Particle image velocimetry technique measurements of the near wake behind a cylinder-pair of unequal diameters

Gao, Yangyang; Wang, Xikun; Tan, Danielle; Keat, Tan Soon

doi:10.1088/0169-5983/45/4/045504

Cited by 10 publications

(13 citation statements)

References 24 publications

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“…Meanwhile, the vortexes from base flow pass through the spacing and also contribute to the high intensity here. The same intensity distribution can be seen in the near wake flow of a cylinder-pair [39]. This formation mechanism for grid-generated turbulence is distinguished from that reflected by smoke wires [40], which is mostly due to the different Reynold number and base flow regime.…”

Section: Turbulent Flowmentioning

confidence: 61%

PIV Study on Grid-Generated Turbulence in a Free Surface Flow

Yao

Cao

et al. 2021

Water

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To investigate the feature of turbulence developing behind the filter device in a current flow, the flow fields at intermediate downstream distance of an immersed grid in an open water channel are recorded using a two-dimensional (2D) Particle Image Velocimetry (PIV) system. The measurements on a series of vertical and horizontal sections are conducted to reveal the stream-wise evolution and depth diversity of grid turbulence in the free surface flow. Unlike the previous experiments by Laser Doppler Velocimetry (LDV) and Hot-Wire Anemometry (HWA), the integral scales and space-time correlations are estimated without using the Taylor hypothesis in this paper. The distributions of mean velocity, turbulence intensity and integral scale show the transition behavior of grid-generated flow from perturbations to fully merged homogenous turbulence. The distributions of velocity and turbulence intensity become more uniform with increasing distance. While the spatial divergence of integral scale becomes more pronounced as the flow structures develop downstream. The vertical distributions of flow parameters reveal the diversity of flow characteristics in the water depth direction influenced by free surface and the outer part of turbulence boundary layer (TBL) from the channel bottom. The applicability of the newly proposed two-order elliptic approximation model for the space-time correlations of the decaying grid turbulence in channel flow is verified at different positions. The calculated convection velocity for large-scale motion and sweep velocity for small-scale motion based on this model bring a new insight into the dynamic pattern of this type of flow.

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Section: Turbulent Flowmentioning

confidence: 61%

PIV Study on Grid-Generated Turbulence in a Free Surface Flow

Yao

Cao

et al. 2021

Water

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“…The LaVision Davis software (Davis 8.2.3) was used to process the raw particle images and determine the flow vector fields through the fast-Fourier-transform (FFT) method based on cross correlation algorithm with the standard Gaussian sub-pixel fit structured as an iterative multi-grid method. The processing procedure included two passes, starting with the interrogation window consisted of 64×64 pixels (8 mm×8 mm or 0.4D×0.4D) with a 50% overlap, and then stepping down to 32×32 pixels (4 mm×4 mm or 0.2D×0.2D) with a 50% overlap, corresponding to 128×128 velocity vectors (namely, spatial resolution of 2 mm×2 mm) obtained in the viewing area, more details of PIV measurement system and data processing were presented by Gao et al (2013). For each case, a total of 6500 PIV images were captured, yielding about 70 vortex shedding cycles for an isolated cylinder (Strouhal number St=0.2) at low Reynolds number Re=1500, and more than 230 vortex shedding cycles at high Reynolds number Re=5000.…”

Section: Experimental Set-upmentioning

confidence: 99%

Experimental investigation on flow past nine cylinders in a square configuration

Gao

Guo

et al. 2018

Fluid Dyn. Res.

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An experimental investigation on flow past nine cylinders in a square configuration was carried out using the particle image velocimetry technique and load cell in a water channel. The center-to-center spacing ratio L/D was in the range of 1.5-3.0 and the Reynolds number Re was varied from 1500 to 5000. The effects of spacing ratio and Reynolds number on the instantaneous timeaveraged flow fields and force coefficients are investigated. The results show that three distinct flow regimes are categorized with variation of the spacing ratios and Reynolds numbers, namely, shielding flow regime, transition flow regime and vortex shedding flow regime. Depending on the interferences of shear layers around the nine cylinders, each flow regime is further divided into two types of flow patterns. An interesting feature of bistable flow pattern with different flow modes is observed at small spacing ratio L/D=1.5. The nondimensional vortex shedding frequencies appear to be more associated with the individual shear layers rather than the multiple cylinders. Moreover, force analysis, streamline topologies and Reynolds stress contours are presented to elucidate the effects of spacing ratio and Reynolds number on the complex wake interference among the nine cylinders. The flow characteristics and force coefficients are found to be more sensitive to L/D rather than Re.

