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.

Heat Transfer Enhancement and Vortex Flow Structure Over a Heated Cylinder Oscillating in the Crossflow Direction

1999 ◽  
Vol 121 (4) ◽  
pp. 789-795 ◽  
Author(s):  
C. Gau ◽  
J. M. Wu ◽  
C. Y. Liang
Keyword(s):  
Heat Transfer ◽  
Stagnation Point ◽  
Flow Structure ◽  
Oscillation Amplitude ◽  
Local Heat Transfer ◽  
Flow Region ◽  
Local Heat ◽  
Dominant Mode ◽  
Enhancement Of Heat Transfer ◽  
Heated Cylinder

Experiments are preformed to study the flow structure and heat transfer over a heated oscillating cylinder. Both flow visualization using a smoke wire and local heat transfer measurements 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. These include excitations at harmonic, subharmonic, superharmonic, and nonharmonic frequencies. Synchronization of vortex shedding with the cylinder excitation occurs not only at Fe/Fn = 1 but also at Fe/Fn = 3, which can greatly enhance the heat transfer. The simultaneous enhancement of heat transfer at the stagnation point, its downstream region, and the wake region of the flow suggests that different modes of instabilities occurring in the shear layer of the near wake are actually initiated and amplified far upstream in the stagnation point, which were suppressed in the accelerated flow region and re-amplified in the decelerated flow region. As long as the dominant mode of the instability is amplified by the excitation of cylinder, enhancement of heat transfer can be obtained. During the experiments, the Reynolds numbers vary from 1600 to 4800, the ratios of oscillation amplitude to diameter of the cylinder from 0.064 to 0.016.

Mechanisms of Enhanced Heat Transfer for Oscillating Circular Cylinders

Volume 1 ◽  
2004 ◽  
Author(s):  
Tait Pottebaum ◽  
Mory Gharib
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Vortex Formation ◽  
Cross Flow ◽  
Circular Cylinders ◽  
Transfer Enhancement ◽  
Heated Cylinder ◽  
Wide Range ◽  
Temperature And Velocity Fields ◽  
Oscillating Circular Cylinders

Experiments were conducted to determine the relationship between wake structure and heat transfer for an oscillating circular cylinder in cross-flow. An internally heated cylinder was suspended in a water tunnel and oscillated transverse to the freestream. The cylinder’s heat transfer coefficient was measured over a wide range of oscillation amplitudes and frequencies. By comparing these results to the known wake mode regions in the amplitude-frequency plane, relationships between wake mode and heat transfer were identified. Representative cases were investigated further by using digital particle image thermometry/velocimetry (DPIT/V) to simultaneously measure the temperature and velocity fields in the near-wake. This revealed more detail about the mechanisms of heat transfer enhancement. The dynamics of the vortex formation process, including the trajectories of the vortices during roll-up, are the primary cause of the heat transfer enhancement.

Lock-On Vortex Shedding Patterns and Bifurcation Analysis of the Forced Streamwise Oscillation of the Cylinder Wake

2015 ◽  
Vol 25 (09) ◽  
pp. 1530022
Author(s):  
N. Nabatian ◽  
N. W. Mureithi
Keyword(s):  
Frequency Ratio ◽  
Transverse Velocity ◽  
Lift Coefficient ◽  
Oscillation Amplitude ◽  
Vortex Formation ◽  
Excitation Frequency ◽  
Immersed Boundary ◽  
Cylinder Wake ◽  
Single Vortex ◽  
Frequency Ratios

The two-dimensional numerical simulation of the flow over a cylinder forced to oscillate in the streamwise direction for Re = 200 is performed in CFX ANSYS. The controlled-vibration comprises of prescribed inline vibration from displacement amplitude-to-cylinder diameter A/D = 0.05 up to 0.5 with the excitation frequency ratios of 1, 1.5 and 2 including the harmonic and superharmonic excitation regions. The immersed boundary method is used to model the cylinder oscillation. Modal decomposition of the transverse velocity field via the proper orthogonal decomposition (POD) method is applied to uncover the interaction of symmetric and antisymmetric modes of the near wake. A model using the first two POD modes is developed based on symmetry group equivariance. The model predicts the mode interactions and bifurcated solution branches for all cases, and is shown to be in good agreement with numerical as well as previous experimental results. Lock-on is determined for a range of values of the oscillation amplitudes and frequency ratios. It is shown that the lock-on range widens with increasing nondimensional oscillation amplitude. The asymmetric 2S, P + S and symmetric pattern S with symbol S for a single vortex and P for a vortex pair shed per cycle, as well as a regime in which vortex formation is not synchronized with cylinder motion are observed in the cylinder wake depending on the combination of oscillation amplitude and frequency ratio. The frequency ratio variation from 1 to 2 leads to the switching from asymmetric to symmetric modes. The symmetric flow pattern corresponds to a near zero lift coefficient on the cylinder.

