2021 Archive

Abstract: Neural networks can be used to gain global situational awareness from local sensor measurements in fluid mechanics.  Although numerous models have been proposed to date for such application, the reconstruction is still challenging when sensors are sparsely populated.  Moreover, when sensors become on-/offline in time while moving their locations in practical applications, developing a robust technique is a major challenge.  In response, we propose a neural network-based flow field reconstruction technique from arbitrary number of sensors in motion.  We use Voronoi tessellation to obtain a structured-grid representation from low-dimensional sensor information.  This geometric data projection enables us to use a simple convolutional neural network for global field reconstruction without machine learning customizations.  We demonstrate the use of the present model for vortical wake flows, sea surface temperature, and wall-bounded turbulence.  The present technique achieves accurate flow field reconstruction from an arbitrary number of moving sensors while having robustness against noisy sensor inputs.  The flexibility of the present framework enables us to expect a broad range of extensions in both numerical and experimental studies.

Abstract: Fluid damping plays an important role in shaping damped oscillations of aeroelastic systems. In this study, we experimentally characterize the nonlinear fluid damping associated with vortices shed from the rounded leading edge and the sharp trailing edge of a rigid but elastically mounted pitching wing in the absence of a free-stream flow. We simulate the dynamics of the elastic mount using a cyber-physical system. We perturb the wing and measure the fluid damping coefficient from damped oscillations over a large range of pitching frequencies, pitching amplitudes, pivot locations and leading-/trailing-edge sweep angles. A universal fluid damping scaling based on the Morison equation is proposed and validated. Within the small-amplitude limit, the scaled non-dimensional fluid damping is found to increase linearly with the pitching amplitude, with a constant slope corresponding to the unsteady drag coefficient. This slope decreases as the pitching amplitude increases, presumably because the shed vortices no longer follow the rotating wing. Flow fields obtained using particle image velocimetry (PIV) are used to explain the nonlinear behavior of the fluid damping.

Abstract: Linear stability analysis has been used for decades to study fluid flows. However, its use in fluid-structure interaction (FSI) problems has only been introduced in the last few years. The flow around an elastically-mounted bluff body has been commonly used as the model problem, and, in general, the investigations focus on the characterization of the least stable modes close to the first instability. In this presentation, a methodology to calculate the sensitivity of the least stable modes of fluid-structure interaction systems with respect to local forces is presented. We make use of the adjoint equations of the flow-structure coupled system to calculate the gradients. The methodology was applied to two-dimensional incompressible laminar steady flows around an elastically-mounted circular cylinder, and we obtained the gradients of the real and imaginary parts of the least stable eigenvalues with respect to forces located at arbitrary points in the flow domain. Selected values of mass ratio and reduced velocity were considered in the simulations, and the results were compared to those obtained for a fixed cylinder at the same Reynolds number. The sensitivity fields of the fluid-structure interaction system can be very different from its fixed structure counterpart, and amongst the cases, with an elastic structure, the fields vary greatly according to the reduced velocity. The sensitivity results were verified against linear and nonlinear simulations of flows with small control cylinders placed at locations selected according to the sensitivity fields. The agreement between the predictions made with the sensitivity analyses and the linear and nonlinear results of the forced flows was excellent. In some cases, it was possible to completely suppress the cylinder vibration.

Abstract: The aerodynamics of inclined flat plates in close ground proximity is experimentally investigated at a chord-based Reynolds number of 50,000 for aspect ratios (AR) of 1 and 2. The minimum ground height is varied between 0.1 to 1.0 chord lengths. Lift is measured directly using a force balance for angles of attack ranging from -90 to 90 degrees. Streamwise and cross plane velocity measurements are conducted using planar and stereo particle image velocimetry, respectively. In all cases, notable changes in loading are observed when the plate is below 0.75 chord lengths from the ground. The results show that the effect of ground proximity on the lift force is dependent on the combination of AR and angle of attack, but the most significant effects are observed for angles between 20 and 40 degrees for both AR. The analysis relates the observed changes in aerodynamic loading to the changes in flow field development, and pressure reconstructions are performed at the centreline plane to elucidate the associated changes in surface stresses.

Abstract: Oscillating foils have shown to be an effective way to extract hydrokinetic energy and offer benefits of shallow water operation, scalability, and low cut-in speeds. For arrays of oscillating foils, the role of wake structure is particularly important since the coherent structures can cause constructive and/or destructive interference with downstream foils. This presentation explores a wide range of oscillating foil kinematics within energy harvesting mode, with the goal of grouping and parameterizing the wake based on the input kinematic variables. A first approach based on the primary vortex strength shed from each oscillating foil kinematics is discussed and it is found three energy harvesting modes with respect to the foil’s relative angle of attack. To fully consider the wake between two foils, an autoencoder and clustering model is developed to capture the pertinent wake features from each kinematics and further cluster foil kinematics based on wake similarity. Four different wake patterns are obtained through this model and when compared with the primary vortex analysis approach, a connection between these wake patterns and foil relative angle of attack is determined, and hence provide insight for optimizing foil array configurations for energy harvesting.

