Imposed Shear Flow Research Articles

Investigating the effects of shear rate on the collapse time in a gas–liquid system by lattice Boltzmann

In order to evaluate the effects of shear rate on the time of collapse process in a two-phase; one component system, a free energy lattice Boltzmann model is developed. Considering a system consists of two droplets in the surrounding vapor it is observed that, for various physical and geometrical conditions, imposing shear flow has similar effects on the collapse time. Actually, for each case there is an optimum shear rate of collapse that is corresponded to highest collapse rate and smallest time. In all test cases, for shear rates smaller than the optimum value, increasing the shear rate raise collapse rate and reduce its time. But for higher shear rates, increasing the shear rate has reverse effect and decreases the collapse rate. The results also show that, higher droplet radius ratio, viscosity ratio, surface tension and interface thickness lead to a higher optimum shear rate of collapse while higher density ratio of liquid and vapor decreases that.

Designing Composite Coatings That Provide a Dual Defense against Fouling.

Inspired by marine organisms that utilize spines and shape changes to prevent the biofouling of their surfaces, we use computational modeling to design a gel-based composite coating that provides a two-pronged defense mechanism against the fouling of the underlying substrate. Using dissipative particle dynamics (DPD) simulations, we construct a coating that encompasses rigid posts embedded in a thermoresponsive gel, which exhibits a lower critical solution temperature (LCST). When the gel is heated above its LCST, it collapses to expose the buried posts, which act as spines or spikes that prevent a solid particle from penetrating the layer. Moreover, we show that an imposed shear flow readily dislodges these particles and washes them away from the coated substrate. As the system dissipates heat and cools, the LCST gel expands, and this dynamic morphological change can also be harnessed to dislodge the adsorbed particles. Thus, both the exposed posts and the swelling gels can provide barriers to the penetration of particulates through the coating. In this manner, the coating provides a dual mechanism against the fouling of the substrate. This physical approach can be particularly beneficial because it does not require the release of any chemical substances that could have detrimental consequences to the environment.

Microphase transitions of block copolymer/homopolymer under shear flow

Cell dynamics simulation is used to investigate the phase behavior of block copolymer/homopolymer mixture subjected to a steady shear flow. Phase transitions occur from transverse to parallel and then to perpendicular lamellar structure with an increase of shear rate and this is the result of interaction between the shear flow and the concentration fluctuation. Rheological properties, such as normal stress differences and shear viscosity, are all closely related with the direction of the lamellae. Furthermore, we specifically explore the phase behavior and the order parameter under weak and strong shear of two different initial states, and realize the importance of the thermal history. It is necessary to apply the shear field at the appropriate time if we want to get what we want. These results provide an easy method to create ordered, defect-free materials in experiment and engineering technology through imposing shear flow.

Proper orthogonal decomposition applied to laminar thermal convection in a vertical two plate channel

This work reports the thermal convection with imposed shear flow in a thin two-plate channel. Flow structures are investigated under heating asymmetric conditions and different laminar flow conditions. The dynamics of heat flow and the energy distribution were determined by visualization with the Schlieren technique and application of the proper orthogonal decomposition (POD) method. The obtained results from the POD mode analysis revealed that for some flow conditions the heat transfer is related to the energy of the POD modes and their characteristic numbers. It was possible to detect periodic motion in the two-plate channel flow from the POD mode analysis. It was also found that when the energy is distributed among many POD modes, the fluid flow is disorganized and unsteady.

Shear-Induced Desorption of Isolated Polymer Molecules from a Planar Wall.

We use Brownian dynamics simulations with hydrodynamic interactions to reconcile the apparent contradictions in experiments on the effect of shear flow on desorption of polymers. Our simulations show that desorption is governed by an interplay between chain-wall attraction and wall-induced hydrodynamic repulsion. When adsorption is weak, the chains form large loops and tails that generate stronger hydrodynamic interactions in the presence of the imposed shear flow, causing longer chains to desorb faster than shorter ones. In contrast, when adsorption is strong, the chains adopt a flattened conformation, weakening hydrodynamic interactions and resulting in shorter chains with fewer sticking points desorbing faster than longer ones.

