and experimental studies have demonstrated that such stroke timeasymmetry could augment the aerodynamic forces. However, the computational overheads are prohibitively high, while experiments are restricted by lab settings. Consequently, only a few cases were investigated, leaving the stroke timeasymmetry not well understood. To efficiently calculate the wing thrust forces with different asymmetry ratios ($\varepsilon )$, a deep neural network (DNN) model was trained using validated numerical data. Specifically, the simulation results with various $\varepsilon $ were verified by experiments and collected as the training data. To improve the practicality, the inputs were limited to the wing kinematics and phase angles, while the outputs were the temporal distributions of the aerodynamic forces and the flow velocities. Subsequently, the wing performances with different $\varepsilon $ can be evaluated by the DNN model. Variations of thrust coefficients and propulsive efficiencies against $\varepsilon $ were used to optimize the wing performances. [Preview Abstract] 

S02.00007: Effect of Phase Difference on the Aperiodic Transition in the FlowField of a PitchingPlunging Foil Dipanjan Majumdar, Chandan Bose, Sunetra Sarkar The present work explores the role of phasedifference on the transitional flow dynamics of a pitchingplunging foil. An extensive parameter space of plunge amplitude ($h$) and phase offset ($\phi$) between pitchplunge motions is considered keeping the pitch amplitude and nondimensional flapping frequency constant ($\alpha = 15^o\ \&\ k=4$). Numerical simulations are performed at a low Reynolds number ($Re = 300$) using an Immersed Boundary Method based inhouse NavierStokes solver. The phase offset is found to be a crucial parameter in determining the onset of the aperiodic transition. In the range of $10\pi/8\le\phi\le14\pi/8$, the flowfield and, therefore, the aerodynamic loads remain periodic even for $h$ values as high as $h=0.475$. Significant enhancement in thrust generation is also observed in this range of $\phi$, at all $h$ values. On the other hand, the flowfield turns aperiodic even at lower $h$ values (quasiperiodic at $h=0.25$ and chaotic at $h=0.375$) approximately in the range of $\pi/8\le\phi\le9\pi/8$. A novel scaling relation is achieved in terms of the effective angleofattack and the Strouhal number (based on the peaktopeak amplitude of the leading edge), which differentiates the distinct dynamical regimes in the parameter space in a robust manner. [Preview Abstract] 

S02.00008: Adaptive numerical simulations of insect flight to study the impact of wing damage Thomas Engels, Henja Wehmann, Fritz Lehmann, Kai Schneider A waveletbased adaptive approach to compute the aerodynamics of flapping insects is presented. Dynamically evolving grids using regular Cartesian blocks allow significant reduction of memory and CPU time requirements while monitoring the precision of the computation. Distributing the blocks among MPI processes permits an efficient parallelization on large scale supercomputers. The numerical approximation uses artificial compressibility to avoid solving elliptic problems and volume penalization is applied to impose boundary conditions on the Cartesian grids. A centered 4th order finite difference discretization is combined with biorthogonal interpolating wavelets as grid refinement indicators. Validation cases are presented to assess the accuracy and performance of the open access code WABBIT on massively parallel computer architectures (arXiv:1912.05371). Computations using realistic fly bodies obtained from mircoCT scans are presented and the impact of wing damage in flapping flight is investigated. [Preview Abstract] 

S02.00009: Transitional Wake Dynamics of a LowAspectRatio FiniteSpan Flapping wing Sunetra Sarkar, CHANDAN BOSE, Sayan Gupta This study investigates the threedimensional flow dynamics of a lowaspectratio flapping wing at a Reynolds number of 250. A discrete immersed boundary method is employed to solve the threedimensional NavierStokes equation. Kinematic parameters, particularly the amplitude and frequency of the flapping motion, are found to have significant effects on the flowfield transition. Even though the threedimensional wake is seen to be more stable as compared to the twodimensional cases, an aperiodic transition is observed beyond a considerably high value of dynamic plunge velocity (\textit{kh}). The periodic bifurcated wake is seen to transition to a very complex aperiodic wake as \textit{kh} is increased to a significantly high value, marking that the aperiodic transition in the wake of a flapping wing is indeed a physical phenomenon. The critical value of \textit{kh} beyond which the aperiodic transition takes place is seen to be much higher than the 2D case. The qualitative nature of the dynamical transitions is seen to be very similar in 2D and 3D with different bifurcation boundaries. The underlying complex interactions among leadingedge, trailingedge and tip vortices generated in each flapping cycle and their role behind the loss of periodicity are investigated in detail. [Preview Abstract] 

