Symmetric Vortex Shedding From a Circular Cylinder Under Periodic Flow Forcing

2006 ◽  
Author(s):  
E. Konstantinidis ◽  
S. Balabani
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Vortex Formation ◽  
Excitation Frequency ◽  
Symmetric Mode ◽  
Near Wake ◽  
Wake Field ◽  
Vortex Pairs ◽  
Probabilistic Function ◽  
Number Of Cycles

This paper describes an experimental study of the near wake of a circular cylinder subjected to streamwise flow forcing. The wake field is examined by PIV and LDV for excitation frequencies in which symmetric shedding is likely. The results show that symmetric formation of vortex pairs occurs close to the cylinder synchronized with the oscillatory component of the flow. The symmetric mode rapidly breaks down and gives rise to an antisymmetric arrangement of single vortices further downstream. The number of cycles for which the symmetrical vortices persist in the near wake is a probabilistic function of the excitation frequency and forcing amplitude. Details of the related wake kinematics and frequencies are shown and the findings are discussed in relation to symmetric vortex formation occurring in self-excited streamwise oscillations.

On vortex formation from a cylinder. Part 1. The initial instability

1988 ◽  
Vol 190 ◽  
pp. 491-512 ◽  
Author(s):  
M. F. Unal ◽  
D. Rockwell
Keyword(s):  
Reynolds Number ◽  
Kinetic Energy ◽  
Circular Cylinder ◽  
Shear Layer ◽  
Vortex Shedding ◽  
Vortex Formation ◽  
Absolute Instability ◽  
Near Wake ◽  
Lower Range ◽  
Fluctuation Amplitude

Vortex shedding from a circular cylinder is examined over a tenfold range of Reynolds number, 440 ≤ Re ≤ 5040. The shear layer separating from the cylinder shows, to varying degrees, an exponential variation of fluctuating kinetic energy with distance downstream of the cylinder. The characteristics of this unsteady shear layer are interpreted within the context of an absolute instability of the near wake. At the trailing-end of the cylinder, the fluctuation amplitude of the instability correlates well with previously measured values of mean base pressure. Moreover, this amplitude follows the visualized vortex formation length as Reynolds number varies. There is a drastic decrease in this near-wake fluctuation amplitude in the lower range of Reynolds number and a rapid increase at higher Reynolds number. These trends are addressed relative to the present, as well as previous, observations.

Three-dimensional vortex formation from an oscillating, non-uniform cylinder

1992 ◽  
Vol 238 ◽  
pp. 31-54 ◽  
Author(s):  
F. Nuzzi ◽  
C. Magness ◽  
D. Rockwell
Keyword(s):  
Flow Structure ◽  
Phase Plane ◽  
Shear Flows ◽  
Vortex Formation ◽  
Excitation Frequency ◽  
Three Dimensional ◽  
Two Dimensional ◽  
Near Wake ◽  
Shedding Frequency

A cylinder having mild variations in diameter along its span is subjected to controlled excitation at frequencies above and below the inherent shedding frequency from the corresponding two-dimensional cylinder. The response of the near wake is characterized in terms of timeline visualization and velocity traces, spectra, and phase plane representations. It is possible to generate several types of vortex formation, depending upon the excitation frequency. Globally locked-in, three-dimensional vortex formation can occur along the entire span of the flow. Regions of locally locked-in and period-doubled vortex formation can exist along different portions of the span provided the excitation frequency is properly tuned. Unlike the classical subharmonic instability in free shear flows, the occurrence of period-doubled vortex formation does not involve vortex coalescence; instead, the flow structure alternates between two different states.

Wake Topology of a Cylinder Undergoing Vortex-Induced Vibrations With Elliptic Trajectories

2016 ◽  
Vol 138 (5) ◽  
Author(s):  
Sina Kheirkhah ◽  
Serhiy Yarusevych ◽  
Sriram Narasimhan
Keyword(s):  
Vortex Shedding ◽  
Vortex Dynamics ◽  
Vortex Formation ◽  
Wake Vortex ◽  
Incoming Flow ◽  
Mass Ratio ◽  
Formation Region ◽  
Vortex Pairs ◽  
Vortex Induced Vibrations ◽  
Spanwise Vortex

