Wyniki wyszukiwania - BazTech

1

Minimal Energy Dissipation Rate and Director Orientation Relative to External Dissipative Fields such as Temperature and Velocity Gradients in Nematic and Cholestric Liquid Crystals

Sarman S., Wang Y-L., Laaksonen A.

Computational Methods in Science and Technology

|

2017

|

Vol. 23, No. 3

239--250

EN

The purpose of this review article is to summarize observations accumulated over the years on director alignment phenomena in nematic and cholesteric liquid crystals by molecular dynamics simulation of molecular model systems and by experiment on real systems. The main focus is on the alignment angle between the director and external dissipative fields such as velocity gradients in various flow geometries and temperature gradients doing irreversible work on the system. A general observation is that the director attains an orientation relative to the field where the energy dissipation rate is minimal in the steady state. In the case of planar elongational flow, it can be proven by using symmetry arguments that the energy dissipation rate must be either maximal or minimal and simulations have shown that is minimal. In planar Couette flow both simulations and experiments imply that the energy dissipation rate is minimal in the steady state. Finally, in the case of heat conduction, symmetry arguments imply that the energy dissipation rate must be either minimal or maximal and simulations and experiments indicate that it is minimal. All these observations can be explained by applying a recently proven theorem according to which the energy dissipation rate is minimal in the steady state in the linear regime at low fields.

2

The flow of viscoelastic fluids in an opposed jets rheometer and a Fano flow rheometer

Wilkes C., Gampert B.

International Journal of Applied Mechanics and Engineering

|

2003

|

Vol. 8, no spec.

319-324

EN

The elongational rheometer used for this work is an opposed jet rheometer, which allows elongational flow measurements of highly dilute polymer solutions. The polymer concentrations can be lower than the critical concentration under steady state conditions and lower than the critical concentration in the case of flow conditions. A further experimental set up was used for characterising the viscoelastic fluids. The working method of this apparatus is the tubeless siphon or Fano flow, which can be understood approximately as the reverse fibre spinning process. The beginning of the fibre spinning process can be applied in order to determine the critical concentration . The experimental investigations undertaken here were carried out using a commercial, anionic polyacrylamide with a relatively high molar mass and hydrolysis factor. The polyacrylamide was dissolved in highly pure water. Sodium chloride with different concentrations was added to the solutions to control the partial neutralisation of negative charges at the ends of the polymer chains.

3

Rheological model and numerical simulation of dispersive mixing of rubber compounds

Petera J., Clarke J.

Applied Mechanics and Engineering

|

1999

|

Vol. 4, no spec.

303-308

EN

With the ultimate aim of improving mixer geometry by using a mathematical model for rubber mixing, this Paper concentrates on the dispersive mixing process of filler disagglomeration. Specially designed elongational flow experiments were used to achieve dispersive mixing under conditions of known stress and strain rate history. It was found that the disagglomeration process is satisfactorily described by a first order differential equation with the rate constant proportional to the power density experienced by the compound during mixing. The kinetic model was implemented in original finite element software to obtain a 2-dimensional simulation of mixing in a twin rotor internal mixer. It was concluded that the 2-dimensional simulation would be useful for relating cross-sectional rotor geometry to efficiency of filler disagglomeration, and hence dispersive mixing.