Fiber Inertia Research Articles

Kinematics of rigid fibers in a turbulent channel flow

Measurements of concentrations, translational velocities, orientations and rotation rates of seven, straight and rigid fiber types in a fully developed turbulent water channel flow (one-way coupled, Reτ=435) are reported. The aim was to study inertial and length effects on fiber kinematics. Fibers were characterized by Stokes numbers and mean, normalized lengths ranging between 0.02 ¡ St ¡ 0.34 and 16 ≤L¯+≤ 66.8. Except for the longest (L¯+= 66.8) fibers, a depletion layer was observed near the wall, its width increasing with decreasing St (for a given length). Away from the wall, fibers lagged the fluid, increasingly so with increasing St and L¯+ due to increased drag. In the buffer layer, fiber excess velocities decreased with L¯+ while inertial effects were small. Irrespective of fiber length, all fibers strongly aligned with the streamwise direction in the near-wall region. Fiber rotation rates peak in the buffer layer, and remain nearly constant in the log-layer and wake region. However, peak values decreased with increasing L¯+. Our results suggest intricate coupling between fiber inertia and length in conjunction with the governing turbulence scales.

Turbulence Modulation by Slender Fibers

In this paper, we numerically investigate the turbulence modulation produced by long flexible fibres in channel flow. The simulations are based on an Euler–Lagrangian approach, where fibres are modelled as chains of constrained, sub-Kolmogorov rods. A novel algorithm is deployed to make the resolution of dispersed systems of constraint equations, which represent the fibres, compatible with a state-of-the-art, Graphics Processing Units-accelerated flow-solver for direct numerical simulations in the two-way coupling regime on High Performance Computing architectures. Two-way coupling is accounted for using the Exact Regularized Point Particle method, which allows to calculate the disturbance generated by the fibers on the flow considering progressively refined grids, down to a quasi-viscous length-scale. The bending stiffness of the fibers is also modelled, while collisions are neglected. Results of fluid velocity statistics for friction Reynolds number of the flow Reτ=150 and fibers with Stokes number St = 0.01 (nearly tracers) and 10 (inertial) are presented, with special regard to turbulence modulation and its dependence on fiber inertia and volume fraction (equal to ϕ=2.12·10−5 and 2.12·10−4). The non-Newtonian stresses determined by the carried phase are also displayed, determined by long and slender fibers with fixed aspect ratio λtot=200, which extend up to the inertial range of the turbulent flow.

Comparison of feature detection and outlier removal strategies in a mono visual odometry algorithm for underwater navigation

This work aims to develop and evaluate a navigation strategy based on optical payloads for Autonomous Underwater Vehicles (AUVs). The use of cameras for navigation purposes can make possible a correct vehicle localization in particular working conditions, where other sensors, such as Doppler Velocity Log (DVL), could not be used. In particular, feature detection and outliers removal algorithms have been chosen as possible critical step in the whole algorithm and have been carefully investigated. Underwater environment introduces challenging conditions for a feature based navigation system and, consequently, the images need to be firstly processed. The developed visual-inertial odometry (VIO) algorithm has been employed for the vehicle translation estimation and this information has been fused with the altimeter, Inertial Measurement Unit (IMU) and Fiber Optic Gyroscope (FOG) measurements. The developed algorithm was tested with an image set acquired by Zeno AUV in the Haifa Bay, Israel (September 2018) and with an image set acquired by FeelHippo AUV in Vulcano, Italy (June 2019) and the results were compared with the path estimated exploiting the other on-board sensors (e.g., the DVL, which has been considered as reference sensor for the benchmark path computation). The algorithm performances are evaluated in both cases, focusing either on the estimate quality and on the requested computational load.

Lagrangian Time Scale of Passive Rotation for Mesoscale Particles in Turbulence

Turbulence induces rotation in the living and the non-living materials in the ocean. The time scale of rotation for a living organism is important in understanding an organism's feeding efficiency, mating, prey capture rate etc. This time scale is also crucial for understanding the migration of non-living materials such as microplastics. Herein, we investigate the tumbling motion of mesoscale particles that resemble organisms of intermediate size, such as zooplankton that appear in the ocean. Using time-resolved measurements of the orientation of rigid inertial fibers in a turbulence-tank, we analyze the autocorrelation of their tumbling rate. The correlation time ($\tau_d$) is well predicted by Kolmogorov inertial-range scaling based on the fiber length ($L$) when the fiber inertia can be neglected. For inertial fibers, we propose a simple model considering fiber inertia (measured by a tumbling Stokes number) and a viscous torque which accurately predicts both the correlation time and the variance of the tumbling rate. Our measurements and the theoretical model provide a basic understanding of the rotational response of an intermediately sized organism to the surrounding turbulence in its non-active state.

