scholarly journals An investigation of the near-wake flow structure of a cylinder with guiding plates

2018 ◽  
Vol 180 ◽  
pp. 02024
Author(s):  
Firat Ekinci ◽  
Erhan Firat ◽  
Göktürk M. Özkan ◽  
Hüseyin Akilli
Keyword(s):  
Large Scale ◽  
Transverse Velocity ◽  
Vortex Formation ◽  
Angular Position ◽  
Wake Flow ◽  
Circular Cylinders ◽  
Near Wake ◽  
Image Velocimetry ◽  
Large Scale Vortex ◽  
Viv Suppression

In this study, the flow behind a circular cylinder with a pair of outer identical guiding plates was investigated using particle image velocimetry (PIV) for various angular positions of the plates (i.e. α=±70°, ±100°, and ±130°). The gaps between these plates and cylinder are equal and are 0.3D. Experiments were carried out at a subcritical Reynolds (Re=ρ·U∞·D/μ) number of 7500, based on the cylinder diameter (D) and the flow velocity (U∞). The features of the near-wake with and without the guiding plates were interpreted in terms of patterns of time-averaged vorticity and streamlines, time-averaged and fluctuating velocity components. The spectral analysis was also carried out to determine the time-dependent variation of the transverse velocity at given locations in the near-wake. Two-dimensional computations of flow around circular cylinders with and without guiding plates have also been performed to predict the timeaveraged and root-mean-square of force coefficients of the various models. It was seen that the guiding plates at an appropriate angular position can lead to substantial attenuation, or retardation, of the process of large-scale vortex formation in the near-wake, thus can lead to vortex-induced vibration (VIV) suppression without any increase in drag.

On vortex formation from a cylinder. Part 2. Control by splitter-plate interference

1988 ◽  
Vol 190 ◽  
pp. 513-529 ◽  
Author(s):  
M. F. Unal ◽  
D. Rockwell
Keyword(s):  
Reynolds Number ◽  
Large Scale ◽  
Vortex Formation ◽  
Effective Control ◽  
Absolute Instability ◽  
Near Wake ◽  
Number Range ◽  
Large Scale Vortex ◽  
The Absolute ◽  
Substantial Control

Control of vortex formation from a circular cylinder by a long plate in its wake is examined over the Reynolds number range 140 < Re < 3600. There are two basic flow regimes: a pre-vortex formation regime, in which the plate precludes formation of a large-scale vortex upstream of the tip of the plate; and a post-vortex formation regime in which one or more large-scale vortices are formed upstream of the edge. The unsteady pressure loading at the tip of the plate increases by over an order of magnitude during transition from the pre- to post-vortex formation regime. If the plate is located near the cylinder, it is possible to more than double the vortex formation length, relative to the case of the free wake. Moreover, these observations suggest that: there is a minimum streamwise lengthscale for development of the absolute instability of the near wake and thereby the large-scale vortex; and the vortex formation length may also be influenced by the downstream vorticity dynamics. When the plate is located downstream of the initially formed vortex, effective control is possible when the near-wake fluctuation level and mean base pressure of the corresponding free (non-impinging) wake are sufficiently small. This occurs in the low and moderate subcritical regimes; the substantial control by the wake-plate interaction in this range of Reynolds number implies low strength of the absolute instability of the near wake. However, in the pure von Kármán regime, selfcontrol of the near wake dominates that imposed by the wake-edge interaction, suggesting a strong absolute instability of the near wake.

Effect of Dissimilar Leading Edges on the Flow Structures Around a Square Cylinder

2015 ◽  
Vol 137 (6) ◽  
Author(s):  
R. Ajith Kumar ◽  
K. Arunkumar ◽  
C. M. Hariprasad
Keyword(s):  
Amplitude Ratio ◽  
Vortex Formation ◽  
Leading Edge ◽  
Wake Flow ◽  
Practical Significance ◽  
Flow Structures ◽  
Square Cylinder ◽  
Near Wake ◽  
Study Results ◽  
Leading Edges

In the present study, results of a flow visualization study on the flow around a square cylinder with dissimilar leading edges are presented. The radii of the leading edges of the cylinder “r1” and “r2” are such that the ratio r1/r2 is systematically varied from 0 to 1. The flow structures around the cylinder with different leading edge radii particularly the vortex shedding mode and mechanism are investigated. For studies with stationary as well as oscillated cylinder cases, the results are taken at a Reynolds number value of 2100. For the oscillated case, a special mechanism is made to oscillate the cylinders at a desired amplitude and frequency. That is, the cylinder undergoes forced oscillation in this case. Results indicate that dissimilar leading edges bring notable changes in the near-wake flow structures of a square cylinder. For the stationary cylinder cases, the vortex formation length decreases with increase in the r1/r2 ratio. Flow structures are also found to be influenced by the amplitude ratio (amplitude to body size ratio); the higher the amplitude, the larger the size of vortices shed per cycle of cylinder oscillation. In view of marine structures and building sections with similar geometries, the present results carry considerable practical significance.