Control of droplet collapse during coarsening process by imposing shear flow: a lattice Boltzmann simulation

The effects of shear flow on droplet collapse during the coarsening process in a vapor–liquid system are investigated by a free energy lattice Boltzmann model. To simulate different viscosity ratios, a local relaxation time parameter is integrated with LBM algorithm. The results show that for zero and small shear rates, droplet coarsening happens in its regular pattern where small droplet is collapsed while greater one grows (sub-critical regime). But if the shear rate be greater than a critical value, collapse is abated, droplet coarsening is inverted and smaller droplet grows (super-critical regime). Therefore during coarsening process, collapse mechanism can be controlled by imposing suitable shear flow. Also it is shown that, higher droplet radius ratio, viscosity ratio and surface tension lead to higher critical shear rate of collapse while higher density ratio of liquid and vapor decreases that.

Effect of flow type, channel height, and viscosity on drop production from micro-pores

The formation and detachment of drops from micron-sized pores into shear flow fields is investigated using numerical simulations. The open source software OpenFOAM is used for the simulations. The numerical algorithm employs the finite volume method for solving the mass and momentum conservation equations with a volume-of-fluid approach for capturing the fluid–fluid interface. In addition, a contact model accounts for the interaction between the fluids and the walls of the channel and pore. After validating the numerical models and methods by comparison to experimental data, a parameter study is performed to investigate the effect of various geometrical, flow and fluid parameters on the characteristics of drop production, in particular, drop size. The effect of the type of imposed channel shear flow (pressure-driven or plane Couette), channel height and fluid viscosity is considered. It is found that, in one range of Reynolds and capillary numbers, the channel wall shear rate is a good indicator of drop sizes, regardless of imposed shear flow type, channel height or viscosity ratio. In this flow regime, a master drop size curve is produced for each viscosity ratio considered, with the curve being shifted lower for the higher viscous disperse phase. In another flow regime, when Reynolds numbers are very large relative to capillary numbers, the average shear rate in the channel was a better drop size indicator, although a different master curve was produced for each channel height.

Simulation of viscous sintering using the lattice Boltzmann method

The viscous flow is one of the important mass transport mechanisms in sintering powder materials such as porous glass. The main underlying driving force here is the tendency of the system to minimize the overall surface free energy. In this work, we first address the fundamental problem of the coalescence of two suspending drops and provide an alternative derivation of Frenkel's formula by explicitly taking account of driving forces which arise from the curvature of the contact zone. The result thus obtained is compared with non-ideal (two phase) fluid lattice Boltzmann simulations. Furthermore, we use this simulation tool to address the more complex case of the sintering of a compact of viscous spherical particles. We observe a significantly faster densification dynamics than in experiments but obtain qualitative agreement upon rescaling the unit of time. We attribute this faster dynamics in simulations to the absence of intergranular friction and discuss a possible improvement of the model. We also propose, for the present LB model, a simple way of imposing shear flow to determine the time dependence of the effective viscosity during the sintering process. The method is benchmarked via a simple analytic case and first simulation results are presented for situations for which no analytic theory exists.

Modeling the dynamic flow–fiber interaction for microscopic biofluid systems

Microscopic flow–fiber interaction is frequently encountered in biofluid systems. In this paper, we propose a three-dimensional numerical model for such systems, including a spring network model for the elastic fiber, the lattice Boltzmann method for the fluid flow, and the immersed boundary method for the flow–structure interaction. Simulations are also conducted to examine the fiber deformation under constant and sinusoidal shear conditions. While an equilibrium state can be reached under a constant shear, periodic oscillation in fiber shape is observed in a sinusoidal shear flow, at a frequency identical to that of the imposed shear flow. A continuous phase shift of the fiber oscillation is also noticed along the fiber. These simulations demonstrate the potential usefulness of this model in microscopic biofluid and other systems, and further studies are necessary to investigate the dynamic flow–fiber interaction in various flow situations.