Abstract: Pulsatile jet propulsion is a highly energy-efficient swimming mode used by various species of aquatic animals and which inspires engineers of underwater vehicles. Here, we present a bio-inspired jet propulsor that combines the flexible hull of a jellyfish with the compression motion of a scallop to create individual vortex rings for thrust generation. Similar to biological jetters, our propulsor generates a nonlinear time-varying exit velocity profile and has a finite volume capacity. The formation process of the vortices generated by this jet profile is analysed using time-resolved velocity field measurements. The transient development of the vortex properties is characterised based on the evolution of ridges in the finite-time Lyapunov exponent field and on local extrema in the pressure field, which is derived from the velocity data. Special attention is directed toward the vortex pairing observed in the trailing shear layer. During vortex pairing, the Lagrangian vortex boundaries first contract in the stream-wise direction before expanding in the normal direction to keep the non-dimensional energy at its minimum value, in agreement with the Kelvin-Benjamin variational principle. The circulation, diameter, and translation velocity of the vortex increase due to pairing. The vortex pairing takes place because the velocity of the trailing vortex is higher than the velocity of the main vortex ring prior to merging. The comparison of the temporal evolution of the Lagrangian vortex boundaries and the pressure based vortex delimiters confirms that features in the pressure field serve as accurate and robust observables for the vortex formation process.

Abstract: The pressure loads induced by vortices on immersed surfaces are central to numerous problems in fluid dynamics. While past studies in unsteady aerodynamics have focused mostly on the role of vortices in force production, we show using a force partitioning method (FPM; Menon & Mittal, J. Fluid Mech., 918, R3, 2021) that strain-dominated regions associated with vortices can in fact have a significant effect on aerodynamic loads in some situations. FPM allows us to quantify the loads induced on immersed surfaces by individual vortices as well as their associated regions of strain. By analyzing the forces on a pitching airfoil undergoing dynamic stall, we show that our current understanding of vortex-dominated phenomena could be incomplete without considering the substantial, and sometimes dominant, effect of strain-dominated regions that are associated with vortices.

Abstract: The onset and evolution of the dynamic stall vortex (DSV) are analyzed by means of large eddy simulations of an SD7003 airfoil undergoing periodic plunging motion in a transitional Reynolds number flow (Re = 6 × 10^4). Interactions between upstream propagating Kelvin-Helmholtz instabilities and a shear layer formed at the leading edge trigger flow separation. The former appear to be related to acoustic waves scattered at the trailing edge due to initial vortex shedding. Two freestream Mach numbers (M = 0.1 and 0.4) are employed to examine the flow differences due to compressibility variations. The existence of a common timing for the acoustic perturbations in both flows suggests a possible Mach number invariance for the birth of the Kelvin-Helmholtz instability. Increasing compressiblity, however, induces earlier spanwise fluctuations, higher flow three-dimensionality and a weaker and more diffuse DSV, which is formed further downstream of the leading edge and has lower residency time. Modal decomposition, performed with both the classical dynamic mode decomposition (DMD) and its multi-resolution variant (mrDMD), highlights key trends and demonstrates the capacity of the mrDMD to extract physically meaningful flow structures related to the stall onset. Such detailed characterization of the shear layer can be used for a systematic exploration of flow control strategies for unsteady airfoils.

Abstract: Fish schools are examples of active systems whose collective dynamics emerge from individual-level interactions. These systems are often modeled with self-propelled particles in unbounded domains subject to behavioral rules based on visual feedback that usually neglect hydrodynamic interactions. Little is known about how geometric confinement together with flow-mediated interactions affect the collective behavior of fish. Here, we combine vision-based rules with hydrodynamic interactions in a circular domain, and we map out the different collective phases that the group of fish can achieve. We show that new collective phases emerge where (1) the group follows the tank wall; (2) the group splits into two groups milling in opposite directions, in a double milling phase; (3) a new bistable regime emerges in which the school intermittently switches from schooling to milling and vice-versa. We analyze the bistable regime by constructing effective potentials on the coarse-grained translational and rotational order parameters. We find that the bistable regime is sensitive to the school size and the geometric confinement.

Abstract: Vertical-axis wind turbines are great candidates for wind energy diversification and could contribute to reaching a near-zero carbon emission electrical grid. The complex aerodynamics of vertical-axis wind turbines have challenged their development and integration into urban infrastructure. The blades of these turbine undergo periodic oscillations in inflow conditions, which continuously change the blade’s effective angle of attack and flow velocity. These oscillations often lead to the formation of large-scale vortices and the occurrence of dynamic stall. These flow structures allow an increase in torque production, but also cause heavy load transients jeopardising the turbine’s structural integrity and potentially leading to premature failure. This talk aims at presenting the dilemma vertical-axis wind turbines face over a wide operation envelope: generating torque while ensuring structural reliability.  Time-resolved particle image velocimetry and load measurements were performed to characterise the occurrence of dynamic stall on a scaled-down vertical-axis wind turbine for a wide range of operating condition.The timescales of vortex formation and the corresponding impact on the unsteady load response are analysed. Close attention is brought to the asymmetry in flow topology and load response found between the first and second half of the blade’s revolution. Future work on using these metrics for active flow control using blade pitching is also introduced.

