Flexible Boundary Research Articles

Micromechanical modeling of hollow cylinder torsional shear test on sand using discrete element method

Previous studies on the hollow cylinder torsional shear test (HCTST) have mainly focused on the macroscopic behavior, while the micromechanical responses in soil specimens with shaped particles have rarely been investigated. This paper develops a numerical model of the HCTST using the discrete element method (DEM). The method of bonded spheres in a hexagonal arrangement is proposed to generate flexible boundaries that can achieve real-time adjustment of the internal and external cell pressures and capture the inhomogeneous deformation in the radial direction during shearing. Representative angular particles are selected from Toyoura sand and reproduced in this model to approximate real sand particles. The model is then validated by comparing numerical and experimental results of HCTSTs on Toyoura sand with different major principal stress directions. Next, a series of HCTSTs with different combinations of major principal stress direction (α) and intermediate principal stress ratio (b) is simulated to quantitatively characterize the sand behavior under different shear conditions. The results show that the shaped particles are horizontally distributed before shearing, and the initial anisotropic packing structure further results in different stress–strain curves in cases with different α and b values. The distribution of force chains is affected by both α and b during the shear process, together with the formation of the shear bands in different patterns. The contact normal anisotropy and contact force anisotropy show different evolution patterns when either α or b varies, resulting in the differences in the non-coaxiality and other macroscopic responses. This study improves the understanding of the macroscopic response of sand from a microscopic perspective and provides valuable insights for the constitutive modeling of sand.

An extended model to assess Jeffery–Hamel blood flow through arteries with iron-oxide (Fe2O3) nanoparticles and melting effects: Entropy optimization analysis

Abstract Nanofluids are utilized in cancer therapy to boost therapeutic effectiveness and prevent adverse reactions. These nanoparticles are delivered to the cancerous tissues under the influence of radiation through the blood vessels. In the current study, the propagation of nanoparticles within the blood in a divergent/convergent vertical channel with flexible boundaries is elaborated computationally. The base fluid (Carreau fluid model) is speculated to be blood, whereas nanofluid is believed to be an iron oxide–blood mixture. Because of its shear thinning or shear thickening features, the Carreau fluid model more precisely depicts the rheological characteristics of blood. The arterial section is considered a convergent or divergent channel based on its topological configuration (non-uniform cross section). An iron oxide ( F e 2 O 3 {\rm{F}}{{\rm{e}}}_{2}{{\rm{O}}}_{3} ) nanoparticle is injected into the blood (base fluid). To eliminate the viscous effect in the region of the artery wall, a slip boundary condition is applied. An analysis of the transport phenomena is preferred using the melting heat transfer phenomena, which can work in melting plaques or fats at the vessel walls. The effects of thermal radiation, which is advantageous in cancer therapy, biomedical imaging, hyperthermia, and tumor therapy, are incorporated in heat transport mechanisms. The governing equation for the flow model with realistic boundary conditions is numerically tickled using the RK45 mechanism. The findings reveal that the flow dynamism and thermal behavior are significantly influenced by melting effects. Higher Re \mathrm{Re} can produce spots in which the track of the wall shear stress fluctuates. The melting effects can produce agitation and increase the flow through viscous head losses, causing melting of the blockage. The maximum heat transfer of 5 % 5 \% is achieved with We {\rm{We}} when the volume friction is kept at 1 % 1 \% . With higher estimation of inertial forces Re \mathrm{Re}\hspace{1em} and same volume friction, the skin drag coefficient augmented to 34 % 34 \% . The overall temperature is greater for the divergent flow scenario.

Empowered or overwhelmed? Procrastination extinguishes the positive effects of work flexibility on work–family conflict

AbstractProviding flexibility at work is the most pervasive tool for organizations to help employees manage the work–family (WF) interface. Extant research, however, indicates that flexibility does not consistently reduce WF conflict. This paper reports on two studies that contribute to our understanding of how, and for whom, perceived work flexibility improves these outcomes. We extend work on the mechanisms by which flexibility influences outcomes and extend conservation of resources theory using choice overload theory to understand the boundaries of flexibility as a positive resource, specifically, in the form of procrastination. In Study 1, we found that perceived work flexibility was negatively related to subjective work demand for those low on procrastination. In Study 2, we replicate these effects and extend them by finding that effects of flexibility on WF conflict were mediated by its influence on subjective work demand. We discuss the implications of these findings for future research and practice around flexibility in the WF interface.

