Transition from Jet Plume Dilution to Ambient Turbulent Mixing

Abstract

The average properties of many turbulent buoyant jet flow configurations have been deduced by the application of simple asymptotic arguments based on dimensional analysis coupled with a general understanding of the flow dynamics. Most of these results have been confirmed by experimental observation and form the basis for validation of the many computer codes that have been developed to define such flows. A key issue in the application of these results to problems of environmental concern is the influence of mixing driven by ambient turbulence levels. Many of the models in use by regulatory agencies ignore ambient turbulence-induced mixing on the basis that to do so will provide conservative estimates of dilution. However, it is now clear from the results of laboratory studies that the estimates of dilution derived by both the asymptotic theories and integral models grossly underestimate the actual concentration levels that exist within the flow. This situation has arisen as a result of using an averaging process the obliterates peak concentration levels. Recent laboratory work that provides detailed spatial and temporal measurements of concentration distributions in jets and plumes discloses peak concentration levels that can exceed fluxweighted average concentrations by a factor of 6. Furthermore, the measurements show regions of high concentration fluid that remains as coherent regions within the flow field well into the domain where jet or plume derived turbulent mixing is significantly reduced. The importance of the level of ambient turbulence in these flow situations becomes clear and it should not be ignored without the realization that effluent concentrations may greatly exceed estimated levels.