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.
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
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,...
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 â1.0 to 2.0 at
0.2 intervals. For negative c/U, we find that flow separation is enhanced and...
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...
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,...
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...
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...
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...
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
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...
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.
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.
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...