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.

Vortex shedding and heat transfer in rotationally oscillating cylinders

2014 ◽  
Vol 748 ◽  
pp. 549-579 ◽  
Author(s):  
Prabu Sellappan ◽  
Tait Pottebaum
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Heat Transfer Rate ◽  
Convective Heat ◽  
Parameter Space ◽  
Vortex Shedding ◽  
Transfer Rate ◽  
Oscillation Amplitude ◽  
Tangential Velocity ◽  
Wake Modes

AbstractWake formation and heat transfer from a rotationally oscillating circular cylinder in cross-flow at $\mathit{Re}= 750$ are studied. Two aspects, the effect of cylinder forcing on vortex shedding and the effect of the wake structures on convective heat transfer, are studied. Cylinder forcing conditions range between $0.09 \leq \theta _{PP} \leq 2.09$, where $\theta _{PP}$ is the peak-to-peak oscillation amplitude in radians and $0.70 \leq F_{R} \leq 3.16$, where $F_{R}$ is the ratio of forcing frequency to natural shedding frequency. Digital particle image velocimetry (DPIV) is used to obtain quantitative wake structure information. Wake modes, and regions of the parameter space in which they occur, are identified for both heated and unheated cylinders. For the heated cylinder, cylinder forcing is found to affect the convective heat-transfer rate. Certain wake modes, including newly discovered wake modes synchronized over multiple oscillation cycles, are found to correlate with significant heat-transfer enhancement. Cylinder tangential velocity is also found to affect the heat-transfer rate in certain regions of the parameter space.

The effect of oscillation on flat plate heat transfer

1971 ◽  
Vol 47 (3) ◽  
pp. 537-546 ◽  
Author(s):  
Hiroshi Ishigaki
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Flat Plate ◽  
Viscous Dissipation ◽  
High Frequency ◽  
Small Amplitude ◽  
Oscillation Amplitude ◽  
Flow Oscillation ◽  
Velocity Amplitude ◽  
Dissipation Effect

The time-mean heat transfer of the incompressible laminar boundary layer on a flat plate under the influence of oscillation is studied analytically. Flow oscillation amplitude outside the boundary layer is assumed constant along the surface and the viscous dissipation effect is considered. First, the small velocity–amplitude case is treated and the approximate formulae are obtained in the extreme cases when the frequency is low and high. Next, the finite velocity–amplitude case is treated under the condition of high frequency and it is found that the formulae obtained for the small amplitude and high frequency case are also valid. These results show that, when the oscillation is of high frequency, the time-mean heat flux to the wall can be several times as large as that without oscillation. This is due wholly to the viscous dissipation effect combined with oscillation.

Heat Transfer on Internal Surfaces of a Duct Subjected to Impingement of a Jet Array with Varying Jet Hole-Size and Spacing

2005 ◽  
Vol 128 (1) ◽  
pp. 158-165 ◽  
Author(s):  
U. Uysal ◽  
P.-W. Li ◽  
M. K. Chyu ◽  
F. J. Cunha
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Local Heat Transfer ◽  
Target Surface ◽  
Hole Size ◽  
Streamwise Direction ◽  
Actual Distribution ◽  
The Heat Transfer Coefficient ◽  
Jet Array

One significant issue concerning the impingement heat transfer with a jet array is related to the so-called “crossflow,” where a local jet performance is influenced by the convection of the confluence from the impingement of the jet∕jets placed upstream. As a result, the heat transfer coefficient may vary along the streamwise direction and creates more or less nonuniform cooling over the component, which is undesirable from both the performance and durability standpoints. Described in this paper is an experimental investigation of the heat transfer coefficient on surfaces impinged by an array of six inline circular jets with their diameters increased monotically along the streamwise direction. The local heat transfer distributions on both the target surface and jet-issuing plate are measured using a transient liquid crystal technique. By varying the jet hole-size in a systematic manner, the actual distribution of jet flow rate and momentum within a jet array may be optimally metered and controlled against crossflow. The effects on the heat transfer coefficient distribution due to variations of jet-to-target distance and interjet spacing are investigated. The varying-diameter results are compared with those from a corresponding array of uniform jet diameter.

