title: 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 mm(3)). 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.
authors.affiliation: UMR 6614 CORIA, Avenue de l’Université, 76801 Saint-Etienne du Rouvray, France.
authors.forename: Daniel
authors.initials: D
authors.lastname: Allano
authors.forename: Mokrane
authors.initials: M
authors.lastname: Malek
authors.forename: Françoise
authors.initials: F
authors.lastname: Walle
authors.forename: Frédéric
authors.initials: F
authors.lastname: Corbin
authors.forename: Gilles
authors.initials: G
authors.lastname: Godard
authors.forename: Sébastien
authors.initials: S
authors.lastname: Coëtmellec
authors.forename: Bertrand
authors.initials: B
authors.lastname: Lecordier
authors.forename: Jean-Marc
authors.initials: JM
authors.lastname: Foucaut
authors.forename: Denis
authors.initials: D
authors.lastname: Lebrun