150 2015,27(1):150-158 DOI: 10.1016/S1001-6058(15)60467-X Ferrofluid measurements of bottom velocities and shear stresses * MUSUMECI Rosaria E. 1 , MARLETTA Vincenzo 2 , ANDÒ Bruno 2 , BAGLIO Salvatore 2 , FOTI Enrico 1 1. Department of Civil Engineering and Architecture, University of Catania, Catania, Italy, E-mail: rmusume@dica.unict.it 2. Department of Electric, Electronic and Computer Science Engineering, University of Catania, Catania, Italy (Received October 9, 2014, Revised November 3, 2014) Abstract: A novel direct measurement strategy of bottom velocities and shear stresses based on the use of ferrofluids is presented. Such a strategy overcomes some of the limits of state-of-the-art instruments. A preliminary experimental campaign has been carried out in the presence of currents in steady flow conditions in order to test the effects of ferrofluid quantity and of the controlling permanent magnetic force. An alternating current (AC) circuit and a direct current (DC) conditioning circuit have been tested. For velocities larger than 0.05 m/s, the near-bottom velocity-output voltage calibration curve has a monotone parabolic shape. The sensitivity of the instrument is increased by a factor of 30 when the DC circuit is used. Key words: magnetofluids, Rosensweig effect, flow resistances, bed shear stresses, bottom velocities Introduction In physical modelling of hydraulic processes, measurements of wall shear stresses are extremely im- portant. Indeed large flow resistances develop right at the boundaries, strongly affecting both the hydrodyna- mics and, in the presence of mobile beds, the sediment transport. In the past, several instruments have been deve- loped and used to measure the shear stresses generated at the bottom in the presence of waves and currents, such as mechanical shear plates or cells located at the bottom, thermal anemometry based on the use of hot- film and hot-wire probes, acoustic and optic methodo- logies, such as ADV, LDA, PIV and PTV [1-4] or un- conventional approaches such as bioluminescence [5] . However, the applications of most of such techniques are affected by several limits. For example, mechani- cal probes can provide only integral measures over areas of O (1 dm 2 ). Thermal probes, which represent the state-of-the-art for measuring bed shear stresses, are extremely sensitive to the operating conditions, e.g. they cannot be effectively used in the presence of im- purities or sediments without damaging the probes. * Biography: MUSUMECI Rosaria E. (1975-), Female, Ph. D., Assistant Professor Finally, the widely spread optoacoustic instruments do not provide very accurate velocity measurements at the wall, due to the bottom-induced reflection. Only very recently a new methodology for the measurement of the bottom shear stresses in a quasi- noninvasive manner based on the use of ferrofluids has been qualitatively tested in the presence of an os- cillatory motion [6] . Ferrofluids are two-state systems made up by small ferromagnetic particles (with size 1 nm-15 nm) dispersed in an organic non-magnetic solvent [7] . A surfactant covers the nano-particles in order to prevent their agglomeration, which would be otherwise indu- ced by the Van Der Waals forces and the magnetic forces. Although their name seems to suggest the op- posite, the ferrofluids are superparamagnetic materials. Indeed, thanks to the nano-scale size of the dispersed particles, remaining at the fluidic state, the ferrofluids are deformed by the application of a magnetic field. Such a deformation disappears as soon as the magne- tic field is removed. At the microscopic scale, long chains of particles are formed in the direction of the magnetic field, whereas at the macro-scale a series of spikes aligned along the magnetic field appears. Such a phenomenon is called “the Rosensweig effect”. Thanks to their properties, the ferrofluids are used in a number of applications, ranging from me- chanics, as lubricants or packings, to biomedics,