S02.00010: Wind Tunnel Measurements of the PrandtlD Research Aircraft in Preparation for a Stereoscopic Particle Image Velocimetry Flow Survey Bradley Zelenka, Xiaofeng Liu, Erik Olson We will present experimental measurements of the PrandtlD, a flying wing type of glider whose wings were designed using Prandtl's minimum induced drag with the bending moment as the design constraint. The PrandtlD exhibits several novel aerodynamic characteristics, including the design's ability to make coordinated turns without the use of a rudder for yaw correction. This yawing behavior is the result of an induced thrust near the wingtips of the design, which our future studies will validate. In preparation for these future studies, a stingmounted 24.4'' wingspan model of the design has been built and tested in the San Diego State University Low Speed Wind Tunnel. Load measurements were taken using an external force balance through a range of angles of attack and sideslip at wind speeds of 100 and 120 mph to fully characterize the PrandtlD's overall aerodynamic behavior. These results will be used for setup and validation of a planned Stereoscopic Particle Image Velocimetry (SPIV) threedimensional flow survey to analyze the fundamental flow structures contributing to the design's novel behaviors. Our SPIV study will serve as ultimate validation of the predicted aerodynamic phenomena that the PrandtlD, and other similar designs, generate. [Preview Abstract] 

S02.00011: Tip vortex characteristics of a hovering multirotor propeller YoungJin Yoon, Haecheon Choi We investigate the tip vortex characteristics of a hovering multirotor propeller using large eddy simulation. Flow field and aerodynamic coefficients from the current simulation show good agreements with those from experiments. We identify instantaneous locations of tip vortices and explore wandering motions at different vortex ages. The magnitude of the wandering motion increases with vortex age and this results in an overall increase of turbulence level along the slipstream boundary. To identify the velocity statistics around the tip vortex and the growth rate of the vortex core, the vortex center is identified during evolution of each tip vortex and surrounding velocity fields are ensembleaveraged. The swirl velocity exhibits a selfsimilar behavior when normalized with the peak velocity and core radius, and the velocity profile is well described by the LambOseen model. The vortex core shows a lower growth rate than that from large scale rotors. The distributions of the Reynolds stresses reveal strong anisotropy with the highest level of turbulent kinetic energy located inside the core. [Preview Abstract] 

S02.00012: An~effective~method to reduce wall interference in subsonic wind tunnels. Narges Tabatabaei, Ricardo Vinuesa, Ramis \"Orl\"u, Philipp Schlatter Wall interference in windtunnel tests is known to be one of the main sources of uncertainty in experimental aerodynamics, reducing the accuracy~and fidelity~of the measurements. Even lowblockageratio test sections require a wall correction if a~faithful~representation of freeflight conditions~is intended. This problem is investigated via Reynoldsaveraged NavierStokes (RANS) simulations for a range of angles of attack. The simulations are validated with Large Eddy Simulation (LES) and experimental windtunnel data. The isolated aerodynamics effect of confinement is analyzed beside the boundary layer growth effect. A simple and efficient~yet effective~method is proposed to design wall inserts, capable of correcting the wall interference in subsonic wind tunnels with moderate blockage ratio. In this method, we propose the use of linear inserts to account for the effect of the windtunnel walls. The use of these inserts leads to very good agreement between freeflight and windtunnel data, while this approach benefits from simple manufacturing and experimentaldeployment processes. Windtunnel experiments with the proposed insert design for validation purpose are currently underway. [Preview Abstract] 

