On modelling turbulent flow in an oscillatory baffled column - RANS model or large-eddy simulation?

Oscillatory flow in a baffled column can achieve intensive and uniform mixing with lower shear rate in comparison to the traditional stirred tank reactors (STR). The flow in an oscillatory baffled column (OBC) is unsteady with periodicity in both time and space. Mixing is achieved by the formation of eddies. It is therefore very important to understand the turbulence in an OBC as it determines the degree of segregation, breakage of droplets and bubbles, and the efficiency of chemical/biochemical reactions. Modelling of turbulence is usually associated with the traditional Reynolds Averaged Navier-Stokes (RANS) model. Our results show that the RANS turbulence models, which are sufficiently good for simulating flows in STR, are very poor in predicting periodic flows in an OBC as the methodology of averaging in time in RANS has effectively removed the turbulence. In OBC, eddies of various sizes are the main ingredient for mixing, the large-eddy simulation (LES) is particularly suitable for such type of flows. In this paper, we describe our understanding on the way in which the LES works. (C) 2003 Society of Chemical Industry.