Role of coherent structures on Reynolds stresses in a 2-D bubble column

The hydrodynamics of two-dimensional bubble columns operated in various flow regimes are studied using particle image velocimetry. Both averaged velocity profiles and Reynolds stress profiles are obtained and discussed in relation to large-scale structures present in the flow. The normal stresses, dominated by large-scale structures, are an order of magnitude higher than the shear stress. It is found that the contribution from the bubbles to the shear to the normal stresses is negligible. A time series of the flow field is studied, demonstrating that the flow could be split into a low-frequency contribution due to the vortical structures and a high-frequency fluctuating part. The latter gives rise to flat normal stress profiles, and the former is responsible for the original form of the normal sh ess profiles. The shear stress in the smaller columns investigated can be related to the averaged vertical velocity profile according to a Boussinesq approximation. Data on the eddy viscosity are presented.