粗糙壁面边界层的亚微米分辨长工作距离显微PIV测量

2017/02/13   下载量： 5 次

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应用领域 其他
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采用ImagerProX4M相机。长工作距离显微镜。测量一个粗糙的砂轮圆盘在液体池中旋转时所产生的流场。

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The prediction of the mean flow and turbulence structure of a boundary layer on surfaces with different type and
shape of roughness is still one of the major questions in fluid mechanics. The specific character of wall roughness in
terms of shape, size and distribution strongly varies from case to case and an equivalent roughness concept was
introduced in order to enable engineers to design systems with rough walls. The current paper shows a method to
obtain the roughness function of different surface roughness by using a small-scale rig with rotating disks and a
long-distance microscopic PIV in order to obtain submicron resolution inside the boundary layer developed on the
disks when they rotate. This rig was designed and constructed for the optical measurements and consists of an
electric motor which drives the disks that are rotating in the middle of a 20-liter water tank. The measurements were
performed on surfaces with different degree of roughness. These are a smooth reference, three antifouling coatings
used in marine applications, two different sand roughness and one case of periodic roughness. All measurements
were performed at two different Reynolds numbers (5.7×105 and 1.15×106), corresponding to disk rotational speeds
of 300 and 600 rpm. Field mean velocity profiles were calculated by averaging vertical bins with 1-pixel (px) width.
Every point of a mean velocity profile was based on near 200 instant velocity values obtained from 2000 PIV image
pairs by using particle tracking approach. The results of a wall shear stress study of the smooth disk case were used
together with torque measurements and the micro-PIV measurements for the different rough surfaces to come up
with their dimensionless velocity profiles. The results show that as expected, the velocity profiles are shifted
downwards due to the roughness presence. Finally, the skin friction coefficients of the different cases are compared
with previous studies and the roughness function is determined for the different surfaces. The roughness function is
very useful in some industries such as in the naval architecture field, where the drag can be better estimated and the
adverse effect can be better counteracted.

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北京欧兰科技发展有限公司
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