1,721,124 research outputs found
Real time estimation of the heaving and pitching motions of a ship using a Kalman filter
Get PDF"May 1982" "Reprinted from Proc. OCEANS' 81, Boston, Mass., September 1981."Bibliography: leaf 1094."Grant NGL-22-009-124"Michael Triantafyllou, Michael Athans
Vortex Dynamics in Flow Instabilities and Biomimetic Flows (Prof. Jin S. Chung Award Lecture, 2015)
No full textFlow patterns around bodies undergoing flow-induced vibrations and biomimetic flows have intriguing similarities, and similar flow
mechanisms can be used to understand their principal properties and find ways to alter their behavior. The long history and extensive
study of flow-structure interaction allowed the rapid development of the new field of biomimetic fluid mechanics. In turn, properties and discoveries that were made in the study of the swimming of fish and cetaceans, and bird flight, are entering now the engineering domain, suggesting new ways to control the flow and how to devise new technology, invigorating our profession
Miniaturized chemical sensor with bio-inspired micropillar working electrode array for lead detection
No full textA disposable, miniaturized and compact microelectromechanical systems (MEMS) chemical sensor incorporated with three-dimensional, free-standing micropillar working electrode array was proposed, fabricated and tested for electrochemical detection of lead ions. Inspiration of designing arch-shaped columnar sensing electrode array originated from the biological imitation of shark’s olfactory sensing system, considering shark has developed ultrasensitive olfactory capacity during the evolutionary processes. The analytical performance of proposed bio-inspired MEMS chemical sensor was comprehensively investigated. Under optimal conditions, high sensitivity of 32 nA/(μg/L) as well as favorable detection limit of 0.2 μg/L was achieved with short deposition time of 30 s. The sensor exhibited linear responses to lead ions in the concentration range from 1 to 130 μg/L with good linearity (correlation coefficient: 0.9994). The collection efficiency towards target ions in the preconcentration step was significantly enhanced by the presence of micropillar electrode array, due to both the enlargement of electrode surface area and the interaction effect between protruding micropillars and moving solution. Proposed MEMS chemical sensor eliminates the involvement of mechanical/forced stirring of testing solution, making it a potential alternative to conventional macro-sized electrochemical sensor for the application of on-site determination of heavy metal contamination.Singapore. Ministry of EducationSingapore. National Research Foundatio
Copper detection with bio-inspired mems-based electrochemical sensor
No full textA bio-inspired, miniaturized, compact and disposable electrochemical sensor
fabricated by microelectromechanical systems (MEMS) technology for copper detection is proposed. The biomimetic design circumvents the usage
of mechanical stirring, rendering the MEMS-based sensor to exhibit high
sensitivity of 32 nA/ppb and low detection limit of 0.4 ppb with short eposition time of 40 s.
.Singapore Ministry of EducationSingapore. National Research Foundatio
U-shaped fairings suppress vortex-induced vibrations for cylinders in cross-flow
Get PDFWe employ three-dimensional direct and large-eddy numerical simulations of the vibrations and flow past cylinders fitted with free-to-rotate U-shaped fairings placed in a cross-flow at Reynolds number 100 ⩽ Re ⩽ 10,000. Such fairings are nearly neutrally buoyant devices fitted along the axis of long circular risers to suppress vortex-induced vibrations (VIVs). We consider three different geometric configurations: a homogeneous fairing, and two configurations (denoted A and AB) involving a gap between adjacent segments. For the latter two cases, we investigate the effect of the gap on the hydrodynamic force coefficients and the translational and rotational motions of the system. For all configurations, as the Reynolds number increases beyond 500, both the lift and drag coefficients decrease. Compared to a plain cylinder, a homogeneous fairing system (no gaps) can help reduce the drag force coefficient by 15 % for reduced velocity U* = 4.65, while a type A gap system can reduce the drag force coefficient by almost 50 % for reduced velocity U* = 3.5, 4.65, 6, and, correspondingly, the vibration response of the combined system, as well as the fairing rotation amplitude, are substantially reduced. For a homogeneous fairing, the cross-flow amplitude is reduced by about 80 %, whereas for fairings with a gap longer than half a cylinder diameter, VIVs are completely eliminated, resulting in additional reduction in the drag coefficient. We have related such VIV suppression or elimination to the features of the wake flow structure. We find that a gap causes the generation of strong streamwise vorticity in the gap region that interferes destructively with the vorticity generated by the fairings, hence disorganizing the formation of coherent spanwise cortical patterns. We provide visualization of the incoherent wake flow that leads to total elimination of the vibration and rotation of the fairing–cylinder system. Finally, we investigate the effect of the friction coefficient between cylinder and fairing. The effect overall is small, even when the friction coefficients of adjacent segments are different. In some cases the equilibrium positions of the fairings are rotated by a small angle on either side of the centreline, in a symmetry-breaking bifurcation, which depends strongly on Reynolds number
Biomimetic survival hydrodynamics and flow sensing
Get PDFThe fluid mechanics employed by aquatic animals in their escape or attack maneuvers, what we call survival hydrodynamics, are fascinating because the recorded performance in animals is truly impressive. Such performance forces us to pose some basic questions on the underlying flow mechanisms that are not yet in use in engineered vehicles. A closely related issue is the ability of animals to sense the flow velocity and pressure field around them in order to detect and discriminate threats in environments where vision or other sensing is of limited or no use. We review work on animal flow sensing and actuation as a source of inspiration and as a way to formulate a number of basic problems and investigate the flow mechanisms that enable animals to perform these remarkable maneuvers. We also describe some intriguing mechanisms of actuation and sensing
