Experiments on a round turbulent buoyant plume

Abstract

This paper reports a comprehensive set of hot-wire measurements of a round buoyant plume which was generated by forcing a jet of hot air vertically up into a quiescent environment. The boundary conditions of the experiment were measured, and are documented in the present paper in an attempt to sort out the contradictory mean flow results from the earlier studies. The ambient temperature was monitored to ensure that the facility was not stratified and that the experiment was conducted in a neutral environment. The axisymmetry of the flow was checked by using a planar array of sixteen thermocouples and the mean temperature measurements from these are used to supplement the hot-wire measurements. The source flow conditions were measured to ascertain the rate at which the buoyancy was added to the flow. The measurements conserve buoyancy within 10%. The results are used to determine balances of the mean energy and momentum differential equations. In the mean energy equation it is found that the vertical advection of energy is primarily balanced by the radial turbulent transport. In the mean momentum equation the vertical advection of momentum and the buoyancy force balance the radial turbulent transport. The buoyancy force is the second largest term in this balance and is responsible for the wider (and higher) velocity profiles in plumes as compared to jets. Budgets of the temperature variance and turbulent kinetic energy are also determined in which thermal and mechanical dissipation rates are obtained as the closing terms. Similarities and differences between the two balances are discussed. It is found that even though the direct effect of buoyancy in turbulence, as evidenced by the buoyancy production term, is substantial, most of the turbulence is produced by shear. This is in contrast to the mean velocity field where the effect of the buoyancy force is quite strong. Therefore, it is concluded that in a buoyant plume the primary effect of buoyancy on turbulence is indirect, and enters through the mean velocity field (giving larger shear production).

The Fluxes and Behaviour of Plumes Inferred from Measurements of Coherent Structures within Images of the Bulk Flow

“…Plume centreline velocities have been measured using more advanced experimental techniques than in the present study (e.g. Ezzamel et al, 2015;Shabbir & George, 1994). These Figure 5b plots the equivalent data for the momentum flux M (z) -these data exhibit similar trends.…”

“…Morton, Taylor & Turner, 1956) tended to favour the representation of the mean profiles as 'top-hat' profiles with a uniform velocity and buoyancy within the plume and zero outside. More recent experimental studies of plumes (Ezzamel, Salizzoni & Hunt, 2015;Shabbir & George, 1994;Wang & Law, 2002) examined the distributions of the time-averaged velocities w(r, z) and buoyancies g ′ (r, z) = g(ρ a − ρ(r, z))/ρ a at points fixed in space, where r and z denote the radial and vertical coordinate, respectively. The radial distributions of w(r, z) and g ′ (r, z) are observed to be typically Gaussian-like and this has influenced the choice in more recent models (Craske & van Reeuwijk, Under consideration).…”

Energy-consistent entrainment relations for jets and plumes

“…While the viscous dissipation associated with the mean-flow components is indeed negligible at high Reynolds number (Re) (see, e.g., Tennekes & Lumley 1972), the component associated with the turbulence cannot be neglected. The latter dissipation rate is crucial in the balance of turbulence kinetic energy, as it is of the same order of magnitude as the turbulence production term (Shabbir & George 1994). However, the balance of turbulence kinetic energy is not considered in this paper, and viscous effects are thus entirely absent from the mathematical description of the mean flow.…”

ASHEE-1.0: a compressible, equilibrium–Eulerian model for volcanic ash plumes

“…This study is mainly aimed at assessing the capability of the numerical model to describe the time-average behavior of a turbulent plume and to reproduce the magnitude of large-scale fluctuations and large-eddy structures. We will mainly refer to laboratory experiments by George et al (1977) and Shabbir and George (1994) and numerical simulations by Zhou et al (2001) for a quantitative assessment of model results.…”