Active vorticity control in a shear flow using a flapping foil

Gopalkrishnan, R.; Triantafyllou, Michael; Triantafyllou, George S.; Barrett, D. S.

doi:10.1017/s0022112094002016

Cited by 236 publications

(159 citation statements)

References 17 publications

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“…In this latter case, the interaction between the foil and vortex can lead to substantially increased or decreased lift, affecting the performance of the foil. This has been demonstrated theoretically by the work of Streitlien [72] and experimentally by Gopalkrishnan [30] where the relative positions were altered via the phase between foil oscillation and vortex shedding. In this case, identifying the relative location of the external vortex to the foil is critically important for predicting the type of affect that will result from the interaction [30].…”

Section: Discussionmentioning

confidence: 85%

Performance analysis for lateral-line-inspired sensor arrays

Fernández¹

2011

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The lateral line is a critical component of the fish sensory system, found to affect numerous aspects of behavior including maneuvering in complex fluid environments, schooling, prey tracking, and environment mapping. This sensory organ has no analog in modern ocean vehicles, despite its utility and ubiquity in nature, and could fill the gap left by sonar and vision systems in turbid cluttered environments. Yet, while the biological sensory system suggests the broad possibilities associated with such a sensor array, nearly nothing is known of the input processing and what information is available via the real lateral line. This thesis demonstrates and characterizes the ability of lateral-line-inspired linear pressure sensor arrays to perform two sensory tasks of relevance to biological and man-made underwater navigation systems, namely shape identification and vortex tracking.The ability of pressure sensor arrays to emulate the "touch at a distance" feature of the lateral line, corresponding to the latter's capability of identifying the shape of objects remotely, is examined with respect to moving cylinders of different cross sections. Using the pressure distribution on a small linear array, the position and size of a cylinder is tracked at various distances. The classification of cylinder shape is considered separately, using a large database of trials to identify two classification approaches: One based on differences in the mean flow, and one trained on a subset which utilizes information from the wake. The results indicate that it is in general possible to extract specific shape information from measurements on a linear pressure sensor array, and characterize the classes of shapes which are not distinguishable via this method.Identifying the vortices in a flow makes it possible to predict and optimize the performance of flapping foils, and to identify imminent stall in a control surface. Vortices in wakes also provide information about the object that generated the wake at distances much larger than the near-field pressure perturbations. Experimental studies in tracking a vortex pair and an individual vortex interacting with a flat plate demonstrate the ability to track vortices with a linear pressure sensor array from both small streamlined bodies and large flat bodies. Based on a theoretical analysis, the relationship between the necessary array parameters and the range of vortices of interest is established.

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Section: Discussionmentioning

confidence: 85%

Performance analysis for lateral-line-inspired sensor arrays

Fernández¹

2011

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“…Red is high thrust and blue is low thrust, white represents the value of a single foil. known to consist of two primary vortices of alternating sign shed by the foil per cycle (Gopalkrishnan et al 1994;Anderson et al 1998;Read et al 2003). This wake structure is similar to a drag-producing von Kármán street of a bluff body, although the signs of the vortices on each side of the 'street' is reversed in the flapping foil case because it produces thrust instead of drag.…”

Section: Results From Full Simulationsmentioning

confidence: 93%

Performance augmentation mechanism of in-line tandem flapping foils

2017

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The propulsive performance of a pair of tandem flapping foils is sensitively dependent on the spacing and phasing between them. Large increases in thrust and efficiency of the hind foil are possible, but the mechanisms governing these enhancements remain largely unresolved. Two-dimensional numerical simulations of tandem and single foils oscillating in heave and pitch at a Reynolds number of 7,000 are performed over a broad and dense parameter space, allowing the effects of inter-foil spacing (S) and phasing (ϕ) to be investigated over a range of non-dimensional frequencies (or Strouhal number, St). Results indicate that the hind foil can produce from no thrust, to twice the thrust of a single foil depending on its spacing and phasing with respect to the fore foil, which is consistent with previous studies that were carried out over a limited parameter space. Examination of instantaneous flowfields indicate that high thrust occurs when the hind foil weaves in between the vortices that have been shed by the fore foil, and low thrust occurs when the hind foil intercepts these vortices. Contours of high thrust and minimal thrust appear as inclined bands in the S − ϕ parameter space and this behaviour is apparent over the entire range of Strouhal numbers considered (0.2 St 0.5). A novel quasi-steady model that utilises kinematics of a virtual hind foil together with data obtained from simulations of a single flapping foil shows that performance augmentation is primarily determined through modification of the instantaneous angle of attack of the hind foil by the vortex street established by the fore foil. This simple model provides estimates of thrust and efficiency for the hind foil, which is consistent with data obtained through full simulations. The limitations of the virtual hind foil method and its physical significance is also discussed.

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“…Gursul & Ho (1992) demonstrated that unsteady motion of airfoils can cause a very high lift coefficient, and work by Anderson et al (1998) shows that oscillating foils are able to produce propulsive thrust very efficiently. Experimental and computational works on the performance of an oscillating lifting surface in the presence of oncoming vorticity (Koochesfahani & Dimotakis 1988;Gopalkrishnan et al 1994;Anderson 1996; reveal that proper phasing between the foil motion and the encounter with oncoming vorticity can yield a significant increase or decrease in efficiency. The principal modes of foil-vortex interaction with a street of alternating-sign vortices can be summarized as (Gopalkrishnan et al 1994): (a) Vorticity annihilation, where foil-generated vorticity interacts destructively with the oncoming vorticity, resulting in the generation of a weak vortex street downstream of the foil.…”

Section: Vorticity Control Mechanismsmentioning

confidence: 99%

Three-dimensional flow structures and vorticity control in fish-like swimming

et al. 2002

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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. IntroductionFish locomotion offers a different paradigm of propulsion than utilized in humanengineered vehicles, employing a rhythmic unsteady motion of the body and fins. It has been shown in the literature that drag reduction and high propulsive efficiency are achievable through it. Gray (1936), in a still controversial paper, estimated that the drag on a swimming dolphin must be lower than on a towed rigid model of the dolphin body by a large factor; Lighthill (1971) used the kinematics measured in a live small fish, together with a large-amplitude slender body theory, and arrived at the opposite conclusion, i.e. that the drag on a swimming fish must be larger than on a rigidly towed fish model, by a factor of about three. Recently, precise measurements on an actively swimming robotic vehicle in the shape of a bluefin tuna show that the power needed to self-propel the robot is reduced by up to about 50% compared to the power needed to tow the robot straight-rigid (Barrett et al. 1999),

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Active vorticity control in a shear flow using a flapping foil

Cited by 236 publications

References 17 publications

Performance analysis for lateral-line-inspired sensor arrays

Performance analysis for lateral-line-inspired sensor arrays

Performance augmentation mechanism of in-line tandem flapping foils

Three-dimensional flow structures and vorticity control in fish-like swimming

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