Experiments on a turbulent plume: Shape analyses

• Published in: Journal of Geophysical Research 2011-03, Vol.116 (B3), p.n/a, Article B03208
• Main Authors: Kitamura, Shogo, Sumita, Ikuro
• Format: Article
• Language: English
• Subjects: Buoyancy; Geophysics; laboratory experiments; Nonlinear systems; Plumes; Remote sensing; shape evolution; volcanic plume; Volcanoes
• ISSN: 0148-0227; 2169-9313; 2156-2202; 2169-9356
• DOI: 10.1029/2010JB007633

We report the results of laboratory experiments on a turbulent plume, a simplified model of a positively buoyant volcanic plume with a small content of fine ash to study how the shape of the plume changes as a function of time. To model such plumes, we continuously inject a dense fluid downward at a constant exit velocity and study how the plume shape changes with time as vortices develop from flow instability and entrainment. Here a series of experiments are conducted with exit velocity and the density difference as the changeable parameters, and the jet‐plume transition is realized in the laboratory. Initially the plume has a “finger‐like” shape which becomes unstable and forms a “plume head” and later transforms into a “cone‐like” self‐similar shape. We also find that when the buoyancy becomes sufficiently large compared to inertia, there is a temporary deviation from the cone‐like shape to form a “headed cone.” We devise new methods to quantitatively characterize these changes of shape and define four regimes as a function of time. We find that the onset times of the flow instability and the regimes have negative power law dependence on the initial Re (or the exit velocity), and that buoyancy causes the regime transitions to become earlier. Our experiments suggest that monitoring the change of the shape of the rising volcanic plumes and analyzing the regime onset times, can be used as a measure to constrain their buoyancy. Key Points Shape change of turbulent plume after its ejection was studied from experiments New methods to characterize plume shape were devised to define 4 regimes For the same Reynolds number regime onset times become earlier with buoyancy