Confinement Ratio Research Articles

Lattice Boltzmann study of effective viscosities of porous particle suspensions

The effective viscosities of dilute and semi-dilute suspensions in a two-dimensional shear flow are studied using the lattice Boltzmann method. The suspensions contain non-Brownian hard circular buoyant porous particles. Here a more accurate formula for intrinsic viscosity as a function of Darcy number (Da) for the whole Da regime is proposed through our numerical result. The effects of fluid inertia, permeability of the particle, and confinement of the bounding walls are investigated. It is found that for the cases with a small Da, the effective viscosity significantly increases with confinement and fluid inertia. However, for the cases with a large Da, the confinement ratio and fluid inertia have very minor effect. Moreover, for semi-dilute suspensions, the permeability of the particle weakens the effect of the hydrodynamic interactions between particles on the relative viscosity ηr and makes ηr decrease. The above phenomena can be well understood through quantifying the disturbance of the porous particle to the flow.

Experimental analysis of the influence of the impinged plate by an impinging jet on the vortex dynamics using PIV

In this experimental study, we are interested in the flow generated by a plane jet impacting on different plates. To analyse these flows and to compare the evolutions of the various parameters (velocities, turbulence rate, vortex structures, etc.), the PIV technique was used. For example, for a confinement ratio of 4 and a Reynolds number of 4838, a comparison is made between the deviated flows for each of the three plates considered in this study. For the smooth slotted plate, it is noted that the large structures created by the shear between the jet and the driving zone are convected and are organized differently according to the initial dynamics of the flow (Re). Indeed, depending on the configuration, these large structures are either deviated laterally in the parietal flow or pass alternately through the slot. With regard to the solid plates, Kelvin Helmholtz’s structures are always deflected, the parietal flow is more nourished but its vortex dynamics is influenced by the surface state. Indeed, for the smooth solid plate we have large swirl structures whereas for the corrugated plate, we notice the appearance of small vivid and intense vortex structures. A quantitative analysis of these phenomena is conducted in order to study and compare parietal flows.

Post-pinch-off relaxation of two-dimensional droplets in a Hele-Shaw cell

We report on the shape relaxation of 2D droplets, formed right after the spontaneous pinch-off of a capillary bridge-droplet confined within a Hele-Shaw cell. An array of bridge-droplets confined within a microchip device first undergo neck thinning due to the evaporation driven volume change. Subsequently, an abrupt topological change transforms each bridge-droplet into a small central satellite droplet and the twin-droplets pinned at the edges of the cell. We monitor the shape relaxation with high temporal resolution optical microscopy. Capillary action drives the 2D shape relaxation, while the viscous dissipation in the film retards it. As a result, the tip of the twin-droplets exhibit a self-similar parabolic shape evolution. Based on these observations, the lubrication-approximation model accurately predicts the internal pressure evolution and the droplet tip displacement. The geometrical confinement substantially slows down the dynamics, facilitating visualization of the capillary-viscous regime, even for low viscosity liquids. The characteristic relaxation timescale shows an explicit dependence on the confinement ratio (width/gap) and the capillary-velocity of liquid. We verify the broad applicability of the model using different liquids.

Predicting strength and strain enhancement ratios of circular fiber-reinforced polymer tube confined concrete under axial compression using artificial neural networks

Numerous research studies experimentally investigated the axial compressive behavior of fiber-reinforced polymer tube confined concrete cylinders in the past two decades. However, only a limited number of research studies developed stress–strain models to predict the strength and strain enhancement ratio of fiber-reinforced polymer tube confined concrete cylinders under axial compression. The available strength and strain enhancement ratio models of fiber-reinforced polymer tube confined concrete cylinders are a function of actual confinement ratio only. This study develops strength and strain enhancement ratio models for circular fiber-reinforced polymer tube confined concrete under axial compression based on artificial neural network analyses using Purelin and Tansig transfer functions. The developed strength and strain enhancement ratio models are functions of actual confinement ratio, orientation of fibers, height to diameter ratio, and axial strain in unconfined concrete at peak axial stress. The formulation and performance evaluation of the developed strength and strain enhancement ratio models are carried out using experimental investigation results of 238 circular fiber-reinforced polymer tube confined concrete under concentric axial compression compiled from a database of 599 fiber-reinforced polymer tube confined concrete specimens. The predictions of the developed strength and strain enhancement ratio models match well with the experimental investigation results of the compiled database. The developed strength and strain enhancement ratio models exhibit smaller statistical errors than the available models in the research studies for predicting the strength and strain enhancement ratios of circular fiber-reinforced polymer tube confined concrete under axial compression.