S02.00013: Optimization and scaling of aerodynamic performance for a flapping wing in hover Alexander Gehrke, Karen Mulleners We experimentally optimise the pitch angle kinematics of a flapping wing system in hover to maximise the stroke average lift and hovering efficiency with the help of an evolutionary algorithm and insitu force and torque measurements at the wing root. Additional flow field measurements are conducted to link the vortical flow structures to the aerodynamic performance. The pitch angle profiles yielding maximum average lift have trapezoidal shapes and high average angles of attack. These kinematics create a strong leading edge vortex early in the cycle which enhances the force production. The most efficient pitch angle kinematics resemble sinusoidal evolutions and have lower average angles of attack. The leading edge vortex grows slower and stays closebound to the wing for the majority of the strokecycle. This increases the efficiency by 93\% but sacrifices 43\% of the lift in the process. We estimate the shearlayer velocity at the leading edge solely from the input kinematics and use it to scale the average and the timeresolved evolution of the circulation and the aerodynamic forces. The experimental data agrees well with the shearlayer velocity prediction, making it a promising metric to quantify and predict the aerodynamic performance of the flapping wing hovering motion. [Preview Abstract] 

S02.00014: Exact solutions for the unsteady motion of multiple wings in an inviscid fluid Peter J. Baddoo, Darren Crowdy, Nick Moore, Anand Oza When two or more fliers or swimmers move together, their interactions can significantly affect the characteristics of the surrounding flow. Indeed, it is well known that many natural swimmers exploit these effects to enhance their propulsive efficiency. This raises the question of when these swimmers are operating in cooperation or competition; i.e. do the interaction effects help or hinder the swimmers. We use conformal maps and multiply connected function theory to build a model for these interactions. Our model is based on thin aerofoil theory and requires equivalent assumptions such as attached flow, smallamplitude motions and linearised wakes. Accordingly, our approach is very general and permits consideration of a range of wing motions (pitching, heaving, undulatory) and configurations (tandem, inline, periodic, ground effect). Unlike previous approaches, our model is not based on the assumption that the swimmers are far apart and thus interact only weakly. We focus on the (doubly connected) case where there are two interacting swimmers and find that our results show excellent agreement with experimental data. Specifically, our model recovers the equilibrium configurations observed in recent experiments and suggests the existence of new stable configurations. [Preview Abstract] 

S02.00015: Prediction of unsteady lift on a pitching foil Shuji Otomo, Karen Mulleners, Kiran Ramesh, Ignazio Maria Viola The ability to accurately predict the forces on an aerofoil in realtime when large flow variations occur is important for improving the manoeuvrability and control of small aerial and underwater vehicles. Closedform analytical formulations are only available for small flow fluctuations. This limits their applicability to gentle manoeuvres. Here we investigate largeamplitude, nonsymmetric pitching motions of a NACA~0018 aerofoil at a Reynolds number of $3.2 \times 10^4$ using timeresolved force and velocity field measurements. We adapt the linear theory of Theodorsen, assuming it gives a normal force as a high angle of attack treatment. The accuracy of the models is remarkably good, including when large leadingedge vortices are present, but not when leading and trailing edge vortices have a strong interaction. In such scenarios, discrepancies between the theoretically predicted and the measured forces are shown to be due to vortex force that is calculated using the impulse method. These results aim to contribute to the development of loworder models to predict unsteady forces for highamplitude manoeuvres characterised by massive separation. [Preview Abstract] 

S02.00016: The influence of blade geometry parameters on the descent performance of smallscale rotors Marcel Veismann, Daniel Yos, Morteza Gharib In axial descent, rotorcraft generally encounter a flow phenomenon called vortex ring state (VRS), during which the rotor downwash is reingested, leading to severe losses in thrust and strong oscillatory airloads on the rotor. In this study, we investigated the effect of specific geometric blade parameters on the descent behavior of smallscale rotors. The experimental approach included 3D printing custom rotorblade designs and evaluating their performance in wind tunnel experiments. Metrics subject to perturbation were the blade's chord length, pitch, taper, and the number of blades. Furthermore, various tip geometries were given consideration. Results indicate that the descent performance is highly correlated to the geometry of the rotor and hover performance parameters. Thus, this study provides a predictive tool for rotorcraft behavior in descent, enabling us to estimate the average thrust losses without the necessity of having to perform actual tests. Complimentary PIV analysis provides further insight into the rotortip's vortex formation characteristics of the various blade designs, which are believed to be the driving factor for VRS behavior. [Preview Abstract] 

