Recursos de colección

DSpace at MIT (104.280 recursos)

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Mostrando recursos 1 - 14 de 14

  1. Active Vorticity Control in a Shear Flow Using a Flapping Foil

    Gopalkrishnan, R.; Triantafyllou, M.S.; Triantafyllou, G.S.; Barrett, D.S.
    It is shown experimentally that free shear flows can be substantially altered through direct control of the large coherent vortices present in the flow.

  2. Design and Projected Performance of a Flapping Foil AUV

    Licht, Stephen; Polidoro, Victor; Flores, Melissa; Hover, Franz S.; Triantafyllou, Michael S.
    The design and construction of a biomimetic flapping foil autonomous underwater vehicle is detailed. The vehicle was designed as a proof of concept for the use of oscillating foils as the sole source of motive power for a cruising and hovering underwater vehicle. Primary vehicle design requirements included scalability and flexibility in terms of the number and placement of foils, so as to maximize experimental functionality. This goal was met by designing an independent self-contained module to house each foil, requiring only direct current power and a connection to the vehicle’s Ethernet local area network for operation. The results of tests on the foil modules in the Massachusetts Institute of Technology...

  3. Review of Experimental Work in Biomimetic Foils

    Triantafyllou, Michael S.; Techet, Alexandra H.; Hover, Franz S.
    Significant progress has been made in understanding some of the basic mechanisms of force production and flow manipulation in oscillating foils for underwater use. Biomimetic observations, however, show that there is a lot more to be learned, since many of the functions and details of fish fins remain unexplored. This review focuses primarily on experimental studies on some of the, at least partially understood, mechanisms, which include 1) the formation of streets of vortices around and behind two- and three-dimensional propulsive oscillating foils; 2) the formation of vortical structures around and behind two- and three-dimensional foils used for maneuvering, hovering, or fast-starting; 3) the formation of leading-edge vortices in flapping foils,...

  4. Turbulent Flow over a Flexible Wall Undergoing a Streamwise Traveling Wavy Motion

    Shen, Lian; Zhang, Xiang; Yue, Dick K.P.; Triantafyllou, Michael S.
    Direct numerical simulation is used to study the turbulent flow over a smooth wavy wall undergoing transverse motion in the form of a streamwise travelling wave. The Reynolds number based on the mean velocity U of the external flow and wall motion wavelength λ is 10 170; the wave steepness is 2πa/λ = 0.25 where a is the travelling wave amplitude. A key parameter for this problem is the ratio of the wall motion phase speed c to U, and results are obtained for c/U in the range of −1.0 to 2.0 at 0.2 intervals. For negative c/U, we find that flow separation is enhanced and...

  5. Three-dimensional flow structures and vorticity control in fish-like swimming

    Zhu, Q.; Wolfgang, M.J.; Yue, D.K.P.; Triantafyllou, M.S.
    We employ a three-dimensional, nonlinear inviscid numerical method, in conjunction with experimental data from live fish and from a fish-like robotic mechanism, to establish the three-dimensional features of the flow around a fish-like body swimming in a straight line, and to identify the principal mechanisms of vorticity control employed in fish-like swimming. The computations contain no structural model for the fish and hence no recoil correction. First, we show the near-body flow structure produced by the travelling-wave undulations of the bodies of a tuna and a giant danio. As revealed in cross-sectional planes, for tuna the flow contains dominant features resembling the flow around a two-dimensional oscillating plate...

  6. Vortex-induced vibrations of a cylinder with tripping wires

    Hover, F.S.; Tvedt, H.; Triantafyllou, M.S.
    Thin wires are attached on the outer surface and parallel to the axis of a smooth circular cylinder in a steady cross-stream, modelling the effect of protrusions and attachments. The impact of the wires on wake properties, and vortex-induced loads and vibration are studied at Reynolds numbers up to 4.6 X 10^4, with 3.0 X 10^4 as a focus point. For a stationary cylinder, wires cause significant reductions in drag and lift coefficients as well as an increase in the Strouhal number to a value around 0.25-0.27. For a cylinder forced to oscillate harmonically, the main observed wire effects are: (a) an earlier onset of frequency lock-in,...

  7. Drag reduction in fish-like locomotion

    Barrett, D.S.; Triantafyllou, M.S.; Yue, D.K.P.; Grosenbaugh, M.A.; Wolfgang, M.J.
    We present experimental force and power measurements demonstrating that the power required to propel an actively swimming, streamlined, fish-like body is significantly smaller than the power needed to tow the body straight and rigid at the same speed U. The data have been obtained through accurate force and motion measurements on a laboratory fish-like robotic mechanism, 1:2m long, covered with a flexible skin and equipped with a tail fin, at Reynolds numbers up to 10^6, with turbulence stimulation. The lateral motion of the body is in the form of a travelling wave with wavelength lambda and varying amplitude along the length, smoothly increasing from the front to the...

