Strain Rate Dependence of Heat Transfer as Applied to Planar Poiseuille Flow
We have continued our work on determining a more realistic way to model heat dissipation in a nonequilibrium computer experiment. We have applied configurational expressions for the temperature to nonequilibrium molecular dynamics (NEMD) simulations. These configurational expressions require only configurational information, i.e. first and second spatial derivatives of the interaction energy, instead of the kinetic energy as in the usual expressions drived from the equipartition principle. They are thus particularily suited to NEMD simulations in which the flow velocity profile is often unknown, hence making the usual expression for the temperature based on kinetic energy impossible to use reliably.
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Principal Investigator Denis EvansPhysical and Theoretical Chemistry, RSC Australian National University |
Project r61 |
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Co-Investigator Jerome DelhommelleLaboratoire de Chimie Theorique Universite Henri Poincare |
RFCD Codes 250600 |
Significant Achievements, Anticipated Outcomes and Future Work
The previous years had been devoted to assess the validity of the configurational expressions for the temperature and their suitability to serve as references for thermostatting mechanisms (the so-called configurational thermostat). This year, we have applied the configurational thermostat to study inhomogeneous systems (for instance, liquids flowing past an interface). In such systems, the flow velocity profile is unknown. Usual simulations methods assume that the Navier-Stokes prediction for an homogeneous liquid (i.e. a linear profile) still holds for these homogeneous systems. This is of course not correct. There is therefore an interplay between the assumed flow profile and the structural and dynamical properties of the system which can lead to artefacts. Using a configurational thermostat, which does not require making any assumption about the flow velocity profile, enables us to let the simulated system develop its own flow profile.
On the example of a liquid flowing past a smooth solid surface, we showed that standard simulation methods, which assumed a linear flow profile, destroyed the physically stable flow profile ("plug-flow" profile) and enforced a linear flow profile. These spurious effects can be seen even at low shear rates (in the molecular dynamics sense). These results have been reported in a paper entitled "On the effects of assuming the flow profile in nonequilibrium simulations".
We also showed that the so-called "string phase" (particles align in strings along the flow direction when subjected to shear) observed in simulations was an artefact arising from the assumption of a linear flow profile. Using a configurational thermostat we do not observe any "string phase". Instead we observe a steep increase of the viscosity with an increase in the shear rate. This phenomenon is well known in the field of colloids (it is refered to as shear thickening). It is generally attributed to lubrication effects. Our work provides another explanation, i.e. thickening is a consequence of the non-Newtonian nature of the flow. This work was published in the paper entitled "Reexamination of string phase and shear thickening in simple fluids" (see list of publications below).
Future work includes further applications to inhomogeneous systems such as shear-induced melting of crystals.
Computational Techniques Used
The algorithms are locally written nonequilibrium molecular dynamics codes.
Publications, Awards and External Funding
External Funding and Awards
None
Publications
J. Delhommelle, J. Petravic and D. J. Evans, Reexamination of string phase and shear thickening in simple fluids, Physical Review E
68 031201 (2003) .
J. Delhommelle, J. Petravic and D. J. Evans, On the effects of assuming the flow profile in nonequilibrium simulations, Journal of
Chemical Physics 119, 11005 (2003) .
J. Delhommelle, J. Petravic and D. J. Evans, Non-newtonian behavior in simple fluids, Journal of Chemical Physics accepted for
publication (2004).