Mode C flow transition behind a circular cylinder with a near-wake wire disturbance

2013 ◽  
Vol 727 ◽  
pp. 30-55 ◽  
Author(s):  
I. Yildirim ◽  
C. C. M. Rindt ◽  
A. A. van Steenhoven
Keyword(s):  
Circular Cylinder ◽  
Particle Image ◽  
Three Dimensional ◽  
Period Doubling ◽  
Feedback Mechanism ◽  
Near Wake ◽  
Number Range ◽  
Image Velocimetry ◽  
The Wire ◽  
Secondary Vortices

AbstractThe three-dimensional transition of the flow behind a circular cylinder with a near-wake wire disturbance has been investigated experimentally. The asymmetric placement of a wire in the near-wake region of the cylinder causes an unnatural mode of shedding to occur, namely mode C. We performed flow visualization and particle image velocimetry (PIV) experiments to investigate the influence of the wire on various properties of the flow, such as the dynamics of the streamwise secondary vortices. Experiments were performed at the Reynolds number range of Re = 165–300. From these experiments, it can be concluded that mode C structures are formed as secondary streamwise vortices around the primary von Kármán vortices. The spanwise wavelength of those mode C structures is determined to be approximately two cylinder diameters. The presence of the wire also triggered the occurrence of period doubling in the wake. Each new set of mode C structures is out of phase with the previous set, i.e. doubling the shedding period. This period-doubling phenomenon is due to a feedback mechanism between the consecutively shed upper vortices.

The unsteady three-dimensional wake produced by a trapezoidal pitching panel

2011 ◽  
Vol 685 ◽  
pp. 117-145 ◽  
Author(s):  
Melissa A. Green ◽  
Clarence W. Rowley ◽  
Alexander J. Smits
Keyword(s):  
Particle Image ◽  
Three Dimensional ◽  
Qualitative Change ◽  
Strong Interactions ◽  
Stream Velocity ◽  
Near Wake ◽  
Three Dimensional Flow ◽  
Trailing Edge ◽  
Image Velocimetry ◽  
Formation And Evolution

AbstractParticle image velocimetry (PIV) is used to investigate the three-dimensional wakes of rigid pitching panels with a trapezoidal geometry, chosen to model idealized fish caudal fins. Experiments are performed for Strouhal numbers from 0.17 to 0.56 for two different trailing edge pitching amplitudes. A Lagrangian coherent structure (LCS) analysis is employed to investigate the formation and evolution of the panel wake. A classic reverse von Kármán vortex street pattern is observed along the mid-span of the near wake, but the vortices realign and exhibit strong interactions near the spanwise edges of the wake. At higher Strouhal numbers, the complexity of the wake increases downstream of the trailing edge as the spanwise vortices spread transversely and lose coherence as the wake splits. This wake transition is shown to correspond to a qualitative change in the LCS pattern surrounding each vortex core, and can be identified as a quantitative event that is not dependent on arbitrary threshold levels. The location of this transition is observed to depend on both the pitching amplitude and free stream velocity, but is not constant for a fixed Strouhal number. On the panel surface, the trapezoidal planform geometry is observed to create additional vortices along the swept edges that retain coherence for low Strouhal numbers or high sweep angles. These additional swept-edge structures are conjectured to add to the complex three-dimensional flow near the tips of the panel.

Experimental Study of the Spanwise Wake Compression of a Trapezoidal Pitching Panel

2015 ◽  
Author(s):  
Justin T. King ◽  
Melissa A. Green
Keyword(s):  
Strouhal Number ◽  
Particle Image ◽  
Three Dimensional ◽  
Stereoscopic Particle Image Velocimetry ◽  
Current Work ◽  
Wake Structure ◽  
Number Range ◽  
Screw Propeller ◽  
Image Velocimetry ◽  
Engineering Designs

Aquatic animals can maneuver and propel themselves through a variety of means. Among these means, are the oscillation and undulation of the flukes and fins of different cetaceans and fishes. The motions of these species can be employed to develop thrust-producing, highly three-dimensional wakes. Recently, a great deal of interest in incorporating certain biological propulsion schemes into engineering designs has been generated. Experiments have shown that bio-inspired propulsors can develop large efficiencies, with some efficiencies being greater than those of a screw-propeller propulsion system. In the current work, stereoscopic particle image velocimetry (PIV) was used to characterize the wake produced by a rigid, trapezoidal pitching panel. Prior work has shown that one of the dominant parameters governing wake structure is the Strouhal number. Detailed analysis in terms of Strouhal number is the focus of the current work, and the Strouhal number range tested was from 0.17 to 0.56.

The Blockage Effects for the Unsteady Characteristics of the Near Wake Flows Behind a Bluff Body

2003 ◽  
Author(s):  
Hiroshi Nakayama ◽  
Masafumi Hirota ◽  
Hiroshi Yamashita ◽  
Yoshio Fukui ◽  
Hideomi Fujita
Keyword(s):  
Bluff Body ◽  
Particle Image ◽  
Flow Characteristics ◽  
Measured Data ◽  
Near Wake ◽  
Number Range ◽  
Wake Flows ◽  
Image Velocimetry ◽  
Unsteady Characteristics ◽  
Proper Orthogonal

Unsteady flow characteristics behind a triangular-shaped bluff body were experimentally investigated in a water flow of the Reynolds number range ReD = 500∼3000. The effect of changing the blockage ratio, BR, from BR = 0.2 to 0.6, was investigated. The experimental data of the velocity were obtained by using cinematic particle image velocimetry (PIV) to map the unsteady velocity field around the vortex-generating region. Based on the measured data, influences of the blockage on time-averaged velocities, turbulence intensity distributions and Strouhal number were examined. Moreover, proper orthogonal decomposition (POD) was performed on fluctuating vorticity fields to identify the most energetic parts of the vorticity fluctuations.

