Near-Wake Flow Structures of a Corner-Chamfered Square Cylinder at Higher Harmonic Excitations

2015 ◽  
Author(s):  
M. S. Aswathy ◽  
K. K. Amrita ◽  
C. M. Hariprasad ◽  
R. Ajith Kumar
Keyword(s):  
Vortex Shedding ◽  
Water Channel ◽  
Wake Flow ◽  
Flow Structures ◽  
Square Cylinder ◽  
Near Wake ◽  
Harmonic Excitations ◽  
Vortex Shedding Frequency ◽  
Higher Harmonic ◽  
Vortex Shedding Modes

In this paper, the results of a flow visualization study on the flow structures around a chamfered square cylinder are presented. Square cylinders having side dimension Bo with corner chamfering ‘b’ are used such that b/Bo ratio assumes values 0, 0.1, 0.2 and 0.3. Corners of the square cylinder are equally cut by a measure ‘b’ both in the stream-wise and cross stream directions. Flow over these cylinders are visualized in a water channel. All the studies correspond to a Reynolds number value of 2100 (based on Bo). Results are taken for two situations (a) cylinders are stationary and (b) cylinders are oscillated at frequency ‘fe’. The main objective of this study is to investigate the near-wake flow structures around the cylinders at harmonic and higher harmonic excitations. Experiments were conducted for fe/fs= 1.0, 1.5, 2.0, 2.5 and 3.0 where fs is the vortex shedding frequency from the stationary cylinder for each b/Bo ratio. Peak-to-peak amplitude of excitation is kept at 1B in all cases. In this investigation, the main focus is on investigating the vortex shedding modes, mechanisms and the number of vortices shed per shear layer as the cylinder completes one oscillatory cycle as a function of fe/fs ratio.

Effect of Corner-Arc on the Flow Structures Around a Square Cylinder

2018 ◽  
Author(s):  
Hariprasad Chakkalaparambil Many ◽  
Vishnu Chandar Srinivasan ◽  
Ajith Kumar Raghavan
Keyword(s):  
Drag Reduction ◽  
Oscillation Amplitude ◽  
Water Channel ◽  
Excitation Frequency ◽  
Flow Structures ◽  
Square Cylinder ◽  
Near Wake ◽  
Vortex Size ◽  
Vortex Shedding Frequency ◽  
Visualization Experiments

In this paper, flow structures around a corner modified square cylinder (side dimension, Bo) are presented and discussed. Cylinders with various corner arcs (circular) were considered (arc radius ‘r’). For various Corner Ratios (CR = r/Bo), values ranging from 0 to 0.5, flow visualization experiments were conducted in a water channel and the results are reported at Re = 2100 (based on Bo). Results presented are for two cases (a) stationary cylinders reporting the values of CD (coefficient of drag), St (Strouhal no.), and D (vortex size) and (b) oscillating cylinders at fe/fs = 1 (fe is the cylinder excitation frequency and fs is the vortex shedding frequency) and a/Bo = 0.8 (a is the cylinder oscillation amplitude). The work is aimed to explore the most effective configuration for drag reduction. Cylinder with corner ratio of 0.2 is proved to be the most effective one among the cases considered in this study with 19.3% drag reduction. As a major highlight, in contrast to the results of the previous studies, current study do not reveal a monotonous decrease of drag with increasing corner modification. Instead, it is shown here that, there is a specific value of CR ratio where the drag is the minimum most. A peculiar type of vortex structure was observed in the cases of stationary cylinders with CR > 0.2, contributing to the increase in drag. In the case of oscillating cylinders, description of one complete cycle for all CR ratios at various time instances are presented. The near-wake structures were observed to be dependent on the CR ratio. Counter intuitively, cylinder oscillation does not bring major difference in vortex size compared to the stationary case.

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.

Effect of Corner Cut on the Near Wake Flow Structures of a Square Cylinder

2014 ◽  
Author(s):  
Hariprasad Chakkalaparambil Many ◽  
Nagella Yashwanth ◽  
Haresh Bhardwaj ◽  
R. Ajith Kumar ◽  
B. H. Lakshmana Gowda
Keyword(s):  
Free Stream ◽  
Wake Flow ◽  
Practical Significance ◽  
Flow Structures ◽  
Stream Velocity ◽  
Square Cylinder ◽  
Sinusoidal Oscillation ◽  
Near Wake ◽  
Almost All ◽  
Sharp Corners

