2022 Archive

Abstract: For many animals, sensing the flow of water or air disturbance is vital to their survival: it helps them locate food, mates, and prey and to escape predators. Across species, many flow sensors take the form of long, flexible cantilevers. These cantilevers exhibit sustained oscillations when interacting with fluid flow. Although much is known about how flexible cantilevers oscillate under the influence of vortex shedding, little is known about the mechanisms governing these structures' oscillatory response without vortex shedding. Our present work employs a high-fidelity numerical solver to examine the dynamics of a long, flexible cylindrical cantilever at Reynolds numbers below the critical Reynolds number of vortex shedding, i.e., Re<45. Of particular interest is investigating the underlying mechanism of the cantilever's sustained oscillations in this Re regime. Results show that frequency lock-in/synchronization is the mechanism that facilitates the transmission of disturbances to the wake and leads to the cantilever's sustained oscillations. We show that the motion of the cantilever during synchronization results in periodic vortex shedding in water for Re>20. For the cantilever in the air, wavy patterns in the shear layer dominate the wake region during the vibrations, indicating that parallel shear layers synchronize with the cantilever's motion. The tip motion trajectory of the cantilever resembles a figure-eight shape in water and an oval shape in the air. The findings of this study lead to a view in which synchronization is regarded as an intrinsic characteristic of fluid-structure systems and suggest possible directions for designing artificial flow sensors.

Abstract: Hydrodynamic pressure is a physical quantity that is utilized by fish and many other aquatic animals to generate thrust and sense the surrounding environment. To advance our understanding of how fish react to unsteady flows, it is necessary to intercept the pressure signals sensed by their lateral line system. In this study, we propose a new, non-invasive technique for reconstructing the instantaneous pressure field around an undulating body from particle image velocimetry (PIV) data. The proposed method utilizes a physics-informed neural network (PINN) to predict an optimized solution for the velocity and pressure fields that simultaneously satisfies the governing equations (i.e., the Navier Stokes equations) and the constraints put forth by the measurements. The method was validated using a direct numerical simulation of a self-propelled fish. The results demonstrate that when compared to the Queen 2.0 algorithm by Dabiri et al. (2014), the PINN is less sensitive to the spatio-temporal resolution of the velocity field measurements and provides a more accurate pressure reconstruction, particularly on the surface of the body. The improved accuracy can be attributed to three main advantages of the PINN method: 1) the kinematics of the undulating body can be directly incorporated into the pressure reconstruction process, 2) the pressure at the fluid-body interface can be directly computed without the need for extrapolation, and 3) the user has the flexibility to only enforce boundary conditions that are physically relevant. These results demonstrate that PINNs are a useful data assimilation tool for studying biolocomotion or fluid-structure interaction.

Abstract: With a growing interest in low Reynolds numbers compressible flows, compressibility effects on the secondary instabilities developing on the circular cylinder periodic wake are investigated. The unsteady and time-averaged two-dimensional flows are characterized for Reynolds numbers from Re=200 to 350 and Mach numbers up to Ma=0.5, revealing different flow structures and characteristic length scales that correlate to the instability wavelengths. The two-dimensional time-periodic solution is used as base state for a global stability analysis performed by means of Floquet theory coupled with a time-stepping finite difference approach of the non-linear operator, identifying Mode A and Mode B secondary instabilities, which are responsible for the three-dimensionalisation of the two-dimensional periodic wake. A stabilizing or a destabilizing effect of compressibility is observed on Mode A, depending on the Reynolds number and the mode wavelength, while Mode B is found to be stabilized by the increase of Mach number.

