The type of flow observed in the first (visual) model resembled closely that seen in the second model (photographic) for which a typical series of pictures is shown in Fig.2 . initially the dye is in the core of the bolus , wherein it is observed to be carried through the bolus at a velocity roughly twice the average (or transport) radially toward the walls of the glass tube (symmetrically , if the dye is exactly on the axis) . Upon reaching the wall , the dye adheres to the glass and there remains until the bottom interface of the bolus arrives.