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.

Turbulent Flows Over Rough Forward Facing Steps

2014 ◽  
Author(s):  
Weijie Shao ◽  
Martin Agelin-Chaab
Keyword(s):  
Turbulent Flows ◽  
Large Scale ◽  
Reattachment Length ◽  
Particle Image Velocimetry Technique ◽  
Large Scale Structures ◽  
Image Velocimetry ◽  
The Mean ◽  
Scale Structures ◽  
Scale Turbulence ◽  
Proper Orthogonal

This paper reports an investigation of the effects of rough forward facing steps on turbulent flows. The surfaces of the rough steps were covered with sandpapers. A particle image velocimetry technique was used to conduct measurements at the mid-plane of the test section and at several locations downstream to 68 step heights. A Reynolds number of Reh = 4800 and δ/h = 4.7 were employed, where h is the mean step height and δ is the incoming boundary layer thickness. The results indicate that mean reattachment length decreases with increasing roughness. In addition, the effect of the step roughness decreases with downstream distance. The proper orthogonal decomposition results showed that the step roughness affects even the large scale structures. Furthermore, the reconstructed turbulence quantities suggest that the step roughness suppresses the large scale turbulence.

Experimental Study of Turbulent Cross-Flow Over Two Circular Cylinders Adjacent to Wall and Free Surface Boundaries

2007 ◽  
Author(s):  
S. S. Paul ◽  
M. F. Tachie ◽  
S. J. Ormiston
Keyword(s):  
Free Surface ◽  
Reynolds Shear Stress ◽  
Cross Flow ◽  
Circular Cylinders ◽  
Particle Image Velocimetry Technique ◽  
Mean Velocity ◽  
Image Velocimetry ◽  
Vertical Location ◽  
Approach Velocity ◽  
Velocity Turbulence

A particle image velocimetry technique was used to study fluid flow around two identical circular cylinders arranged side-by-side with a fixed center-to-center distance. The vertical location of the cylinder pair was varied. The cylinders spanned the tunnel test-section and were oriented normal to the streamwise direction. The Reynolds number based on the cylinder diameter and the approach velocity was 2030. The mean velocity, turbulence intensities, Reynolds shear stress, and momentum flux distributions are used to study the influence on the flow of the free surface at the top of the tunnel and the wall at the bottom of the tunnel.

Flow-pattern identification for two staggered circular cylinders in cross-flow

2000 ◽  
Vol 411 ◽  
pp. 263-303 ◽  
Author(s):  
D. SUMNER ◽  
S. J. PRICE ◽  
M. P. PAÏDOUSSIS
Keyword(s):  
Particle Image ◽  
Cross Flow ◽  
Flow Patterns ◽  
Shear Layers ◽  
Circular Cylinders ◽  
Angle Of Incidence ◽  
Vortex Pairing ◽  
Image Velocimetry ◽  
Pitch Ratio ◽  
Flow Pattern Identification

The flow around two circular cylinders of equal diameter, arranged in a staggered configuration, was investigated using flow visualization and particle image velocimetry for centre-to-centre pitch ratio P/D = 1[ratio ]0 to 5.0 and angle of incidence. α = 0° to 90°. Experiments were conducted within the low subcritical Reynolds number regime, from Re = 850 to 1900. Nine flow patterns were identified, and processes of shear layer reattachment, induced separation, vortex pairing and synchronization, and vortex impingement, were observed. New insight was gained into previously published Strouhal number data, by considering the flow patterns involved. The study revealed that vortex shedding frequencies are more properly associated with individual shear layers than with individual cylinders; more specifically, the two shear layers from the downstream cylinder often shed vortices at different frequencies.

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.

scholarly journals Experimental investigations on the sharp leading-edge separation over a flat plate at zero incidence using particle image velocimetry

2020 ◽  
Vol 61 (9) ◽  
Author(s):  
K. Fujiwara ◽  
R. Sriram ◽  
K. Kontis
Keyword(s):  
Reynolds Number ◽  
Large Scale ◽  
Particle Image ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Reattachment Length ◽  
Large Scale Structures ◽  
Image Velocimetry ◽  
Scale Structures ◽  
Sharp Leading Edge