Abstract: Fish modulates the caudal fin flexibility and curvature during cruising by activating the muscles connected to fin-rays. For this purpose fish uses the bilaminar structure of the fin-ray consisting of two bony layers (hemitrich) and one soft layer (intraray). However, the effect of fish muscle activation on propulsion performance is not fully known. Therefore, a two-dimensional realistic bilaminar sunfish caudal fin-ray model is developed to study the effect of fin-ray muscle control on propulsion performance. Specifically, first the material properties of the fin-ray are inversely optimized to match the experimental data. Then a parametric study is developed by considering a sinusoidal muscle activation function for the ray and changing the muscle activation magnitude and phase. The ray is implicitly coupled with a flow field to study the fluid structure interaction. The result shows that the muscle activation cannot both increase thrust force and propulsion efficiency. It also shows that the activation phase is a critical factor affecting propulsion performance in which the maximum propulsion efficiency occurs when the activation phase is 270°, accompanied by a weak leading edge vortex. It shows that increasing the thrust is accompanied by an increase in trailing edge vortex strength and trailing edge-leading edge amplitude ratio. The wake behavior of the flow field is also studied thoroughly. A new definition for the angle of attack is introduced based on the relative velocity of the center of mass of the ray and the chord line orientation. It is showed that the lower angle of attack leads to a more slicing-like motion, resulting in a weaker leading edge vortex and finally a higher the propulsion efficiency.

Abstract: The ability of membrane wings to adapt their shape passively in unsteady flow conditions enables several aerodynamic advantages over rigid wings. In pursuit of a theoretical model for evaluating these benefits, a theoretical framework is developed to predict the two-dimensional membrane wing response to unsteady flow conditions in an inviscid flow. Our model assumes linear deformations of an extensible membrane under constant tension, which are coupled aeroelastically to external aerodynamic loads using the unsteady thin airfoil theory. The structural and aerodynamic membrane responses are investigated for harmonic heave oscillations, an instantaneous change in angle of attack, sinusoidal transverse gusts, and a sharp-edged gust. The unsteady lift responses for these scenarios produce aeroelastic extensions to the classic Theodorsen, Wagner, Sears, and Kussner functions, respectively, for a membrane airfoil. These extensions incorporate for the first time membrane fluid-structure interaction into the expressions for the unsteady lift response of a flexible airfoil. In this talk, we will explore how the membrane elasticity affects the unsteady lift and how these affects could be exploited for gust mitigation strategies in future applications.

Abstract: This work proposes a novel approach to characterize unsteady, undulating propulsion components through evaluating the momentum and energy equations using detailed control volume analyses. Specifically, it is difficult in such conditions to separate thrust from drag as pressure components are inseparable. In general, the goal is to link physical processes to equation-specific control volume assessments measured within Computational Fluid Dynamics (CFD) models. Findings indicate that the energy equation uniquely highlights lift-work in the energy budget and can separate propulsive and drag forces. The effort expands from previously validated CFD studies of thrust resulting from heaving and pitching foils; results indicate that the presented approach provides a novel method to separate the axial force into thrust and drag components. Overall, the results indicate promise to isolate loss mechanisms from propulsive ones along with novel metrics of efficiencies useful to measuring self-propelling vehicles. 

Abstract: An axisymmetric body at the sufficiently high angle of attack shows separated flow with a set of vortices that remain attached to the body.  A cone is a self-similar canonical representation of such asymmetric structures with a peculiar asymmetric wake and body pressure distribution. In this talk, we will examine the contributions of different flow features in the wake of a perfect cone by relating the localized surface pressure to coherent flow structures. Direct numerical simulation is used to solve the flow over the axisymmetric cone for a wide range of angles of attack. In order to ensure higher accuracy and to achieve the required resolution near the boundaries, the immersed boundary method with pseudo-body-conformal grids is employed. We will discuss how two near-wake stable primary vortices originate from the separated shear layer, and how they induce reverse flow in the wake and initiate other strong secondary vortical structures very close to the surface. At a higher angle of attack, the primary vortices become asymmetric inducing substantial side forces. The role of major vortical structures on the force generation and pressure distribution on the cone surface is investigated to find the origin of this force asymmetry.

Abstract: A propeller design method has been developed that passively mitigates the formation of coherent tip vortex structures in the near-field of the rotor wake. Using the blade root bending moment coefficient (C_B) as a surrogate variable, gradients in circulation in the radial direction are avoided in a constrained optimization problem. A series of wind tunnel experiments are utilized to verify design prediction and diagnose topological characteristics of the propeller vortex wakes for a conventional power-optimized design and a vortex-attenuated design. Each set of blades designs were optimized for the same specified thrust coefficient, freestream velocity, and design RPM. Phase-locked stereoscopic particle image velocimetry (stereo-PIV) was utilized to visualize the shape and behavior of both sets of rotor blades in the axial configuration. It can be shown through the stereo-PIV that the resulting equally loaded blades produce significantly different wake shapes and behaviors. The wake optimized propellers can be shown to be more advantageous in developing quite blade technologies.

