Can Air Resist a Piston? A Serious Discussion

[jbriggs444]

@jbriggs444 Once again a skeptical emoji. Please put up or....

It is rude to tell others how to respond.

We know that a flat plate moving through the air broadside can exceed the speed of sound. We know that the disturbance from this flat plate exists upstream of the plate. You might quibble that this is not really "exceeding the speed of sound" because the "speed of sound" is locally elevated due to heating and, perhaps, non-ideality of the fluid. You might quibble that this is not really "exceeding the speed of sound" because there is fluid flow in the direction of motion carrying the disturbance along with it.

But we see no such nuance in your postings. It's just "you can't hear the jet until it hits you". Or "Wikipedia is wrong".

I dislike posting in a thread where experts such as @boneh3ad are already participating because they've forgotten more abound fluid dynamics than I've ever learned.

 

[Sailor Al]

It is rude to tell others how to respond.

Yes, that would be rude. I asked, I didn't tell.

 

[Sailor Al]

We know that a flat plate moving through the air broadside can exceed the speed of sound. We know that the disturbance from this flat plate exists upstream of the plate.

But that's exactly my point, it doesn't.
If the plate is exceeding the speed of sound then the disturbance does not exist upstream of the plate.
If you're upstream of the plate, just like being in front of the supersonic plane, you won't hear the plate till it hits you.
No nuance is needed, it's basic physics.
The speed of sound in a medium is the speed that a disturbance will proceed through the medium.
I have provided the reference above:
Young H. Freedman R. (2018) University Physics with Modern Physics, 15th Edition , Sears & Zemansky

 

[A.T.]

If you're upstream of the plate, just like being in front of the supersonic plane, you won't hear the plate till it hits you.

A supersonic flat plate and other blunt objects will produce a bow shock in front of them, which travels at the same supersonic speed as the object. You will hear that before the objects hits you or passes close to you.

https://en.wikipedia.org/wiki/Bow_shock_(aerodynamics)

 

[Sailor Al]

A supersonic flat plate and other blunt objects will produce a "bow shock-wave" in front of them, which travels at the same supersonic speed as the object. You will hear that before the objects hits you or passes close to you.

That's a pretty picture, but there's nothing in the text that indicates the shock wave is propagating at >Mach 1.0. The caption says the flow is supersonic in the wind tunnel, so that shockwave is clearly propagating slower than the flow. If it were going faster its curve would be convex, not concave as in the pic!
Please, check your physics books. Mach 1.0 is the speed that disturbances propagate through a medium.

 

[A.T.]

That's a pretty picture, but there's nothing in the text that indicates the shock wave is propagating at >Mach 1.0.

The bow shock is in front of the supersonic object, so it will obviously reach you before the object.

The caption says the flow is supersonic in the wind tunnel, so that shockwave is clearly propagating slower than the flow.

No idea what you mean here. Maybe you have problems understanding/accepting Galilean Invariance:

https://en.wikipedia.org/wiki/Galilean_invariance

Relative to the wind tunnel walls the bow shock and object are both stationary, which means that both are moving faster than Mach 1 relative to the air in the wind tunnel.

 

[Sailor Al]

The bow shock is in front of the supersonic object, so it will obviously reach you before the object.

Like I said, there's nothing in the text to support your claim.

Relative to the wind tunnel walls the bow shock and object are both stationary, which means that both are moving faster than Mach 1 relative to the air in the wind tunnel.

Please refer to your physics textbook:
Mach 1.0 is the speed that a disturbance will propagate in a medium.
You will not hear the supersonic jet plane before it hits you.

 

[A.T.]

The bow shock is in front of the supersonic object, so it will obviously reach you before the object.

Like I said, there's nothing in the text to support your claim.

Because it is trivially obvious. How would you hit the object from the front, without first passing through the bow shock in front of it?

You will not hear the supersonic jet plane before it hits you.