Inertial effects on the dynamics of rigid heavy fibers in isotropic turbulence

Holographic cinematography is used to measure rotational and translational dynamics of inertial fibers in isotropic turbulence. Results show that for the present parameter range, translational motion is similar to that for spheres. In contrast, tumbling rates peaked at an intermediate Stokes number.

Orientation, distribution, and deformation of inertial flexible fibers in turbulent channel flow

In this paper, we investigate the dynamics of flexible fibers in turbulent channel flow. Fibers are longer than the Kolmogorov length scale of the carrier flow, and their velocity relative to the surrounding fluid is non-negligible. Our aim is to examine the effect of local shear and turbulence anisotropy on the translational and rotational behavior of the fibers, considering different elongation (parameterized by the aspect ratio, $$\lambda $$ ) and inertia (parameterized by the Stokes number, St). To these aims, we use a Eulerian–Lagrangian approach based on direct numerical simulation of turbulence in the dilute regime. Fibers are modeled as chains of sub-Kolmogorov rods (referred to as elements hereinafter) connected through ball-and-socket joints that enable bending and twisting under the action of the local fluid velocity gradients. Velocity, orientation, and concentration statistics, extracted from simulations at shear Reynolds number $$Re_{\tau }=150$$ (based on the channel half height), are presented to give insights into the complex fiber–turbulence interactions that arise when non-sphericity and deformability add to inertial bias. These statistical observables are examined at varying aspect ratios (namely $$\lambda _{r}=l_{r}/a=2$$ and 5, with $$l_{r}$$ the semi-length of each rod-like element r composing the fiber and a its cross-sectional radius) and varying fiber inertia (considering values of the element Stokes number, $$St_{r}=1$$ , 5, 30). To highlight the effect of flexibility, statistics are compared with those obtained for fibers of equal mass that translate and rotate as rigid bodies relative to the surrounding fluid. Flexible fibers exhibit a stronger tendency to accumulate in the very-near-wall region, where they appear to be trapped by the same inertia-driven mechanisms that govern the preferential concentration of spherical particles and rigid fibers in bounded flows. In such region, the bending of flexible fibers increases as inertia decreases, and fiber deformation appears to be controlled by mean shear and turbulent Reynolds stresses. Preferential segregation into low-speed streaks and preferential orientation in the mean flow direction is also observed.

Tumbling of Inertial Fibers in Turbulence.

Anisotropic particles play a major role in many environmental and industrial turbulent flows. The modeling of their rotation dynamics is a fundamental challenge which has some important consequences in industrial processes, such as in the paper making industry. This study investigates the rotation rate of neutrally buoyant fibers longer than the Kolmogorov length η_{K}. We show that the fiber inertia is at the origin of a decrease of the rotation rate. We propose a model which describes this phenomenon. We introduce also a new Stokes number which defines the limit of validity of the classical slender body approximation.

Large eddy simulation of inertial fiber deposition mechanisms in a vertical downward turbulent channel flow

The deposition pattern of elongated inertial fibers in a vertical downward turbulent channel flow is predicted using large eddy simulation and Lagrangian particle tracking. Three dominant fibers deposition mechanisms are observed, namely, diffusional deposition for small inertial fibers, free‐flight deposition for large inertial fibers, and the interception mechanism for very elongated fibers. The fibers are found to exhibit orientation anisotropy at impact, which is strongly dependent on the fiber elongation. An increase in the fiber elongation increases the wall capture efficiency by the interception mechanism. The diffusional deposition mechanism is shown to dominate for fibers with large residence time, , in the accumulation zone and small deposition velocities, , while the free‐flight mechanism governs deposition for fibers with small and large . This study describes how particles deposit on a surface and, ultimately for many practical applications, how such deposition may promote fouling. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1451–1465, 2017

Particle-level simulations of flocculation in a fiber suspension flowing through a diffuser

We investigate flocculation in dilute suspensions of rigid, straight fibers in a decelerating flow field of a diffuser. We carry out numerical studies using a particle-level simulation technique that takes into account the fiber inertia and the non-creeping fiber-flow interactions. The fluid flow is governed by the Reynolds-averaged Navier-Stokes equations with the standard k-omega eddy-viscosity turbulence model. A one-way coupling between the fibers and the flow is considered with a stochastic model for the fiber dispersion due to turbulence. The fibers interact through short-range attractive forces that cause them to aggregate into flocs when fiber-fiber collisions occur. We show that ballistic deflection of fibers greatly increases the flocculation in the diffuser. The inlet fiber kinematics and the fiber inertia are the main parameters that affect fiber flocculation in the prediffuser region.

