Observations in Turbulent Buoyant Jets by Use of Laser-Induced Fluorescence

Citation

Papantoniou, Dimitris Antony (1986) Observations in Turbulent Buoyant Jets by Use of Laser-Induced Fluorescence. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/SX4D-3555. https://resolver.caltech.edu/CaltechETD:etd-03142008-142149

Abstract

The entrainment mechanism and mixing process are investigated in the far field of a liquid phase buoyant jet issuing into an unconfined, quiescent medium, by an experimental technique based on laser-induced fluorescence (LIF). Visualization experiments show the existence of a large scale organization in the far field, with structures spanning the radial extent of the conical flow region. Quantitative, high-resolution measurements of scalar concentration were performed along the radial direction in the far field region. For each data set, a large number of successive instantaneous concentration profiles were obtained by combining LIF techniques with linear photodiode array imaging and high speed data acquisition. The measurements revealed that the instantaneous profile bears no resemblance to the time-averaged profile. The flow interior is characterized by large spatial gradients of concentration, associated with interfaces between mixed jet fluid and fresh, entrained ambient fluid transported to regions deep into the flow. This is inconsistent with the description of transport by gradient diffusion concepts. The probability of finding unmixed ambient fluid and the concentration variance are greatly increased under the action of buoyancy. At any axial location, the arrival of a structure front is marked by a spatially coherent (along the radial direction) increase in the local concentration level. It is found that, within the structure, values of the concentration are generally decreasing in the upstream direction; substantial uniformity within the mixed fluid portion is observed along the radial direction. In the conical flow field of the momentum jet, a central region (in fixed spatial coordinates) may be identified within which the local mixed fluid composition is relatively uniform. This is not the case for the buoyancy driven plume, due to a greater variance in the position of the large structure and the high value of the intermittency. It is suggested that fluid is entrained by vortical motions mainly from the back and side regions of the large structure. Flow visualization reveals vorticity in the axial direction which enhances the mixing process; this vorticity appears stronger in the buoyancy driven flow. The results of these experiments are interpreted through a simple conceptual model of entrainment and mixing that encompasses the observed large scale organization of the buoyant jet flow.

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