Three-Dimensional Measurement of Near-Wall Velocity in Millimeter Channel by a Single View Imaging

2015 ◽  
Author(s):  
Yoshiyasu Ichikawa ◽  
Kojiro Nishiwake ◽  
Hiromu Wakayama ◽  
Yuki Kameya ◽  
Makoto Yamamoto ◽  
...  
Keyword(s):  
Velocity Field ◽  
Velocity Distribution ◽  
Three Dimensional ◽  
Wall Region ◽  
Measured Velocity ◽  
Wall Velocity ◽  
Wall Area ◽  
Micro Piv ◽  
Wall Flow ◽  
Near Wall

It is well known that there is a strong correlation between heat transfer and near-wall flow. It is important to obtain the detailed near-wall flow field, but it has a lot of difficulties to measure near-wall region by traditional approaches for example hot wire anemometry and particle image velocimetry (PIV). The purpose of this study is to determine the three-dimensional velocity field at near-wall area in micron resolution by the astigmatism particle tracking velocimetry (APTV). In this study, an estimation of depth location of tracer particles by applying a specialized imaging optics controlling the astigmatism [1] was employed. We have developed a measurement system to get the particle location within 15 μm from wall using a long-working-distance microscope with astigmatic optics. As a proof-of-concept, near-wall velocity field in a millimeter-ordered parallel plate channel was measured with low Reynolds numbers (Re = 1 ∼ 5) Poiseuille flow to confirm the validity of it. As a result, we can obtain the near-wall velocity within 15 μm from the wall precisely. From the velocity distribution, the standard deviation of the velocity at each location was calculated and the dispersion of velocity was evaluated. As a result, it was confirmed that the measurement was carried out more accurately in high-speed area. Comparison of the measured velocity distribution with a theoretical calculation and micro-PIV results were also done. From these velocity distributions, the wall shear stress on the wall was determined.

Accurate PIV Measurement on Slip Boundary using Single-Pixel Algorithm

2021 ◽  
Author(s):  
Hongyuan Li ◽  
Yufan Cao ◽  
Xiangyu Wang ◽  
Xia Wan ◽  
Yaolei Xiang ◽  
...  
Keyword(s):  
Boundary Condition ◽  
Slip Boundary Condition ◽  
Complex Flow ◽  
Wall Velocity ◽  
Slip Boundary ◽  
Micro Piv ◽  
Piv Measurement ◽  
Wall Flow ◽  
Single Pixel ◽  
Near Wall

Abstract To accurately measure the near-wall flow by particle image velocimetry (PIV) is a big challenge, especially for the slip boundary condition. Apart from high-precision measurements, an appropriate PIV algorithm is important to resolve the near-wall velocity profile. In our study, single-pixel algorithm is employed to calculate the near-wall flow, which is demonstrated to be capable of accurately resolving the flow velocity near the slip boundary condition. Based on the synthetic particle images, the advantages of the single-pixel algorithm are manifested in comparison with the conventional window correlation algorithm. Specially, the single-pixel algorithm has higher spatial resolution and accuracy, and lower systematic error and random error for the case of slip boundary condition. Furthermore, for experimental verification, micro-PIV measurements are conducted over a liquid-gas interface and the single-pixel algorithm is successfully applied to the calculation of near-wall velocity under the slip boundary condition, especially the negative slip velocity. The current work demonstrates the advantage of the single-pixel algorithm in analyzing the complex flow under the slip boundary condition, such as drag reduction, wall skin friction evaluation and near-wall vortex structure measurement.

A Comparison Between Experimental and Predicted Flow Profiles Beneath a Semi-Confined Axisymmetric Impinging Jet

2003 ◽  
Author(s):  
C. Y. Cheong ◽  
P. T. Ireland ◽  
S. Ashforth-Frost
Keyword(s):  
Viscous Flow ◽  
Flow Model ◽  
Three Dimensional ◽  
Impinging Jet ◽  
Theoretical Models ◽  
Velocity Profiles ◽  
Axisymmetric Case ◽  
Air Jet ◽  
Wall Flow ◽  
Near Wall

Theoretical predictions have been compared with experiment for a single semi-confined impinging jet. The turbulent air jet discharged at Re = 20 000 and impinged at nozzle-to-plate spacings (z/d) of 2 and 6.5. Experimental velocity profiles were obtained using hot-wire anemometry. Theoretical velocity profiles were derived using stagnation three-dimensional flow model and viscous flow model for an axisymmetric case. For z/d = 2, velocity profiles in the inviscid region of the near wall flow can be predicted accurately using the stagnation flow model. As the edge of the jet is approached, the flow becomes complex and, as expected, cannot be predicted using the model. Prediction of boundary layer profiles using the viscous flow solution for an axisymmetric case is also reasonable. For z/d = 6.5, the developing impinging jet is essentially turbulent on impact and consequently predictions of near wall flow field, using both the theoretical models, are inappropriate.

