The transition to unsteady flow and the dynamics of moderate-Reynolds-number flows in unbounded and wall-bounded periodic arrays of aligned cylinders are examined using lattice-Boltzmann simulations. The simulations are compared to experiments, which necessarily have bounding walls. With bounding walls, the transition to unsteady flow is accompanied by a loss of spatial periodicity, and the temporal fluctuations are chaotic at much smaller Reynolds numbers. The walls, therefore, affect the unsteady flows everywhere in the domain. Consistency between experiments and simulations is established by examining the wake lengths for steady flows and the fundamental frequencies at higher Reynolds numbers, both as a function of the Reynolds number. Simulations are used to examine the velocity fluctuations, flow topologies, and the fluctuating forces on the cylinders.