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“…It was also found that the vortical wake of the upstream cylinder causes a restoring force on the downstream cylinder, which is equivalent to a linear spring, hence affecting its natural frequency [3]. In the few studies conducted with interfering cylinders of unequal diameter [115,82,40,41,39], it was found that the upstream cylinder also affects the response of the downstream cylinder, reducing its amplitude relative to the single cylinder case, while the frequency of oscillation of the downstream cylinder was found to be either equal to, or twice the value of the frequency of the upstream cylinder. Only one published study [91] has begun to show the harbor seal whiskers' response under WIV conditions.…”

Section: Motivationmentioning

confidence: 99%

Passive wake detection using seal whisker-inspired sensing

Beem¹

2015

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This thesis is motivated by a series of biological experiments that display the harbor seal's extraordinary ability to track the wake of an object several seconds after it has swum by. They do so despite having auditory and visual cues blocked, pointing to use of their whiskers as sensors of minute water movements. In this work, I elucidate the basic fluid mechanisms that seals may employ to accomplish this detection. Key are the unique flow-induced vibration properties resulting from the geometry of the harbor seal whisker, which is undulatory and elliptical in cross-section.First, the vortex-induced vibration (VIV) characteristics of the whisker geometry are tested. Direct force measurements and flow visualizations on a rigid whisker model undergoing a range of 1-D imposed oscillations show that the geometry passively reduces VIV (factor of > 10), despite contributions from effective added mass and damping. Next, a biomimetic whisker sensor is designed and fabricated. The rigid whisker model is mounted on a four-armed flexure, allowing it to freely vibrate in both in-line and crossflow directions. Strain gauges on the flexure measure deflections at the base. Finally, this device is tested in a simplified version of the fish wake -seal whisker interaction scenario. The whisker is towed behind an upstream cylinder with larger diameter. Whereas in open water the whisker exhibits very low vibration when its long axis is aligned with the incoming flow, once it enters the wake it oscillates with large amplitude and its frequency coincides with the Strouhal frequency of the upstream cylinder. This makes the detection of an upstream wake as well as an estimation of the size of the wake-generating body possible. A slaloming motion among the wake vortices causes the whisker to oscillate in this manner. The same mechanism has been previously observed in energy-extracting foils and trout actively swimming behind bluff cylinders in a stream. Dave, Jason, Gabe, Pablo, Remi, and Yahya were postdocs when we first met.Thank you for being approachable and sharing much needed advice all along the way.Your love of science has inspired me to be more inquisitive.Brandon, Chris, Matthew, and Patrick were undergrads or high school students who spent their summers working in the Tow Tank with me. Thanks for giving me the chance to practice being a mentor, moving this research forward a lot faster than I could do on my own, and helping me lift the mega-whisker in and out of the tank! May you all keep on building and exploring.Thanks to Kathy Streeter and the New England Aquarium staff for helping me understand more about real harbor seals. They graciously collected shed whisker specimens and allowed me to take many close-up pictures of their seals.Thanks to all the people who helped me prepare for and make it through research cruises: Eric Hayden, Terry Hammar, and Hanu Singh for contributing their expertise 5 to help me make my first ocean-ready sensor, all parties involved in the first UNOLS Chief Scientist Training Cruise se...

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Particle image velocimetry technique measurements of the near wake behind a cylinder-pair of unequal diameters

Cited by 10 publications

References 24 publications

PIV Study on Grid-Generated Turbulence in a Free Surface Flow

PIV Study on Grid-Generated Turbulence in a Free Surface Flow

Experimental investigation on flow past nine cylinders in a square configuration

Passive wake detection using seal whisker-inspired sensing

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