Abstract: This talk presents new methods for modeling and simulation of the aortic and mitral heart valves and use of these methods to study congenital heart disease. To construct model heart valves, we specify that the heart valve supports a pressure and derive an associated system of partial differential equations for its loaded state. Using the solution to this system, we then derive reference geometry and material properties. By tuning the parameters in this process, we design the model valves. This process produces material properties that are consistent with known values, yet also includes material heterogeneity. When used in fluid-structure interaction simulations, these models are highly effective, producing realistic flow rates and robust closure under physiological driving pressures. Using these models, we study flows through the bicuspid aortic valve. Simulations show that a bicuspid valve, without alterations to the aorta anatomy, alters blood flow patterns dramatically. These flows suggest that hemodynamics may play a strong role in aortic dilation and aneurysm formation.

Abstract: We present spectral analysis modal methods (SAMMs) to perform POD in the frequency domain using non-time-resolved Particle Image Velocity (PIV) data combined with unsteady surface pressure measurements. In particular, time-resolved unsteady surface pressure measurements are synchronized with non-time-resolved planar PIV measurements acquired at 15 Hz in a Mach 0.6 cavity flow. Leveraging the spectral linear stochastic estimation (LSE) method of Tinney et al. (2006), we first estimate the cross correlations between the velocity field and the unsteady pressure sensors via sequential time shifts, followed by a Fast Fourier transform to obtain the pressure-velocity cross spectral density matrix. This leads to a linear multiple-input / multiple-output (MIMO) model that determines the optimal transfer functions between the input cavity wall pressure and the output velocity field. Two variants of SAMMs are developed and applied. The first, termed “SAMM-SPOD”, combines the MIMO model with the SPOD algorithm of Towne et al. (2018). The second, called “SAMM-RR”, adds independent sources and uses a sorted eigendecomposition of the input pressure cross-spectral matrix to enable an efficient reduced-rank eigendecomposition of the velocity cross-spectral matrix. In both cases, the resulting rank-1 POD eigenvalues associated with the Rossiter frequencies exhibit very good agreement with those obtained using independent time-resolved PIV measurements. The results demonstrate that SAMMs provide a methodology to perform space-time POD without requiring a high-speed PIV system, while avoiding potential pitfalls associated with traditional time-domain LSE.

Abstract: In this work, we present an approach to obtain a desired leading-edge vortex (LEV) shedding pattern from an unsteady airfoil through the execution of a suitable motion kinematics. Previous research revealed that LEV shedding is associated with the  leading-edge suction parameter (LESP) exceeding a maximum threshold. A low-order method called LESP-modulated discrete vortex method (LDVM) was also developed to predict the onset and termination of LEV shedding from an airfoil undergoing a prescribed motion kinematics. In the current work we present an inverse-aerodynamic formulation based on the LDVM to generate an appropriate motion kinematics to achieve a prescribed LESP variation and thus the desired LEV shedding characteristics from the airfoil. The algorithm identifies the kinematic state of the airfoil required to attain the target LESP value through an iterative procedure performed inside the LDVM simulation at each time step. Several case studies are presented to demonstrate events such as initiation and termination of LEV shedding at prescribed instants of time, inducing LEV shedding from the chosen surface of the airfoil, inducing or deterring LEV shedding during an unsteady motion on demand, and achieving similar LEV shedding patterns using different maneuvers. The kinematic profiles generated by the low-order formulation are also simulated using a high-fidelity unsteady RANS method. The flowfield results from CFD confirm  the occurrence of the desired  events at the prescribed  time instants.

Abstract: While hunting for prey, seals are able to use their whiskers for hydrodynamic trail following, a skill partly attributed to the unique shape of the whiskers themselves. The whisker’s undulated topography has thus been investigated for its hydrodynamic properties, demonstrating reduced drag and oscillating lift forces when compared to flow over a smooth cylinder however the exact mechanisms are not well understood. The current investigation parameterizes the seal whisker inspired undulated geometry into seven non-dimensional parameters and shows that individual modifications to the geometric parameters can strongly impact the overall flow response. In particular, simulations are performed for flow over five geometries with various undulation wavelengths (λ=1, 2, 3.4, 5, 6.9) and a comparable smooth elliptical cylinder. The effect on wake patterns and spanwise variation is investigated through comparison of Reynolds stresses and turbulent kinetic energy calculations. Changes in the flow response vary nonlinearly with respect to topography wavelength with minimal drag and oscillating lift occurring at a biologically relevant wavelength, λ=3.4. The analysis highlights the redirection of energy along the span and the resulting effect on wake structure and vorticity.

Abstract: Water channel experiments are presented for a pair of NACA 0012 pitching hydrofoils of aspect ratio 3. One foil is fixed, while the other is completely free to move in the horizontal plane. A side-by-side arrangement is found to be two-dimensionally stable to perturbations away from this equilibrium arrangement, and arises naturally from purely hydrodynamic forces. This provides the first experimental evidence of two-dimensional stability and supports the Lighthill conjecture, which is that hydrodynamics forces may be sculpting the structure of fish schools.  This minimal schooling arrangement also increases the swimming speed by 20% compared to an isolated swimmer.  These findings are supported by force measurements, trajectory measurements, and free-swimming simulations of two-dimensional pitching foils. Moreover, previously discovered one-dimensionally stable equilibria driven by wake vortex interactions are shown to be, in fact, two-dimensionally unstable, at least for an out-of-phase synchronization.  These newfound schooling performance and stability characteristics suggest that fluid-mediated equilibria may play a role in the control strategies of schooling fish and fish-inspired robots.

