2023 Archive

Abstract: Bubbly turbulent flows occur in many engineering and environmental applications. In these flows, an important aspect is the interaction of bubbles with vortical structures, where bubble deformability and size play a crucial role. To understand these complex interactions, we experimentally study an idealization, namely, the interaction of a single air bubble with a single water vortex ring, with the focus being on the effects of bubble deformability and bubble-to-vortex size ratio on both bubble dynamics and vortex ring dynamics. During these interactions, the bubble dynamics are captured using high-speed imaging, while the effects on the ring’s vortex core are captured using time-resolved PIV. 

In the talk, I will focus on the interaction of a deforming air bubble and a rigid buoyant particle (a rigid bubble), with a single water vortex ring, with deformability being the distinct difference between the two. Both the buoyant particle/ deforming bubble are captured by the vortex ring, due to the low pressure within its core, leading to a strongly coupled interaction between the bubble/buoyant particle and the vortex ring. The deforming bubble undergoes elongation both outside the ring during its capture, and within the ring after capture, with the latter stage leading to bubble break-up within the ring. In contrast, the rigid bubble remains spherical during capture, and stays more localized within the ring. These differences in deformability lead to distinct differences in the ring’s convection speed, azimuthal vorticity, and enstrophy. These results could have implications in bubbly turbulent flows such as in bubble drag reduction where many studies indicate that deforming bubbles are better for drag reduction than nondeforming ones.

Abstract: Cyclorotors are a propulsion system that use several rotating, periodically pitching blades to produce a net force in a single direction. Previous studies have found that they exhibit improved performance at low Reynolds Numbers compared to conventional rotors, making them of much interest for the field of Micro Unmanned Aerial Vehicles. Due to the curvilinear flow they are subjected to, the blades experience an effect known as virtual camber which has an impact upon cyclorotor efficiency. In theory, this effect could be countered by cambering the blades in the opposite direction, but previous studies have found this to have detrimental effects on the blade’s capability to generate a leading edge vortex (LEV), which can aid their force production. As an alternative, this study looks into using blades that only camber their trailing edge portion, leaving the leading edge portion intact for LEV production.  The effects of this camber on force production, efficiency and flow structures were investigated on a cyclorotor in the hover configuration, through a combination of dynamic force measurements coupled with particle image velocimetry.  The results show that the leading edge vortex is preserved when only the trailing-edge is cambered as opposed to the fully cambered blade. However, this still leads to an overall reduction in force production and efficiency compared to the symmetric blades.

Abstract: While several previous studies considered tandem cylinders coupled through flow between them, a hitherto unexplored elastic coupling with fluid flow between them significantly influences FIV. Therefore, we numerically study the transverse flow-induced vibration (FIV) of elastically coupled tandem cylinders at Reynolds number 100 using ViCar3D. A systematic comparison between the classic elastically mounted tandem cylinders and elastically coupled cylinders is presented. The latter configuration exhibits two vibration modes, in-phase and out-of-phase, with corresponding natural frequencies approaching the Strouhal frequency of the system. Using the obtained results, we answer the following questions. (a) What are the FIV regimes associated with elastically coupled tandem cylinders? (b) What is the mechanism for lock-in of classic and elastically coupled tandem cylinders? (c) What are the participating wake modes in a quasi-periodic FIV regime? (d) Does the gap vortex formation modify by the small amplitude FIV response? (e) What is the effect of the relative motion of the cylinders on galloping response? (f) Can the system be used for undamped FIV suppression/energy harvesting applications?

Abstract: The drag force on an accelerating object is usually described by a quasi-steady force that scales with the square of the instantaneous velocity and an added mass force due to the acceleration. This description could lead to a significant underestimation of the actual drag force. We aim to find a better description of the drag force on a flat plate in an unsteady flow by measuring both the drag force and velocity field for a large range of constant accelerations and velocities. Our experiments show that the force due to acceleration does not scale linearly with acceleration, contrary to what is expected from added mass. We associate this force to the generation and advection of vorticity at the plate surface and combine this into a single scaling law model, based on the history force for unsteady flow. This scaling avoids previous inconsistencies in using added mass forces in the description of forces on accelerating plates. This new scaling law has proved useful in predicting the drag force and total circulation in the wake of different plate geometries and for non-constant accelerations.