We aren't talking about a jet plane with a pointy nose, but flat plate which generates a bow shock in front of it.

Mach 1.0 is the speed that a disturbance will propagate in a medium.

What you are missing is that medium itself is moving at different speeds. A blunt object like a plate or piston will push some air in front of it, and move that air along. So a disturbance traveling at a certain speed relative to that local air can be traveling much faster relative to the surrounding air.

 

[renormalize]

Mach 1.0 is the speed that a disturbance will propagate in a medium.
You will not hear the supersonic jet plane before it hits you.

Your statement is simply wrong.

Please take a look at this recent experimental paper: Measurement of unsteady shock standoff distance around spheres flying at Mach numbers near one, T.Kikuchi & K.Ohtani (2022)
The authors fired plastic, aluminum and steel ball-bearings of diameter $\sim0.8\text{cm}$ through air at speeds spanning Mach 1.0 and observed the shocks produced. For example, here is a photo of a plastic bearing traveling from left to right at Mach 1.03:

The detached bow shock wave clearly leads (stands-off) the projectile by about 2 ball diameters. If you do the math, you find that the shock reaches your ear about $45\mu\text{s}$ before the projectile does. So much for your claim.

Moreover, for all projectile speeds from Mach 0.95 to 1.25, the speed of the shock is equal-to or greater-than that of the projectile:

High-amplitude, non-linear disturbances in air such as shock waves most certainly can travel at supersonic speeds! Perhaps you're the one who needs to refer to research literature?

 

[Sailor Al]

Because it is trivially obvious. How would you hit the object from the front, without first passing through the bow shock in front of it?

Like I said: there's nothing in the text that supports your argument.

What you are missing is that medium itself is moving at different speeds. A blunt object like a plate or piston will push some air in front of it, and move that air along. So a disturbance traveling at a certain speed relative to that local air can be traveling much faster relative to the surrounding air.

But if there's a small region of compressed air in front of the object, then in that compressed region the speed of sound will be higher than the speed of sound in the surrounding air. The shock wave may be travelling faster there but it's still Mach 1.0 in that medium.
From anywhere else in the surrounding air, the shock wave will move at Mach 1.0

 

[Frabjous]

All I can say is, check your physics textbooks.

Looking at a 2012 edition, the couple of pages in Freedman do not address shock velocity. You are reading in your own biases.

 

[A.T.]

But if there's a small region of compressed air in front of the object, then in that compressed region the speed of sound will be higher than the speed of sound in the surrounding air. The shock wave may be travelling faster there but it's still Mach 1.0 in that medium.

The air in front of the blunt object is not just compressed but also moving along with the object, so the speed of the local medium adds to the speed through the local medium (see Galilean Transformation). But whatever the contributions here are, the final result is that the bow shock right in front of the supersonic blunt object moves at the same speed as the object, and stays at a constant distance ahead of it. So it will pass you before the object hits you.

 

[boneh3ad]

By "shocks" do you mean "shock waves?
According to my Physics text* :
"A shock wave is produced continuously by any object that moves through the air at supersonic speed"
As noted my piston is not supersonic.
And BTW, I think the Wikipedia page is incorrect when it claims, in the opening sentence:
"In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium."
The speed of sound is a medium is the speed that a pressure wave travels in that medium. A shockwave is a pressure wave - it travels at the speed of sound in that medium, not "faster than the local speed of sound in the medium."
So much misinformation!
*Young H. Freedman R. (2018) University Physics with Modern Physics, 15th Edition , Sears & Zemansky

I've provided you with several references that corroborate my analysis and my claims. They've been supported by 100 years of aerodynamics observations/experiments and have contributed to the designs of aircraft that fly millions of miles per year.

If you want to be a skeptic in the face of a preponderance of evidence, be my guest. I will not be providing further guidance for someone who simply refuses to engage seriously.

 

[berkeman]

I will not be providing further guidance for someone who simply refuses to engage seriously.

And with that, this thread is done.