  The motion of an assembly of rigid fibers is investigated for different
classes of closed streamline flows, steady or time dependent, two dimensional
or three dimensional. In our study, the dynamics of the fiber assembly is
fully-coupled to the flow field by means of a state-of-the-art immersed
boundary method. We show that, for sufficiently small Stokes times of the
assembly, the whole flow gradient tensor can be accurately reconstructed by
simply tracking the fiber assembly and measuring suitable fiber velocity
differences evaluated at the fiber ends. Our results strongly suggest the
possibility of using rigid fibers (or assemblies of them) to perform
multi-point flow measures, either in laboratory or in field: future experiments
are therefore mandatory to inquire the feasibility of a new `Fiber Tracking
Velocimetry' technique.