Buoyancy-driven convective heat transfer from a semi-circular cylinder for various confinements

Buoyancy-driven convective heat transfer from a semi-circular cylinder for various confinements has been studied using numerical simulations for wide ranges of parameters, Reynolds numbers (1 ≤ Re ≤ 50), Richardson numbers (0 ≤ Ri ≤ 2), Prandtl numbers (0.7 ≤ Pr ≤ 50) and confinement ratios (0.2 ≤ β ≤ 0.8). A hot semi-circular cylinder is symmetrically kept in a 2D rectangular confinement. The circular side of the cylinder faces the upstream flow and the fluid flows against gravity in the channel. The governing equations are numerically solved using FLUENT and the results obtained are presented in the form of isotherms, streamlines, pressure coefficients, drag coefficients, Nusselt numbers, etc. The highest value of pressure coefficient increases with blockage ratio for all cases. The drag coefficient decreases with Re and shows complex phenomena with change in Ri and blockage ratio of the channel. Pressure drag has contributed more as compared with viscous drag in all cases. The curved surface showed more heat transfer than the flat surface of the semi-circular cylinder. The value of β also has great influence at large value of Peclect numbers (= 2500). Overall average heat transfer in terms of average Nusselt number is a function of Ri, Re, Pr and β.

Formation number of confined vortex rings

This paper investigates the formation number of vortex rings generated by a piston-cylinder mechanism in a confined tube. We use Direct Numerical Simulations (DNS) of axisymmetric confined vortex rings to study the influence of different parameters on the separation (or pinch-off) of the vortex ring from the trailing jet. It is shown that the structure of the vortex ring at pinch-off depends on the type of injection program (pulse dominated by either positive or negative acceleration ramps) and the confinement ratio D w /D , where D w is the inner diameter of the tube and D the diameter of the cylinder). For low confinement ratios ( D w /D ≤ 2), a vortex of opposite sign generated at the lateral wall strongly interacts with the vortex ring and the pinch-off is not clearly observed. The pinch-off is observed and analysed for confinement ratios D w /D ≥ 2 . 5. DNS data are used to estimate the value of the formation time, which is the time necessary for the vortex generator to inject the same amount of circulation as carried by the detached vortex ring. The confined vortex ring at pinch-off is described by the model suggested by Danaila, Kaplanski and Sazhin [A model for confined vortex rings with elliptical core vorticity distribution, Journal of Fluid Mechanics, 811 :67-94, 2017]. This model allows us to take into account the influence of the lateral wall and the elliptical shape of the vortex core. The value of the formation time is predicted using this model and the slug-flow model.

Optical properties of frustrated cholesteric liquid crystals in planar cell

ABSTRACTWe investigated the alignment director of a frustrated cholesteric liquid crystal (CLC) confined in a planar cell. Three cells with different confinement ratio (c = d/p) (where p is the pitch and d is the cell thickness) are prepared. Under an electric field, the CLC planar texture is transformed into a cholesteric fingerprint (CF). The results showed that CF contrast depends on c. When c ≈ 2, CLC stripes are formed by a periodic CF, with a period equal to the CLC pitch. The CF is developed and slowly extended to the whole cell surface along the rubbing direction and the contrast of the grating stripes keeps unchangeable. Yet, the CLC finger borders have a different light intensity. However, when c ≈ 1, the CF contrast increases with time. When c < 1, an isolated CF is obtained whose borders contrast change by varying the angle θ between the polarizer and the CFs.

