When Chaos Meets Chaos - Turbulent Entrainment from a Turbulent Background

Lead Research Organisation: Imperial College London
Department Name: Dept of Aeronautics

Abstract

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.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509486/1 01/10/2016 30/09/2021
2091392 Studentship EP/N509486/1 01/10/2017 31/07/2021 Krishna Kankanwadi
 
Description The aim of this research project was to investigate the effects of background turbulence on the process of entrainment. Entrainment can be regarded as the process by which turbulent bodies of fluid grow. With regards to the effect of background turbulence on this process, there was a lack of consensus in the per-existing literature. Therefore, in this project a parametric study that independently varied all of the relevant parameters was conducted in order to ascertain the effects of background turbulence on entrainment. The two main parameters that were investigated were the intensity of the background turbulence and the length scale of the same. The project investigated this through the analysis of a cylinder wake.

In the far wake region, where the wake lacks significant coherence, we were able to show that an increase in background turbulence intensity resulted in an increased inter-facial surface area. This would intuitively lead us to a conclusion that the expected entrainment rate should be increased. However, it is found that the increased entertainment that should be expected due to the increased inter-facial surface area, is overbalanced by the action of extreme yet intermittent events that result in a negative entrainment rate. An increase in the intensity of the background turbulence led to a reduction in net entrainment rate. Given that the background was sufficiently intense, such that it overpowered the wake, the wake experienced net detrainment (negative entrainment). The length scale of incoming turbulence was shown to have a negligible effect on the entrainment process in the far wake.

Along with the entrainment rate, which has a direct engineering application, the structure of the inter-facial region was also investigated. It was found that regardless of the level of turbulence available in the background, the classical signature of an enstrophy jump at the interface was still present when turbulence was available on both sides of the interface. This is a very important result as it proves the existence of a turbulent-turbulent interface, something that was questioned in previous literature.

One of the branches of ongoing research focuses on the studying the turbulent-turbulent interface in detail. Whereas, another focuses on the near field effects of free-stream turbulence on the shedding of a circular cylinder.
Exploitation Route These findings can potentially used to better design and plan for engineering applications that directly involve the interaction of several bodies of turbulent fluid. As an example, the design of a new wind farm would largely benefit with a better understanding of the growth of the wake behind each turbine.
Sectors Aerospace, Defence and Marine,Environment

URL http://www.tsfp-conference.org/proceedings/2019/105.pdf