Scale dependence, correlations, and fluctuations of stresses in rapid granular flows

It is shown that, unlike in simple molecular fluids, the stress field in gr anular fluids may be strongly scale, or resolution, dependent. This is a re sult of the intrinsic lack of scale separation in these fluids. Another con sequence of the lack of scale separation in granular fluids is that microsc opic stress fluctuations, whose origin (like in molecular fluids) is the un derlying discreteness of the system, may appear as observables in macroscop ic measurements; the correlation (or decay) time of the stress fluctuations is of the order of the mean-free time, which is also a macroscopic time. A ll of these properties are intrinsic to granular fluids and not (for exampl e) results of the practical lack of scale separation that is dictated by th e fact that grains are of macroscopic dimensions or the limited statistics in simulations. Numerical evidence, based on molecular-dynamic simulations of shear flows of smooth disks in a two-dimensional enclosure, serves to de monstrate the above phenomena. (C) 2001 American Institute of Physics.