2020 Archive

Abstract: The current understanding of biologically inspired aquatic propulsion is extensive for simple two-dimensional models but is lacking for more complex three-dimensional models. This experimental work used a novel two degree-of-freedom full fish platform to investigate the relationships between model kinematics and system performance. This platform was used to understand which conclusions from simple models can be extended to full fish models. Phase-averaged torque input, thrust output, and kinematics were acquired for 567 cases that spanned a motion parameter space that included the maximum trailing edge excursion of the caudal fin, the phase offset between the tail and caudal fin motion, and the heave-to-pitch ratio. We have shown that thrust and efficiency are maximized when the phase offset is between 90 and 115 degrees. The optimal heave-to-pitch ratio was highly dependent on the trailing edge excursion and ranged between 0.43 and 0.78. The optimal parameters shown here are different than those for simplified two-dimensional models and warrant further investigation. We have shown that efficiency is maximized when the peak in power input is temporally aligned with the peak in power output. Together, these results hint at strategies for anticipating or designing model kinematics for specific performance goals.

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

Abstract: Potential flow plays an important role in many applications, including flow estimation in aerodynamics. For the models in these applications to work efficiently, it is best to avoid Biot-Savart interactions between the potential flow elements, particularly for 3D models. This work addresses grid-based computations for planar potential flows and their implementation in a low-order vortex model for fast modeling of separated aerodynamic flows and gust interactions. The model uses the immersed boundary projection method to solve for the vector potential field subject to the constraints introduced by the presence of a body, any edge conditions, and Kelvin’s circulation theorem, with each constraint adding a Lagrange multiplier to the overall saddle point system. Sharp edges are treated by decomposing the body forcing Lagrange multiplier into a singular and non-singular part. To enforce the Kutta condition, the non-singular part can then be tuned to remove the singularity introduced by the sharp edge. The equations are discretized on a staggered Cartesian grid and solved using the lattice Green’s function. The accuracy of these computations is demonstrated for a flat plate shedding singular vortex elements in 2D and the extension to 3D flows will be discussed.

Abstract: Visual observations of fluid-structure interactions (e.g. the swaying of trees in the wind) encode information about the surrounding flow conditions. We present a physics-based model that leverages the relationship between the force on a structure due to incident flow and the resulting structural deflection to measure normalized flow speeds. Visually-measured wind speeds are compared to ground truth, anemometer-measured values for wind tunnel experiments on flexible cantilevered cylinders and trees. Results suggest the potential for flow-speed inference using measurements of structural deflections, which can reduce the need to further instrument or visualize the flow in order to measure its speed.

Abstract: The fluid-structure interaction of an inverted flag in a uniform flow exhibits three different vibration modes: the stationary, flapping and deflected modes. The vortex-induced vibration of such a flag is studied both numerically and experimentally to identify its flapping behavior. Experiments of flags made of spring steel were conducted in a wind tunnel, where the wind speed was swept up and down through the various oscillatory modes of the inverted flags, while the numerical simulation of the same problem was thoroughly studied. It was noticed that during its mode transition, a difference in critical velocity occurs during upsweep and downsweep, creating a hysteresis that was shown to be controlled by the combination of the leading edge and the trailing edge vortices. To better understand this, we adapted a recently proposed force partitioning method technique and extended the method for flexible structures using integral boundary formulation of thin structures to identify the modal force contributions from the major flow features. We will discuss the connection between hysteretic behavior and the leading coherent vortices from the fluttering flags and explain how the flag aspect ratio affects its bistable vibration.

Abstract: Transcatheter heart valves (THV) suffer from clinically silent complications like subclinical leaflet thrombosis which may result in fatal outcomes for the patient. Such malfunction is detected incidentally during post-implant follow-up, and common imaging techniques are either invasive or expose the patient to radiation and are cost prohibitive. This informs a critical need for a novel, non-invasive and non-toxic continuous monitoring modality of THVs which can provide persistent and longitudinal monitoring of prosthesis function. We conduct a data-driven, in-silico investigation into the viability of wireless, remote monitoring of prosthetic aortic valve health using pressure microsensors. To do this, we developed a versatile reduced-order valve model capable of simulating a wide range of valve conditions. The strong coupling between leaflet mobility and downstream hemodynamics facilitates correlating pressure measurements at strategic locations in the vicinity of the THV with leaflet status. High-fidelity simulations of transvalvular flow in a canonical aorta model with various valve conditions allow us to construct hemodynamic “signatures” of healthy and dysfunctional leaflets. These signatures can be analyzed using supervised learning methods to determine optimal sensor configuration, detect the presence of reduced leaflet motion (RLM) and quantify its severity. Preliminary results demonstrate pressure measurements at as few as two discrete locations per valve leaflet can be used for accurate retrospective as well as prospective prediction of leaflet status (“Healthy”/ “RLM”) and its range-of-motion.