Abstract: Cross-flow turbines, also known as vertical-axis turbines, convert the kinetic energy in moving fluid to mechanical energy using blades that rotate about an axis perpendicular to the incoming flow. In this work, the performance and wake of a two-turbine array in a fence configuration (side-by-side) were experimentally measured. The turbines were operated under coordinated control, characterized by synchronous rotation rates with a constant phase difference. The array was measured with turbines co-rotating, counter-rotating with the blades traveling upstream at the array midline, and counter-rotating with the blades traveling downstream at the array midline. From the performance data, we found significant dependence between phase difference and the array efficiency. From the wake data we hypothesize how phase influences interactions between turbines.

Abstract: When two or more cylinder-like structures are placed in close proximity, the resulting interactions are known to alter the wake patterns significantly from those behind a single cylinder. One commonly investigated arrangement of two cylinders is the side-by-side configuration, wherein two identical cylinders are placed parallel to the incoming flow. In this work, the flow past two elliptic cylinders is studied experimentally in a water-tunnel. We consider two oval-shaped (elliptical) cylinders with large values of eccentricities (e = 0.89, 0.99), at varying gap ratios between the cylinders (0 < G < 2.0) and Reynolds numbers (200 < Re < 2000). The three well-known wake regimes (single-body wake, asymmetric wake, two parallel wakes) are quantified by dye visualization and planar particle image velocimetry (PIV). The formation length is observed to increase with decreasing gap ratio, and to decrease with increasing Reynolds number. In the case of the parallel coupled wake anti-phase regime, logarithmic spiral structures are observed in the outermost vortices. These outermost vortices are characterized by fitting logarithmic spirals to the flow visualization data, and evaluating the corresponding logarithmic spiral exponent, k. For an ellipse with e = 0.99 and G = 2.0, the value of k in the far-wake is shown to decrease with increasing Re.

Abstract: From air-sea gas exchange, oil pollution, to bioreactors, the ubiquitous fragmentation of bubbles/drops in turbulence has been modelled by relying on the classical Kolmogorov-Hinze paradigm since the 1950s. This framework hypothesizes that bubbles/drops are broken solely by eddies of the same size, even though turbulence is well known for its wide spectrum of scales. Here, by designing an experiment that can physically and cleanly disentangle eddies of various sizes, we report the experimental evidence to challenge this hypothesis and show bubbles are preferentially broken by the sub-bubble-scale eddies. Our work also highlights that fragmentation cannot be quantified solely by the stress criterion or the Weber number; The competition between different time scales is equally important. Instead of being elongated slowly and persistently by flows at their own scales, bubbles are fragmented in turbulence by small eddies via a burst of intense local deformation within a short time.

Abstract: In this talk, we numerically study the combined vortex and cavity synchronization/lock-in in a flexibly mounted hydrofoil at high Reynolds number. The coupled cavitation/fluid-structure system is solved using in-house body-fitted framework based on a variational finite element formulation. We identify a frequency lock-in phenomenon as the primary source of sustained large-amplitude vibrations of hydrofoil. We find that the unsteady lift forces lock into a sub-harmonic of the hydrofoil's natural frequency. During the cavity collapse and shedding, we observe a periodic generation of spanwise clockwise vorticity, leading to unsteady lift generation. We determine the origin of this flow unsteadiness near the trailing edge of the hydrofoil via the interplay between the growing cavity and adverse pressure gradient. In the frequency lock-in regime, large coherent cavitating structures are seen over the hydrofoil suction surface accompanied by a cavity growth-detachment-collapse cycle. For the post-lock-in regime, cavity shedding is primarily limited to the cavity trailing end and the attached cavity is observed to undergo high frequency spatially localized oscillations.