Abstract: Water tunnel experiments were conducted to study the interaction of gusts with airfoils as well as swept and unswept finite wings that are loaded or unloaded. A single vortex filament was generated by plunging an upstream airfoil at zero angle of attack in transient motion. The details of the interactions were documented by means of phase-locked force measurements and particle image velocimetry (PIV) measurements in multiple spanwise planes. The vortex filament, which was initially quasi-two-dimensional, deformed, diffused, and lost coherence as it interacted with the wing. The results show that when the counter-clockwise vortex passes the wing in proximity, the lift coefficient exhibits a sharp rise followed by a sharp decrease before recovering to the static value. The first peak is observed before the vortex arrives near the leading-edge, and is quasi-steady in nature. For the unloaded swept wing, the local structure of the vortex only depends on the relative location of the wing cross-section at that particular spanwise plane. For the loaded swept wing, the wing sweep has the largest effect at a post-stall angle of attack. Close interactions may produce vortex shedding parallel to the leading-edge of the swept wing, increasing the magnitude of the lift peak relative to the unswept wing.

Abstract: Multi-vessel coordination and controlled maneuvering through upstream wakes is important to a wide range of applications; from surface ships to autonomous underwater vehicles. In this work we study the predictive performance of physics-based and machine-learning (ML) models for unsteady inflow maneuvering forces using tandem flapping foils as a model system, in an effort to replicate the ability of fish to manoeuvre and obtain performance gains from schooling formations. Two physics-based approaches, one following simple quasi-steady assumptions and another using the classical Theodorsen have been modified to account for the flow unsteadiness, and are found to perform fairly well when there are only mild interactions with the upstream wake, with minimum error levels of around 6%. However, this error increases to 40% when there is strong wake interaction. Three ML models were trained and tested; a Long Short-Term Memory (LSTM) model, a Neural Ordinary Differential Equations (NODE) model, and a Sparse Identification of Nonlinear Dynamics (SINDy) approach. We find that all three models can match the low error of the physics-based for mild inflow unsteadiness and are capable of improving the predictions in the case of strong interactions, reducing the error levels below 20%. While these ML models require substantial training data and care in choosing their meta-parameters, their predictions do prove to be more reliable for a wider range of unsteadiness conditions as well as potentially still producing human-interpretable models (in the case of SINDy), making them an interesting research direction for further study.

Abstract: The stability characteristics of a trailing vortex pair interacting with its image in the ground plane is formulated analytically and studied numerically. The theoretical framework models the inviscid interaction between a counter-rotating pair of perturbed finite-core vortices near a planar surface. The stability equations are derived by matching the Biot-Savart integrals of the vortices with their temporally-varying position vectors, which are subject to constraints that represent a ground-image system. The stability problem is then cast into an optimal perturbation analysis to deduce the maximum growth rate for a prescribed disturbance as a function of time and the induced vortex trajectories.

Abstract: We numerically investigate linear modal instabilities of flow over finite-span untapered NACA 0015 wings at Reynolds number of 400 and a range of angles of attack and sweep on two wings having aspect ratios 4 and 8. Base flows were generated by direct numerical simulation, marching the unsteady incompressible three-dimensional Navier-Stokes equations to a steady state, or using selective frequency damping to obtain stationary linearly unstable flows. Global stability analysis is performed to identify the instabilities that lead to complex wake structures seen behind the wing. Unstable three-dimensional linear global modes of swept wings are identified for the first time using spectral-element time-stepping solvers. On unswept wings, stability analysis revealed that the most unstable global mode peaks in the midspan region of the wake with the peak of the mode structure moving towards the tip as sweep is increased. We show that the growth of these modes leads to the formation of vortical structures observed in the flow. In addition, data-driven modal analysis is employed to identify the most energetic structures of the nonlinear wake. On unswept wings, the dominant mode at low angles of attack is a Kelvin-Helmholtz-like instability, qualitatively analogous with global modes of infinite-span wings under same conditions. At higher angles of attack and moderate sweep angles, the dominant mode is a structure denominated the interaction mode. At high sweep angles, this mode evolves into elongated streamwise vortices on higher aspect ratio wings, while on shorter wings it is similar to a tip-vortex instability.

