Steady three-dimensional shock wave reflection transition phenomena.
Date
2008-06-05T10:10:42Z
Authors
Baloyi, Jeffrey
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Abstract
Abstract
Shockwave reflection has in recent times been investigated as a threedimensional
phenomenon where geometrical effects on the reflection patterns
have been given more attention than previously. A typical example is that of a
supersonic body flying over a ground plane, in which the bow wave reflects off
the ground surface. Depending on the Mach number, the reflection can be
regular below the body, but will then make a transition to the three-shock
Mach reflection pattern at some lateral position. In this report symmetrically
arranged wedges with a finite span (i.e. one above the other) were modeled
and visualised in CFD in order to investigate the three-dimensional steady
state transition from regular reflection to Mach reflection. This follows on the
work done by Skews (2000) where it was observed from shadowgraph
pictures that there seems to be a sudden jump at the transition point in the
growth of the Mach stem.
Contrary to what was observed by Skews (2000), the transition was found to
be gradual and smooth in the current CFD simulations. High visual clarity from
the CFD results could not be achieved, even after successive grid refinements
were performed on and around the shockwaves, because of the averaging
technique of fluid property values in cells performed by CFD codes. The flows
in the vicinity of the transition are examined, with particular attention to the
shear layers that are generated from the triple point lines. Because of the
inclination of the Mach stem surface to the oncoming flow the Mach number
behind this surface can be supersonic, in contrast to the two-dimensional
case.
The steady state reflection phenomenon where there is transition from Mach
reflection to regular and then back to Mach reflection when moving laterally
outward from the vertical symmetry plane was also investigated using the
same CFD setup, but with a much wider wedge span. This particularly
interesting situation suggests the existence of complex transition criteria. The aim was to reproduce numerically this phenomenon observed experimentally
by Ivanov et al. (1999), and to see if these results can be replicated for a
lower Mach number attainable using a local wind tunnel. Both aims were
achieved, but with the same limitation mentioned above of the averaging
technique of fluid property values by CFD codes. There are currently no
analytical criteria for the prediction of shock wave reflection transition in the
three-dimensional case, nor for the possible existence of a dual solution
domain, as exist for two-dimensional flows. Parametric studies of the type
discussed in this report should lead to a fuller understanding of the flow
conditions of importance.