When Chaos Meets Chaos - Turbulent Entrainment from a Turbulent Background

Turbulent flows are very abundant in nature and can be illustrated by a wake behind objects such as trees and buildings. A wake typically grows as fluid particles move downstream of the object. This increase in wake volume must be accompanied by a net mass transfer into the wake from the background fluid. The overarching term used to define all physical processes that lead to this mass transfer is defined as entrainment. Entrainment can be classed into engulfment (large scale corrugations 'engulfing' a significant mass of background fluid) and small scale nibbling at the interface between the wake and the free-stream. Nibbling references the diffusion of vorticity at the interface, and occurs due to the action of viscosity at the smallest physical length scale. Despite a large amount of work going into the understanding of entrainment processes from a non-turbulent environment, entrainment when the background fluid is itself turbulent is largely unexplored and poorly understood. This research project aims to set the foundation and make initial strides into gaining a universal understanding into the physics that govern turbulent-turbulent entrainment (TTE), by examining entrainment from turbulent free-streams into turbulent wakes.
Applications of TTE are vast as a majority of industrial and environmental flows exist in a turbulent background. The recent natural disaster involving Deepwater Horizon is a good example. The ocean is a highly turbulent environment and better understanding of TTE could have led to more accurate estimations of growth of the oil spill. Further research can lead to better strategies for damage limitation caused by any such catastrophes. Better understanding of TTE can also be a great advisory asset to policy makers in metropolitan cities; air pollution is approaching concerning levels in several cities around the world. Exhaust gasses from vehicles such as buses are released in turbulent backgrounds and therefore understanding how the pollutants are dispersed in this environment can be very helpful. The critical nature of this project is very apparent and potential applications can indeed be very extensive.
In this project, entrainment from a turbulent environment is explored experimentally by observing the behaviour of a wake behind a cylinder when subjected to a turbulent free-stream. A fully characterised turbulent free-stream is subjected to a circular cylinder through the use of several turbulence generating grids that are specifically designed to produce varying levels of turbulence intensity (u), integral length scale (L) and turbulent kinetic energy dissipation rate (e) downstream of the grids. The few studies that are present in the current literature have shown the importance of each of these three parameters in a solitary sense. Although, there is no consensus on the influence of each parameter and their respective sensitivities. This is largely due to the lack of technology available to vary all three parameters independently. However, with the recent development of space filling fractal grids, we are able to accomplish just that and assess the separate effects of each parameter on TTE. To this end, experimental campaigns employing simultaneous particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) have been conducted in a 0.6m x 0.6m water flume located at Imperial College London.
Preliminary results have highlighted two main mechanisms by which entrainment is affected by free-steam turbulence (FST). Firstly, an investigation on near field effects of FST with regards to the shedding mechanism of the cylinder will be conducted. Secondly, we will aim to ascertain the direct effects of parameters, (u, L & e) in the FST, on entrainment into the wake downstream of the near-wake region. This dual-pronged approach will provide the basis to greatly enhance our understanding of the TTE phenomenon.