Abstract: An experimental investigation is conducted on a three-dimensional laminar separation bubble forming on the suction surface of a finite wing at a chord Reynolds number of 125 000. The rectangular semispan wing has a NACA 0018 airfoil section and an aspect ratio of 2.5. Measurements are performed using surface pressure taps and particle image velocimetry. Over a majority of the wingspan, the separated shear layer of the three-dimensional bubble rolls up into spanwise uniform vortices which behave similarly to the vortices in the two-dimensional separation bubble. Near the wing tip, where the spanwise pressure and convection velocity gradients are strongest, the vortices display a higher degree of three-dimensionality. The wing tip vortex inhibits boundary layer separation and transition in proximity to the wing tip, and the amplification of disturbances in the shear layer is reduced. Unlike a canonical two-dimensional separation bubble, the three-dimensional separation bubble does not form a closed region of recirculating flow. Fluid enters the recirculation region of the three-dimensional separation bubble at the wing tip, where it is drawn towards the wing root. The three-dimensional separation bubble is classified as a crossflow separation. The results show that away from the wing tip, the three-dimensional separation bubble is essentially spanwise uniform, whereas near the wing tip, three-dimensional effects substantially modify the bubble's mean structure and dynamics.

Abstract: Flow interactions are thought to be beneficial for animals flying and swimming in groups, and fish located behind but laterally displaced from upstream neighbours seem to exploit neighbour-induced vortices to save energy. However, it is not clear if fish actively seek these favorable positions or whether flow interactions create conditions in which flapping swimmers lock into stable formations that lead to energy savings. Recent experiments with pairs of self-propelled robotic flappers indicate that followers in any position relative to the neighbor ahead obtain hydrodynamic benefits if they match their tailbeat phase with the flow velocity of the leader's wake, a strategy that freely swimming fish also seem to exhibit even in the absence of visual and lateral line sensing. Meanwhile, pairs of hydrofoils, positioned in tandem with no means of adjusting their flapping motions, were shown to swim together cohesively, even when flapping at dissimilar kinematics, due to the interaction of the follower with the leader's wake. Are the hydrodynamic mechanisms at play in these distinct arrangements, in tandem hydrofoils and laterally-displaced fish, the same? To address this question, we employed a minimal vortex sheet model that captures salient features of the flow interactions among flapping swimmers, and we analyzed the free swimming of a pair of in-line and laterally-displaced flapping swimmers. We found that flow interactions stabilize formations of flapping swimmers at lateral positions that favor vortex phase matching for any flapping phase of the follower. Our results are consistent with the hypothesis that fish, although may choose to, do not need to actively match their tailbeat phase with the local vorticity to save energy and that hydrodynamics alone could create these energetically favorable formations.

Abstract: In the unsteady flow fields generated by pitching airfoils and plates, vortex wakes and coherent vortex interactions have been a constant focus because vortex behaviors such as formation, shedding, merging, and breaking, have been shown to be relevant to the performance dynamics such as lift, drag, or propulsion. In order to identify the important vortex structures and investigate their dynamics, the topological data analysis tool Persistent Homology (PH) was applied to both numerical and experimental flow fields generated by pitching flat plates. Several possible ways of using PH in this context were compared. The first comparison investigated point vortices shed by a pitching plate, as generated by the Discrete Vortex Method (DVM). The results show how the choice between the “standard” Euclidean distance and a vorticity-adapted Euclidean distance affects the resulting Vietoris–Rips (VR) complexes and their Betti numbers (number of connected components and topological holes). In both cases the PH was computed snapshot-by-snapshot. The second comparison investigated a set of phased-averaged experimental PIV vorticity fields of a pitching trapezoid with strouhal number St = 0.24, recorded on an Eulerian rectangular grid. The results show the difference between PH of the grid points treated as point vortices, and PH of the vorticity field itself computed using the cubical complex. In both cases, the changes in persistence diagrams and other representative diagnostics were correlated to the lift and drag felt by the pitching trapezoid, and interpreted in terms of the physical mechanism of vortex formation and evolution.

Abstract: When an object is accelerated in a fluid, a primary vortex is formed through the roll-up of a shear layer. This primary vortex does not grow indefinitely and will reach a limiting size and strength. Additional vorticity beyond the critical limit will end up in a trailing shear layer and accumulate into secondary vortices. The secondary vortices are typically considerably smaller than the primary vortex. In this paper, we focus on the formation, shedding, and trajectory of secondary vortices generated by a rotating rectangular plate in a quiescent fluid using time-resolved particle image velocimetry. The Reynolds number Re based on the maximum rotational velocity of the plate and the distance between the centre of rotation and the tip of the plate is varied from 840-11150. At low Re, the shear layer is a continuous uninterrupted layer of vorticity that rolls up into a single coherent primary vortex. At Re = 1955, the shear layer becomes unstable and secondary vortices emerge and subsequently move away from the tip of the plate. For Re > 4000, secondary vortices are discretely released from the plate tip and are not generated from the stretching of an unstable shear layer. First, we demonstrate that the roll-up of the shear layer, the trajectory of the primary vortex, and the path of secondary vortices can be predicted by a modified Kaden spiral for the entire Re range considered. Second, the timing of the secondary vortex shedding is analysed using the swirling strength criterion. The separation time of each secondary vortex is identified as a local maximum in the temporal evolution of the average swirling strength close to the plate tip. The time interval between the release of successive secondary vortices is not constant during the rotation but increases the more vortices have been shed. The shedding time interval also increases with decreasing Reynolds number. The increased time interval under both conditions is due to a reduced circulation feeding rate.

