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A low-profile wall shear comparator to mount and test surface samples

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Abstract

Accurate measurement of shear stress on a solid surface is a crucial but challenging task in fluid mechanics. Different sensors are usually used for different experimental settings: water channel, wind tunnel, towing tank, watercraft, aircraft, etc. This paper presents a direct shear sensor designed to work for varying test objects and flow conditions. Designed to compare two different sample surfaces, the shear sensor is comprised of two floating elements, whose displacement is proportional to the shear stress they experience, and two optical encoders, which measure the displacements precisely, right under the floating elements. The main plate includes two identical sets of floating elements and flexure beams machined monolithically from a thick piece of metal, allowing displacements in only one in-plane direction. The side-by-side arrangement allows the two floating elements to experience essentially the same flow conditions, regardless of test condition, enabling the comparative sensing. The method of machining these folded-beam flexures, whose width is on the scale of micrometers, while thickness and length are in millimeters and centimeters, respectively, is presented. The main plate is designed with the help of finite element analysis to ensure dynamic response of the floating elements is appropriate for target flow conditions. The utility of the shear sensor is verified in three different flow settings, i.e., water tunnel, boat in open water, and wind tunnel. A miniature underwater camera system is also developed to observe the sample surfaces during testing on a moving object, such as a boat.

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Acknowledgements

This work was supported by DARPA HR0011-15-2-0021 and NSF 1336966 as well as Volgenau Endowed Chair in Engineering (C.-J.K.). Editing by Danning Yu is appreciated.

Author information

Author notes

  1. Muchen Xu

    Present address: ASML, 17082 Thornmint Ct, San Diego, CA, 92127, USA

  2. Hao Tong

    Present address: Tsinghua University, Beijing, China

  3. Yuta Ujiie

    Present address: Nagoya University, Nagoya, Japan

Authors and Affiliations

  1. Mechanical and Aerospace Engineering Department, University of California, Los Angeles (UCLA), Los Angeles, CA, 90095, USA

    Muchen Xu, Blaine Arihara, Hao Tong, Ning Yu, Yuta Ujiie & Chang-Jin Kim

  2. Bioengineering Department, University of California, Los Angeles (UCLA), Los Angeles, CA, 90095, USA

    Chang-Jin Kim

  3. California NanoSystems Institute (CNSI), University of California, Los Angeles (UCLA), Los Angeles, CA, 90095, USA

    Chang-Jin Kim

Authors
  1. Muchen Xu
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  2. Blaine Arihara
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  3. Hao Tong
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  4. Ning Yu
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  5. Yuta Ujiie
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  6. Chang-Jin Kim
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Correspondence to Muchen Xu or Chang-Jin Kim.

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Xu, M., Arihara, B., Tong, H. et al. A low-profile wall shear comparator to mount and test surface samples. Exp Fluids 61, 82 (2020). https://doi.org/10.1007/s00348-020-2922-z

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  • DOI: https://doi.org/10.1007/s00348-020-2922-z

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