In this paper, results of a flow visualization study on the flow around a square section cylinder with corner chamfering are presented. The corners of the cylinder are chamfered so that the each corner forms a triangle with horizontal (stream-wise) and cross stream (perpendicular to the free stream velocity) dimension ‘b’. Experiments are conducted for b/B0 ratios of 0.05, 0.1, 0.2 and 0.3 where ‘B0’ is the side dimension of the uncut square cylinder. The flow structures, particularly the vortex shedding mode and mechanism around the cylinder with chamfered corners are investigated in order to deduce the effect of corner modifications on the flow. For studies with stationary cylinder (case (a)), the results are taken at Reynolds number values of 1500, 2100 and 2800. For sinusoidally oscillated cylinder case (case (b)), the studies are restricted to Re=2100. To bring out the effect of corner chamfering more clearly, experiments are also conducted with a square cylinder without corner cuts, i.e., with sharp corners. For the case (b), a special mechanism is made to oscillate the cylinders at a desired amplitude and frequency. That is, the cylinder undergoes forced sinusoidal oscillation in case (b). It is found that drag decreases and Strouhal number increases with b/B0 ratio. Quite uniquely, at b/B0=0.2, cross-stream convection of vortices have been observed. Vortex coalescence is observed in almost all cases. Results indicate that corner chamfering brings notable changes in the near-wake flow structures of a square section cylinder. In view of marine structures and building sections with similar geometries, the present results carry considerable practical significance.

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.

Numerical analysis of bluff body wakes under periodic open-loop control

2013 ◽  
Vol 739 ◽  
pp. 94-123 ◽  
Author(s):  
Derwin J. Parkin ◽  
M. C. Thompson ◽  
J. Sheridan
Keyword(s):  
Strouhal Number ◽  
Cross Section ◽  
Vortex Shedding ◽  
Open Loop ◽  
Wake Flow ◽  
Dynamic Mode ◽  
Two Dimensional ◽  
Near Wake ◽  
Recirculation Bubble ◽  
Number Range

AbstractLarge eddy simulations at$Re= 23\hspace{0.167em} 000$are used to investigate the drag on a two-dimensional elongated cylinder caused by rear-edge periodic actuation, with particular focus on an optimum open-loop configuration. The 3.64 (length/thickness) aspect-ratio cylinder has a rectangular cross-section with rounded leading corners, representing the two-dimensional cross-section of the now genericAhmed-body geometry. The simulations show that the optimum drag reduction occurs in the forcing Strouhal number range of$0. 09\leq S{t}_{act} \leq 0. 135$, which is approximately half of the Strouhal number corresponding to shedding of von Kármán vortices into the wake for the natural case. This result agrees well with recent experiments of Henninget al. (Active Flow Control, vol. 95, 2007, pp. 369–390). A thorough transient wake analysis employing dynamic mode decomposition is conducted for all cases, with special attention paid to the Koopman modes of the wake flow and vortex progression downstream. Two modes are found to coexist in all cases, the superimposition of which recovers the majority of features observed in the flow. Symmetric vortex shedding in the near wake, which effectively extends the mean recirculation bubble, is shown to be the major mechanism in lowering the drag. This is associated with opposite-signed vortices reducing the influence of natural vortex shedding, resulting in an increase in the pressure in the near wake, while the characteristic wake antisymmetry returns further downstream. Lower-frequency actuation is shown to create larger near-wake symmetric vortices, which improves the effectiveness of this process.

Stability properties of forced wakes

2007 ◽  
Vol 579 ◽  
pp. 137-161 ◽  
Author(s):  
B. THIRIA ◽  
J. E. WESFREID
Keyword(s):  
Vortex Shedding ◽  
Mean Flow ◽  
Near Wake ◽  
Global Mode ◽  
Spatial Properties ◽  
Moderate Reynolds Number ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency ◽  
The Stability ◽  
Selection Of

Thiria, Goujon-Durand & Wesfreid (J. Fluid Mech. vol. 560, 2006, p. 123), it was shown that vortex shedding from a rotationally oscillating cylinder at moderate Reynolds number can be characterized by the spatial coexistence of two distinct patterns, one of which is related to the forcing frequency in the near wake and the other to a frequency close to the natural one for the unforced case downstream of this locked region. The existence and the modification of these wake characteristics were found to be strongly affected by the frequency and the amplitude of the cylinder oscillation. In this paper, a linear stability analysis of these forced regimes is performed, and shows that the stability characteristics of such flows are governed by a strong mean flow correction which is a function of the oscillation parameters. We also present experiments on the spatial properties of the global mode and on the selection of the vortex shedding frequency as a function of the forcing conditions for Re = 150. Finally, we elucidate a diagram of locked and non-locked states, for a large range of frequencies and amplitudes of the oscillation.

