On the transition from finite-volume negatively buoyant releases to continuous fountains

An experimental investigation to identify the source conditions that distinguish finite-volume negatively buoyant fluid projectile behaviour from fountain behaviour in quiescent environments of uniform density is described. Finite-volume releases are governed by their source Froude number Fr-D and the aspect ratio L/D of the release, where L denotes the length of the column of fluid dispensed vertically from the nozzle of diameter D. We establish the influence of L/D on the peak rise heights of a release formed by dispensing saline solution into fresh water for 0 less than or similar to Fr-D less than or similar to 6 and 0 less than or similar to L/D less than or similar to 34. Within these ranges, we determine the source conditions for which a flow may be regarded, in terms of the initial rise height attained, as either finite-volume or continuous flux. The critical aspect ratio. (L/D)(f), for a given Fr-D, which when exceeded no longer influenced release behaviour, led to the determination of {Fr-D, (L/D)(f)} paired source conditions that give rise to solely Froude-number-dependent, i.e. fountain-like, behaviour. As such, we make the link between finite-volume releases and continuous fountains. The {Fr-D, (L/D)(f)} pairs led us directly to the classification of a {Fr-D, L/D} space from which source conditions giving rise to either negatively buoyant projectiles or fountains may be readily identified. The variation of (L/D)(f) with Fr-D corresponds closely to established fountain regimes of very weak, weak and forced fountains. Moreover, our results indicate that the formation or otherwise of a primary vortex, as fluid is ejected, has a profound influence on the length of the dispensed fluid column that is necessary to achieve rise heights equal to fountain rise heights.