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Title: Entrainment of free-stream vortical disturbances in the entrance region of confined flows
Author: Alvarenga, Claudia
ISNI:       0000 0004 6494 2662
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2018
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The entrainment of free-stream vortical disturbances in the entry region of developing pressure-driven confined flows is studied analytically and numerically, as a contribution to the understanding of laminar-to-turbulent transition. The topic has attracted wide and long-lasting attention since the work of Reynolds (1883), as the understanding of transition is a crucial aspect in the successful design of ducts and pipes. Given the analytical difficulties that it poses, the transition mechanism is also a topic of fundamental interest per se. The focus is on low-frequency/long-wavelength disturbances, which, for a flatplate boundary layer, evolve into streamwise elongated structures known as Klebanoff modes or laminar streaks. It is assumed that the amplitude of the oncoming fluctuations is much smaller than the amplitude of the mean flow, so that the relevant equations can be linearized. The streaks dynamics is governed by the linear unsteady boundary region equations, that is the rigorous asymptotic limit of the Navier-Stokes equations for low-frequency perturbations. They are derived here for the first time for non-parallel plane channel and pipe flows. Physically realistic initial conditions at the inlet are derived rigorously by the method of matched asymptotic expansions. The relevant equations are solved analytically, when possible, and by finite differences when an analytical solution cannot be found. Theoretical and analytical results are shown for channel flows for physically realistic disturbances representative of low-speed water channels and wind tunnels. Theoretical results are shown for pipe flows.
Supervisor: Ricco, Pierre ; He, Shuisheng Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available