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Evaluation of methods for viscosity simulations of lubricants at different temperatures and pressures: a case study on PAO-2

Evaluation of methods for viscosity simulations of lubricants at different temperatures and pressures: a case study on PAO-2
Evaluation of methods for viscosity simulations of lubricants at different temperatures and pressures: a case study on PAO-2
The behavior of lubricants at operational conditions, such as at high pressures, is a topic of great industrial interest. In particular, viscosity and the viscosity-pressure relation are especially important for applications and their determination by computational simulations is very desirable. In this study we evaluate methods to compute these quantities based on fully atomistic molecular dynamics simulations which are computationally demanding but also have the potential to be most accurate. We used the 9,10-dimethyloctadecane molecule, main component of PAO-2 base oil as the lubricant for our tests. The methods used for the viscosity simulations are the Green-Kubo equilibrium molecular dynamics (EMD-GK) and non-equilibrium molecular dynamics (NEMD), at pressures of up to 1.0 GPa and various temperatures (40-150 degrees Celsius). We present the theory behind these methods and investigate how the simulation parameters affect the results obtained, to ensure viscosity convergence with respect to the simulation intervals and all other parameters. We show that by using each method in its regime of applicability, we can achieve good agreement between simulated and measured values. NEMD simulations at high pressures captured zero shear viscosity successfully, while at 40 degrees Celsius EMD-GK is only applicable to pressures up to 0.3 GPa, where the viscosity is lower. In NEMD, longer and multiply repeated simulations improve the confidence interval of viscosity, which is essential at lower pressures. Another aspect of these methods is the choice of the utilized force field for the atomic interactions. This was investigated by selecting two different commonly used force fields.
Non-Newtonian Behavior, Viscosity-pressure, Viscosity-temperature, Synthetic Base Stocks, Lubricant Physical Analysis
1040-2004
Mathas, Dimitrios
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Holweger, Walter
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Wolf, Marcus
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Bohnert, Christof
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Bakolas, Vasilios
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Procelewska, Joanna
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Wang, Ling
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Bair, Scott
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Skylaris, Chris-Kriton
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Mathas, Dimitrios
aee09891-6e68-489b-982a-cf73908c16d2
Holweger, Walter
97dc70d7-c418-430b-8f43-424983c07e8d
Wolf, Marcus
0903c887-eefe-43f7-9173-efce9c865624
Bohnert, Christof
f4dda3a3-b0bc-42a7-9efd-43d2fab9e2d3
Bakolas, Vasilios
15abf440-f075-4705-be98-39e110c33345
Procelewska, Joanna
1139b924-fc8c-413f-a970-7b21d1cc88c1
Wang, Ling
c50767b1-7474-4094-9b06-4fe64e9fe362
Bair, Scott
d0f78d11-966d-4dcc-824b-cd8c44801626
Skylaris, Chris-Kriton
8f593d13-3ace-4558-ba08-04e48211af61

Mathas, Dimitrios, Holweger, Walter, Wolf, Marcus, Bohnert, Christof, Bakolas, Vasilios, Procelewska, Joanna, Wang, Ling, Bair, Scott and Skylaris, Chris-Kriton (2021) Evaluation of methods for viscosity simulations of lubricants at different temperatures and pressures: a case study on PAO-2. Tribology Transactions. (doi:10.1080/10402004.2021.1922790).

Record type: Article

Abstract

The behavior of lubricants at operational conditions, such as at high pressures, is a topic of great industrial interest. In particular, viscosity and the viscosity-pressure relation are especially important for applications and their determination by computational simulations is very desirable. In this study we evaluate methods to compute these quantities based on fully atomistic molecular dynamics simulations which are computationally demanding but also have the potential to be most accurate. We used the 9,10-dimethyloctadecane molecule, main component of PAO-2 base oil as the lubricant for our tests. The methods used for the viscosity simulations are the Green-Kubo equilibrium molecular dynamics (EMD-GK) and non-equilibrium molecular dynamics (NEMD), at pressures of up to 1.0 GPa and various temperatures (40-150 degrees Celsius). We present the theory behind these methods and investigate how the simulation parameters affect the results obtained, to ensure viscosity convergence with respect to the simulation intervals and all other parameters. We show that by using each method in its regime of applicability, we can achieve good agreement between simulated and measured values. NEMD simulations at high pressures captured zero shear viscosity successfully, while at 40 degrees Celsius EMD-GK is only applicable to pressures up to 0.3 GPa, where the viscosity is lower. In NEMD, longer and multiply repeated simulations improve the confidence interval of viscosity, which is essential at lower pressures. Another aspect of these methods is the choice of the utilized force field for the atomic interactions. This was investigated by selecting two different commonly used force fields.

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Evaluation of methods for viscosity simulations of lubricants at different temperatures and pressures a case study on PAO 2 - Accepted Manuscript
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Accepted/In Press date: 22 April 2021
e-pub ahead of print date: 1 May 2021
Published date: 1 May 2021
Keywords: Non-Newtonian Behavior, Viscosity-pressure, Viscosity-temperature, Synthetic Base Stocks, Lubricant Physical Analysis

Identifiers

Local EPrints ID: 449231
URI: http://eprints.soton.ac.uk/id/eprint/449231
ISSN: 1040-2004
PURE UUID: 4410d688-39d2-4799-934a-c90c0c63cad4
ORCID for Dimitrios Mathas: ORCID iD orcid.org/0000-0003-0116-0156
ORCID for Ling Wang: ORCID iD orcid.org/0000-0002-2894-6784
ORCID for Chris-Kriton Skylaris: ORCID iD orcid.org/0000-0003-0258-3433

Catalogue record

Date deposited: 20 May 2021 16:31
Last modified: 29 May 2021 01:53

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Contributors

Author: Dimitrios Mathas ORCID iD
Author: Walter Holweger
Author: Marcus Wolf
Author: Christof Bohnert
Author: Vasilios Bakolas
Author: Joanna Procelewska
Author: Ling Wang ORCID iD
Author: Scott Bair

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