Choosing Turbulence Schemes

Most flows of interest in the water and wastewater industry are turbulent. The unsteady and irregular behavior of turbulent flows are their defining characteristic. Thus, the complete solution of the Navier-Stokes equations is inherently transient. However, most of the time the CFD engineer can capture sufficient information from a CFD analysis using mean quantities of the flow field. This chapter describes three turbulent resolving strategies for the N-S equations. Each strategy is introduced by presenting the assumptions and changes employed in the solution of the Navier-Stokes equations. The chapter briefly describes applications of RANS closure models. Recommendation for a generic turbulent closure model is difficult because it requires an understanding of the relevant flow features of the problem of interest. In the activated sludge model, RANS simulation is used to predict the distribution of water flow velocities, pressure, and volume fractions in the activated sludge tank.