Host: The Japan Society of Mechanical Engineers
Name : [in Japanese]
Date : September 13, 2020 - September 16, 2020
When an object is immersed in a gas in the presence of thermal gradient whose scale is comparable to the molecular mean free path, a tangential Knudsen force can be generated. This thermal force can be exploited to set objects in motion even without the presence of external forces and with nearly frictionless contact1. This paper presents a numerical analysis of novel configurations of surface microstructures shown in Fig.1, that can induce tangential Knudsen forces. As this phenomenon appears in rarefied gas conditions, or when the Knudsen number is not vanishingly small, conventional analyses based on the continuum hypothesis are inadequate and must be treated by the Boltzmann equation rather than the Navier–Stokes equations2. In this study, the direct simulation Monte Carlo (DSMC) method is used and performed for different microstructures and different Knudsen numbers. Results show that the tangential Knudsen force varies in each microstructure and in different Knudsen numbers. It is found that certain shapes and geometric parameters induce large tangential Knudsen forces. Furthermore, the mechanism of the tangential Knudsen force is also found to be different depending on the range of the Knudsen number. For Knudsen numbers of Kn < 0.1, i.e., near the continuum limit, the flow in the bulk of the gas caused by thermally driven flows, drags the upper surface through viscous shear, inducing a tangential Knudsen force. However, from the transition flow regime and above, i.e., Kn > 0.1, molecular flow mechanism appear and therefore the mechanism of the tangential Knudsen force cannot be described by viscous flow mechanism alone.
