Shear-dominated Regime Research Articles

Importance of liquid bridge forces in dynamics of rock-ice avalanches: insights from discrete element simulations

Rock-ice avalanches imminently threaten high-mountain and glacierized regions. The wet rock-ice particle flow exhibits dependence on pendular liquid bridges; however, its mechanisms are unclear. This paper focuses on the influence of interstitial water in the pendular regime on the dynamics of rock-ice avalanches. A scheme was proposed to incorporate capillary and viscous liquid bridge forces into the discrete element method (DEM) for simulating the wet rock-ice particle flow. Numerical results indicate that the proposed scheme could well reproduce the key experimental outcomes of small flume tests. The liquid bridges could reduce the mixture mobility remarkably via pseudo-cohesion, which was mainly attributed to the capillary force. The pseudo-cohesion effect could alter the collision-dominant flow regime to the shear-dominant regime through buffering collisions and suppress the segregation through impeding the downward penetration of gravels. A very small amount of water, too little to form liquid bridges, could lubricate the sliding surface, and increase the mixture mobility. The role of the critical inter-element separation distance was deterministic in DEM for wet particles. This paper demonstrated the crucial impact of the liquid bridges in the dynamics of rock-ice avalanches and offered a promising scheme for deeply understanding the solid–liquid interactions in wet rock-ice avalanches.

Investigating Diurnal and Seasonal Turbulence Variations of the Martian Atmosphere Using a Spectral Approach

We use a spectral approach to analyze the pressure and wind data from the InSight mission and investigate the diurnal and seasonal trends. Our analyses show that the daytime pressure and wind spectra have slopes of approximately −1.7 and −1.3 and, therefore, do not follow the Kolmogorov scaling (as was also previously reported for a reduced data set in Banfield et al.). We find that the nighttime pressure spectral slope is close to −1 (as reported in Temel et al.), and that the wind speed spectral slope is close to −0.5, flatter than the theoretical slope expected for the shear-dominated regime. We observe strong nocturnal (likely shear-generated) turbulent behavior starting around L s = 150° (InSight sol 440) that shifts to progressively earlier local times before reaching the “5th season” (InSight sols 530–710) identified by Chatain et al.. The diurnal spectral slope analyses indicate an asymmetry in the diurnal behavior of the Martian boundary layer, with a slow growth and fast collapse mechanism. Finally, the low-frequency (5–30 mHz) pressure data exhibit large spectral slope oscillations. These occur particularly during the periods with a highly stable atmosphere and, therefore, may be linked to gravity wave activity.

Forced motion of a cylinder within a liquid-filled elastic tube – a model of minimally invasive medical procedures

This work analyses the viscous flow and elastic deformation created by the forced axial motion of a rigid cylinder within an elastic liquid-filled tube. The examined configuration is relevant to various minimally invasive medical procedures in which slender devices are inserted into fluid-filled biological vessels, such as vascular interventions, interventional radiology, endoscopies and laparoscopies. By applying the lubrication approximation, thin shell elastic model, as well as scaling analysis and regular and singular asymptotic schemes, the problem is examined for small and large deformation limits (relative to the gap between the cylinder and the tube). At the limit of large deformations, forced insertion of the cylinder is shown to involve three distinct regimes and time scales: (i) initial shear dominant regime, (ii) intermediate regime of dominant fluidic pressure and a propagating viscous-peeling front, (iii) late-time quasi-steady flow regime of the fully peeled tube. A uniform solution for all regimes is presented for a suddenly applied constant force, showing initial deceleration and then acceleration of the inserted cylinder. For the case of forced extraction of the cylinder from the tube, the negative gauge pressure reduces the gap between the cylinder and the tube, increasing viscous resistance or creating friction due to contact of the tube and cylinder. Matched asymptotic schemes are used to calculate the dynamics of the near-contact and contact limits. We find that the cylinder exits the tube in a finite time for sufficiently small or large forces. However, for an intermediate range of forces, the radial contact creates a steady locking of the cylinder inside the tube.

