Turbulent ‘stopping plumes’ and plume pinch-off in uniform surroundings

Observations of turbulent convection in the environment are of variously sus- tained plume-like flows or intermittent thermal-like flows. At different times of the day the prevailing conditions may change and consequently the observed flow regimes may change. Understanding the link between these flows is of practical importance meteorologically, and here we focus our interest upon plume-like regimes that break up to form thermal-like regimes. It has been shown that when a plume rises from a boundary with low conductivity, such as arable land, the inability to maintain a rapid enough supply of buoyancy to the plume source can result in the turbulent base of the plume separating and rising away from the source. This plume ‘pinch-off’ marks the onset of the intermittent thermal-like behavior. The dynamics of turbulent plumes in a uniform environment are explored in order to investigate the phenomenon of plume pinch-off. The special case of a turbulent plume having its source completely removed, a ‘stopping plume’, is considered in particular. The effects of forcing a plume to pinch-off, by rapidly reducing the source buoyancy flux to zero, are shown experi- mentally. We release saline solution into a tank filled with fresh water generating downward propagating steady turbulent plumes. By rapidly closing the plume nozzle, the plumes are forced to pinch-off. The plumes are then observed to detach from the source and descend into the ambient. The unsteady buoyant region produced after pinch-off, cannot be described by the power-law behavior of either classical plumes or thermals, and so the terminology ‘stopping plume’ (analogous to a ‘starting plume’) is adopted for this type of flow. The propagation of the stopping plume is shown to be approximately linearly dependent on time, and we speculate therefore that the closure of the nozzle introduces some vorticity into the ambient, that may roll up to form a vortex ring dominating the dynamics of the base of a stopping plume

Government, management and the environmental crisis

In case there should be any doubt that the human predicament will soon be a global crisis the reasons why this must be so are discussed. The increased use of resources and the growing dependence on a fuel-based technology are crisis enough, but to this is added the growing population of the part of the world less advanced in technology, where attitudes are different and cannot be changed to conform to the richer population's idea of what is what in the world. Most of our problems are created because the total effect of an operation is not properly considered when it is undertaken for the part that is wanted. Thus the rich countries have got themselves into a predicament the fault for which the poor see lying entirely with the rich, while the rich see the danger in the growing poor populations. The mechanism of evolution by selection from spontaneous variety must be followed because we cannot foresee needs. To keep the process healthy we must retain room for manoeuvre and choice and we must studiously avoid trying to operate monolithic technological ways of maintaining our society. Policies should seek to anticipate the imminent pressures of the future so as to retain freedom of movement, and the most obvious policy should be to make the price of fuel artificially high to conserve resources and stimulate the right kind of new technologies.

Scientific method and the environmental scene: the need for an experimenting, evolving society

The idea is abroad that there is a technique, called the scientific method, which solves problems: this is a fallacy: and there is no certainty that solutions are even possible. Malthus's warning was correct and growth of population and consumption is now almost out of control. Evolutionary mechanisms have been stultified so that they no longer guarantee the creation of higher forms, and the variety on which it thrives has been destroyed. Planners plan as if they were planning the result of an election instead of merely the election itself. They do not understand freedom nor its value for a healthy society. Instead they have organised the proliferation of the illiterate and the unemployed.

On swirl development in a square cross-sectioned, S-shaped duct

10.1007/s00348-006-0216-8Experiments in Fluids416975-989EXFL