Measurement of bottom velocities and shear stresses by ferrofluids at the sea bottom

Abstract

A novel direct measurement strategy of bottom velocities and shear stresses based on the use of ferrofluids is presented. The proposed methodology is aimed at overcoming some of the limits of state-of-the-art instruments, for use in presence of both surface waves and sediments. An inductive readout strategy is proposed along with experimental results which indicate the applicability of the proposed strategy.

Measurement of Wave Near-Bed Velocity and Bottom Shear Stress by Ferrofluids

“…Another advantage of ferrofluid based sensors is the electrical insulation between the readout electronics and the liquid medium. This gives the possibility to easily control fluids and to implement measurements in liquids [29], [41]- [46].…”

“…As it will be more clear in the following, because the position of the ferrofluid spike is controlled only by the intensity of the magnetic field and by the action of the flow, the proposed technique is designed to overcome some of the limits of the available instrumentation, and it can be also used in the presence of sediments without damages to the measuring system. To detect the displacement of the center of mass of the ferrofluid spike induced by the flow, an inductive read-out strategy was proposed [29].…”

“…As already stated, this makes it possible to setup the measurement system by removing the output voltage offset before starting a new measurement session with different initial water-level conditions. Actually a relationship between the water level in static conditions and the output voltage of the circuit has been observed [29] and investigated. A modeling of this relationship is presented in the following section.…”

“…Following such two preceding seminal works, the characteristics of ferrofluids was exploited to perform quasi-noninvasive measurements of near-bed velocities and shear-stresses in the presence of waves and sediments and preliminary results were presented in [29]. The developed sensing system aims at overcoming some of the above limits such as use in the presence of surface waves, sediments, suspended load (either moderate or intense), and a moderate bed load transport, closeness to the wall and reduction of dimensions of the measuring area as well as minimization of the risk of perturbing the original flow conditions.…”

“…The developed sensing system aims at overcoming some of the above limits such as use in the presence of surface waves, sediments, suspended load (either moderate or intense), and a moderate bed load transport, closeness to the wall and reduction of dimensions of the measuring area as well as minimization of the risk of perturbing the original flow conditions. With respect to the results presented in [29], in this paper a new optimized version of the conditioning electronic is proposed together with an extended analysis of the obtained experimental results. The new electronics makes it possible to setup the measurement system by removing the output voltage offset before starting a new measurement session with different initial conditions (different water levels).…”