Quote:
Originally Posted by AdrianK
<<the nose of the bike would have dynamic pressure, which is lower than static pressure and dependent upon the speed of the air, so in effect the air behind the slots (covered) would have an overpressure, compared to the air flowing past it..>>
..... could you explain that a bit more? It seems a little counter-intuitive  . |
er if you really want me to?
This is what I learnt when studying for my pilots license.
Introduce an object into a body of moving air (or move that body through the air) and things happen to the air molecules. For some reason, unkown to me 'cos I is thick, when the air molecules are parted by the object they will meet at the same time when past that object.
I will give the example of an aircraft wing, or the spoiler on a F1 car (It snt really a spoiler as that is designed to 'spoil' the air flow and induce drag, the F1 cars have an upside down wing or aerofoil)
The upper surface of a wing (lower for F1) is usually more curved than the lower surface (upper for F1). when the air molecules meet the wing they part and go over and under the wing, when they get to the trailing edge of the wing they will get there at the same time (thats the bit I dont understand why).
The air molecules running over the top of the wing have a further distance to 'run' before they reach the trailing edge so to meet the lower wing air at the same time they move faster. This gives a lower (dynamic) pressure.
The air molecules under the wing, have a lesser distance to travel so are moving slower and have a higher (static) pressure. This is what gives the wing lift, and in formula 1 its this 'lift' upside down which presses the car against the tarmac.
This is also by the way how a carburettor venturi works, faster air running through the squeezed space gives a lower pressure, lower pressure means that it sucks the air in faster.
And also how carburettor icing works, even on a fairly warm day as lower pressure means lower temperature, which means air that is moist loses its ability to retain that moisture as the air cools, the moisture turns to water which can be cooled below freezing point and thus form ice on the carburettor butterfly valves, this is the main reason why light aircraft crash, carburettor icing, loss of engine power, loss of forward thrust, loss of lift, glide to a good landing point or die!
Now a bike is not a wing, so a lot of the air that the bike is moving through will be 'battered' out of the way and becomes turbulent very rapidly (this is form drag because the bike is not a clean smooth aeordynamic object). Whatever clean air flow that there is will become turbulent once it is past the smooth part of the bike that is moving through it.
So from the above, the air behind the nose fairing, if it were blocked would have a higher (slow moving static) pressure than the air moving past the nose fairing., there wouldnt be a suction effect, it would be the opposite, however watch a jet taking off in the drizzle and you see wing tip vortices, this is the higher static pressure spilling over the top of the wing and becoming turbulent when it meets the lower dynamic pressure (hence the vertical winglets on the ends of a lot of jet wings, even the humble old 737 has 'em, to reduce this turbulence and reduce drag, increasing efficincy and saving fuel, whilst still charging you more for your ticket)
I digress..ahem.. this turbulence is what is happening when the lower pressure air is meeting the higher pressure air from behind the fairing, so there never is an overpressure as such.
right you can wake up now