Abstract: We investigate theoretically and experimentally the stability of three interlaced helical vortices with respect to displacement disturbances whose wavelengths are large compared to the size of the vortex cores. A space-time analogy is used to present a model for the spatial evolution of semi-infinite helical vortices subjected to time-periodic disturbances. This model is used to investigate the spatially evolving wake of a three-bladed turbine, which is subject to the global or local pairing instability that represents different wake control strategies. Perturbations modify the minimal distance between neighboring pairs, but also their relative orientation, i.e., locally parallel, antiparallel, or perpendicular.  This is relevant for vortex merging, but also for the development of elliptical and Crow instabilities.

Abstract: Seals use their specialized whiskers to detect and follow prey underwater. Previous studies have revealed that the harbor seal whisker morphology suppresses lift oscillation and vortex introduced vibration through features including flattened cross section, undulatory thickness, and a unique 180° frontal-dorsal phase difference in the undulation. These features have inspired many flow sensor designs. However, to understand the broad range of flow and signal correlation experienced by the seal whisker and to optimize the design and expand the application of inspired sensors, systematic investigations into the vortex-introduced vibration (VIV) of seal whiskers are needed. In this study, the single degree-of-freedom (cross flow) VIV of a harbor seal whisker is solved using direct numerical simulation for parametrically varied reduced velocity and angle of attack (AOA) at a constant Reynolds number of 300. The whisker is modeled as a rigid body with a mass ratio of 1 and a damping ratio of 0.02. The results have revealed an abundance of modes of response in the parametric space. In this talk, we will elaborate on the vortex shedding modes and how they compare to those from basic shapes like circular and elliptical cylinders. Particularly, we will show how the dynamic mode decomposition (DMD) of the flow field helps to identify dominant modes from complex three dimensional wake structures.

Abstract: The influence of initial geometry on the formation, development, and decay of a vortex loop behind a polygonal disk at a Reynolds number Re = 20, 000 is investigated. Using particle image velocimetry in 5° azimuthal increments, we reconstruct a 360° visualization of the vortex loop generated behind square and circular disks starting impulsively from rest. The coherence of the loop is strongest behind a circular disk, remaining strong and maintaining its shape far downstream. Meanwhile, as the number of sides of the polygonal disk decrease, topological changes to the vortex loop are observed, and a faster transition to turbulence, as compared to the disk.

Abstract: Many engineering applications involve flow past a moving boundary or interface, which can be challenging to simulate with a body fitted mesh. Immersed methods avoid the need for mesh generation altogether, but these methods have so far been limited to second order accuracy when applied to 3D flows with moving boundaries. In this talk we present our progress towards an immersed interface method that achieves high order spatial and temporal accuracy in 3D simulations with complex geometries and moving boundaries. Our spatial discretizations combine dimension-split finite differences with high order weighted least squares interpolants near immersed boundaries, yielding third order spatial accuracy for advection terms and up to sixth order spatial accuracy for diffusion terms or elliptic PDEs. We also discuss the issue of "freshly-cleared cells" in sharp interface methods with moving boundaries, and present a treatment that maintains the temporal order of accuracy of arbitrary explicit Runge-Kutta schemes (as well as preliminary results for diagonally implicit schemes). We conclude by applying this approach to a 3D immersed interface discretization of the advection-diffusion equation on a multiresolution grid, which captures thin boundary layers on immersed surfaces through a combination of high order discretizations and grid adaptivity.

Abstract: Modern engineering devices such as wind/tidal turbines and micro air vehicles experience unsteady loads due to turbulence and gusts. This unsteady loading may fatigue turbine blades or the vehicle's wings over time and therefore the mitigation of unsteady load is critical for these devices. Natural flyers such as insects and birds acquire both active and passive systems to control their position and velocity, and to mitigate the impact of gusts and lulls.  Active control systems are established in engineering, whereas passive control systems are not fully understood. To address this, we investigate the unsteady load mitigation mechanism of an aerofoil made of a passive trailing-edge, which spans 2/3 of the chord. The aerofoil undergoes sinusoidal heaving motion at a mean angle of attack of 6 degrees and Reynolds number of the order of 10 thousand. This study reveals that the deflection and unsteady load mitigation is scaled with the product of two different Cauchy numbers, which are the non-dimensional number comparing the fluid force and elastic force. These results will pave the way for the development of low-order models to better predict passive deflection and unsteady loading.