Impact of nanoparticles migration on mixed convection and entropy generation of a $$\hbox {Al}_{2}\hbox {O}_{3}$$–water nanofluid inside an inclined enclosure with wavy side wall

A numerical study based on the non-homogeneous model of the mixed convection of $$\hbox {Al}_{2}\hbox {O}_{3}$$–water nanofluid in an inclined enclosure is made. The heated side wall of the enclosure is considered to be wavy while the top wall is made to translate horizontally. The wavy physical domain is transformed to a square computational domain through a suitable coordinate transformation. The transformed governing equations are integrated based on the control volume approach in a staggered grid arrangement. A third-order accurate upwind scheme QUICK is used to discretize the convective terms while a central difference type scheme is used to approximate the diffusive terms. The discretized equations along with the specified boundary conditions are solved through a pressure correction-based SIMPLE algorithm. The impact of the inclination angle of the enclosure and surface waviness on the nanofluid mixed convection is elucidated. The parameters governing the Brownian motion, thermophoresis, Lewis number and buoyancy ratio parameter are determined based on the nanofluid thermophysical properties. The effect of the nanoparticle bulk volume fraction and nanoparticle diameter on the mixed convection is analyzed for different choices of nanofluid-to-base fluid thermal buoyancy ratio in the buoyancy-dominated regime as well as shear-dominated regime. A comparison of the non-homogeneous model for the nanofluid with the homogeneous model is also made in this study. The inclination angle of the enclosure is found to have an impact on the mixed convection when buoyancy force is dominant. Heat transfer augmentation occurs as the wave number, and/or wave amplitude of the wavy side wall is increased. The thermodynamic optimization is studied by analyzing the average Nusselt number and the total entropy generation.

Cells Under Stress: An Inertial-Shear Microfluidic Determination of Cell Behavior

The deformability of a cell is the direct result of a complex interplay between the different constituent elements at the subcellular level, coupling a wide range of mechanical responses at different length scales. Changes to the structure of these components can also alter cell phenotype, which points to the critical importance of cell mechanoresponse for diagnostic applications. The response to mechanical stress depends strongly on the forces experienced by the cell. Here, we use cell deformability in both shear-dominant and inertia-dominant microfluidic flow regimes to probe different aspects of the cell structure. In the inertial regime, we follow cellular response from (visco-)elastic through plastic deformation to cell structural failure and show a significant drop in cell viability for shear stresses >11.8 kN/m2. Comparatively, a shear-dominant regime requires lower applied stresses to achieve higher cell strains. From this regime, deformation traces as a function of time contain a rich source of information including maximal strain, elastic modulus, and cell relaxation times and thus provide a number of markers for distinguishing cell types and potential disease progression. These results emphasize the benefit of multiple parameter determination for improving detection and will ultimately lead to improved accuracy for diagnosis. We present results for leukemia cells (HL60) as a model circulatory cell as well as for a colorectal cancer cell line, SW480, derived from primary adenocarcinoma (Dukes stage B). SW480 were also treated with the actin-disrupting drug latrunculin A to test the sensitivity of flow regimes to the cytoskeleton. We show that the shear regime is more sensitive to cytoskeletal changes and that large strains in the inertial regime cannot resolve changes to the actin cytoskeleton.

Population splitting of rodlike swimmers in Couette flow.