On fiber behavior in turbulent vertical channel flow

In the present work, the dynamic behavior of inertial fibers suspended in a turbulent vertical channel flow has been investigated. The three-dimensional turbulent flow field was obtained from the Navier–Stokes equations by means of direct numerical simulation in an Eulerian reference frame. The fibers were modeled as prolate spheroidal particles in a Lagrangian frame and characterized by their inertia and shape. The translation and rotation of the individual fibers were governed by viscous forces and torques as well as by gravity and buoyancy according to Newton’s laws of motion. The test matrix comprised four different Stokes numbers (inertia) and three different aspect ratios (shape). The twelve different fiber types were suspended both in a downward and in an upward channel flow. Fiber orientation and velocity statistics were compared with channel flow results in absence of gravity.The results showed that gravity has a negligible effect for fibers with modest inertia, i.e. low Stokes numbers, whereas gravity turned out to have a major impact on the dynamics of highly inertial fibers. Irrespective of the bulk flow direction, a preferential alignment of the inertial fibers with the gravity force was found in the channel center where fibers have been known to orient randomly in absence of gravity. In the downward channel flow, the drift velocity of the fibers towards the walls was substantially higher for fibers than for spheres and also higher than when gravity was neglected. In the upward flow configuration, the modest drift velocity of inertial spheres was totally quenched for all fibers irrespective of shape. The suppressed drift velocity resulted in a more uniform fiber distribution throughout the channel as compared to the distinct near-wall accumulation in downward flow and in absence of gravity. This suggests that an upward flow configuration should be the preferred choice if a uniform fiber distribution is desired, as in a biomass combustion reactor.

Gravity Effects on Fiber Dynamics in Wall Turbulence

The present study investigated gravity effects on the dynamical behavior of inertial fibers suspended in a vertical channel flow. Direct numerical simulations were performed to obtain the turbulent flow field and the fibers were modelled as prolate spheroidal point particles. For each of the four fiber classes, three different gravity configurations were considered: upward flow with gravity opposing, downward flow with aiding gravity, and channel flow in absence of gravity. Results for the fiber distribution and the translational and rotational fiber motion were reported. In the near-wall region, the presence of gravity resulted in an increased fiber density in the downward flow but a nearly uniform distribution of fibers in upward flow. However, the preferential clustering of fibers in near-wall low-speed streaks was unaffected by gravity. The mean wall-normal or drift velocity of the fibers was higher in the downward flow and lower in the upward flow as compared to the case with no gravity. The suppressed drift velocity in the upward flow resulted in a more uniform fiber distribution throughout the channel in contrast to the near-wall accumulation of fibers in the two other cases. Overall gravity turned out to have negligible effects on some of the statistics of the least inertial fibers whereas the inclusion of gravity had a strong impact for heavier fibers.

Slip velocity of rigid fibers in turbulent channel flow

In this study, the slip velocity between rigid fibers and a viscous carrier fluid is investigated for the reference case of turbulent channel flow. The statistical moments of the slip velocity are evaluated modelling fibers as prolate spheroids with Stokes number, St, ranging from 1 to 100 and aspect ratio, λ, ranging from 3 to 50. Statistics are compared one-to-one with those obtained for spherical particles (λ = 1) to highlight effects due to fiber elongation. Comparison is also made at different Reynolds numbers (Reτ =150, 180, and 300 based on the fluid shear velocity) to discuss effects due to an increase of turbulent fluctuations. Results show that elongation has a quantitative effect on slip velocity statistics, particularly evident for fibers with small St. As St increases, differences due to the aspect ratio tend to vanish and the relative translational motion between individual fibers and surrounding fluid is controlled by fiber inertia through preferential concentration. A clear manifestation of inertial effects is the different distribution of slip velocities for fibers trapped in sweep/ejection events and for fibers segregated in near-wall fluid streaks. The corresponding conditional probability distribution functions, shown here for the streamwise and wall-normal slip velocity components, are found to be non-Gaussian, thus suggesting that fiber motion relative to the fluid in high-shear flow regions may not be modelled as a pure diffusion process with constant diffusion coefficient. For the range of simulation parameters investigated, no significant Reynolds number effects are observed, indicating that fiber dynamics exhibit a scaling behavior with respect to the shear velocity up to Reτ =300.

