Author(s): M. Rouzes; F. Y. Moulin; S. Cazin; M. Marchal; O. Eiff
Keywords: Stereoscopic PIV; Light intensity-based methodology; In situ experiments
Abstract: A full characterization of turbulent boundary layer flows over natural beds requires high-resolve measurements over the whole water column and close to the bed. In this paper, optical-based SPI (Stereoscopic Particle Image Velocimetry) was setup enabling non-intrusive velocity measurements wit high resolution close to the bed by using two cameras located above and relatively far from the bed However, and in contrary to ADV or acoustic measurements techniques, optical systems such as SPIV ar extremely dependent of turbidity conditions, measurement quality dropping quickly with increasin turbidity. Experiments in a hydraulic flume (12 x 0.50 x 0.20 m3) over a bed covered with 4-cm diamete hemispheres were performed (Rouzes et al., 2014). The influence of turbidity and camera angle on th quality of recorded images was studied for three different angles (θ=15°, 25°, 35°) and three differen levels of ambient turbidity (Nephelometric Turbidity Unit NTU≈3,15,25). The results suggest that ligh intensity follows a law depending on inherent SPIV system parameters as well as physical phenomen involved (such as laser sheet divergence, light diffusion absorption, reflection and diffusion by turbidit and PIV particles). It is presented here a light intensity-based model that has been developed as a guidin tool to help the design of in situ SPIV measurement system.