Kelvin-Helmholtz Instability on the Mach reflection shear layer.
Date
2013-07-22
Authors
Rubidge, Sidney Richard
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Abstract
The Kelvin-Helmholtz Instability (KHI) is an instability that takes the form of
repeating wave like structures. It forms on a shear layer where two fluids are moving
at a relative velocity to one another. Such a shear layer forms as part of the Mach
reflection. This dissertation focuses on experimentally visualising the presence of
the KHI on the Mach reflection shear layer, as well as its evolution.
Experimentation was performed at Mach numbers of 1.34, 1.46 and 1.61. Plain and
parabolic entrance test pieces with varying wedge angles of 30◦ to 38◦ were tested.
Flow field visualisation was performed with a schlieren optical system. The camera-
side knife edge orientation was altered to vary the axis of sensitivity of the optical
system.
It was found that the KHI did form on the Mach reflection shear layer; more readily
with increasing Mach number and wedge angle. The evolution of the instability was
found to be almost scalable, with M0 = 1.46 and M0 = 1.61 achieving the best
scaling of the length and average width of the KHI respectively. The KHI was best
visualised with the camera-side knife edge perpendicular to the shear layer (i.e. the
axis of sensitivity along the length of the shear layer), the visualisation of the KHI
was completely obscured when camera-side knife edge was parallel to the shear layer. Second order Euler, Navier-Stokes and k-ǫ numerical simulations of the flow field
were also conducted using FLUENT 13. It was found that the Euler and Navier-
Stokes solvers achieved very similar results, both producing the KHI but at a much
less developed state than the experimental cases. The k-ǫ solver, however, did not
produce the KHI.