Abstract: Active flow control is a technology that is becoming increasingly more compelling for a multitude of aerodynamic applications. One of the implementation problems faced by the community is related to finding good locations to place the microjet actuators such that their effect is maximized with minimal input. Due to the vastness and complexity of the parameter space in the highly non-linear Navier-Stokes plant, this problem has only been explored computationally to date, with several restrictions such as the linearization of the plant. In this work, we will discuss a fully experimental implementation of the actuator placement problem in the fully non-linear context of the experiment, which provides a powerful platform for practical optimization to find effective actuator patterns in complex geometries, given a goal function of engineering relevance. The solutions encountered will then be briefly discussed for physical insight to understand the mechanisms leveraged by the optimization algorithm.

Abstract: This research adopts a bioinspired approach to improve how unmanned aerial vehicles (UAVs) and small aircraft fly by studying the aerodynamic responses of a red–tailed hawk (Buteo jamaicensis) when flying through a vertical gust. Red–tailed hawks remain stable and mitigate strong wind gusts that small aircraft struggle to fly through. However, the specific maneuvers that hawks perform to stabilize within natural or artificial wind gusts are not fully understood. To study these aerodynamic responses in a controlled environment, a flight–testing arena was developed. Four industrial fans placed perpendicular to the hawk’s flight path were used to produce an average vertical gust velocity of 6.5 – 8.5 m/s at 0.8 m above the fans, the location the hawk flies at. This uniform gust region was introduced in the flight path, encompassing the entire wingspan of the hawk. Hawks normally fly at around 10 m/s at level flight, making the gust magnitude between 65% and 85% of the hawk’s flight speed. The gust responses were recorded using calibrated multi–camera videography from two GoPro Hero 6 Black cameras at 240 frames per second. The hawk’s beak, tail, wings, and wrist were tracked in 3D to study the pitching response of the hawk when flying through the vertical gust. Tracking these specific points on the hawk will provide knowledge about how red–tailed hawks can morph their wings and tail to mitigate strong wind gusts while their body remains stable. This presentation reviews the experiment methodology and discusses some preliminary tracking data of the hawk’s responses to vertical gust events.

Abstract: Neutrally-buoyant near ground swimmers experience alterations in their added mass, quasi-steady, and wake-induced forces compared to swimming far from a ground plane.  In fact, using a simple freely-swimming pitching hydrofoil as a model near-ground swimmer it has been shown that a hydrofoil will be attracted to a stable equilibrium altitude due to competing hydrodynamic forces.  Here, a potential flow decomposition method using the unsteady Bernoulli equation is presented and applied to understand the competing forces that give rise to equilibria.  It is shown that many previous hypotheses do not hold and that equilibria are a balance between negative time-averaged quasi-steady lift and positive time-averaged wake-induced lift, while the time-averaged added mass lift is nearly zero across all ground proximities.  Results that run counter to previous hypotheses are examined in detail. 

Abstract: An aspect ratio 9.5 rectangular wing is revolved in a cylindrical domain at 45-degree angle of incidence and a Reynolds number Re = 300, based on the wing velocity two chord lengths from the axis of rotation. Four cases are considered. Case A represents the physical problem in which the approach velocity varies linearly with the distance from the axis of rotation and Coriolis and centripetal accelerations are active in the non-inertial reference frame attached to the wing. Case B implements the same reference frame attached to the wing, 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 leading-edge vortex (LEV), demonstrating that inflow shear is an important factor governing LEV behavior, in addition to the rotational accelerations. However, the mechanisms are active at different times in the vortex evolution. Vorticity transport analyses were conducted in chordwise planar control regions, at z/C = 2.0 (measured from the axis of rotation), revealing distinct differences in the contributions of the vorticity sources governing leading-edge vortex development for the four cases investigated.

Abstract: The Betz efficiency, since its derivation over one hundred years ago, has served as an upper bound on the power-conversion efficiency of wind-energy systems in theory and in practice. Recently, however, Dabiri (Phys. Rev. Fluids, 2020) suggested that relaxing the steady-flow assumption of the Betz derivation can lead to efficiencies that exceed the Betz limit. We thus seek to determine the effect of unsteady streamwise motion on the efficiency of wind-energy devices both analytically and experimentally. We first model the influence of periodic streamwise motion of an actuator disc using time-varying velocity potentials, which allow us to quantify the effects of various motion profiles on the theoretical efficiency. We find that certain classes of velocity-potential models and streamwise-motion waveforms yield time-averaged efficiencies above the Betz limit. These analytical results then motivate the construction of an experimental apparatus to investigate the applicability of the theory to a horizontal-axis wind turbine actuated in surge motions. The planned experiments will clarify the relationship between unsteady streamwise motion and turbine efficiency, and will inform the design and control of both existing and novel energy-harvesting systems.

Abstract: The encounter of a vortex gust with an aerodynamic body is a canonical fluid–structure interaction with implications for the prediction of transient loads on fliers and swimmers and their generation of vortex sound. In this talk, we will present an analytical model that is used to investigate the dynamically-coupled interactions of vortex gusts encountering a symmetric Joukowski airfoil on linear elastic supports. The model is solved as a potential flow problem using a time-dependent conformal mapping. The Brown and Michael framework models the unsteady shedding of vorticity from the airfoil into the wake, and the aeroelastic motion of the airfoil is analyzed using unsteady airfoil theory. The proposed model may be used to investigate the combined effects of airfoil thickness, airfoil motions and shedding of vorticity on gust-airfoil interactions problems. Special attention is paid to the effects of gust position and strength alongside structural parameters on direct vortex impingement.

