Force Analysis Of Micro/Nano Particles In Flow Field With Molecular Dynamics Simulation

Micro/nano particle two-phase flow widely exists in nature and engineering. At the same time, it causes many problems such as haze, car exhaust and spread of disease. The force acting on the particle determines its motion. Thus it is very important to make a detailed research on the force that the fluid molecules act on the micro/nano particles. In present work,particle in the stationary gas environment and in the stable flow field were investigated by using molecular dynamics method.For Brownian particle in the stationary gas environment, the force acting on the particle was studied over a wide range of Knudsen number(Kn), the ratio between the free path and particle diameter. The spectrum analysis shows that, the force have the feature of "preferential frequency", namely the frequency of the forces acting on the Brownian particle is mainly concentrated around a certain frequency.Meanwhile, the forces also show obvious size effect, namely the magnitude of the force significantly influenced by the particle size:with the decrease of Kn, the force increases to a peak value and then gradually decreases. The size effect can directly affect the motion of the asymmetrical particle, whichfits experimentally recorded particle motion in the literature well.For micro/nano size particles in steady flow field, the drag force was usually calculated with slip correction of the Stokes law, such as the Cunningham slip correction. Although it has been widely use in practice, the Cunningham slip correction was obtained in the high speed circumstance and the performance in low velocity flow field is still need to be verified. In this paper, the drag force is firstly studied for particle in high-speed flow fields, the obtained numerical results fit well with the Cunningham’s slip correction. The further studies on the drag force were carried out for the micro/nano particle in low speed flow fields, which are commonly met in practice. The results show the Cunningham correction can no more make a accurate calculation of the drag force. A new correction method was given based on the numerical results in our work.Due to the large number of molecules in the modelling, computation cost increases exponentially with the increase of number of molecules. In this work, we take advantage of the high performance in data transmission and operation of the GPU card, carried out the simulation with the CUDA based computational code to speed up the computation.