How does a diffuser/ride-height create drag?

[raymondu999]

Hey all. Just wondering about something. I remember reading that any and all downforce has a drag penalty, just that some can produce a higher lift/drag ratio, being more efficient. Wouldn't this then mean that ride height, which should help the diffuser produce more downforce, produces more drag as you go shorter? And wouldn't it mean that the diffuser also produces drag? Wouldn't it mean (to a certain extent) that the DDD has more drag than an SDD?

How does the diffuser produce drag in the first place, too?

[xpensive]

The function of the diffuser is to help speeding up the air under the car in order to create a lower static pressure according to Bernoulli. But this "suction" effect does not come for free, it will cost power, in this case in the shape of drag.
Power is Force times Speed, which we can translate to Drag is Power over Speed.

At the same time, Power is volumetric Flow times Pressure, why the Power would be the volumetric Flow under the car times the static Pressure differential between the front edge of the floor and diffuser. There you get the Power, then divide that with car speed and you get Drag.

Something like that anyway, are you there kilco?

[xpensive]

When I find this question interesting, I would like to keep it on the board, input welcome?

[ringo]

The diffuser is dependent on drag of the car's body to work. It works best when it exits in low pressure areas. These low pressure areas on a car are always areas that create the most drag. Like behind the car, under the wing etc. So the diffuser is a device that also makes use of existing drag, somewhat making the car more aerodynamically efficient.
Another thing to take note of is gurneys on diffusers. These are basically shields to make low pressure areas, which increase drag of course, so the diffuser can "scavenge" to increase it's performance.
This is all how i see part of the relationship with diffuser drag anyway. Indirectly drag is required for the device to work well.

The other part concerning the expansion of the air in the diffuser. A diffuser is a steady state device so it does not consume work. So it does not consume power.

q = (delta pressure)/density + 1/(2 x gravity constant) x delta (V^2)

q is usually set to zero, but since there are slight density changes and other factors like height, it has a value but is usually negligible.

It does not really cost power since all it does is change area to control velocity. Other parts of the car though have to have a cost for the diffuser to get that pressure difference to work how it is intended though.

[autogyro]

Come on no more excuses.
Increase DF and you increase Drag, decrease DF and you decrease Drag.
Maybe through design and better streamlining but less drag for sure.
Also the 'grail' less turbulent wake.

[Shrek]

Come on no more excuses.
Increase DF and you increase Drag, decrease DF and you decrease Drag.

Actually in NASCAR you see them taping up the nose inlets because it can lead to an increase of downforce while dcreasing drag, but yeah after that i don't know of any that can be free

[autogyro]

Come on no more excuses.
Increase DF and you increase Drag, decrease DF and you decrease Drag.

Actually in NASCAR you see them taping up the nose inlets because it can lead to an increase of downforce while dcreasing drag, but yeah after that i don't know of any that can be free

Taping up the inlets reduces the amount of air going under the car.
That increase DF from the under floor but it also increases drag from the under floor. This drag increase is larger than the reduction in drag on the body from the taped up and smoother nose.
Arguing the reverse is simply aero merchants continuing to con people by over complicating the issue.

[kilcoo316]

I remember reading that any and all downforce has a drag penalty, just that some can produce a higher lift/drag ratio, being more efficient.

Mmmmm, mostly. But there are exceptions.

Wouldn't this then mean that ride height, which should help the diffuser produce more downforce, produces more drag as you go shorter?

Yeap, but you could reduce your wing AoA, reducing overall drag levels while maintaining a constant (more or less) downforce.

And wouldn't it mean that the diffuser also produces drag? Wouldn't it mean (to a certain extent) that the DDD has more drag than an SDD?

Yes and yes.

However for the reason mentioned above (as well as more downforce than current cars can generate resulting in a faster laptime) diffuser drag is not a problem.

How does the diffuser produce drag in the first place, too?

Mainly pressure drag (the diffuser surface has a component in the longitudinal as well as vertical direction) and a (very) little bit of viscous drag.

[kilcoo316]

Come on no more excuses.
Increase DF and you increase Drag, decrease DF and you decrease Drag.
Maybe through design and better streamlining but less drag for sure.
Also the 'grail' less turbulent wake.

You go away.


You've already demonstrated you've no idea what your talking about so leave the thread and don't mislead people.

[Just_a_fan]

Come on no more excuses.
Increase DF and you increase Drag, decrease DF and you decrease Drag.

Wing end plates say otherwise...

[xpensive]

The other part concerning the expansion of the air in the diffuser. A diffuser is a steady state device so it does not consume work. So it does not consume power.

Speeding up the air under the car does not consume power? I think not.

Power is Volumetric flow times Pressure differential, that rarely comes for free. If air-speed under the car is 20 m/s higher than above, there's a price for it, one way or the other,

[autogyro]

Come on no more excuses.
Increase DF and you increase Drag, decrease DF and you decrease Drag.

Wing end plates say otherwise...

Again misleeding. To start with wing end plates say what?

Add them to an inefficient wing with high drag and they increase DF which also increases drag if compared to the original wing. However there is actualy no comparison, because without the end plates the wing would be an incomplete design and not subject to comparison in any meaningful way.

[kilcoo316]

At the same time, Power is volumetric Flow times Pressure, why the Power would be the volumetric Flow under the car times the static Pressure differential between the front edge of the floor and diffuser. There you get the Power, then divide that with car speed and you get Drag.

I'd go with velocity differential and massflow.


Where exactly you'd measure V at... at (1) the floor leading edge or the diffuser leading edge and (2) at the diffuser exit.

Then a fudgy result with the ram drag eqn will give you a number of sorts...

[kilcoo316]

However there is actualy no comparison, because without the end plates the wing would be an incomplete design and not subject to comparison in any meaningful way.

[...]
When is a formula 1 car ever a complete design?

[kilcoo316]

Please do not avoid the obvious kilco and do not call me a silly little child.

You actually think a front wing without endplates would produce less drag?

I take it you've never heard of a term like "spanwise lift distribution" either?