Abstract: A stationary wing placed in a wake generated upstream experiences lift enhancement and stall delay. This effect is the most predominant in the post-stall conditions and when the wing is placed at an optimal offset distance from the wake centerline. The von Karman vortex street in the wake causes excitation of the separated flow in the post-stall conditions. The increase in the time-averaged lift force is associated with the flow separation, leading-edge vortex formation, and subsequent reattachment in a process similar to the dynamic stall of oscillating wings. At the optimal location, direct impingement of large vortices is avoided, and the velocity fluctuations in the wake are much smaller than those at the wake centerline. The small-amplitude excitation of the shear layer separated from the leading-edge, coupled with the appropriate wake frequency, lead to large vortices and separation bubbles, providing increased lift in the time-averaged sense. In contrast, the large-amplitude velocity fluctuations at the wake centerline lead to alternating partially attached and totally separated flow (deep stall), which is not as effective for the lift enhancement. The mechanism of delayed flow separation is similar to that of active flow control separation, with similar optimal frequencies. The degree of the lift enhancement is remarkable, given that the wake at these Reynolds numbers is turbulent, with relatively small spanwise correlation length.

Abstract: Ring vortices are efficient at transporting fluid across long distances. They are observed in nature in various ways: they propel squids, inject blood in the heart, and entertain dolphins. These vortices are generally produced by ejecting a volume of fluid through a circular orifice and have been widely studied and characterised. After four convective times, three events happen simultaneously: the vortex moves faster than the shear layer it originates from, it separates from the shear layer, and the circulation and non-dimensional energy of the vortex converge. The simultaneity of these three events obfuscates the causality between them. To analyse the temporal evolution of the vortex independently of the separation, we analyse the development of vortices generated in the wake of cones. The vortex rings that form behind the cones have a self-induced velocity that causes them to follow the cone. They continue to grow as the cone travels well beyond the limiting vortex formation times observed for vortices generated by pistons. The non-dimensional circulation, based on the vortex diameter, and the non-dimensional energy of the vortex rings converge after three convective times. This result proves that the convergence of non-dimensional quantities is not just a consequence of the separation. In addition, the evolution of the vortex is modelled with an axisymmetric discrete vortex method. The model predicts accurately the evolution of the vortex.

Abstract: This talk will report on the development of a (semi) analytic method to approximate resolvent (pseudospectral) mode shapes and their amplification levels. These modes seek to reproduce wall-normal vorticity modes of maximal amplification of the linearized Navier-Stokes equations in wall-bounded parallel shear flows. The method assumes that mode shapes can be accurately approximated by a sum of suitably-defined wavepackets. The small number of parameters prescribing the shape of these wavepackets may be found by solving a low-dimensional optimization problem, thus eliminating the need to form and decompose discretized linear operators. We demonstrate the applicability and assess the performance of this method on the simplified scalar Squire operator in laminar plane Couette and plane Poiseuille flow. In particular, we show that the method can be applied to cases where leading resolvent modes are affected by boundary conditions and/or multiple critical layers. We next introduce a modified Laplacian inner product in order to apply this method to the full Navier-Stokes system for parallel shear flows. We additionally explore the capabilities of this method to compute suboptimal mode shapes and amplification levels, and discuss prospects for applying this method to more complex systems.

Abstract: Peripheral pulmonary artery stenosis (PPAS) is the most common cardiovascular abnormality associated with Williams and Alagille syndromes, two congenital disorders for which structural cardiovascular abnormalities comprise the leading cause of morbidity and mortality. Narrowing of the central and peripheral pulmonary arteries (PAs) in PPAS results in lung perfusion disparity, right ventricular (RV) hypertension, RV hypertrophy, and ultimately RV failure. No clinical consensus exists regarding the optimal treatment strategy for Williams and Alagille patients with PPAS, as transcatheter interventions limited to the central PAs have been associated with unfavorable outcomes, and extensive surgical repair requires long operating hours and specialized expertise. In this work, we lay the foundations for a virtual treatment planning platform capable of identifying lesions most critical for normalizing pulmonary vascular resistance (PVR) and thus PA pressures. We demonstrate our ability to accurately predict post-stent PA hemodynamics by incorporating the relevant autoregulatory physiology into our CFD simulations with fluid-structure interaction. Based on controlled comparisons of PA hemodynamics following different stenting plans within the same patient cohort, we further develop preliminary clinical recommendations. We note that our methods are broadly applicable to CFD investigations of other cardiovascular interventions.