Influence of Wall Proximity and Free Surface on Wake of Two Side-by-Side Circular Cylinders: PIV Measurement and POD Analysis

2010 ◽  
Author(s):  
S. S. Paul
Keyword(s):  
Free Surface ◽  
Large Scale ◽  
Circular Cylinders ◽  
Particle Image Velocimetry Technique ◽  
Large Scale Structures ◽  
Turbulent Statistics ◽  
Piv Measurement ◽  
Image Velocimetry ◽  
Pitch Ratio ◽  
Pod Analysis

The paper reports on an experimental study of turbulent flow around a pair of circular cylinders with a pitch ratio of two. The cylinders were located in the vicinity of a plane wall, in a uniform stream, and adjacent to a free surface in an open channel. The Reynolds number based on depth of flow and fresstream velocity was 30300 while the Froude number was 0.3. A particle image velocimetry technique was used to conduct detailed velocity measurements around and in the near wake region of the cylinder pairs, from which contours and profiles of the mean velocities and turbulent statistics were obtained and discussed. The proper orthogonal decomposition was then applied to provide an insight into the structure of the flow. Reconstruction of the fluctuating velocity components for various numbers of modes were also shown to investigate the role of large-scale structures.

Viscous Free-Surface Flow Past Twin Vertical Cylinders

2003 ◽  
Author(s):  
Tong Chen ◽  
Allen T. Chwang
Keyword(s):  
Free Surface ◽  
Large Scale ◽  
Free Surface Flow ◽  
Surface Flow ◽  
Circular Cylinders ◽  
Small Scale ◽  
The Finite Element Method ◽  
Suction Force ◽  
Large Scale Vortex ◽  
Vertical Cylinders

The laminar flow behaviors around two vertical circular cylinders (in a tandem arrangement) that pierce a free surface are investigated by the finite element method in this paper. The computational results exhibit two major free-surface effects: the presence of a free surface allows the occurrence of small-scale Kelvin-Helmholtz instabilities, but suppresses the onset of large-scale vortex alternating behavior. It is also found that the vorticity will expand in a necklace shape adjacent to the free surface. The second cylinder may experience a persisting suction force due to “trapped” vortices in the gap between the two cylinders, which may not happen in the absence of a free surface.

Experimental Study on the Near Wake Behind Two Staggered Cylinders of Unequal Diameters

2012 ◽  
Author(s):  
Yangyang Gao ◽  
Xikun Wang ◽  
Soon Keat Tan
Keyword(s):  
Reynolds Stress ◽  
Incident Angle ◽  
Reynolds Shear Stress ◽  
Flow Characteristics ◽  
Circular Cylinders ◽  
Mean Flow ◽  
Near Wake ◽  
Particle Image Velocimetry Technique ◽  
Image Velocimetry ◽  
Spacing Ratio

The wake structure behind two staggered circular cylinders with unequal diameters was investigated experimentally using the particle image velocimetry technique (PIV). This investigation was focused on the variations of flow patterns in terms of incident angle at Reynolds number Re = 1200. Comparisons of the time-averaged flow field of two staggered cylinders with unequal diameters at different angles were made to elucidate the mean flow characteristics. The characteristics of Reynolds shear stress contours at different incident angles and spacing ratios were also investigated. The results showed that with increasing of incident angle, the scale of Reynolds stress contours behind the upstream cylinder becomes larger, as well as the effect of spacing ratio on Reynolds stress contours.

