The flow of granular materials is an economically important phenomenon which is poorly understood. Granular materials may flow in several regimes: gas-solid two phase flow, rapid flow, or frictional flow. The purpose of this report is to develop a constitutive relation for the frictional flow regime in which the grains slide or roll over one another. Ultimately this relation will be incorporated into a large computer code which will be able to simulate all aspects of the motion of bed material in fluidized bed combustors. 13 refs., 6 figs., 2 tabs.