Off-lattice simulation algorithms for athermal chain molecules under extreme confinement

A new algorithmic approach is presented for the generation and successive equilibration of polymer configurations under conditions of extreme confinement where the inter-wall distance, in at least one dimension, approaches the diameter of the spherical monomers. It significantly improves on the Monte Carlo (MC) protocol described in Karayiannis and Laso (2008) [126]. The algorithm is designed to generate highly confined packings of freely-jointed chains of hard spheres of uniform size. Spatial confinement is achieved by including flat, parallel impenetrable walls in one or more dimensions of the simulation box. The present MC scheme allows the systematic study of the effect of chain length, polydispersity, volume fraction, bond tolerance (gap), cell aspect ratio and level of confinement on the short- and long-range structure of polymer chains near and far from the confining planes. In the present study we focus on the efficiency of the MC protocol in generating, equilibrating, and configurationally decorrelating chain assemblies with average lengths ranging from N = 12 to 1000 monomers and at volume fractions from dilute up to the maximally random jammed (MRJ) state. Starting from cubic amorphous cells filled with polymer chains, the MC algorithm is able to reach quasi 2-d (plate-like) and 1-d (tube-like) states under conditions of extreme confinement and/or cell aspect ratio where the inter-wall distance approaches the diameter of beads forming the chains. A comparison with corresponding bulk packings shows the similarities and differences produced by extreme spatial confinement.

Stress and deformation characteristics of sea ice in a high-resolution, anisotropic sea ice model.

The drift and deformation of sea ice floating on the polar oceans is caused by the applied wind and ocean currents. Over ocean basin length scales the internal stresses and boundary conditions of the sea ice pack result in observable deformation patterns. Cracks and leads can be observed in satellite images and within the velocity fields generated from floe tracking. In a climate sea ice model the deformation of sea ice over ocean basin length scales is modelled using a rheology that represents the relationship between stresses and deformation within the sea ice cover. Here we investigate the link between emergent deformation characteristics and the underlying internal sea ice stresses using the Los Alamos numerical sea ice climate model. We have developed an idealized square domain, focusing on the role of sea ice rheologies in producing deformation at spatial resolutions of up to 500 m. We use the elastic anisotropic plastic (EAP) and elastic viscous plastic (EVP) rheologies, comparing their stability, with the EAP rheology producing sharper deformation features than EVP at all space and time resolutions. Sea ice within the domain is forced by idealized winds, allowing for the emergence of five distinct deformation types. Two for a low confinement ratio: convergent and expansive stresses. Two about a critical confinement ratio: isotropic and anisotropic conditions. One for a high confinement ratio and isotropic sea ice. Using the EAP rheology and through the modification of initial conditions and forcing, we show the emergence of the power law of strain rate, in accordance with observations.This article is part of the theme issue ‘Modelling of sea-ice phenomena’.

FE modeling of Partially Steel-Jacketed (PSJ) RC columns using CDP model

This paper deepens the finite element modeling (FEM) method to reproduce the compressive behavior of partially steel-jacketed (PSJ) RC columns by means of the Concrete Damaged Plasticity (CDP) Model available in ABAQUS software. Although the efficiency of the CDP model is widely proven for reinforced concrete columns at low confining pressure, when the confinement level becomes high the standard plasticity parameters may not be suitable to obtain reliable results. This paper deals with these limitations and presents an analytically based strategy to fix the parameters of the Concrete Damaged Plasticity (CDP) model. Focusing on a realistic prediction of load-bearing capacity of PSJ RC columns subjected to monotonic compressive loads, a new strain hardening/softening function is developed for confined concrete coupled with the evaluation of the dilation angle including effects of confinement. Moreover, a simplified efficient modeling approach is proposed to take into account also the response of the steel angle in compression. The prediction accuracy from the current model is compared with that of existing experimental data obtained from a wide range of mechanical confinement ratio.

