Grouping is the attachment of particles as a result of fluctuation in the carrying-fluid's velocity field. A mathematical model for grouping in standing wave and moving wave velocity field were build and the characterization for creation of grouping were identified. Grouping was also modeled for nanoparticles so that Brownian motion should be taken for account. We add particle motion equation a stochastic term which considered Brownian motion as a white noise and checked grouping in different particles size in order to find the size where grouping become insignificant and Brownian motion become the dominant factor in particles motion. Recently the study of grouping has led to the development of a novel Diesel exhaust system which induces the smoke particles to group and hence reduce the risk associated with those particles.