Flexible Fibers in Turbulence

Simulation of a single flexible fiber suspension solved using partitioned fluid structure interaction (FSI) strategy is presented in this paper. The study of fiber motion during injection modeling process in composite material manufacturing is the motivation of this research. Fluid field is solved by mixed finite element method (FEM). Stokes equation is used as governing equation due to low Reynolds number characteristics. Total Lagrangian (TL) incremental FEM is used for calculating flexible fiber translation, rotation and deformation. Bathe time integration method is used for solving fiber dynamics. The interaction between the fiber (solid) and fluid results from forces on the surface of fiber, which are extracted from fluid solver and are exerted on fiber, solved as the natural boundary condition. After solid solver running forward a certain time step, new positions and velocities of each node on fiber surface are obtained which can be used for updating fluid mesh domain and served as essential boundary condition of fluid field. The process continues until the motion and deformation of fiber are obtained. Example of rigid and flexible fiber motion in a simple shear flow and a Poiseuille flow are presented in this paper. The simulation of rigid fiber are very close to the result of Jeffery’s theory. Snake rotation can be observed when the flexible fiber was tested in Poiseuille flow.

Simulation of shape dynamics of a long flexible fiber in a turbulent flow in the hydroentanglement process

Heritage buildings are a crucial aspect of a country’s cultural heritage, serving as a means of preserving and passing down its history and traditions to future generations. The heritage buildings in southern China possess significant conservation, utilization, and research value. However, research is lacking on the spatial distribution characteristics and subdivision types of these buildings in the region. This study aimed to investigate the spatial agglomeration and distribution characteristics of heritage buildings in southern China, as well as the factors contributing to the formation of these spatial distribution patterns. This article focused on the protection of 981 heritage buildings in southern China since the founding of China. The study examined the buildings’ spatial agglomeration and distribution characteristics from various dynasties and subdivided types. It utilized the average nearest neighbor analysis, unbalance index, and kernel density estimation to analyze this distribution. Additionally, this study also investigated the primary factors influencing the spatial distribution and differentiation of these buildings. The results demonstrated the following: (1) In general, the spatial distribution of heritage buildings in southern China is characterized by unevenness and clustering, with a concentration in the eastern coastal and Sichuan provinces. (2) In terms of temporal dimension, the spatial distribution of heritage buildings exhibits unique characteristics in various dynastic zones. (3) In the type dimension, the number of different types of heritage buildings varies greatly. (4) Further analysis of the distribution and types of heritage buildings indicates that quantitative differences are primarily influenced by natural, human, and socio-economic factors. This research was unique as it explored the geospatial distribution characteristics and determinants of heritage buildings. It offers a valuable perspective on the spatial distribution of heritage buildings and can serve as a reference for future studies on the preservation and protection of such buildings in China. Additionally, the findings can provide guidance for the management and rational use of heritage buildings in southern China.

In this paper, a novel technique for drag reduction in turbulent flows is presented. The technique involves the modification of the large scales of turbulent flows and is a passive approach. The lateral transport of momentum, which is a dominant mechanism in turbulence, is attenuated by the introduction of moving shear-free surfaces (SFSes). This brings about a reduction in the drag. 2D simulations have been carried out for a turbulent channel flow using shear stress transport (SST) Reynolds-averaged Navier–Stokes (RANS) model and validated with the available experimental results. The interaction between the plates and the fluid is two way, and is enforced either by the use of a rigid body solver with moving mesh, or by considering the SFSes to be fixed at particular locations and then updating the velocities of the plates at those locations. The latter is equivalent to solving a fully developed flow in the moving mesh case. The number, shape, size and placement of the SFSes strongly influence the amount of drag reduction. The phenomenon is confirmed to be governed by a ‘slow’ turbulent time scale. Further, the efficacy of the method is seen to depend on the ratio of two time scales – an advection time scale indicating the ‘resident time’ near an SFS, and the turbulent time scale. In addition, the effectiveness of the approach is improved by judicious placement of multiple SFSes in the flow.

