Author(s): Maxime Rouzes; Frederic Moulin; Olivier Eiff
Keywords: Benthic boundary layers; Turbidity; Stereoscopic PIV; Optical access
Abstract: Stereoscopic Particle Image Velocimetry (SPIV) measurements were undertaken in the laboratory under controlled hydraulic and water quality conditions to provide a guideline for the design of an in situ stereoscopic PIV system. The objectives are to understand how the experimental in situ conditions limit the access to velocity measurements and how to improve the access, in particular close to the natural bed where many hydrobiological processes take place. Indeed, optical-based PIV in situ measurements can be severely limited by the light intensity drop associated with naturally occurring turbid conditions. In this context, a stereoscopic PIV configuration is proposed which permits steeply inclined camera viewpoints in order to improve the image quality and optical access in the bed canopy. The effect of camera view angle and turbidity on the intensity variation in a vertical image plane is examined. A relationship between the turbidity (NTU) of the sampled water and the attenuation laser light in the water column as a function of viewing angle was found. It is shown that increased mean and RMS intensity levels are obtained by decreased camera angles (steeper viewpoints) and increase of turbidity. The positive angle effect is shown to be due to the anisotropic angular light scattering distribution. The response to the turbidity on the other hand is due to the increased domination of the turbidity particles relative to the PIV seeding particles through backscattering. The increased light intensities also increase the PIV correlation values, confirming the benefit of steeply inclined SPIV configurations under natural turbid conditions. These conclusions, in combination with the attenuation measurements, therefore provide a tool to help design and optimize a SPIV system under known in situ NTU and water depth conditions.