Lattice Boltzmann simulations of sedimentation of a single fiber in a weak vertical shear flow

Instability of a suspension is directly related to the problem of the cross-stream migration of a particle relative to its neighboring particle suspension. Such cross-stream or lateral migration of a single non-spherical particle (fiber) settling in a bounded weak shear flow with vertical streamlines produced by a perturbation to the fiber number density is studied using lattice Boltzmann simulations. The present simulation results demonstrate that at a given shear rate, the lateral migration can be divided into three phases depending on settling Reynolds number Rsd and particle aspect ratio κ. At a low settling Reynolds number Rsd, the suspension becomes more stable in phase 1. As Rsd increases and excesses a critical settling Reynolds number Rsd1, the fiber suspension becomes unstable in phase 2. In phase 3, at an enough large Rsd, the inertia dominates the weak shear flow and it may have little effect on stability. A mechanism of the instability induced by an inertial fiber orientation drift and a shear induced cross-streamline drift, recently proposed by Shin, Koch, and Subramanian [“Structure and dynamics of dilute suspensions of finite reynolds number settling fibers,” Phys. Fluids 21, 123304 (2009)], is examined and confirmed.

On inertial effects of long fibers in wall turbulence: fiber orientation and fiber stresses

Inertia effects of large-aspect-ratio fibers have been investigated in wall turbulence. The turbulent flow field in a plane channel was obtained from a direct numerical simulation. The translational and rotational motion of the rigid fibers were obtained by a Lagrangian approach, first for inertial fibers with Stokes number St = 10, 1.0 and 0.1 and thereafter for massless fibers, which correspond to St = 0. All simulations were one-way coupled. The fiber orientation statistics and the normal components of the fiber stress tensor turned out to be almost independent of the fiber inertia all the way from the channel wall to the center for St ≤ 1.0. This observation suggested that fiber inertia plays a negligible role for Stokes number below unity and the gap between inertial fibers and massless fibers has been bridged. The massless particle approach appears as a viable alternative to mimic the orientation and stress tensor of fibers with only modest inertia.

A study of a flexible fiber model and its behavior in DNS of turbulent channel flow

The dynamics of individual flexible fibers in a turbulent flow field have been analyzed, varying their initial position, density and length. A particle-level fiber model has been integrated into a general-purpose, open source computational fluid dynamics code. The fibers are modeled as chains of cylindrical segments connected by ball and socket joints. The equations of motion of the fibers contain the inertia of the segments, the contributions from hydrodynamic forces and torques, and the connectivity forces at the joints. Direct numerical simulation of the incompressible Navier–Stokes equations is used to describe the fluid flow in a plane channel, and a one-way coupling is considered between the fibers and the fluid phase. We investigate the translational motion of fibers by considering the mean square displacement of their trajectories. We find that the fiber motion is primarily governed by velocity correlations of the flow fluctuations. In addition, we show that there is a clear tendency of the thread-like fibers to evolve into complex geometrical configurations in a turbulent flow field, in fashion similar to random conformations of polymer strands subjected to thermal fluctuations in a suspension. Finally, we show that fiber inertia has a significant impact on reorientation timescales of fibers suspended in a turbulent flow field.

Effect of Rotary Inertia on the Orientational Behavior of Dilute Brownian and Non-Brownian Fiber Suspensions

The orientational behavior of a dilute suspension of slender Brownian and non-Brownian fibers with rotary inertia in simple shear and turbulent channel flows is numerically investigated. The translational inertia of fibers is neglected. The equations describing the evolution of fibers orientation are integrated along the Lagrangian paths of the fluid elements. The fully developed turbulent channel flow at Re τ = 180 is provided by a direct numerical simulation (DNS). The coupling between the flow field and the fiber dynamics is one way. The Brownian motion is modeled by a stochastic Wiener process. The results are compared with those of inertia-free particles. In simple shear flow, the inertial non-Brownian fibers align slower than the inertia-free fibers to the shear direction while they tend to the same steady state orientation. For Brownian fibers, the steady state orientation of inertial and inertia-free fibers differ. In turbulent channel flow, the second moment of the orientation distribution function shows an oscillatory behavior at high values of inertia for non-Brownian fibers while the oscillations disappear at lower inertia. For Brownian fibers, the oscillations are weaker due to the damping effect of the Brownian diffusivity.