Abstract: This study examined lift forces on two nonslender delta wings with sweep angles of Λ = 40° and 50°, both having flags attached to the leading edge. Notable increases in lift were observed in the post-stall regime for the clean wings, as well as delay in stall. When a flag was present, flow field measurements indicated that the shear layer reattached to the wing surface and the leading-edge vortex re-formation was observed. In contrast, wings without flags experienced a completely stalled flow under the same free stream conditions. Across various angles of attack, mass ratios, and flag lengths, maximum lift improvement was achieved when the flag oscillated at an optimal dimensionless frequency range, and the tip velocity of the flag had a sufficient amplitude. This optimal frequency range was consistent with the natural shear layer instabilities found for the clean wings. Excitation provided by the flag oscillations within this range significantly improved the coherency of the otherwise separated flow. The main mechanism in increasing lift appears to be the excitation of the shear layer, which exhibits a convective instability. This differs substantially from lift enhancements on airfoils, where flags lock-in to the wake instability, which is known to have an absolute instability at the post-stall angles of attack for the clean airfoil.

Abstract: The stomach is responsible for mixing, grinding, sieving, and chemically breaking down food. The peristaltic motion of the stomach walls combines with the secreted gastric enzymes to physically and chemically breakdown the ingested food. However, experimental investigation of these phenomena is challenging and cost-intensive. This study presents a computational model of the stomach based on imaging data. An enzyme is secreted from the stomach walls that mixes with the contents and hydrolyzes the protein in the liquid meal. We also use the model to study the dissolution of an orally ingested pill and the subsequent delivery of the dissolved drug into the intestines. The effects of weaker motility, caused by disorders such as gastroparesis, on the flow field inside the stomach and on the rate of protein hydrolysis and drug dissolution are analyzed. The findings demonstrate the potential of a computational approach in this field in quantifying the effects of disease and dysfunction.

Abstract: We investigate the hydrodynamic implications of organised surface textures on aquatic animals. This study considers a modified NACA foil with an egg-carton type roughness at Re=100,000. On the foil, we prescribe general undulatory kinematics that result in zero net thrust. In this presentation, we investigate a reduction in the required power for a roughness wavelength corresponding to λ=L/128. First, we use dynamic mode decomposition (DMD) to identify coherent flow structures; we stabilise the boundary layer by applying the body motion's inverse map to the flow field. The DMD analysis identifies distinct flow structures present in the smooth and power-reducing roughness topology that can be likened to Helmholtz rollers. Next, we identify sensitivities in the cases by using the DMD basis to perform resolvent analysis. Finally, we correlate resolvent modes with power frequency spectra, enabling targeted visualisation of flow structures at specific frequencies. The study concludes that optimally tuned rough surfaces do more than merely increase frictional drag; they also alter flow structures, occasionally increasing the performance of the swimmer.

Abstract: There are over 3000 snake species, each exhibiting various lifestyles, including terrestrial, amphibious, and marine behaviors. Throughout more than 100 million years of evolution, their elongated and limbless body structure has remained remarkably preserved. Despite their seemingly simple morphology, snakes have adapted and developed more than 10 distinct gaits to navigate diverse environments. For instance, some snakes like the dice snake (Natrix tesselatta) prefer swimming fully submerged to forage, while others like the garter snake (Thamnophis sauritus) are more inclined to swim on the water's surface.

We proposed to visualize the wake produced by these two swimming snakes using two different methods. We used Defocused Digital Particle Tracking Velocimetry (DDPTV) to observe the vortex structures created by Natrix tesselatta swimming underwater. The waves produced by Thamnophis sauritus swimming on the surface were measured using synthetic Schlieren imaging.

The PIV revealed the creation of multiple vortices along the body of the snake due to its undulation. The 3D structure of the vortices generally consisted of paired vortex tubes, some of which were linked together to form a hairpin structure. The observations match predictions from computational fluid dynamic studies of other anguilliform swimmers. The Schlieren imaging allowed us to observe that the snake was simultaneously producing dragging and thrusting waves, contributing to the general propulsion of the animal. 

Abstract: Passive flow control via fluid-structure interaction (FSI) is a promising control paradigm for unmanned aerial vehicles operating in vortex-dominated low Reynolds number regimes. The associated aerodynamic flows are unsteady, high dimensional and nonlinear. A flexible structure therefore has the potential to passively alter key unsteady vortex structures through its vibrations, if its intrinsic modal dynamics are carefully aligned with the driving flow processes. Towards this aim, we perform high-fidelity numerical FSI simulations to study the dynamic interplay between a separated aerodynamic flow with vortex shedding content (Re=500 and AOA 15) and a flexible flat plate modeled using linear Euler-Bernoulli beam theory. We focus on how the flow dynamics and associated lift are informed by the structural modal shape(s) and deformation timescales of the structural response, and how in turn this is informed by the structural parameters relative to the vortex shedding behavior of the baseline reference (rigid plate) case. The ending long-term dynamics are then found to be characterized by the natural frequencies of the beam, taking appropriate amount of fluid effects. These long-term limit cycle dynamics are described in detail by this characterization, drawing connections to the baseline rigid case as appropriate.