S02.00017: A novel model to capture the flow asymmetry of an accelerating plate at incidence Adam DeVoria, Kamran Mohseni An inviscid model for the flow over a flat plate accelerating at constant angle of attack is presented. The separated flows at the leading and trailing edges are represented by vortexentrainment sheets (DeVoria \& Mohseni 2019, JFM, 866) and the rollup process is assumed to be self similar. The flow asymmetry is captured in the governing equation by an additional term that is equal to the first nonsingular member of the Laurent series expansion of the complex potential for the flow around a sharp edge. The coefficient quantifying this higherorder flow component is a new similarity parameter that represents the timedependent effects of the vortex structure growth and the angle of attack. The value of the parameter is obtained from known kinematic input and is used to compute a selfsimilar solution for a given instance of time. The timeevolution of the physical solution is then obtained from a series of such parameterized selfsimilar solutions. The asymmetric flow structure is well represented up to times prior to the appearance of secondary flow structures. Similarly, the forces exerted on the plate are captured with much improved accuracy as compared to previous work. [Preview Abstract] 

S02.00018: CyberPhysical Approach to a SelfPropelled Flapping Airfoil Jay Young, James Luo, CHK Williamson Traditional flapping airfoil studies tether the airfoil in place and fix the imposed freestream velocity. However, this approach does not accurately reflect practical conditions in which the propulsor would be free to accelerate if thrust is generated. Using the CyberPhysical Fluid Dynamics (CPFD) Facility (Mackowski {\&} Williamson 2011), a closedloop forcefeedback system, we study a flapping airfoil undergoing selfpropulsion. The airfoil freely accelerates from rest until an equilibrium cruising velocity is achieved wherein the net thrust and drag forces are balanced. We explore the optimal combination of heave and pitch amplitudes to minimize energy expenditure for a given cruising velocity and examine the underlying vortex dynamics that generate efficient propulsion. With CPFD, the airfoil accelerates in response to the net thrust generated. We study the vortex dynamics giving rise to the unsteady aerodynamics of the selfpropelled airfoil. [Preview Abstract] 

S02.00019: Instantaneous pressure field and aerodynamic loads across a harmonically pitching airfoil Jibu Jose, Subhra Shanka Koley, Christopher Williams, Ayush Saraswat, Jin Wang, Joseph Katz Experimental studies of complex unsteady aerodynamic loads on an airfoil undergoing dynamic stall were performed using a harmonically pitching airfoil. The experiments were performed at a Reynolds number of 45,000 in a refractive index matched water tunnel using a NACA 0015 airfoil with 50mm chordlength, oscillating harmonically between 5$^{\mathrm{o}}$ and 20$^{\mathrm{o}}$ at a reduced frequency of 0.411. Time resolved stereo PIV data were acquired at 1250 frames/s covering the flow on both sides of the foil simultaneously. Assuming a 2D flow, the pressure field around the airfoil was computed by direct integration of material acceleration calculated from the timeresolved velocity field, using an inhouse developed, GPU based, parallelline, omnidirectional code. Surface integration of the pressure field was used for computing the lift and pitching moment on the airfoil. The formation and development of Leading Edge Vortex, and subsequent dynamic stall vortex, and the existence of a phase lag between the incidence angle and the development of suction side structures during upstroke and downstroke were evident from the data. Growth and migration of the pressure minima from the leading to the trailing edge induced pitch up and pitch down moments, respectively. [Preview Abstract] 