Abstract: This talk will report on the results obtained from applying resolvent analysis on two-dimensional incompressible laminar and turbulent flows through square and rectangular ducts. We identify the dominant linear energy-amplification mechanisms present in such flows, and in particular study the effects of both aspect ratio and the secondary mean flows that are present in turbulent cases (Prandtl's secondary flows of the second kind). We find that the presence of such secondary flows can either amplify or suppress amplification in different regimes, and we identify cases for which small amplitude secondary flows (approximately 1% of the streamwise velocity) can lead to entirely distinct energy amplification mechanisms, with resolvent gains that are approximately twice as large.

Abstract: In this talk, time-resolved particle image velocimetry (PIV) of the flow over stalled symmetric and cambered aerofoils at a chord-based Reynolds number Rec=71,000 will be used to elucidate low-order dynamics and determine noise characteristics. At this Reynolds number, the flow exhibits a combination of intense turbulent fluctuations and strong coherent 2D leading-edge vortex roll-up. The ability to capture the stall state from just 3 pressure probes using linear stochastic estimation (LSE) is explored. Further, the universality of the low-order dynamics between the symmetric and stalled aerofoils will be discussed, with important consequences for real-time sensing of aerofoil stall for any foil geometry. Finally, the LSE framework is demonstrated for determining the instantaneous flow structures correlated with noise generation.

Abstract: The high Reynolds number turbulent separated flow over a Gaussian speed-bump has presented turbulence modelling challenges for predicting flow separation, reattachment, turbulent transition, and relaminarization. This has motivated the computational fluid dynamics community to accelerate progress in this area. Unfortunately, the lack of time-resolved (TR) experimental data on the Bump has limited progress on understanding the link between the unsteady dynamics and energy transfer mechanisms that lead to flow separation, which would provide substantial insight for the advancement of turbulence models. The above challenges motivate the present work, which attempts to provide TR estimates of the velocity field from under-sampled particle image velocimetry (PIV) Bump data. We propose flow estimation techniques that utilize surface-mounted pressure sensors and recurrent neural network architectures to predict transient dynamics that are inherently missing from the under-sampled PIV time-series. Results after up-sampling the 15 Hz original PIV time-series to 3000 Hz reveal the complex unsteady dynamics characterizing the Bump flow. We observe a very low-frequency breathing mode that is likely associated with the contraction and expansion of the separation bubble, coupled with unsteadiness at higher frequencies linked with the flapping of the shear layer.

Abstract: The laminar-turbulent transition often involves the evolution of Tollmien-Schlichting (T-S) waves of small amplitude. Real surfaces are not perfectly smooth and even small imperfections can affect this process. Here, we investigate the two-dimensional scenario of T-S waves interacting with rectangular bumps using a compressible DNS code. The bumps used had sharp corners which represents the most critical bump shape. Effects of Mach and frequency were analyzed for bump heights varying from 5% to 40% of the displacement thickness. The effect of the bump varied linearly with bump height only up to 10%. In the linear regime, the effect amounts to less than a 5% increase in wave amplitude. It was found that the waves required a substantial extent of the plate to return to the smooth surface growth rate. The bump effect strongly increased with height, but, somewhat unexpectedly, the region extension was almost independent on the bump height. For heights below 10% the effect of the bump was almost independent on frequency, but for heights above it, the bump effect increased with frequency. Up to M = 0.6, compressibility effects were small. At M = 0.9 these effects increased sharply because the downstream regions affected by the bump increased substantially. Since the mesh required to simulate such small elements is demanding, an approximate method to include the bump effect in a coarser mesh was developed. This reduced the computational costs by an order of magnitude. It constitutes an improvement on previous methods that use a Taylor expansion to determine a boundary condition at the wall that causes the same effect of a surface irregularity. The improvement involves corrections based on the Stokes’ layer and on a second order approximation.

Abstract: The collective movement of fish in schools is driven by behavioral imperatives such as safety from predators, improved foraging and increased hydrodynamic efficiency. While the movement of each fish is powered by propulsive forces generated by its fin(s), patterns that emerge in fish schools are modulated not just by behavior but also by the complex flow fields encountered by fish swimming within a school. Hydrodynamics also plays a key role in enabling a fish to sense the position/velocity of neighbors through its lateral line system, and to control its own velocity and heading. A model of collective swimming that allows the investigation of behavior, as well as hydrodynamic sensing and energetics, could answer questions related to the behavioral ecology of fish as well as to inform the design of bioinspired swimming systems. In the current study, we present a new dynamical model of collective swimming of fish that has three key features:(a) the model is based on Newtonian dynamics; (b) the model includes vortex wakes as well as the interaction of fish with these vortex wakes; (c) the model is parameterized with data from direct numerical simulations (DNS) of swimming fish. The complex collective swimming patterns that emerge from this model are analyzed and recapitulated against observations. The model is also used to examine the effect of factors such as turbidity and swimming speed on school topology and stability.

