Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Kryptid on 07/11/2011 22:29:24

Title: Can spinning torus generate a sonic boom?
Post by: Kryptid on 07/11/2011 22:29:24

Will a spinning torus (a doughnut-shaped object) generate a sonic boom if its angular velocity exceeds the speed of sound? I suppose the same question could be asked of a simple disk.

In accordance with my understanding, sonic booms originate from forcing the air in front of an object to move aside quickly in the form of a shockwave. With a spinning torus, there is no air "in front" of the direction of motion and would not be pushed out of the way.

Title: Can spinning torus generate a sonic boom?
Post by: Soul Surfer on 07/11/2011 23:12:28

The limits on helicopter performance are when the rotor blade tip speed in the direction of travel approaches the velocity of sound and the incipient shock wave is the reason for the chopping sound of a fast approaching helicopter.

A smooth rotating torus flywheel would not generate a shock wave but I am not sure if it would be possible to get it up to that speed without it falling apart

Title: Can spinning torus generate a sonic boom?
Post by: imatfaal on 08/11/2011 09:48:13

From a browse on internet - some highspeed fly wheel with magnetic bearings will have a peripheral speed of over 1300m/s (ie ovver 3 times speed of sound).  But the limiting factor is as Soulsurfer said the danger of flying apart - the wheel rated at 1310m/s was a highly specialised one-off, created from exotic materials, and designed to maximise energy storage density. 

Title: Can spinning torus generate a sonic boom?
Post by: syhprum on 08/11/2011 09:59:28

I presume this high speed flywheel was running a vacuum enclosure hence the speed of sound was irrelevant 

Title: Can spinning torus generate a sonic boom?
Post by: wolfekeeper on 08/11/2011 15:16:53

Well, there would be a shockwave, because the air molecules near to the torus would have to travel faster than the speed of sound, and ones further away would be stationary; so there must be a region where they go supersonic, and hence there would be a shockwave, but the pressure pattern would be stationary.

Presumably it would make a white noise type sound, like a hurricane or something; but you wouldn't get the BOOM noise.

Title: Can spinning torus generate a sonic boom?
Post by: Bored chemist on 08/11/2011 18:59:18

The edges of the rotors of the "high pressure" ends of turbomolecular pumps are supersonic, but they are not smooth. The fact that they exist shows that you certainly could spin a disk that fast.

Title: Can spinning torus generate a sonic boom?
Post by: wolfekeeper on 08/11/2011 19:28:54

The characteristic speed of the better aluminium alloys is about 450 m/s, so you can't spin a simple loop faster than about Mach 1.5 without it exploding.

However it turns out that a wheel that tapers from the hub can spin with a rim speed about 3 times faster before exploding.

Other materials like Kevlar can spin about 4 times faster still.