S02.00020: Effects of Inflow Velocity Profile and Rotational Accelerations on LEV Formation for a Revolving Wing James Paulson, Thierry Jardin, James Buchholz An aspect ratio 10 rectangular wing is revolved in a cylindrical domain at 45 degree angle of incidence, and Reynolds number $Re=O$(1000). Four cases are considered. Case A represents the physical problem in which the approach velocity varies linearly with distance from the axis of rotation, and Coriolis and centripetal accelerations are active in the noninertial reference frame attached to the wing. Case B implements the same inflow but without rotational accelerations. In cases C and D, the rotational accelerations are the same as A and B, respectively; however, the inflow is uniform along the span. Each case exhibits a strikingly different behavior of the leadingedge vortex (LEV), demonstrating that inflow shear is an important factor governing LEV behavior, in addition to the rotational accelerations. A conical, attached vortex is observed only for case A. Vorticity transport analyses were conducted in chordwise planar control regions, at $z/C=2.0$ (measured from the axis of rotation). In all cases, the leadingedge shearlayer vorticity flux and the diffusive flux from the wing surface provide opposing contributions to the measured circulation; however, they fluctuate significantly for all except case A. [Preview Abstract] 

S02.00021: Dynamic stall on an airfoil pitching at very high amplitudes and Reynolds numbers Claudia E. Brunner, Janik Kiefer, Martin O. L. Hansen, Marcus Hultmark The blades of a vertical axis wind turbine undergo large variations in angle of attack as it rotates around its axis. At low tip speed ratios, the angle of attack can exceed the stall angle of the blade and induce dynamic stall, an unsteady flow phenomenon that leads to significant hysteresis in the lift and drag forces. Accurate predictions of the forces acting on the blades are necessary to predict the performance of the turbine. At moderate Reynolds numbers, dynamic stall on vertical axis wind turbines has been widely studied, but due to the experimental challenges of investigating unsteady flows at high Reynolds numbers, dynamic stall in this regime is less well understood. In the current study, a NACA 0021 airfoil is sinusoidally oscillated at very high amplitudes, such that the stall angle is exceeded and the airfoil experiences dynamic stall with sufficient time to reattach during the downstroke. Reynolds numbers upwards of 10\textasciicircum 6 are achieved using a highpressure wind tunnel, and a range of reduced frequencies are tested. The phaseaveraged pressure distribution around the surface of the airfoil provides insight into the forces and moments acting on the airfoil, as well as the timeresolved separation and reattachment behavior. [Preview Abstract] 

S02.00022: Effect of the Sweep Angle on the Hydrodynamic Performance of a FullyPassive OscillatingPlate Hydrokinetic Turbine Waltfred Lee, Guy Dumas, Peter Oshkai The influence of the sweep angle on the performance of a fullypassive oscillatingplate hydrokinetic turbine prototype was investigated experimentally. The sweep angle was introduced to promote spanwise flow along the plate in order to delay the shedding of the leading edge vortex (LEV). We considered two configurations: a plate with 6 degrees sweep angle and an unswept plate (control), which were undergoing fullypassive pitch and heave motions in a uniform inflow at the Reynolds number of 19000. The resulting kinematic parameters and the energy extraction performance were evaluated for both plates. 2D particle image velocimetry (PIV) was used to obtain patterns of the phaseaveraged outofplane vorticity during the oscillation cycle. The circulation in the wake was related to the loading on the plate by calculating the moments of vorticity with respect to the pitching axis of the plate. Tomographic (3D) PIV was implemented to evaluate the spanwise variation of the vortex structure. The results show evidence of the delay of the shedding of the LEV on the swept plate, which leads to the improvement of energy extraction performance of the fullypassive hydrokinetic turbine. [Preview Abstract] 