Orientation, distribution, and deposition of elongated, inertial fibers in turbulent channel flow

In this paper, the dispersion of rigid, highly elongated fibers in a turbulent channel flow is investigated. Fibers are treated as prolate ellipsoidal particles which move according to their inertia and to hydrodynamic drag and rotate according to hydrodynamic torques. The orientational behavior of fibers is examined together with their preferential distribution, near-wall accumulation, and wall deposition: all these phenomena are interpreted in connection with turbulence dynamics near the wall. In this work a wide range of fiber classes, characterized by different elongation (quantified by the fiber aspect ratio, λ) and different inertia (quantified by a suitably defined fiber response time, τp) is considered. A parametric study in the (λ,τp)-space confirms that, in the vicinity of the wall, fibers tend to align with the mean streamwise flow direction. However, this aligned configuration is unstable, particularly for higher inertia of the fiber, and can be maintained for rather short times before fibers are set into rotation in the vertical plane. A more complex situation is observed in the spanwise and wall-normal flow directions, where fiber inertia and elongation destabilize near-wall alignment in a nontrivial fashion. Fiber orientational behavior and fiber translational behavior are observed to influence the process of fiber accumulation at the wall. Comparing the behavior of fibers with that of spherical particles, it is observed that the aspect ratio has little or no effect on clustering, preferential distribution, and segregation; yet it does affect the wallward drift velocity of the fibers in such a way that longer fibers tend to deposit at higher rates. No preferential orientation and no significant segregation is observed in the channel centerline, confirming that the role of inertia and, in particular, of elongation becomes less important in and beyond the logarithmic layer.

Deposition of fiber in a human airway replica

Fiber is a notorious occupational hazard. Exposures to airborne asbestos fiber in the workplace increase the incidence of lung cancer for asbestos workers. Due to the lack of experimental data, the nature of fiber deposition in the human airway is unclear. In this study, a set of experiments were carried out to investigate the effect of fiber dimension and fiber inertia on the deposition pattern in the human airway replica. The deposition study was conducted by delivering aerosolized carbon fibers into the replica at constant inspiratory flow rates of 15–60 l/min. The results showed that impaction is the dominant deposition mechanism in this study. Most of the high-inertia fibers deposited in the oropharynx and the carina ridges of the bifurcations in tracheobronchial airways. A series of fiber deposition patterns were obtained and the deposition efficiencies were acquired for certain regions in the human airway replica.

The Effect of Fiber Inertia on Its Orientation in a Shear Flow with Application to Lung Dosimetry

The effect of fiber inertia on its orientation in a flow field was studied. Expressions for fluid dynamic torque given by Jeffery (1923) were used in the Euler equations of motion to determine fiber orientation. Fiber orientation was found to depend on two nondimensional parameters; its aspect ratio and a nondimensional parameter related to the fluid viscosity, flow velocity gradient, and fiber mass density and diameter. Fiber orientation was calculated for different fiber diameters and was compared with that obtained by Jeffery (1923) for normal breathing in humans and rats. The inertial effects are significant when fiber diameter exceeds lμm.

Sedimentation of Fibers from Laminar Flows in a Horizontal Circular Duct

An analysis is presented to describe the motion of a fiber under gravity in laminar flows within a horizontal circular duct. The trajectory and orientation of the fiber were determined by simultaneously solving three equations for translations and two equations for rotation. Neglecting fiber inertia and the effect of Brownian diffusion, the limiting trajectories of the fiber for which the fiber just penetrates the duct without being collected were obtained for a variety of conditions of fiber size, flow pattern and velocity, and duct dimension. The limiting trajectories were then used to calculate the deposition efficiency of a fibrous aerosol passing through the duct. Assuming a uniform fiber concentration and a random fiber orientation at the duct entrance, numerical results and empirical expressions were obtained for deposition efficiency in terms of the fiber aspect ratio, the aspect ratio of the duct, and a sedimentation aprameter which represents the ratio of the mean residence time of the fiber in ...