We present a quantitative analysis on the response of a dilute active suspension of self-propelled rods (swimmers) in a planar channel subjected to an imposed shear flow. To best capture the salient features of the shear-induced effects, we consider the case of an imposed Couette flow, providing a constant shear rate across the channel. We argue that the steady-state behavior of swimmers can be understood in the light of a population splitting phenomenon, occurring as the shear rate exceeds a certain threshold, initiating the reversal of the swimming direction for a finite fraction of swimmers from down- to upstream or vice versa, depending on the swimmer position within the channel. Swimmers thus split into two distinct, statistically significant and oppositely swimming majority and minority populations. The onset of population splitting translates into a transition from a self-propulsion-dominated regime to a shear-dominated regime, corresponding to a unimodal-to-bimodal change in the probability distribution function of the swimmer orientation. We present a phase diagram in terms of the swim and flow Péclet numbers showing the separation of these two regimes by a discontinuous transition line. Our results shed further light on the behavior of swimmers in a shear flow and provide an explanation for the previously reported non-monotonic behavior of the mean, near-wall, parallel-to-flow orientation of swimmers with increasing shear strength.

Three-dimensional dynamic cutting forces prediction model during micro-milling nickel-based superalloy

Nowadays, there are urgent demands of micro structure/parts which have high strength in high-temperature environment in the fields such as aerospace, energy, power, biomedical, etc. Nickel-based superalloy with high strength and high hardness under high temperature is the suitable material for manufacturing this kind of micro parts. Aimed at the problem of the complicated cutting force variation rule when micro-end milling nickel-based superalloy, the cutting forces model during micro-end milling of nickel-based superalloy processing is studied. Firstly, micro-end milling hole experiments are carried out to establish the radical run-out prediction model of cutting edge, which lays the foundation for establishing the cutting thickness calculation model during micro-end milling. Then, based on the minimum cutting thickness value, micro-end milling of nickel-based superalloy process is divided into two different cutting processes: shear-dominant regime cutting process and ploughing-dominant regime cutting process. Moreover, cutting forces prediction model during shear-dominant regime cutting process is developed based on the cutting forces in proportion to cutting layer area, which takes the effect of ploughing into account. Meanwhile, cutting forces prediction model during ploughing-dominant regime cutting process is developed based on the cutting force in proportion to interference volume between the flank surface of cutting tool and the workpiece. The experiment results verify that the cutting forces prediction results and experiment results are well matched.

A unifying framework for parameterizing stably stratified shear-flow turbulence

An alternative framework for parameterizing stably stratified shear-flow turbulence is presented. Using dimensional analysis, four non-dimensional parameters of interest are identified that consider the independent effects of stratification, shear, viscosity, and scalar diffusivity. In the interest of geophysical applications, the problem is further simplified by considering only high Reynolds number flow. This leads to a two-dimensional parameter space based on a buoyancy strength parameter (i.e., an inverse Froude number) and a shear strength parameter. Consideration for the gradient Richardson number allows the space to be divided into an unforced regime, a shear-dominated regime, and a buoyancy-dominated regime. On this basis, a large database of direct numerical simulation and laboratory data from various sources is evaluated. Of particular interest is the observed length scale of overturning. Overturns are found to scale with k1/2/N in the buoyancy-dominated regime, k1/2/S in the shear-dominated regime, and k3/2/ε in the unforced regime, where k, N, S, and ε are the turbulent kinetic energy, buoyancy frequency, mean shear rate, and turbulent kinetic energy dissipation rate, respectively. Implications for estimates of diapycnal mixing in the ocean are discussed and a new parameterization for eddy diffusivity is presented.

Damping Models for Shear Beams with Applications to Spacecraft Wiring Harnesses

Spacecraft wiring harnesses can fundamentally alter a spacecraft’s structural dynamics, necessitating a model to predict the coupled dynamic response of the structure and attached cabling. Although a beam model including first-order transverse shear can accurately predict vibration resonance frequencies, current time domain damping models are inadequate. For example, common proportional damping models result modal damping that depends unrealistically on the frequency. Inspired by a geometric rotation-based viscous damping model that provides frequency independent modal damping in an Euler–Bernoulli formulation, a viscous damping model with terms associated with the shear and bending angles is presented. The model provides modal damping that is approximately constant in the bending-dominated regime (low mode numbers), increasing by at most 6% for a particular selection of bending and shear angle-based damping coefficients. In the shear-dominated regime (high mode numbers), damping values increase linearly with mode number and in proportion to the shear angle-based damping coefficient. A key feature of this shear beam damping model is its ready finite element implementation using only matrices commonly developed for an Euler–Bernoulli beam. Such an analysis using empirically determined damping coefficients generates damping values that agree well with existing spacecraft wiring harness cable data.