S02.00023: Aeroelastic Flutter of an Airfoil in the Presence of an Active Flap TsoKang Wang, Kourosh Shoele Aeroelastic motion is a ubiquitous instability observed in various applications including aircraft design, renewable energy extraction, animal locomotion, and more. However, the control of the aeroelastic response has been mostly limited to using linear potential flow models. In this work we introduce a novel computational algorithm that enables the use of high fidelity simulations of a fluidstructure interaction (FSI) system with a deforming body to investigate the fluttering phenomena and form a reducedorder model of the system. A spatiotemporal modal analysis technique will be used to understand the effect of prescribed control surface motion on the fluttering response of the airfoil. It will be shown how the results from this study about the role of the structural deformation on the flow can be employed to form a better control method for the aeroelastic problems. [Preview Abstract] 

S02.00024: Scaled Experiments on Rigid and Flexible Wings for Microrobots Emma Singer, Geoffrey Spedding Subgram FlappingWing MicroAir Vehicles (FWMAVs) in $Re < 1000$ flow regimes have unsteady liftgenerating mechanisms that are extremely sensitive to changes in wing topology and actuation timing. At scale, practical testing is difficult, constrained by factors such as human fabrication error, material inhomogeneity, and long assembly times. To isolate a single wingrelated variable in such a system, we instead employ a dynamicallyscaled experiment in water that contains a single 2DOF robotic wing, thus eliminating the wingpair and body dynamics of a complete FWMAV. The timeresolved forces are measured with a 6axis forcetorque sensor at the wing root, and these are related to wake kinematics measured with a TomographicPIV system. Results are grouped by a dimensionless wing parameter \textit{effective stiffness}, which is the bending rigidity normalized by the dynamic fluid pressure. Three examples are compared, from rigid to flexible, and the influence of the effective stiffness on vortex shedding and wake structure are outlined, with a view to understanding the major factors that determine flight efficiency in FWMAVs. [Preview Abstract] 

S02.00025: Instantaneous thrust production mechanisms and vortex shedding dynamics of oscillatory propulsors in inviscid flows Firas Siala, Melissa Green The inviscid propulsive performance and vortex shedding dynamics of purely heaving and pitching panels operating at Strouhal numbers of $St_{A}$ = 0.05 – 0.35 are numerically investigated using discrete vortex modeling. We explore the mechanisms that these two generic types of kinematics use to produce thrust. Of particular interest is the relationship between timedependent thrust production and wake structure. In agreement with the literature, our results show that thrust produced by purely pitching panels is primarily from addedmass, whereas thrust produced by heaving panels is entirely circulatorybased. Furthermore, it is observed that for a given Strouhal number, the heaving and pitching panels have almost identical wake structures, while the timeresolved and timeaveraged thrust coefficients considerably differ. To further examine the correlation of wake vortex shedding and instantaneous thrust production, Lagrangian analysis using the finitetime Lyapunov exponent (FTLE) field was carried out using the dynamics of discrete point vortices generated by the inviscid flow simulations. The release of bounding Lagrangian saddles, identified as intersections of positive and negativetime FTLE ridges, from the panel trailingedge is shown to be highly correlated with the [Preview Abstract] 

S02.00026: Performance Estimation of a Flapping Foil Under Unsteady Upstream Flow Conditions Rodrigo Vilumbrales Garcia, Gabriel D. Weymouth, Bharathram Ganapathisubramani For a system of two flapping foils in tandem formation, the follower can achieve a surplus if its location and trajectory are adapted to the wake of the leader. The most common approach to evaluating the performance of the back foil for a range of conditions is to carry out a large number of numerical simulations, which can lead to accurate solutions but, on the other hand, can be computationally expensive. Although there is a large existing literature about the topic, the physics of the operation are not completely understood, especially when the foils are subjected to highlyunstable flow conditions or unusual kinematics, such as nonsinusoidal motions. In this study, the authors aim to estimate the thrust and efficiency performance of a foil subjected to an inline tandem arrangement using a range of theoretical and Machine Learning approximations. This could lead to a quick evaluation method that would provide deep information about the physical characteristics that are responsible for the performance augmentation, helping with the understanding of atypical cases, which would lead to a great increase in the range of applications of tandem flapping foil arrangements. [Preview Abstract] 

