Abstract
Observations of turbulent convection in the environment are of variously sustained 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 experimentally. 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.
Highlights
Many investigations of turbulent thermal convection have reported structures that can be well-approximated by turbulent plumes
Turbulent plumes that persisted for a short period of time before they pinched-off into thermals were observed by glider pilots and ornithologists
Motivated by the work of Hunt et al [22], the present study aimed to obtain a better understanding of the dynamics of the plume-thermal transition
Summary
Many investigations of turbulent thermal convection have reported structures that can be well-approximated by turbulent plumes (see e.g., [4, 11, 21]). In the late afternoon, the amount of wing-flapping was observed to increase when the ambient convection patterns were more intermittent Such unsteady thermal convection phenomena have been observed in patterns of high Reynolds number, Re ≫ 104, convection and they play a central role in many atmospheric, oceanographic and geological processes (see e.g., [13], [22]). If the thermal conductivity of the ground is low, there will be insufficient heat flux to the warm patch leading to a pinch-off, and the buoyant region will rise as a thermal [15]. The first explanations of the physical processes behind plume-thermal transition were due to Hunt [20] and Hunt et al [22] They noted that when the plume source has low thermal conductivity plume pinch-off can occur. We show that the tail of the pinched-off plume rises approximately linearly in time
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