Abstract: Recent advancements in small size Unmanned Aerial Vehicles (UAVs) have led to their increased popularity due to their versatility, including vertical takeoff and landing (VTOL) capabilities and low operating costs. Although UAVs emit less noise than helicopters, their increased use, especially in densely populated areas at lower altitudes, will likely lead to stricter noise regulations. As the rotors still remain as one of the major noise source, understanding rotor noise generation at these low rotational speeds is crucial for developing certified systems that meet these standards. To gain insight on the underlying noise generation mechanisms, high-fidelity numerical simulation is employed to study the aeroacoustics of a rotor operating in the transitional regime. The investigation focuses on a two bladed NACA0012 rotor with a constant pitch angle. Result shows distinctive flow structures along its span. Inboard, attached laminar flow is observed, while midspan to the blade tip region experiences flow separation with 2D and 3D flow structures.

These distinct regions along the span leave characteristic signatures on the near and far field acoustic spectra. Spectral analysis of pressure fluctuations at the blade surface and in the near wake reveals tonal peaks at the blade passing frequency and harmonics, along with a broadband hump at higher frequencies. These findings are in line with the experimental observation. The broadband hump originates from separated shear layers on both the pressure and suction sides, while peaks within the hump also arise from the leading and trailing edges due to shear layer instabilities and their interaction with the following blade's edges, including the tip vortex and wake.

Abstract: During the COVID-19 pandemic, face masks were the first line of defense for the global population to reduce the spread of the virus. However, the effectiveness of face masks can be affected by a number of factors, including how the mask is worn, the type of mask, and the wearer's activities. One of the most common activities that can affect the effectiveness of a face mask is talking. When people talk, the morphology of their faces changes, which can cause the mask to move and create gaps that allow respiratory droplets to escape. This is especially true for people who wear cloth masks, which are less effective at filtering out respiratory droplets than surgical masks or N95 respirators. To explore this problem systematically, we have developed a new method for modeling the movement of facial features during talking. This method, called geometrically weighted principal component analysis (GWPCA), can be used to create a low-complexity model of the face features. The GWPCA model can then be used to create a dynamically moving facial shape for identified categories of talking syllabi. This information will be used to explore the fluid dynamics of facemasks.

Modeling the airflow between the face and a mask is very complex. This is because it depends on the shape of the face, the type of mask, the wearer's activities, and other factors. An analytical integral boundary layer model is pursued here to quantify the flow in the interface region between the face and mask using mask shape on a moving face from a detailed deployment mask mode. An analytical model is validated with a detailed flow simulation and then employed to find the relationship between the fitness of the mask during talking, mask porosity, and its level of leakage. The research outcome will be a fast  model that enables improved operational forecasting of respiratory disease spread. It will directly lead to better epidemic planning in the future and provide a technique to quantify the efficacy of mask recommendation strategies for the diverse population in real time during future pandemics. 

Abstract: Impulsive flows happen in various circumstances of interest to engineering applications. However, the direct measurement of the hydrodynamic load acting on a body is not always possible for different reasons, for example the wake sensitivity of sensors. Moreover, this force measurement does not allow to correlate the story of the force with the physics of the flow. As an alternative, the feasibility of applying a planar control-volume approach to time-resolved PIV (TR-PIV) experimental data is investigated. The focus is on reconstructing the unsteady load acting on an accelerated flat plate normal to the flow. Although the flow is in the turbulent regime (Re = 18×10^3), the assumption of two-dimensional flow is made at the mid-plane of a plate with aspect ratio AR = 5. Three different kinematic cases have been considered by varying the value of acceleration and keeping the same target velocity. The method adopted here consists of using the control volume approach to estimate the force, in combination with a Poisson solver for the pressure field. The method was tested on direct numerical simulation (DNS) data of an impulsively accelerated flat plate where different experimental uncertainties are taken into account to mimic real PIV data. This allows us to identify sources of error and how they contribute to the final result. In both the numerical and experimental data the pressure term appears to give the largest contribution to the noise in the total force. Then, the force estimation based on experimental PIV data is shown in comparison with the force measured by a force sensor. The results match the measured force for the first part of the motion, where the unsteady term and the pressure term dominate the drag force.

