Underbody Diffuser Theory explanation please

[soumyakd]

I have been digging into the exact step by step process of how an underbody diffuser works, but every theory leaves a flaw behind which i dont understand clearly nor i can solve it out by myself. So, can anyone help me with a clear cut explanation of how the expanding area of diffusers help in gaining downforce ( I do understand that the increasing area fills up the wake behind the car) ?
CAn anyone help me with this ?

[Huntresa]

I have been digging into the exact step by step process of how an underbody diffuser works, but every theory leaves a flaw behind which i dont understand clearly nor i can solve it out by myself. So, can anyone help me with a clear cut explanation of how the expanding area of diffusers help in gaining downforce ( I do understand that the increasing area fills up the wake behind the car) ?
CAn anyone help me with this ?

You mean this ? "A diffuser, in an automotive context, is a shaped section of the car underbody which improves the car's aerodynamic properties by enhancing the transition between the high-velocity airflow underneath the car and the much slower freestream airflow of the ambient atmosphere. It works by providing a space for the underbody airflow to decelerate and expand (in area, density remains constant at the speeds that cars travel) so that it does not cause excessive flow separation and drag, by providing a degree of "wake infill" or more accurately, pressure recovery. The diffuser itself accelerates the flow in front of it, which helps generate downforce."

[hollus]

http://www.f1technical.net/forum/viewto ... t=Diffuser

[Pup]

The diffuser is an elegant way of moving this partial vacuum from behind the car, where it causes drag, to below the car, where it creates downforce.

And an elegant way of stating it, too. Thanks for the link - I'd missed that thread.

[coaster]

I'll take guess that the beginning of the diffuser angles away from the floor at 8 degrees or less, and tries to mate up with lowest rear wing element to pull the air out of the tunnel, creating a vacuum under the floor in front of the diffuser entry, pulling the back of the car down? Maybe? Dunno!

[soumyakd]

The diffuser itself accelerates the flow in front of it, which helps generate downforce."[/quote]

I dint get ur point here abt the diffuser accelerating the flow !
How come it adds to the downforce ? I mean it just expands the air there.

[soumyakd]

I'll take guess that the beginning of the diffuser angles away from the floor at 8 degrees or less, and tries to mate up with lowest rear wing element to pull the air out of the tunnel, creating a vacuum under the floor in front of the diffuser entry, pulling the back of the car down? Maybe? Dunno!

How does mating with the wing bottom create a vaccum ? I understand that the diff does create a vacuum effect which sucks in more air from the underbody. But how ?

[stefan_]

Maybe this will clear some things about how the diffuser works.

[youtube]http://www.youtube.com/watch?v=DVHR8_B7bfg[/youtube]

[Ogami musashi]

There two conservation principle involved there:

Conservation of mass
Conservation of energy


The complete assembly of undertray+diffuser form a tube of section S1 and S2 respectively.

The quantity of air that enters S1 per second must be the same that exits S2 per sec.
Since we're dealing with particle moving we can also say that the speed at which the quantity flows through S1 must be proportional to the speed that flows through S2 i.e S1V1=S2V2.

Since S2 is larger that S1, V1 is greater than V2 so the speed in the undertray is the greater than in the diffuser.

Now why does the diffuser accelerates the flows in the undertray? This is because the air is slowed down in S2...but the density stays the same (at the speed a car travels); the word "expand" is misleading, the air is not stretching is actually just slows down...and thus a "vacuum" is created and since nature doesn't like it it will be filled...by air coming from the undertray...in other words the diffuser will pump air from the undertray to fill the whole S2 section and the S1V1=S2V2 principle will be verified, only that V1 will be even greater than if the diffuser wasn't there and S2 will be slower.

The peak acceleration at the location where the gradient of section is the greatest, since the difference of section will dictate the speed, the quickest and most abrupt change of section will lead the to steepest change in speed and this is where the diffuser starts.


Now, speed are defined, and they're changing, but energy's not. So basically the three (negating the internal energy of the system) forms of energies (derived from static pressure, height and speed) will varying to conserve the total energy of the system.

If speed increase, either potential energy (from height) or static pressure derived energy will need to compensate.
If we take one particle (in fluids dynamics a particle is not one molecule but a bunch of molecules, that is a small volume of air), its potential energy will not change (the particle will stay at the same height) so only the static pressure will decrease.