Abstract: The choice and placement of sensors and actuators is an essential factor determining the performance that can be realized using feedback control. This determination is especially important, but difficult, in the context of controlling transitional flows.  The highly non-normal nature of the linearized Navier-Stokes equations makes the flow sensitive to small perturbations, with potentially drastic performance consequences on closed-loop flow control performance. Full-information controllers, such as the linear quadratic regulator~(LQR), have demonstrated some success in reducing transient energy growth and suppressing transition; however,  sensor-based output feedback controllers with comparable performance have been difficult to realize. In this study, we propose two methods for sensor selection that enable sensor-based output feedback controllers to recover full-information control performance: one based on a sparse controller synthesis approach, and one based on a balanced truncation procedure for model reduction. Both approaches are investigated within linear and nonlinear simulations of a sub-critical channel flow with blowing and suction actuation at the walls. We find that sensor configurations identified by both approaches allow sensor-based static output feedback LQR controllers to recover full-information LQR control performance, both in reducing transient energy growth and suppressing transition. Further, our results indicate that both the sensor selection methods and the resulting controllers exhibit robustness to Reynolds number variations.

Abstract: We develop a cluster-based feedback control strategy to characterize the complex dynamics of cavity flow. This probabilistic model is utilized to modify the evolution of flow states to a desired dynamics using limited sensor measurements for capturing the attractor physics. The flow state is encoded into a low-dimensional feature space, which is partitioned into a set of clusters, where the occurrence of each flow state is represented by a cluster probability vector. The evolution of the flow-state probability distribution is given based on the transition probability matrix, which encapsulates the intra-cluster and inter-cluster transitions. We perform community detection on the transition probability to identify and characterize the flow physics among the most probable inter-cluster transitions. Further, we exploit the linearity of the probability transitions and formulate an LQR problem to control the cluster probability vector to a desired probability distribution. We first demonstrate this probabilistic control approach on the canonical Lorenz-63 system with bistability. We then employ the cluster based characterization of dynamics exhibited by the 2D laminar cavity flow. The feature space is chosen as the limited sensor measurements along the cavity wall. The clusters are further divided into communities and we identify a probable transition path from the low rms pressure fluctuations to large rms pressure fluctuations. We discuss our efforts to reduce the pressure fluctuations in the cavity using this cluster based approach.

Abstract: The wind gusts are known to compromise the quality of commercial aircraft. They have, however, more dramatic effects on vehicles that operate at significantly lower speeds such as Micro Air Vehicles (MAVs). Due to their low operational speeds, most gusts MAVs encounter are large-amplitude. Deepening the fundamental understanding of the flow physics of large-amplitude gust encounters will provide valuable information for the development of aerodynamic models that can quickly predict the unsteady forcing for gust flows of various types. The current work aims to study and compare the response of flat plate wings in both transverse and vortex gust encounters. A sine-squared transverse gust was created in a water towing tank using a water jet, whilst a vortex gust was created in a water channel by clockwise half rotation of an upstream gust generator plate. Both gust encounters resulted in large transients in the lift force and the unsteady forcing was found to depend on the circulation shed from the leading edge of the wing. The lift force experienced and the circulation shed showed a steeper increase for the transverse gust encounters despite the same gust ratio of 1. Moreover, average velocity variations in time were obtained from the flowfield measurements along an upstream line for both gust types, and variations in the gust-induced angle of attack were computed. The Helmbold equation was then used to predict the lift coefficients based on the calculated gust-induced angle of attack. The trends in the measured and predicted lift coefficients were found to compare well with each other, but the values were found to agree well only when the gust-induced angle of attack is low.

