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Experimental Investigation of Flexible Filaments in Fluid Flow

Silva Leon, Jorge Francisco

[Thesis]. Manchester, UK: The University of Manchester; 2019.

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Abstract

In recent years there has been an increased interest on flexible fluid-structure interaction problems with applications to flow control (reduction of drag and lift fluctuations) and energy harvesting. Particularly, studies have suggested that a hairy coating (poroelastic coating) may help reduce drag and lift fluctuations of a bluff body (cylinder) by around 15% and 40%, respectively. However, these studies have focused on two-dimensional setups, therefore real effects such as three-dimensional vortex shedding in the wake of a cylinder have not been considered. For instance, preliminary experiments carried out in a wind tunnel revealed that the motions of a single filament in the wake of a cylinder are complex due to the influence of the cylinder wake flow and the outer crossflow impinging on the hanging filament (sagged due to gravity). For this reason, this work was set to study experimentally the fundamental behaviour of filaments alone, hanging from a vertical support tube (i.e. not attached to a bluff body). This simple configuration is ideal to analyse the fundamental dynamics of flexible filaments in flow and provide insights for future investigations of hairy coatings. Noteworthy, the filaments hanging in crossflow were free to move in three dimensions, in contrast to the previously existent studies which come from two-dimensional studies, and thus provides unprecedented data valuable for validating fluid-structure interaction simulation codes. At low wind speeds the filaments bent and remained in static equilibrium, similar to the reconfiguration of plants. Beyond this condition, at a certain wind speed, the filaments started to vibrate and in certain cases entered into large-amplitude three-dimensional flutter motions which became more complicated as the wind speed was further increased. Through the use of stereoscopic non-contact high-speed imaging, hotwire anemometry, smoke visualizations and the recourse to linear and nonlinear time-series analysis techniques, the full range of filament behaviours were studied in detail. In particular, the results from this research provided unprecedented data and empirical correlations for the filament static reconfiguration and fluid loading at previously unexplored conditions. Also, the fluid mechanisms responsible for the onset of filament motion were investigated. Additionally, the vortex shedding from reconfigured filaments was for the first time experimentally studied and characterized. This work also provided the first documentation of the three-dimensional flutter motions of filaments, and the effects of turbulence intensity and filament attachment angle on the filaments flapping motions dynamics. Finally, the experimental methodologies (data acquisition, image processing and time-series analyses of motion) developed during this research were also applied for studying other fluid-structure interactions problems: the flow-induced vibration of cantilever rods in axial flow for nuclear reactor applications, and the dynamics of inverted flags for energy harvesting applications.

Bibliographic metadata

Type of resource:
Content type:
Administered thesis
Form of thesis:
Alternative format
Type of submission:
Doctoral level ETD - final
Thesis title:
Experimental Investigation of Flexible Filaments in Fluid Flow
Degree type:
Doctor of Philosophy
Degree programme:
PhD Mechanical Engineering
Publication date:
2019-04-17T17:22:04
Institution:
The University of Manchester
Location:
Manchester, UK
Total pages:
434
Abstract:
In recent years there has been an increased interest on flexible fluid-structure interaction problems with applications to flow control (reduction of drag and lift fluctuations) and energy harvesting. Particularly, studies have suggested that a hairy coating (poroelastic coating) may help reduce drag and lift fluctuations of a bluff body (cylinder) by around 15% and 40%, respectively. However, these studies have focused on two-dimensional setups, therefore real effects such as three-dimensional vortex shedding in the wake of a cylinder have not been considered. For instance, preliminary experiments carried out in a wind tunnel revealed that the motions of a single filament in the wake of a cylinder are complex due to the influence of the cylinder wake flow and the outer crossflow impinging on the hanging filament (sagged due to gravity). For this reason, this work was set to study experimentally the fundamental behaviour of filaments alone, hanging from a vertical support tube (i.e. not attached to a bluff body). This simple configuration is ideal to analyse the fundamental dynamics of flexible filaments in flow and provide insights for future investigations of hairy coatings. Noteworthy, the filaments hanging in crossflow were free to move in three dimensions, in contrast to the previously existent studies which come from two-dimensional studies, and thus provides unprecedented data valuable for validating fluid-structure interaction simulation codes. At low wind speeds the filaments bent and remained in static equilibrium, similar to the reconfiguration of plants. Beyond this condition, at a certain wind speed, the filaments started to vibrate and in certain cases entered into large-amplitude three-dimensional flutter motions which became more complicated as the wind speed was further increased. Through the use of stereoscopic non-contact high-speed imaging, hotwire anemometry, smoke visualizations and the recourse to linear and nonlinear time-series analysis techniques, the full range of filament behaviours were studied in detail. In particular, the results from this research provided unprecedented data and empirical correlations for the filament static reconfiguration and fluid loading at previously unexplored conditions. Also, the fluid mechanisms responsible for the onset of filament motion were investigated. Additionally, the vortex shedding from reconfigured filaments was for the first time experimentally studied and characterized. This work also provided the first documentation of the three-dimensional flutter motions of filaments, and the effects of turbulence intensity and filament attachment angle on the filaments flapping motions dynamics. Finally, the experimental methodologies (data acquisition, image processing and time-series analyses of motion) developed during this research were also applied for studying other fluid-structure interactions problems: the flow-induced vibration of cantilever rods in axial flow for nuclear reactor applications, and the dynamics of inverted flags for energy harvesting applications.
Keyword(s):
Thesis main supervisor(s):
Thesis co-supervisor(s):
Funder(s):
Degree grantor:
Language:
en

Institutional metadata

University researcher(s):
Academic department(s):

Record metadata

Manchester eScholar ID:
uk-ac-man-scw:319228
Created by:
Silva Leon, Jorge
Created:
17th April, 2019, 16:22:04
Last modified by:
Silva Leon, Jorge
Last modified:
1st May, 2020, 11:32:40

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