Wake vortex shedding topology of a cylinder undergoing vortex-induced vibrations (VIV) is investigated experimentally. Vibration measurements and flow visualization are utilized to study the connection between the cylinder response and the wake topology. The experiments were performed for two different orientations of the elliptic trajectories relative to the incoming flow at a fixed Reynolds number, moment of inertia ratio, mass ratio, and reduced velocity. Similar to the classical 2P regime, two counter-rotating vortex pairs are produced per oscillating cycle for both cases of elliptic trajectories examined here. However, significant changes in wake vortex dynamics are observed along the cylinder span. These changes include merging of vortices, which leads to shedding patterns similar to 2S and P + S modes downstream of the vortex formation region. The observed changes in vortex dynamics are accompanied by splitting of spanwise vortex filament and are attributed primarily to the changes in the local amplitude of vibrations along the span of the pivoted cylinder. It is shown that, being dependent on both the local amplitude of vibrations and vortex dynamics, the observed wake topology cannot be captured by the classical map of shedding regimes developed for VIV of one degree-of-freedom (DOF) cylinders.

Suppression of Vortex Shedding Downstream of a Circular Cylinder Using Permeable Cylinders in Shallow Water

2013 ◽  
Author(s):  
Göktürk Memduh Özkan ◽  
Hüseyin Akıllı
Keyword(s):  
Circular Cylinder ◽  
Flow Control ◽  
Shallow Water ◽  
Flow Structure ◽  
Vortex Shedding ◽  
Outer Cylinder ◽  
Vortex Formation ◽  
Stream Velocity ◽  
Bluff Bodies ◽  
Different Diameters

The characteristics of the flow around a 50mm circular cylinder surrounded by a permeable outer cylinder were investigated by Particle Image Velocimetry (PIV) and flow visualization techniques in order to control the unsteady flow structure downstream of the cylinder in shallow water. The effect of outer permeable cylinder with a porosity of β = 0.4 on the flow control was studied using five different diameters; D = 60, 70, 80, 90, 100mm. Depth-averaged free stream velocity was kept constant as U = 170mm/s corresponding to a Reynolds number of Re = 8500 and the water height was adjusted to hw = 25mm throughout the study. The results clearly showed that the outer permeable cylinder significantly affects the flow structure of the inner cylinder. It was found that by the existence of outer cylinder, the frequency of unsteady vortex shedding is reduced, vortex formation region is elongated and fluctuations are attenuated which are good indications of effective flow control. Owing to the results, optimum parameters were defined and suggested for the suppression of vortex-induced vibrations on bluff bodies.

Vortex Formation in the Wake of a Vibrating, Flexible Cable

1974 ◽  
Vol 96 (4) ◽  
pp. 317-322 ◽  
Author(s):  
S. E. Ramberg ◽  
O. M. Griffin
Keyword(s):  
Vortex Shedding ◽  
Vortex Formation ◽  
Reynolds Numbers ◽  
Hot Wire ◽  
Near Wake ◽  
Formation Region ◽  
Vortex Streets ◽  
Region Length ◽  
Karman Vortex ◽  
Flexible Cables

The von Karman vortex streets formed in the wakes of vibrating, flexible cables were studied using a hot-wire anemometer. All the experiments took place in the flow regime where the vibration and vortex-shedding frequencies lock together, or synchronize, to control the wake formation. Detailed measurements were made of the vortex formation flow for Reynolds numbers between 230 and 650. As in the case of vibrating cylinders, the formation-region length is dependent on a shedding parameter St* related to the natural Strouhal number and the vibrational conditions. Furthermore, the near wake configuration is found to be dependent on the local amplitude of vibration suggesting that the vibrating cylinder rseults are directly applicable in that region.

High Frequency Characteristics of the Near Wake and Vortex Past a Triangular Cylinder

2020 ◽  
Vol 143 (3) ◽  
Author(s):  
Lei Sun ◽  
Yong Huang ◽  
Xiwei Wang ◽  
Xiang Feng ◽  
Wei Xiao
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Vortex Core ◽  
High Frequency ◽  
Vortex Formation ◽  
Flame Stabilization ◽  
Near Wake ◽  
Freestream Velocity ◽  
Formation Region ◽  
Flow Past