Abstract: Generally, natural fliers demonstrate greater flight control than manufactured aerial vehicles. Elastic joints allow natural wings to passively respond to gusts enabling steady flight in turbulence. To investigate the underlying physical principle, we consider the gust response of a fixed foil and a foil that can passively and elastically pitch. The incompressible Navier-Stokes equations are weakly coupled with a rotational harmonic oscillator within the open-source toolbox OpenFOAM. A 2D NACA0012 foil is modelled at an initial chord-based Reynolds number Re = 10^3, at an angle of attack of 5°. The initial spring moment is set to balance the fluid dynamic torque. The inflow velocity is doubled within one convective time following a hyperbolic tangent law. The results show that a passively-pitching foil experiences a total load fluctuation that is up to seven times lower than that of a fixed-pitch foil. Different gust types, including reducing inflow speed and changing inflow direction, have been examined, and results show similar effect on mitigating unsteady loads through passive pitch. We demonstrate through an analytical model that the quasi-steady variation of any force component, e.g. the lift, can be cancelled if the pitch axis lies along a semi-infinite line from the foil. We show how the pitching axis location influences the efficacy of the unsteady load mitigation for different gust types. We extend these results by testing selected cases at Re = 5*10^4 and Re = 10^6, where the same trends are observed. Furthermore, when the gust triggers vortex shedding on the fixed foil, this is suppressed if the foil is allowed to passively pitch. These results provide new insights into the design of a passive system to mitigate unsteady loads; a system that may enhance the controllability and resilience of aerial and underwater vehicles, and turbine blades.

Abstract: In this study, we introduce a cluster-based decomposition technique to create data-driven, coarse-grained representations of nonlinear dynamics in complex systems subjected to external forcing. We collect time-series measurements by simulating the system with added external forcing and apply k-means clustering for coarse-graining. We represent the measurement data as a weighted sum of the centroids associated with the clusters and train the corresponding weights using sparse regression. This results in a deterministic, nonlinear predictive model for the time-series measurements' evolution, contrasting with probabilistic linear Markov models found in previous studies. We integrate this approach with model-predictive control (MPC) to guide the system towards favorable states. We first demonstrate the method using the Lorenz system, then apply it to control the laminar flow over a flat plate at a high angle of attack (35 deg) by employing momentum injection. This approach effectively achieves high lift and low drag states.

Abstract: The hydrodynamic interactions between individual swimmers can lead to the formation of stable schools. These interactions, which occur between fish bodies and vortices shed by other fishes, are determined by the kinematics and spacing of neighboring swimmers. In this experimental study, we investigate how these interactions are affected by tailbeat frequency for a pair of pitching hydrofoils. We found that equilibrium constellations of the two-fish school can be manipulated by changing the pitching frequency. We analyzed these constellations using multi-layer stereo PIV to capture three-dimensional flow structures. Understanding schooling mechanisms at high frequencies can provide insights into the schooling of high-speed fish species such as tuna, as well as benefit the design and control of future high-speed multi-agent bio-inspired robotic platforms.

Abstract: We will explain the basic concepts of network theory and discuss the benefits of applying network-based analysis to fluid flows. We will review recent advances in the field, including the use of network measures to identify important flow structures, detect transitions in flow behavior, and analyze interactions in laminar and turbulent flows as well as fluid-structure interaction systems. We will also highlight some challenges in applying the network-based methodology, including the need for appropriate data acquisition and processing methods, and the need to develop more sophisticated network models that can capture the full complexity of fluid dynamics.

Biography:  Aditya G. Nair joined the Department of Mechanical Engineering at the University of Nevada, Reno in August 2020. His research interests are in the areas of computational fluid dynamics, unsteady aerodynamics, high-performance computing, data science, and control theory focused on modeling and control of high-dimensional fluid flow physics. Aditya G. Nair received his Ph.D. in mechanical engineering from Florida State University (Tallahassee) in 2018. Prior to this, he completed his M.S. in mechanical engineering from the University of Michigan (Ann Arbor) in 2013 and a B.E. in mechanical engineering from the University of Mumbai in 2011. Following his Ph.D., he served as a post-doctoral research associate in the mechanical engineering department at the University of Washington (Seattle) till July 2020. 

Abstract: The role of compressibility on airfoil flutter in the transonic regime is investigated with a series of two-dimensional direct numerical simulations based on an immersed boundary method. An energy map approach based on forced kinematics is used to identify limit cycle oscillations and quantify the influence of Mach number and spring stiffness on pitching amplitude fluctuations. A subcritical instability is observed for Mach numbers close to 0.7 which covers a range of frequencies. Three shock-induced mechanisms are identified that influence the extent of energy imparted from the flow to the airfoil. The primary mechanism is flow separation triggered by the generation of a lambda shock. The influence of the lambda-shock dynamics on airfoil flow separation is characterized for different Mach numbers providing insight into the "transonic dip" observed under the current flow conditions.  

