Poiseuille flow to measure the viscosity of particle model fluids.Backer, J.A.; Lowe, C.P.; Hoefsloot, H.C.J.; Iedema, P.D. General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. The most important property of a fluid is its viscosity, it determines the flow properties. If one simulates a fluid using a particle model, calculating the viscosity accurately is difficult because it is a collective property. In this article we describe a new method that has a better signal to noise ratio than existing methods. It is based on using periodic boundary conditions to simulate counter-flowing Poiseuille flows without the use of explicit boundaries. The viscosity is then related to the mean flow velocity of the two flows. We apply the method to two quite different systems. First, a simple generic fluid model, dissipative particle dynamics, for which accurate values of the viscosity are needed to characterize the model fluid. Second, the more realistic Lennard-Jones fluid. In both cases the values we calculated are consistent with previous work but, for a given simulation time, they are more accurate than those obtained with other methods.
When a particle model simulates fluid behavior, the calculation of all particle interactions causes long computation times. Especially in mesoscale simulations, the bulk areas can be computationally demanding. To reduce the time spent on such regions, we propose a model that combines different length scales in one system. This is a particle analog to mesh refinement in, for instance, finite-element methods. To this end, we define particles of a coarse-grained scale within the framework of dissipative particle dynamics. These particles have a lower number density, but the same mass density, pressure, temperature, and viscosity as the original description. Furthermore, the coarse-grained particles can directly interact with the "normal" particles. The two length scales are combined in one system, coupled by an overlap region. At the edges of this region, particles transform into the other scale, through local refining or coarse graining. The resulting combined system adequately reproduces the properties and flow behavior of a normal system. When half the system is coarse grained, the computation time reduces by a factor of two. Thus, computational efficiency can be greatly increased for a variety of mesoscale applications.
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