S02.00027: Inverse Airfoil Design Using Generative Adversarial Network Priyam Gupta, Prince Tyagi, Raj Kumar Singh This research proposes Deep Learningbased Inverse Airfoil Design framework using Generative Adversarial Networks.The objective of the inverse design problem in this study is to design airfoil shapes which produce desired Pressure Distribution at given flow conditions.The Convolutional Neural Network based Generator extracts features from the pressure coefficient profiles and predicts the corresponding airfoil shape coordinates.The Discriminator then attempts to differentiate between the actual and the predicted airfoils.A Bezier layer is embedded in the generator to ensure smoothcontoured aerodynamic surfaces without any sharp deformities.The GANS are trained on a database of airfoil shapes and pressure coefficient distribution obtained for Reynold's number of 100,000 and a range of angle of attacks.The trained generator efficiently generated the desired airfoil with an L2 error of less than 1.5\%.The results show that the GAN based framework is computationally time efficient and highly accurate. [Preview Abstract] 

S02.00028: Evolution of threedimensional flow structures over the rotating wing Abbishek Gururaj, Mahyar Moaven, Zu Puayen Tan, Brian Thurow, Vrishank Raghav The development of the leadingedge vortex (LEV) and associated flow structures over surfaces undergoing unsteady maneuvers like rotation and/or pitching is advantageous in some cases while detrimental in others. As such, the comprehensive understanding of these flow phenomenon is paramount. In this study, a novel singlecamera plenoptic 3D velocimetry technique in the rotating frame of reference is employed to characterize the threedimensional flow over a rotating wing. Unlike past fixedframe 3D velocimetry, this approach allows prolonged flow measurement across multiple complete rotations. Preliminary analysis show that multiple vortices are shed from the leadingedge and the LEV developed stronger and closer to the wing surface than secondary vortices, consistent with observations in the literature on rotating wings. It was also observed that after some time has elapsed after the start of rotation, the convection speed of LEV became higher towards the root relative to the tip in the measurement volume considered. To further understand the dynamics of these flow structures, the components of the vorticity equation in the rotating frame of reference will be quantified to assess their contributions to the evolution of flow structures over the rotating wing. [Preview Abstract] 

S02.00029: Effect of Axial Casing Grooves on the Structure of Turbulence in the Tip Region of an Axial Compressor rotor passage Subhra Shankha Koley, Huang Chen, Ayush Saraswat, Joseph Katz Stereo PIV measurements performed in a refractive index matched facility examine the effect of axial casing grooves (ACGs) on the structure of turbulence in the tip region of an axial compressor rotor. The ACGs delay the onset of stall at low flowrates by entraining the Tip Leakage Vortex (TLV), and by causing periodic changes to incident angle as their outflow impinges on the rotor blade. However, ACGs typically cause undesirable loss of efficiency at design flowrates. Interactions of the tip flow with ACGs modifies the magnitude and spatial distribution of the highly anisotropic and inhomogeneous components of the turbulent kinetic energy (TKE). Owing to TLV entrainment, at low flowrate the ACGs reduce the turbulence in the passage compared to that of the smooth endwall, but the anisotropy varies with the groove geometry. Still, the TKE is high in the TLV, the shear layer separating the backward leakage flow with the main passage, flow and near the corner of the grooves. At high flowrates, interactions of the TLV with secondary flows generated by typical grooves increase the tip region turbulence. This adverse effect and associated efficiency loss can be mitigated using grooves that minimize the injection of secondary flows into the passage at high flowrates. [Preview Abstract] 

S02.00030: Transient flow around an airfoil at increasing angle of attack Olaf Marxen Flow around the wings of an unmanned aerial vehicle may be subject to rapidly varying flow conditions. A particularly interesting case occurs when this change causes a sudden increase of angle of attack (AoA) for the airfoil. However, at present there is very limited knowledge of the underlying flow physics for airfoils subject to nonperiodic transient conditions. Due to this lack of knowledge, accurate and general yet efficient calculation methods for the forces generated by an airfoil subject to transient flow conditions are presently lacking. In order to improve our knowledge of transient aerodynamics, the flow around a NACA0015 airfoil subject to a rapidly increasing AoA is investigated experimentally for constant wind tunnel speeds and hence Reynolds numbers (approximately Re=140,000 to 570,000). The lift during dynamic testing was found to be higher than that from static testing at the same AoA. For the lowest Reynolds number considered, a significant lift overshoot could be observed, reaching values beyond the global maximum lift achieved during static testing. Analysis of transient results indicates that the movement of the location of laminarturbulent transition as well as local boundarylayer separation occurring on the suction side of the airfoil play a key role. [Preview Abstract] 

