When Chaos Meets Chaos - Turbulent Entrainment from a Turbulent Background

Lead Research Organisation: Imperial College London
Department Name: Aeronautics

Abstract

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Publications

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Kankanwadi K (2020) Turbulent entrainment into a cylinder wake from a turbulent background in Journal of Fluid Mechanics

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509486/1 30/09/2016 30/03/2022
2091392 Studentship EP/N509486/1 30/09/2017 30/07/2021 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 and even speculated that it may break down and cease to exist. Following this major finding, this interface was further investigated through the use of the enstrophy budget equation. This is a common tool of analysis when examining interfaces. It was shown that the role of viscosity, which is the most crucial method by which entrainment occurs in a non-turbulent environment, is actually subdued and not so crucial in a turbulent/turbulent interface. A turbulent/turbulent interface has rotational fluid on both sides of the interface and hence strain, through the medium of the inertial term, does most of the heavy lifting when it comes to enstrophy production in the vicinity of the interface.

The final branch of our research focuses on the near field effects of free-stream turbulence on the shedding of a circular cylinder. Experimental data for this branch has been collected, although the data analysis is still ongoing and in its infancy.
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

Energy

Environment

URL https://doi.org/10.1017/jfm.2020.755
 
Description Work on turbulent/turbulent entrainment will now be used to develop models for wind turbine wakes in an industrial setting. This is reflected through the collaborations with Industrial partners.
First Year Of Impact 2021
Sector Aerospace, Defence and Marine,Energy,Environment
Impact Types Cultural

Societal

Economic

 
Description Accurate modelling of wind turbine wake spreading through consideration of realistic turbulent entrainment: revolutionising wind farm optimisation
Amount £1,612,676 (GBP)
Funding ID EP/V006436/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 06/2021 
End 06/2026
 
Description Callaboration with Vestas 
Organisation Vestas Technology UK Ltd
Country United Kingdom 
Sector Private 
PI Contribution We are developing models for wind turbine wakes based on my work in turbulent/turbulent entrainment and multi-scale generated turbulence
Collaborator Contribution Providing relevant data to develop and tune the models we are developing
Impact N/A
Start Year 2021
 
Description Collaboration with Frazer Nash Consultancy 
Organisation Frazer-Nash Consultancy
Country United Kingdom 
Sector Private 
PI Contribution We are developing models for wind turbine wakes which are of interest to clients of Frazer Nash building offshore wind farms
Collaborator Contribution They will assist with disseminating our findings to industry to generate immediate impact
Impact N/A
Start Year 2021
 
Description Collaboration with University of Oldenburg 
Organisation Carl von Ossietzky University of Oldenburg
Country Germany 
Sector Academic/University 
PI Contribution We will use the world-class facilities at Oldenburg and in return I will assist them with the implementation of my multi-scale triple decomposition analysis, published in Baj, Bruce & Buxton (2015), Baj & Buxton (2017), Baj & Buxton (2019)
Collaborator Contribution Use of their world leading facilities for wind energy research
Impact N/A
Start Year 2021