A GPU accelerated three-dimensional ghost cell method with an improved implicit surface representation for complex rigid or flexible boundary flows

This paper presents an efficient and reliable three-dimensional (3D) ghost cell method for complex rigid or flexible boundary flows. An improved implicit interface representation method is developed to treat the arbitrary complex interfaces including the thin or sharp boundaries. This method is robust and avoids the solving of a large dense matrix, and thus the computational efficiency is greatly enhanced. The ghost cell method with a unified eight-node interpolation scheme is adopted to enforce the no-slip boundary conditions on complex boundaries. To perform 3D high-resolution simulation on a desktop computer, a GPU parallel framework in CUDA environment is developed to significantly accelerate the computational efficiency of the present solver. The horizontal oscillations of a two-dimensional (2D) cylinder and a 3D sphere in the rest fluid are simulated to validate the accuracy and computational performance of the present model. Spatial accuracy of near second-order on the boundary treatment and good grid convergence are confirmed. Also, the present solver can achieve hundreds of acceleration ratios for 2D grids and thousands of acceleration ratios for 3D grids. Then, uniform flows around a 3D arbitrary rigid or flexible object such as a stationary cylindrical cylinder, a stationary square cylinder, an oscillating airfoil, and a swimming carangiform are simulated. The accuracy and capability of the present model for 3D complex geometries with large-amplitude movement or deformation is satisfactorily validated. Moreover, typical 3D wake structures are well captured.

Lattice Boltzmann simulation of deformable fluid-filled bodies: progress and perspectives.

With the rapid development of studies involving droplet microfluidics, drug delivery, cell detection, and microparticle synthesis, among others, many scientists have invested significant efforts to model the flow of these fluid-filled bodies. Motivated by the intricate coupling between hydrodynamics and the interactions of fluid-filled bodies, several methods have been developed. The objective of this review is to present a compact foundation of the methods used in the literature in the context of lattice Boltzmann methods. For hydrodynamics, we focus on the lattice Boltzmann method due to its specific ability to treat time- and spatial-dependent boundary conditions and to incorporate new physical models in a computationally efficient way. We split the existing methods into two groups with regard to the interfacial boundary: fluid-structure and fluid-fluid methods. The fluid-structure methods are characterised by the coupling between fluid dynamics and mechanics of the flowing body, often used in applications involving membranes and similar flexible solid boundaries. We further divide fluid-structure-based methods into two subcategories, those which treat the fluid-structure boundary as a continuum medium and those that treat it as a discrete collection of individual springs and particles. Next, we discuss the fluid-fluid methods, particularly useful for the simulations of fluid-fluid interfaces. We focus on models for immiscible droplets and their interaction in a suspending fluid and describe benchmark tests to validate the models for fluid-filled bodies.

A self-sustaining mechanism for plane Couette flow with a flexible lower boundary

Abstract The stability of plane Couette flow to travelling-wave disturbances is studied asymptotically at high Reynolds numbers Re when the lower boundary possesses a degree of flexibility modelled by a spring-backed plate. First, it is shown that a three-dimensional (3D) linear instability exists, with streamwise and spanwise wavelengths comparable with the channel width. Building on this, nonlinear effects from the self-interaction of the wave are introduced, leading to a self-sustaining interaction between a roll/streak flow and the 3D wave. Governing nonlinear vortex-wave interaction (VWI) equations are derived and a perturbation analysis is carried out to guide a numerical investigation of the equations. The co-existence of two families of finite-amplitude solutions, each with different flow structures, is found. Numerical solutions of the VWI equations in each case show that a small wave amplitude of O(Re−1(log Re)−1/2) is all that is necessary to provoke an O(1) change to the basic Couette flow.

Enhancing tool dynamics and stability in internal turning with an adjustable clamping device under variable cutting conditions

This paper presents a novel method for suppressing the vibration of internal turning tools by adjusting clamping conditions. First, an adjustable device, incorporating multiple clamping modes, is developed to shift the tool frequency and improve the tool stiffness and damping. A multitude of clamping schemes can be generated by combining different clamping modes, and these schemes are subsequently quantitatively characterized as a group of multi-sourced boundary constraints. Building upon this foundation, the dynamical model for slender turning tools equipped with the designed device incorporates the effect of flexible boundary constraints. This incorporation is achieved through an inversion method that considers the uniqueness of the solution, facilitating accurate identification. Then, a cutting conditions-dependent adjustment strategy is developed to enhance tool dynamics and turning stability. The simulation results clearly illustrate the device’s capability to continuously and effectively adjust tool dynamics over a wide range. Consequently, the optimized clamping scheme can constantly adapt to the variable cutting conditions and the tool stability domain is further improved. Finally, a series of internal turning tests are carried out on difficult-to-machine materials. The experimental results demonstrate the effectiveness of the clamping device and the corresponding adjustment strategy in suppressing tool vibration stemming from various sources, no matter what tool structures and cutting parameters are pre-selected. As a result, the surface quality is improved, and the tool life is extended.

Assessing flexibility in networked multi-energy systems: A modelling and simulation-based approach

In response to the uncertainty and volatility arising from renewable sources, there is a growing need for enhanced flexibility within the energy system to maintain a continuous balance between power generation and demand. In this context, interest is growing around the so-called Multi-Energy Systems (MES) where different energy vectors coexist and optimally interact through conversion technologies and energy networks, creating additional flexibility opportunities. Nevertheless, there exists a gap in research regarding the impact of the network on flexibility availability. Typically, these complex systems are treated as power nodes or energy hubs without comprehensive network considerations. For this reason, the paper aims to propose a methodology and a tool to evaluate flexibility in a Multi-Energy System, considering not only the individual devices in place and the users' demands but also their interactions with the physical energy network. In detail, a simulation-based methodology is developed and described, and finally tested on a Case Study. As a result, both the physical and operational flexibilities (UP-flex and DOWN-flex) of the system regarding the electrical vector were obtained analytically and graphically. Particular attention was given to the evolution of key temperatures within the district heating network and the thermal power produced by the central unit in various flexibility scenarios. The outcomes demonstrate the utility of this tool for defining flexibility boundaries and profiles, as well as for assessing whether the flexibility demanded by grid operators aligns with the physical constraints of the network.

The linear stability of plane Couette flow with a compliant boundary

The linear stability of plane Couette flow subject to one rigid boundary and one flexible boundary is considered at both finite and asymptotically large Reynolds number. The wall flexibility is modelled using a very simple Hooke-type law involving a spring constant K and is incorporated into a boundary condition on the appropriate Orr–Sommerfeld eigenvalue problem. This problem is analyzed at large Reynolds number by the method of matched asymptotic expansions and eigenrelations are derived that demonstrate the existence of neutral modes at finite spring stiffness, propagating with speeds close to that of the rigid wall and possessing wavelengths comparable to the channel width. A large critical value of K is identified at which a new short wavelength asymptotic structure comes into play that describes the entirety of the linear neutral curve. The asymptotic theories compare well with finite Reynolds number Orr–Sommerfeld calculations and demonstrate that only the tiniest amount of wall flexibility is required to destabilize the flow, with the linear neutral curve for the instability emerging as a bifurcation from infinity.

Bistable morphology analysis of the flexible single-vertex origami unit cell

In origami structures, it is necessary to study the novel properties of the single-vertex origami unit cell first, which is the basic unit that could be assembled into a large-scale origami tessellation with complex patterns. When the elasticity of the sheet and the kinematics of the crease are combined, nearly all single-vertex origami unit cells with an arbitrary crease pattern are foldable and bistable. In this case, the stable morphology of the unit cell is dominated by both the crease rotation and the shell deformation. In this work, two models are proposed for the solution of the bistability of the flexible origami unit cell. First, a flexible boundary conical shell model is proposed, by introducing crease mechanics based on the previous work. This linear model can be easily solved for flexible f-cones with arbitrary crease patterns by a certain assembly scheme. Moreover, to consider the nonlinearity, the discrete creased model (DCM) is further developed for the stable state solution of the arbitrary origami unit cell. Thereafter, the results obtained by the two models are verified with the corresponding FE simulations. Based on the normalized crease stiffness, the coupling effect between the crease and the shell is analyzed. The core deformation mechanism of flexible origami unit cells is classified into three cases: the crease-dominated case, the crease-shell coupled case, and the shell-dominated case. Additionally, the energy ratio of crease rotation to shell bending at the snapping state is found to be approximately proportional to the normalized crease stiffness.

Influence of microscopic parameters on the macroscopic mechanical response of sand

In order to study the influence of microscopic parameters on the macroscopic mechanical response of sand, the discrete element software PFC3D is used to establish the numerical model of the triaxial test of sand in this paper, the indoor triaxial test of sand is carried out to calibrate the numerical model. The triaxial tests are finished under the conditions of confining pressures of 100 kPa, 200 kPa, 300 kPa, and 400 kPa, respectively. The influence of the microscopic parameters on the macroscopic mechanical response of the sand is analyzed, the orthogonal experimental method is used to study the sensitivity of the numerical model to microscopic parameters such as porosity, flexible boundary stiffness, normal tangential stiffness ratio, and friction coefficient. The results show that the peak strength and residual strength of the stress-strain curve of sand are most affected by the friction coefficient, they are positively correlated with the friction coefficient under different confining pressures. The normal tangential stiffness ratio is negatively correlated, it is less affected by porosity and boundary flexibility stiffness, the sensitivities of different microscopic parameters are analyzed through the orthogonal tests, the results of orthogonal tests and numerical simulations show the consistent patterns in terms of sensitivity to microscopic parameters. The results can provide the reference for the influence of microscopic parameters and the macroscopic mechanical response of sand.

