Wall Force Research Articles

Structure and dynamics of hydrodynamically interacting finite-size Brownian particles in a spherical cavity: Spheres and cylinders.

The structure and dynamics of confined suspensions of particles of arbitrary shape are of interest in multiple disciplines from biology to engineering. Theoretical studies are often limited by the complexity of long-range particle-particle and particle-wall forces, including many-body fluctuating hydrodynamic interactions. Here, we report a computational study on the diffusion of spherical and cylindrical particles confined in a spherical cavity. We rely on an immersed-boundary general geometry Ewald-like method to capture lubrication and long-range hydrodynamics and include appropriate non-slip conditions at the confining walls. A Chebyshev polynomial approximation is used to satisfy the fluctuation-dissipation theorem for the Brownian suspension. We explore how lubrication, long-range hydrodynamics, particle volume fraction, and shape affect the equilibrium structure and the diffusion of the particles. It is found that once the particle volume fraction is greater than 10%, the particles start to form layered aggregates that greatly influence particle dynamics. Hydrodynamic interactions strongly influence the particle diffusion by inducing spatially dependent short-time diffusion coefficients, stronger wall effects on the particle diffusion toward the walls, and a sub-diffusive regime-caused by crowding-in the long-time particle mobility. The level of asymmetry of the cylindrical particles considered here is enough to induce an orientational order in the layered structure, decreasing the diffusion rate and facilitating a transition to the crowded mobility regime at low particle concentrations. Our results offer fundamental insights into the diffusion and distribution of globular and fibrillar proteins inside cells.

A head-to-head comparison of wall motion score index, force, strain, and ejection fraction for the prediction of SYNTAX and Gensini coronary scores by dobutamine stress echocardiography.

Predicting the severity of coronary artery disease (CAD) may be possible during dobutamine stress echocardiography (DSE) with various indices of left ventricular function. We assessed the relative value of ejection fraction (EF), force, global longitudinal strain (GLS), and wall motion score index (WMSI) in predicting SYNTAX and Gensini scores in patients with known or suspected CAD. We prospectively enrolled 223 patients (120 men; mean [SD] age, 62 [9] years) and assessed the following indices: 1) EF (with triplane imaging); 2) force, calculated as the ratio of systolic blood pressure to left ventricular end‑systolic volume; 3) GLS; 4) WMSI. All patients underwent coronary angiography within 12 weeks with SYNTAX and Gensini scores evaluation. The correlation of SYNTAX and Gensini scores was highest with peak WMSI (SYNTAX, rho = 0.591; Gensini, rho = 0.612; P<0.001), intermediate with peak force (SYNTAX, rho = -0.346; Gensini, rho = -0.377; P<0.001) and GLS (SYNTAX, rho = -0.205; P = 0.002 and Gensini rho = -0.216; P = 0.001), and the weakest for EF (SYNTAX, rho = -0.149; P = 0.03 and Gensini, rho = -0.191; P = 0.006). The similar hierarchy of variables was detected for changes during DSE. In the subgroup after myocardial infarction (n = 66 [27%]), GLS outperformed the force. In patients with CAD at DSE peak, theWMSI and force were better predictors of the coronary SYNTAX and Gensini scores and CAD severity than GLS or EF. However, in patients after myocardial infarction, the GLS correlation with coronary scores improved and got closer to the visual assessment.

Experimental Study on the Deformation of a Cut-Off Wall in a Landfill

To effectively solve the leakage problem of landfill leachate, it is necessary to establish a vertical cut-off wall around the landfill site for seepage control. To accurately analyze the deformation of the cut-off wall, a 1/8-scale landfill cut-off wall miniature model is designed and fabricated. The actual force of the cut-off wall is simulated by filling sand on the outer side of the model wall. Meanwhile, a nonlinear finite element model of the wall is established using the ANSYS to simulate the deformation of the seepage wall. The results show that the cut-off wall made from a polyvinyl alcohol-bentonite-fly ash-cement (PBFC) anti-seepage slurry has a high ultimate bearing capacity, and its ultimate strain is approximately 5%, which is much higher than the limit strain of the actual force conditions of the cut-off wall. The horizontal displacement of the cut-off wall is approximately linear and decreases gradually from the top to the bottom, and no reverse displacement is observed. The main stress of the wall is smaller than that of the slurry, and there is no tensile stress. The simulated results are similar to the measured values of the cut-off wall, which indicates the accuracy of the simulation analysis.

