196,907 research outputs found
Ultra-fast escape of a deformable jet-propelled body
No full textIn this work a cephalopod-like deformable body that fills an internal cavity with fluid and expels it to propel an escape manoeuvre, while undergoing a drastic external shape change through shrinking, is shown to employ viscous as well as mainly inviscid hydrodynamic mechanisms to power an impressively fast start. First, we show that recovery of added-mass energy enables a shrinking rocket in a dense inviscid flow to achieve greater escape speed than an identical rocket in a vacuum. Next, we extend the shrinking body results of Weymouth & Triantafyllou (J. Fluid Mech., vol. 702, 2012, pp. 470–487) to three-dimensional bodies and show that three hydrodynamic mechanisms must be combined to achieve rapid escape performance in a viscous fluid: added-mass energy recovery; flow separation elimination; and an optimized energy storage and recovery. In particular, we show that the mechanism of separation elimination achieved through rapid body shrinking, coordinated with the mechanism of recovering the initially imparted added-mass energy, is critical to achieving a high escape speed. Hence a flexible, collapsing body can be vastly superior to a rigid-shell jet-propelled body
Developing business establishment surveys to understand reverse logistics processes within a multi-retailer shopping environment
No full text
Vortex-induced vibrations of a long flexible cylinder in shear flow
No full textWe investigate the in-line and cross-flow vortex-induced vibrations of a long cylindrical tensioned beam, with length to diameter ratio L/D = 200, placed within a linearly sheared oncoming flow, using three-dimensional direct numerical simulation. The study is conducted at three Reynolds numbers, from 110 to 1100 based on maximum velocity, so as to include the transition to turbulence in the wake. The selected tension and bending stiffness lead to high-wavenumber vibrations, similar to those encountered in long ocean structures. The resulting vortex-induced vibrations consist of a mixture of standing and travelling wave patterns in both the in-line and cross-flow directions; the travelling wave component is preferentially oriented from high to low velocity regions. The in-line and cross-flow vibrations have a frequency ratio approximately equal to 2. Lock-in, the phenomenon of self-excited vibrations accompanied by synchronization between the vortex shedding and cross-flow vibration frequencies, occurs in the high-velocity region, extending across 30% or more of the beam length. The occurrence of lock-in disrupts the spanwise regularity of the cellular patterns observed in the wake of stationary cylinders in shear flow. The wake exhibits an oblique vortex shedding pattern, inclined in the direction of the travelling wave component of the cylinder vibrations. Vortex splittings occur between spanwise cells of constant vortex shedding frequency. The flow excites the cylinder under the lock-in condition with a preferential in-line versus cross-flow motion phase difference corresponding to counter-clockwise, figure-eight orbits; but it damps cylinder vibrations in the non-lock-in region. Both mono-frequency and multi-frequency responses may be excited. In the case of multi-frequency response and within the lock-in region, the wake can lock in to different frequencies at various spanwise locations; however, lock-in is a locally mono-frequency event, and hence the flow supplies energy to the structure mainly at the local lock-in frequency.United States. Office of Naval Research (Grant N00014-07-1-0135)United States. Office of Naval Research (Grant N00014-07-1-0446)BP (Firm) (MIT Major Projects Research Program
Chaotic flow generated by an oscillating foil
No full textThe present paper describes the results of a numerical study
of the flow field generated by the oscillations of a thrusting foil.
The investigation is carried out to see whether a transition from a
periodic flow to an aperiodic chaotic flow is present as observed around
oscillating cylinders. Moreover, the transition process is analysed in detail
and the characteristics of the system attractor are determined as function
of the parameters of the problem. The approach considers a two-dimensional
problem. Such purely two-dimensional method of calculation describes
the main features of the phenomenon but does not provide a complete
and accurate description of the flow, because it takes into account only
the cross-stream wake vorticity (at right angles to the direction of motion)
although in reality trailing vorticity (parallel to the direction of motion)
can appear even when a 2-D foil is considered. Notwithstanding the
above limitations, a two-dimensional approach can provide useful
information on the overall vorticity dynamics and on the forces acting
on the foil and it is the necessary introduction to full 3-D simulations
Signature Based Analysis of <i>m</I>-consecutive-<i>k< <i>f</I> Systems With Exchangeable Components
No full textTriantafyllou, Ioannis S./0000-0002-7512-5217; Eryilmaz, Serkan/0000-0002-2108-1781; Koutras, Markos/0000-0001-5160-2405In this article, we study reliability properties of m-consecutive-k-out-of-n: F systems with exchangeable components. We deduce exact formulae and recurrence relations for the signature of the system. Closed form expressions for the survival function and the lifetime distribution as a mixture of the distribution of order statistics are established as well. These representations facilitate the computation of several reliability characteristics of the system for a given exchangeable joint distribution or survival function. Finally, we provide signature-based stochastic ordering results for the system's lifetime and investigate the IFR preservation property under the formulation of m-consecutive-k-out-of-n: F systems. (C) 2011 Wiley Periodicals, Inc. Naval Research Logistics 58: 344-354, 201
A Class of Distribution-Free Exponentially Weighted Moving Average Schemes for Joint Monitoring of Location and Scale Parameters
No full textIn this chapter, we investigate and compare six distribution-free exponentially
weighted moving average (EWMA) schemes for simultaneously monitoring the lo-
cation and scale parameters of a univariate continuous process. More precisely, we
consider a well-known distribution-free EWMA scheme based on the Lepage statis-
tic, and we propose ve new EWMA schemes for the same purpose. One of the ve
new schemes is based on the maximum of EWMA of two individual components,
one for the location parameter and the other for the scale parameter, of the Lepage
statistic. Such a component-wise combined EWMA is referred to as the cEWMA.
Further, we consider an EWMA scheme based on the Cucconi test statistic. We
show that it is possible to express the Cucconi statistic as a quadratic combina-
tion of two orthogonal statistics, one of which is useful for monitoring the location
parameter and the other for monitoring the scale parameter. Such decomposition
of the Cucconi statistic is not unique, and one can split it in three dierent ways.
Therefore, we design three more cEWMA schemes corresponding to the decompo-
sitions of the Cucconi statistic. We discuss the implementation steps along with
an illustration. We perform a detailed comparative study based on Monte-Carlo
simulation. We observe that the three cEWMA-Cucconi schemes perform very well
for various location-scale models
Nasal juvenile angiofibroma: Current perspectives with emphasis on management
No full textLópez, F., Triantafyllou, A., Snyderman, C.H., Hunt, J.L., Suárez, C., Lund, V.J., Strojan, P., Saba, N.F., Nixon, I.J., Devaney, K.O., Alobid, I., Bernal–Sprekelsen, M., Hanna, E.Y., Rinaldo, A., Ferlito, A
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