SSD: [quote]Thanks for this, CliveL. Could you please explain 'expansion field over the wing '?
Thanks.
Also, how was the flow re-compressed at the tail? What is the aerodynamic explanation of this?
As the intakes produced shocks, what about other protruberances such a aerials and drain masts etc? Did these also produce (small) shocks?[unquote]
I was generalising a bit and referring to the shocks as heard on the ground which take the form of a classic 'N' wave. This is a sudden rise in static pressure followed by a drop in static pressure to below atmospheric and then another sharp pressure rise. In supersonic flow any increase in static pressure is associated usually described as some sort of compression and conversely a drop in static pressure as an expansion. Since this happens over the region where the wing sits I described it as an expansion field over the wing.
The N wave is OK for the 'far field' shock characteristics, but as you hint, the flow near the aircraft is more complex than that - all the separate shocks gradually. merge into the bow and tail shocks. In the near field all the aerials, drain masts etc have their own little shock waves of course.
I have never seen a completely satisfactory explanation for the drop in static pressure, but if you will accept a much simplified description .....
I think there are two parts to the explanation.
The pressure over the upper wing surface will be below atmospheric static in the usual sense when lift is being generated - this will depend on the amount of lift (and hence weight)
Besides that there is a term related to the volume of the aircraft. One graphic description I have seen is that flying along at 2.0M the aircraft 'tears a hole' in the atmosphere that the surrounding air, being limited to 1.0M cannot fill. Consequently there is a drop in static pressure around the airframe. When the aircraft is past the 'void' is filled up and the resulting rapid increase in static pressure goes with the rearmost shock. This tail shock is sometimes described as the rarefaction shock.
This may not be scientifically accurate, but it satisfies me
CliveL