Phase behaviors of diblock copolymer/nanorod composites under oscillatory shear flow

AbstractThe phase behaviors of lamellar diblock copolymers (DBCPs) melts and lamellar DBCPs/nanorods (NRs) composites subjected to oscillatory shear flow, have been investigated using dissipative particle dynamics. The oscillatory shear is a quite common shearing mode used in experiments and manufacturing processes. The rich lamellar (LAM) reorientation and morphological transition of systems strongly depend on the shear amplitude and shear frequency. At very low frequency, the amplitude‐induced phase behaviors of DBCPs melts or DBCPs/NRs composites are quite similar to those subjected to a steady shear. For DBCPs/NRs case, we control NRs concentration low 15% to preserve LAM morphology in the nanocomposites, and simultaneously consider both types of selective and nonselective NRs. Our aim is to compare the different inductions on DBCPs melts or nanocomposites caused by shear amplitude and frequency, and observe how the NRs are oriented and dispersed in phase‐separated copolymers matrix while under oscillatory shear, and how the presence of selective/nonselective NRs affects the shear‐induced LAM reorientations and rheological properties of systems. Our results show the NRs orientation not only directly depend on the imposed shear flow but also is interfered by the alignment of shear‐stretched copolymer molecules. The shear viscosity decreases with the frequency decreases, also influenced by the NRs concentration and surface property. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Three-dimensional flow of colloidal glasses

Recent experiments performed on a variety of soft glassy materials have demonstrated that any imposed shear flow serves to simultaneously fluidize these systems in all spatial directions [Ovarlez \textit{et al.} (2010)]. When probed with a second shear flow, the viscous response of the experimental system is determined by the rate of the primary, fluidizing flow. Motivated by these findings, we employ a recently developed schematic mode-coupling theory [Brader \textit{et al.} (2009)] to investigate the three dimensional flow of a colloidal glass, subject to a combination of simple shear and uniaxial compression. Despite differences in the specific choice of superposed flow, the flow curves obtained show good qualitative agreement with the experimental findings and recover the observed power law describing the decay of the scaled viscosity as a function of the dominant rate. We then proceed to perform a more formal analysis of our constitutive equation for different kind of `mixed' flows consisting of a dominant primary flow subject to a weaker perturbing flow. Our study provides further evidence that the theory of Brader \textit{et al.} (2009) reliably describes the dynamic arrest and mechanical fluidization of dense particulate suspensions.

Experimental constraints on the deformation and breakup of injected magma

The injection, breakup and stirring of dikes entering convecting silicic magma chambers can govern how they grow and differentiate, as well as influence their potential for eruption at the surface. Enclaves observed in plutons may preserve a record of this process and, thus, identifying and understanding the physical processes underlying their formation is a crucial issue in volcanology. We use laboratory experiments and scaling theory to investigate the mechanical and rheological conditions leading to the deformation and breakup of analog crystal-rich dikes injected as discrete plumes that descend into an underlying imposed shear flow. To scale the experiments and map the results across a wide range of natural conditions we define the ratio S of the timescale for the growth of a gravitational Rayleigh–Taylor (R–T) instability of the sheared, injected material to the timescale for settling through the fluid layer and the ratio Y of the timescales for shearing and lateral disaggregation of the particle–fluid mixture (yielding). At low S (<3) and high Y (>40), descending plumes are stretched and tilted before undergoing R–T instability, forming drips with a wavelength that is comparable to the initial diameter of the injection. At low Y (<40) and S values that increase from ∼3 as Y →0, an injection yields in tension before a R–T instability can grow, forming discrete particle-fluid blobs that are much smaller than the initial injection diameter and separated by thin filaments of the original mixture. At high S (>3) and high Y (>40), injections remain intact as they settle through the layer and pond at the floor. Applied to magma chambers, our results do not support the production of a continuum of enclave sizes. Indeed, from scaling analyses we expect the two breakup regimes to form distinct size populations: Whereas enclaves formed in the R–T regime will be comparable to the injection size, those formed in the tension regime will be much smaller. We show that enclave size distributions observed in the field can potentially be used to infer the Y−S conditions for the magma chamber at the time of injection. In addition, these observations can constrain aspects of the styles of flow, stirring, and mixing within the magma chamber, as well as the rheological contrast between the injected and host magma at the time of enclave formation. Our work shows that the contrast in composition between the injected and host magmas will have a strong effect on the mingling structures that are likely to be generated.

Correlation length scaling laws in drift-Alfvén edge turbulence computations

The effect of changes in plasma parameters, that are characteristic near or at an L–H transition in fusion edge plasmas, on fluctuation correlation lengths are analysed by means of drift-Alfvén turbulence computations. Scalings by density gradient length, collisionality, plasma beta and by an imposed shear flow are considered. It is found that strongly sheared flows lead to the appearance of long-range correlations in electrostatic potential fluctuations parallel and perpendicular to the magnetic field.