The static pressure will decrease and thus hopefully the pressure above the car will be higher thus creating a pressure differential and thus downforce.

Hope this helps.

[coaster]

This is why I'm a tradesman and not a designer, it's very hard to visualise what was described here and probably so more with the necessary algebra to fill the description with facts.
If I was born in ancient Rome, I'd strangle Pythagoras and make visual diagrams standard over friggin algebra!

[Kiril Varbanov]

This is why I'm a tradesman and not a designer, it's very hard to visualise what was described here and probably so more with the necessary algebra to fill the description with facts.
If I was born in ancient Rome, I'd strangle Pythagoras and make visual diagrams standard over friggin algebra!

Here's an old article from my blog, explained rather simple, particularly about F1 diffusers.

Some visualization of this with Venturi pressure gradients from Symscape:

[Ogami musashi]

This is why I'm a tradesman and not a designer, it's very hard to visualise what was described here and probably so more with the necessary algebra to fill the description with facts.
If I was born in ancient Rome, I'd strangle Pythagoras and make visual diagrams standard over friggin algebra!

Here's an old article from my blog, explained rather simple, particularly about F1 diffusers.

Some visualization of this with Venturi pressure gradients from Symscape:
http://4.bp.blogspot.com/-yBL4Z78fMsE/U ... effect.png

While you graphics are correct, i'm afraid your blog explanation is not or not correctly worded.

The diffuser area has higher pressure than the undertray area..in addition, "negative pressure" is not a correct term, the static pressure is always positive. It can be lower than a pressure above or below it but it is positive.



I'll try to build on your graphics to explain it in a simplier form:

1/Why the air accelerates it the blue section?

A certain amount of air is flowing through a section of the entrance volume (the most upstream volume) per second.
That very same amount of air must still pass through the smallest section (the blue one) per second.
Since the section is smaller for that amount to flow it has two possibilities:

Compress so that all particle can flow at once, but the air at those speeds (the speed of an F1 car) is not compressible;
Flow faster: if you want 10 particles flowing through a section per second, you can have either 5 particles per half second through a small section or 10 particles per second through a 2 times as big section.

This is what happens there, the air is accelerated.

And there's another law of conservation (the energy conservation ) that says that if speed increases, the pressure will decrease. So we have now in the smallest section less pressure. Above the car the pressure is supposed to be higher so when you do the difference between the two, the pressure above the car being greater it pushes the car down creating downforce.


2/okay so why do we have a diffuser at the back?

The first reason is that when the amount of air flows from the blue section into the diffuser it tends to accumulate there that is you have an increase in the amount of air flowing through a section in the diffuser. Since matter can't be created it necessarily means that this amount of air comes from the undertray. So the amount of air flowing through a section in the diffuser has increased (we say that the mass flow has increased) and the matter must come from the undertray so the mass flow in the undertray has increased too. And since the section in the undertray is lower, to verify to that the mass flow is the same as in the diffuser the air must accelerate even more. It accelerates even more so the pressure drops even more.

In other words, the diffuser is pumping the air from the undertray (the blue section) thus accelerating it.

The diffuser has some other reasons to be here both for flow preservation (making the flows in the undertray stable) and for drag reduction (have a better transition between the air that flows in the undertray and the air that is at the back of the car)


Hope i was a bit clearer now.

[soumyakd]

Now why does the diffuser accelerates the flows in the undertray? This is because the air is slowed down in S2...but the density stays the same (at the speed a car travels); the word "expand" is misleading, the air is not stretching is actually just slows down...and thus a "vacuum" is created and since nature doesn't like it it will be filled...by air coming from the undertray...in other words the diffuser will pump air from the undertray to fill the whole S2 section and the S1V1=S2V2 principle will be verified, only that V1 will be even greater than if the diffuser wasn't there and S2 will be slower.
and thus downforce.
Hope this helps.

agreed. but How is vacuum produced when the air slows down ? Slowing down suggests low velocity..so higher pressure

[Ogami musashi]

The "vacuum" means that the amount of air that enters the diffuser is not sufficient to fill the diffuser. As i said earlier the air is not compressible at those speeds so the air doesn't "expand" like some gaz. So to fill the diffuser and conform to the pressure requirement it must pump air from the undertray.

[soumyakd]

Bang on !!!! Thank you so much guys Finally diffusers made sense to me. The diffuser is an add on for the undertray, which maximises the downforce for the undertray ! right ?