This review delves into the dynamics of fibre-laden turbulent flows, a field that has garnered substantial attention due to its relevance in both natural and engineering contexts. The focus here is mainly on finite-size fibres, those exceeding the Kolmogorov scale, diverging from the commonly studied smaller ones. The study synthesises current understanding of the behaviour and organisation of both rigid and flexible finite-size fibres within turbulent flows, underscoring the added complexity these anisotropic particles introduce compared to their spherical counterparts. The influence of the length, the curvature and the inertia on the dynamics of rigid and flexible fibres is addressed. Fibre-based novel experimental methods, such as Fibre Tracking Velocimetry, are highlighted. Ultimately, this paper seeks to provide a clearer picture of the intricate dynamics at play in fibre-laden turbulent flows and their practical implications in various fields.

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.

The DOE National Nuclear Security Administration (NNSA) Stockpile Stewardship Program (SSP) is an integrated technical program for maintaining the safety, security, and reliability of the U.S. nuclear stockpile. The SSP incorporates nuclear test data, computational modeling and simulation, and experimental facilities to advance understanding of nuclear weapons. The suite of data analyzed comes from activities including previous nuclear tests, stockpile surveillance, experimental research, and development and engineering programs. This integrated national program requires the continued use of experimental facilities and the computational capabilities to support the SSP missions. These component parts, in addition to an appropriately scaled production capability, enable NNSA to support stockpile requirements. The ultimate goal of the SSP, and thus of the Advanced Simulation and Computing (ASC) program, is to ensure that the U.S. maintains a safe, secure, and effective strategic deterrent. The ASC program is a cornerstone of the SSP, providing simulation capabilities and computational resources to support the annual stockpile assessment and certification process, study advanced nuclear weapons design and manufacturing processes, analyze accident scenarios and weapons aging, and provide the tools to enable stockpile Life Extension Programs (LEPs) and the resolution of Significant Finding Investigations (SFIs). This work requires a balance of resources, including technical staff, hardware, simulation software, and computer science solutions. The ASC program focuses on increasing the predictive capabilities in a three-dimensional (3D) simulation environment while maintaining support to the SSP. The Program continues to improve its unique tools for understanding and solving progressively more difficult stockpile problems (sufficient resolution, dimensionality, and scientific details), and quantifying critical margins and uncertainties. Resolving each issue requires increasingly difficult analyses because the aging process has progressively moved the stockpile further from the original test base. While the focus remains on the U.S. nuclear weapons program, where possible, the Program also enables the use of high-performance computing (HPC) and simulation tools to address broader national security needs, such as foreign nuclear weapon assessments and nuclear counterterrorism. The 2022 Nuclear Posture Review (NPR) calls for NNSA to “deliver a modern, adaptive nuclear security enterprise based on an integrated strategy for risk management, production-based resilience, science and technology innovation, and workforce initiatives.” Furthermore, “NNSA will establish a Science and Technology Innovation Initiative to accelerate the integration of science and technology (S&T) throughout its activities.” Executing this strategy necessitates the continued emphasis on developing and sustaining high-quality scientific and engineering staff, as well as supporting computational and experimental capabilities. These components constitute the foundation of the nuclear weapons program. The continued success of the SSP and LEPs is predicated upon the ability to credibly certify the stockpile, without a return to underground nuclear tests (UGTs). Shortly after the nuclear test moratorium entered into force in 1992, the Accelerated Strategic Computing Initiative (ASCI) was established to provide an extensive simulation capability to underpin stockpile certification. While computing and simulation have always been essential to the success of the nuclear weapons program, the program goal of ASCI was to execute NNSA’s vision of using these tools in support of the stockpile stewardship mission. The ASCI program was essential to the successful demonstration of the SSP, providing critical nuclear weapons simulation and modeling capabilities. ASCI officially evolved into the ASC program in fiscal year (FY) 2005, but the mission remains essentially the same: provide the simulation and computational capabilities that underpin the ability to maintain a safe, secure, effective nuclear weapon stockpile, without returning to underground nuclear testing. The capabilities that the ASC program provides at the national laboratories play a vital role in the nuclear security enterprise and are necessary for fulfilling the stockpile stewardship and life extension requirements outlined for NNSA. The Program develops modern simulation tools that provide insights into stockpile aging issues, provide the computational and simulation tools that enable designers and analysts to certify the current stockpile and life-extended nuclear weapons, and inform the decision-making process when any modifications in nuclear warheads or the associated manufacturing processes are deemed necessary. Furthermore, ASC is enhancing the predictive simulation capabilities that are essential to evaluate weapons effects, design experiments, and ensure test readiness. The ASC program continues to improve its unique tools to solve stockpile problems— with a focus on sufficient resolution, dimensionality, and scientific detail—to enable Quantification of Margins and Uncertainties (QMU) and to resolve the increasingly difficult analyses needed for stockpile stewardship. The needs of the Stockpile Management and Production Modernization programs (formerly Directed Stockpile Work) also drive the requirements for simulation and computational resources. These requirements include planned LEPs, stockpile support activities, and mitigation efforts against the potential for technical surprise. All of the weapons within the current stockpile are in some stage of the life extension process. The simulation and computational capabilities are crucial for successful execution of these life extensions and for ensuring NNSA can certify these life-extended weapons without conducting a UGT.

