25 research outputs found

    Vortex detection and tracking in massively separated and turbulent flows

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    The vortex produced at the leading edge of the wing, known as the leading edge vortex (LEV), plays an important role in enhancing or destroying aerodynamic force, especially lift, upon its formation or shedding during the flapping flight of birds and insects. In this thesis, we integrate multiple new and traditional vortex identification approaches to visualize and track the LEV dynamics during its shedding process. The study is carried out using a 2D simulation of a flat plate undergoing a 45 degree pitch-up maneuver. The Eulerian 1 function and criterion are used along with the Lagrangian coherent structures (LCS) analyses including the finite-time Lyapunov exponent (FTLE), the geodesic LCS, and the Lagrangian-Averaged Vorticity Deviation (LAVD). Each of \h{these} Lagrangian methods \h{is} applied at the centers and boundaries of the vortices to detect the vortex dynamics. The techniques enable the tracking of identifiable features in the flow organization using the FTLE-saddles and -saddles. The FTLE-saddle traces have shown potential to identify the timing and location of vortex shedding, more precisely than by only studying the vortex cores as identified by Eulerian techniques. The traces and the shedding times of the FTLE-saddles on the LEV boundary matches well with the plate lift fluctuation, and indicates a consistent timing of LEV formation, growth, shedding. The formation number and vortex shedding mechanisms are compared in the thesis with the shedding time and location by the FTLE-saddle, which validates the result of the FTLE-saddles and provide explanations of vortex shedding in different aspects (vortex strength and flow dynamics). The techniques are applied to more cases involving vortex dominated flows to explore and expand their application in providing insight of flow physics. For a set of experimental two-component PIV data in the wake of a purely pitching trapezoidal panel, the Lagrangian analysis of FTLE-saddle tracking identifies and tracks the vortex breakdown location with relatively less user interaction and provide a more direct and consistent analysis. For a simulation of wall-bounded turbulence in a channel flow, tracking FTLE-saddles shows that the average structure convection speed exhibits a similar trend as a previously published result based on velocity and pressure correlations, giving validity to the method. When these Lagrangian techniques are applied in a study of the evolution of an isolated hairpin vortex, it shows the connection between primary and secondary hairpin heads of their circulation and position, and the contribution to the generation of the secondary hairpin by the flow characteristics at the channel wall. The current method of tracking vortices yields insight into the behavior of the vortices in all of the diverse flows presented, highlighting the breadth of its potential application

    A quasi-objective single-buoy approach for understanding Lagrangian coherent structures and sea ice dynamics

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    Sea ice drift and deformation, namely sea ice dynamics, play a significant role in atmosphere–ice–ocean coupling. Deformation patterns in sea ice can be observed over a wide range of spatial and temporal scales, though high-resolution objective quantification of these features remains difficult. In an effort to better understand local deformation of sea ice, we adapt the trajectory-stretching exponents (TSEs), quasi-objective measures of Lagrangian stretching in continuous media, to sea ice buoy data and develop a temporal analysis of TSE time series. Our work expands on previous ocean current studies that have shown TSEs provide an approximation of Lagrangian coherent structure diagnostics when only sparse trajectory data are available. As TSEs do not require multiple buoys, we find they have an expanded range of use when compared with traditional Eulerian buoy-array deformation metrics and provide local-stretching information below the length scales possible when averaging over buoy arrays. We verify the ability of TSEs to temporally and spatially identify dynamic features for three different sea ice datasets. The ability of TSEs to quantify trajectory stretching is verified by concurrent ice fracture in buoy neighborhoods ranging from tens to hundreds of kilometers in diameter, as well as the temporal concurrence of significant storm events.</p

    Analysis of the flow structure and reattachment over accelerating non-slender delta-wing planforms

