Optical measurements of bottom shear stresses by means of ferrofluids

The research is focused on the development and on the assessment of a measurement technique for bottom shear stresses. In particular, the wall–fluid interaction is analyzed adopting ferrofluids and using an optical readout system. The principle of operation of this technique is based on the capability of ferrofluids to react to external magnetic field changing their shape and their viscosity. The proposed magneto-rheological sensor, consisting in a magnetized drop of ferrofluid located at the channel bottom, is exposed to different flow conditions and its deformations are video-recorded. Thanks to the application of image analyses processes, the relation between shear stresses and magneto-rheological sensor deformation is investigated. The assessment of the measuring technique is carried out in the presence of different sandy bottoms and by considering several hydraulic (steady current) conditions. The range of measured bottom shear stress is 0.01–0.20 N/m$$^2$$. Tests carried out with different sandy bottoms characterized by different roughness provide insights about the high sensitivity of the sensor, which is able to detect slight changes in the sandy bottom mixture (less than 10$$\%$$ concentration in volume). Statistical analysis on the ferrofluid deformation shows that the sensor deformation is strictly related to the local hydrodynamics. For higher Re number we observed larger mean displacement in the direction of the flow and bigger oscillations. Power spectral densities of the ferrofluid displacement and of the velocity fluctuations measured at the ferrofluid apex point show how the two signals are characterized by the same slope in log–log graph for intermediate and high frequencies (> 0.2 Hz) representative of small-scale eddies.