scholarly journals Quasi-Periodic Oscillating Flows in a Channel with a Suddenly Expanded Section

Symmetry ◽  
2019 ◽  
Vol 11 (11) ◽  
pp. 1403 ◽  
Author(s):  
Masuda ◽  
Tagawa
Keyword(s):  
Oscillatory Flow ◽  
Harmonic Component ◽  
Periodic Oscillation ◽  
Reynolds Numbers ◽  
Flat Plates ◽  
Downstream Region ◽  
Harmonic Components ◽  
Pass Through ◽  
High Reynolds ◽  
Volume Method

In this study, two-dimensional numerical simulation was carried out for an oscillatory flow between parallel flat plates having a suddenly expanded section. Governing equations were discretized with the second-order accuracy by a finite volume method on an unequal interval mesh system resolving finer near walls and corners to obtain the characteristics of the oscillatory flow accurately. Amplitude spectrums of a velocity component were obtained to investigate the periodic characteristics of the oscillatory flow. At low Reynolds numbers, the flow is periodic because the spectrum mostly consists of harmonic components, and then at high Reynolds numbers, it transits to a quasi-periodic flow mixed with non-harmonic components. In conjunction with the periodic oscillation of a main flow, separation vortices that are not uniform in size are generated from the corner of a sudden contraction part and pass through a downstream region coming into contact with the wall. The number of the vortices decreases rapidly after they are generated, but the vortices are generated again in the downstream region. In order to specify where aperiodicity is generated, the turbulent kinetic energy is introduced, and it is decomposed into the harmonic and non-harmonic components. The peaks of the non-harmonic component are generated in the region of the expanded section.

The local and global stability of confined planar wakes at intermediate Reynolds number

2011 ◽  
Vol 686 ◽  
pp. 218-238 ◽  
Author(s):  
M. P. Juniper ◽  
O. Tammisola ◽  
F. Lundell
Keyword(s):  
Recirculation Zone ◽  
Numerical Study ◽  
Reynolds Numbers ◽  
Shear Layers ◽  
Flat Plates ◽  
Apparent Contradiction ◽  
Wake Flows ◽  
Flow Parameters ◽  
High Reynolds ◽  
Global And Local

AbstractAt high Reynolds numbers, wake flows become more globally unstable when they are confined within a duct or between two flat plates. At Reynolds numbers around 100, however, global analyses suggest that such flows become more stable when confined, while local analyses suggest that they become more unstable. The aim of this paper is to resolve this apparent contradiction by examining a set of obstacle-free wakes. In this theoretical and numerical study, we combine global and local stability analyses of planar wake flows at $\mathit{Re}= 100$ to determine the effect of confinement. We find that confinement acts in three ways: it modifies the length of the recirculation zone if one exists, it brings the boundary layers closer to the shear layers, and it can make the flow more locally absolutely unstable. Depending on the flow parameters, these effects work with or against each other to destabilize or stabilize the flow. In wake flows at $\mathit{Re}= 100$ with free-slip boundaries, flows are most globally unstable when the outer flows are 50 % wider than the half-width of the inner flow because the first and third effects work together. In wake flows at $\mathit{Re}= 100$ with no-slip boundaries, confinement has little overall effect when the flows are weakly confined because the first two effects work against the third. Confinement has a strong stabilizing effect, however, when the flows are strongly confined because all three effects work together. By combining local and global analyses, we have been able to isolate these three effects and resolve the apparent contradictions in previous work.

Planar Oscillatory Flow Forces at High Reynolds Numbers

1993 ◽  
Vol 115 (1) ◽  
pp. 31-39 ◽  
Author(s):  
J. R. Chaplin
Keyword(s):  
Surface Roughness ◽  
Reynolds Number ◽  
Circular Cylinder ◽  
Oscillatory Flow ◽  
Reynolds Numbers ◽  
Number Range ◽  
High Reynolds Numbers ◽  
Reynolds Number Range ◽  
Fine Surface ◽  
High Reynolds

