Why do planes have these clouds around them when they break the sound barrier?
Author: admin Category: 5
Like in this picture: http://www.defenselink.mil/specials/images/sightofsound.jpg
I didn’t really think that there was a physical sound barrier… or is it just some sort of side effect?
It is called a Prandtl–Glauert singularity.
For understanding it, you need to know two things: What happens when a plane approaches the sound barrier, and what the effect of a shock wave is on air.
When the speed of air over an object reaches or exceeds the speed of sound, all changes in the direction of flow happen abrupt in a shock wave. At lower speeds, the Brownian movement of the molecules allows some sort of a warning for the following molecules in the flow, that a change of speed or direction will take place - the sound makes some molecules change their speed and direction already before the object. At the speed of sound or faster, the air flows faster than the warning could get passed: The air molecules suddenly collide in something very similar to a typical rush-hour traffic jam - the shock wave.
As the air moves at different speeds over the surface of a plane, you have points, where shock waves form first - the canopy, the top of the wings, rudder, etc. There are also always two shock waves, one in front, caused by the air having to move around the object, the other after the air passed around the object and returns to it’s normal flow.
Behind the front shock wave, the air is typically slower (lower Mach number), has a lower density and lower temperatures. This lower temperature is now the key behind the PGS: When the temperature drops below the dew point, the moisture inside the air condenses and forms clouds. When the air passes the aft shock wave, the temperature, velocity and density return to normal - the cloud disappears.
That relation to moisture is also the reason why you can see such clouds only in moist air - like close to the coast or on wet days.
July 12th, 2009
water molecules tiny ones that build up at that speed stacking up
References :
July 12th, 2009
It’s a side-effect. It’s caused by the rapid compression and expansion of the air and any water vapor that may be there.
That one is travelling at almost exactly Mach 1. At around 1.3 or 1.4 the shock wave doesn’t begin to break up until it’s 20 or 30 meters behind the aircraft. And it looks for all the world as if the airplane was jumping through a ‘hoop’ in the sky.
Doug
References :
July 12th, 2009
It is called a Prandtl–Glauert singularity.
For understanding it, you need to know two things: What happens when a plane approaches the sound barrier, and what the effect of a shock wave is on air.
When the speed of air over an object reaches or exceeds the speed of sound, all changes in the direction of flow happen abrupt in a shock wave. At lower speeds, the Brownian movement of the molecules allows some sort of a warning for the following molecules in the flow, that a change of speed or direction will take place - the sound makes some molecules change their speed and direction already before the object. At the speed of sound or faster, the air flows faster than the warning could get passed: The air molecules suddenly collide in something very similar to a typical rush-hour traffic jam - the shock wave.
As the air moves at different speeds over the surface of a plane, you have points, where shock waves form first - the canopy, the top of the wings, rudder, etc. There are also always two shock waves, one in front, caused by the air having to move around the object, the other after the air passed around the object and returns to it’s normal flow.
Behind the front shock wave, the air is typically slower (lower Mach number), has a lower density and lower temperatures. This lower temperature is now the key behind the PGS: When the temperature drops below the dew point, the moisture inside the air condenses and forms clouds. When the air passes the aft shock wave, the temperature, velocity and density return to normal - the cloud disappears.
That relation to moisture is also the reason why you can see such clouds only in moist air - like close to the coast or on wet days.
References :
http://en.wikipedia.org/wiki/Prandtl%E2%80%93Glauert_singularity