Diffuser Confusion

[hardingfv32]

"Let's just all agree it's magic."

It is only magic if you are not willing to learn what in fact is the actual science.

Brian

[n_anirudh]

OK, here is a simpler single diffuser from this year. Ferrari /Massa 2011 Spain.
there is no step change and is a smooth transition

OK this is from 2010 oz Gp when PdlRosa stopped in front of me..sauber 2010 double diffuser:




This should pretty much be convincing.

[vonk]

hollowBallistix and vonk

You should both read:

Experimental study of multiple-channel automotive underbody diffusers

http://pid.sagepub.com/content/224/7/865.full.pdf+html

hollowBallistix, at the start of the study it states how/why the diffuser functions. It differs from your ideas.
vonk, in the main body of the report there are actual pressure tap test results that show, that in fact, the diffuser functions as/in open system.

Brian

Brian, I read that paper before.



While this isn’t the place to nit pick it, the diffuser used a flat upper surface, and the bluff bodies provided funneled inflow under the floor. I would have been more comfortable with a horizontal splitter (orange). But all of this is not relevant to our concave diffuser debate. Also, I'm not implying that an open system diffuser can't work.

vonk

[vonk]

OK, here is a simpler single diffuser from this year. Ferrari /Massa 2011 Spain.
there is no step change and is a smooth transition

OK this is from 2010 oz Gp when PdlRosa stopped in front of me..sauber 2010 double diffuser:




This should pretty much be convincing.

n_anirudh, these pictures are fantastic. Do you have any of the 2011 Red Bull? Ferrari’s looks almost like a horizontal diffuser.

What makes the Sauber a “double diffuser”? Is it the very low second airfoil?

Interesting variety of approaches. But of what is this convincing. I’d still like to know what that 2009 Red Bull diffuser was all about.

vonk

[xpensive]

"Let's just all agree it's magic."

It is only magic if you are not willing to learn what in fact is the actual science.

Brian

Xcellent post harding, those kind of one-liners are rather useless and doesn't contribute whatsoever.

There's nothing magic about the diffuser, its function is to help speeding up the air under the car in the most effective way to create downforce, the FIA has been trying to limit the xit cross-section area of the same to limit said downforce.

Btw, those scoops on the RBR you asked about are the intakes for the "double diffuser", intended to to further speed up the air under the floor, in order to create even lower static pressure and thus downforce.

[hardingfv32]

I would say the radius at the throat of the diffuser is the same on the 2011 car and the original photo that started this discussion. The general shape seems the same with both.

Also note how the older car's diffuser is almost 2X deeper. Is that a DDD diffuser in fact? So is the depth a function of the rules or the availability of the DDD?

Brian

[hardingfv32]

xpensive

How does the concave shape of the diffuser roof specifically "help speeding up the air under the car"

Brian

[PlatinumZealot]

This would break the definition of a diffuser though. You would see massive pressure loss with this abrupt expansion.

I agree about the definition of a diffuser. The term diffuser for devices of this kind has been around for a long time. Has it become a misnomer for what the picture shows? But that’s sacrilege, I know.

Could you explain what pressure loss you are referring to?

Total energy loss of the gas as it flows through the diffuser.

Energy in the form of pressure. Total pressure = static pressure + dynamic pressure. I mean loss in dynamic pressure, (sorry for the confusion) and a rise in static pressure, but overall a loss in total pressure in the form of heat.

[PlatinumZealot]

n smikle

"This would break the definition of a diffuser though. You would see massive pressure loss with this abrupt expansion."

Agreed, but why would a competitive F1 car have this shape diffuser then. With further research I have found other examples of concave diffusers.

What other parts of the system are making this viable?

Brian

What applications? Any photos. I am reserved about this because I know it can work for one reason, but it will have energy losses for another.

[godlameroso]

xpensive

How does the concave shape of the diffuser roof specifically "help speeding up the air under the car"

Brian

It speeds up the air under the car relative to the top. For argument's sake let's assume you could keep air perfectly attached to a pair of surfaces(I know it's impossible please bear with me). Viewing one side of the surface you notice it's smooth the other has a curve, assuming the air stays attached, the air on the smoother side of the surface will travel relatively faster than the air that has to go around the curve. Just like it's easier to pedal a bike on a flat road than uphill even though the same distance is covered.

Let's stay with the bike metaphor, lets say you pedal your bike to 15kph, and you just coast on a flat road, eventually you stop, but you tend to keep going, now imagine the same scenario but there is a ramp, you climb the ramp you notice yourself slowing down, once you pass the apex of the ramp you begin to pick up speed again. If the ramp is properly designed, you can almost travel the same distance as if you coast on a flat road all other conditions being equal. That is essentially what you try to do with an aerodynamic surface. However, F1 is not an ideal world and almost every design is a compromise one way or another.