Heat Transfer on Internal Surfaces of a Duct Subjected to Impingement of a Jet Array With Varying Jet Hole-Size and Spacing

2005 ◽  
Author(s):  
U. Uysal ◽  
P.-W. Li ◽  
M. K. Chyu ◽  
F. J. Cunha
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Local Heat Transfer ◽  
Target Surface ◽  
Hole Size ◽  
Streamwise Direction ◽  
Actual Distribution ◽  
The Heat Transfer Coefficient ◽  
Jet Array

One significant issue concerning the impingement heat transfer with a jet array is related to the so-called “crossflow”, where a local jet performance is influenced by the convection of the confluence from the impingement of the jet/jets placed upstream. As a result, the heat transfer coefficient may vary along the streamwise direction and creates more or less nonuniform cooling over the component, which is undesirable from both the performance and durability standpoints. Described in this paper is an experimental investigation of the heat transfer coefficient on surfaces impinged by an array of six inline circular jets with their diameters increased monotically along the streamwise direction. The local heat transfer distributions on both the target surface and jet-issuing plate are measured using a transient liquid crystal technique. By varying the jet hole-size in a systematic manner, the actual distribution of jet flow rate and momentum within a jet array may be optimally metered and controlled against crossflow. The effects on the heat transfer coefficient distribution due to variations of jet-to-target distance and inter-jet spacing are investigated. The varying-diameter results are compared with those from a corresponding array of uniform jet diameter.

Vortex Laden Flows in a Micro-Gap for Enhanced Direct Chip Cooling

2016 ◽  
Author(s):  
Amir Gorodetsky ◽  
Herman D. Haustein
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Pressure Drop ◽  
Vortex Shedding ◽  
Excitation Frequency ◽  
Flow Patterns ◽  
Flow Pulsation ◽  
Additional Advantage ◽  
Micro Channels ◽  
Wide Range

Heat dissipation in modern high-power electronics require high performance cooling, which traditional air-based systems cannot provide. Rather, novel systems using liquids, which have inherently better heat transfer characteristics, must be used. Therefore, these have recently been extensively examined. The present study aims to identify the liquid flow patterns which significantly increase heat transfer, examine them through simulation (transient 2D laminar DNS) and experimentally realize the most promising configuration. Any such flow patterns should target a major inhibitor of heat transfer, namely, the development of the thermal boundary layer. From the literature, it was seen that traveling vortices should meet this demand, due to generation of perpendicular unsteady or periodic flows, and consequently significant disruption of the boundary layer. Traditionally, micro-channels have been widely employed for micro-electronics cooling. However, the generation and persistence of the desired vortices over longer distances, as well as a desired lower pressure drop can be obtained in micro-gaps, which have inherently overall lower wall-fluid friction. The desired vortices can be further enhanced by active methods such as inlet flow pulsation. In the present study, based on numerical simulations (grid-independent and validated against an analytical solution) a suitable micro-gap geometrical configuration was chosen, while the flow rate (Re) and excitation frequency (Strouhal number around the well-known resonance, St = 0.3) with low amplitude, were examined over a wide range. Further examination led to the choice of two methods for vortex generation. The first is a use of bluff bodies as flow obstructers in the micro-gap, whereby vortex shedding (von Karman street) occurs already at low Reynolds numbers (Re>50). A preliminary experimental device was constructed with side and top view capabilities, for flow visualization, as well as the possibility of wall temperature measurement by IR thermography. Preliminary simulations and experiments showed that Vortex shedding onset was only mildly affected in the micro-scale (200 micron obstruction in 600 micron channel), while heat transfer was seen to increase three-fold over obstruction-free gap, with only mild pressure drop increase. The second method has additional advantage of imposed perpendicular flow. The model consists of a row of slot-jets in a micro-gap with cross-flow. Recent experimental and numerical studies employing a similar hybrid cooling scheme, showed significant heat flux dissipation (305 W/cm2). Here too, significant increase of the heat transfer was found, with additional increase associated with flow pulsation. In future experimental work, the intention is to include MEMS based actuators for individual control of the jets’ excitation ability and effective slot width.

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