Similitude Considerations for Aerosol Diffusion

For an external flow, it is as.sumed that the fluid has a uniform velocity, U, except in the region disturbed by the body. If the concentration in the mainstream of the fluid is a dimensionless concentration can be defined as follows  

Limtitng consideration to the steady state, the equation of convective diffiLsion in the absence of an external force field can be expressed in dimensionless fomi as follows 

In many cases, the velocity field can be assumed to be independent of the diffusional field. The steady isothermal flow of a viscous fluid, such as air, in a system of given geometry depends only on the Reynolds number when the velocity is small compared with the speed of sound. 

The boundary condition for particle diffusion differs from the condition for molecular dilTusion becau.se of the finite diameter of the particle. For certain classes of problems, such as flows around cylinders and spheres, the particle concentration is assumed to vanish at one particle radiu.s from the surface  

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