Bed surface and ripple migration observations in a small oscillatory flow tunnel

A Bed LAser Surface Tracking (BLAST) system measured ripple profiles at 30 Hz and sub-millimeter spatial resolution to quantify the intra-wave bed surface profile dynamics occurring during ripple migration. Ripples were generated with oscillatory flow in a 2 m long, 0.25 m wide flow tunnel filled with narrowly distributed sand (d₅₀=0.65 mm). The BLAST system includes a 520 nm, 75-degree fan beam laser projecting a 1 m laser line on the bed in the flow direction and a digital SLR camera capturing high definition (HD) video (1080p) at 30 frames per second. A Nortek Vectrino Profiler measured the three-component fluid velocity in a one-dimensional profile just above the bed. A piston drove a slightly asymmetrical flow with maximum free stream velocity of 25 cm/s and a 4.5 second period. The flow ran continuously until the ripples were in equilibrium with lengths of ~30 cm and heights of ~6 cm and were migrating at a steady rate of ~6 mm/min. Image frames were extracted from the HD video and processed in Matlab. Algorithms then determined the bed elevation from the location of the laser line in each image. This technique allowed for the measurement of the sub-millimeter intra-wave oscillation of the ripple profiles. The results show an expansion of the same flank on adjacent ripples during flow acceleration and reversal.