Interplay of wall force field and wall physical characteristics on interfacial phenomena of a nano-confined gas medium

The effect of the wall force field and the wall physical characteristics on the interfacial phenomena of a 5.4 nm nanoconfined gas medium are investigated by applying three-dimensional molecular dynamics simulations. Assuming that a range of 1 nm from each wall is affected by the wall force field, the gas distribution changes notably in this region which covers 40% of the channel height. Therefore, a combined effect of the wall force field and the wall stiffness, its mass as well as the interaction strength determines the interfacial phenomena such as interfacial thermal resistance (ITR) at the gas/solid interface. The increment in interaction strength of the gas/solid atoms leads to an increase in the amount of adsorbed gas on the wall which reduces the temperature jump and ITR notably. When the ratio between the interaction strengths of the gas/solid to the gas/gas atoms exceeds 4, the temperature jump and the ITR reduce to zero. As the wall becomes stiffer, the temperature jump and consequently the ITR are increased considerably. Simultaneously, the temperature distribution in the gas medium along the channel height becomes more and more independent of the gas density as the wall stiffness increases. It is observed that as the ratio between the mass of the wall and the gas atoms increases up to 2, the temperature jump reduces notably. Meanwhile, as the wall atoms become heavier, an increment in the temperature jump is observed. Consequently, the minimum ITR occurs at a ratio of 2. It is also shown that the temperature profile in the bulk region of the channel is approximately independent of the wall mass. Considering several metals as the wall material, our analysis reveals that silver, copper, and nickel experience the least ITR among the others.

Multi-Fluid Computational Fluid Dynamic Predictions of Turbulent Bubbly Flows Using an Elliptic-Blending Reynolds Stress Turbulence Closure

The accurate prediction of bubbly flows is critical to many areas of nuclear reactor thermal hydraulics, mainly, but not only, in relation to the key role bubble behaviour plays in boiling flows. Large scale computations of flows with hundreds of thousands of bubbles are possible at a reasonable computational cost using computational fluid dynamic, multi-fluid Eulerian-Eulerian models. The main limitation of these models is the need to entirely model interfacial transfer processes with proper closure relations. Here, the capabilities and advantages provided by a model that includes an elliptic-blending Reynolds stress turbulence closure (EB-RSM), allowing fine resolution of the velocity field in the near-wall region, are tested over a large database. This database includes mostly monodispersed bubbly flows over a wide range of operating conditions and geometrical parameters, including upward and downward pipe flows, large diameter pipes and a square duct. The model shows encouraging accuracy and robustness, with good agreement over most void fraction distributions and accurate prediction of the magnitude and position of the near-wall void fraction peak. The model does not include any wall force, avoiding all the related uncertainties, and the prediction of the void fraction peak relies on the fine resolution of the near-wall pressure gradient induced by the turbulence field. Overall, the EB-RSM allows accurate resolution of the velocity and turbulence field near the wall, and the transition to this and similar turbulence closures is of value in assisting the ongoing quest for thermal hydraulic models that are accurate and of general applicability. Additional modifications to the near-wall modelling approach, which is still based on its single-phase counterpart, may be required to deal with high void fraction conditions and, in the overall model, additional improvements to momentum and, most importantly, bubble-induced turbulence closures are desirable.