Effect of transverse temperature gradient on the migration of a deformable droplet in a Poiseuille flow

Intricate manipulation of droplets in fluidic confinements may turn out to be critically important for achieving their controlled transverse distributions. Here, we study the migration characteristics of a suspended deformable droplet in a parallel plate channel under the combined influence of a constant temperature gradient in the transverse direction and an imposed pressure driven flow. An outstanding question concerning the resultant non-trivial dynamical features that we address here pertains to the nonlinearity that results as a consequence of the shape deformation, which does not permit us to analyse the combined transport as a mere linear superposition of the results for the thermocapillary and imposed flow driven droplet migration in an effort to obtain the final solution. For the analytical solution, an asymptotic approach is used, where we neglect any effect of inertia or thermal convection of the fluid in either of the phases. To obtain a numerical solution, we use the conservative level set method. We perform numerical simulations over a wide range of governing parameters and obtain the dependence of the transverse steady position of the droplet on different parameters. In order to address practical microfluidic set-ups, the influence of a bounding wall as well as the effect of thermal convection and finite shape deformation on the cross-stream migration of the droplet is investigated through numerical simulations. Increase in the thermal Marangoni stress shifts the steady-state transverse position of the droplet further away from the channel centreline, for any particular value of the capillary number (which signifies the ratio of the viscous force to the surface tension force). The confinement ratio, which is the ratio of the droplet radius to the channel height, plays an important role in predicting the transverse position of the droplet and thus has immense consequences for the design of droplet-based microfluidic devices with enhanced functionalities. A large confinement ratio drives the droplet towards the channel centre, whereas a smaller confinement ratio causes the droplet to move towards the wall. Moreover, for a fixed droplet radius and constant imposed temperature gradient, an increase in the channel height results in an increase in the time required for the droplet to reach the steady-state position. However, the final steady-state position of the droplet is independent of its initial position but at the same time dependent on the droplet phase thermal conductivity. A larger droplet thermal conductivity compared with the carrier phase results in a steady-state droplet position closer to the channel centreline. A higher fluid inertia, on the other hand, shifts the steady-state position towards the channel wall.

Effect of Physical Nanoconfinement on the Viscosity of Unentangled Polymers during Capillary Rise Infiltration

We investigate the role of physical confinement on the polymer viscosity and the glass transition temperature (Tg) of unentangled polymers undergoing capillary rise infiltration (CaRI). CaRI thermally drives polymer infiltration into the voids of densely packed nanoparticle films via capillarity, inducing extreme nanoconfinement of the polymer. We tune the confinement ratio (CR), defined as the ratio of the polymer radius of gyration to the average pore radius in the nanoparticle packing, by using different polymer molecular weights and by varying the nanoparticle size constituting the packing, respectively. We show that physical confinement of unentangled polymers in the interstices of weakly interacting nanoparticles leads to increased viscosity by more than 2 orders of magnitude relative to the bulk viscosity and to increased polymer Tg by 32 K. The increase in both viscosity and Tg increases with CR and saturates at CR ∼ 1. The correlation between the viscosity and Tg increase suggests that the slowdo...

A constitutive model for gfrp-wire-confined concrete with consideration of low and high confinement ratios

Fiber-reinforced polymer (FRP) wire has been applied to confine concrete through a device that controls the confining pressure during installation. Thus, the tightness of jackets is controlled. Experiments conducted on FRP-wire–confined concrete in previous studies have shown that the method provides better confinement effects on concrete than the use of FRP sheets. Despite numerous experimental studies, there are few models available that can predict the stress–strain relation of FRP-wire–confined concrete. This study utilized test results from experiments reported in the literature, which were conducted on 36 specimens with various amounts of FRP wire and various strengths of concrete. Based on those results, an empirical model that is able to predict the stress–strain behavior of FRP-wire–confined concrete is proposed. Note that this study considers both low and high confinement ratios. New peak-stress and peak-strain equations are derived and validated based on the experimental results. The limit values between low, and high confinement cases are also presented. The proposed model achieves acceptable result in comparison to experimental results and a literature model.

