Why Does Air Compress Under Shockwaves?

[kevjcarvalho]

can anyone tell me why air gets compressed due to a shockwave...

 

[russ_watters]

The object moving through the air moves faster than the air can get out of the way.

 

[kevjcarvalho]

that i know...but how does compressible flow come into picture?

 

[redargon]

I thought the shockwave was caused because the air is compressed.

 

[russ_watters]

that i know...but how does compressible flow come into picture?

Since air can't get out of the way, it piles up - it compresses.

 

[FredGarvin]

Think of a snow plow...

 

[mikeph]

#4 is correct in that shock waves are a consequence, not cause, of compression of air. The air in your room will be slightly compressed when you slam the door shut, but this is negligible and certainly causes no shock wave! As the flow velocity increases to near a critical value, air compression becomes less negligible, and the measure of this is the Mach number M, the magnitude of the velocity flow divided by the speed of sound of the flow (depends on temperature: Mach 1, M=1, at sea level is 340 m/s, but 11km up it is only 295 m/s). Although the velocity is a field, M only takes a single value for the entire flow region in most cases, so a flow can be "supersonic" even though a small part of the flow is still less than the speed of sound.Essentially, in a subsonic flow past a solid body there are pressure waves moving upstream and downstream- since the flow is slow, the sound wave can travel upstream- why you can hear a commercial plane coming towards you, for example. However, as the flow approaches Mach 1, the velocity upstream is the same as the speed of sound, and the pressure waves do not travel upstream, but are stationary, where they are super positioned. This amplification is the shock wave, physically a region of very high pressure, followed by a region of very low pressure slightly downstream. I believe this low pressure region is responsible for the spontaneous vapourisation of water molecules in the atmosphere, the white cloud you see surrounding the aircraft.

As you pass Mach 1, the sound waves all move downstream, and this is why you would not be able to hear a plane traveling at M>1 until a while after it passes you!

 

[mugaliens]

I thought the shockwave was caused because the air is compressed.

It's caused by the air being compressed faster than the speed of sound. As a result, a shock wave is created.

If the air were being compressed slower than the speed of sound, it would simply result in a pressure wave, not a shock wave.