Dr. K. R. Rajagopal, Dept. of Mechanical and Biomedical Engineering, Texas A&M What do we mean by a fluid and how do we model them? 3PM Friday, Nov. 5, MEC 4-3 While we have some sort of intuitive feel for what we mean by a fluid, when asked to give a precise definition of the same, we are hard pressed to do so. After a brief discussion of the difficulty of providing a precise definition I outline the quintessential characteristics of a fluid-like body. I then discuss the numerous assumptions that underlie the Navier-Stokes model which is incapable of describing the behavior of many real fluids and contrary to popular belief incapable of describing turbulence. I then discuss different classes of fluid models that include fluids models of the differential, rate and integral type. I conclude the talk with a discussion of some important open problems both from the perspective of mathematics and mechanics. I end the talk by discussing the modelling of turbulence and try to provide a rationale for the need for a different perspective for developing models for turbulence, models that are totally different from the ones that are in vogue.