A ferrofluid-based sensor to measure bottom shear stresses under currents and waves

Abstract

The measurement of the near-bottom flow characteristics is crucial to correctly understand coastal processes. Aiming at overcoming some of the limits of present state-of-the-art measuring instruments, we propose a novel approach to measure bottom shear stress under currents and waves based on the exploitation of magneto-rheological fluids, named ferrofluids. In particular the deformation of a magnetically controlled ferrofluid drop O(0.01 ml) is transduced by a conditioning circuit into an output voltage which is proportional to the bottom shear stress. Calibration curves are presented for both steady-current and regular wave conditions, over fixed and weakly mobile beds, by showing that the behaviour of the proposed measuring system can be assumed linear. In the range of the investigated parameters, it is shown that the working range is comprised between 0.08 N/m 2 and an upper limit which is a function of the controlling magnetic field and of the flow type.

Wave-Current Interaction at a Right Angle Over Rough Beds: Turbulence Analysis

“…Future experimental studies should focus on: (i) extend the range of the experiments for the current U, (ii) recover direct measurements of bottom shear stresses rather than inferring via indirect methods, for instance by using innovative techniques, such as the one of Musumeci et al (2018); (iii) further analysis on turbulence, including a phase-averaged Reynolds stress analysis, in order to better understand how the current bottom turbulent stress is altered during the different phases of the wave cycle.…”

“…Direct measurements of shear stresses has not been performed. Although advanced techniques and instruments exist (Musumeci et al, 2018;Stancanelli et al, 2020), most of them are not usable in large facilities such the one used in the current work, as they require close optical or electromagnetic monitoring which is not feasible in the present experimental setup. The best fit procedure is different depending if the flow is hydraulically smooth, i.e.…”

Magnetowetting dynamics of sessile ferrofluid droplets: a review

“…187 Later on, the results from the above two studies were extended to measure the shear stress under both steady-and unsteady-flow conditions over uniform sandy-bottom surfaces. 188 The operating range of the FF sensors using the inductive readout method was found to be between the lower limit of 0.08 N m À2 and an upper limit depending on the flow type and the magnetic field strength. Interestingly, the proposed method of shear stress measurement was suggested as an alternative to the existing MEMS or hot filmbased methods, primarily considering the cost and durability of the systems.…”

“…185 Recently, ferrofluidic sensors have been brought into the limelight for measuring the bottom shear stress under steadycurrent and wavy conditions over any steady surface or moving or oscillating surface, which exploits the FF configurations engendered by the Rosensweig instability. [186][187][188][189] Fundamentally, the Rosensweig instability or the normal-field instability is employed to generate a single peak from a meager quantity of FF (BO [10 À2 ] ml), deposited on the bottom wall of a tank. In the presence of a permanent magnet, the height of the peak is BO [1] mm, which subsequently acts as the sensor for the shear stress measurement.…”

The role of alongshore flows on inner surf and swash zone hydrodynamics on a dissipative beach

“…However, τ is difficult to quantify even for immobile beds. Some approaches in mobile bed scenarios include the quadratic drag law (Barnes et al, 2009;Masselink et al, 2009;Puleo et al, 2000), the von Karman-Prandtl relationship or Law of the Wall (Austin et al, 2011;Cox et al, 2000;Inch et al, 2015;Miles et al, 2006;Puleo et al, 2012), shear plates (Barnes et al, 2009;Jiang and Baldock 2015), hot film anemometers (Conley and Griffin, 2004;Gust, 1988) or more recently the use of ferrofluids (Musumeci et al, 2018).…”