Drag Forces on Nonspherical Aerosol Particles

This report is a review of current knowledge, based on the modified Stokes drag, of drag forces on nonspherical aerosol particles. The Knudsen numbers (Kn) for the aerosol studies are between 0 and 150, ranging from the continuum regime (Kn = 0) to the transition regime (0<Kn <20) to the free molecule regime (Kn > 20). The drag force on an aerosol particle can be expressed in terms of Stokes drag, dynamic shape factor, slip correction factor and non-Stokesian factor. The dynamic shape factor accounts for shape effects, the slip correction factor for the slip condition of gas-particle interaction, and the non-Stokesian effect for increased drag at a Reynolds number greater than 0.05. The dynamic shape factors of many shapes were measured in a sedimentation tank, using macroscopic objects, and also in aerosol systems. Theoretical and semi-theoretical equations have been developed for spheroids, cylinders, aggregates and rectangular prisms. The Knudsen-Weber equation with the adjustment for sphere diameter can be used to determine the slip correction factor for nonspherical particles. The non-Stokesian drag for rectangular prisms has been calculated numerically, and a general equation was formulated. The theory agrees well with the experimental data for cubes, tetrahedrons and aggregates.