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Title: Shock-induced energy transfers in MHD
Author: Nwobu, Francis Obiora
ISNI:       0000 0004 8504 7034
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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In magnetohydrodynamic (MHD) flow control, an applied magnetic field can manipulate the flow properties of an ionised gas in order to improve vehicle performance in high-speed flight applications. Shock-wave turbulence interactions (SWTIs) are a fundamental feature of high-speed flows. They play a key role in predictions of transition, drag and heat transfer in flow control configurations. In hydrodynamics, linear theory is a good predictor of turbulent kinetic energy amplification in SWTIs, however, at present similar linear analysis is missing from MHD. In the present study, linear theory is developed and combined with direct numerical simulations (DNS) to investigate the production of vorticity (an indicator of turbulent kinetic energy) by small-amplitude entropy disturbances incident on an isolated fast shock in two- and three-dimensions. Entropy disturbances in the form of planar waves and Gaussian-distributed spots are considered. The angle of an imposed mean magnetic field is varied in order to study downstream vorticity production for thermodynamic and magnetic pressure-dominated regimes respectively. Linear theory indicates total vorticity production is suppressed for magnetic pressure-dominated flows, however, vorticity transport in the fastest refracted mode is amplified. DNS results of two-dimensional planar waves (of amplitude 1% the base flow) incident on a fast shock at angles above and below a critical value compare well with linear theory predictions. DNS results near the critical angle noticeably diverge from the linear theory. However, the linear reconstruction of the two-dimensional entropy spot-fast shock interaction shows very good agreement with DNS for 1% and 10% disturbances - a 10% upstream turbulence intensity is common in SWTIs studies. DNS of the three-dimensional entropy spot-fast shock interaction shows in-plane vorticity production similar to the two-dimensional case with the three-dimensional distribution appearing to constitute an extrusion of in-plane structures.
Supervisor: Touber, Emile ; Navarro-Martinez, Salvador Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral