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-   -   Help on sonic boom (http://www.physicsforums.com/showthread.php?t=107033)

 goozooloo Jan16-06 10:52 PM

Help on sonic boom

Could some one tell me what is it called in physics (name of property etc. ) that does not allow air particles to move past the speed of sound?

 FredGarvin Jan17-06 08:09 AM

Air can travel faster than the local speed of sound given the proper conditions. The first example I can think of is the exhaust gases out of the space shuttle and jet engines. This is accomplished by using converging-diverging nozzles.

You may be thinking of a choked flow condition where a fluid speed is limited to the maximum of speed of sound. It is however not an absolute and can be overcome with differernt flow geometry.

Perhaps you can restate your question?

 goozooloo Jan17-06 08:14 AM

Well basically, when an aircraft moves faster than the speed of sound it tries to push the air along that same speed right? but the reason for the sonic boom is that the air cannot excede that limit. what causes this to occur?

 jaap de vries Jan18-06 11:42 PM

I think the basic idea is the translational speed of the air molecules (about 400 m/s and the mean free path on a molecular level. were you more thinking along these lines? Basically the air molecules are not "communicating" fast enough.

 russ_watters Jan19-06 12:05 AM

Quote:
 Quote by goozooloo Well basically, when an aircraft moves faster than the speed of sound it tries to push the air along that same speed right? but the reason for the sonic boom is that the air cannot excede that limit. what causes this to occur? thanks for ur reply
That isn't really what is happening. The shock wave is a plain, ordinary sound wave that due to the speed of the aircraft, can't outrun the aircraft. Sub-sonic aircraft push pressure waves out in a circular (from the ground) pattern. Pile these circles up and you get a cone, with all the waves concentrated on the "surface" of the cone.

Quote:
 Quote by Me The best explanation I've seen is using water drops: if you drip water into a pool, concentric circles form as the wave expands from where the drop hit. If you do a steady drip, drip, drip, these waves will continually move away from the impact point. If you start to move the impact point slowly, the waves will still be able to move forward away from the impact point and you'll end up with circles that are not concentric, but still moving away from the impact point in all directions. If you move faster than the waves can move, all the circles you make will be behind the impact point, and their diameters will be larger the further from the impact point, forming a cone. HERE is a picture of the phenomena using a plane, but the circles are there...
Remember also that all motion is relative, and except in the passing of the wave and friction from the plane, the air molecules don't actually need to be moving much relative to the ground. In fact, if air molecules are being pushed at high speed, that's an indicator of high drag. Still, when the Space Shuttle blasts into the atmosphere at 10,000 mph, air molecules are going to bounce off that blunt nose at 10,000 mph. In fact, that's what keeps it cool: the bluntness pushes air out in front of it like a snow-plow, keeping the shock wave (and it's enormous pressure induced temperature rise) away from the space shuttle).

 brewnog Jan19-06 12:36 PM

Had never thought about the cooling effect the bow wave had on things like the shuttle, thanks Russ.

 goozooloo Jan22-06 10:26 PM

Thanks for the info

however you sed that the air molecule bouncing off the nose of the space shuttle is still travelling at 10,000 km/h, if so then why doesnt the shockwave travel at that speed since its carried by those molecules. I think the effect here is friction, thats why the speed of sound becomes higher and higher as air density gets lower with gains in altitude. Just what i think, if you would please correct me if im wrong.
Thanks

 jaap de vries Jan23-06 11:14 AM

Quote:
 Quote by goozooloo thats why the speed of sound becomes higher and higher as air density gets lower with gains in altitude. Just what i think, if you would please correct me if im wrong. Thanks
Actually this is wrong. By gaining altitude the speed of sound decreases because of decrease in temperature a = (T*gamma*R)^1/2. I don't see how the soundspeed would go up with decrease in density where did you read that??

 Clausius2 Jan24-06 03:22 AM

Quote:
 Quote by jaap de vries Actually this is wrong. By gaining altitude the speed of sound decreases because of decrease in temperature a = (T*gamma*R)^1/2. I don't see how the soundspeed would go up with decrease in density where did you read that??
Actually temperature arises from upper stratosphere layers on.

 jaap de vries Jan24-06 11:04 AM

Quote:
 Quote by Clausius2 Actually temperature arises from upper stratosphere layers on.
Very true Claussius,
Only I don't think goozooloo had this in mind
by that time the air density is so low that conventional gas dynamics can no longer be used and one has to refer to kinetic theory and individual molucules hitting the surface of an object.

Jaap

 |Orion's Thought| Feb3-06 08:44 PM

Isnt it- its "hot" in the upper atmosphere because the air molecules are moving so fast, however, they are so far and few between, it feels very cold. Yes? No? Maybe so?

 russ_watters Feb3-06 10:37 PM

Sorry, missed this before...
Quote:
 Quote by goozooloo however you sed that the air molecule bouncing off the nose of the space shuttle is still travelling at 10,000 km/h, if so then why doesnt the shockwave travel at that speed since its carried by those molecules. I think the effect here is friction, thats why the speed of sound becomes higher and higher as air density gets lower with gains in altitude. Just what i think, if you would please correct me if im wrong. Thanks
Yes, the molecules would be moving fast and transfer all that energy (the speed of sound depends on temperature), but then those molecules would need to do a big billiards game to send all that energy through the atmosphere. The atmosphere is big, so the energy quickly gets absorbed into a standard shockwave based on the prevaling temperature.
Quote:
 Isnt it- its "hot" in the upper atmosphere because the air molecules are moving so fast, however, they are so far and few between, it feels very cold. Yes? No? Maybe so?
No: temperature is average kinetic energy of the particles, not kinetic energy per unit volume - so the density doesn't matter.

 goozooloo Feb5-06 10:19 PM

THANKS, i think i got the basic idea now :smile:

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