In the present paper,we theoretically study the drag force on a particle moving towards a wall in the free molecule regime.Based on the gas kinetic theory,the formula for the force acting on a near-wall spherical particle is derived,which is consistent with Direct Simulation Monte Carlo results.It is found that the near-wall effect cannot take effect for the vanishing momentum accommodation coefficient.If the wall temperature is higher than the gas temperature,then the drag force can be enhanced by the near-wall effect.For a lower wall temperature,the drag can be suppressed,and even change its direction(in the same direction with the particle velocity).
free molecule regimedrag forcenear-wall effectgas kinetic theory
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