Abstract: We develop an adaptive mesh refinement strategy compatible with the lattice Green’s function (LGF) technique for solving viscous, incompressible flows on unbounded domains.   The LGF method exploits the regularity of a finite-volume scheme on a formally unbounded Cartesian mesh to yield robust (conservative, stable) and computationally efficient (linear complexity) solutions.  The original method is spatially adaptive, but embedded mesh refinement is challenging to integrate with the underlying LGF which is only defined for a fixed resolution.  We present a strategy for mesh refinement where the solution to the pressure Poisson equation is approximated using the LGF technique on a composite mesh constructed from a series of infinite lattices of differing resolution.  For the incompressible Navier-Stokes equations, this is further combined with an integrating factor for the viscous terms and an appropriate Runge Kutta scheme for the resulting differential-algebraic equations. The parallelized algorithm is validated with numerical simulations of vortex rings.  The collision of vortex rings at high Reynolds number is simulated to highlight the reduction in computational cells achievable with both spatial and the refinement adaptivity. 

Abstract: The aeroelastic effect is an important research topic covering aircraft stability, renewable energy extraction, animal locomotion, and more. In this work, we study the flutter response of an airfoil with an active flap using a high-fidelity FSI algorithm. Through various analysis tools, including a modal analysis technique incorporating the structural response with the flow, the connections between the airfoil fluttering motion and the flap oscillation amplitude and frequency are revealed. The flutter behavior is shown to be determined by the relative intensity of the geometry-induced natural frequency shift and the flap-induced vortex shedding mechanism. The finding of the current research provides insight into how the structure and flow interact with each other and could be utilized to form better control methods of the aeroelastic problems.

Professor Jeff D. Eldredge, University of California, Los Angeles

Abstract: Time-periodic flows are ubiquitous in fluid mechanics, and they often exhibit complex dynamics arising from the nonlinear interaction between harmonics of the fundamental frequency. We propose a framework that elucidates the input-output characteristics of these flows in the proximity of a time-periodic solution of the governing equations. More specifically, we linearize the dynamics about the aformentioned orbit, and we compute the harmonic resolvent operator, which is a frequency-domain linear operator that governs the dynamics of small time-periodic perturbations about a periodically time-varying base flow, in response to some time-periodic forcing input. The singular value decomposition of the harmonic resolvent sheds light on the dominant input-output structures of the flow. The right and left singular vectors are the optimal forcing and response modes, respectively, and they can be understood as full spatio-temporal signals that reveal the space-time amplification mechanisms that dominate the flow. The singular values can be understood as gains on their corresponding forcing-response pairs, and they can reveal whether the underlying physical mechanisms can be described via a low-rank reconstruction. Furthermore, the harmonic resolvent operator provides insight into the leading-order interactions between structures at different frequencies, and we will see that perturbations at frequency ω interact with perturbations at frequency α through the base flow at frequency ω − α. We first demonstrate the method on a system of three ordinary differential equations. We then apply it to the two-dimensional flow over an airofil at near-stall angle of attack and, finally, we use it to understand the input-output dynamics of perturbation in a separated turbulent boundary layer at Reθ = 490.

Abstract: Unlike most biological swimmers that rely on body/caudal fin (BCF) type of locomotion, a California sea lion produces thrust by moving its large foreflippers from above its head into a position abducted against its abdomen, a motion called a ‘clap’. This is followed by a long glide in a streamlined position. The flow structures resulting from this motion will not resemble the traditionally seen structures during BCF swimming, namely the reverse von Kármán street. Here, we use soft robotics and particle image velocimetry (PIV) to study the flow around an anatomically correct silicone flipper that is actuated by a servo motor. The flipper is mounted on a robotic platform and is programmed to clap into a flat plate that represents the body of the sea lion. The resulting data indicates that thrust is not produced through compression of fluid between the ventral side of the flipper and the body. Instead, the surrounding fluid is entrained by the upper surface of the flipper, producing vortices that run along the span and directly off the tip of the flipper. We also notice a cutoff frequency after which the efficiency of velocity production diminishes, which indicates the existence of an ideal ratio between rotational velocity and tip speed.

