Due to the increasing importance of bubble column reactors in most industrial sectors, the study of their hydrodynamics acquired significant relevance in recent years. The design of bubble columns depends on the quantification of three main phenomena: (1) mixing characteristics, (2) heat and mass transfer properties, (3) chemical kinetics in case of reactants systems.
As a consequence, the correct design and operation relies on the precise knowledge of the fluid dynamics phenomena on different scales: (1) molecular scale, (2) bubble scale, (3) reactor scale, and (4) industrial scale. The fluid dynamics properties in bubble columns depend on the interaction between the gas and liquid phases, which are related to the prevailing flow regime.  
The description of the hydrodynamics of bubble columns required the definition of some parameters. The superficial gas and liquid velocities are defined as the ratio between the volumetric flow rate of the gas and liquid, respectively, divided by the column cross-sectional area.  Although the superficial velocity concept is based on a simple one-dimensional flow assumption, it can be used to characterize and determine the hydrodynamics in bubble columns since an increase in its value can determine a flow regime transition.    
Concerning global flow properties, a fundamental aspect which is helpful in describing the bubble column design process is the global gas holdup. It is defined as the ratio of the volume occupied by the gas phase and the sum of the volume occupied by the gas and liquid phases: 
Where: 
*  is the volume occupied by the gas phase.
*  is the volume occupied by the liquid phase.
The gas holdup provides information about the mean residence time of bubbles inside the column. Combined with bubble dimensions ( a fundamental local flow property), it determines the interfacial area for the heat and mass transfer rate between the phases.