The Flow Around an Impulsively Started Square Prism

2010 ◽  
Author(s):  
Nelson Tonui ◽  
David Sumner
Keyword(s):  
Circular Cylinder ◽  
Recirculation Zone ◽  
Particle Image ◽  
Reynolds Numbers ◽  
Temporal Development ◽  
Near Wake ◽  
Towing Tank ◽  
Streamwise Location ◽  
Square Prism ◽  
Image Velocimetry

The flow around a square prism impulsively set into motion was studied experimentally using particle image velocimetry (PIV). The experiments were conducted in an X-Y towing tank for Reynolds numbers from Re = 200 to 1000 and dimensionless acceleration parameters from a* = 0.5 to 10. The temporal development of the near-wake recirculation zone, and its pair of primary eddies, was examined from the initial start until the wake became asymmetric. When considering the time elapsed from the start of motion, the temporal development of the wake was sensitive the initial acceleration. “Impulsively started” conditions were effectively attained for a* ≥ 3. However, when considering the distance traveled from the start of motion, the wake parameters were sensibly independent of a* for a* ≥ 0.5. Concerning the temporal development of the recirculation zone, the length of the recirculation zone, the streamwise location of the primary eddies, and the strength of the primary eddies increased with time following the impulsive start, while the cross-stream spacing of the eddy centres remained nearly constant. The recirculation zone of the square prism was longer than that of the impulsively started circular cylinder but shorter than an impulsively started flat plate. For t* > 2, the primary eddy strength, maximum vorticity, and cross-stream spacing of the primary eddies were the same for both the square prism and circular cylinder.

scholarly journals Experimental Validation of Falling Liquid Film Models: Velocity Assumption and Velocity Field Comparison

Polymers ◽  
2021 ◽  
Vol 13 (8) ◽  
pp. 1205
Author(s):  
Ruiqi Wang ◽  
Riqiang Duan ◽  
Haijun Jia
Keyword(s):  
Experimental Data ◽  
Particle Image Velocimetry ◽  
Velocity Field ◽  
Experimental Validation ◽  
Particle Image ◽  
Velocity Fields ◽  
Number Range ◽  
Comparison Results ◽  
Image Velocimetry ◽  
Experimental Particle

This publication focuses on the experimental validation of film models by comparing constructed and experimental velocity fields based on model and elementary experimental data. The film experiment covers Kapitza numbers Ka = 278.8 and Ka = 4538.6, a Reynolds number range of 1.6–52, and disturbance frequencies of 0, 2, 5, and 7 Hz. Compared to previous publications, the applied methodology has boundary identification procedures that are more refined and provide additional adaptive particle image velocimetry (PIV) method access to synthetic particle images. The experimental method was validated with a comparison with experimental particle image velocimetry and planar laser induced fluorescence (PIV/PLIF) results, Nusselt’s theoretical prediction, and experimental particle tracking velocimetry (PTV) results of flat steady cases, and a good continuity equation reproduction of transient cases proves the method’s fidelity. The velocity fields are reconstructed based on different film flow model velocity profile assumptions such as experimental film thickness, flow rates, and their derivatives, providing a validation method of film model by comparison between reconstructed velocity experimental data and experimental velocity data. The comparison results show that the first-order weighted residual model (WRM) and regularized model (RM) are very similar, although they may fail to predict the velocity field in rapidly changing zones such as the front of the main hump and the first capillary wave troughs.

A Method for Measuring Deformation Rates in Hydrogel Structures: Multi-Plane Imaging and Measurement Limitations

2000 ◽  
Author(s):  
Joseph M. Bauer ◽  
David J. Beebe
Keyword(s):  
Cross Sections ◽  
Particle Image ◽  
Three Dimensional ◽  
Local Deformation ◽  
Dimensional Representation ◽  
Volume Changes ◽  
Mechanical Constraints ◽  
Seed Particles ◽  
Three Dimensional Representation ◽  
Image Velocimetry

Abstract A technique for determining the three dimensional motions of hydrogel structures in microchannels is introduced. In developing this technique, we have adapted microscopic particle image velocimetry (μPIV), a method for measuring velocity fields in microfluidic devices. The motions of 1 μm fluorescent seed particles that are incorporated into a hydrogel microstructure (200 μm tall × 400 μm diameter) are tracked over several expansion cycles using microscopy. Combining measurements taken in different planes produces a three-dimensional representation of the motions present during volume changes can be reconstructed. By providing cross sections of the local deformation rates in hydrogel microstructures, this technique allows for the optimization of device designs as well as providing a better understanding of the processes by which hydrogels change volume under mechanical constraints.

Sign in / Sign up

Export Citation Format

Share Document