ROLE OF COHERENT STRUCTURES ON REYNOLDS STRESSES IN A 2-D BUBBLE-COLUMN

The hydrodynamics of two-dimensional bubble columns operated in variou s flow regimes are studied using particle image velocimetry. Both aver aged velocity profiles and Reynolds stress profiles are obtained and d iscussed in relation to large-scale structures present in the flow. Th e normal stresses, dominated by large-scale structures, are an order o f magnitude higher than the shear stress. It is found that the contrib ution from the bubbles to the shear to the normal stresses is negligib le. A time series of the flow field is studied, demonstrating that the flow could be split into a low-frequency contribution due to the vort ical structures and a high-frequency fluctuating part. The latter give s rise to flat normal stress profiles, and the former is responsible f or the original form of the normal sh ess profiles. The shear stress i n the smaller columns investigated can be related to the averaged vert ical velocity profile according to a Boussinesq approximation. Data on the eddy viscosity are presented.