Search results for "Shear flow"

2015

In many technical applications, but also in natural processes like ice nucleation in clouds, crystallization proceeds in the presence of stresses and flows, hence the importance to understand the crystallization mechanism in simple situations. We employ molecular dynamics simulations to study the crystallization kinetics of a nearly hard sphere liquid that is weakly sheared. We demonstrate that shear flow both enhances and suppresses the crystallization kinetics of hard spheres. The effect of shear depends on the quiescent mechanism: suppression in the activated regime and enhancement in the diffusion-limited regime for small strain rates. At higher strain rates crystallization again become…

Relaxation phenomena in mixed isomeric alcohols by Mandelstam-Brillouin scattering

1991

Mandelstam-Brillouin scattering data in mixed isomeric alcohols n-pentanol (nPe-OH) and 2-methyl-2-butanol (2Me-2BuOH) are presented. The hypersonic velocity and normalized absorption are measured as a function of the scattering angle, in the temperature range from - 15-degrees-C to + 45-degrees-C, and as a function of n-PeOH molar fraction going from the pure n-PeOH to the pure 2Me-2BuOH. The experimental results confirm the existence of a shear relaxation phenomenon in the GHz region, that has been previously detected in pure liquids. The temperature dependence of the relaxation time tau-s and of the shear modulus G-infinity evaluated within viscoelastic liquid models, support the existen…

Simulations of non-spherical particles suspended in a shear flow

2000

The lattice-Boltzmann method was used to investigate the effects of the shape and concentration of the particles on the rheological properties of non-Brownian suspensions for non-zero Reynolds numbers. Several case studies were analyzed and the methods used were found to give accurate predictions for these systems. The viscosity of suspensions of both spherical and non-spherical particles was determined as functions of shear rate and concentration of particles. It was shown that, for high shear rates, shear thickening appears. This phenomenon is particularly pronounced for particles of irregular shape.

Stability of three-dimensional relativistic jets: implications for jet collimation

2010

The stable propagation of jets in FRII sources is remarkable if one takes into account that large-scale jets are subjected to potentially highly disruptive three-dimensional (3D) Kelvin-Helmholtz instabilities. Numerical simulations can address this problem and help clarify the causes of this remarkable stability. Following previous studies of the stability of relativistic flows in two dimensions (2D), it is our aim to test and extend the conclusions of such works to three dimensions. We present numerical simulations for the study of the stability properties of 3D, sheared, relativistic flows. This work uses a fully parallelized code Ratpenat that solves equations of relativistic hydrodynam…

Local simulations of the magnetized Kelvin-Helmholtz instability in neutron-star mergers

2010

Context. Global MHD simulations show Kelvin-Helmholtz (KH) instabilities at the contact surface of two merging neutron stars. That region has been identified as the site of efficient amplification of magnetic fields. However, these global simulations, due to numerical limitations, were unable to determine the saturation level of the field strength, and thus the possible back-reaction of the magnetic field onto the flow. Aims. We investigate the amplification of initially weak fields in KH unstable shear flows, and the back-reaction of the field onto the flow. Methods. We use a high-resolution ideal MHD code to perform 2D and 3D local simulations of shear flows. Results. In 2D, the magnetic …

An experimental investigation of the shear behaviour of polyethylenes with different structures

1985

Rheological properties in shear flow are presented for four different polyethylene samples: a high density, a linear low density and two low density polyethylenes manufactured using different techniques. Tests have been performed with the aid of capillary types of instrument equipped with capillaries of various lengths at three different temperatures. End correction factors have been determined and true flow curves obtained. Swelling ratios for both unannealed and annealed samples have been determined as well as the shear rate and shear stress at which irregularities begin. In some cases generalized plots have been prepared and in all cases the rheological response is discussed in terms of …

DIRECT NUMERICAL SIMULATION OF MOTION OF FERROMAGNETIC PARTICLES IN MAGNETORHEOLOGICAL SUSPENSION

2008

ABSTRACT Results simulation of magnetorheological suspension at particle level are reported. The present approach accounts for a better description of hydrodynamic interaction between close spheres. Development of lamellar structures similar to those obtained by other researchers in Poiseuille flow is observed in shear flow. Studies of single layer lamellar structures reveal presence of short chains and more complex aggregates.

Experimental and Theoretical Study of the Field Induced Phase Separation in Electro- and Magnetorheological Suspensions

1999

We present here the study of field induced phase separation in E.R. and M.R. fluids. Two thermodynamic models — one for the formation of chais of particles and the other for phase separation are presented and compared with experimental results obtained with two kinds of suspensions. One was made of silica particles in silicone oil and the other was made of magnetic polystyrene particles in water. In the presence of a flow the phase separation occurs with the dense phase forming a regular pattern of stripes. The dependence of the period of these stripes on the intensity of the magnetic field is well reproduced by the same kind of thermodynamic model if we add the effect of normal stresses i…

Polymer-brush lubricated surfaces with colloidal inclusions under shear inversion.

2011

We characterize the response of compressed, sheared polymer-brush bilayers with colloidal inclusions to highly nonstationary inversion processes by means of molecular dynamics simulations and scaling theory. Bilayers with a simple (dimeric) solvent reveal an overshoot for the shear stress, while simulations of dry brushes without explicit solvent molecules fail to display this effect. We demonstrate that mechanical instabilities can be controlled by the inclusion of macromolecular structures, such as colloids of varying softness. Based on a recently developed theory, we suggest a scaling approach to determine a characteristic time for conformational and collective responses.

Grafted polymer layers under shear: A Monte Carlo simulation

1993

Endgrafted polymers at surfaces exposed to a shear flow are modeled by a nonequilibrium Monte Carlo method where the jump rate of effective monomers to neighboring lattice sites against the flow direction is smaller than in the flow direction, assuming that this difference in jump rates is proportional to the local velocity of the flowing fluid. In the dilute case of isolated chains, the velocity profile is assumed linearly increasing with the distance from the surface, while for the case of polymer brushes the screening of the velocity field is calculated using a parabolic density profile for the brush whose height is determined self‐consistently. Linear dimensions of isolated chains are o…