Instability in a viscous flow driven by streamwise vortices

2001 ◽  
Vol 432 ◽  
pp. 127-166 ◽  
Author(s):  
K. W. BRINCKMAN ◽  
J. D. A. WALKER
Keyword(s):  
Boundary Layer ◽  
Numerical Solutions ◽  
Three Dimensional ◽  
Wall Layer ◽  
External Flow ◽  
Three Dimensional Flow ◽  
Dimensional Flow ◽  
Wall Flow ◽  
Turbulent Wall ◽  
Near Wall

Unsteady separation processes at large finite, Reynolds number, Re, are considered, as well as the possible relation to existing descriptions of boundary-layer separation in the limit Re → ∞. The model problem is a fundamental vortex-driven three-dimensional flow, believed to be relevant to bursting near the wall in a turbulent boundary layer. Bursting is known to be associated with streamwise vortex motion, but the vortex/wall interactions that drive the near-wall flow toward breakdown have not yet been fully identified. Here, a simulation of symmetric counter-rotating vortices is used to assess the influence of sustained pumping action on the development of a viscous wall layer. The calculated solutions describe a three-dimensional flow at finite Re that is independent of the streamwise coordinate and consists of a crossflow plane motion, with a developing streamwise flow. The unsteady problem is constructed to mimic a typical cycle in turbulent wall layers and numerical solutions are obtained over a range of Re. Recirculating eddies develop rapidly in the near-wall flow, but these eddies are eventually bisected by alleyways which open up from the external flow region to the wall. At sufficiently high Re, an oscillation was found to develop in the streamwise vorticity field near the alleyways with a concurrent evolution of a local spiky behaviour in the wall shear. Above a critical value of Re, the oscillation grows rapidly in amplitude and eventually penetrates the external flow field, suggesting the onset of an unstable wall-layer breakdown. Local zones of severely retarded streamwise velocity are computed which are reminiscent of the low-speed streaks commonly observed in turbulent boundary layers. A number of other features also bear a resemblance to observed coherent structure in the turbulent wall layer.

On the 3D Structure of Elasticity-Induced Unstable Flow in the Curved Microchannel by Using Confocal Micro-PIV and Polarized Camera

2015 ◽  
Author(s):  
Xiao-Bin Li ◽  
Masamichi Oishi ◽  
Marie Oshima ◽  
Feng-Chen Li ◽  
Song-Jing Li
Keyword(s):  
Flow Rate ◽  
High Speed ◽  
3D Structure ◽  
Three Dimensional ◽  
Working Fluid ◽  
Wall Region ◽  
Unstable Flow ◽  
Solution Flow ◽  
Micro Particle Image Velocimetry ◽  
Micro Piv

In this paper, the three-dimensional (3D) structures of a micellar solution flow in the curvilinear microchannel have been investigated by means of confocal micro particle image velocimetry (PIV). The working fluid is aqueous solution of CTAC/NaSal (cetyltrimethylammonium / Sodium Salysilate). As the flow rate increases, the flow gradually gets into the irregular motion. It is found that the inside flow seems not completely chaotic, but in a manner of oscillation. To be specific, the flow nonlinearity grows as the flow rate increases, the inside flow shows different structures near the wall region and in the bulk due to the elongation of viscoelastic surfactant. Typically, two sub-streams were twisted together, and their flow directions change at the locations where the signs of geometric curvature change. The oscillation stripes represented the area of high extensional stress in the viscoelastic fluid, and were further identified by using polarized high-speed camera. Moreover, statistics shows that the viscoelastic flow field inside the curved microchannel shares the main features of elastic turbulence.

scholarly journals Asymmetrical Velocity Distribution in the Drag-Reducing Channel Flow of Surfactant Solution Caused by an Injected Ultrathin Water Layer

Symmetry ◽  
2020 ◽  
Vol 12 (5) ◽  
pp. 846
Author(s):  
Zaiguo Fu ◽  
Xiaotian Liang ◽  
Kang Zhang
Keyword(s):  
Velocity Distribution ◽  
Channel Flow ◽  
Reynolds Shear Stress ◽  
Surfactant Solution ◽  
Water Layer ◽  
Statistical Characteristics ◽  
Wall Region ◽  
Experimental Conditions ◽  
Mean Velocity ◽  
Near Wall

Although the turbulent intensity is suppressed in the drag-reducing channel flow by viscoelastic additives, the mean velocity distribution in the channel flow is symmetrical and tends to be similar to the laminar flow. In the study of near-wall modulation of the drag-reducing flow with an injected ultrathin water layer, an asymmetrical mean velocity distribution was found. To further investigate this phenomenon and the underlying cause, an experiment was carried out with the water injected from a porous channel wall at a small velocity (~10−4 m/s) into the drag-reducing flow of surfactant solution. The instantaneous concentration and flow fields were measured by using planar laser-induced fluorescence (PLIF) and particle imaging velocimetry (PIV) techniques, respectively. Moreover, analyses on turbulent statistical characteristics and spatial distribution of viscoelastic structures were carried out on the basis of comparison among various flow cases. The results showed that the injected ultrathin water layer under present experimental conditions affected the anisotropy of the drag-reducing flow. The characteristics, such as turbulence intensity, showed the zonal feature in the wall-normal direction. The Reynolds shear stress was enhanced in the near-wall region, and the viscoelastic structure was modified severely due to the redistributed stress. These results may provide experimental supports for the near-wall modulation of turbulence and the exploration of the drag-reducing mechanism by viscoelastic additives.

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