Optimisation of lighting techniques for photometric measurement of sediment-laden flows

Richard Wilson

Wednesday 1 july 2015

8:45 - 9:00h at Africa (level 0)

Themes: (T) Water engineering, (ST) Experimental facilities and instrumentation

Parallel session: 8F. Engineering - Instrumentation

The use of photometry is becoming a more prevalent method for analysing the dynamics of sediment-laden flows in the laboratory. This is due to advances in technology and its non-intrusive nature on flow propagation. When coupled with established velocity and density measurement techniques, significant data can be obtained and analysed. For photometric data to be accurately captured, a robust lighting configuration must be installed. It must ensure that current boundaries have a strong contrast with the surrounding ambient environment. The following study compares three different lighting configurations of a lock-exchange turbidity current interacting with a rectangular obstacle in a flume: (a) side-view illumination using halogen lamps combined with overhead illumination using LED strips; (b) side-view illumination using halogen lamps without any overhead illumination; (c) side-view and overhead illumination using halogen lamps. Photometry is used to delineate the current boundary and produce high-resolution spatio-temporal contour plots. Visual influence of a UVP and siphon instrument rack on the delineation process is also taken into account. The contour plots provided detailed insights into the dynamic processes of the current interaction, such as reduction in velocity of the approaching current head, vortices formed above and after the obstacle and the formation of Kelvin-Helmholtz instabilities at the rear of the head upon collision with the obstacle. Investigation of the delineated tests showed that the LED overhead lamp provided an even illumination distribution above the current, however its weak illuminance caused unwanted noise to be present on the flume bed. The lighting configuration without overhead illumination resulted in low contrast of the current boundary and isolated patches of delineation within the current body due to shadows. The configuration using halogen lamps from the side and overhead provided strong contrast and eliminated noise, however the overhead lamps provided poor light distribution due to their point-source nature, which also resulted in isolated areas of delineation. The results showed using either LED or halogen overhead lamps is promising for future tests, however it is recommended that additional LED strips are installed to improve contrast, or additional halogen lamps are installed overhead to improve light distribution.