||September 28, 2012 18:31
What happens to the Mach number when a normal shock appears? A normal shock causes the flow to decelerate from M > 1 to M < 1 over a very small distance. So how would that look on a Mach contour plot? Across an oblique shock, the velocity component normal to the shock drops across a small distance, while the tangential velocity is unchanged. What would this look like if you plot the velocity vectors? The Mach number contours won't be as tightly clustered, but a look at the velocity vectors can clarify what you are seeing.
In general shocks will look like a tight clustering of Mach contours on a contour plot, but understanding the flow physics will help you understand why certain flow features look like they do in post-processing.