S02.00031: Threedimensional study of the near wake of a smallscale coaxial rotor system using tomographic particle image velocimetry Lokesh Silwal, Zu Puayen Tan, Vrishank Raghav In recent years, the popularity of coaxial rotor has increased rapidly with its applications extending from smallscale UAVs and Mars rotor to concepts for the air taxi model. Thus, the comprehensive understanding of coaxial rotor flowfield under various operating conditions has become vital to ensure its smooth operation. Previous twodimensional studies have demonstrated the complex nature of the coaxial rotor wake which is dominated by mutual interactions between the tip vortices. However, the threedimensional evolution and interaction of helical vortices in the near wake is yet to be investigated. With the current study, we aim to employ tomographic PIV enabled by a single camera quadscope system to accomplish a threedimensional and timeresolved study of a smallscale (0.2m span) coaxial rotor wake. The focus of the study will be to elucidate the effect of tip Reynolds number on the threedimensional interaction of the upper and lower rotor helical vortex structures in the near wake at fixed rotor spacing. Here, the tip Reynolds number is varied between 19,000 to 40,000 which corresponds to the operational range of smallscale UAVs. [Preview Abstract] 

S02.00032: On the energy harvesting performance of oscillating airfoils with different geometries using modified discrete vortex method Kiana Kamrani Fard, Vickie Ngo, James Liburdy The energy harvesting performance of flapping airfoils with different geometries is studied in reduced frequencies of k$=$fC/U $=$0.060.16, pitching amplitude of $\theta \quad =$ 70\textdegree and heaving amplitude of h\textunderscore 0/C$=$0.5. A low order discrete vortex model with a vortex shedding criterion at the leadingedge is used to estimate the transient lift force and the model results are compared to 2D CFD results. The location of the leadingedge separation point as well as the instant that the leadingedge vortex starts to form are identified from 2D CFD wall shear stress results. It is found from 2D CFD results that the instant at which the leadingedge vortex is shed can be defined as a function of the reduced frequency and the leadingedge geometry and is independent of airfoil motion kinematics. This instant is then used to find the critical leadingedge suction parameter (LESP) from thin airfoil theory which is included in the low order model. The parameters of interest to study the energy harvesting performance of different airfoil shapes include transient lift force, total power coefficient and total efficiency. [Preview Abstract] 

S02.00033: Towards Understanding of LeadingEdge Separation for Energy Harvesting with an Oscillating Foil Vickie Ngo, Holly Manjarrez, James Liburdy The energy harvesting performance of an oscillating foil is studied by conducting a 2D turbulent model simulation in ANSYS FLUENT. The simulation objective is to identify the key flow physics associated with the leadingedge separation of an airfoil with variable leadingedge thickness undergoing a sinusoidal oscillating motion at low freestream Reynolds numbers. Of the key characteristics are the wall shear stress, vorticity, pressure field, and leadingedge flow separation. The leadingedge separation was detected from an abrupt drop in wall shear stress along the foil surface and is used as a primary indicator of vortex shed. These vortices have shown to be essential components of the oscillating foil's energy harvesting performance. The results of this study reveal a correlation that exists between leadingedge geometry and the reduced frequency that can predict flow separation. The time at which separation occurs is a function of the reduced frequency and the geometric parameters of the leading edge. The position on the foil at separation is independent of reduced frequency but is a function of geometric parameters of the leading edge. Further, the results of varying geometric and motion parameters were evaluated to inform a panelbased discrete vortex model to evaluate the overall energy harvesting potential. [Preview Abstract] 
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