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

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.