Abstract: Direct numerical simulations are performed for leading-edge momentum injection control of flow over a NACA0009 airfoil at post-stall angles of attack. The response to momentum-injection can be decomposed into two components: a short-time response that is characterized by an initial decrease followed by an increase in the lift, and a long-time response that is sensitive to the instantaneous wake state at the onset of actuation. We develop a theoretical model following classical unsteady aerodynamic theory where the effect of actuation is modeled as a combination of source/sink, doublet, and vortex elements to capture the short-time response to actuation. We demonstrate the capabilities of data-driven reduced-order models, such as dynamic mode decomposition (DMD) and DMD with control, to model both the short- and long-time behavior of the system. To this end, we utilize insight gained from the theoretical models derived to specify and interpret the form that this model takes. We find that the lift response consists of a component directly proportional to the rate of change of actuation strength, and a circulatory contribution that persists after the actuation burst.

Abstract: Voice production is a result of fluid-structure interaction (FSI) in the larynx between glottal airflow and a pair of vocal fold whose vibration is induced by the flow. Computational modeling of the FSI process can be used for treatment of voice disorders, e.g., planning of medialization thyroplasty, the surgical procedure for unilateral vocal fold paralysis. In this project, we aim to develop a computer software suite that consists of several tools, e.g., the FEM model of the vocal fold, 1D flow-3D tissue FSI model, as well as full 3D FSI model, that can be collectively used for increasing fidelity of modeling. In particular, the 1D flow-3D tissue model may be used to identify patient-specific tissue properties and also design optimization for the surgical implant. In this study, we describe a 1D unsteady and viscous flow model that is derived from the momentum and mass conservation equations. To enhance the model, we use a machine learning approach (SINDY) to determine the free modeling parameters with a series of 3D FSI simulation results as the training data. After training, we apply the enhanced 1D flow model coupled with the 3D tissue model in the FSI simulation of both idealized vocal fold geometries and subject-specific anatomical geometries reconstructed from the MRI images of rabbits' larynges. For the idealized geometries, we compare the simulation results from the simplified FSI model with those from the full 3D FSI simulation. For the subject-specific geometries, we compare the results with those from the high-speed imaging experiment of in vivo phonation. The 1D flow model is validated in both of these setups and is shown to have robust performance.

Abstract: The partial circulation approach described by Eldredge (2020) provides a way to estimate the leading-edge suction parameter LESP on airfoils with finite thickness or in cases where the bound circulation/velocity potential is not known (e.g. in PIV results with laser shadows). A relation between the LESP and the leading-edge partial circulation is developed from a potential flow model, in which the LESP is proportional to the partial circulation at a fixed location with first-order approximation. The method is tested on a fixed airfoil undergoing intermittent separation in a wind tunnel with a randomly surging flow, and on viscous and inviscid numerical simulations. The estimated LESP correlates with the degree of flow separation. Comparisons between the different cases show that in general, the larger the contour used to integrate the partial circulation, the more accurate the measurement of LESP. Boundary layer vorticity near the leading edge contaminates the partial circulation estimation and the dependency upon the locations of the starting and ending points of the integration contour.

Abstract: Bioprosthetic heart valves are used in the surgical repair of a variety of congenital heart defects, including Tetralogy of Fallot (ToF) which affects 1 in every 2500 newborns annually. The longevity of bioprosthetic valves is highly variable and there is currently little understanding of what hemodynamic factors may lead to early valve dysfunction. In this work, we analyze the flow in a 3D printed model of the pulmonary outflow tract representative of ToF anatomy, with a 25mm valve implanted, at cardiac outputs of 2 L/min, 3.5 L/min, and 5 L/min. The full 3D, three-component, phase-averaged velocity fields for each case were obtained over the cardiac cycle using magnetic resonance velocimetry (MRV). In addition, images of instantaneous valve leaflet motion were captured with a high-speed camera at 1500Hz. The velocity fields revealed key differences among all cases in the location of reverse flow regions, systolic jet shape, vorticity patterns, and flow asymmetry. High-speed camera images showed that effective valve orifice area, leaflet closing dynamics, and the flutter frequency of the leaflet tips also varied with cardiac output. In particular, the 2 L/min case produced more asymmetry, stronger recirculation regions, and a smaller orifice area than the other cases, which could contribute to uneven leaflet fatigue and allow for calcification that may lead to early valve dysfunction. 