Predictions from an anisotropic inflationary era

This paper investigates the predictions of an inflationary phasestarting from a homogeneous and anisotropic universe of theBianchi I type. After discussing the evolution of the background spacetime, focusing on the number ofe-folds and the isotropization, we solve the perturbation equations and predict the powerspectra of the curvature perturbations and gravity waves at the end of inflation.The main features of the early anisotropic phase is (1) a dependence of the spectra onthe direction of the modes, (2) a coupling between curvature perturbations andgravity waves and (3) the fact that the two gravity wave polarizations do not sharethe same spectrum on large scales. All these effects are significant only on largescales and die out on small scales where isotropy is recovered. They depend on acharacteristic scale that can, but a priori must not, be tuned to some observable scale.To fix the initial conditions, we propose a procedure that generalizes the one standardlyused in inflation but that takes into account the fact that the WKB regime is violated atearly times when the shear dominates. We stress that there exist modes that do not satisfythe WKB condition during the shear-dominated regime and for which the amplitude at theend of inflation depends on unknown initial conditions. On such scales, inflation loses itspredictability.This study paves the way for the determination of the cosmological signature of a primordial shear, whateverthe Bianchi I spacetime. It thus stresses the importance of the WKB regime to drawinflationary predictions and demonstrates that, when the number ofe-folds is large enough, the predictions converge toward those of inflation in a Friedmann–Lemaîtrespacetime but that they are less robust in the case of an inflationary era with a small number ofe-folds.

Combined Effect of Spin Speed and Ionic Strength on Polyelectrolyte Spin Assembly

Polyelectrolyte spin assembly (PSA) of multilayers is a sequential process featuring adsorption of oppositely charged polyelectrolytes from dilute solutions undergoing spin-coating flow. Here, we report on the dependence of PSA multilayer buildup of poly(sodium 4-styrenesulfonate) and poly(allylamine hydrochloride) on solution ionic strength and spin speed. We observed that at a given spin speed, the PSA coating growth rate (thickness/bilayer) and polymer surface coverage shows a nonmonotonic dependence on salt concentration, first increasing and then decreasing with increasing solution ionic strength. This is argued to be a manifestation of two competing mechanisms responsible for the layer formation. At low salt concentrations, the electrostatic interactions control the multilayer assembly process, while at high salt concentrations it is dominated by shear flow. We explain this nonmonotonic behavior in the framework of a Flory-like theory of multilayer formation from polyelectrolyte solution under shear flow. Additionally, the PSA process led to multilayer coatings with a radial dependence on thickness at lower spin speed in the shear-dominated regime. On increasing spin speed, such radial dependence subsided, eventually leading to uniform coatings by planarization. The surface topography of the multilayered coatings adsorbed at salt concentration less than 0.1 M was flat and featureless for all studied spin speeds. Unique morphological features in the films were formed at salt concentration higher than 0.1 M, the size of which depended on the spin speed and solution ionic strength.

Activated Dynamics and Effective Temperature in a Steady State Sheared Glass

We conduct nonequilibrium molecular dynamics simulations to measure the shear stress sigma, the average inherent structure energy E{IS}, and the effective temperature T{eff} of a sheared model glass as a function of bath temperature T and shear strain rate gamma. For T above the glass transition temperature T0, the rheology approaches a Newtonian limit and T{eff}-->T as gamma-->0, while for T<T0, sigma approaches a yield stress and T{eff} approaches a limiting value near T0. In the shear-dominated regime at high T, high gamma or at low T, we find that sigma and E{IS} each collapse onto a single curve as a function of T{eff}. This indicates that T{eff} is controlling behavior in this regime.

