BY: D R Glynn, FLOWSOLVE, London. Tel: +44 181 944 0940
The flow is choked (Mach 1) at the throat, then continues to accelerate as it expands. Halfway along the curve of the valve, the expansion can no longer be sustained, and the flow decelerates through a shock and becomes subsonic.
(2) CASE 1 - upstream pressure 5 bars. Mach number contours.
The shock can be seen by the bunched contours at the location where the vectors indicated deceleration in the previous plot.
Use of a finer grid (and maybe a higher-order scheme) would give a sharper prediction of the shock.
Mach number contour
(3) CASE 2 - upstream pressure 7 bars. Mach number contours.
Increasing the pressure causes the shock to move downstream. In this case it has moved beyond the tip of the valve, so that the flow in the solution domain is entirely supersonic - with one exception. Note the small subsonic region at the valve tip.
Mach contours 2