Velocity and density measurements in forced fountains with negative buoyancy

Fabio Addona,Renata Archetti,Luca Chiapponi

doi:10.1063/5.0048012

Abstract

In fluid mechanics, fountains take place when a source fluid is driven by its own momentum into a surrounding ambient fluid, and it is counterbalanced by buoyancy. These phenomena are largely encountered in nature and human activities. Despite the numerous studies on the subject, few experimental data are available about the internal structure of turbulent fountains. Here, we present a set of laboratory experiments with the aim to (i) get direct velocity and density measurements of fountains in a controlled environment and (ii) obtain insights about the basic physics of the phenomenon. The results concern the characteristics of the mean and turbulent flow: we report the analysis of the turbulent kinetic energy, the velocity skewness, and the Reynolds stresses, including a quadrant analysis of the fluctuating velocities. For some tests, the correlation between density and vertical velocity is investigated for both mean and fluctuating values. We have quantified the momentum transport, which is mainly out-downward at the nozzle axis with peaks at the mean rise height, where also maximum levels of the buoyancy and mass fluxes are present. The ability of acoustic Doppler current profilers to identify the rise height of the fountain and to measure the velocity field is also discussed.

Highlights

Fountains take place when a source fluid is driven by its own momentum into a surrounding ambient fluid, and it is counterbalanced by buoyancy
The results concern the characteristics of the mean and turbulent flow: we report the analysis of the turbulent kinetic energy, the velocity skewness, and the Reynolds stresses, including a quadrant analysis of the fluctuating velocities
Turbulent fountains are generally defined as localized vertical flows of a source fluid into an ambient fluid with different density

Summary

Introduction

Turbulent fountains are generally defined as localized vertical flows of a source fluid into an ambient fluid with different density. The result is a jet with opposing buoyancy invested by a return flow.[1]. One of the main examples is the role of fountains in heating and cooling within buildings (e.g., in air conditioning).[2,3] Fountains form as cool air is mechanically injected upward through a floor-level cooling diffuser[4] and as warm air is injected downward to form curtains, which are commonly used in tunnels and shop entrances as a means of segregating regions of fluid.[5] Fountains in the built environment include those that may form more naturally (e.g., during an enclosure fire)

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