Study on the hydraulic characteristics of the gradually expanding and gradually contracting watercourse with different width-narrow ratios

The characteristics of the water flow in the gradually expanding and gradually contracting watercourse are very complicated, and the width-to-narrowness ratio has a significant influence on the hydraulic characteristics of the gradually expanding and gradually contracting watercourse. Five three-dimensional hydraulic calculation models with different width-to-narrowness ratios were established to study its hydraulic characteristics. The results show that: (1) river flow gradually appears as the recirculation zone with the increase of the width-to-narrowness ratio. The recirculation zone increases gradually with the increase of the width-to-narrowness ratio and the shapes are different. The flow velocity in the recirculation zone is negatively correlated with the width-to-narrowness ratio. (2) When the width-to-narrowness ratio is small, the shearing force distribution of gradually expanding and gradually contracting sections are basically in “Hump-shape” along the Y-axis; When the width-to-narrowness ratio is large, the section basically shows a trend of “parabola-like” change that increases first and then decreases near the shore, and the average shearing force of the expanding section is larger than that of the contracting section. (3) As the width-to-narrowness ratio of the watercourse increases, the shearing force at the widest part of the river section gradually decreases, and the shearing force of the side wall at the contracting section is larger than that of the expanding section.

Comparison of DEM predictions and measured wall-media contact forces and work in a vibratory finisher

Discrete element methods (DEM) have been widely used in predicting the behavior of vibrated granular media, including that in vibratory finishing (VF) processes. In VF loose abrasive particles are fluidized by a vibrating container and the particle vibration work on the surface of parts entrained in the granular bed. In the present work, DEM was used to predict the forces exerted by the container walls on the granular media and the results were compared with previous experimental measurements performed with varying frequencies of vibration and amounts of media in the container. It was found that the overall trends in the DEM simulations were comparable to the experiments results, in that wall forces increased significantly with frequency but less so with increasing media mass. This was attributed to the viscous forces of granular media deformation, which generated wall forces that tended to increase with increasing media deformation rate and with increasing media pressure, although these trends were less pronounced in the DEM models than in the experiments. The DEM predictions of the contact force magnitudes were, however, well predicted in only two of the three container locations. The DEM predictions were also characterized by much larger cycle-to-cycle variations in the wall-media contact behavior than was observed experimentally. This included the presence of apparently random vibration cycles without any wall-media contact, which were not present in the actual force measurements.

Contrastive study on the granular avalanche dynamic process using the depth-integrated continuum and discrete element methods

This paper carried out simulations of a channel flow experiment and a case study of granular avalanche flow using two kinds of methods, depth-integrated continuum method and discrete element method. In the simulation of the channel flow experiment, we presented a method for calculating normal force and bending moment of the rigid wall based on the depth-integrated continuum method. Although the peak value of the normal force and bending moment cannot be obtained using the depth-integrated continuum method, the residual value can be obtained rapidly and accurately. Then, Frank landslide in Canada was simulated using the two methods. Calculation results of the extent and duration of the granular avalanche were in good agreement with actual situation, which validated the correctness and effectiveness of the two methods. The velocity process, energy process, and risk zoning of the granular avalanche were obtained. Calculation results showed that the maximum average velocity of the granular avalanche was about 68 m/s at 20 s. The computational efficiency of the depth-integrated continuum method was better than that of the discrete element method, while the energy process of the granular avalanche could be better traced using the discrete element method. Detailed comparison and discussion were carried out in the simulations of the experiment and the case study which can provide a scientific reference and basis for the simulation of the granular avalanche flow.

Reduction of asymmetric wall force in JET and ITER disruptions including runaway electrons

It was shown that asymmetric vertical displacement event disruptions in JET and ITER do not produce a large asymmetric force on conducting structures surrounding the plasma, provided that the current quench (CQ) time is less than the resistive wall penetration time. This is verified with JET data. In ITER, the CQ is expected to be comparable to the resistive wall penetration time, which gives a small wall force. It is shown that runaway electrons (REs) in JET do not produce a large wall force, and this is verified with JET data. This occurs if the RE current is about half of the initial plasma current. In ITER, the RE current might be comparable to the initial current, and REs might lengthen the CQ time sufficiently to produce a large wall force.

Silica nanoparticles inhibit arsenic uptake into rice suspension cellsviaimproving pectin synthesis and the mechanical force of the cell wall

We investigated the mechanism explaining why silica nanoparticles inhibit arsenic uptake into rice at the single-cell level.

