Buoyant Plumes from Multiport Diffuser Discharge in Deep Coflowing Water

An experimental study of the central portion of a unidirectional multiport diffuser discharging in the direction of a uniform ambient current in deep nonturbulent water was conducted by means of a towed model in a laboratory tank. The trajectories of the two-dimensional plane plumes that result after merging of the individual jets can be divided into weakly deflected plumes ( F_a < 0.60), with upstream spreading upon surface interaction, and strongly deflected plumes ( F_a > 1.0), with an intermediate range that contains an upstream blocked wedge. The dominant parameters F_a is an ambient/discharge Froude number, F_a =u _a/${\mathit{j}}_{\mathit{o}}^{1/3}$, where u _a is the ambient current and j _o is the two-dimensional buoyancy flux per unit diffuser length. In each plume regime the plume trajectory is a straight line, but with a distinctly different functional dependence of the trajectory slope on F_a . For the strongly deflected case, the plume behavior, its rate of rise, its tendency to attach to the bottom, and the distance for merging of the individual jets, also depend on the detailed three-dimensional diffuser geometric characteristics, including port spacing and port height. A “leakiness parameter” has been defined that characterizes these three-dimensional influences, in particular the ability of the ambient flow to pass through the merging diffuser flow field and allow the diffuser plume to stay aloft downstream. This behavior is different from earlier slot diffuser experiments by Cederwall (1971) and Roberts (1979) with zero leakiness in which a fully mixed, bottom-attached plume was observed.