Abstract: Propulsive flapping foils are widely studied in the development of animal-like autonomous systems and other engineering applications.  In this work, we carry out extensive numerical simulations to address various aspects of flapping propulsion. First, we explore the validity and applicability of strip theory to model flapping foils. We show that there exists an intermediate range of Strouhal numbers where the strip theory can be applied and that 3D effects dominate outside of this range. Second, we examine the possibility of predicting cycle-averaged peak forces of 3D foils based on 2D simulations. We show that an aspect-ratio based-correction, analogous to Prandtl finite wing theory, enables the use of strip theory in finite flapping-foil design. Finally, we study the importance of the leading-edge sweep angle to the performance of flapping foils. We carefully control the foil parameters of tail-like and flipper-like kinematics for a range of sweep angles. We observe no significant change in mean force, power, moment and efficiency for tail-like and flipper-like motions as the sweep angle increase, leading to a conclusion that fishtails, flukes and flippers can have a large range of potential sweep angles without a negative impact on hydrodynamic performance.

Abstract: Sensor placement and feature selection are critical steps in engineering, modeling, and data science that share a common mathematical theme: the selected measurements should enable solution of an inverse problem. Most real-world systems of interest are nonlinear, yet the majority of available techniques for feature selection and sensor placement rely on assumptions of linearity or simple statistical models. We show that when these assumptions are violated, standard techniques can lead to costly over-sensing without guaranteeing that the desired information can be recovered from the measurements. In order to remedy these problems, we introduce a novel data-driven approach for sensor placement and feature selection for a general type of nonlinear inverse problem based on the information contained in secant vectors between data points. Using the secant-based approach, we develop three efficient greedy algorithms that each provide different types of robust, near-minimal reconstruction guarantees. We demonstrate them on a problem where linear techniques consistently fail: sensor placement to reconstruct a fluid flow formed by a complicated shock-mixing layer interaction.

Abstract: Investigations into the biomechanics of cervical cerclage. Premature cervical remodeling, or cervical insufficiency, is a medical condition during pregnancy in which the uterine cervix softens and dilates before full term, usually between 18 and 22 weeks gestation, such that a preterm birth occurs. It is a common cause of second trimester pregnancy loss. Part of the clinical treatment of this condition is to perform a cervical cerclage (a purse string suture to close the cervix). Studies on the efficacy of this procedure are conflicting and mostly rely on statistical investigations. The importance of biomechanical factors such as cervical length, canal geometry and tissue softness are not as well documented. The purpose of this investigation is to examine the mechanical limitations of the cerclage. Working with physicians from The George Washington University Hospital, we create generalized, synthetic models of the cervix from ultrasound images and fabricate them with silicone to mimic physiological, softening cervical tissue. Aspects of the cervical geometry (length of cervix, shape and width of dilatation) and suture material used in the cerclage are varied. The synthetic cervices are stitched by physicians according to clinical techniques. Each synthetic cervix is placed into a capsule designed to contain the material while it is being compressed. Using a custom steel insert attached to a 5kN load cell, force is applied directly to the suture and measured as a function of time until failure (designated as when the cervical tissue begins to rip). The results of this study will provide insight into the most effective clinical interventions and the mechanism of their success.

Abstract: Zebrafish is used by biologists as an animal model to study the effects of neurotoxicants and drugs on locomotion and develop pharmacological treatments. Very few fish swimming simulations have been derived from real body deformations to investigate the complex and stereotyped escape response of zebrafish and support animal experimentation. This experiment-driven numerical approach combined experimental imaging for modeling body deformations and three-dimensional (3D) numerical simulations to compute the actual energetic performances. To this end, a novel 3D reconstruction of the zebrafish shape was generated and deformed according to experimental data. As a first application, three escape locomotion were recorded and simulated across six high-viscosity fluids. In addition to kinematic data such as traveled distance, velocity, and bending amplitude, the expended energy and cost of transport were computed based on the power output. Mean power and expended energy seemed to remain steady over viscosity, while the cost of transport was found highly correlated to fluid viscosity despite considerably altered escape phenotypes. Eventually, fictitious simulations were performed by combining body deformations and fluid viscosity, especially for challenging the experimental escape motions. Such simulations have revealed zebrafish was particularly efficient to escape and energetic expenditure could be emphasized by increasing fluid viscosity. These results provided preliminary insights for the implementation of an effort test, involving zebrafish experiments, viscous fluids, and energetic performances from numerical simulations.

Abstract: We study the kinematics and dynamics of a highly compliant membrane disk placed head-on in a uniform flow. With increasing flow velocity, the membrane disks deform nonlinearly into increasingly parachute-like profiles. The experiments were carried out in a closed-loop low-speed wind tunnel with Reynolds number in the range of 10^4 -10^5. Remarkably, these aerodynamically sustained membrane disks show a higher flow resistance (drag) than similarly shaped rigid concave bodies. We model the steady structural response of the membranes using a nonlinear aeroelastic model. The predictions of the model agree well with the mean deformations of the membrane disks for the full range of experimental parameters studied. Through a simultaneous quantification of the unsteady membrane kinematics and forces, we detect the onset of large amplitude membrane fluctuations, match with the observed drag modulation and have their origins in the resonance between the flow structures and the membrane’s natural frequency. A drum model with anisotropic spring-stiffness is proposed, which quantitatively captures the observed resonant response. Further, PIV experiments are being conducted to yield deeper insights into the steady and transient fluid-structure interactions.