Plume merger: double rows, area sources and ambient stratification

The merger of adjacent plumes is observed in many situations, such as multiport diffusers, natural ventilation and cooling towers. In the latter case, plume merger impacts the dilution rate and the likelihood for fog formation. Having the ability to estimate correctly visible plume lengths is crucial in evaluating possible risks to nearby infrastructure, e.g. due to ice accretion or reduced visibility.

By combining turbulent plume theory with potential flow theory, this talk will explore plume merger with particular emphasis on the number of plumes, the source conditions and the ambient stratification. In the former case, we will contrast a single-row vs. double-rows of plumes. The double-row geometry admits substantially different patterns of ambient entrainment depending on whether plumes are aligned or staggered. Notwithstanding these differences, double rows of plumes will be shown to exhibit remarkably similar profiles of vertical velocity. Our assessment of the impact of source conditions will focus primarily on the source radii, i.e. we will present a novel extension to earlier theories that permits these source radii to be non-vanishing. The adaptation in question allows us to consider more carefully the impact of source separation e.g. on the vertical distance necessary for merging plume first contact, full merger or the eventual relaxation to a circular cross-sectional shape. Of course, the vertical distances just described may also be influenced by density variations in the ambient. In the final part of this talk, therefore, we shall consider the evolution of merging plumes through ambients with stratifications characterized as uniform or non-uniform. The significance of our work in the context of cooling tower plume abatement will be briefly highlighted.