SIMULATING SHEAR-FLOW

In the present article, we have analyzed to which extent the steady st ates produced in simulations of fluids undergoing shear flow, can trul y be representations of experimental steady states. For this purpose, we have performed nonequilibrium molecular dynamics (NEMD) simulations of two different fluid systems undergoing shear how. One system is a Lennard-Jones (LJ) fluid where the viscous heat produced by shearing t he system is eliminated only in certain regions of the simulation box. The other system is a polymer immersed in an atomic solvent. In this case, the viscous heat was removed by coupling a homogeneous thermosta t to different degrees of freedom in the system. The results of these simulations show that at the shear rates commonly produced in simulati on, the rate of production of viscous heat is very large. This heat is eliminated by the thermostat at rates that are higher than the rates of transport of heat across the fluid. Moreover, the heat has no time to redistribute into the different degrees of freedom of the system, a nd different steady states are reached depending on to which degrees o f freedom the thermostat is coupled. As a conclusion of this investiga tion we believe that the efforts of simulating fluids undergoing shear flow should be directed to simulate lower shear rates. (C) 1996 Ameri can Institute of Physics.