Quantifying the impact of source variability on unsteady buoyant jet behaviour

Buoyant jets are ubiquitous in both naturally- and industrially-derived environmental flows (e.g. volcanic eruptions, marine wastewater discharges, industrial atmospheric emissions), leading to significant and wide-ranging societal, economic, and environmental impacts. For example, during the 2010 eruption of Eyjafjallajökull in Iceland, European and North American airspace was closed for over a month, causing societal disruption and costing the aviation industry millions of dollars per day whilst flight restrictions were in place. Understanding the fundamental behaviour of buoyant jets is therefore crucial to minimising their potential impacts. A buoyant jet can be divided into two regions: a momentum-driven jet region close to the source, and a buoyancy-driven plume region further away from the source. Well-established integral model theories have been developed that are based on detailed knowledge of how the time-averaged behaviour in the plume region is affected by steady source conditions in the jet region. These steady-state theories underpin many of the numerical models used to predict the evolutionary behaviour of buoyant jets, particularly when quantitative data is difficult to obtain directly from the source conditions, due to physical and practical limitations. As such, the assumption of time-averaged conditions at the source eliminates any variability in the downstream plume behaviour associated with source unsteadiness. Observations of evolving buoyant jets at field scales, such as during pulsatory volcanic eruptions, indicates a potential disconnect between these well-established steady-state theories and reality.

The current study aims to address this disconnect by evaluating the impact of source unsteadiness on the evolving downstream plume behaviour by conducting a series of scaled parametric experiments of buoyant jets discharged vertically into both homogeneous and stratified ambient water bodies. The fresh water source fluid of density ρ₀ = 1000 kg.m^-3, with a known concentration of fluorescent dye or seeding particles added, was pumped into a stagnant, homogeneous or stratified saline water ambient volume with density ranging from ρ₁ = 1010 – 1030  kg.m^-3. Unsteady buoyant jet source conditions were achieved using an electronically operated solenoid valve to control the rate of valve opening and closing, thus creating pulsatory discharge conditions with a known frequency. These unsteady source conditions could then be compared directly with equivalent steady discharges, permitting a comprehensive evaluation of the evolving plume behaviour (e.g. geometry, velocity structure, dye concentration, and entrainment characteristics) in response to source variability. A range of measurement techniques, including particle image velocimetry, ultrasonic velocity profiling and laser-induced fluorescence, was adopted in the study. The implications of the experimental results comparing steady versus unsteady plume dynamics will be discussed in the context of the evolution of volcanic plumes.