  We report an experimental study of the dynamics of an air-fluidized thin
granular layer. Near-onset behavior of this shallow fluidized bed was described
in the earlier paper (Tsimring et al, 1999). Above the threshold of
fluidization the system exhibits a Hopf bifurcation as the layer starts to
oscillate at a certain frequency due to a feedback between the layer dilation
and the airflow drag force. After application of temporal band-pass filtering
of this frequency we discovered the spatio-temporal dynamics in the form of
defect turbulence. This type of dynamics is natural for spatio-temporal systems
close to the threshold of a Hopf bifurcation. At high flow rates, low-frequency
short-wavelength structures appear in addition to the long-wavelength
excitations. A simple model describing the instability and occurrence of
oscillations in a shallow fluidized bed, is proposed.
