We present the results of an experimental and theoretical study on the atomization process of high viscosity and polymeric fluids. We have development a model for the atomization process in swirl and fan nozzles. The primary atomization in such nozzles results in the formation of filaments and long ligaments, which breakup into droplets. We propose a primary atomization model based on the interaction of two breakup mechanisms: the growth of surface wave and the formation of perforations. At the nozzle orifice, small scale surface waves are formed due to a high relative velocity between the liquid sheet and ambient gas. As the surface waves grow, the liquid sheet forms alternating thick and thin regions due to the nonlinearity of the surface wave. The thin region becomes thinner as the sheet expands and perforations appear. As these perforations expand, streamwise filaments form as the boundaries of these perforations approach each other. Close to the breakup position, the growth of these perforations will eventually be stopped by the thick regions on the liquid sheet. As these streamwise filaments detach from the liquid sheet, the liquid sheet breaks up and forms filaments. As a result, there are two types of filaments formed: the thin streamwise filaments formed from the breakup of thin regions due to perforations, and the thick spanwise filaments formed from the thick regions due to the surface wave. These filaments become thinner due to the lateral velocity and eventually these two types of filaments break up into droplets due to surface waves and form the spray with a wide range of droplet sizes. Theoretical models are developed to predict the droplet size distribution for this breakup for both Newtonian and Viscoelastic fluids.