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    Delta wings are triangular-shaped lifting surfaces, and in past decades, they have been found to have important applications in maneuvering combat aircraft and supersonic aircraft. Slender, or high swept, delta wings have been widely studied in early investigations since they suffer less wave drag in supersonic environments. Recently, however, with more and more low-aspect ratio wing applications on UCAVs (Unmanned Combat Aerial Vehicles) and MAVs (Micro Air Vehicles), non-slender delta wing configurations (low-sweep angle) begin to raise interest. The collaborative investigation of flow coherent structure, suction side surface pressure, and aerodynamic forces of non-slender delta wings presented here provides critical insight for effective flow-control development, especially for non-slender delta wings at high angles of attack, or encountering unsteady aerodynamic or atmospheric phenomena. As a baseline for studying non-slender delta wings under axial or vertical acceleration, experiments of steady translation with fixed wings under multiple angles of attack were conducted both in the Center of Excellence at Syracuse University at Re ≈ 20, 000, and in the OTTER lab at Queen’s University at Re ≈ 300, 000. According to the comparison of experimental results from both labs, 3D reconstruction of the flow field exhibits the tendency a \u27\u27conical flow structure departing the wing surface at high angles of attack, and the flow fully stalling. Force measurements confirmed the static stall angle for both tested Lambda = 45-deg non-slender delta wings in two groups. Similar lift and drag behavior is observed for two non-slender delta wings at Re of 20,000 and 300,000. For the collaborative project, table 3.1, table 4.1 and table 5.1 give detailed information of experimental datasets and corresponding sections in each chapter. Chapters based on collaborator’s experimental results comprise data analysis conducted in the Green Fluid Dynamics Lab. Axial and vertical accelerated translation experiments were conducted at pre- and post-stall angles of attack in the OTTER lab by that research group. FTLE analysis of this data, and its comparison with surface pressure and aerodynamic forces, were conducted in the Green Fluid Dynamics Lab in the Syracuse University. Sufficiently strong axial accelerations are shown to enable reattachment at the post-stall angle of attack. Meanwhile, the surface pressure distribution reveals a high pressure region created by the axial acceleration, whose motion from leading edge to trailing edge can be indicated by the topology change of nFTLE ridges. This reveals a direct connection of the kinematics (FTLE scalar field) to the aerodynamic performance (surface pressure). The high pressure is followed by a strong leading edge suction, which further confirms the establishment of flow reattachment at the leading edge. The motion of the high pressure region also potentially causes the coefficient of pitching moment to fluctuate under certain circumstances. Hence, the axial acceleration also brings challenge for the flow control along with the increased lift. With a limited magnitude, the tested vertical acceleration does not contribute to a clear flow reattachment. However, it induces more rolled-up coherent structures in the leading edge shear layer. The surface pressure distribution on the suction side exhibits no obvious evolution through the tested vertical acceleration

    Doctor of Philosophy

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    dissertationWith modern computational resources rapidly advancing towards exascale, large-scale simulations useful for understanding natural and man-made phenomena are becoming in- creasingly accessible. As a result, the size and complexity of data representing such phenom- ena are also increasing, making the role of data analysis to propel science even more integral. This dissertation presents research on addressing some of the contemporary challenges in the analysis of vector fields--an important type of scientific data useful for representing a multitude of physical phenomena, such as wind flow and ocean currents. In particular, new theories and computational frameworks to enable consistent feature extraction from vector fields are presented. One of the most fundamental challenges in the analysis of vector fields is that their features are defined with respect to reference frames. Unfortunately, there is no single ""correct"" reference frame for analysis, and an unsuitable frame may cause features of interest to remain undetected, thus creating serious physical consequences. This work develops new reference frames that enable extraction of localized features that other techniques and frames fail to detect. As a result, these reference frames objectify the notion of ""correctness"" of features for certain goals by revealing the phenomena of importance from the underlying data. An important consequence of using these local frames is that the analysis of unsteady (time-varying) vector fields can be reduced to the analysis of sequences of steady (time- independent) vector fields, which can be performed using simpler and scalable techniques that allow better data management by accessing the data on a per-time-step basis. Nevertheless, the state-of-the-art analysis of steady vector fields is not robust, as most techniques are numerical in nature. The residing numerical errors can violate consistency with the underlying theory by breaching important fundamental laws, which may lead to serious physical consequences. This dissertation considers consistency as the most fundamental characteristic of computational analysis that must always be preserved, and presents a new discrete theory that uses combinatorial representations and algorithms to provide consistency guarantees during vector field analysis along with the uncertainty visualization of unavoidable discretization errors. Together, the two main contributions of this dissertation address two important concerns regarding feature extraction from scientific data: correctness and precision. The work presented here also opens new avenues for further research by exploring more-general reference frames and more-sophisticated domain discretizations

    Experimental studies on dispersion processes in periodic flows

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    The aim of this thesis Experimental studies on dispersion processes in periodic flows is to study the hydrodynamics and asses the mixing properties of periodic flows, through extensive experimental campaigns. With mixing, we refer to the field of environmental fluid mechanic that seeks to provide tools to assess the flow of nutrients needed for the survival of an ecosystem, limit toxic pollutants and minimize the anthropic impact. Over the last twenty years, a lot of literature had been devoted to asses mixing processes occurring in uniform flows but less on periodic ones. A periodic flow is an oscillatory flow whose characteristics assume the same sequence of values exactly after a fixed length of time, known as the period. These repetitive velocity patterns produce in the hydrodynamics a periodical occurrence and, eventually, destruction of flow structures responsible for the dispersion or the entrainment of pollutants/ nutrients within the domain considered. Coastal areas provide a typical example of regions dominated by period flows, such as those induced by tidal currents and sea waves. These areas are also characterised by a massive human development, and biodiverse ecosystems, making the study of the mixing of paramount importance for their sustainment and preservation