Abstract: In this presentation, I will talk about closed-loop flow control studies applied to flows with transition. High-fidelity numerical simulators are leveraged for testing techniques applied to trailing edge noise attenuation and transition delay. The extremum seeking control technique is applied for online minimization of acoustic noise produced in airfoil flows at two differents Reynolds setups, where distinct noise generation mechanisms take place. We also present the application of deep learning for control design tested with the Kuramoto-Sivashinsky equation, which is able to model convective instabilities.

Abstract: Offshore wind turbines and wind farms are increasingly gaining popularity. When installed in deep waters, bottom mounted turbines are no longer possible and must be installed on platforms, whose design is inspired from naval architecture. For this reason, the aerodynamic analysis for Floating Offshore Wind Turbines (FOWTs) has to take into account factors such as platform motion, which triggers vortex instabilities, modifying the wake structure, influencing the flow reaching downstream wind turbines. We investigate this effect here by the aid of numerical simulation. The fluid flow is modeled by Large-Eddy Simulations (LES) and the turbine is modeled by Actuator Lines. The platform motion is imposed, at a given amplitude and frequency. We find that the frequency of the imposed motion strongly affects the structure of the wake. For example, when excited with half of the rotation frequency, those movements favour the coalescence of six tip vortices, generating large structures, possibly increasing alternate loads on downstream turbines. For larger excitation frequencies, for example 1.5 of the rotation frequency, the more classical out-of-phase vortex pairing mechanism comes into play. Those findings are compared with results from stability analyses.

Abstract: This work constructs three open-loop pitch maneuvers with the objective of mitigating the lift transient on a wing during a large-amplitude, transverse gust encounter. The open-loop maneuvers were experimentally implemented for different gust ratios, and force, pitch moment, and flowfield measurements were collected. The lift histories of all the cases are decomposed and compared to identify the flow mechanisms responsible for lift attenuation. The pitch maneuvers were found to mitigate a significant portion of the circulatory contribution to the lift force. Accurate modeling of added-mass was found to be critical for lift mitigation maneuver design. Pitch control input was found to double the range of the pitching moment coefficients during the gust encounters relative to the uncontrolled case. The source of the pitching moment fluctuations between the pitching and non-pitching cases differed. Quantification of the total momentum variation in the measured flowfield showed that pitch maneuvers reduced the disturbance to the gust's flowfield, and thereby reduced the momentum transfer between the gust and the passing wing. Leading-edge vortices shed from the wing during the encounters were found to have an initial linear growth region, followed by a plateau corresponding to vortex detachment. Comparison of vortex circulation strengths between experiments and unsteady discrete vortex model (DVM) simulations demonstrated the viability of using DVMs for maneuver design.

Abstract: Understanding the hydrodynamics of fish swimming is crucial to the study of interactions of fish with their surroundings. Hydrodynamic models based on potential flow theory, in particular the vortex dipole-based models, constitute a viable approach to analytically study fish swimming, offering a mathematically tractable representation of the flow physics. Despite their promise, the accuracy of the dipole-based modeling paradigms has never been validated. Here, we bridge this gap through a computational fluid dynamics campaign informed by experimental data. We demonstrate that dipole-based hydrodynamic models can capture the major characteristics of the flow around a swimming fish, while they cannot predict the geometric effect of the elongated fish body. To address this limitation, we propose an alternative model that assimilates the fish body by a pair of vortex sheets, which demonstrates an improved accuracy with a marginal increase in the computational cost.

Abstract: An overview of the suite of methodologies collectively known as global linear (modal and non-modal) flow instability will be presented and the relation of global linear theory to resolvent analysis and theoretical flow control will be highlighted. Aspects of the theoretical foundation and numerical implementation will be briefly discussed. Representative solutions of two-dimensional (BiGlobal) and three-dimensional (TriGlobal) eigenvalue and initial value problems will be presented at incompressible, supersonic and hypersonic flow conditions.

