On the use of bioluminescence for estimating shear stresses over a rippled seabed

Abstract

The bioluminescence emitted by micro-planktonic organisms has been adopted to detect shear stresses within oscillating flows over small scale bedforms. To this aim, an experimental campaign has been planned in order to optimize optical detectors and to address peculiar needs of biological organisms. Oscillating flow over ripples has been simulated in an annular cell, an apparatus that permits one to carry out experiments on ripple formation under well controlled conditions. Bioluminescence emission within the annular cell ascertains the existence of a high shear stress regime induced by the presence of the rippled bed in the range of Reynolds numbers between 2000 and 11000. Stress maps obtained from the experimental campaign showed the stronger stresses close to the rippled bed, with maximum values ranging between 15 and 35 Pa. 2D numerical simulations of oscillating flows over a rippled bed have been also run reproducing the experimental conditions, showing a good agreement on the position of the high stress regions, but underpredicting the maximum shear stress. Bioluminescence appears to be a valid instrument to characterize the flow and in particular to visualize high stress areas, leading to the possibility to measure shear stresses directly. Though limits occur, mostly due to the difficulties of dealing with biological organisms, such an approach can be considered as a starting point for the development of a bio-inspired visually based system for flow measurements.