A model for the particle-level simulation of multiple flexible fibers moving in a wall-bounded fluid flow

It is of great significance to study the spatial and temporal distribution characteristics and influencing factors of national key cultural relics protection units in the Yangtze River Delta region. Taking the national key cultural relics protection units in the Yangtze River Delta as an example, the spatial and temporal distribution characteristics and influencing factors of the sites were studied by using the methods of nearest proximity index, spatial density, standard deviation ellipse, and geographic detector. It can be concluded that the number of key cultural relics under protection in the Yangtze River Delta region is concentrated in the Song, Yuan, Ming and Qing Dynasties and since modern times, with the main types of ancient buildings, and the largest number in the 100-year time scale since modern times. On the whole, the spatial density of the national insurance unit has formed contiguous areas dominated by “Shanghai, Suzhou, and Wuxi,”“Nanjing, Yangzhou,”“Hangzhou and Shaoxing” and single core areas dominated by Ningbo and Huangshan, with the spatial distribution characteristics of “three contiguous areas and two single cores.” During the eight periods from prehistoric period to modern times, the national security units showed a state of agglomeration in spatial distribution, showing the transfer trend of “Southwest-northwest-southeast-southeast-southeast-northwest-northeast” and the evolution process of “agglomeration—dispersion—agglomeration—dispersion—agglomeration—agglomeration.” Natural environmental factors such as altitude and river network density, and socio-economic factors such as economic development level, abundance of scenic spots, abundance of intangible cultural heritage resources and total population have a significant impact on the spatial distribution of national protection units in the Yangtze River Delta.

A viscoelastic model for flexible fibers with material damping

The fuzzy, flexible fibers attached to biological surfaces not only reduce drag but also undergo nonlinear deformation under shear flow, and the deformation of these fibers in turn affects fluid flow. This phenomenon is increasingly attracting attention. Due to the complexity of fluid–structure interaction problems, existing analytical solutions such as perturbation methods, integral solutions, and series expansions are still unable to fully address this issue. To tackle this problem, this paper uses COMSOL finite element software for numerical calculations, effectively simulating the interaction between fibers and fluid flow. The results show that this method matches experimental data within a certain velocity range, extending the scope of previous studies and confirming the −1/2 scaling law at higher velocities. Furthermore, the study explores the bending deformation of elastic fibers and the flow field under different fluid loads. This paper provides a new approach for using finite element simulations to model the nonlinear deformation of such fibers, along with visual analysis, which is applicable to similar fluid–structure interaction problems and can provide valuable insights for future scientific research and engineering applications.

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.

Two-dimensional simulation of turbulent solid-liquid flow is carried out. The modeling is established for a two-phase flow of solid particles in a vertical pipe water flow. Governing equations of flow and turbulence field are solved in an Eulerian-Lagrangian approach by the use of k-τ (turbulence time scale) model and trajectories of the particles are obtained using the Lagrangian method with a deterministic inter-particle collision model. Comparison between the results of the model for mean and r.m.s velocities of the liquid and solid phase with the experimental results shows a good agreement. The effect of variation of particle density and concentration are studied.

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.

Anthropogenic fibres in the Baltic Sea water column: Field data, laboratory and numerical testing of their motion