New indirect method for calculation of flow forces in molecular dynamics simulation

Abstract

A new method for calculation of fluid flow forces in molecular dynamics (MD) simulations is proposed herein. We call this method Molecular Momentum Balance method. This method together with two other methods reported in literature are used in an in-house, parallel MD code to calculate drag force exerted on a carbon nanotube in a uniform flow of liquid argon, as well as skin wall friction in argon channel flow. The two methods used as a benchmark are a method based on direct summation of forces exerted by fluid molecules, called the Direct Method herein, and a method based on the flow momentum balance obtained from the mean velocity profile, called the momentum balance method herein. Carbon nanotube is modeled as a rigid cylinder, and the channel walls are modeled with two layers of carbon atoms. All the interactions are modeled using Lennard-Jones potential function. In the case of carbon nanotube, results indicate that at best, the momentum balance method generates 20% error compared to the drag force calculated by the direct method. This error does not noticeably decrease by increasing the number of bins used for capturing the mean velocity profile. In contrast to this, the present method, i.e. the molecular momentum balance generates a very small error compared to the results from the direct method. This error can be reduced to less than 1% by increasing the time period in which data are averaged. Based on this experience, the present method was applied to calculate skin friction drag in two channel flows, i.e. a channel with spring-mounted carbon wall molecules, and a channel with rigid carbon wall molecules. The results were within 0.1% of those from the direct method. This confirms that the new method works properly to predict flow forces in different MD simulations.