Diagnosing tracer transport in convective penetration of a stably stratified layer
arxiv(2023)
摘要
We use large-eddy simulations to study the penetration of a buoyant plume
carrying a passive tracer into a stably stratified layer with constant buoyancy
frequency. Using a buoyancy-tracer volume distribution, we develop a method for
objectively partitioning buoyancy-tracer space into three regions, each of
which corresponds to a coherent region in physical space. Specifically, we
identify a source region where undiluted plume fluid enters the stratified
layer, a transport region where much of the transition from undiluted to mixed
fluid occurs in the plume cap, and an accumulation region corresponding to the
radially spreading intrusion. This method enables quantification of different
measures of turbulence and mixing within each of the three regions, including
potential energy and turbulent kinetic energy dissipation rates, an activity
parameter, and the instantaneous mixing efficiency. We find that the most
intense buoyancy gradients lie in a thin layer at the cap of the penetrating
plume. This forms the primary stage of mixing between plume and environment and
exhibits a mixing efficiency around 50
environmental and plume fluid joining the intrusion are subjected to relatively
weak turbulence and weaker buoyancy gradients as mixtures are homogenised. As
the intrusion spreads radially, environmental fluid surrounding the intrusion
is mixed into the intrusion with moderate mixing efficiency. This dominates the
total entrainment of environmental fluid into the plume as a whole. However,
the 'strongest' entrainment, as measured by the specific entrainment rate, is
largest in the plume cap where the most buoyant environmental fluid is
entrained.
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