Abstract: Unsteady separated flow is present on many physically large objects. Low-fidelity simulations cannot accurately estimate the influence of separated flow. The physical scale of some objects precludes full-scale testing in wind tunnels and complicates the acquisition of performance data in operational conditions. Small-scale wind tunnel tests can provide an estimate of the aerodynamic loading. Tufts can be used to qualitatively validate the surface flow between different investigations and the full-scale object in operational conditions. In the current investigation it is shown, with computational and experimental data, that tufts can provide a quantitative estimate of the unsteady wing loading. Unsteady 3D simulations of a NACA0012 wing, at different angles of attack and Reynolds numbers, are used to obtain unsteady surface flow and lift coefficient data beyond stall. The computational data provides a proof-of-concept by using a linear surrogate model based on pseudo tuft orientations, this model is then extended with a non-linear component. Experimental data of a NACA0012 wing, equipped with tufts and a force balance, has been used in combination with neural networks to infer quantitative information about the unsteady wing loading. This results in the ability to capture non-periodic lift and pitching moment fluctuations based on visual tuft observations.

Abstract: Vortex tubes with initial axial core-size variations develop twist waves, and collision of such twist can lead to a sudden radial expansion of the vortex core known as vortex bursting. For sufficiently large initial core-size variations and Reynolds numbers, bursting leads to significant increases in enstrophy and has the potential to destabilize the vortex core. Using direct numerical simulations of the 3D Navier-Stokes equations, we have simulated and analyzed vortex bursting on tubes with circulation-based Reynolds number of 5000. For a baseline analysis, we have considered bursting on rectilinear vortex tubes. Currently, we are probing the stability of the mechanisms involved when bursting occurs on vortices with curved centerline geometries, such as helical vortices and vortex rings. The results indicate that interaction between the bursting structure and the core dynamics affects the details of the flow evolution and further destabilizes the bursting structure and the core structure. In this talk I will present an overview of our results of vortex bursting on straight and curved vortices.

Abstract: We present a series of Direct Numerical Simulations of the planar flow past an impulsively started cylinder at Reynolds numbers up to 1,000,000. An intriguing portrait of unsteady separation is revealed; vorticity generation and vortex shedding entail a cascade of separation events on the cylinder surface that are reminiscent of Kelvin-Helmholtz instabilities. Primary vortices roll-up along the cylinder surface as a result of instabilities of the initially attached vortex sheets, followed by vortex eruptions, creation of secondary vorticity and formation of dipole structures that are subsequently ejected from the surface of the cylinder. The vortical structures and their relationship to the forces experienced by the cylinder are analyzed.  

Abstract: Characterizing the aerodynamic response of wings to oncoming gusts is critical to maintaining stability and efficiency of aircraft. In this study, surface pressure and particle image velocimetry measurements are used to analyze the unsteady flow physics of a NACA 0015 wing in a time-varying freestream flow. Unsteadiness exhibited in the wing's aerodynamic response to velocity acceleration and deceleration can be attributed to the dynamics of developing vortical structures. Whether the flow is accelerating or decelerating determines the temporal and spatial scales of the vortical structures, including the convective time, size, and location from which vortical structures develop and shed. These scales determine the degree to which vortical structures interact with each other and with the wing surface, thereby also influencing the unsteady loading on the wing.

Abstract: Mechanical Aortic Valves (MAVs) are routinely implanted as permanent replacements for malfunctioning or diseased human valves.Compared with bioprosthetic ones, MAVs are more durable and insusceptible to tearing and calcification, nevertheless they promote non-physiological hemodynamics which might lead to platelet activation and mechanical hemolysis. Previous research has correlated the blood damage to augmented levels of turbulent stress downstream of MAVs when compared to bioprosthetic devices. In this scenario, we  numerically investigate two emerging technologies proposed for mitigating such detrimental effects on hemodynamics: a tri-leaflet configuration and a bi-leaflet valve with vortex generators. Simulations are carried out by means of a finite difference flow solver with immersed boundary forcing. When compared to the baseline design, vortex generators are found to anticipate the break-up of the shear layers downstream of the leaflets, lowering the overall turbulent shear stress at peak flow rate. Conversely, the trileaflet design provides comparable haemodynamics at peak flow rate, but further reduced stress levels in the deceleration phase. The findings of this study could be used to improve the design of next-generation MAVs in order to reduce the risk of thromboembolic complications.

