# Why doesn’t the air expanding behind a shockwave reverse the effects of wave drag?

Wave drag is (from my understanding) the difference in pressure between the front and rear of an object due to shockwaves, making an opposing force. This needs higher pressure in front of said object and lower pressure behind.

Now shocks themselves aren’t high pressure, but high pressure occurs right after the shock.

(Again from my understanding), higher pressure air will expand and cause lower pressure. Same thing that happens with an airfoil.

So if wave drag is from there being higher pressure on the front of an object, and higher pressure air will expand making lower pressure air, why wouldn’t the two pressure differentials just cancel each other out?

To make things simpler just imagine you have a square plate angled at 45 degrees going into the free stream.

So to recap, the air will contact the 45 degree plate, make a shockwave which will have high pressure after it, then that higher pressure air would expand and make low pressure. Wouldn’t this high pressure then low pressure cancel the net force out? Am I missing or misunderstanding something?

My guess is that this effect does happen on say an airfoil, but by the time the high pressure air expands and turns low pressure, it's already passed the wing or where it won't change wave drag. Not sure though.

Also, since shockwaves aren't always right close to the surface, the expansion of the high pressure might still make low pressure, but it will not be contacting the object. See the second picture in the answer here to know what is meant by "the shocks aren't always close to the surface".

• You're forgetting the expansion fan on the upper part of the plate Commented Apr 29 at 17:42
• @sophit Oh okay, so how might that change the wave drag amount? Commented Apr 29 at 17:49
• @sophit Sorry if I’m not understanding something obvious, but isn’t the expansion fan because of the low pressure? The expansion fan doesn’t do anything on its own, right? Commented Apr 30 at 3:28
• This is what happens at supersonic speed on a flat plate Commented Apr 30 at 4:58
• I see. So I was basically forgetting that the back side of said object is also lower pressure, so even if the front of the object had high then low pressure (like I mentioned above) it wouldn’t cancel anything out? Hope that makes sense haha Commented Apr 30 at 14:55