I wouldn't be working this way if I had a family - Differences in remote workers' needs for supervisor's family-supportiveness depending on the parental status

This study investigates how working remotely blurs the boundaries between work and non-work domains by contrasting the experiences of employees with different parental status. The study further shows how leaders can mitigate this blurring via family-supportive supervisor behaviours (FSSB), and extends the concept to encompass non-work roles beyond the family. Working from home leads to an increasing intertwining of work and non-work roles, with family status playing a significant role in shaping boundary challenges and support needs. Through semi-structured interviews with 89 employees working from home in various industries, the study reveals that parents and non-parents, distinct in their challenges and requirements, exhibit varied demonstrated needs from their leaders. As parent employees require flexible boundaries to attend to their family responsibilities, non-parent employees need safeguards to maintain boundaries around their private life. The results underscore that FSSB benefit employees regardless of parental status. This study emphasizes the importance of employers tailoring their work-life programs to accommodate the diverse needs of employees, and recognizes the pivotal role of supervisors in attuning their supportive behaviours to employees' work-nonwork boundary needs and preferences.

SAE-PPL: Self-guided attention encoder with prior knowledge-guided pseudo labels for weakly supervised video anomaly detection

Recently, many weakly supervised video anomaly detection (WS-VAD) methods focus on generating reasonable pseudo-labels for frames by using video-level annotations. To achieve this goal, some existing label-noise cleaning techniques and pseudo-label generators are used by them, but their performance is limited. The main reason is that the useful prior knowledge, i.e., the graduality of anomaly events, is ignored. Here, we propose a Self-Training Framework, which assumes flexible soft boundaries between abnormal and normal clips. The framework consists of: (1) A prior knowledge guided pseudo-label generator that incorporates prior knowledge of video segment distributions into the MIL framework to generate high-confidence pseudo labels; (2) An improved self-guided attention encoder is developed to capture multiscale long-term spatiotemporal features, where dependencies among anomaly frames are preserved. Moreover, a pseudo-label based self-training scheme is adopted to supervise the encoder. Experimental results verify the superiority of our method over baseline approaches.

Proximity-based density description with regularized reconstruction algorithm for anomaly detection

This study addresses unsupervised anomaly detection using one-class classification, which constructs a decision boundary to determine if a new instance belongs to the target class. Existing one-class classification methods often fail in real-world scenarios due to their sensitivity to noise and inability to handle complex structures. We propose a proximity-based density description with a regularized reconstruction algorithm to overcome these limitations. Our method defines density-descriptive coefficients to reconstruct initial density and derives optimal coefficients by minimizing reconstruction error subject to sparsity and smoothness constraints. The sparsity constraint reduces noise effects, while the smoothness constraint encourages a flexible decision boundary. We evaluate our algorithm on benchmark datasets and compare it to existing methods, demonstrating superior performance.

National-scale bi-directional EV fleet control for ancillary service provision

Deploying real-time control on large-scale fleets of electric vehicles (EVs) is becoming pivotal as the share of EVs over internal combustion engine vehicles increases. In this paper, we present a Vehicle-to-Grid (V2G) algorithm to simultaneously schedule thousands of EVs charging and discharging operations, that can be used to provide ancillary services. To achieve scalability, the monolithic problem is decomposed using the alternating direction method of multipliers (ADMM). Furthermore, we propose a method to handle bilinear constraints of the original problem inside the ADMM iterations, which changes the problem class from Mixed-Integer Quadratic Program (MIQP) to Quadratic Program (QP), allowing for a substantial computational speed up. We test the algorithm using real data from the largest carsharing company in Switzerland and show how our formulation can be used to retrieve flexibility boundaries for the EV fleet. Our work thus enables fleet operators to make informed bids on ancillary services provision, thereby facilitating the integration of electric vehicles.