Effect of Mass Ratio on the Vortex-Induced Vibrations of a Long Tensioned Beam in Shear Flow
Get PDFThe flow past a cylindrical tensioned beam of aspect ratio 200 is predicted by direct numerical simulation of the threedimensional Navier-Stokes equations. The beam is free to oscillate in inline and crossflow directions and submitted to a linearly sheared oncoming flow. The ratio between high and low inflow velocities is 3.67, with a maximum Reynolds number of 330. Two structure/fluid mass ratios are considered, 6 and 3. Structure vortex-induced vibrations are characterized by mixed standingtraveling wave patterns. A reduction of mass ratio from 6 to 3 leads to purer, more pronounced traveling wave responses and larger amplitude vibrations in both directions. While multifrequency structure vibrations are observed at m = 6, case m = 3 exhibits monofrequency responses. A large zone of synchronization between vortex shedding and structure vibration (lock-in) is identified in the high velocity region. The topology of fluidstructure energy exchanges shows that the flow can excite the structure at lock-in and damps its vibrations in non-lock-in region. Inline/crossflow motion synchronization is monitored. Similar zigzagging patterns of inline/crossflow motion phase difference are put forward for both mass ratios, highlighting a predominant character of counterclockwise orbits in the excitation region. Topics: Shear flow, Vortex-induced vibrationBP-MIT Major Projects Progra
Shape of retracting foils that model morphing bodies controls shed energy and wake structure
Get PDFThe flow mechanisms of shape-changing moving bodies are investigated through the simple model of a foil that is rapidly retracted over a span wise distance as it is towed at constant angle of attack. It is shown experimentally and through simulation that by altering the shape of the tip of the retracting foil, different shape-changing conditions may be reproduced, corresponding to: (a) a vanishing body, (b) a deflating body, and (c) a melting body. A sharp-edge, ‘vanishing-like’ foil manifests strong energy release to the fluid; however it is accompanied by an additional release of energy, resulting in the formation of a strong ring vortex at the sharp tip edges of the foil during the retracting motion. This additional energy release introduces complex and quickly-evolving vortex structures. By contrast, a streamlined, ‘shrinking-like’ foil avoids generating the ring vortex, leaving a structurally simpler wake. The ‘shrinking’ foil also recovers a large part of the initial energy from the fluid, resulting in much weaker wake structures. Finally, a sharp-edged but hollow, ‘melting-like’ foil provides an energetic wake while avoiding the generation of a vortex ring. As a result, a melting-like body forms a simple and highly energetic and stable wake, that entrains all of the original added mass fluid energy. The three conditions studied correspond to different modes of flow control employed by aquatic animals and birds, and encountered in disappearing bodies, such as rising bubbles undergoing phase change to fluid
Fluid-Structure Energy Transfer of a Tensioned Beam Subject to Vortex-Induced Vibrations in Shear Flow
Get PDFThe fluid-structure energy transfer of a tensioned beam of length to diameter ratio 200, subject to vortex-induced vibrations in linear shear flow, is investigated by means of direct numerical simulation at three Reynolds numbers, from 110 to 1,100. In both the in-line and cross-flow directions, the high-wavenumber structural responses are characterized by mixed standing-traveling wave patterns. The spanwise zones where the flow provides energy to excite the structural vibrations are located mainly within the region of high current where the lock-in condition is established, i.e. where vortex shedding and cross-flow vibration frequencies coincide. However, the energy input is not uniform across the entire lockin region. This can be related to observed changes from counterclockwise to clockwise structural orbits. The energy transfer is also impacted by the possible occurrence of multi-frequency vibrations. Topics: Energy transformation, Fluids, Shear flow, Vortex-induced vibrationBP America Production CompanyBP-MIT Major Projects Progra
High Harmonic Forces and Predicted Vibrations from Forced In-line and Cross-flow Cylinder Motions
No full textString-like ocean structures, such as deep water marine risers are susceptible to a condition of dual lock-in, where both the in-line and transverse natural frequencies are excited due to vortex shedding in the wake. This type of excitation can result in dominant, large amplitude third harmonic forces in the cross-flow direction that do not exist in conditions allowing only cross-flow motion. Forced motions of a rigid cylinder in both the in-line and cross-flow directions are performed to obtain coefficients defining the magnitude of third harmonic lift forces for given cylinder motions. In-line motion amplitude, cross-flow amplitude, phase between in-line and cross-flow motion, and reduced velocity are varied, producing a four-dimensional matrix of data points, at a Reynolds number of 8800. In free vibrations, variation of the effective added mass drives the system to specific steady-state oscillations, under lock-in conditions. These free vibration steady-state oscillations are successfully predicted with the new forced oscillation data set, using the simplifying assumption that lock-in occurs in both the in-line and cross-flow directions, and the necessary assumption that the normalized average power over one cycle must be zero for a free vibration. The new data set and our procedure will allow a more accurate strip-theory approach to marine riser VIV analysis and design
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