Effects of initial geometry on the development of thermals

• Published in: Experiments in fluids 2005-09, Vol.39 (3), p.591-601
• Main Authors: BOND, D, JOHARI, H
• Format: Article
• Language: English
• Subjects: Aspect ratio; Buoyancy; Circulation; Convection and heat transfer; Exact sciences and technology; Fluid dynamics; Fluid flow; Fluids; Fundamental areas of phenomenology (including applications); Instrumentation for fluid dynamics; Near fields; Physics; Self-similarity; Turbulent flows, convection, and heat transfer
• ISSN: 0723-4864; 1432-1114
• DOI: 10.1007/s00348-005-0997-1

A series of experiments were performed in a water tank to examine the effects of the initial geometry on the development of buoyant thermals in the near field. The flows were produced by releasing buoyant fluid from cylindrical tubes of aspect ratios 2 to 8 into a uniform density environment. In the near field, a vortical cap forms, followed by a column of buoyant fluid, for the larger aspect ratios. The flow development can be divided into two phases; an initial acceleration phase and a subsequent phase where the flow decelerates and displays thermal-like characteristics. In the first phase, the leading edge of the flow accelerates and the length and time scales are dependent on the initial geometry. In the second phase, the flow attains a constant circulation and the appropriate length and time scales are the cube root of the initial volume and a buoyant time scale. Circulation due to baroclinic torques and the hydrodynamic impulse in the second phase are correlated with the initial buoyant force. The data indicate that the self-similar thermal characteristics are applicable to the buoyant flows in the near field beyond the initial phase, even though the flow may not resemble a spherical thermal.