Abstract: This study is an extension of our previous work on surface actuation as a lift improvement strategy for flow past an airfoil at Re 1000. In the current work, we employ gradient based optimization to determine the spatio-temporal properties of actuation. With actuation introduced on the entire surface of the airfoil, we explore the configuration of actuation for lift and drag improvements separately. It is found that actuation has distinct behavior for the two performance imperatives. Further, actuation for lift improvement results in a drag penalty and vice versa. Apart from highlighting the differences in temporal variation of the aerodynamic coefficients for each case, our investigation explores the critical spatial locations on the airfoil surface, the temporal variation of actuation in relation to the underlying shedding process and key flow features for each performance goal. 

Abstract: Helical vortices, such as those generated in the wake of a rotor, are subject to various instabilities including displacement instabilities, which occur when the vortex core is shifted from its baseline position. After being perturbed, the vortices deform and begin to break down. The zero-wavenumber displacement instability mode can be triggered by introducing an asymmetry to the rotor producing the vortices. The vortex dynamics in this case are highly complex, so a simplified model based on an infinitely-repeating strip of point vortices is developed to reproduce the nonlinear instability evolution. The model is validated against a more sophisticated filament model and water channel experiments, showing remarkable agreement for a range of parameters relevant to industrial rotors. It is then used to investigate the effectiveness of different types of rotor asymmetries at accelerating vortex breakdown. Even small initial displacements around 5% of the vortex spacing substantially disturb the vortices, and the direction of the perturbation plays an important role in the speed of the instability development. These findings can then be used to design wind turbine rotors that minimize the detrimental effects of their wakes on downstream turbines within a wind farm.

Abstract: It is well established that a flat plate impulsively set into linear motion will result in the formation of a clockwise vortex, also known as the starting vortex. This study is aimed at characterizing the resulting starting vortex and shear layer in the wake of the airfoil based on three initial conditions, namely, the angle of attack, the surge speed and the surge distance. It will be shown that the circulation of such a starting vortex is a fraction of Wagner’s prediction. Further, it was observed that the shear layer breaks into secondary vortices beyond a critical surge speed. The primary vortex in this case is also characterized.

Abstract: Dynamic stall is known to be a ubiquitous phenomenon in rotary flows, leading initially to an overshoot in performance followed by a dramatic loss in such performance. Being able to prevent or delay this performance loss via flow control mechanisms is highly dependent on understanding the underlying flow physics at play, including identifying the dominant unsteady scales associated with the dynamic stall vortex shedding process.  Experimental particle image velocimetry data from a series of water tunnel experiments is used to study the flow evolution around a dynamically pitching NACA 0012 airfoil at low Reynolds number. The airfoil underent a linear pitch ramp maneuver at a fixed dimensionless pitch rate of 0.05 across three Reynolds number to reproduce a canonical light dynamic stall process. The primary objective of this study is to assess the scalability of the dominant scales associated with the flow perturbations across multiple Reynolds numbers. The wavenumber scales associated with this canonical dynamic stall process were extracted using a combination of two-dimensional Log Gabor filtering scheme and Riesz transform. From the transverse velocity spectra, the fluctuations in the flow were observed to reach an amplified state during the initial ejection of vorticity from the leading-edge region of the airfoil.

Abstract: Wing-gust encounters cause harmful lift transients that can be mitigated through maneuvering of the wing. This work presents a method to generate an open-loop (i.e., prescribed) maneuver that optimally regulates the lift on the wing during a transverse gust encounter. Obtaining an optimal maneuver is important for laboratory experiments on the physics of wing-gust interactions and may be useful for the future design of feedback controllers. Prior work of us has shown that an Iterative Maneuver Optimization (IMO) framework can generate an optimal maneuver by using a surrogate model to propose a control signal that is then tested in experiment or high-fidelity simulation. The input to the surrogate model is updated to account for differences between the test data and the expected output. The optimal maneuver is obtained through iteration of this process. This paper simplifies the IMO method by replacing the surrogate model with the classical lift model of Theodorsen, removing the process of optimization over the surrogate model, and removing the requirement to know the time-averaged profile of the gust. The proposed method, referred to as Simplified IMO (SIMO), only requires input and output data collected from simulations or experiments that interact with the gust. Numerical simulations using a Leading Edge Suction Parameter modulated Discrete Vortex Model (LDVM) are presented to generate the input and output data of the wing-gust encounters for this paper. The results show an optimal pitch maneuver and an optimal plunge maneuver that can each regulate lift during a transverse gust encounter.