Measurements of pressures around a circular cylinder with fine surface roughness in planar oscillatory flow reveal considerable changes in drag and inertia coefficients over the Reynolds number range 2.5 × 105 to 7.5 × 105, and at Keulegan-Carpenter numbers between 5 and 25. In most respects, these results are shown to be compatible with previous measurements in planar oscillatory flow, and with previous measurements in which the same 0.5-m-dia cylinder was tested in waves.

scholarly journals Comparison of 2D Grid Simulations for Flow Past Cylinder at High Reynolds Numbers

2019 ◽  
Vol 15 (1) ◽  
pp. 70-78 ◽  
Author(s):  
Lenka Lausová ◽  
Ivan Kološ ◽  
Vladimíra Michalcová
Keyword(s):  
Reynolds Numbers ◽  
Wake Region ◽  
Mean Velocity ◽  
Ansys Fluent ◽  
Flow Past ◽  
Different Types ◽  
Fluent Software ◽  
The Mean ◽  
High Reynolds ◽  
Volume Method

Abstract The paper focuses on the verification of the suitability of the SST k - ω model on the flow past a circular cylinder in 2D for a high Reynolds number. The study compares the results of drag and lifts coefficients with respect to different types of meshes and time steps. The mean velocity field in the wake region behind the cylinder is evaluated and compared to experimental data available from literature. The numerical simulations are solved using CFD codes in the ANSYS Fluent software and use the finite volume method.

scholarly journals Experimental PIV investigation of the PZT fans array coupling effect at high Reynolds numbers

2021 ◽  
Vol 2108 (1) ◽  
pp. 012010
Author(s):  
Yiyang Chen ◽  
Hao Ding ◽  
Ziwen Wang ◽  
Jianxin Li ◽  
Jiahuan Cui
Keyword(s):  
Flow Field ◽  
Heat Dissipation ◽  
Coupling Effect ◽  
Saddle Points ◽  
Periodic Oscillation ◽  
Reynolds Numbers ◽  
Field Pattern ◽  
Vibration Modes ◽  
High Reynolds Numbers ◽  
High Reynolds

Abstract Piezoelectric fan arrays are being increasingly emphasized for heat dissipation in small-sized electronic devices. In this study, PIV experiments were conducted to investigate the flow fields induced by piezoelectric fan arrays with different vibration modes and pitches at high Reynolds numbers (324< Re <509) in a stationary air environment. As a result, when the PZT fan array is vibrating in-phase, the saddle points in the time averaged flow field are formed and separated gradually as the pitch increases, the remnant vortex and the induced vortex interact to form a jet with a periodic oscillation in the direction. Jet velocity reaches a maximum at P = 3A. In counter-phase vibration, saddle points are separated from one region under large pitches, the interaction of counter-rotating induced vortices forms a vertical upward jet. The morphology of induced and remnant vortices with different vibration modes and array pitches are responsible for the jet formation and flow field pattern. The interaction of counter-rotating vortices in counter-phase vibration leads the jet intensity higher than in-phase vibration induced jet, the optimal setting of the PZT fan under this study is determined as P = 2.5A with counter-phase vibration. The experimental results provide validation for the simulation study and give guidance to the application

Flow Characteristics of Freely Rotating Rectangular Cylinders With Different Width to Height Ratios

2011 ◽  
Author(s):  
Y. G. Park ◽  
H. S. Yoon ◽  
M. Y. Ha
Keyword(s):  
Strouhal Number ◽  
Flow Characteristics ◽  
Periodic Oscillation ◽  
Reynolds Numbers ◽  
Immersed Boundary ◽  
Cylinder Surface ◽  
Oscillation Regime ◽  
Height Ratio ◽  
Volume Method ◽  
Rectangular Cylinders

The present study numerically investigates flow past freely rotating rectangular cylinders with different width to height ratios. The immersed boundary method (IBM) to model the rectangular cylinder based on the finite volume method is used to study a two-dimensional (2-D) laminar fluid flow for different Reynolds numbers of 50, 100, and 150 in the range of 0.2 ≤ W/H ≤ 1.0, where W/H is the width to height ratio. There are three different regimes of motion. The first one is the periodic oscillation regime. The second one is rotation with reversal of direction regime. The last one is the rotation with one direction regime. All the cases are periodic oscillation regime except the cases of Re = 100 and 150 with W/H = 1.0. For Re = 100 and 150 with W/H = 1.0, the regimes are the rotation with reversal of direction regime and the rotation with one direction regime, respectively. The Strouhal number decreases with increasing the width to height ratio for Re = 50, 100 and 150. However, for Re = 100 and 150, the Strouhal number disappears at a width to height ratio of 1.0. The present study reports the detailed information of flow structure on the cylinder surface at different width to height ratio.

