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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenTue, 16 Apr 2024 15:15:20 +0000Large-amplitude flapping of an inverted flag in a uniform steady flow – a vortex-induced vibration
https://resolver.caltech.edu/CaltechAUTHORS:20160422-150517531
Authors: {'items': [{'id': 'Sader-J-E', 'name': {'family': 'Sader', 'given': 'John E.'}, 'orcid': '0000-0002-7096-0627'}, {'id': 'Cossé-J', 'name': {'family': 'Cossé', 'given': 'Julia'}}, {'id': 'Kim-Daegyoum', 'name': {'family': 'Kim', 'given': 'Daegyoum'}}, {'id': 'Fan-Boyu', 'name': {'family': 'Fan', 'given': 'Boyu'}}, {'id': 'Gharib-M', 'name': {'family': 'Gharib', 'given': 'Morteza'}, 'orcid': '0000-0003-0754-4193'}]}
Year: 2016
DOI: 10.1017/jfm.2016.139
The dynamics of a cantilevered elastic sheet, with a uniform steady flow impinging on its clamped end, have been studied widely and provide insight into the stability of flags and biological phenomena. Recent measurements by Kim et al. (J. Fluid Mech., vol. 736, 2013, R1) show that reversing the sheet's orientation, with the flow impinging on its free edge, dramatically alters its dynamics. In contrast to the conventional flag, which exhibits (small-amplitude) flutter above a critical flow speed, the inverted flag displays large-amplitude flapping over a finite band of flow speeds. The physical mechanisms giving rise to this flapping phenomenon are currently unknown. In this article, we use a combination of mathematical theory, scaling analysis and measurement to establish that this large-amplitude flapping motion is a vortex-induced vibration. Onset of flapping is shown mathematically to be due to divergence instability, verifying previous speculation based on a two-point measurement. Reducing the sheet's aspect ratio (height/length) increases the critical flow speed for divergence and ultimately eliminates flapping. The flapping motion is associated with a separated flow – detailed measurements and scaling analysis show that it exhibits the required features of a vortex-induced vibration. Flapping is found to be periodic predominantly, with a transition to chaos as flow speed increases. Cessation of flapping occurs at higher speeds – increased damping reduces the flow speed range where flapping is observed, as required. These findings have implications for leaf motion and other biological processes, such as the dynamics of hair follicles, because they also can present an inverted-flag configuration.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/5kb75-avy13Coupled motion of two side-by-side inverted flags
https://resolver.caltech.edu/CaltechAUTHORS:20171206-103904472
Authors: {'items': [{'id': 'Huertas-Cerdeira-C', 'name': {'family': 'Huertas-Cerdeira', 'given': 'Cecilia'}}, {'id': 'Fan-Boyu', 'name': {'family': 'Fan', 'given': 'Boyu'}}, {'id': 'Gharib-M', 'name': {'family': 'Gharib', 'given': 'Morteza'}, 'orcid': '0000-0003-0754-4193'}]}
Year: 2018
DOI: 10.1016/j.jfluidstructs.2017.11.005
The interaction and coupling between two inverted flags that are placed side-by-side in a uniform flow is investigated in an effort to determine the behavior of systems that are formed by arrays of cantilevered plates. Inverted flags are elastic plates that are free to move at their leading edge and clamped at their trailing edge. We show that placing two inverted flags of equal dimensions side-by-side will cause their motion to couple. In-phase, anti-phase, staggered, alternating and decoupled flapping modes are present, with the anti-phase mode being predominant at small flag distances and low wind speeds. Increases both in amplitude and frequency of flapping are observed in the two flag system with respect to a single flag. Two side-by-side inverted flags of different lengths are found to interact for small length differences, with the longer flag being able to induce a motion on the shorter flag even when the latter is outside of its flapping wind speed range.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/h351p-mw484Effect of morphology on the large-amplitude flapping dynamics of an inverted flag in a uniform flow
https://authors.library.caltech.edu/records/24vvw-k1f65
Authors: {'items': [{'id': 'Fan-Boyu', 'name': {'family': 'Fan', 'given': 'Boyu'}, 'orcid': '0000-0002-1743-5225'}, {'id': 'Huertas-Cerdeira-Cecilia', 'name': {'family': 'Huertas-Cerdeira', 'given': 'Cecilia'}, 'orcid': '0000-0003-4553-0470'}, {'id': 'Cossé-Julia', 'name': {'family': 'Cossé', 'given': 'Julia'}}, {'id': 'Sader-J-E', 'name': {'family': 'Sader', 'given': 'John E.'}, 'orcid': '0000-0002-7096-0627'}, {'id': 'Gharib-M', 'name': {'family': 'Gharib', 'given': 'Morteza'}, 'orcid': '0000-0003-0754-4193'}]}
Year: 2019
DOI: 10.1017/jfm.2019.474
