Influence of flexible fins on vortex-induced load over a circular cylinder at low Reynolds number
TL;DR: In this paper, the effect of fin flexibility on the performance with regard to vortex-induced forces by varying non-dimensional flexural rigidity, KB∈ [0.01, 10], of the fin was analyzed.
Abstract: Fins or fairings are typically streamlined structures employed to reduce the vortex-induced unsteady forces acting on a bluff body by preventing shear layer roll-up in the near-wake region. In this work, fins would refer to thin plate-like structures attached tangentially to the bluff body's top and bottom surfaces. Of particular interest here are flexible fins that can undergo static deformation or coupled fluid-elastic vibrations due to the non-linear interactions with the shear layer from a circular cylinder and the roll-up of shear layers at the trailing edge of the fin. We present a numerical analysis to realize the effect of fin flexibility on the performance with regard to vortex-induced forces by varying non-dimensional flexural rigidity, KB∈ [0.01, 10], of the fins. Two-dimensional simulations are carried out for a fixed non-dimensional fin mass ratio, m*=0.1, and Reynolds number, Re = 100. In this study, we consider two fins attached tangentially to the top and bottom surfaces of a fixed circular cylinder. We show that as the flexibility of the fins increases progressively, the stability of the fins is lost and the fins undergo a coupled flapping motion. As a function of KB, three distinct dynamic response regimes of the flexible fins are identified: (i) fixed-point stability for KB>1, (ii) periodic outward flapping 0.025≤KB≤0.1, and (iii) periodic flapping about the initial position with large amplitudes KB<0.025. Flexibility and inclination angle of fins are observed to be effective in minimizing the vortex-induced forces.
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TL;DR: In this article , the hydrodynamic mechanism of drag reduction by a rotationally oscillating cylinder with a flexible filament was explored using the penalty immersed boundary method, and the effects of the filament length, bending rigidity, oscillatory frequency, and oscillatory amplitude on drag reduction were systematically examined.
Abstract: The hydrodynamic mechanism of drag reduction by a rotationally oscillating cylinder with a flexible filament was explored using the penalty immersed boundary method. A simulation of a stationary cylinder without a filament was also performed for comparison. The effects of the filament length, bending rigidity, oscillatory frequency, and oscillatory amplitude on drag reduction were systematically examined. The underlying mechanism of drag reduction was characterized in terms of the shape deformation of the filament, wake pattern, pressure distribution, and flapping dynamics. Two dominant flapping modes were observed: an oscillation mode with less than half a wave on the filament, and an undulation mode with more than one wave on the filament. In the oscillation mode, drag reduction is mainly achieved by the thrust generated by the filament, accompanied by an increase of lift fluctuations. The pressure difference caused by the flapping motion between the upper and lower sides of the filament is the main cause of the thrust. In the undulation mode, drag reduction is realized by both the thrust generated by the filament and the decreased form drag of the cylinder. A filament flapping in the oscillation mode can generate greater thrust than a filament flapping in the undulation mode. A long undulatory filament with relatively low oscillatory amplitude effectively stabilizes the wake, resulting in a decrease of the lift fluctuations.
8 citations
TL;DR: In this paper , the effect of a non-uniform bending rigidity of the hairy coating on drag reduction was explored and a stable streamline shape was found to delay the vortex formation and stabilize the recirculation zone, resulting in decreased form drag.
Abstract: Abstract Abstract The hydrodynamic mechanism of drag reduction by a flexible hairy coating was explored using the penalty immersed boundary method. A two-dimensional flexible hairy coating is constituted by multiple flexible filaments. A simulation of a cylinder without a hairy coating at a Reynolds number of 100 was also performed for comparison. The results of the simulations show good agreement with the experimental data by Niu & Hu (Phys. Fluids, vol. 23, 2011, 101701), where maximum drag reduction of 22% was attained at a particular length, bending rigidity, coating density and coating angle of the hairy coating. The hydrodynamic mechanism of drag reduction was characterized in terms of the wake pattern, shape deformation and kinetic energy of the hairy coating. The effect of a non-uniform bending rigidity of the hairy coating on drag reduction was explored. A stable streamline shape of the hairy coating was found to delay the vortex formation and stabilize the recirculation zone, resulting in decreased form drag. Active flapping of the hairy coating with enhanced vortex shedding is adverse to drag reduction. A hairy coating with a stiff base and flexible trailing edge is beneficial to maintaining a stable shape.
2 citations
TL;DR: In this article , a hybrid rigid-flexible thin plate is proposed to study the VIV caused by trailing vortices shedding in viscous flows, and a series of numerical simulations are presented to study fluid-solid interaction of blunt bodyflexible/rigid plate in uniform flows, considering with and without flexible plate, different velocities and different blunt body shapes.
1 citations
TL;DR: In this article, the effects of flexible plates on the hydrodynamic forces acting on a circular cylinder in turbulent flows were evaluated using the k − ω S S T model.
