# Will air in a shockwave flow outwards?

It's well known that shockwaves extend outward from a plane, as seen in this picture.

My question is, does the high-pressure air in the shockwave extend outward? This is hard to explain so I'll let the picture do the work.

Basically I'm asking if air will flow where the black arrow is pointing, as the shock is high pressure and I don't see anything stopping it. I know that the shockwave won't go past the supersonic pocket of air, but will the air in the shockwave flow in the vector the arrow is pointing at?

Imagine that the shock was just a high pressure, thin line of gas and nothing more. The air would try to flow in the direction of the arrow, right?

• In reality, the air isn't going Mach 0.8, the airfoil is. The accelerated flow runs into the boundary layer attached to the airfoil. This is where the shockwave forms. Air may indeed be deflected in the direction of the arrow, but the normal shockwave may not be stable at all. Early pioneers of this flight regime described shockwaves as "dancing across" the wing surface. Area ruling was developed to literally give the compressed air somewhere to go. Commented Mar 15 at 13:21

No.

The shock is not high pressure.

The shock is a jump in properties. The flow is higher pressure behind the shock than in front. Think of the shock as an artificial boundary where the properties change.

Like the place where water meets air -- that interface is a rapid change in properties.

In the case of your picture, the shock does not turn the flow.

• Oh okay interesting. Would you mind telling me some of the properties that change? Commented Mar 14 at 22:12
• Mainly pressure, density, and temperature. Commented Mar 14 at 22:22
• One of the fundamental principles of aerodynamics is that particles of air remain in their streamline. This is not strictly true in turbulent error where air is mixed. But it is an important aspect of understanding the answer. Commented Mar 14 at 22:28
• @CamilleGoudeseune Does the pressure go up or down? (compared to ambient) Commented Mar 14 at 23:19
• Rob McDonald, you should get payed by @Wyatt 😅 Commented Mar 15 at 6:57

It's not the air in the shockwave flowing in the direction of the vector, it's the energy being propagated in the form of a wave.

Waves transfer energy through a media. The water in a wave that travels across an ocean does not come from it's source, only it's energy.

Waveforms can hold their shape a long distance. One can transfer energy from you voice into paper cup onto a string, and the string will vibrate for many feet and can be heard as sound by attaching it to the bottom of another paper cup.

Shorter wavelengths, such as visible light, can carry even greater distances.

Sonic waves created by an aircraft reaching Mach 1 can be 120 decibels or louder. These sound waves, in a similar range to thunder, can carry for miles.

• The physics of light don't apply directly to aerodynamics. I don't think mentioning them is helpful here. Additionally, the shockwave is a nonlinear boundary in wave behavior. So almost everything you say about waves does not hold equally on both sides of the shock wave. In fact, sound stops propagating normally at the shockwave. Commented Mar 14 at 22:26
• @MarkJonesJr. Well, radio waves apply to aviation, don't they? What I am trying to illustrate, as the OP has asked multiple questions about this subject, is that wave energy is not about movement of one parcel of air in the direction the sound is traveling. The energy (that creates "sound" against our eardrums) is transferred through air as a high and low pressure "pulse". Correct me if I'm wrong: through the same medium speed of sound does not diminish over distance, amplitude of the pressure pulse does. Commented Mar 15 at 1:45
• @MarkJonesJr. "Sound stops propagating normally at the shockwve". Would it hold that the normal shockwave ends where the air flow is no longer supersonic? The OP has the shockwave extending past it. Perhaps you can help with an answer. Does the freestream disrupt this wave, preventing it from propagating? I'm seeing airflow (and turbulence) over the wing aft of the shockwave. Commented Mar 15 at 2:04