Tribological characterization of hybrid aluminum composite under boundary lubricating sliding conditions

Tribological behaviour of hybrid aluminium composite is studied under lubricated sliding conditions. The scope of this study is to determine the fundamental tribological properties of grapheme as secondary reinforcement and to evaluate its performance under boundary lubricating conditions. The hybrid aluminium composite is fabricated by rapid spark plasma sintered processing route. Variable concentration of Graphene Nano platelets (GNP) [0.5, 1, 2, 3, 4, 5 wt%] is added to the Al-Si-6 wt% Al2O3 composite. The lubricant used is PAO base oil and SAE20W50 Engine oil. The effect of the load and sliding distance is studied on the ball-on-disc tribometer. It is reported that COF remain stable with the increasing sliding distance (90 m–450 m) and decrease with the increasing load (10 N −40 N) and then attain a steady stable value after 40 N to 80 N load. The possibility behind the low friction and wear is the exceptional lubricating effect of the GNP particles layer which mixes with the lubricating oil. The GNP particle layer are the 2D Nano particles that form a low shear stress junction protective layer at the sliding interface, which cause easy shear because of weak Vander wall force drastically reduce friction and wear of the hybrid composite.

Postural and respiratory function of the abdominal muscles: A pilot study to measure abdominal wall activity using belt sensors

BACKGROUND: The abdominal muscles play an important respiratory and stabilization role, and in coordination with other muscles regulate intra-abdominal pressure (IAP) to stabilize the spine. OBJECTIVE: To examine a new, non-invasive method to measure activation of the abdominal wall and compare changes in muscle activation during respiration while breathing under a load, and during instructed breathing. METHODS: Thirty-five healthy individuals completed this observational crossover study. Two capacitive force sensors registered the abdominal wall force during resting breathing stereotype, instructed breathing stereotype and under a load. RESULTS: Mean abdominal wall force increased significantly on both sensors when holding the load compared to resting breathing (Upper Sensor: P&lt; 0.0005, d=-0.46, Lower Sensor: P&lt; 0.0005, d=-0.56). The pressure on both sensors also significantly increased during instructed breathing compared to resting breathing (US: P&lt; 0.0005, d=-0.76, LS: P&lt; 0.0005, d=-0.78). CONCLUSIONS: The use of capacitive force-sensors represent a new, non-invasive method to measure abdominal wall activity. Clinically, belts with capacitive force sensors can be used as a feedback tool to train abdominal wall activation.

Jet development and impact load of underwater explosion bubble on solid wall

The damage effects of an underwater explosion have always been a crucial problem in the ship mechanics. Notably, the bubble evolution and the jet impact load are one of the most difficult parts in the shock-resistance design of ship structures due to the discontinuities and significant nonlinear deformation. In this paper, the Eulerian finite-element method is introduced to continuously simulate the shock wave and non-spherical bubble evolution stages near a horizontal solid wall with the volume of fluid method and pressure balance technique used to resolve the multi-medium flow. The numerical model is established in a cylindrical coordinate system and validated by comparing the results with a spark-generated bubble experiment. After that, based on the present model, the shock wave propagation and the bubble evolution are simulated to study the characteristics of the impact loads of an underwater explosion. Besides, the influences of the wall location (upside or downside) and the stand-off distance from the wall are also analyzed. The results show that the features of the jet impact load are much more complicated than those of the shock wave. Nearby a downside wall, the buoyancy and Bjerknes force compete to dominate the bubble motion with opposite influences. By contrast, They enhance the effect of each other to develop a liquid jet towards the upside wall. The pressure peak, impact range, and duration time nonlinearly depend on the combination of the case parameters and are not monotonic to a single one. Within a proper range of the parameter combination, the jet impact load can reach its maximum and be more destructive than the shock wave because of a comparable pressure peak and a much longer duration.

