Application of optical techniques to turbulent/turbulent entrainment

Turbulent flows are observed to grow with distance downstream, as illustrated by the spreading of a volcanic plume. It is observed that the interfacial layer demarcating the turbulent portion of a flow and the background is extremely thin. Entrainment describes the process by which quantities such as mass/energy/vorticity are transferred from the background into the turbulent portion of the flow across this interface. Classically, entrainment has been considered from a non-turbulent (or even quiescent) background in which vorticity is only present in the turbulent portion of the flow. In such circumstances the interface is known as the turbulent/non-turbulent interface. In reality, however, the vast majority of environmental (and industrial) flows exist within a background that is itself turbulent. Entrainment from a non-turbulent background is therefore simply a special case of the more general turbulent/turbulent entrainment in which there is vorticity present on both sides of the turbulent/turbulent interface. Entrainment into a cloud is a good example of turbulent/turbulent entrainment due to the turbulent nature of the atmospheric boundary layer.

Identification of this turbulent/turbulent interface is non-trivial since vorticity magnitude thresholds, classically used to identify the turbulent/non-turbulent interface, are no longer appropriate due to the presence of vorticity on both sides of the interface. It is therefore convenient to use passive scalar to mark the primary turbulent portion of fluid for a flow developing in a turbulent background, although this scalar will only act as a faithful marker of the turbulent fluid in the limit of high Schmidt number. For practical reasons this favours the use of experimental methods to explore turbulent/turbulent entrainment at anything other than low Reynolds number. In this talk we will learn about the application of combined planar laser induced fluorescence (PLIF) and particle image velocimetry (PIV) experiments to explore turbulent/turbulent entrainment into turbulent wakes.

Structure identification in turbulent flows

What is a coherent flow structure in a turbulent flow? This deceptively simple question is one that fluid mechanics has wrestled with for a century and has important implications for how we conceptualize eddy-resolving numerical methods such as large-eddy simulation, and how we think about dissipation. As will be discussed in Oliver Buxton’s lecture, criteria for identifying entrainment across a turbulent/non-turbulent interface are based in some way on vorticity, so rotation is clearly important, but straining is also often closely associated, spatially, with rotation, implying a need to consider both of these quantities. As will be discussed, criteria for local coherent flow structure identification are therefore often based on the velocity gradient tensor, which provides a simple means to extract rotation and strain statistics, but there are a number of such criteria, and this is still a local calculation. Use of the velocity gradient tensor implies one has access to all nine velocity derivatives in the flow and this is often not practical in environmental fluid mechanics. Therefore, there is a long history to using the notion that Reynolds stresses contain implicit information on coherent structure. However, while fluid ejections and sweeps both contribute positively to Reynolds stress, they are very different physically and play a different role in processes such as sediment entrainment. Thus, in addition to an examination of velocity gradient tensor-based methods of relevance for numerical studies and high resolution experiments, we will also look at modern approaches to extracting information on coherent flow structures from single-point data – an approach of relevance to field- and high Reynolds number experiments.