Transport of nano-objects in narrow channels: influence of Brownian diffusion, confinement and particle nature

This paper presents experimental results about transport of dilute suspensions of nano-objects in silicon-glass micrometric and sub-micrometric channels. Two kinds of objects are used: solid, rigid latex beads and spherical capsule-shaped, soft polymersomes. They are tracked using fluorescence microscopy. Three aspects are studied: confinement (ratio between particle diameter and channel depth), Brownian diffusion and particle nature. The aim of this work is to understand how these different aspects affect the transport of suspensions in narrow channels and to understand the different mechanisms at play. Concerning the solid beads we observe the appearance of two regimes, one where the experimental mean velocity is close to the expected one and another where this velocity is lower. This is directly related to a competition between confinement, Brownian diffusion and advection. These two regimes are shown to be linked to the inhomogeneity of particles distribution in the channel depth, which we experimentally deduce from velocity distributions. This inhomogeneity appears during the entrance process into the sub-micrometric channels, as for hydrodynamic separation or deterministic lateral displacement. Concerning the nature of the particles we observed a shift of transition towards the second regime likely due to the relationships between shear stress and polymersomes mechanical properties which could reduce the inhomogeneity imposed by the geometry of our device.

Laminar Mixed Convection of Non-Newtonian Nanofluids Flowing Vertically Upward Across a Confined Circular Cylinder

Numerical results on laminar mixed convective heat transfer phenomenon between a confined circular cylinder and shear-thinning type nanofluids are presented. The cylinder is placed horizontally in a confined channel through which nanofluids flow vertically upward. The effect of buoyancy is same as the direction of the flow. Because of existence of mixed convection, governing continuity, momentum, and energy equations are simultaneously solved within the limitations of Boussinesq approximation. The ranges of parameters considered are: volume fraction of nanoparticles, ϕ = 0.005–0.045; Reynolds number, Re = 1–40; Richardson number, Ri = 0–40; and confinement ratio of circular cylinder, λ = 0.0625–0.5. Finally, the effects of these parameters on the streamlines, isotherm contours, individual and total drag coefficients, and local and average Nusselt numbers are thoroughly delineated. The individual and total drag coefficients decrease with the increasing both ϕ and Re; and/or with the decreasing both Ri and λ. The rate of heat transfer increases with the increasing Re, ϕ, Ri, and λ; however, at Re = 30–40, when ϕ &gt; 0.005 and Ri &lt; 2, the average Nusselt number decreases with the increasing Richardson number. Finally, correlations for the total drag coefficient and average Nusselt number are proposed as functions of pertinent dimensionless parameters on the basis of present numerical results.

Understanding the compressive behavior of shape memory alloy (SMA)-confined normal- and high-strength concrete

This paper presents the first experimental study on the axial compressive behavior of high-strength concrete (HSC) confined by shape memory alloy (SMA) wire. Concrete cylinders that were prepared using two different grades of concrete, namely normal-strength concrete (NSC) and HSC, were confined with Nitinol (Ni-Ti) SMA spirals having a pitch spacing of 36 and 20 mm, respectively. Preliminary material tests were performed on SMA wires to investigate the tensile strength and strain of SMA in martensitic and austenitic phases and the effect of the temperature on the recovery stress. The confining pressure was applied on concrete cylinders by SMA spirals that were prestrained at 0, 5.5, and 9.5%. The material test results show that an increase in the prestrain level from 5.5% to 9.5% leads to an increase in the recovery stress of SMA wire at temperatures higher than the austenitic finish temperature (90 °C). The compression test results of SMA-confined concrete specimens show that an increase in the prestrain level leads to an increase in the peak axial stress and corresponding axial strain of SMA-confined concrete. Confinement of NSC and HSC specimens by 9.5% prestrained SMA spirals results in a 38.1% and 23.6% higher peak axial stress and a 333% and 346% higher corresponding axial strain, respectively, compared to those of unconfined specimens. It is also shown that because of the more brittle behavior of HSC specimens, SMA-confined HSC fails at a lower axial strain compared to SMA-confined NSC. The lower ultimate axial strain of HSC specimens is also attributed to the lower confinement ratio of the HSC specimens of the current study compared to that of NSC specimens. The promising findings of this study point to the significant potential for the use of SMA spirals in the construction industry for the development of high-performance composite structural members and for strengthening and retrofit of existing concrete members.

