Ph.D Thesis | |

Ph.D Student | Victoria Suponitsky |
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Subject | The Generation of Streaks and Hairpin Vortices from a Localized Vortex Disturbance Embedded in Unbounded Uniform Shear Flow |

Department | Department of Aerospace Engineering |

Supervisors | Full Professors Cohen Jacob |

Professor Emeritus Bar-Yoseph Pinhas |

Since
the coherent structures (streaks and hairpin vortices) have been discovered in
turbulent boundary layers, an enormous amount of experimental, numerical and
theoretical work has been carried out to investigate their generation and
regeneration mechanisms. This thesis demonstrates the capability of a *simple
model of interaction between a localized vortical disturbance and laminar
uniform unbounded shear flow, *to reproduce the generation mechanism and
characteristics of these coherent structures that naturally occur in fully
developed turbulent boundary layers. The effects of the disturbance ‘localized
character’ in streamwise and spanwise directions plus its initial orientation
relative to the base flow are investigated by using several geometries of the
initial disturbance. The results demonstrate that a small amplitude initial
disturbance (linear case) eventually evolves into a streaky structure
independent of its initial geometry and orientation, whereas, a large amplitude
(strongly non-linear case) disturbance evolves into a hairpin vortex (or a
packet of hairpin vortices) that is independent of its geometry over a wide
range of the initial disturbance orientations. The main non-linear effects are:
(i) self-induced motion, which results in movement of the vortical structure
relative to the base flow and the destruction of its streamwise symmetry, (ii)
the coincidence between the direction of the vorticity lines and the vortical
structure. This is unlike the linear case, where there is a strong deviation of
the vorticity vector from the direction of the vortical structure, i.e. the
vortical structure cannot be represented as a vortex filament. The quantitative
evolution of the disturbance has been found to be sufficiently independent of
the initial disturbance geometry, whereas the quantitative characteristics,
i.e. inclination angle, center and strength (which is governing by the
transient growth mechanism), strongly depend on the disturbance geometry. The
Reynolds number has been found to have a negligible effect on the kinematics of
the vortical structure, but does have a significant effect on its transient
growth. Finally, the formation of the asymmetric hairpin vortex is
demonstrated.