Chaotic rotation of a towed elliptical cylinder

Weymouth, Gabriel

doi:10.1017/jfm.2014.42

Cited by 9 publications

(6 citation statements)

References 30 publications

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“…Far less computing power is required to solve the equations compared to the original two-domain problem, so fast and accurate solutions can be obtained. This method has been shown to give accurate results for a variety of problems including towed cylinders (Weymouth 2014), boundary layer instabilities (Maertens & Triantafyllou 2014), vorticity shedding of shrinking cylinders (Weymouth et al 2012) and unsteady dynamics of perching manoeuvres (Polet et al 2015). This method has second-order convergence, and can predict the aerodynamic forces on flapping foils to a high accuracy (Maertens & Weymouth 2015).…”

Section: Computational Setupmentioning

confidence: 99%

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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Section: Computational Setupmentioning

confidence: 99%

Performance augmentation mechanism of in-line tandem flapping foils

2017

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“…Also, a significant torsional excitation was measured, especially at certain speeds. The excitation of this mode may be driven by the elliptical cross section of the geometry [137].…”

Section: C4 Discussion and Future Workmentioning

confidence: 99%

“…The reason for this may be that the coupling attempts to induce a torsional motion, which is what was observed in a separate experiment conducted with a flexible whisker model (Appendix C). A torsional response may be a result of the elliptical cross section [137].…”

Section: Versionmentioning

confidence: 99%

Passive wake detection using seal whisker-inspired sensing

Beem¹

2015

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This thesis is motivated by a series of biological experiments that display the harbor seal's extraordinary ability to track the wake of an object several seconds after it has swum by. They do so despite having auditory and visual cues blocked, pointing to use of their whiskers as sensors of minute water movements. In this work, I elucidate the basic fluid mechanisms that seals may employ to accomplish this detection. Key are the unique flow-induced vibration properties resulting from the geometry of the harbor seal whisker, which is undulatory and elliptical in cross-section.First, the vortex-induced vibration (VIV) characteristics of the whisker geometry are tested. Direct force measurements and flow visualizations on a rigid whisker model undergoing a range of 1-D imposed oscillations show that the geometry passively reduces VIV (factor of > 10), despite contributions from effective added mass and damping. Next, a biomimetic whisker sensor is designed and fabricated. The rigid whisker model is mounted on a four-armed flexure, allowing it to freely vibrate in both in-line and crossflow directions. Strain gauges on the flexure measure deflections at the base. Finally, this device is tested in a simplified version of the fish wake -seal whisker interaction scenario. The whisker is towed behind an upstream cylinder with larger diameter. Whereas in open water the whisker exhibits very low vibration when its long axis is aligned with the incoming flow, once it enters the wake it oscillates with large amplitude and its frequency coincides with the Strouhal frequency of the upstream cylinder. This makes the detection of an upstream wake as well as an estimation of the size of the wake-generating body possible. A slaloming motion among the wake vortices causes the whisker to oscillate in this manner. The same mechanism has been previously observed in energy-extracting foils and trout actively swimming behind bluff cylinders in a stream. Dave, Jason, Gabe, Pablo, Remi, and Yahya were postdocs when we first met.Thank you for being approachable and sharing much needed advice all along the way.Your love of science has inspired me to be more inquisitive.Brandon, Chris, Matthew, and Patrick were undergrads or high school students who spent their summers working in the Tow Tank with me. Thanks for giving me the chance to practice being a mentor, moving this research forward a lot faster than I could do on my own, and helping me lift the mega-whisker in and out of the tank! May you all keep on building and exploring.Thanks to Kathy Streeter and the New England Aquarium staff for helping me understand more about real harbor seals. They graciously collected shed whisker specimens and allowed me to take many close-up pictures of their seals.Thanks to all the people who helped me prepare for and make it through research cruises: Eric Hayden, Terry Hammar, and Hanu Singh for contributing their expertise 5 to help me make my first ocean-ready sensor, all parties involved in the first UNOLS Chief Scientist Training Cruise se...

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“…In general, period doubling/quadrupling is a common phenomenon when a dynamic system with periodicity (not only limited to fluid mechanics) is about to transition to chaos (e.g. Pourazarm et al, 2015;Weymouth, 2014;Moskalik and Buchler, 1990). However, such a classical period doubling route to chaos is not observed for the wake transition to chaos through the Mode B flow for flow past an isolated circular/square cylinder, since the Mode B structure is neither uniform along the spanwise direction nor periodic over time (see e.g.…”

Section: Summary On the Wake Transition To Chaosmentioning

confidence: 99%

Transition to chaos in the cylinder wake through the Mode C flow

Jiang

Cheng

2020

Physics of Fluids

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The evolution of Mode C wake characteristics with the Reynolds number (Re) for Re up to 400 is investigated numerically. The Mode C wake instability is generated by placing a small wire in the near wake of a main circular cylinder. This setup ensures that the wake is unstable to Mode C only (without potential mode interactions), as demonstrated by Floquet stability analysis. Based on three-dimensional direct numerical simulations, three evolution regimes are identified for the fully developed Mode C flow. In the uniform periodic regime (Re = 166.4-210), the Mode C structure is uniformly distributed along the spanwise direction. The flow structure is 2T-periodic (T being the vortex shedding period) but retains a spatiotemporal symmetry every 1T. In the non-uniform periodic regime (Re = 220-230), the slightly non-uniform Mode C structure remains 2T-periodic at Re = 220 but undergoes a period quadrupling to 4T-periodic at Re = 230 before transitioning to chaos. In the chaotic regime (Re ≥ 240), the flow loses periodicity and becomes increasingly chaotic with increasing Re. The progressive wake transition to chaos is found to originate from the instability in the braid shear layer region, through the uneven growth in strength and the consequent non-uniformity of the Mode C streamwise vortices. The wake transitions to chaos through the routes of Mode C and Mode B (for an isolated circular/square cylinder) are compared.

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Chaotic rotation of a towed elliptical cylinder

Cited by 9 publications

References 30 publications

Performance augmentation mechanism of in-line tandem flapping foils

Performance augmentation mechanism of in-line tandem flapping foils

Passive wake detection using seal whisker-inspired sensing

Transition to chaos in the cylinder wake through the Mode C flow

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