Diffuser design factors

[mach11]

what all factors go in to designing the diffuser of a F1 car???

[Confused_Andy]

How much downforce it can produce surely....

[BreezyRacer]

You take the basic goal of creating a low pressure region under the chassis. That is what provides the downforce .. the low pressure region. Literally everything else on car affects how the optimal diffuser is designed, including the overall wake, interactions with wings, splitters, and tires, as well as pitch sensitivity to these things, and the rules you have to comply with, such as the stepped bottom rule and you've got the basic ingredients for your design consideration.

The whole issue revolves around maximizing that low pressure region under the car. The diffuser itself is not where the downforce is created. That occurs ahead of the diffuser. Really good designs can put the low pressure area well ahead of the diffuser.

Probably the biggest issue these days is how the diffuser exit interacts with the rear wing elements. The double decker layout is quite useful for that. Also quite common these days is the use of vortex generators to redirect as much airflow around the sides of the cars to help isolate airflow away from the floor area.

[tahadar]

Also quite common these days is the use of vortex generators to redirect as much airflow around the sides of the cars to help isolate airflow away from the floor area.

I think you'll find that it is more complex then that. Vortices from the front wing strakes, the splitter, bargeboards, etc will be ingested under the floor and through the diffuser. and with votices having low pressure cores you will find that they actually draw more air under the car rather than sealing off the floor completely.

[BreezyRacer]

Also quite common these days is the use of vortex generators to redirect as much airflow around the sides of the cars to help isolate airflow away from the floor area.

I think you'll find that it is more complex then that. Vortices from the front wing strakes, the splitter, bargeboards, etc will be ingested under the floor and through the diffuser. and with votices having low pressure cores you will find that they actually draw more air under the car rather than sealing off the floor completely.

DUH! Yes, it's quite complex, I agree. But based on the simple, wide ranging scope of the question I'm not ready to submit my 30 page dissertation on the subject just yet.

[n smikle]

Well the diffuser is basically a smooth pressure transition.
All you have to do is make an expanding Cross sectional area that follows a certain slope. (or even curvature). From where you want a desired lower pressure to another fixed higher pressure (e.g the atmosphere). The diffuser makes the fluid loose less kinetic energy so that the low pressure parts stay low for a longer length along the diffuser before they reach the higher exit pressure.

If you look in a Fluid mechanics book.. you can see some experimental results of which shape produces the best pressure transition.

In F1 I think there is a maximum length, height and width of the diffuser.. so the creativity is in the different shapes(tricks) that can fit those constraints. The double diffuser for example.

The diffuser is a simple thing really. It is only as good as the pressure at the exit and entrance of it. You need a good front air splitter too.

[ESPImperium]

Once the season is finished im gonna try and get a race by race evolution of the diffusers for each team, just so we can get a idea on what the ideal or optimim design is/was.

[xpensive]

The purpose of the diffuser is to help speed up the air under the car to a level higher than above the same.

Bernoulli takes care of the rest, when static- plus dynamic pressure stays constant:
ps1 - ps2 = pd2 - pd1, where pd = Rho * v^2/2.

[kilcoo316]

I think you'll find that it is more complex then that. Vortices from the front wing strakes, the splitter, bargeboards, etc will be ingested under the floor and through the diffuser. and with votices having low pressure cores you will find that they actually draw more air under the car rather than sealing off the floor completely.

You want to seal off the sides of the floor.

You want to "open up" the front of the floor.



Everything upstream of the rear wheels is geared to those two goals.

[tahadar]

You want to seal off the sides of the floor.

like I said, this is not entirely true. Mclaren featured large cut-outs ahead of their rear wheels in pre-season testing to promote more airflow into the diffuser by entering under the sides of the floor. Some teams also feature floor 'curls' for the same reason.

[kilcoo316]

like I said, this is not entirely true. Mclaren featured large cut-outs ahead of their rear wheels in pre-season testing to promote more airflow into the diffuser by entering under the sides of the floor. Some teams also feature floor 'curls' for the same reason.

They screwed up their diffuser.

Where are the holes now?



The floor curls you speak of (the small kinda "gurney flaps" upstream the rear tyre) are more about the stagnation point infront of the rear wheel than the floor.


The vortex sheets generated by most of the appendages on an F1 car cannot be compared to that from a LERX on say, an F-16 or MiG-29. The vortices here are much weaker and prone to vortex burst much earlier.


Again - you want to seal off the sides of the floor. They do strive to try and achieve that.

[BreezyRacer]

As long as we're getting more detailed here about diffusers, also note that there have been some moves to use the underside to create VGs too, in the outer splitter area. Williams has a nice setup, modified again at Singapore and McLaren have a very detailed VG (perhaps everyone does .. we really don't get to see these details often) in their floor. These VGs in these locations are designed to "bleed off" turbulent air, thus increasing undertray velocity and lowering pressure as far forward as possible. In a similar vein Red Bull have been doing this type of thing with a curved up floor section since China I believe.

It's difficult to separate out wings from diffusers from splitters from vanes these days. They are all interdependent. In general terms for instance the height of the rear bodywork has an effect on diffuser strength, as does of course the lower and even upper rear wing elements. Take away the rear wing and I imagine that the undertray based DF drops substantially. A diffuser needs a wake (low pressure area behind the vehicle) to operate effectively.

[tahadar]

Again - you want to seal off the sides of the floor. They do strive to try and achieve that.

My whole point is that the "they do strive to achieve that" comment may or may not be applicable depending on which aerodynamicist you speak to and that it is not a steadfast rule that will result in failure if you do not keep it your primary objective. It is simply a matter of varying philosophies. The (experienced) people I have discussed this with view their design ideas as 'supporting' the floor seal theory but not explicitly what they were trying to achieve, just a 'by-product' in a way.

If floor sealing was the be-all and end-all of floor aerodynamics then I believe we wouldn't have seen McLaren even bother with the floor cutouts i mentioned previously.

[kreuzberg]

I'm not sure if anyone has asked this before but does anyone know what the benefit of an asymmetrical diffuser is over a symmetrical one?

[Michiba]

In terms of 'sealing' the floor, doesn't the front wheel rims help with this?