Aging under shear: Structural relaxation of a non-Newtonian fluid

The influence of an applied shear field on the dynamics of an aging colloidal suspension has been investigated by the dynamic light-scattering determination of the density autocorrelation function. Though a stationary state is never observed, the slow dynamics crosses between two different nonequilibrium regimes as soon as the structural relaxation time tau(s) approaches the inverse shear rate gamma (-1) . In the shear-dominated regime (at high gamma values) the structural relaxation time is found to be strongly sensitive to the shear rate ( tau(s) approximately gamma (-alpha) , with alpha approximately 1 ) while aging proceeds at a very slow rate. The effect of the shear on the detailed shape of the density autocorrelation function is quantitatively described, assuming that the structural relaxation process arises from the heterogeneous superposition of many relaxing units, each one independently coupled to shear with a parallel composition rule for time scales: 1/tau-->1/tau+A gamma .

Post-Permian evolution of the Central North Sea: a numerical model

A dynamic thermomechanical model is presented for the post-Permian evolution of the Central North Sea including the Central Graben. Permo-Carboniferous and Jurassic thermal events, as well as Triassic and Jurassic extensional phases and Late Cretaceous shortening, are considered. The equations of force balance for a continuum and the transient heat equation are solved using the finite element method. The lithosphere responds to applied forces by elastic deformation at low deviatoric stress levels, fracture at low temperature and pressure conditions and nonlinear viscous flow at high temperature and pressure. A first-order rheological strain softening is incorporated to simulate a transition from strong to soft material behaviour, such as a change from dislocation creep to diffusion creep. Seismic refraction and reflection data indicate crustal thinning below the Central Graben by a factor of ∼2. While the Triassic rifting appears to have been associated with a more distributed deformation pattern, the Upper Jurassic rifting is more localised. Pure shear extensional models have been advocated to explain this. However, dipping seismic reflectors in the upper mantle indicate that simple shear could be important. The strain softening of the present model allows for modelling of strain localisation during the Upper Jurassic rifting phase and detailed modelling of Moho structures. The present model accounts for the main structural and stratigraphic observations in the Central North Sea. Furthermore, the results indicate that in the part of the Central North Sea investigated in this paper, a pure shear regime in Triassic changed into a simple shear-dominated regime with mantle shear zones developing during Late Jurassic extension.

A numerical study of the evolution and structure of homogeneous stably stratified sheared turbulence

The structure and evolution of homogeneous stably stratified sheared turbulence have been investigated through direct numerical simulation. In these simulations the primary dimensionless parameter is the Richardson number which measures the relative importance of stratification and mean shear.For Richardson numbers less than the transition value the Reynolds stress and vertical density flux are down-gradient. Some of the vertical kinetic energy gained indirectly through production is expended in creating potential energy. Included in this shear-dominated regime is the stationary Richardson number at which the turbulent kinetic energy is constant in time although the spectra are evolving. At low dimensionless shear rate the stationary Richardson number increases with increasing Reynolds number.At the transition Richardson number the maximum anisotropy and energy partition are achieved. For larger Richardson numbers potential energy is released into vertical kinetic energy and the vertical density flux becomes counter-gradient. The associated production reversal enhances the decay rate of the turbulent kinetic energy.The effects of other dimensionless parameters have been investigated. After initial transients the developed flow is rather insensitive to the presence of significant initial potential energy. An increase in the Schmidt number increases the effect of stable stratification, e.g. the counter-gradient vertical density flux occurs earlier.In the shear dominated case the down-gradient fluxes are produced by the pumping of fluid through coherent hairpin-shaped vorticity. In the buoyancy dominated flow the counter-gradient fluid parcels induce helical vorticity structures as they move toward a position of neutral buoyancy.