A Statistical Damage Constitutive Model of Hydrate-Bearing Sediments Based on DEM Tests with Flexible Membrane Boundaries

A Statistical Damage Constitutive Model of Hydrate-Bearing Sediments Based on DEM Tests with Flexible Membrane Boundaries

Numerical study on the flow-induced noise from waterjet-propelled ship regarding a flexible boundary

In hydroacoustics, it is generally acknowledged that a half-space is bounded by a flexible boundary with non-constant reflection coefficients. However, limited research has focused on the impact of the flexible boundary on noise characteristics. In this paper, a hybrid method combining the unsteady Reynolds-averaged Navier-Stokes (URANS) equations with the Ffowcs-Williams–Hawkings (FW–H) equation is presented. Firstly, the hybrid method coupled with the mirror source method (MS method) is applied to predict noise in a half-space with a rigid boundary for validation. Secondly, the proposed method is extended to predict the underwater noise generated by a medium-speed, mono-hull ship. Specifically, the differences between the noise fields in a half-space and free-space are considered and investigated comprehensively. The results reveal that: (1) the hybrid method performs well in predicting the overall noise induced by complex-shaped configurations in a half-space bounded by different boundaries, and (2) the half-space boundary can influence the noise field due to constructive\\destructive interferences. Additionally, (3) the quadrupole noise generated in the non-uniform wake of a waterjet-propelled ship should be considered. The proposed method has provided an efficient approach to predict the flow-induced noise radiated from a waterjet-propelled ship regarding a flexible boundary.

Numerical multi‐span segmented trapdoor test with viscoelastic boundary for soil arching within pile‐supported embankment under cyclic loading

AbstractDynamic wave reflections would interfere with the laboratory observation of the soil arching under cyclic loading when a traditional trapdoor model with rigid boundaries was used. This problem would be prominent when investigating the soil arching in pile‐supported embankments where the stationary support (pile) would be small, making the rigid boundary close to the domain of interest affected by dynamic loads. To avoid dynamic wave reflections and narrow the calculation domain, soil arching within a pile‐supported embankment under cyclic loading is numerically investigated in a multi‐span segmented trapdoor model with flexible boundaries by the particle flow code (PFC). In addition, a pavement structure is included and the springs supporting the trapdoor follow a bi‐linear constitutive model. The energy‐absorbing flexible boundary at the bottom weakens the dynamic wave reflections and is in favor of passing vertical dynamic waves to compact the fill and weaken the influence of cyclic loading on the soil arching effect during the loading stage. Lateral flexible boundaries would facilitate realistically reproducing the effect of cyclic loading on the soil arching because they could consider the weakening of the plugging effect that occurs during the lateral expansion. The development of soil arches of mid‐span and side‐span are different when the load applied at the mid‐span is laterally transferred to adjacent spans by the arching effect.

Vibration Control of Marine Flexible Riser with Uncertain Model

When flexible risers in the ocean are subjected to various environmental loads in the ocean, vibration will inevitably occur. Due to the nonlinearity and uncertainty of the actual riser system, it is impossible to obtain an accurate mathematical model of the riser system. The existing control algorithms designed based on the precise model of the riser system cannot meet the actual control requirements. A flexible riser boundary control method based on fuzzy control algorithm is proposed to suppress the vibration of marine flexible riser systems with uncertain models. Firstly, the dynamic models of existing marine flexible risers were studied. In response to the uncertainty of the riser system model, the control experience was transformed into an automatic control strategy in the form of fuzzy language control rules to improve the adaptability of the controller. Then, combining boundary control technology with fuzzy control, a boundary controller is constructed to stabilize the riser in a small neighborhood of its original position and reduce fatigue damage to the riser. Finally, the effectiveness of the designed fuzzy boundary control algorithm was verified through MATLAB simulation results, and the vibration suppression effect of the algorithm on the vertical tube was better than that of traditional PD control.

Numerical simulation of biaxial undrained shear test based on fluid–solid coupling DEM method

The undrained shear test plays a crucial role in comprehending the deformation and failure mechanisms inherent in rock and soil. While the test offers a macroscopic perspective, numerical simulations can delve into the microscopic aspects of sample behavior, thereby compensating for this limitation. Drawing inspiration from the measurement sphere fluid–solid coupling method and the discrete element method, this paper proposes a tailored numerical simulation approach for the biaxial undrained shear test. This approach is implemented in the high-performance discrete element software MatDEM. In this study, a rapid division method for the fluid network is presented, along with a detailed explanation of the control equation governing excess pore water pressure and fluid–solid interaction during shear. Subsequently, A custom flexible boundary with a relatively smooth surface is introduced to simulate the constraint of the rubber membrane, which helps reduce simulation errors. Furthermore, the specific process of the numerical simulation is introduced, and an effective method for generating samples with high void ratios is proposed. Finally, three different cases are presented, involving varying confining pressures, different void ratios, and specific shear paths, respectively. The results indicate that the proposed method can effectively simulate the process of undrained shear tests and accurately capture the excess pore water pressure responses, and lateral deformation of the samples during shearing.