Diffuser design factors

[BreezyRacer]

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?

If you're referring to a vehicle that turns left and right, there is presumably isn't any advantage except to compensate for non optimal packaging or some other portion of asymmetrical shapes.

If a vehicle were to always turn left a diffuser could be optimal and asymmetrical.

As for tires isolating the sides of an undertray, the big problem is the wake and turbulence that comes off the tires. Turbulence is the enemy of aerodynamics. Open wheels, in general, increase the challenges of aero engineering.

[fastback33]

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.

My understanding is that, you don't want the distance from the edge of the diffusor to the ground to be very large, BUT, you do want it open so that the air flowing in from the sides can help to speed up the air moving longitudinally.

[tahadar]

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

The main purpose of the wheel covers is to control the wheel wake and generally divert it away from the sidepods. They do not have a direct impact on the floor from what I understand.

[Michiba]

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

The main purpose of the wheel covers is to control the wheel wake and generally divert it away from the sidepods. They do not have a direct impact on the floor from what I understand.

watch this as posted by di44ety

http://www.youtube.com/watch?v=oImPiK4l ... r_embedded

[tahadar]

seeing as the bargeboards and podvanes and front wing endplates all try and divert the wheel wake away from the sidepods, I can't say i agree with what was being shown in that video at all. The flow paths shown in the video appear completely implausible, given the effect of the wheel cover 'shoulder' which has a significant impact on moving the wheel wake outwards. The 'first impulse' of the brake cooling exit air will be to follow the path being promoted by the tyre cover shoulders and given the path of the flow in front of the sidepods, i cant see how it would be possible for the flow to turn inboard and go under the floor.

Although I do have to say that the animations in that vid were pretty impressive!

[Michiba]

seeing as the bargeboards and podvanes and front wing endplates all try and divert the wheel wake away from the sidepods, I can't say i agree with what was being shown in that video at all. The flow paths shown in the video appear completely implausible, given the effect of the wheel cover 'shoulder' which has a significant impact on moving the wheel wake outwards. The 'first impulse' of the brake cooling exit air will be to follow the path being promoted by the tyre cover shoulders and given the path of the flow in front of the sidepods, i cant see how it would be possible for the flow to turn inboard and go under the floor.

Although I do have to say that the animations in that vid were pretty impressive!

Maybe the effect is more prominent during high speed turns (where the wheels are slightly turned).

Now I'm not expert on aero, but I thought the diffuser worked to 'suck' air out from under the car by reducing pressure their, hence air can be sucked in from the sides too. Anyone else care to comment? That's why side skirts were so effective because they prevented air from the sides going under the floor thus maintaining the low pressure under the car. Or is this all just nonsense?

[BreezyRacer]

A diffuser is not a vacuum cleaner .. it does not suck air. The low pressure wake area behind the car does that by itself. Any car has a turbulent low pressure wake behind it. And there's a lot of air competing to fill in the low pressure wake area, from all parts of the car .. sides, top, and bottom. We want to fill that wake with airflow from the bottom, thus the diffuser shape.

For a diffuser to work it has to minimize turbulence while that event occurs. It's the turbulent airflow that you are trying to get rid of with these appendages and shapes.

As for the spillage from the sides .. You are always going to get some spillage from the sides but it's probably not quite like you envision it is. Imagine the airflow entering straight under the front of the car .. now imagine that same airflow actually bending outwards between the tires because the low pressure of the air under the car is trying to balance pressure with the normal airflow along the sides of the car. So that gives it a curve outwards, kind of like a bulge. When the airflow gets closer to the diffuser it starts curving back inwards towards the diffuser because it's being pulled into the wake at the back of the car.

Ironically enough this curving outwards towards the sides of the car and then back inwards again towards the diffuser actually helps the downforce because the airflow is traveling a greater distance than if it had just flowed straight under the car and into the diffuser. That means the airflow is traveling at a higher speed, and it's high speed airflow that produces the low pressure and consequently the down force. If I get time I'll dig up a pic or two of an under tray with flows and you'll see what I mean. It's hard to just explain it I guess ..

[kilcoo316]

watch this as posted by di44ety
http://www.youtube.com/watch?v=oImPiK4l ... r_embedded

I don't see a reason for air exhausting at > 200 degC from the brake ducting to turn, then continue to flow downward underneath the floor of the car.


Nice graphics, but I don't think there is any CFD or experiments done to produce those 'pathlines'.



Good point by Breezy with regards using VGs at or around the leading edge of the floor to try and move the underfloor aero centre forward. Definitely something the teams would be playing with.

[xpensive]

Imagine that all of a sudden the sliding skirts of 78-80 were allowed again, with the same flat-bottom rules of today.
I wonder how that would affect downforce and the design-approach on the diffuser?

[Michiba]

watch this as posted by di44ety
http://www.youtube.com/watch?v=oImPiK4l ... r_embedded

I don't see a reason for air exhausting at > 200 degC from the brake ducting to turn, then continue to flow downward underneath the floor of the car.


Nice graphics, but I don't think there is any CFD or experiments done to produce those 'pathlines'.



Good point by Breezy with regards using VGs at or around the leading edge of the floor to try and move the underfloor aero centre forward. Definitely something the teams would be playing with.

yeah, I don't see how that could happen either, but I'm not aero expert. I did read earlier on, when the rim shields were first implemented, that one of the functions was to 'seal' the air under the car. Not sure what that meant, but surely there is a reason for directing the air downwards. Anyone care to explain why?

[Conceptual]

Wouldn't the low pressure area under the floor be pulling in the air released from the brake ducts? When I watched the video, my forst thought was that it was just getting sucked under by the low pressure. Exactly what the sliding skirts were made to prevent!

[Michiba]

Wouldn't the low pressure area under the floor be pulling in the air released from the brake ducts? When I watched the video, my forst thought was that it was just getting sucked under by the low pressure. Exactly what the sliding skirts were made to prevent!

yes, that's exactly what I thought, but breezyracer had other ideas. Would anyone have an CFD images to show how they could possibly work?

The fact that those rims are only on the front wheel and vastly different to the rear wheel rims makes me think they have more of a role than just diverting air away.

[Mystery Steve]

The fact that those rims are only on the front wheel and vastly different to the rear wheel rims makes me think they have more of a role than just diverting air away.

The front brakes generally create more heat than the rear due to dissipating more energy, so cooling requirements wouldn't necessarily be the same.

[xpensive]

Wouldn't the low pressure area under the floor be pulling in the air released from the brake ducts? When I watched the video, my forst thought was that it was just getting sucked under by the low pressure. Exactly what the sliding skirts were made to prevent!

yes, that's exactly what I thought, but breezyracer had other ideas. Would anyone have an CFD images to show how they could possibly work?

Agreed, it is indeed difficult to comprehend how a low-pressure under the car could send the airflow outwards? There must be a serious misunderstanding here, which is why I brought op the anology with the sliding skirts. As for the function of the diffuser, it is obviously designed to help speeding up the air under the car, no semantics will ever change that.

[Mystery Steve]

As for the function of the diffuser, it is obviously designed to help speeding up the air under the car, no semantics will ever change that.

My understanding is that at the front of the body, the air is accelerated under the body as cross-sectional area decreases, creating an area of relatively lower pressure and creates downforce due to the pressure differential. The rear diffuser then allows the air to slow down gradually as it exits the body (like a subsonic nozzle) so that the pressure differential between the air exiting the diffuser and the air aft of the body is minimized, which would reduce drag.

Am I incorrect or are we both saying the same thing, but saying it different ways?

And a lower pressure under the car would definitely not cause an outward flow. The second law of thermo pretty much takes care of that.