So, how fast is the airflow ??
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So, how fast is the airflow ??
Being a sad sort of devil, I was watching Discovery Wings and an Air Canada 767 on its way to Europe from Toronto, while pondering the 'how close would you have to stand' thread.
The question that came out of this is just how fast is the airflow into a typical (typical ??) modern jet engine. Say a 767 ??
The question that came out of this is just how fast is the airflow into a typical (typical ??) modern jet engine. Say a 767 ??
Depends on where you are standing.
The idea is to keep the airflow at a constant pressure (ambient) while feeding the engine all it wants. Not a difficult task when you're moving into the air. However when standing still you have to suck the air which is akin to dropping the pressure below ambient inside the inlet and letting the air move from high pressure (ambient) to the lower pressure inside the inlet. Thus if you are standing in the way you will have a pressure drop across you which has to be reacted out by the friction in your feet.
The idea is to keep the airflow at a constant pressure (ambient) while feeding the engine all it wants. Not a difficult task when you're moving into the air. However when standing still you have to suck the air which is akin to dropping the pressure below ambient inside the inlet and letting the air move from high pressure (ambient) to the lower pressure inside the inlet. Thus if you are standing in the way you will have a pressure drop across you which has to be reacted out by the friction in your feet.
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I suspected when there were few responses, it was not a simple answer.
Ok, let me try to set some conditions.
Say a Boeing 767, at take off power.
The observed position being 3 feet from the front of the engine cowling at a pont 5 ft vertically from the ground (there is a point, I guess there is the possibility that it may vary depending across the engine radius)?
Some weeks ago, there was a thread on jet engine noise . I was given some book references re jet engine design. I really must try to find them again because this is a facinating subject.
Ok, let me try to set some conditions.
Say a Boeing 767, at take off power.
The observed position being 3 feet from the front of the engine cowling at a pont 5 ft vertically from the ground (there is a point, I guess there is the possibility that it may vary depending across the engine radius)?
Some weeks ago, there was a thread on jet engine noise . I was given some book references re jet engine design. I really must try to find them again because this is a facinating subject.
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The diffuser does slow the air flow in flight so that air hits the fan at about .4 to .5 Mach number rather than the .8 to .85 Mach number of the aircraft. This is done with over 99% isentropic efficiency, i.e. the actual static pressure rise is 99% of the theoretical amount if all the kinetic energy loss of the air is converted to static pressure according to Bernoulli's equation.
When an engine is running statically on the ground, the diffuser does not work in the same way as the air is being accelerated rather than decellerated. There is a "sink flow" into the engine, so in the absence of the ground or other obstacles, the velocity would be roughly inversely proportional to the square of the distance from the intake. In practice the flow is more complicated; usually a "ground vortex" is generated due to ground friction. I've known this to pull in a lump of loose tarmac 15 feet up into the intake of an engine running on a static test bed.
I don't know if this answers your question, but its the best I can do.
When an engine is running statically on the ground, the diffuser does not work in the same way as the air is being accelerated rather than decellerated. There is a "sink flow" into the engine, so in the absence of the ground or other obstacles, the velocity would be roughly inversely proportional to the square of the distance from the intake. In practice the flow is more complicated; usually a "ground vortex" is generated due to ground friction. I've known this to pull in a lump of loose tarmac 15 feet up into the intake of an engine running on a static test bed.
I don't know if this answers your question, but its the best I can do.
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I don't know if this answers your question, but its the best I can do.
Last edited by phnuff; 23rd Jul 2003 at 01:41.
