1,024 research outputs found
Lock-in of the vortex-induced vibrations of a long tensioned beam in shear flow
The occurrence of lock-in, defined as the local synchronization between the vortex shedding frequency and the cross-flow structural vibration frequency, is investigated in the case of a tensioned beam of length to diameter ratio 200, free to move in both the in-line and cross-flow directions, and immersed in a linear shear current. Direct numerical simulation is employed at three Reynolds numbers, from 110 to 1100, so as to include the transition to turbulence in the wake. The Reynolds number influences the response amplitudes, but in all cases we observed similar fluid-structure interaction mechanisms, resulting in high-wavenumber vortex-induced vibrations consisting of a mixture of standing and traveling wave patterns. Lock-in occurs in the high oncoming velocity region, over at least 30% of the cylinder length. In the case of multi-frequency response, at any given spanwise location lock-in is principally established at one of the excited vibration frequencies, usually the locally predominant one. The spanwise patterns of the force and added mass coefficients exhibit different behaviors within the lock-in versus the non-lock-in region. The spanwise zones where the flow provides energy to excite the structural vibrations are located mainly within the lock-in region, while the flow damps the structural vibrations in the non-lock-in region
Distributed lock-in drives broadband vortex-induced vibrations of a long flexible cylinder in shear flow
A slender flexible body immersed in sheared cross-flow may exhibit vortex-induced vibrations (VIVs) involving a wide range of excited frequencies and structural wavenumbers. The mechanisms of broadband VIVs of a cylindrical tensioned beam of length-to-diameter aspect ratio 200 placed in shear flow, with an exponentially varying profile along the span, are investigated by means of direct numerical simulation. The Reynolds number is equal to 330 based on the maximum velocity, for comparison with previous work on narrowband vibrations in linear shear flow. The flow is found to excite the structure at a number of different locations under a condition of wake–body synchronization, or lock-in. Broadband responses are associated with a distributed occurrence of the lock-in condition along the span, as opposed to the localized lock-in regions limited to the high inflow velocity zone, reported for narrowband vibrations in sheared current. Despite the instantaneously multi-frequency nature of broadband responses, the lock-in phenomenon remains a locally mono-frequency event, since the vortex formation is generally synchronized with a single vibration frequency at a given location. The spanwise distribution of the excitation zones induces travelling structural waves moving in both directions; this contrasts with the narrowband case where the direction of propagation toward decreasing inflow velocity is preferred. A generalization of the mechanism of phase-locking between the in-line and cross-flow responses is proposed for broadband VIVs under the lock-in condition. A spanwise drift of the in-line/cross-flow phase difference is identified for the high-wavenumber vibration components; this drift is related to the strong travelling wave character of the corresponding structural waves
Multi-frequency vortex-induced vibrations of a long tensioned beam in linear and exponential shear flows
The multi-frequency vortex-induced vibrations of a cylindrical tensioned beam of aspect ratio 200, free to move in the in-line and cross-flow directions within first a linearly and then an exponentially sheared current are investigated by means of direct numerical simulation, at a Reynolds number equal to 330. The shape of the inflow profile impacts the spectral content of the mixed standing traveling wave structural responses: narrowband vibrations are excited within the lock-in area, which is limited to a single region lying in the high flow velocity zone, for the linear shear case; in contrast, the lock-in condition occurs at several spanwise locations in the exponential shear case, resulting in broadband responses, containing a wide range of excited frequencies and spatial wavenumbers. The broadband in-line and cross-flow vibrations occurring for the exponential shear current have a phase difference that lies within a specific range along the entire span; this differs from the phase drift noted for narrowband responses in linear shear flow. Lower vibration amplitudes, time-averaged and fluctuating in-line force coefficients are observed for the exponential shear current. The cross-flow force coefficient has comparable magnitude for both inflow profiles along the span, except in zones where the broadband vibrations are under the lock-in condition but not the narrowband ones. As in the narrowband case, the fluid forces associated with the broadband responses are dominated by high frequencies related to high-wavenumber vibration components. Considerable variability of the effective added mass coefficients along the span is noted in both cases
Ultra-fast escape maneuver of an octopus-inspired robot
We design and test an octopus-inspired flexible hull robot that demonstrates
outstanding fast-starting performance. The robot is hyper-inflated with water,
and then rapidly deflates to expel the fluid so as to power the escape
maneuver. Using this robot we verify for the first time in laboratory testing
that rapid size-change can substantially reduce separation in bluff bodies
traveling several body lengths, and recover fluid energy which can be employed
to improve the propulsive performance. The robot is found to experience speeds
over ten body lengths per second, exceeding that of a similarly propelled
optimally streamlined rigid rocket. The peak net thrust force on the robot is
more than 2.6 times that on an optimal rigid body performing the same maneuver,
experimentally demonstrating large energy recovery and enabling acceleration
greater than 14 body lengths per second squared. Finally, over 53% of the
available energy is converted into payload kinetic energy, a performance that
exceeds the estimated energy conversion efficiency of fast-starting fish. The
Reynolds number based on final speed and robot length is .
We use the experimental data to establish a fundamental deflation scaling
parameter which characterizes the mechanisms of flow control via
shape change. Based on this scaling parameter, we find that the fast-starting
performance improves with increasing size.Comment: Submitted July 10th to Bioinspiration & Biomimetic
Simulation-Based Comparative Analysis of Nonparametric Control Charts with Runs-Type Rules
In this chapter, we study well-known distribution-free Shewhart-type monitoring schemes based on order statistics. In order to empower the in- and out-of-control performance of the control charts being under consideration, several runs-type rules are enhanced. The simulation-based experimentation carried out reveals that the proposed schemes achieve remarkable efficiency for detecting possible shifts in the distribution of the underlying process
Efficiency of Fish Propulsion
It is shown that the system efficiency of a self-propelled flexible body is
ill-defined unless one considers the concept of quasi-propulsive efficiency,
defined as the ratio of the power needed to tow a body in rigid-straight
condition over the power it needs for self-propulsion, both measured for the
same speed. Through examples we show that the quasi-propulsive efficiency is
the only rational non-dimensional metric of the propulsive fitness of fish and
fish-like mechanisms. Using two-dimensional viscous simulations and the concept
of quasi-propulsive efficiency, we discuss the efficiency two-dimensional
undulating foils. We show that low efficiencies, due to adverse body-propulsor
hydrodynamic interactions, cannot be accounted for by the increase in friction
drag
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