Back to the original point, the complex and draggy surfaces that adorn the upper side of F1 cars are to slow the air relative to the floor, while keeping the flow as attached as possible. In the past the diffuser regulations made attaching the two airflows easier, now teams must resort to using the diffuser, beam wing, and main rear wing, in different ways to achieve the same result.

So technically the air underneath is sped up, but only relative to the rest of the car, remember when an F1 car moves, it's whole chassis is affected by the airstream, not an isolated component.

[xpensive]

xpensive

How does the concave shape of the diffuser roof specifically "help speeding up the air under the car"

Brian

I wouldn't know what specific effect the concave diffuser would have, in all honesty I had no idea they used that shape.

But to understand the diffuser's function in general, just look at the profile of floor, diffuser and road, then consider that
the volumetric flow of air under the car is constant, then the air under the floor has to increase in order to keep up.

Higher air-speed gives higher dynamic pressure, which in turn means lower static pressure than on the top of the car.

[shelly]

Diffuser is not just about smooth transition of fast flow under the flat floor.

Suction peak is on the kink line; after that, a concave shape will give you a more aggressive pressure recovery.
The two dirvers of underbody performance aree the two suction peaks on the underbody leading edge and on the kink line.
One must take into account th 3dimensionality of flow and the presence of strakes which generate vortices at thier edges. The fences start at the kinkline and enhance the low pressure in the channels, abit upstream and a bit downstream of their leading edge

[vonk]

This would break the definition of a diffuser though. You would see massive pressure loss with this abrupt expansion.

I agree about the definition of a diffuser. The term diffuser for devices of this kind has been around for a long time. Has it become a misnomer for what the picture shows? But that’s sacrilege, I know.

Could you explain what pressure loss you are referring to?

Total energy loss of the gas as it flows through the diffuser.

Energy in the form of pressure. Total pressure = static pressure + dynamic pressure. I mean loss in dynamic pressure, (sorry for the confusion) and a rise in static pressure, but overall a loss in total pressure in the form of heat.

I believe you are talking about a massive increase of turbulence. This would be true if it weren’t for the (possible) Kamm Effect.
I wish I could find again the pictures I remember from smoke trace tests of a car with Kamm tail. The streamlines looked much like this cartoon I made up.





Don’t ask me to explain the fluid dynamics of the Kamm Diffuser though. At least not yet.

[vonk]

Diffuser is not just about smooth transition of fast flow under the flat floor.

Suction peak is on the kink line; after that, a concave shape will give you a more aggressive pressure recovery.
The two dirvers of underbody performance aree the two suction peaks on the underbody leading edge and on the kink line.
One must take into account th 3dimensionality of flow and the presence of strakes which generate vortices at thier edges. The fences start at the kinkline and enhance the low pressure in the channels, abit upstream and a bit downstream of their leading edge

Shelly,

Could you be more specific?

What are “suction peaks”? And, why are they where you say?

Please define “aggressive pressure recovery”.

What do you mean by “enhance the low pressure in the channels”?

What’s “a bit”?

How do you know all this?

vonk

[godlameroso]

Not shelly but...

Suction peaks corresponds to the area on the diffuser where the air volume is expanded, it's not exactly where the diffuser opens up but slightly before. Using my bike analogy of a few posts back, suppose you're on your bike about to go down a ramp. The suction peak would be essentially where your potential energy is the highest in relation to the ramp, ie right before your front wheel goes over the edge. There is also a suction peak at the tea tray and leading edge of the floor, this is because the diffuser not only expands the volume of air at the end of the floor, but also pulls air(like a bellow) that's under the nose and into the floor area(the main reason behind the high noses). (Finally there is also a suction peak underneath the first element of the front wing)

With the diffuser it's similar, except it's air instead of someone on a bike.

Not sure about aggressive pressure recovery, but an analogy might be when you take a submerged bucket, and try to pull it out of the water while upside down, since there is no air in the bucket, the static pressure of the atmosphere is higher than the static pressure in space inside the bucket, hence the resistance in pulling the bucket out of the water. However, once you let in air, the pressure equalizes and the bucket pops right out.

The fins on the diffuser help the air stay attached in the diffuser but that's not all they do, if you notice a lot of these fins curve radially outward toward the rear of the diffuser, this expands the diffuser air even more than it would otherwise be able to.

Almost all the body work on an F1 car makes the air underneath faster than the air on top. The faster the difference in laminar flow, the more extreme the up wash.




See how the flame goes up and not straight back?