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Three-dimensional velocity near-wall measurements by digital in-line holography: calibration and results

Abstract : Velocity measurements in the vicinity of an obstacle remain very complicated even when optical diagnostics based on displacement of micrometric tracers are considered. In the present paper, digital in-line holography with a divergent beam is proposed to measure the three-dimensional (3D) velocity vector fields in a turbulent boundary layer and, in particular, on the near wall region of a wind tunnel. The seeding droplets (1-5 μm) transported by a turbulent airflow are illuminated by a couple of laser pulses coming from a fiber coupled laser diode. These double exposure holograms are then recorded through a transparent glass reticle specially designed for this application with an accurate surface positioning combined with a particularly attractive in situ calibration method of the investigation volume (less than 10 mm3). The method used for processing holograms recorded in such a configuration is detailed. Our original calibration procedure and the assessment of its accuracy are presented. Our holographic probe has been tested in a wind tunnel for a large range of different velocities. Then 3D velocity vector fields extracted from more than 13000 holograms are analyzed. Statistical results show the capability of our approach to access in a turbulent boundary layer. In particular, it leads to relevant measurements for fluid mechanics such as velocity fluctuation and the shear stress in the very close vicinity of a wall.
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https://hal-normandie-univ.archives-ouvertes.fr/hal-01657641
Contributor : Bertrand Lecordier <>
Submitted on : Thursday, December 7, 2017 - 12:59:54 AM
Last modification on : Tuesday, April 16, 2019 - 3:22:04 PM

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Daniel Allano, Mokrane Malek, Françoise Walle, Frédéric Corbin, Gilles Godard, et al.. Three-dimensional velocity near-wall measurements by digital in-line holography: calibration and results. Applied optics, Optical Society of America, 2013, 52 (1), pp.A9-A17. ⟨10.1364/AO.52.0000A9⟩. ⟨hal-01657641⟩

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