  The present work investigates the mechanical behaviour of finite-size,
elastic and inertial fibres freely moving in a homogeneous and isotropic
turbulent flow at moderate Reynolds number. Fully-resolved, direct numerical
simulations, based on a finite difference discretisation and the immersed
boundary method, are performed to mutually couple the dynamics of fibres and
fluid turbulence, allowing to account for the backreaction of the dispersed
phase to the carrier flow. An extensive parametric study is carried out over
the characteristic properties of the suspension, i.e., fibre's linear density
(from neutrally-buoyant to heavy fibres), length (from short fibres comparable
with the dissipative scale to long fibres comparable with the integral scale)
and bending stiffness (from highly flexible to almost rigid fibres), as well as
the concentration (from dilute to non-dilute suspensions). Results reveal the
existence of a robust turbulence modulation mechanism that is primarily
controlled by the mass fraction of the suspension (with only a minor influence
of the fibre's bending stiffness), which is characterised in detail by means of
a scale-by-scale analysis in Fourier space. Despite such alteration with
respect to the single-phase case due to the non-negligible backreaction, fibres
experience only two possible flapping states (previously identified in the very
dilute condition) while being transported and deformed by the flow.
Additionally, we show that the maximum curvature obeys to different scaling
laws that can be derived from the fibre dynamical equation. Finally, we explore
the preferential concentration and alignment of fibres within the flow,
highlighting the peculiar role of inertia and elasticity.
