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opps......you're right about the perfect gas equation......

lol

PS-don't have to be rude and call people stupid....everyone makes mistakes....especially when they only started studying fluids 2 months ago.....

Just to back up the correct ideal(or perfect) gas equation that is Pv=RT, with v = specific volume (1/density).

This subject can be attacked from a different perspective...

It can be seen with the basic conservation of energy equation (applied to a diffuser) and some basic assumptions (one-dimensional flow, negligible heat transer, negligible changes in potential energy, and non-flow Work, as well as steady state flow conditions)

The equation simplifies to...

mfr-in * (h1 + Vin^2/2) = mfr-out * (h2 + Vout^2/2)

with mfr = mass flow rate, and with the steady state assumption, these terms cancel because mfr-in=mfr-out,

then the equation becomes...

h1 + Vin^2/2 = h2 + Vout^2/2

now, h = enthalpy, which is defined as h = u + Pv
with ..

u = internal energy
P= PRESSURE (what we're looking at
v= specific volume

now looking at our equation... h1 + Vin^2/2 = h2 + Vout^2/2
with Vout decreasing significantly at the exit of the diffuser, this drop in magnitude has to be made up by h2 = u + Pv, this correction is mainly due to a RISE in pressure, as "Guest" mentioned before.


Now, this is a somewhat simplified look, but this is how fluids is generally taught at the basic level, to understand the basic charactersistics of a diffuser. I know this stuff didn't add much to the aerodynamic discussion, but i thought a different look at the subject apart from the perfect gas equation might interest some of you. If you have some questions on some of the terms or things i've said, let me know.

If you see a big mistake, let me know (i'm supposed to know this stuff, being an engineering student, lol)

one other thing, Vout/Vin in these equations is VELOCITY, not volume

Anonymous wrote:Once more, as in the MP4/19 topic, someone has to go back to the books and study some more before posting some theories. Monstrobolaxa, the ideal gas law that you state is wrong. It is not p*V=rho*T but p*V=R*T where R is the gas-constant. Furthermore V is not volume but specific volume (this is 1/rho). So unfortunately the ideal gas law doesn't help you with pressures in moving situations and you'll have to go back to conservation of mass and Bernoulli. And once again: if the area increases, the velocity lowers and hence the pressure rises. And before you say that the density changes... please do the following little exercise: use the ideal gas law to calculate the density at say 15 deg C, 101325 Pa (R=287.15). Now lower the pressure by 5000 Pa (roughly Cp of -2 at say 250 pkh, which would already be quite a challenge to achieve over a large part of the car) and calculate the density again (T and R haven't changed after all). Low and behold the density variation is small (~5%). That's why for aero-problems at these speeds density is always treated a constant (if the air temperature isn't changed). This means that the area increase in the diffuser is not creating downforce as such. It is merely helping to create downforce in a different way. So please start using the brain again and think how a diffuser can help the car create downforce. (And just as a final helping hand: fluids are not only flowing from high to low pressures. It is more willing to do so, but unfortunately for the aero guys it also has to flow back from the low pressure to a higher pressure again... and that's where the design problems start)

Excellent description.

To get rid of confusion between V=velocity and V=volume why don't we just use U as velocity.This is accepted in standard x-axis velocity component.So as long as we don't involve discussion with y-direction if velocity component,which is v,there would be no more confusion with this.