Flowing fibers as a proxy of turbulence statistics

Abstract

The flapping states of a flexible fiber fully coupled to a three-dimensional turbulent flow are investigated via state-of-the-art numerical methods. Two distinct flapping regimes are predicted by the phenomenological theory recently proposed by Rosti et al. [Phys. Rev. Lett. 121, 044501, 2018]: the under-damped regime, where the elasticity strongly affects the fiber dynamics, and the over-damped regime, where the elastic effects are strongly inhibited. In both cases we can identify a critical value of the bending rigidity of the fiber by a resonance condition, which further provides a distinction between different flapping behaviors, especially in the under-damped case. We validate the theory by means of direct numerical simulations and find that, both for the over-damped regime and for the under-damped one, fibers are effectively slaved to the turbulent fluctuations and can therefore be used as a proxy to measure various two-point statistics of turbulence. Finally, we show that this holds true also in the case of a passive fiber, without any feedback force on the fluid.

Numerical study of filament suspensions at finite inertia

“…In this approach, two sets of grid points are needed: a fixed Eulerian grid x for the fluid and a moving Lagrangian grid X for the flowing deformable structure. The volume force arising from the action of the filaments on the fluid is obtained by the convolution onto the Eulerian mesh of the singular forces estimated on the Lagrangian nodes; these are computed using the fluid velocity interpolated at the location of the Lagrangian points (Rosti et al 2018a(Rosti et al , 2019b. These so-called interpolation and spreading operations are usually performed by means of regularised delta functions, in our case the one proposed by Roma, Peskin & Berger (1999).…”

The assembly of freely moving rigid fibres measures the flow velocity gradient tensor

“…Motivated by the evidence reported by Rosti et al (2018aRosti et al ( , 2020, the goal of this work is to investigate similar possibilities for the case of rigid fibres in laminar flow. Rigid fibres are indeed easier to fabricate than elastic ones and are good candidates for novel experimental, non-invasive techniques able to access small-scale, multi-point properties of fluid flows.…”

“…For flexible fibres with finite size, some recent contributions have considered this kind of object as the key ingredient for a novel way of flow measurement. In particular, the possibility of using flexible fibres to quantify two-point statistics has been highlighted by Rosti et al (2018aRosti et al ( , 2020 in the case of homogeneous isotropic turbulence by means of fully resolved direct numerical simulation. This latter case is very far from the realm of application of Jeffery's model for at least three main reasons.…”

Fluid dynamics of COVID-19 airborne infection suggests urgent data for a scientific design of social distancing

“…The Poisson equation for the pressure is solved using the 2decomp library coupled with a fast and efficient FFT-based approach. The solver is parallelized using the MPI protocol and has been extensively validated in a variety of problems [37][38][39][40][41] . See also: https://groups.oist.jp/cffu/code.…”