    Phenomenology and scaling of optimal flapping wing kinematics

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    Biological flapping wing fliers operate efficiently and robustly in a wide range of flight conditions and are a great source of inspiration to engineers. The unsteady aerodynamics of flapping-wings are dominated by large-scale vortical structures that augment the aerodynamic performance but are sensitive to minor changes in the wing actuation. We experimentally optimise the pitch angle kinematics of a flapping wing system in hover to maximise the stroke average lift and hovering efficiency using a 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 for the Pareto-optimal kinematics. The optimised pitch angle profiles yielding maximum stroke-average lift coefficients have trapezoidal shapes and high average angles of attack. These kinematics create strong leading-edge vortices early in the cycle which enhance the force production on the wing. 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 throughout the majority of the stroke-cycle. This requires less aerodynamic power and increases the hovering efficiency by 93% but sacrifices 43% of the maximum lift. In all cases, a leading-edge vortex is fed by vorticity through the leading edge shear-layer which makes the shear-layer velocity a good indicator for the growth of the vortex and its impact on the aerodynamic forces. 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

    Insights into Leading Edge Vortex Formation and Detachment on a Pitching and Plunging Flat Plate

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    The present study is a prelude to applying different flow control devices on pitching and plunging airfoils with the intention of controlling the growth of the leading edge vortex (LEV); hence, the lift under unsteady stall conditions. As a pre-requisite, the parameters influencing the development of the LEV topology must be fully understood, and this constitutes the main motivation of the present experimental investigation. The aims of this study are twofold. First, an approach is introduced to validate the comparability between flow fields and LEV characteristics of two different facilities using water and air as working media by making use of a common baseline case. The motivation behind this comparison is that with two facilities the overall parameter range can be greatly expanded. This comparison includes an overview of the respective parameter ranges, control of the airfoil kinematics and careful scrutiny of how post-processing procedures of velocity data from time-resolved particle image velocimetry (PIV) influence the integral properties and topological features used to characterise the LEV development. Second, and based on results coming from both facilities, the appearance of secondary structures and their effect on LEV detachment over an extended parameter range is studied. A Lagrangian flow field analysis, based on finite-time Lyapunov Exponent (FTLE) ridges, allows precise identification of secondary structures and reveals that their emergence is closely correlated to a vortex Reynolds number threshold computed from the LEV circulation. This threshold is used to model the temporal onset of secondary structures. Further analysis indicates that the emergence of secondary structures causes the LEV to stop accumulating circulation if the shear layer angle at the leading edge of the flat plate has ceased to increase

    Insights into leading edge vortex formation and detachment on a pitching and plunging flat plate

    Get PDF
    The present study is a prelude to applying different flow control devices on pitching and plunging airfoils with the intention of controlling the growth of the leading edge vortex (LEV); hence, the lift under unsteady stall conditions. As a pre-requisite the parameters influencing the development of the LEV topology must be fully understood and this constitutes the main motivation of the present experimental investigation. The aims of this study are twofold. First, an approach is introduced to validate the comparability between flow fields and LEV characteristics of two different facilities using water and air as working media by making use of a common baseline case. The motivation behind this comparison is that with two facilities the overall parameter range can be significantly expanded. This comparison includes an overview of the respective parameter ranges, control of the airfoil kinematics and careful scrutiny of how post-processing procedures of velocity data from time-resolved particle image velocimetry (PIV) influence the integral properties and topological features used to characterise the LEV development. Second, and based on results coming from both facilities, the appearance of secondary structures and their effect on LEV detachment over an extended parameter range is studied. A Lagrangian flow field analysis based on finite-time Lyapunov Exponent (FTLE) ridges allows precise identification of secondary structures and reveals that their emergence is closely correlated to a vortex Reynolds number threshold computed from the LEV circulation. This threshold is used to model the temporal onset of secondary structures. Further analysis indicates that the emergence of secondary structures causes the LEV to stop accumulating circulation if the shear layer angle at the leading edge of the flat plate has ceased to increase. This information is of particular importance for advanced flow control applications, since efforts to strengthen and/or prolong LEV growth rely on precise knowledge about where and when to apply flow control measures
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