COHERENT STRUCTURES IN MULTIPHASE FLOWS

The occurrence of coherent structures in multiphase flows is widesprea d: quantitative experimental data with respect to their size, their ve locities, and their frequency in both bubble columns and fluidized bed s are presented; these data have been collected by such different tech niques as Laser-Doppler Velocimetry, Partice Image Velocimetry, Optica l Bubble Probes, and Gamma-Ray Densitometry. In addition, results are discussed from Computational Fluid Dynamics simulations as to particle swarms in the freeboard beyond bubbling fluidized beds. The question is raised whether there is a common explanation for the appearance of coherent structures in such different systems. The similarity of multi phase flows and natural convection phenomena is discussed, while some features of turbulent multiphase flows resemble those in shear-induced turbulence in single-phase flows. Phase locking, on the basis of inte rplaying time scales, and an analogy with the vortices in a Karman vor tex street are suggested as possible mechanisms behind the occurrence of coherent structures. (C) 1998 Elsevier Science S.A. AU rights reser ved.