Wake Flow Behind a Cylinder With a Detached Splitter Plate

2005 ◽  
Author(s):  
Minter Cheng
Keyword(s):  
Strouhal Number ◽  
Vortex Shedding ◽  
Formation Process ◽  
Vortex Formation ◽  
Splitter Plate ◽  
Wake Flow ◽  
Plate Length ◽  
Splitter Plates ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

Fluid flow across a bluff body can induce a series alternating vortices in the downstream flow field. The vortex flow can produce adverse effects on many engineering applications. A number of studies have shown that the wake splitter plate is one of the means to stabilize the vortex formation process. However, most of the previous studies are confined to cylinders with attached splitter plates. Very few studies investigate the effects of the spacing between the cylinder and the splitter plate on the formation of wake vortices. In the present study, the effects of the splitter plate length as well as the gap distance between the splitter plate and the cylinder on the wake flow behind a cylinder have been studied experimentally for low Reynolds number of 400. Both circular and square cylinders are studied in this research. Four splitter plates with different length, 1 ≤ L/D ≤ 4, have been used and a range of cylinder and splitter plate gap distance, 0 < G/D < 6, have been studied. By using flow visualization technique and hot-film anemometer measurement, detailed measurements of the velocity distribution, the vortex shedding frequency, the wake width, and the wake formation length are carried out in order to get a clear understanding of the flow interference behavior. The experimental results indicate that splitter plates alter the vortex formation process in the wake causing a decrease in vortex shedding frequency. The Strouhal number decreases with increasing the splitter plate length as well as the gap distance between the cylinder and the splitter plate. It is shown that a jump in Strouhal number occurs at G/D of 3 to 6. The jump is splitter plate length dependent, and generally the gap distance at which jump takes place increases as the splitter plate length increases.

scholarly journals Visualization of flow past square cylinders with corner modification

2020 ◽  
Vol 4 (3) ◽  
pp. 285-294
Author(s):  
Ch. Krishnappa Vikram ◽  
H. V. Ravindra ◽  
Y. T. Krishnegowda
Keyword(s):  
Aluminum Powder ◽  
Experimental Work ◽  
Vortex Shedding ◽  
Test Section ◽  
Square Cylinder ◽  
Square Cylinders ◽  
Flow Past ◽  
Spacing Ratio ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

This article presents the results for flow past a square cylinder and two square cylinders of the same and different sizes with corner modifications by varying the spacing ratio. Here, experimental work is conducted in a recirculatory channel filled with water. A set of aluminum discs made to rotate to create the flow in the test section. The motor is used to vary the speed of the water. Fine aluminum powder is used as a tracer medium. It is observed that vortex shedding frequency decreases by placing the second cylinder in the downstream of the first cylinder. For similar size cylinders, the width of the eddy in the middle of the cylinders increases with an increase in spacing ratio. With the increase of spacing ratio to 6, the flow past each cylinder behaves like a single square cylinder. If the upstream square cylinder size is smaller than the downstream square cylinder, the eddy size is reduced in between the cylinder compared to the downstream of the second cylinder. If the upstream square cylinder size is bigger than the downstream square cylinder, the eddy size is larger in between the cylinder compared to the downstream of the second cylinder.

Vortex-Induced Vibration Characteristics of an Elastic Square Cylinder on Fixed Supports

2004 ◽  
Vol 127 (2) ◽  
pp. 241-249 ◽  
Author(s):  
Z. J. Wang ◽  
Y. Zhou
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Flow Separation ◽  
Natural Frequencies ◽  
Separation Point ◽  
Square Cylinder ◽  
Vortex Induced Vibration ◽  
The Third ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

The vortex-induced structural vibration of an elastic square cylinder, on fixed supports at both ends, in a uniform cross flow was measured using fiber-optic Bragg grating sensors. The measurements are compared to those obtained for an elastic circular cylinder of the same hydraulic diameter in an effort to understand the effect of the nature (fixed or oscillating) of the flow separation point on the vortex-induced vibration. It is found that a violent vibration occurs at the third-mode resonance when the vortex-shedding frequency coincides with the third-mode natural frequency of the fluid-structure system, irrespective of the cross-sectional geometry of the cylinder. This is in distinct contrast to previous reports of flexibly supported rigid cylinders, where the first-mode vibration dominates, thus giving little information on the vibration of other modes. The resonance behavior is neither affected by the incidence angle (α) of the free stream, nor by the nature of the flow separation point. However, the vibration amplitude of the square cylinder is about twice that of the circular cylinder even though the flexural rigidity of the former is larger. This is ascribed to a difference in the nature of the flow separation point between the two types of structures. The characteristics of the effective modal damping ratios, defined as the sum of structural and fluid damping ratios, and the system natural frequencies are also investigated. The damping ratios and the system natural frequencies vary little with the reduced velocity at α=0deg, but appreciable at α⩾15deg; they further experience a sharp variation, dictated by the vortex-shedding frequency, near resonance.

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