Laboratory observations of the gravitational convection resulting from ’’point’’ source buoyancy (heat) injection in a stationary, unstratified fluid are described. These include various formation stages of buoyant vortex rings (thermals), the effect of rigid boundaries on a single thermal, and the motion of a sequence of two thermals, an infinite sequence of thermals, and the starting, steady, and interrupted plume. In addition, evaporative convection and boiling are briefly described. The stability of the starting plume cap and steady plume are compared by considering the effect of a perturbation of the bulk convective velocity. Perhaps, the main finding of the investigation is evidence that the starting plume cap can be treated as a thermal whose buoyancy increases at a constant rate.
By measuring the heat dispersion behind a heated wire stretched across a wind tunnel (Taylor 1921, 1935), the Lagrangian velocity autocorrelation was determined in an approximately isotropic, grid-generated turbulent flow. The techniques were similar to previous ones, but the scatter is less. Assuming self-preservation of the Lagrangian velocity statistics in a form consistent with recent measurements of decay in this flow (Comte-Bellot & Corrsin 1966, 1971), a stationary and an approximately self-preserving form for the dispersion were derived and approximately verified over the range of the experiment.Possibly the most important aspect of this experiment is that data were available in the same flow on the simplest Eulerian velocity autocorrelation in time, the correlation at a fixed spatial point translating with the mean flow (Comte-Bellot & Corrsin 1971). Thus, the Lagrangian velocity autocorrelation coefficient function calculated from the dispersion data could be compared with this corresponding Eulerian function. It was found that the Lagrangian Taylor micro-scale is very much larger than the analogous Eulerian microscale (76 ms compared with 6.2ms), contrary to an estimate of Corrsin (1963). The Lagrangian integral time scale is roughly equal to the Eulerian one, being larger by about 25 %.
A novel technique for injecting buoyancy (heat) into a liquid is described and demonstrated. When buoyancy was injected for a short time a laminar vortex ring formed. Its vertical displacement was found to be only approximately proportional to the square root of time (measured from an apparent initial time). Approximate geometrical similarity was also observed although the Reynolds number decreased from 28 to about 14.
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