If the depth and diameter of bubbling bed is larger, the bubbles rise and merge, and grow up to almost occupy the whole bed section, the bed layer on upper part of air bubbles will be raised like  plunger, then the particles  will fall down from the top of the plunger downward, at last the plunger disperses, and from bottom to top, another plunger forms to repeat the above mentioned process. This flow phenomenon of stable pulsation and repeating in turn inside the bed layer is known as flow of "Plunger ". Many situations, people call this phenomenon more section "slugging".