By considering the evolution of a localized voidage disturbance imposed on an otherwise uniformly fluidized bed we are able to determine the dominant effects of the many terms in the continuum equations of motion governing a fluidized bed. For small perturbations a linearized theory is developed, showing that the stability of the uniform state is critically dependent upon the particle-phase collisional pressure and the flow rate of the uniform state, while the effect of particle phase viscosity is shown to be purely dispersive. When the uniform state is stable, the disturbance is shown to develop into a decaying pulse followed by a decaying wavetrain.

For finite-amplitude disturbance, nonlinear effects are considered. These are shown to give rise to the propagation of high voidage gradients through the bed. Having established that such voidage fronts will develop, a detailed study of their structure is made. This gives strong indications that, for flow rates at which the uniform state is unstable, the bed will restabilize into a quasisteady periodic state.