  8. Mechanics of nonlinear short-wave generation by a moored near-surface buoy

    Zhu, Q.; Liu, Y.; Tjavaras, A.A.; Triantafyllou, M.S.; Yue, D.K.P.
    We consider the nonlinear interaction problem of surface waves with a tethered near-surface buoy. Our objective is to investigate mechanisms for nonlinear short surface wave generation in this complete coupled wave-buoy-cable dynamical system. We develop an effective numerical simulation capability coupling an efficient and high-resolution high-order spectral method for the nonlinear wave-buoy interaction problem with a robust implicit finite-difference method for the cable-buoy dynamics. The numerical scheme accounts for nonlinear wave-wave and wave-body interactions up to an arbitrary high order in the wave steepness and is able to treat extreme motions of the cable including conditions of negative cable tension. Systematic simulations show that beyond a small threshold value...

  9. Forces on oscillating uniform and tapered cylinders in a crossflow

    Hover, F. S.; Techet, A. H.; Triantafyllou, M.S.
    Forces are measured at both ends of rigid cylinders with span 60 cm, performing transverse oscillations within an oncoming stream of water, at Reynolds number Re ~3800. Forced harmonic motions and free vibrations of uniform and tapered cylinders are studied. To study free motions, a novel force-feedback control system has been developed, consisting of: (a) a force transducer, which measures forces on a section of a cylinder moving forward at constant speed; (b) a computer using the measured force signal to drive in real time a numerical simulation of an equivalent mass-dashpot-spring system; (c) a servomotor and linear table which impose, also in real time, the numerically calculated...

  10. Vortical patterns behind a tapered cylinder oscillating transversely

    Techet, A. H.; Hover, F. S.; Triantafyllou, M.S.
    Visualization studies of the flow behind an oscillating tapered cylinder are performed at Reynolds numbers from 400 to 1500. The cylinder has taper ratio 40:1 and is moving at constant forward speed U while being forced to oscillate harmonically in the transverse direction. It is shown that within the lock-in region and above a threshold amplitude, no cells form and, instead, a single frequency of response dominates the entire span. Within certain frequency ranges a single mode dominates in the wake, consisting of shedding along the entire span of either two vortices per cycle (`2S' mode), or four vortices per cycle (`2P' mode); but within specific parametric ranges...

  11. Oscillating foils of high propulsive efficiency

    Anderson, J. M.; Streitlien, K.; Barrett, D.S.; Triantafyllou, M.S.
    Thrust-producing harmonically oscillating foils are studied through force and power measurements, as well as visualization data, to classify the principal characteristics of the flow around and in the wake of the foil. Visualization data are obtained using digital particle image velocimetry at Reynolds number 1100, and force and power data are measured at Reynolds number 40 000. The experimental results are compared with theoretical predictions of linear and nonlinear inviscid theory and it is found that agreement between theory and experiment is good over a certain parametric range, when the wake consists of an array of alternating vortices and either very weak or no leading-edge vortices form. High...

  12. Calculation of Dynamic Motions and Tensions in Towed Underwater Cables

    Hover, Frank S.; Grosenbaugh, Mark A.; Triantafyllou, Michael S.
    A matrix method for mooring system analysis is extended to address the dynamic response of towed underwater systems. Key tools are equivalent linearization and small perturbation theory, and a pitching towfish model. Two examples of application of the technique are provided. The first studies a fundamental limitation to constrained passive heave compensation, while the second concerns the use of floated tethers as a means for dynamic decoupling.

  13. Robust Control For Underwater Vehicle Systems With Time Delays

    Triantafyllou, Michael S .; Grosenbaugh, Mark A.
    Presented in this paper is a robust control scheme for controlling systems with time delays. The scheme is based on the Smith controller and the LQG/LTR (Linear Quadratic Gaussian/Loop Transfer Recovery) methodology. The methodology is applicable to undenvater vehicle systems that exhibit time delays, including tethered vehicles that are positioned through the movements of a surface ship and autonomous vehicles that are controlled through an acoustic link. An example, using full-scale data from the Woods Hole Oceanographic Institution’s tethered vehicle ARGO, demonstrates the developments.

  14. Real Time Estimation of Ship Motions Using Kalman Filtering Techniques

    Triantafyllou, Michael S.; Bodson, Marc; Athans, Michael
    The estimation of the heave, pitch, roll, sway, and yaw motions of a DD-963 destroyer is studied, using Kalman filtering techniques, for application in VTOL aircraft landing. The governing equations are obtained from hydrodynamic considerations in the form of Linear differential equations with frequency dependent coefficients. In addition, nonminimum phase characteristics are obtained due to the spatial integration of the water wave forces. The resulting transfer matrix function is irrational and nonminimum phase. The conditions for a finite-dimensional approximation are considered and the impact of the various parameters is assessed. A detailed numerical application for a DD-963 destroyer is presented and simulations of the estimations obtained...

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