Effect of pillars on the mixing efficiency of a peristaltically-driven Bingham fluid within a closed channel: A LBM simulation

We have numerically studied the effect of a fluid’s yield stress on the performance of a peristaltic mixer reinforced by cylindrical pillars, typical of those used in microfluidic systems. A multiple-relaxation-time, Lattice Boltzmann code was used to investigate the effect of a fluid’s yield stress on mixing efficiency for a viscoplastic fluid obeying the bi-viscous model. A Lagrangian particle-tracking method called “boxcounting” has been utilized for evaluating the mixing performance of the channel through introducing a stirring index parameter. Numerical results indicate that for both Newtonian and Bingham fluids, wave amplitude is the most potent tool for controlling the mixing performance of the mixer. It is shown that while for Newtonian fluids the mixing index is dropped if the channel is equipped with circular pillars, for Bingham fluids pillars can play a slightly positive role in enhancing the mixing efficiency of the mixer, and this is particularly so when the confinement ratio is sufficiently large. A more dramatic increase in the mixing efficiency can be attained when only one of the membranes is vibrating (i.e., the asymmetric case).

The influence of spanwise confinement on round fountains

We study experimentally the effects of spanwise confinement on turbulent miscible fountains issuing from a round source of radius $r_{0}$. A dense saline solution is ejected vertically upwards into a fresh-water environment between two parallel plates, separated by a gap of width $W$, which provide restraint in the spanwise direction. The resulting fountain, if sufficiently forced, rapidly attaches to the side plates as it rises and is therefore ‘confined’. We report on experiments for five confinement ratios $W/r_{0}$, spanning from strongly confined ($W/r_{0}\rightarrow 2$) to weakly confined ($W/r_{0}\approx 24$), and for source Froude numbers $Fr_{0}$ ranging between $0.5\leqslant Fr_{0}\leqslant 96$. Four distinct flow regimes are observed across which the relative importance of confinement, as manifested by the formation and growth of quasi-two-dimensional structures, varies. The onset of each regime is established as a function of both $W/r_{0}$ and $Fr_{0}$. From our analysis of the time-averaged rise heights, we introduce a ‘confined’ Froude number $Fr_{c}\equiv Fr_{0}(W/r_{0})^{-5/4}$, which encompasses the effects of confinement and acts as the governing parameter for confined fountains. First-order statistics extracted from the flow visualisation, such as the time-averaged rise height and lateral excursions, lend further insight into the flow and support the proposed classification into regimes. For highly confined fountains, the flow becomes quasi-two-dimensional and, akin to quasi-two-dimensional jets and plumes, flaps (or meanders). The characteristic frequency of this flapping motion, identified through an ‘eddy counting’ approach, is non-dimensionalised to a Strouhal number of $St=0.12{-}0.16$, consistent with frequencies found in quasi-two-dimensional jets and plumes.

Mechanism-based strength criterion for 3D needled C/C–SiC composites under in-plane biaxial compression

ABSTRACTBiaxial compressive experiments for 3D needled C/C–SiC composites parallel to the nonwoven cloth were conducted. The failure mechanisms and mechanical properties of the composites were greatly related to the biaxial compressive stress confinement ratio, R. It was found that out-of-plane shear failure controlled the failure of the composites. The failure shear plane was aligned with one of the major loading axes for R (0:1 or 1:3), while the failure shear plane occurred along both loading axes for R (1:2 or 1:1). Compared with uniaxial strength, the biaxial compressive strength increased obviously, which also significantly depended on the value of R. Based on the failure modes, a modified twin-shear strength criterion was established to predict the failure surface of 3D needled C/C–SiC composites under biaxial compressive loading.

Particle-Size-Exclusion Clogging Regimes in Porous Media.

From observations of the progressive deposition of noncolloidal particles by geometrical exclusion effects inside a 3D model porous medium, we get a complete dynamic view of particle deposits over a full range of regimes from transport over a long distance to clogging and caking. We show that clogging essentially occurs in the form of an accumulation of elements in pore size clusters, which ultimately constitute regions avoided by the flow. The clusters are dispersed in the medium, and their concentration (number per volume) decreases with the distance from the entrance; caking is associated with the final stage of this effect (for a critical cluster concentration at the entrance). A simple probabilistic model, taking into account the impact of clogging on particle transport, allows us to quantitatively predict all these trends up to a large cluster concentration, based on a single parameter: the clogging probability, which is a function of the confinement ratio. This opens the route towards a unification of the different fields of particle transport, clogging, caking, and filtration.