Effect of wall stiffness, mass and potential interaction strength on heat transfer characteristics of nanoscale-confined gas

The interactive thermal wall model is applied in three-dimensional molecular dynamics simulations to investigate the combined effect of the wall force field, the wall stiffness, the wall atom mass and the wall/gas interaction potential strength on the heat transfer characteristics of static rarefied argon gas within a nanochannel. By increasing the wall stiffness, a reduction in the heat flux through the gas medium occurs which leads to a higher temperature jump. As the wall atom mass is increased up to twice the argon atom mass, the heat flux is enhanced notably and a minimum temperature jump can be found at this point. Further increase in the wall atom mass results in reducing the heat flux and consequently increasing the temperature jump. The increment of the wall/gas interaction potential strength up to four times the one of gas/gas interactions is shown to enhance the heat flux and to reduce the temperature jump until it eventually vanishes. Furthermore, it is found that under such conditions, the density profile experiences a second peak. A further increase of this parameter is found to have a negligible effect on the heat flux through the gas medium and it only increases the second peak in the density profile.

Measurements of wall-media contact forces and work in a vibratory finisher

The bulk flow and local impact velocities of loose abrasive particles in vibratory finishing operations have been explained in the literature in terms of the forces exerted by the vibrating container walls. In the present work, these forces were measured using a force sensor embedded in the walls of a vibrating tub finisher loaded with various amounts of steel media and subject to different vibration frequencies. It was found that wall-media contact forces increased with the depth of media above the sensor and with the normal wall velocity, which explained why wall forces remained approximately unchanged with varying media mass, but increased with increasing tub frequency. The average impulse of the wall on the media increased with increasing media mass and decreased with increasing frequency of vibration. The work done by the walls was also measured and was found to increase with increasing tub frequency for a fixed media mass.

Inertial focusing of a neutrally buoyant particle in stratified flows

Particles in microfluidic channels experience two dominant lift forces in the direction transverse to the flow—the shear gradient lift force and the wall lift force. These forces contribute to the lift experienced by the particle and cause their cross stream migration until they attain an equilibrium position where the net lift force in the transverse direction is zero. Stratified coflow of two liquids with different viscosities is a stable flow-regime observed under some operating conditions. The presence of the second fluid alters the shear gradient induced lift force and the wall force acting on the particle at each point, changing the final equilibrium position. These positions can be tuned and controlled by altering the viscosity or the flow rates of the two fluids so that the particles focus in one fluid. A numerical method based on the combined Immersed Boundary-Lattice Boltzmann Method is used to study inertial focusing of neutrally buoyant particles in stratified Couette flows and pressure driven flows. We analyze how different factors such as the Reynolds number, flow rate ratio, viscosity ratio of the fluids, and particle size affect the particle migration in two-dimensional (2D) and three-dimensional (3D) geometries. Our study shows that in Couette flows, the particle focuses in the low viscosity fluid when the interface is at the center. We also found that a critical viscosity ratio exists beyond which particle focusing in low viscous fluid is guaranteed, for a given flow rate ratio in pressure driven flows.

Computational analysis of a flapping uvula on aerodynamics and pharyngeal wall collapsibility in sleep apnea

Studying the airflows and the resultant aerodynamic pressure/force in the pharyngeal airway is critical for understanding the pathophysiology of snoring and sleep apnea. In this work, an experiment-driven computational study was conducted to examine the aerodynamics in human pharyngeal airway. An anatomically accurate pharynx model associated with different uvula kinematics was reconstructed from human magnetic resonance image (MRI) and high-speed photography. An immersed-boundary-method (IBM)-based direct numerical simulation (DNS) flow solver was adopted to simulate the corresponding unsteady flows in all their complexity. Analyses were performed on vortex dynamics and pressure fluctuations in the pharyngeal airway and force oscillations on the pharyngeal wall under the influence of varying airway obstructions, uvula flapping mode, and uvula flapping frequencies. It was found the vortex formation, aerodynamic pressure, and pharyngeal wall force were significantly affected by the width of the pharyngeal airway. By contrast, the influences from the uvula flapping mode were insignificant when other parameters were similar. Fast Fourier transformation (FFT) and continuous wavelet transform (CWT) analysis of the pressure time history revealed the existence of higher order harmonics of base frequency with significant pressure amplitudes and energy intensities. It was also found the airway pressure and pharyngeal wall force oscillate more dramatically at higher uvula flapping frequencies, which tends to promote the collapse of pharyngeal wall and initiates sleep apnea.

