Near Field Flow Dynamics of Concentrate Discharges and Diffuser Design

The major physical aspects of near field mixing of dense jets resulting from diffuser discharges of concentrate are presented. It is proposed that any environmental impacts of such discharges will be local rather than regional, so initial mixing processes are an essential component of an effective disposal scheme. Typical international environmental criteria for concentrate are summarized; these can be readily met by well-designed diffusers. The major features of dense jets are presented, beginning with the simplest case of an inclined jet into deep stationary water, followed by the effects of shallow water. We then discuss merging jets from multiport and rosette diffusers and it is shown that their dynamic interaction can be critical and lead to significant changes in flow patterns and reduction in dilution. Design criteria are suggested to avoid impaired dilution. The effects of currents on single and multiport diffusers are then discussed. It is shown that small modifications in diffuser design can lead to significant changes in flow field and dilution. Some issues and difficulties with mathematical modeling of near field flows are discussed and how entrainment models may not adequately represent critical phenomena including dynamic jet and boundary interaction, re-entrainment, density current dynamics, and turbulence collapse. Finally, some open research issues are discussed.