This paper describes an investigation of structure in moderately dilut
e three-dimensional shear flows. Structure is defined as a dynamic inh
omogeneity or fluctuation in the spatial concentration field. Numerica
l experiments are performed with large numbers of identical, frictionl
ess, inelastic spheres. The spheres are contained in a fully periodic
cubic control volume. A state of shear is maintained in the control vo
lume by moving the upper periodic image in one direction and the lower
image in the opposite direction. As the coefficient of restitution of
the spheres is lowered, conditions in the control volume deviate from
a state of simple shear, exhibiting strong wavelike fluctuations in t
he concentration, stress, and velocity fields. Visual inspection of th
e spatial concentration field reveals a strong tendency for spheres wi
th a low coefficient of restitution to form dense elongated clouds. Th
e major axis of the clouds tends to align itself in the direction of t
he mean velocity and perpendicular to the direction of variation in th
e mean velocity created by the moving periodic images of the control v
olume.