In-Line and Transverse Forces on Cylinders in Oscillatory Flow at High Reynolds Numbers

1977 ◽  
Vol 21 (04) ◽  
pp. 200-216 ◽  
Author(s):  
Turgut Sarpkaya
Keyword(s):  
Oscillatory Flow ◽  
Least Squares Method ◽  
Reynolds Numbers ◽  
Transverse Force ◽  
Circular Cylinders ◽  
Relative Roughness ◽  
High Reynolds Numbers ◽  
Smooth Cylinder ◽  
New Interpretation ◽  
High Reynolds

This paper presents the results of an extensive experimental investigation of the in-line and transverse forces acting on smooth and sand roughened circular cylinders placed in oscillatory flow at Reynolds numbers up to 1.5 × 106, Keulegan Carpenter numbers up to 100, and relative roughnesses from 1/800 to 1/50. The drag and inertia coefficients have been determined through the use of the Fourier analysis and the least-squares method. The transverse force (lift) has been analyzed in terms of its maximum, semi peak-to-peak, and root-mean-squarevalues. In addition, the frequency of vortex shedding and the Strouhal number have been determined. The results have shown that (a) for smooth cylinders, all of the coefficients just cited are functions of the Reynolds and Keulegan-Carpenter numbers, particularly for Reynolds numbers larger than about 20 000; (b) for rough cylinders, the force coefficients also depend on the relative roughness k/D and differ significantly from corresponding to the smooth cylinder; and that (c) the use of the frequencyparameter' D2/vT and the roughness Reynolds number Umk/vallows a new interpretation of the present as well as the previously obtained data.

Effect of a plane boundary on oscillatory flow around a circular cylinder

1991 ◽  
Vol 225 ◽  
pp. 271-300 ◽  
Author(s):  
B. M. Sumer ◽  
B. L. Jensen ◽  
J. Fredsøe
Keyword(s):  
Circular Cylinder ◽  
Flow Visualization ◽  
Pressure Distribution ◽  
Oscillatory Flow ◽  
Reynolds Numbers ◽  
Diameter Ratio ◽  
Plane Boundary ◽  
Pressure Measurements ◽  
Visualization Experiments ◽  
High Reynolds

This study deals with the flow around a circular cylinder placed near a plane wall and exposed to an oscillatory flow. The study comprises instantaneous pressure distribution measurements around the cylinder at high Reynolds numbers (mostly at Re ∼ 105) and a flow visualization study of vortex motions at relatively smaller Reynolds numbers (Re ∼ 103–104). The range of the gap-to-diameter ratio is from 0 to 2 for the pressure measurements and from 0 to 25 for the flow visualization experiments. The range of the Keulegan–Carpenter number KC is from 4 to 65 for the pressure measurements and from 0 to 60 for the flow visualization tests. The details of vortex motions around the cylinder are identified for specific values of the gap-to-diameter ratio and for the KC regimes known from research on wall-free cylinders. The findings of the flow visualization study are used to interpret the variations in pressure with time around the pipe. The results indicate that the flow pattern and the pressure distribution change significantly because of the close proximity of the boundary where the symmetry in the formation of vortices breaks down, and also the characteristic transverse vortex street observed for wall-free cylinders for 7 < KC < 13 disappears. The results further indicate that the vortex shedding persists for smaller and smaller values of the gap-to-diameter ratio, as KC is decreased. The Strouhal frequency increases with decreasing gap-to-diameter ratio. The increase in the Strouhal frequency with respect to its wall-free-cylinder value can be as much as 50% when the cylinder is placed very close to the wall with a gap-to-diameter ratio of O(0.1).

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