A new algorithm for imposing the contact angle on solid surfaces is proposed in the Lattice Boltzmann color‐gradient model. The capability and accuracy of this algorithm are validated by simulation of contact angles for a droplet resting on a flat surface and on a cylindrical surface. The color‐gradient model with the proposed contact angle algorithm is then used to study the capillary valve effect in porous media. As a preliminary study, the capillary valve effect is explained by simulating immiscible two‐phase displacement within a single‐pore geometry. It is shown that the capillary valve effect is accurately captured by the present simulations. Further simulations of drainage and imbibition are also conducted to understand the capillary valve effect in an experiment‐matched pore‐network micromodel. The simulated results are found to agree quantitatively with the experimental results reported in literature, except for a few differences which result from the exclusion of contact angle hysteresis in the proposed algorithm.

No abstract

No abstract

No abstract

This paper develops a maintenance model for mission-oriented systems subject to natural degradation and external shocks. For mission-oriented systems which are used to perform safetycritical tasks, maintenance actions need to satisfy a range of constraints such as availability/reliability, maintenance duration and the opportunity of maintenance. Additionally, in developing maintenance policy, one needs to consider the natural degradation due to aging and wearing along with the external shocks due to variations of the operating environment. In this paper, the natural degradation is modeled as a Wiener process and the arrival of random shock as a homogeneous Poisson process. The damage caused by shocks is integrated into the degradation process, according to the cumulative shock model. Improvement factor model is used to characterize the impact of maintenance actions on system restoration. Optimal maintenance policy is obtained by minimizing the long-run cost rate. Finally, an example of subsea blowout preventer system is presented to illustrate the effectiveness of the proposed model.