Abstract
Abstract. The behavior of a forced plume is mainly controlled by the source buoyancy and momentum fluxes and the efficiency of turbulent mixing between the plume and the ambient fluid (stratified or not). The interaction between the plume and the ambient fluid controls the plume dynamics and is usually represented by the entrainment coefficient αE. Commonly used one-dimensional models incorporating a constant entrainment coefficient are fundamental and very useful for predictions in geophysical flows and industrial situations. Nevertheless, if the basic geometry of the flow changes, or the type of source or the environmental fluid conditions (e.g., level of turbulence, shear, ambient stratification, presence of internal waves), new models allowing for variable entrainment are necessary. The presented paper is an experimental study based on a set of turbulent plume experiments in a calm unstratified ambient fluid under different source conditions (represented by different buoyancy and momentum fluxes). The main result is that the entrainment coefficient is not a constant and clearly varies in time within the same plume independently of the buoyancy and the source position. This paper also analyzes the influence of the source conditions on the mentioned time evolution. The measured entrainment coefficient αE has considerable variability. It ranges between 0.26 and 0.9 for variable Atwood number experiments and between 0.16 and 0.55 for variable source position experiments. As is observed, values are greater than the traditional standard value of Morton et al. (1956) for plumes and jets, which is about 0.13.
Highlights
Forced plumes play a fundamental role in a large variety of natural phenomena and industrial processes
Understanding the dynamics of plumes issuing from industrial chimneys or those generated by forest fires or volcanoes is a major goal for environmental sciences because they are able to transport toxic gas and fine particles into the high atmosphere
Further work should analyze intermittency and extend the entrainment assumptions to include higher order moments of the velocity and density moments and structure functions (Vindel at al., 2008) following an experimental procedure described in Mahjoub et al (1998). In this experimental study of a turbulent plume, our main result is that the entrainment coefficient is not a constant and clearly varies in time within the same plume independently of the buoyancy and the source position
Summary
Forced plumes play a fundamental role in a large variety of natural phenomena and industrial processes. Understanding the dynamics of plumes issuing from industrial chimneys or those generated by forest fires or volcanoes is a major goal for environmental sciences because they are able to transport toxic gas and fine particles into the high atmosphere. River plumes are another natural plume phenomenon. These are turbid freshwaters flowing from land and generally in the distal part of a river outside the bounds of an estuary or river channel. In engineering, turbulent plumes are involved in building ventilation processes to supply fresh and cool air and are essential to evaluate quality of air in rooms (Nielsen, 1993)
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