The effect of esophageal cooling on esophageal injury during radiofrequency catheter ablation of atrial fibrillation.

Catheter ablation of atrial fibrillation (AF) may lead to collateral damage to the esophagus. We tested the hypothesis that luminal esophageal temperature (LET)-guided esophageal cooling might reduce the incidence of esophageal thermal lesions (ETL). Seventy-six patients from August 2015 to March 2017 with paroxysmal or persistent AF underwent a first-time catheter ablation procedure with or without LET-guided active esophageal cooling through an orogastric tube placed in the esophagus. Esophageal cooling occurred if and only if LET exceeded 0.5°C from baseline while ablating the LA posterior wall. All patients underwent esophagogastric endoscopy the next day. Of the 76 patients studied, 38 (50%) patients underwent esophageal cooling. Baseline characteristics of the non-cooled and cooled groups were comparable. Of these, 59% of patients had ETL. There was a non-significant trend for more severe lesions (grades 3, 4) in the non-cooled group (29% vs. 13.5%, p = 0.10). Average power delivered on the left atrial posterior wall (27 ± 1.8W vs. 27 ± 3.8W, p = 0.34) and average force of contact (10.1g vs. 9.8g, p = 0.38) were similar in both groups while more time was spent ablating on the posterior wall in the non-cooled group (24.6 ± 7.3min vs. 20.4 ± 5.9min, p = 0.014). In a multivariate analysis, esophageal cooling had no significant effect on the esophageal lesion grade post-ablation. The incidence of ETL in patients undergoing left atrial posterior wall isolation is substantial. Our method of esophageal cooling did not decrease the incidence of ETL. There was a non-significant trend toward fewer severe lesions with cooling, but one cannot conclude the value of cooling from this pilot study.

Local stress and pressure in an inhomogeneous system of spherical active Brownian particles

The stress of a fluid on a confining wall is given by the mechanical wall forces, independent of the nature of the fluid being passive or active. At thermal equilibrium, an equation of state exists and stress is likewise obtained from intrinsic bulk properties; even more, stress can be calculated locally. Comparable local descriptions for active systems require a particular consideration of active forces. Here, we derive expressions for the stress exerted on a local volume of a systems of spherical active Brownian particles (ABPs). Using the virial theorem, we obtain two identical stress expressions, a stress due to momentum flux across a hypothetical plane, and a bulk stress inside of the local volume. In the first case, we obtain an active contribution to momentum transport in analogy to momentum transport in an underdamped passive system, and we introduce an active momentum. In the second case, a generally valid expression for the swim stress is derived. By simulations, we demonstrate that the local bulk stress is identical to the wall stress of a confined system for both, non-interacting ABPs as well as ABPs with excluded-volume interactions. This underlines the existence of an equation of state for a system of spherical ABPs. Most importantly, our calculations demonstrated that active stress is not a wall (boundary) effect, but is caused by momentum transport. We demonstrate that the derived stress expression permits the calculation of the local stress in inhomogeneous systems of ABPs.

An improved hybrid continuum-atomistic four-way coupled model for electrokinetics in nanofluidics.

In this study, an efficient hybrid continuum-atomistic method is proposed to study electrokinetic transport of aqueous solutions in nanofluidics. The aqueous phase is considered as a continuous phase containing immersed ion particles. The behavior of the system is then simulated through utilization of an improved hybrid continuum-atomistic four-way coupled approach, including the MultiPhase Particle-In-Cell method for the short-ranged interaction between the ion particles, the Brownian force for the collision between the aqueous phase molecules and the ion particles, and a wall force accounting for the short-ranged interaction of ions and walls. The validation of the proposed model with the results of Molecular Dynamics simulations suggests that this model can be a promising approach for studying the electrokinetic phenomena in more complicated geometries where the Molecular Dynamics approach is computationally prohibitive. Finally, the effects of electrokinetic parameters, such as the height of the channel, the external electric field, and bulk ionic concentration, on the electroosmotic flow in a nanochannel are investigated and discussed.