NONLINEAR-WAVES AND THE ORIGIN OF BUBBLES IN FLUIDIZED-BEDS

Early experiments in the mid-1940s established two different regimes o f behavior of fluidized systems. These are broadly classified into sys tems that exhibit massive phase segregation, leading to particle-free regions called 'bubbles', and those that do not. Explaining the origin of bubbles and of these two regimes has represented both a technologi cal and scientific challenge since then. The late 1960s through the 19 70s saw a series of illuminating experiments that established many fea tures of the flow regimes and their characteristics through both Bow v isualizations and quantitative measurements. Recent numerical and theo retical work has come close the resolving the problem. This paper repr esents the written version of the talk given at the Symposium in honor of Leen van Wijngaarden's retirement. In it, I review the history of progress on the problem in two giant 25-year steps.