Unsteady Performance of Finite-Span Pitching Propulsors in Side-by-Side Arrangements

Kurt, Melike; Moored, Keith W.

doi:10.2514/6.2018-3732

Cited by 4 publications

(5 citation statements)

References 21 publications

(28 reference statements)

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“…Yet, both the spatial organization (3,8) and temporal synchronization (9)(10)(11) have emerged as major drivers of the hydrodynamic interactions, and, consequently, the energetic cost of locomotion and traveling speed of individuals in a collective. Still, our understanding of the force production and energetics of schooling swimmers is mostly limited to canonical spatial arrangements such as a leader-follower in-line arrangement (12)(13)(14)(15) and a side-by-side arrangement (11,(16)(17)(18), while there are fewer studies of staggered arrangements (19)(20)(21)(22).…”

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confidence: 99%

Two-dimensionally stable self-organization arises in simple schooling swimmers through hydrodynamic interactions

Ormonde¹,

Kurt²,

Mivehchi³

et al. 2021

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We present new experiments and free-swimming simulations of a pair of pitching hydrofoils interacting in a simple school. The hydrofoils have an out-of-phase synchronization and their arrangement is varied from in-line to side-by-side arrangements through a series of staggered arrangements representing the two-dimensional interaction plane. It is discovered that there is a two-dimensionally stable equilibrium point for a side-by-side arrangement. In fact, this arrangement is super-stable meaning that hydrodynamic forces will passively maintain this arrangement even under external perturbations and the school as a whole has no net forces acting on, drifting it to one side or the other. Moreover, previously discovered onedimensionally stable equilibria driven by wake vortex interactions are shown to be, in fact, two-dimensionally unstable, at least for an out-of-phase synchronization. Additionally, the stable equilibrium arrangement is verified for freely-swimming foils undergoing dynamic recoil motions. When constrained, the swimmers experience a collective thrust and efficiency increase up to 100% and 40%, respectively, in a side-by-side arrangement. However, in a staggered arrangement where there is direct vortex impingement on a follower, an even higher efficiency improvement of 87% is observed, which is coupled with a 94% increase in the thrust. For freely-swimming foils, the recoil motion attenuates the performance improvements showing a more modest speed and efficiency enhancement of up to 9% and 6%, respectively, when the swimmers are at their stable equilibrium. These newfound schooling performance and stability characteristics suggest that fluid-mediated equilibria may play a role in the control strategies of schooling fish and fish-inspired robots.collective locomotion | hydrodynamic interactions | fish schooling | pattern formation | collective performance M. K. helped design the study, gathered and processed experimental measurements, and drafted the manuscript. A. M. helped design the study, gathered and processed the numerical data, and helped revise the manuscript. K. M. helped design the study, and helped revise the manuscript.

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confidence: 99%

Two-dimensionally stable self-organization arises in simple schooling swimmers through hydrodynamic interactions

Ormonde¹,

Kurt²,

Mivehchi³

et al. 2021

Preprint

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“…An inappropriate selection of these factors might result in 50% performance reduction. Furthermore, several studies (Dewey et al, 2014;Dong and Lu, 2007;Huera-Huarte, 2018;Kurt and Moored, 2018) have reported an enhancement in thrust generation with an improvement in propulsive efficiency when two propellers were arranged in staggered or side-by-side formations with in-phase pitching. With their out-of-phase or anti-phase pitching, hydrodynamic efficiency remained almost the same with greater thrust forces and more power consumption for Reynolds number between 4000-11000.…”

Section: Introductionmentioning

confidence: 99%

“…It was also observed that the transition from drag to thrust producing wakes occurred at smaller Strouhal numbers for shorter spacing distance between foils. In another study, Kurt and Moored (2018) presented that propellers pitching synchronously attained a better performance in terms of 20% more thrust and 17% better efficiency with phase differences of π/2 and 3π/2 when the separation distance was equal to one chord-length. In their study, trends in the variation of power did not coincide with those for thrust, thus leading to a disagreement with the observations by Dewey et al (2014) in this context.…”

Section: Introductionmentioning

confidence: 99%

How does switching synchronization of pitching parallel foils from out-of-phase to in-phase change their wake dynamics?

