Modelling of turbulent wakes

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Title: Modelling of turbulent wakes
Authors: Rigas, Georgios
Item Type: Thesis or dissertation
Abstract: The dynamics of the turbulent three-dimensional wake generated by an axisymmetric bluff body with blunt trailing edge are experimentally and theoretically investigated at a diameter based Reynolds number of 188,000. A detailed analysis of the base pressure measurements shows that the large scale structures of the turbulent three-dimensional wake retain the structure of the laminar instabilities observed in the transitional regimes, in a statistical sense. These persisting instabilities at the turbulent regime, are associated with spatial and temporal symmetry breaking, giving rise to spatial reflectional symmetry and quasi-periodic vortex shedding. The influence of turbulence recovers the lost temporal and spatial symmetries in the long-time average. It is shown that the turbulent spatial dynamics are reproduced by a simple stochastic model the deterministic part of which accounts for the spatial symmetry breaking and gives rise to steady large scale structures through a supercritical pitchfork bifurcation, and the stochastic part modelling in a phenomenological sense the turbulent fluctuations acting on the large scale structures. The axisymmetric body wake is further investigated when axisymmetric slot-jet zero-net-mass-flux forcing is applied on the rear base. Landau-like models that capture the weakly nonlinear interaction between the global vortex shedding mode and axisymmetric forcing are derived from the phase-averaged Navier-Stokes equations. The Landau-like models describe accurately the forced response by means of measured base pressure of the global vortex shedding mode. With the present analysis it is demonstrated that the concept of weakly nonlinear global modes can be extended to a fully turbulent flow, far from the critical bifurcation Reynolds number, and a general framework for the description of systems with broken symmetries---giving rise to global dynamics---and turbulent dynamics is provided. The novel results presented here advance the understanding of the dynamics of three-dimensional turbulent wakes and pave the way for turbulence prediction and control.
Content Version: Open Access
Issue Date: Oct-2014
Date Awarded: Apr-2015
Supervisor: Morgans, Aimee
Sponsor/Funder: Engineering and Physical Sciences Research Council
Funder's Grant Number: EP/I005684/1
Department: Aeronautics
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Aeronautics PhD theses

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