https://hdl.handle.net/1721.1/18156 Sun, 05 Apr 2026 08:16:13 GMT 2026-04-05T08:16:13Z https://hdl.handle.net/1721.1/25621 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 a large drag force is produced. For positive c/U, the results show that as c/U increases from zero, the separation bubble moves further upstream and away from the wall, and is reduced in strength. Above a threshold value of c/U ≈ 1, separation is eliminated; and, relative to small- c/U cases, turbulence intensity and turbulent shear stress are reduced significantly. The drag force decreases monotonically as c/U increases while the power required for the transverse motion generally increases for large c/U; the net power input is found to reach a minimum at c/U ≈ 1.2 (for fixed U). The results obtained in this study provide physical insight into the study of fish-like swimming mechanisms in terms of drag reduction and optimal propulsive efficiency. Wed, 01 Jan 2003 00:00:00 GMT https://hdl.handle.net/1721.1/25621 2003-01-01T00:00:00Z https://hdl.handle.net/1721.1/25620 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 over most of the length of the fish body. For the giant danio, on the other hand, a mixed longitudinal-transverse structure appears along the hind part of the body. We also investigate the interaction of the body-generated vortices with the oscillating caudal fin and with tail-generated vorticity. Two distinct vorticity interaction modes are identified: the first mode results in high thrust and is generated by constructive pairing of body-generated vorticity with same-sign tail-generated vorticity, resulting in the formation of a strong thrust wake; the second corresponds to high propulsive efficiency and is generated by destructive pairing of body-generated vorticity with opposite-sign tail-generated vorticity, resulting in the formation of a weak thrust wake. Tue, 01 Jan 2002 00:00:00 GMT https://hdl.handle.net/1721.1/25620 2002-01-01T00:00:00Z https://hdl.handle.net/1721.1/25619 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, when compared with the smooth cylinder case; (b) at moderate amplitude/cylinder diameter (A=D) ratios (0.2 and 0.5), changes in the phase of wake velocity and of lift with respect to motion are translated to higher forcing frequencies, and (c) at A=D = 1:0, no excitation region exists; the lift force is always dissipative. The flow-induced response of a flexibly mounted cylinder with attached wires is significantly altered as well, even far away from lock-in. Parameterizing the response using nominal reduced velocity Vrn = U/fnD, we found that frequency lock-in occurs and lift phase angles change through 180deg at Vrn=4.9; anemometry in the wake confirms that a mode transition accompanies this premature lock-in. A plateau of constant response is established in the range Vrn = 5.1-6.0, reducing the peak amplitude moderately, and then vibrations are drastically reduced or eliminated above Vrn = 6.0. The vortex-induced vibration response of the cylinder with wires is extremely sensitive to angular bias near the critical value of Vrn = 6.0, and moderately so in the regime of suppressed vibration. Mon, 01 Jan 2001 00:00:00 GMT https://hdl.handle.net/1721.1/25619 2001-01-01T00:00:00Z https://hdl.handle.net/1721.1/25618 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 tail end. A parametric investigation shows sensitivity of drag reduction to the non-dimensional frequency (Strouhal number), amplitude of body oscillation and wavelength lambda, and angle of attack and phase angle of the tail fin. A necessary condition for drag reduction is that the phase speed of the body wave be greater than the forward speed U. Power estimates using an inviscid numerical scheme compare favourably with the experimental data. The method employs a boundary-integral method for arbitrary flexible body geometry and motions, while the wake shed from the fish-like form is modelled by an evolving desingularized dipole sheet. Fri, 01 Jan 1999 00:00:00 GMT https://hdl.handle.net/1721.1/25618 1999-01-01T00:00:00Z https://hdl.handle.net/1721.1/25617 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 of the incident wave amplitude, the buoy performs chaotic motions, characterized by the snapping of the cable. The root cause of the chaotic response is the interplay between the snapping of the cable and the generation of surface waves, which provides a source of strong (radiation) damping. As a result of this interaction, the chaotic buoy motion switches between two competing modes of snapping response: one with larger average peak amplitude and lower characteristic frequency, and the other with smaller amplitude and higher frequency. The generated high-harmonic/short surface waves are greatly amplified once the chaotic motion sets in. Analyses of the radiated wave spectra show significant energy at higher frequencies which is orders of magnitude larger than can be expected from nonlinear generation under regular motion. Fri, 01 Jan 1999 00:00:00 GMT https://hdl.handle.net/1721.1/25617 1999-01-01T00:00:00Z https://hdl.handle.net/1721.1/25616 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 