The perforated plate is composed of 62 1-mm diameter holes arranged in a circular pattern. The CT images clearly show that not all aeration holes are active at low superficial gas velocities and the number of active aeration holes increases with increasing Ug. The more dense glass bead bed has less active holes at the same Ug when compared to the less dense beds of ground walnut shell or ground corncob. However, all aeration holes are active, for all bed materials, at the highest superficial gas velocities of this study. The CT images also show that the jets in the center of the bed dissipate before those near the bed walls. Eventually, all jets lose their identify.