Abstract The flow past a triangular cylinder is one of the fundamental flows and widely utilized in flame stabilization and heat transfer. In this study, the near wake and vortex characteristics of the flow past an equilateral triangular cylinder are experimentally measured by a high frequency particle image velocimetry (PIV) system at 3 kHz. The triangular cylinder is installed in a wind tunnel with Reynolds numbers ranging from 10,700 to 17,700. The Reynolds-averaged and phase-averaged methods are utilized to analyze the flow field. Based on the flow fields, the length of the vortex formation region is about 1.5 times of the length of the equilateral triangle side. The residence time of a vortex in the vortex formation region is equal to a vortex shedding period. The stream wise velocity of the vortex core center downstream the vortex formation is about 0.8 times of the freestream velocity, which is slightly larger than the value about 0.7 for the flow past a circular cylinder at the same Reynolds number. The maximum tangential velocity at the periphery of the vortex core maybe occurs slightly in advance of the vortex reaching the boundary of the vortex formation region. The normalized lengths of the recirculation zone of the triangular cylinder keep nearly unchanged and are about 1.55 to 1.9 times of those of the circular cylinder at the same Reynolds number. The normalized normal wise instead of stream wise turbulence intensity has stronger effects on the distribution of the normalized turbulent kinetic energy.

The effect of buoyancy on vortex shedding in the near wake of a circular cylinder

1990 ◽  
Vol 220 ◽  
pp. 253-266 ◽  
Author(s):  
Keun-Shik Chang ◽  
Jong-Youb Sa
Keyword(s):  
Nusselt Number ◽  
Circular Cylinder ◽  
Drag Coefficient ◽  
Transport Equation ◽  
Fourth Order ◽  
Vortex Shedding ◽  
Near Wake ◽  
Periodic Flows ◽  
Vortex Pattern ◽  
Vorticity Transport

The phenomenon of vortex shedding from a heated/cooled circular cylinder has been investigated numerically in the mixed natural and forced convection regimes. Accuracy of the computation was achieved by the fourth-order Hermitian relation applied to the contravariant velocity components in the convection terms of the vorticity transport equation, and by the far-boundary stream-function condition of an integral-series form developed by the authors. Purely periodic flows at Re = 100, efficiently established through the use of a direct elliptic solver called the SEVP, was found to degenerate into a steady twin-vortex pattern at the critical Grashof number 1500, confirming an earlier experimental observation identified as ‘breakdown of the Kármán vortex street’. Various other buoyancy effects about the heated/cooled cylinder are discussed by means of the flow patterns, the Nusselt number and the drag coefficient curves.

Synchronization of Vortex Shedding and Heat Transfer Enhancement Over a Heated Cylinder Oscillating With Small Amplitude in Streamwise Direction

2000 ◽  
Vol 123 (6) ◽  
pp. 1139-1148 ◽  
Author(s):  
C. Gau ◽  
S. X. Wu ◽  
H. S. Su
Keyword(s):  
Heat Transfer ◽  
Vortex Shedding ◽  
Small Amplitude ◽  
Oscillation Amplitude ◽  
Vortex Formation ◽  
Excitation Frequency ◽  
Local Heat Transfer ◽  
Dominant Mode ◽  
Streamwise Direction ◽  
Heated Cylinder

Experiments are performed to study the flow structure and heat transfer over a heated cylinder oscillating radially with small amplitude in streamwise direction. Both flow visualization using a smoke wire in the upstream and the local heat transfer measurements based on wall temperatures around the cylinder were made. The excitation frequencies of the cylinder are selected at Fe/Fn=0, 0.5, 1, 1.5, 2, 2.5, and 3. The oscillation amplitude selected is less than a threshold value of A/D=0.06 where synchronization of vortex shedding with the cylinder excitation was not expected. However, experiments indicate that synchronization still occurs which stimulates a great interest to study its enhancement in the heat transfer. Synchronization occurred at Fe/Fn=2 is antisymmetric vortex formation while synchronization at Fe/Fn=2.5 and 3 is symmetric type. The forward motion (advancing into the cross flow) of the cylinder during one cycle of oscillation has an effect to suppress the instability and the vortex formation. This leads to the occurrence of a smaller and symmetric vortex formation and a less enhancement of heat transfer than the case of antisymmetric type Fe/Fn=2. For excitations at lower frequencies Fe/Fn⩽1.5, all the vortex formations occurred are mostly antisymmetric. The dominant mode of the instability in the shear layer is actually the natural shedding frequency Fn of the vortex. A closer excitation frequency to 2Fn causes a greater enhancement in the heat transfer. During the experiments, the Reynolds numbers varies from 1600 to 3200, the dimensionless amplitude A/D from 0.048 to 0.016.

Near wake field of circular cylinder with a forward splitter plate

2018 ◽  
Vol 173 ◽  
pp. 28-38 ◽  
Author(s):  
Kiran Chutkey ◽  
P. Suriyanarayanan ◽  
L. Venkatakrishnan
Keyword(s):  
Circular Cylinder ◽  
Splitter Plate ◽  
Near Wake ◽  
Wake Field
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