The stability of a cantilevered elastic sheet in a uniform flow has been studied extensively due to its importance in engineering and its prevalence in natural structures. Varying the flow speed can give rise to a range of dynamics including limit cycle behaviour and chaotic motion of the cantilevered sheet. Recently, the 'inverted flag' configuration – a cantilevered elastic sheet aligned with the flow impinging on its free edge – has been observed to produce large-amplitude flapping over a finite band of flow speeds. This flapping phenomenon has been found to be a vortex-induced vibration, and only occurs at sufficiently large Reynolds numbers. In all cases studied, the inverted flag has been formed from a cantilevered sheet of rectangular morphology, i.e. the planform of its elastic sheet is a rectangle. Here, we investigate the effect of the inverted flag's morphology on its resulting stability and dynamics. We choose a trapezoidal planform which is explored using experiment and an analytical theory for the divergence instability of an inverted flag of arbitrary morphology. Strikingly, for this planform we observe that the flow speed range over which flapping occurs scales approximately with the flow speed at which the divergence instability occurs. This provides a means by which to predict and control flapping. In a biological setting, leaves in a wind can also align themselves in an inverted flag configuration. Motivated by this natural occurrence we also study the effect of adding an artificial 'petiole' (a thin elastic stalk that connects the sheet to the clamp) on the inverted flag's dynamics. We find that the petiole serves to partially decouple fluid forces from elastic forces, for which an analytical theory is also derived, in addition to increasing the freedom by which the flapping dynamics can be tuned. These results highlight the intricacies of the flapping instability and account for some of the varied dynamics of leaves in nature.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/24vvw-k1f65Effect of morphology on the large-amplitude flapping dynamics of an inverted flag in a uniform flow
https://resolver.caltech.edu/CaltechAUTHORS:20181210-110613444
Authors: {'items': [{'id': 'Fan-Boyu', 'name': {'family': 'Fan', 'given': 'Boyu'}}, {'id': 'Huertas-Cerdeira-C', 'name': {'family': 'Huertas-Cerdeira', 'given': 'Cecilia'}}, {'id': 'Cossé-J', 'name': {'family': 'Cossé', 'given': 'Julia'}}, {'id': 'Sader-J-E', 'name': {'family': 'Sader', 'given': 'John E.'}, 'orcid': '0000-0002-7096-0627'}, {'id': 'Gharib-M', 'name': {'family': 'Gharib', 'given': 'Morteza'}, 'orcid': '0000-0003-0754-4193'}]}
Year: 2019
DOI: 10.1017/jfm.2019.474
The stability of a cantilevered elastic sheet in a uniform flow has been studied extensively due to its importance in engineering and its prevalence in natural structures. Varying the flow speed can give rise to a range of dynamics including limit cycle behaviour and chaotic motion of the cantilevered sheet. Recently, the 'inverted flag' configuration – a cantilevered elastic sheet aligned with the flow impinging on its free edge – has been observed to produce large-amplitude flapping over a finite band of flow speeds. This flapping phenomenon has been found to be a vortex-induced vibration, and only occurs at sufficiently large Reynolds numbers. In all cases studied, the inverted flag has been formed from a cantilevered sheet of rectangular morphology, i.e. the planform of its elastic sheet is a rectangle. Here, we investigate the effect of the inverted flag's morphology on its resulting stability and dynamics. We choose a trapezoidal planform which is explored using experiment and an analytical theory for the divergence instability of an inverted flag of arbitrary morphology. Strikingly, for this planform we observe that the flow speed range over which flapping occurs scales approximately with the flow speed at which the divergence instability occurs. This provides a means by which to predict and control flapping. In a biological setting, leaves in a wind can also align themselves in an inverted flag configuration. Motivated by this natural occurrence we also study the effect of adding an artificial 'petiole' (a thin elastic stalk that connects the sheet to the clamp) on the inverted flag's dynamics. We find that the petiole serves to partially decouple fluid forces from elastic forces, for which an analytical theory is also derived, in addition to increasing the freedom by which the flapping dynamics can be tuned. These results highlight the intricacies of the flapping instability and account for some of the varied dynamics of leaves in nature.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/fp5rh-grm86