A simulation study on the effects of shear flow on the microstructure and electrical properties of carbon nanotube/polymer composites

We employ a fiber-level simulation technique to simulate carbon nanotube (CNT)/polymer composites in simple shear flow. This model incorporates CNT flexibility, irregular CNT equilibrium shapes and CNT interactions. Electrical conductivity of the composites is determined using a resistor network algorithm. Tunneling resistance of the insulating matrix film between nanotubes is also considered. We show that the rate of imposed shear flow influences the composite conductivity by facilitating the formation or destruction of the conductive aggregates. In addition, the conductivity evolution during shearing for different concentrations is investigated. At low concentration, percolating clusters form and break simultaneously which causes large conductivity fluctuations during the simulations. When sufficiently large concentrations are reached, percolating clusters persist during shearing and the conductivity fluctuations decrease. In agreement with previous research we determine that increasing the shear rate causes alignment of the nanotubes in the flow direction. We show that upon shearing at constant shear rate, the system attains a state with substantially constant electrical conductivity, nanotube orientation and agglomerate size that is a function of the applied shear rate. The state reached for a given shear rate is independent of the initial state of orientation and aggregation.

A simulation study on the combined effects of nanotube shape and shear flow on the electrical percolation thresholds of carbon nanotube/polymer composites

Here we investigate the combined effects of carbon nanotube (CNT) properties such as aspect ratio, curvature, and tunneling length and shear rate on the microstructure and electrical conductivities of CNT/polymer composites using fiber-level simulations. Electrical conductivities are calculated using a resistor network algorithm. Results for percolation thresholds in static systems agree with predictions and experimental measurements. We show that imposed shear flow can decrease the electrical percolation threshold by facilitating the formation of conductive aggregates. In agreement with previous research, we find that lower percolation thresholds are obtained for nanotubes with high aspect ratio. Our results also show that an increase in the curvature of nanotubes can make more agglomeration and reduce the percolation threshold in sheared suspensions.

Instability regimes in flowing suspensions of swimming micro-organisms

The effects of an external shear flow on the dynamics and pattern formation in a dilute suspension of swimming micro-organisms are investigated using a linear stability analysis and three-dimensional numerical simulations, based on the kinetic model previously developed by [D. Saintillan and M. J. Shelley, Phys. Fluids 20, 123304 (2008)]. The external shear flow is found to damp the instabilities that occur in these suspensions by controlling the orientation of the particles. We demonstrate in our simulations that the rate of damping is direction-dependent: it is fastest in the flow direction, but slowest in the direction perpendicular to the shear plane. As a result, transitions from three- to two- to one-dimensional instabilities are observed to occur as shear rate increases, and above a certain shear rate the instabilities altogether disappear. The density patterns and complex flows that arise at long time in the suspensions are also analyzed from the numerical simulations using standard techniques from the literature on turbulent flows. The imposed shear flow is found to have an effect on both density patterns and flow structures, which typically align with the extensional axis of the external flow. The disturbance flows in the simulations are shown to exhibit similarities with turbulent flows, and in particular two of the seemingly universal characteristics of turbulent flows also occur, namely: (i) the bias of Q-R plots toward the second and fourth quadrants, corresponding to stable focus/stretching and unstable node/saddle/saddle flow topologies, respectively, and (ii) the alignment of the vorticity vector with the intermediate strain-rate eigenvector. However, the flows described herein also significantly differ from turbulent flows owing to the strong predominance of large scales, as exemplified by the very rapid decay of the kinetic energy spectrum, an effect further enhanced after the transitions to two- and one-dimensional instabilities.