Experimental Analysis of the Near Wake from a Ducted Thruster at True and Near Bollard Pull Conditions Using Stereo Particle Image Velocimetry (SPIV)

2005 ◽  
Vol 127 (3) ◽  
pp. 191-196 ◽  
Author(s):  
S. El Lababidy ◽  
N. Bose ◽  
P. Liu ◽  
D. Walker ◽  
F. Di Felice
Keyword(s):  
Particle Image Velocimetry ◽  
Particle Image ◽  
Practical Interest ◽  
Stereoscopic Particle Image Velocimetry ◽  
Wake Flow ◽  
Hydrodynamic Characteristics ◽  
Near Wake ◽  
Image Velocimetry ◽  
Wide Range ◽  
Marine Applications

Thrusters working at low advance coefficients are employed in a wide range of offshore and marine applications on Floating, Production, Storage, and Offloading (FPSO) systems; shuttle tankers; tug boats; and mobile offshore units. Therefore, an understanding of the flow around the thrusters is of great practical interest. Despite this interest, there is lack of knowledge in the description of the hydrodynamic characteristics of a ducted thruster’s wake at bollard pull and low advance coefficient values. This work was aimed at providing detailed data about the hydrodynamic characteristics of a Dynamic Positioning (DP) thruster near wake flow at different low advance coefficient values. Wake measurements were made during cavitation tunnel tests carried out on a ducted propeller model at the Italian Ship Model Basin (INSEAN), Rome, Italy. Through these experiments, the DP thruster near wake velocity components at different downstream axial planes, up to 1.5 diameters downstream, were obtained using a Stereoscopic Particle Image Velocimetry (SPIV) system. These experiments were carried out at different advance coefficient (J) values [bollard pull (J=0), J=0.4 and J=0.45].

Near-wake structure of an oscillating cylinder: effect of controlled shear-layer vortices

1996 ◽  
Vol 322 ◽  
pp. 21-49 ◽  
Author(s):  
C. K. Chyu ◽  
D. Rockwell
Keyword(s):  
Shear Layer ◽  
Vortex Formation ◽  
Small Scale ◽  
Asymptotic State ◽  
Near Wake ◽  
Wake Structure ◽  
Image Velocimetry ◽  
Image Density ◽  
Karman Vortex ◽  
Limiting Case

The instantaneous structure of the near wake of a cylinder subjected to small-amplitude perturbations is characterized using high-image-density particle image velocimetry. Emphasis is on control of the small-scale shear-layer vortices, which feed into the Kármán vortices. Modifications of the Kármán vortex formation are classified according to patterns of modulated and locked-on shear-layer vortices. The formation length of the Kármán vortices can be dramatically shortened and, in the limiting case, occur adjacent to the base of the cylinder when it is perturbed at the inherent instability frequency of the shear layer and its subharmonics. Moreover, the induced shear-layer vortices can lead to large-amplitude transverse undulations of the entire near-wake region during formation of the Kármán vortices.These variations of the near-wake structure are further elucidated by considering the transient response of the wake, induced by abrupt cessation and onset of periodic motion of the cylinder. Distinctive intermediate states of the wake arise during relaxation to its asymptotic state; such relaxation requires a very large number of periods of the inherent instability of the shear layer.

Effect of Dissimilar Leading Edges on the Flow Structures Around a Square Cylinder

2014 ◽  
Author(s):  
Arunkumar Kumaran Nair ◽  
R. Ajith Kumar ◽  
Hariprasad Chakkalaparambil Many
Keyword(s):  
Amplitude Ratio ◽  
Vortex Formation ◽  
Leading Edge ◽  
Wake Flow ◽  
Practical Significance ◽  
Flow Structures ◽  
Square Cylinder ◽  
Near Wake ◽  
Study Results ◽  
Leading Edges

In the present study, results of a flow visualization study on the flow around a square cylinder with dissimilar leading edges are presented. The radii of the leading edges of the cylinder ‘r1’ and ‘r2’ are such that the ratio r1/ r2 is systematically varied from 0 to 1. The flow structures around the cylinder with different leading edge radii particularly the vortex shedding mode and mechanism are investigated. For studies with stationary as well as oscillated cylinder cases, the results are taken at a Reynolds number value of 2100. For the oscillated case, a special mechanism is made to oscillate the cylinders at a desired amplitude and frequency. That is, the cylinder undergoes forced oscillation in this case. Results indicate that dissimilar leading edges bring notable changes in the near-wake flow structures of a square cylinder. For the stationary cylinder cases, the vortex formation length decreases with increase in the r1/ r2 ratio. Flow structures are also found to be influenced by the amplitude ratio (amplitude to body size ratio); the higher the amplitude, the larger the size of vortices shed per cycle of cylinder oscillation. In view of marine structures and building sections with similar geometries, the present results carry considerable practical significance.

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