Granular Materials. A granular material is a collection of distinct macroscopic particles, such as sand in an hourglass or peanuts in a container. The evolution of the particles follows Newton's equations, with repulsive forces between particles that are non-zero only when there is a contact between particles.onstitutive laws for granular flows. A constitutive relation is an equation that relates the stress in a material to other known variables. The common constitutive relation in fluid dynamics sets the stress proportional to the strain rate.