Abstract: In this study, the k − ω S S T model is employed to evaluate the effects of flexible plates on the hydrodynamic forces acting on a circular cylinder in turbulent flows. In this regard, four flexible plates are attached to the circular cylinder. Besides, the finite volume method and finite element method have been utilized to discretize the governing equations of the fluid flow and the flexible plates, respectively. The Reynolds number based on the cylinder diameter is kept constant at R e = 10 5 . The effects of fin angle ( θ = 0 ° and 45 ° ) and the flexural rigidity ( 3 ≤ E I ∗ ≤ 20 ) on the wake structure, displacement amplitude, Strouhal number as well as lift and drag coefficients are investigated. The results showed that the flexural rigidity of plates had a significant influence on the hydrodynamic coefficients. Indeed, an increase in the non-dimensional flexural rigidity enhanced the drag coefficient, and then it approximately remained constant. For θ = 0 ° a n d E I ∗ ≤ 5 , the averaged drag coefficient is approximately constant and equal to 6.1. As the E I ∗ increases, the drag coefficient increases rapidly and reaches the value of 14.37 for E I ∗ = 20. In the second configuration ( θ = 45 o ) , values of the drag coefficient for low ( E I ∗ = 5.33 ) and high ( E I ∗ = 20 ) values of flexural rigidity are equal to 7.81 and 8.79, respectively.
1 citations
TL;DR: In this article , a finite volume-based open-source computational toolbox, OpenFOAM, was used to compute the flow field in the Reynolds number (Re) range of 60-180.
Abstract: A numerical study of incompressible and unsteady flow past a slotted circular cylinder was carried out in a laminar regime. Three different-shaped slits (converging, diverging, and parallel) were installed symmetrically about the cylinder's horizontal axis. A finite volume-based open-source computational toolbox, OpenFOAM, was used to compute the flow field in the Reynolds number (Re) range of 60–180. The results showed the presence of the periodic vortex shedding for all Re ranges and the slotted cylinders were effectively able to suppress it. The surface vorticity over the slotted cylinder surface increased by 16%–23% as compared to the normal cylinder. The momentum injection to the cylinder base reduced the strength of vortex shedding and shedding frequency for the slotted cylinders. Pressure recovery was improved due to an increase in the pressure coefficient which further led to a decrease in the pressure drag. The total drag coefficient for the slotted cylinders was lower than the normal cylinders for Re > 70. Also, a comparatively lower Strouhal number was observed for the slotted cylinders.
1 citations
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TL;DR: Gmsh as mentioned in this paper is an open-source 3D finite element grid generator with a build-in CAD engine and post-processor that provides a fast, light and user-friendly meshing tool with parametric input and advanced visualization capabilities.
Abstract: Gmsh is an open-source 3-D finite element grid generator with a build-in CAD engine and post-processor. Its design goal is to provide a fast, light and user-friendly meshing tool with parametric input and advanced visualization capabilities. This paper presents the overall philosophy, the main design choices and some of the original algorithms implemented in Gmsh. Copyright (C) 2009 John Wiley & Sons, Ltd.
5,322 citations
TL;DR: In this paper, a review summarizes fundamental results and discoveries concerning vortex-induced vibration (VIV) that have been made over the last two decades, many of which are related to the push to explore very low mass and damping, and to new computational and experimental techniques that were hitherto not available.
Abstract: This review summarizes fundamental results and discoveries concerning vortex-induced vibration (VIV), that have been made over the last two decades, many of which are related to the push to explore very low mass and damping, and to new computational and experimental techniques that were hitherto not available. We bring together new concepts and phenomena generic to VIV systems, and pay special attention to the vortex dynamics and energy transfer that give rise to modes of vibration, the importance of mass and damping, the concept of a critical mass, the relationship between force and vorticity, and the concept of "effective elasticity," among other points. We present new vortex wake modes, generally in the framework of a map of vortex modes compiled from forced vibration studies, some of which cause free vibration. Some discussion focuses on topics of current debate, such as the decomposition of force, the relevance of the paradigm flow of an elastically mounted cylinder to more complex systems, and the relationship between forced and free vibration.
1,943 citations
TL;DR: A comprehensive review of the progress made during the past two decades on vortex-induced vibration (VIV) of mostly circular cylindrical structures subjected to steady uniform flow is presented in this article.
1,368 citations
TL;DR: In this article, the authors examined the characteristic lengths of the oscillating wakes of bluff bodies and concluded that these are equivalent at high Reynolds number, leading to the conclusion that there are two simultaneous characteristic lengths; the scale of the formation region and the width to which the free shear layers diffuse.
Abstract: The characteristic lengths of the oscillating wakes of bluff bodies is discussed; in particular, those used in the universal non-dimensional frequencies proposed by Roshko (1954b) and Goldburg, Washburn & Florsheim (1965). It is concluded that these are equivalent at high Reynolds number. A closer examination leads to the conclusion that there are two simultaneous characteristic lengths; the scale of the formation region, and the width to which the free shear layers diffuse. Discussion of the mechanics of the formation region results in a physical basis for the determination of the frequency by these two characteristic lengths. The ideas developed are applied to the effects of splitter plates in the wake. The possibility of a high-Reynolds-number symmetrical formation region is suggested as an explanation of the very small lift values observed in the absence of free-stream disturbances.
921 citations
TL;DR: In this article, the principal mechanism for producing propulsive and transient forces in oscillating flexible bodies and fins in water, the formation and control of large-scale vortices, was identified.
Abstract: Interest in novel forms of marine propulsion and maneuvering has sparked a number of studies on unsteadily operating propulsors. We review recent experimental and theoretical work identifying the principal mechanism for producing propulsive and transient forces in oscillating flexible bodies and fins in water, the formation and control of large-scale vortices. Connection with studies on live fish is made, explaining the observed outstanding fish agility.
816 citations