Abstract:  Surfaces undergoing rotation are common in rotorcrafts, wind turbines, and bio-mimicking MAVs among others and exhibit highly temporal and three-dimensional flow. As such, the quantification of the time-resolved flow over rotating wings is crucial to assess aerodynamic performance. In this study, we present preliminary results of the evolution of flow over a rotating wing using a novel rotating volumetric velocimetry technique to enable measurements in the rotating frame of reference. First, the design and implementation of the new methodology is discussed. Experiments to measure time-resolved velocity field in the rotating frame of reference over an impulsively started rotating wing were conducted using this methodology. Leading-edge vortices (LEV) depicted by Q-criterion are observed to have a highly three-dimensional and temporal nature, with the primary LEV having higher strength and size compared to the subsequent vortices. Furthermore, the contributions of vortex tilting/stretching and convection components of the vorticity equation on the dynamics of these leading-edge vortices will be quantified. 

Abstract: Wall-bounded turbulent flows can be challenging to measure within experiments due to the breadth of spatial and temporal scales inherent in such flows. Instrumentation capable of obtaining time-resolved data (e.g., hot-wire anemometers) tend to be restricted to spatially-localized point measurements; likewise, instrumentation capable of achieving spatially-resolved field measurements (e.g., particle image velocimetry) tend to lack the sampling rates needed to attain time-resolution in such flows. In this study, we propose to fuse measurements from multi-rate and multi-fidelity sensors with predictions from a physics-based model to reconstruct a wall-bounded turbulent flow. A "fast" filter is formulated to assimilate high-rate point measurements with estimates from a physics-based model---derived using rapid distortion theory (RDT). Additionally, a "slow" filter is used to update the reconstruction every time a new field measurement becomes available. By marching through the data both forward and backward in time, we are able to reconstruct the turbulent flow with greater spatiotemporal resolution than either sensing modality alone. We demonstrate the approach using direct numerical simulations of a turbulent channel flow from the Johns Hopkins Turbulence Database. 

Abstract:  We present preliminary results for resolvent analysis of laminar separated flows over spanwise periodic swept wings at high angle of attack. Sweep is used in aircraft design to improve overall aerodynamic performance. However, the post-stall flow physics around swept wings is still largely unexplored. In this configuration, the presence of a persistent spanwise flow affects the flow structures. Results from resolvent analysis show how three-dimensional modes are amplified as we increase the sweep angle. The present study serves as a stepping stone for analyzing turbulent and three-dimensional flows over swept wings.

Abstract:  We experimentally study the effect of wing inertia on the flow-induced oscillations of a cyber-physically mounted pitching wing. We report two distinct oscillation modes, namely a structural mode, occurred via a subcritical bifurcation, associated with a high inertia; and a hydrodynamic mode, occurred via a supercritical bifurcation, associated with a low inertia. We characterize these two oscillation modes by analyzing the corresponding amplitude, frequency, force and flow response of the wing, and lastly use an energy approach to explain the existence of these two modes.

Abstract:  In this study, we compare the lift regulation of two open-loop control and one closed-loop control pitch maneuvers during a large-amplitude transverse gust encounter. The first maneuver is calculated to directly negate the effective angle of attack imposed by the transverse gust, the second maneuver is calculated by modeling the encounter using Wagner and Küssner’s unsteady aerodynamic models, and the third maneuver is achieved using a closed-loop feedback control law based on real-time lift measurements.

Abstract:  In flow over an inclined spinning disk, low to moderate tip speed ratios can weaken or even suppress periodic vortex shedding. At higher tip speed ratios, a distinct short-wave instability forms in the advancing tip vortex. We use Spectral Proper Orthogonal Decomposition (SPOD) on the flow field to identify spatially and temporally coherent modes in the flow and compare this short-wave instability to elliptic instabilities in the literature.

Abstract: We show that an inexpensive ensemble of low-dimensional vortex models can accurately estimate low Reynolds number aerodynamic flows, even with unknown flow disturbances, through the assimilation of surface pressure measurements.  We look at two scenarios: an impulsively translating plate subject to flow actuation or placed in a cylinder wake. 

Abstract: The focus of this work is to uncover the physical mechanisms that initiate and sustain flow-induced vibration of cylinders. The influence of different mechanisms is quantified by using a method to partition the force and work done on the cylinder into distinct, physical components. The analysis suggests that vortex shedding in the wake might not be the primary driver of so-called “vortex-induced vibrations.”