Abstract Leading-edge separated flow field over a sharp flat plate is experimentally investigated in Reynolds numbers ranging from 6.2 × 103 to 4.1 × 104, using particle image velocimetry (PIV) and its statistics. It was observed that the average reattachment length is nearly independent of Reynolds number and the small secondary bubble observed near the leading edge was found to shrink with increasing Reynolds number. The wall-normal profiles of the statistical values of kinematic quantities such as the velocity components and their fluctuations scaled well with average reattachment length lR and freestream velocity U∞. Their magnitudes compare well with previous investigations even though the current triangular shaped sharp leading edge is different from previous flat-faced or semi-circular ones. The shear layer was observed to exhibit 2 different linear growth rates over 2 distinct regions. Instantaneous PIV realizations demonstrate unsteady nature of the separation bubble, whose origins in the upstream portion of the bubble are analysed. Bimodal nature of the probability density function (PDF) of fluctuating streamwise velocity at around x/lR = 0.08–0.15 indicates successive generation and passage of vortices in the region, which subsequently interact and evolve into multiscale turbulent field exhibiting nearly Gaussian PDF. Shedding of vortices with wide range of scales are apparent in most of the instantaneous realizations. Proper Orthogonal Decomposition (POD) of the velocity fluctuation magnitude field revealed that the flow structures of the dominant modes and their relative energies are independent of Reynolds number. In each of the dominant modes (first 3 modes), the length scales corresponding to the large scale structures and their spacing are the same for all Reynolds numbers, suggesting that their Strouhal number (observed to be ~ 0.09–0.2 at Reynolds number of 6.2 × 103) of unsteadiness should also be independent of Reynolds number. A single large structure- comparable in size to lR—was apparent well before reattachment in a few instantaneous realizations, as compared to multiple small-scale structures visible in most realizations; at Reynolds number of 6.2 × 103, realizations with such large-scale structures occurred approximately after every 20–30 realizations, corresponding to non-dimensional frequency of 0.4–0.6, which is identified to be the “regular shedding”. It was possible to reconstruct the large-scale structure during the instances from just the first 3 POD modes, indicating that the Strouhal number of regular shedding too is independent of Reynolds number. Graphic abstract

scholarly journals Dynamics of Large Scale Turbulence in Finite-Sized Wind Farm Canopy Using Proper Orthogonal Decomposition and a Novel Fourier-POD Framework

Energies ◽  
2020 ◽  
Vol 13 (7) ◽  
pp. 1660
Author(s):  
Tanmoy Chatterjee ◽  
Yulia T. Peet
Keyword(s):  
Power Generation ◽  
Proper Orthogonal Decomposition ◽  
Large Scale ◽  
Wind Farm ◽  
Wind Farms ◽  
Orthogonal Decomposition ◽  
Large Scale Structures ◽  
Scale Structures ◽  
Pod Analysis ◽  
Proper Orthogonal

Large scale coherent structures in the atmospheric boundary layer (ABL) are known to contribute to the power generation in wind farms. In order to understand the dynamics of large scale structures, we perform proper orthogonal decomposition (POD) analysis of a finite sized wind turbine array canopy in the current paper. The POD analysis sheds light on the dynamics of large scale coherent modes as well as on the scaling of the eigenspectra in the heterogeneous wind farm. We also propose adapting a novel Fourier-POD (FPOD) modal decomposition which performs POD analysis of spanwise Fourier-transformed velocity. The FPOD methodology helps us in decoupling the length scales in the spanwise and streamwise direction when studying the 3D energetic coherent modes. Additionally, the FPOD eigenspectra also provide deeper insights for understanding the scaling trends of the three-dimensional POD eigenspectra and its convergence, which is inherently tied to turbulent dynamics. Understanding the behaviour of large scale structures in wind farm flows would not only help better assess reduced order models (ROM) for forecasting the flow and power generation but would also play a vital role in improving the decision making abilities in wind farm optimization algorithms in future. Additionally, this study also provides guidance for better understanding of the POD analysis in the turbulence and wind farm community.

Structure Analysis of a Low Reynolds Number Turbulent Submerged Jet Interacting With a Free Surface

2014 ◽  
Vol 136 (10) ◽  
Author(s):  
Qian Wen ◽  
Hyun Dong Kim ◽  
Ying Zheng Liu ◽  
Kyung Chun Kim
Keyword(s):  
Reynolds Number ◽  
Free Surface ◽  
Large Scale ◽  
Reverse Flow ◽  
Decomposition Analysis ◽  
Flow Structures ◽  
Spatial Structures ◽  
Particle Image Velocimetry Technique ◽  
Surface Vibration ◽  
Restructuring Process

In this study, the spatial structures of a submerged turbulent jet interacting with a free surface were investigated experimentally. The jet axis was located at three different depths (H/D = 2, H/D = 4 and H/D = 6) beneath the free surface and the Reynolds number was fixed as 3480. Laser-induced fluorescence technique was used for qualitative visualization and the time-resolved particle image velocimetry technique was used for the quantitative measurements. The dynamics of the flow structures were examined further using the proper orthogonal decomposition analysis technique. The results revealed that the dynamic characteristics of large-scale turbulent motions were significantly different with the submerged depths. In case of H/D = 2, the dominant spatial structures displayed a surface vibration induced reverse flow along the boundary, and its subsequent deflection changed the flow structures in the horizontal center plane. The violent free surface vibration caused an unsteady up-and-down motion of the flow structures and had a “squeeze effect” on the flow structures. In case of H/D = 4, the upwelling motion of some vortices in the jet and their subsequently downward entrainment motion significantly changed the dominant spatial structures both in the vertical and horizontal central planes. When the jet was fully attached to the free surface, the vortical structures underwent a merging and restructuring process due to the vertical confinement of the free surface. In case of H/D = 6, the dominant spatial structures both in the vertical and horizontal central planes showed an approximately symmetric pattern, indicating that the dominant structures were not changed by the free surface. After attached to the free surface, the jet did not undergo a merging and restructuring process as shown in case of H/D = 4.

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