A Simple Model for Non-Topological Defects in Sheared Nematic Polymer Monodomains

Nematic fluids have a propensity for orientational defects. In Leslie-Ericksen theory for small molecule liquid crystals, defects are topological in nature and easily visible in textural snapshots. However, kinetic and mesoscopic theories of nematic polymers possess a richer class of defects, including non-topological defects in which the second moment tensor of the orientational probability distribution fails to uniquely define a major director. These defects include the isotropic phase, where the major director lies anywhere on the unit sphere, and so-called oblate defect phases where the major director lies on a circle in the plane normal to a well-defined minor axis. These isotropic and oblate defect phases are independent of the physical space dimension, and while they have appeared in dynamics of heterogeneous attractors [5, 12, 13], a simple monodomain model of defect phases has not previously been developed. Here we present such a model by restricting to the in-plane monodomain response to an imposed shear flow. The orientational dynamics are described by a three-dimensional dynamical system on a compact domain. We show that the set of isotropic and oblate defect phases separates this compact orientational space into two sets of well-defined orientational distributions, one parallel and another orthogonal to the vorticity axis. We identify orbits that pass through the oblate defect set, and how often they do so. Finally, we observe intriguing behavior at the tumbling-wagging bifurcation where limit cycles pass infinitely often through the oblate defect set. 2000 Mathematics Subject Classification. 76A15, 82D60.

Observability of viscoelastic fluids

We apply the observability rank condition to study the observability of various viscoelastic fluids under imposed shear or extensional flows. In this paper the observability means the ability of determining the viscoelastic stress from the time history of the observations of the first normal stress difference. We consider four viscoelastic models: the upper convected Maxwell (UCM) model, the Phan–Thien–Tanner (PTT) model, the Johnson–Segalman (JS) model and the Giesekus model. Our study reveals that all of the four models have observability for all stress components almost everywhere under shear flow whereas under extensional flow most of the models have no observability for the shear stress component. More specifically, for UCM and JS models under imposed shear flow, the observations of the first normal stress difference allow the reconstruction of all components of viscoelastic stress. For UCM and JS models under extensional flow, the two normal stress components can be determined from the measurements of the first normal stress difference; the shear stress component does not affect the evolution of the normal stress components and consequently it cannot be extracted from the observations. Under shear flow, the PTT and Giesekus models have observability almost everywhere. That is, all components of the viscoelastic stress can be determined from the observations when the vector formed by the components of viscoelastic stress does not lie on a certain surface. Under extensional flow, the PTT model has observability almost everywhere for normal stress components whereas the Giesekus model has observability almost everywhere for all stress components. We also run simulations using the unscented Kalman filter (UKF) to reconstruct the viscoelastic stress from observations without and with noises. The UKF yields accurate and robust estimates for the viscoelastic stress both in the absence and in the presence of observation noises.

Sliding, pinch-off and detachment of a droplet on a wall in shear flow

We investigate here what happens beyond the onset of motion of a droplet on a wall by the action of an imposed shear flow, accounting for inertial effects and contact-angle hysteresis. A diffuse-interface method is used for this purpose, which alleviates the shear stress singularity at a moving contact line, resulting in an effective slip length. Various flow regimes are investigated, including steadily moving drops, and partial or entire droplet entrainment. In the regime of quasi-steadily moving drops, the drop speed is found to be linear in the imposed shear rate, but to exhibit an apparent discontinuity at the onset of motion. The results also include the relation between a local maximum angle between the interface and the wall and the instantaneous value of the contact-line speed. The critical conditions for the onset of entrainment are determined for pinned as well as for moving drops. The corresponding critical capillary numbers are found to be in a rather narrow range, even for quite substantial values of a Reynolds number. The approach to breakup is then investigated in detail, including the growth of a ligament on a drop, and the reduction of the radius of a pinching neck. A model based on an energy argument is proposed to explain the results for the rate of elongation of ligaments. The paper concludes with an investigation of detachment of a hydrophobic droplet from the solid wall.

Investigation into the Dual Role of Shear Flow in 2D MHD Turbulence

The turbulent diffusion eta_{T} of a large-scale magnetic field B0 is numerically studied in two-dimensional magnetohydrodynamic turbulence with an imposed shear flow. We demonstrate that a shear flow plays a dual role, quenching transport through shear destruction and enhancing it via resonance. Specifically without resonance eta_{T} proportional, variantB_{0};{-4} with no shear (rms shearing rate=Omega=0) and eta_{T} proportional, variantOmega;{-2.7} for B_{0}=0, while with resonance eta_{T} proportional, variantB_{0};{-2} proportional, variantOmega;{-2}. These results indicate that the absence of resonance is responsible for the most catastrophic reductions in transport.