2021

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This study was inspired from the swimming habits of red nose tetra fish that prefer side-by-side configurations and exhibit changes to their synchronization mid-swimming. Using numerical simulations, alterations to the unsteady wake dynamics imposed by abrupt changes in the phase angle between two pitching side-by-side foils were examined at Reynolds number of 4, 000 and Strouhal number of 0.50. Four hybrid modes were considered in this study with two modes representing an abrupt phase change by π during the 20 th cycle. The other two modes represented a simplified case of burst-and-coast swimming, in which there was a brief (2 oscillation periods) suspension of oscillations before imposing a change in phase angle. In all cases, the foils initially performed out-of-phase pitching, and then they started their in-phase motion by either performing the upstroke or down-stroke first. This kinematic change resulted in the formation and growth of a secondary vortex street in between two primary streets, which enabled and maintained a split wake configuration. Furthermore, the phase switching altered the pressure levels on the top and bottom surfaces of both foils to almost similar levels, which attributed to a reduction in the side-force. The growth rate of the secondary vortex street remained consistent for all four hybrid modes.

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“…These highly three-dimensional spatial configurations found within collectives can be decomposed into canonical in-line, side-by-side, or tip-to-tip arrangements as presented in Figure 1 . To date, these interactions have mostly been studied for propulsors in in-line arrangements [ 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ], although a few efforts have been made to understand interactions in side-by-side arrangements [ 20 , 21 , 22 , 23 ], as well as staggered arrangements [ 24 , 25 , 26 , 27 ]. Here, our focus is on the propulsive performance and flow interactions in in-line , and staggered arrangements.…”

Section: Introductionmentioning

confidence: 99%

“…Including hydrodynamic interactions in a model of a fish school is a challenging task without simplifications made with theoretical approximations [ 28 ]. Our current understanding of flow interactions and energetics within fish schools is mostly based on two-dimensional flow analyses [ 10 , 11 , 12 , 13 , 14 , 20 , 21 , 24 , 25 , 29 , 30 ], but there are far fewer three-dimensional studies focusing on these interactions that employ finite-span wings or hydrofoils, or fish-like bodies [ 15 , 19 , 22 , 25 , 31 , 32 ]. Amongst these studies, some consider the interaction of an oscillatory body with a classic drag-producing von Kármán street [ 30 , 33 ], and the others consider an interaction with a thrust-producing reverse von Kármán street [ 12 , 19 , 20 ].…”

Section: Introductionmentioning

confidence: 99%

Flow Interactions Between Low Aspect Ratio Hydrofoils in In-line and Staggered Arrangements

2020

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Many species of fish gather in dense collectives or schools where there are significant flow interactions from their shed wakes. Commonly, these swimmers shed a classic reverse von Kármán wake, however, schooling eels produce a bifurcated wake topology with two vortex rings shed per oscillation cycle. To examine the schooling interactions of a hydrofoil with a bifurcated wake topology, we present tomographic particle image velocimetry (tomo PIV) measurements of the flow interactions and direct force measurements of the performance of two low-aspect-ratio hydrofoils ( A R = 0.5 ) in an in-line and a staggered arrangement. Surprisingly, when the leader and follower are interacting in either arrangement there are only minor alterations to the flowfields beyond the superposition of the flowfields produced by the isolated leader and follower. Motivated by this finding, Garrick’s linear theory, a linear unsteady hydrofoil theory based on a potential flow assumption, was adapted to predict the lift and thrust performance of the follower. Here, the follower hydrofoil interacting with the leader’s wake is considered as the superposition of an isolated pitching foil with a time-varying cross-stream velocity derived from the wake flow measurements of the isolated leader. Linear theory predictions accurately capture the time-averaged lift force and some of the major peaks in thrust derived from the follower interacting with the leader’s wake in a staggered arrangement. The thrust peaks that are not predicted by linear theory are likely driven by spatial variations in the flowfield acting on the follower or nonlinear flow interactions; neither of which are accounted for in the simple theory. This suggests that unsteady potential flow theory that does account for spatial variations in the flowfield acting on a hydrofoil can provide a relatively simple framework to understand and model the flow interactions that occur in schooling fish. Additionally, schooling eels can derive thrust and efficiency increases of 63-80% in either a in-line or a staggered arrangement where the follower is between two branched momentum jets or with one momentum jet branch directly impinging on it, respectively.

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Unsteady Performance of Finite-Span Pitching Propulsors in Side-by-Side Arrangements

Cited by 4 publications

References 21 publications

Two-dimensionally stable self-organization arises in simple schooling swimmers through hydrodynamic interactions

Two-dimensionally stable self-organization arises in simple schooling swimmers through hydrodynamic interactions

How does switching synchronization of pitching parallel foils from out-of-phase to in-phase change their wake dynamics?

Flow Interactions Between Low Aspect Ratio Hydrofoils in In-line and Staggered Arrangements

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