Publications
http://hdl.handle.net/1721.1/18157
2018-04-25T06:56:49ZTurbulent Flow over a Flexible Wall Undergoing a Streamwise Traveling Wavy Motion
http://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 âˆ’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.
2003-01-01T00:00:00ZThree-dimensional flow structures and vorticity control in fish-like swimming
http://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.
2002-01-01T00:00:00ZVortex-induced vibrations of a cylinder with tripping wires
http://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.
2001-01-01T00:00:00ZDrag reduction in fish-like locomotion
http://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.
1999-01-01T00:00:00Z