Biography: Vassilis Theofilis obtained his MSc in Applied Mathematics and Fluid Mechanics and PhD in Aerospace Engineering at the University of Manchester, UK. After a post-doc at the Department of Applied Mathematics of University of Twente, the Netherlands, he has been Alexander von Humboldt research fellow at DLR Goettingen, Germany and Ramon y Cajal Research Professor at the School of Aeronautics, Technical University of Madrid, Spain. He held visiting appointments at Caltech, Arizona, Maryland, USA and the Universidad Federal Fluminense (Rio de Janeiro), Brazil. In 2016 he was appointed at the Chair of Aerospace Engineering at the University of Liverpool, UK and since 2019 he is also Full Professor at the Escola Politecnica of Universidade São Paulo, Brazil. His research interests lie in fluid flow instability from the incompressible to the hypersonic regime, including development and application of accurate numerical methods for the solution of the pertinent large-scale eigenvalue and singular value problems.

Abstract: An overview of the suite of methodologies collectively known as global linear (modal and non-modal) flow instability will be presented and the relation of global linear theory to resolvent analysis and theoretical flow control will be highlighted. Aspects of the theoretical foundation and numerical implementation will be briefly discussed. Representative solutions of two-dimensional (BiGlobal) and three-dimensional (TriGlobal) eigenvalue and initial value problems will be presented at incompressible, supersonic and hypersonic flow conditions.

Biography: Vassilis Theofilis obtained his MSc in Applied Mathematics and Fluid Mechanics and PhD in Aerospace Engineering at the University of Manchester, UK. After a post-doc at the Department of Applied Mathematics of University of Twente, the Netherlands, he has been Alexander von Humboldt research fellow at DLR Goettingen, Germany and Ramon y Cajal Research Professor at the School of Aeronautics, Technical University of Madrid, Spain. He held visiting appointments at Caltech, Arizona, Maryland, USA and the Universidad Federal Fluminense (Rio de Janeiro), Brazil. In 2016 he was appointed at the Chair of Aerospace Engineering at the University of Liverpool, UK and since 2019 he is also Full Professor at the Escola Politecnica of Universidade São Paulo, Brazil. His research interests lie in fluid flow instability from the incompressible to the hypersonic regime, including development and application of accurate numerical methods for the solution of the pertinent large-scale eigenvalue and singular value problems.

Abstract: We study the stability and non-linear behavior of a compressible flow in an open cavity. First, global stability analysis was performed in a region of the parameter space where both Rossiter and centrifugal modes are unstable, with the Rossiter modes being substantially more unstable. Next, DNS was carried out investigating the interaction of these modes. Without the centrifugal modes, the non-linear regime of the Rossiter modes has sharp spectral peaks that diverge substantially from Linear Stability Theory (LST) predictions and approach empirical predictions of Rossiter modes. In the presence of centrifugal modes, the Rossiter modes are more broad band in spectra, but frequency and other aspects are substantially closer to LST predictions.

Abstract: We used the Force Partitioning Method (FPM, Menon and Mittal, JFM 907, A37, 2021) to decompose the hydrodynamic drag in a rough wall turbulent channel flow. The contributions of vortex and strain dominated regions on the pressure drag are quantified using an auxiliary surface-dependent potential field \phi. We have identified different sources of drag and quantified their relative importance. These sources are: Q-induced force (where Q is the second invariant of the velocity gradient tensor), viscous momentum diffusion induced pressure force, and viscous shear forces on the solid walls. Results have shown that the Q-induced force is responsible for about 50% of the pressure drag and is mainly generated by the strain-dominated (Q <0) regions before each roughness element. We also explored characterizing the equivalent sandgrain height k_s for 21 fully rough channel flows (DNS data provided by Aghaei-Jouybari et al., JFM, 912, A8, 2021) using \phi-dependent norms, and found an empirical correlation that can predict k_s with rms and maximum errors of 12 and 30 percent, respectively, satisfying the expected universality and accuracy of such predictions.