Abstract: The optimal stiffness for soft swimming robots depends on swimming speed, which means no single stiffness can maximise efficiency in all swimming conditions. Tunable-stiffness would produce an increased range of high-efficiency swimming speeds for robots with flexible propulsors and enable soft control surfaces for steering underwater vehicles. We propose and demonstrate a method for tunable soft robotic stiffness using inflatable rubber tubes to stiffen a silicone foil through pressure and second moment of area change. We achieved double the effective stiffness of the system for an input pressure change from 0 to 0.8 bar.  We achieved a resonant amplitude gain of 5 to 7 times the input amplitude and tripled the high-gain frequency range compared to a foil with fixed stiffness. These results show that changing second moment of area is an energy effective approach to tunable-stiffness robots. We have carried out underwater force measurements and flow visualisation for pitching foils and static foils subject to disturbances with the goal of demonstrating the ability to use the foil as a propulsor and as a control surface. 

Abstract: The use of fixed and rear flaps (boat-tail) on heavy road vehicles is an established method to suppress vortex shedding and improve aerodynamic performance. In nature, jellyfish use moving and flexible flaps to swim with unmatched efficiency. Is it possible to use nature-inspired moving flaps to further improve the aerodynamic performance of bluff bodies? To answer this question, we have numerically studied the flapping dynamics of two rear pitching flaps in the presence of a laminar bluff body wake for the first time. The study has uncovered the fundamental physical mechanisms which influence the aerodynamic performance of bluff bodies and the strategies capable of producing net-energy-savings.

Abstract: Forced motion of membranes in a fluid offers various fascinating problems. A scarcely documented forcing consists in placing a submerged membrane in a water wave field, thus imposing the excitation frequency along the whole membrane. This type of interaction has first been studied for potential applications as a wave barrier. It also has been shown that a submerged membrane attached to the seafloor could be an efficient and robust wave energy harvester. So far, the interaction between waves and a submerged membrane has been studied mostly analytically and numerically, with a strong focus on applications, while only few experimental works have been performed to characterize it. In this study, we adress the problem of the interaction between water waves and a submerged membrane, by means of physical experiments, with a view to highlighting and quantifying the various physical phenomena that contribute to the interaction. A thin flexible membrane, clumped at one end, is placed horizontally in a wave field. The simultaneous measurement of the waves (using full reconstructions of the wave field based on top view visualizations of the experimental flume) and of the deformation of the elastic plate (using side view video recording) provides many key parameters for an in-depth understanding of the interaction, such as the energy reflected and transmitted by the structure, or the membrane deflection modes. It is found that the membrane reflects little energy but tends to withdraw energy from the waves, for wavelengths close to the membrane length. The mechanism for wave energy attenuation is further investigated using Particle Image Velocimetry. Observations suggest that a significant part of membrane momentum is transferred to the fluid, in the form of a local horizontal stream near the free edge of the membrane, instead of radiated waves.

Abstract: Many tasks in fluid mechanics, such as design optimization and control, are challenging because fluids are nonlinear and exhibit a large range of scales in both space and time. This range of scales necessitates exceedingly high-dimensional measurements and computational discretization to resolve all relevant features, resulting in vast data sets and time-intensive computations. Indeed, fluid dynamics is one of the original big data fields, and many high-performance computing architectures, experimental measurement techniques, and advanced data processing and visualization algorithms were driven by decades of research in fluid mechanics. Machine learning constitutes a growing set of powerful techniques to extract patterns and build models from this data, complementing the existing theoretical, numerical, and experimental efforts in fluid mechanics. In this talk, we will explore current goals and opportunities for machine learning in fluid mechanics, and we will highlight a number of recent technical advances. Because fluid dynamics is central to transportation, health, and defense systems, we will emphasize the importance of machine learning solutions that are interpretable, explainable, generalizable, and that respect known physics. 

Biography: Dr. Steven L. Brunton is a Professor of Mechanical Engineering at the University of Washington.  He is also Adjunct Professor of Applied Mathematics and Computer science, and a Data Science Fellow at the eScience Institute.  Steve received the B.S. in mathematics from Caltech in 2006 and the Ph.D. in mechanical and aerospace engineering from Princeton in 2012.  His research combines machine learning with dynamical systems to model and control systems in fluid dynamics, biolocomotion, optics, energy systems, and manufacturing.  He received the Army and Air Force Young Investigator Program (YIP) awards and the Presidential Early Career Award for Scientists and Engineers (PECASE). Steve is also passionate about teaching math to engineers as co-author of three textbooks and through his popular YouTube channel, under the moniker “eigensteve”.