motion on the cylinder section. The apparatus allows very low equivalent system damping and strict control of the parametric values and structure of the equivalent system. Calculation of the cross-correlation coefficient between forces at the two ends of the uniform cylinder reveals five distinct regimes as a function of the nominal reduced velocity Vrn: two regimes, for low and high values of Vrn, and far away from the value of VrS corresponding to the Strouhal frequency, show small correlation; two regimes immediately adjacent to, but excluding, VrS show strong correlation, close to 1; surprisingly, there is a regime containing the Strouhal frequency, within which correlation is low. Free vibrations with a 40:1 tapered cylinder show that the regime of low correlation, containing the Strouhal frequency, stretches to higher reduced velocities, while lock-in starts at lower reduced velocities. When comparing the amplitude and phase of the lift coefficient measured for free and then for forced vibrations, we obtain close agreement, both for tapered and uniform cylinders. When comparing the cross-correlation coefficient however, we find that it is much higher in the forced oscillations, especially for the uniform cylinder. Hence, although the force magnitude and phase may be replicated well in forced vibrations, the correlation data suggest that differences exist between free and forced vibration cases. Thu, 01 Jan 1998 00:00:00 GMT https://hdl.handle.net/1721.1/25616 1998-01-01T00:00:00Z https://hdl.handle.net/1721.1/25615 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 a hybrid mode is observed, consisting of a `2S' pattern along the part of the span with the larger diameter and a `2P' pattern along the part of the span with the smaller diameter. A distinct vortex split connects the two patterns which are phaselocked and have the same frequency. The hybrid mode is periodic, unlike vortex dislocations, and the location of the vortex split remains stable and repeatable, within one to two diameters, depending on the amplitude and frequency of oscillation and the Reynolds number. Thu, 01 Jan 1998 00:00:00 GMT https://hdl.handle.net/1721.1/25615 1998-01-01T00:00:00Z https://hdl.handle.net/1721.1/25614 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 propulsive efficiency, as high as 87%, is measured experimentally under conditions of optimal wake formation. Visualization results elucidate the basic mechanisms involved and show that conditions of high efficiency are associated with the formation on alternating sides of the foil of a moderately strong leading-edge vortex per half-cycle, which is convected downstream and interacts with trailing-edge vorticity, resulting eventually in the formation of a reverse Karman street. The phase angle between transverse oscillation and angular motion is the critical parameter affecting the interaction of leading-edge and trailing-edge vorticity, as well as the efficiency of propulsion. Thu, 01 Jan 1998 00:00:00 GMT https://hdl.handle.net/1721.1/25614 1998-01-01T00:00:00Z https://hdl.handle.net/1721.1/25613 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. Sat, 01 Jan 1994 00:00:00 GMT https://hdl.handle.net/1721.1/25613 1994-01-01T00:00:00Z https://hdl.handle.net/1721.1/25612 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 (MIT) Marine Hydrodynamics Water Tunnel and the MIT Ship Model Testing Tank are both used to demonstrate fundamental properties of flapping foils and to predict the performance of the specific vehicle design based on the limits of the actuators. The maximum speed of the vehicle is estimated based on the limitations of the specific actuator and is shown to be a strong function of the vehicle drag coefficient. When using four foils, the maximum speed increases from 1 m/s with a vehicle Cd of 1.4 to 2 m/s when Cd = 0.1, where Cd is based on vehicle frontal area. Finally, issues of vehicle control are considered, including the decoupling of speed and pitch control using pitch-biased maneuvering and the tradeoff between actuator bandwidth and authority during both the cruising and hovering operation. Thu, 01 Jul 2004 00:00:00 GMT https://hdl.handle.net/1721.1/25612 2004-07-01T00:00:00Z https://hdl.handle.net/1721.1/25611 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, under steady flapping or transient conditions; 4) the interaction of foils with oncoming, externally generated vorticity; multiple foils, or foils operating near a body or wall. Thu, 01 Jul 2004 00:00:00 GMT https://hdl.handle.net/1721.1/25611 2004-07-01T00:00:00Z https://hdl.handle.net/1721.1/18537 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 from Kalman filters are discussed. Sat, 01 Jan 1983 00:00:00 GMT https://hdl.handle.net/1721.1/18537 1983-01-01T00:00:00Z https://hdl.handle.net/1721.1/18539 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. Sat, 01 Jan 1994 00:00:00 GMT https://hdl.handle.net/1721.1/18539 1994-01-01T00:00:00Z https://hdl.handle.net/1721.1/18538 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. Tue, 01 Jan 1991 00:00:00 GMT https://hdl.handle.net/1721.1/18538 1991-01-01T00:00:00Z