Transport, dispersion and mixing in quasi-two-dimensional steady turbulent jets

By Julien Landel
(DAMPT, University of Cambridge, UK)

 
 
 
Wednesday 30th of May 2012 – 11:15 – Room MEB10
 
 
Abstract :
The study of quasi-two-dimensional jets is relevant to rivers flowing into lakes or oceans. In the event of a spillage of pollutants into a river, it is critical to understand how these agents disperse with the flow in order to assess damage to the environment. For such flows, characteristic streamwise and cross-stream dimensions can be much larger than the fluid-layer thickness, and so the flow develops in a confined environment. When the distance away from the discharge location is larger than ten times the fluid-layer thickness, the flow is referred to as a quasi-two-dimensional jet. From experimental observations using dyed jets and particle image velocimetry, we find that the structure of a quasi-two-dimensional jet consists of a high-speed meandering core with large counter-rotating eddies developing on alternate sides of the core. To understand the transport and dispersion properties of quasi-two-dimensional jets we use a time-dependent advection–diffusion equation, with a mixing length hypothesis accounting for the turbulent eddy diffusivity. Our analytical solution to this model is supported by experimental releases of dye in jets or numerical releases of virtual passive tracers in experimentally-measured jet velocity fields. Due to the dispersion mechanisms, we find that a significant amount of passive tracers can be transported faster than the advection speed predicted using a top-hat velocity profile in the jet. We discuss the implications for pollution control in river flows.