Question about Diffuser

[walkeriot]

Thank you so much!!! I'm reading it!

i was just little confused from its shape, now im clear

Thanks again!

[walkeriot]

by the way, i've read some articles about the car diffuser. they say a 'suction force' can be produced by the diffuser in order to lower the boundary layer of the air flow.

if the pressure becomes larger at the exit of the diffuser, the direction of the force (due to the pressure different) should be inverse to the car's moving direction. so how does the force suck the air out?

[manchild]

if the pressure becomes larger at the exit of the diffuser, the direction of the force (due to the pressure different) should be inverse to the car's moving direction. so how does the force suck the air out?

It enters below cars floor at larger pressure, speeds up below the floor and exits same way it entered.

Like this:

< car moving direction

'''''''''\......................../'''''''''''''

If there was no diffuser than under body of the car would work as Venturi tube only half way and at the rear end you'd get huge turbulence and huge drag.

Like this:

< car moving direction

''''''''''\.....................@@@@@

Even more, the capacity of under body depends on capacity of diffuser which means that a car could not let under the same amount of air if it had no diffuser relative to car with a diffuser.

[syguy]

Exactly, as manchild, describes.

Air flow is driven by pressure gradients. Assuming subsonic flow, facing a restriction such as the underbody gap on a racecar or a nozzle the air must accelerate to maintain the same mass-flow rate (conservation of mass). If the air increases in speed then the pressure will decrease, following Bernoulli's principle. Such a favorable pressure gradient (high at the entrance and low at the restriction) will not separate and the boundary layer will remain thin.

As the cross-sectional area increases again, either abruptly or gradually, the air will decelerate and therefore the pressure will increase. Now we face an adverse pressure gradient (low at the restriction and high at the area increase). Presented with an abrupt, strong adverse pressure gradient such as the case without a diffuser, as manchild points out, the flow will reverse direction resulting in turbulence and drag - causing the air speed under the car at the restriction to drop and the pressure to increase. However, by ensuring a gradual increase in cross-sectional area (using a diffuser), though still under the influence of an adverse pressure gradient (it's much weaker), the air can be coaxed to remain attached (though the boundary layer will thicken) and retain a relatively high mass-flow rate.

[RH1300S]

Can I just add..........don't forget the rear wing in the scheme of things.

The rear wing works WITH the diffuser and helps get air out from the underbody. Quite likely, the rear wing lower element underside has a pretty low pressure and that will encourage air out of the diffuser (may even accelerate it creating even lower underbody pressures?)

[mini696]

Can I just add..........don't forget the rear wing in the scheme of things.

The rear wing works WITH the diffuser and helps get air out from the underbody. Quite likely, the rear wing lower element underside has a pretty low pressure and that will encourage air out of the diffuser (may even accelerate it creating even lower underbody pressures?)

Also the 'Deck-wing' does this too.