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Thermodynamic Entropy from Sadi Carnot’s Cycle using Gauss’ and Doll’s-Tensor Molecular Dynamics

Hoover William Graham, Hoover Carol Griswold

Computational Methods in Science and Technology

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2022

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Vol. 28, No. 2

41--46

EN

Carnot’s four-part ideal-gas cycle includes both isothermal and adiabatic expansions and compressions. Analyzing this cycle provides the fundamental basis for statistical thermodynamics. We explore the cycle here from a pedagogical view in order to promote understanding of the macroscopic thermodynamic entropy, the state function associated with thermal energy changes. From the alternative microscopic viewpoint the Hamiltonian H(q, p) is the energy and entropy is the (logarithm of the) phase-space volume Ω associated with a macroscopic state. We apply two novel forms of Hamiltonian mechanics to Carnot’s Cycle: (1) Gauss’ isokinetic mechanics for the isothermal segments and (2) Doll’s Tensor mechanics for the isentropic adiabatic segments. We explore the equivalence of the microscopic and macroscopic views of Carnot’s cycle for simple fluids here, beginning with the ideal Knudsen gas and extending the analysis to a prototypical simple fluid.

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Influence of addition of carbon nanotubes on rheological properties of selected liquid lubricants - a computer simulation study

Chopra Anjali, Winczewski Szymon

TASK Quarterly : scientific bulletin of Academic Computer Centre in Gdansk

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2020

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Vol. 24, No 4

345--388

EN

This work is motivated by the improvement of anti-friction properties of lubricants by addition of CNTs proved experimentally in literature. In particular, a methodology is developed to compute the shear viscosity of liquid lubricants (Propylene Glycol) based on Molecular Dynamics simulation. Non-Equilibrium molecular dynamics (NEMD) approach is used with a reactive force field ReaxFF implemented in LAMMPS. The simulations are performed using the canonical (NVT) ensemble with the so-called SLLOD algorithm. Couette flow is imposed on the system by using Lees-Edwards periodic boundary conditions. Suitable parameters such as simulation time and imposed shear velocity are obtained. Using these parameters, the influence of addition of 27 wt% CNTs to Propylene Glycol on its viscosity is analyzed. Results show that 3.2 million time-steps with a 0.1 fs time-step size is not sufficient for the system to reach equilibrium state for such calculations. With the available computational resources, a shear velocity of 5 × 10−5 Å/fs was observed to give viscosity value with approximately 43% error as compared to the experimental value. Moreover, the lubricant exhibited a shear thinning behaviour with increasing shear rates. CNTs enhanced the lubricant's viscosity by 100-190% depending upon the averaging method used for calculation.

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Non-equilibrium Computer Simulations of Coupling Effects under Thermal Gradients

Bresme F.

Computational Methods in Science and Technology

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2017

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Vol. 23, No. 3

165--173

EN

In this work, we discuss recent developments in the computer simulations of molecular fluids under thermal gradients. Non-equilibrium simulations allow performing numerical tests of fundamental questions of non-equilibrium thermodynamics. These tests show that non-equilibrium simulations provide an efficient approach to quantify within a single simulation the thermophysical properties of fluids along an isobar. We discuss aspects connected to the computation of local temperatures in systems under the influence of heat fluxes, and how the combination of non-equilibrium molecular dynamics and non-equilibrium thermodynamics allows understanding phenomena arising from the coupling of internal molecular variables and heat fluxes, which lead, e.g. to thermo-molecular orientation. The behavior of these orientational effects near a fluid critical point is also discussed.