Ferrofluids for a novel approach to the measurement of velocity profiles and shear stresses in boundary layers

This paper addresses the problem of measuring the velocity profile and the shear stresses in the boundary layer that forms when a fluid is flowing over a fixed surface. A novel measurement strategy is presented here. It is based on the use of ferrofluidic materials that are deposited over the surface where the fluid velocity profile has to be estimated, sense the shear stresses to be measured, induce minimal disturbance to the flow and, finally, do not mix with the fluid itself. An experimental setup that reproduces controlled fluid velocity conditions over a flat surface has been realized. Measures have been obtained by using a calibrated videocamera that observes the deformations induced into the ferrofluid by the flowing fluid. An image processing procedure has been developed to derive the velocity profile and then the shear stress in the boundary layer. The results are in accordance with the mathematical models of the system thus demonstrating the suitability of the approach proposed.