Ground Effect Tunnel Designs

[OO7]

Whenever I contemplate venturi tunnels, Group-C and IMSA GTP cars immediately spring to mind. With these cars, the leading edge/throat of the tunnels was placed at the same height as the reference plane/bottom of the car. Up until 1993, my understanding is that there were no rules mandating any form of flat bottom or reference plane in the IMSA GTP series, unlike Group-C. Despite this the throat of the tunnels still formed the lowest point of the car.




When compared to the implementation of tunnels in single seaters, we can observe the following with raised entrances, that illustrate how the venturi throat in this instance is not the lowest part of the bodywork:




My understanding has always been that where the entrance to the tunnel has been raised, this has been achieved to meet a regulated minimum height, in order to curtail excessive downforce production.
McLaren's MP4-X concept car has forced me to re-evaluate and question my original stance. One assumes the McLaren concept is designed to a near unlimited formula that wouldn't regulate tunnel entrance height. If this is the case, why style a design to mimic the raised throat philosophy?

[bhall II]

Generally speaking, increased ride height reduces sensitivity to variation. For instance, an aerodynamic element with a nominal ride height of 5cm loses 20% of its ride height if it's lowered 1cm. For an element with a nominal ride height of 4cm, however, the same 1cm drop amounts to a 25% reduction. Since underbody downforce is, for the most part, proportional to ride height (h/c), a higher ride height is more consistent at the expense of lower peak downforce.

Also, a venturi with a throat ride height that's not as low as the rest of the floor won't stall if the car bottoms out.

Another factor is that, regardless of anything else, diffuser angle is largely defined by the airspeeds it will most often experience. That means a lower throat doesn't necessarily pave the way to more aggressive diffusers.

EDIT: The McLaren design actually seems to have two diffusers with the main one located at the front of the floor.

[Facts Only]

I think part of your confusion is caused by the fact that you are considering that Mclaren to be a "designed" car. Its just a pretty rendering to distract the fans from the abysmal season and create a talking point. Its no more of a design than the Batmobile, or a picture done by a 5 year old of a car with jet engine on the roof and laser guns.

Its never been evolved in CFD, its never been in a wind tunnel and its never been tested and it never will be so they could do anything. Its just a pretty picture.

[OO7]

Generally speaking, increased ride height reduces sensitivity to variation. For instance, an aerodynamic element with a nominal ride height of 5cm loses 20% of its ride height if it's lowered 1cm. For an element with a nominal ride height of 4cm, however, the same 1cm drop amounts to a 25% reduction. Since underbody downforce is, for the most part, proportional to ride height (h/c), a higher ride height is more consistent at the expense of lower peak downforce.

Also, a venturi with a throat ride height that's not as low as the rest of the floor won't stall if the car bottoms out.

Yes, I considered that ride height sensitivity could play a part, but noticed that no pre-1993 IMSA GTPs (prior to the mandated flat floor section under the driver) were designed with raised tunnel throats. There is a video interview with the designer of the Porsche 956, where he stated that unlike single seaters, it was necessary for the throat of the tunnel in this specification of car, to be fed from airflow along the sides of the car. That simply feeding from air entering under the nose had pretty much no impact. Perhaps this may be one of the reasons that a lower throat was necessary?

Also IMSA GTP and Group-C cars seem to run greater ride heights when compared to single seaters, so perhaps this was the trade-off?

Another factor is that, regardless of anything else, diffuser angle is largely defined by the airspeeds it will most often experience. That means a lower throat doesn't necessarily pave the way to more aggressive diffusers.

It's interesting that you mention diffuser angle because of the variations seen in the early 90's IMSA series. Cars such as the Toyota MKII had relatively shallow diffusers, whereas the Nissan GTP ZX-T's and Mazda RX-792P's were very aggressive. One trade off would be packaging around the suspension components and drive shafts and I would have thought going as steep as possible would yield the best results, but clearly this isn't the case, so what are the other trade offs?

EDIT: The McLaren design actually seems to have two diffusers with the main one located at the front of the floor.

I think I see what you mean. The fifth vane around the tunnel entrance, closest to the centreline of the car? The air flowing under the front floor into this region has to make a complete, abrupt, step as it transitions. If I'm reading it correctly, the channels over the top of the floor will provide a flow of air necessary to keep the air transitioning into the stepped region attached?
It also seems as if the floor in the middle if the car is slightly raised (lighter colour), but this wouldn't make much sense.

[toraabe]

As long F1 is not pemitting this tunnels, the problems following the other cars will continue.
Solution in F1, ban the current front wing, and instead introduce full groundeffect.

[OO7]

I think part of your confusion is caused by the fact that you are considering that Mclaren to be a "designed" car. Its just a pretty rendering to distract the fans from the abysmal season and create a talking point. Its no more of a design than the Batmobile, or a picture done by a 5 year old of a car with jet engine on the roof and laser guns.

Its never been evolved in CFD, its never been in a wind tunnel and its never been tested and it never will be so they could do anything. Its just a pretty picture.

I was of the impression that while it hasn't been CFD tested, it was created by engineers based on fundamental aerodynamic principles (excluding the plasma field ).
Going back to bhall's explanation, if this car is capable of tremendous speeds and the diffuser design point focused around the most common airspeeds, a raised tunnel throat may be a necessary solution.

[NOT A TA]

I've never heard of a fundamental principal that would favor channeling airflow directly at the contact patch of the front tires.

Looks to me like someone said "Hey new guy, come up with something cool and futuristic looking to keep the promotion and marketing guys happy."

[variante]

I was of the impression that while it hasn't been CFD tested, it was created by engineers based on fundamental aerodynamic principles (excluding the plasma field ).

Trust me, no aero engineer was involved in the creation of that thing, nor any proper designer...

Anyway, placing the floor as low as possible, even at a constant ride height, doesn't necessarily give you more downforce. In fact, besides sensitivity to ride height variations, another reason for the raised floor could be the presence of a thick boudary layer. Thus, that floor height could be a tradeoff between Venturi effect and boudary layer thickness (and ride variation sensitivity as well).

In particular, if the ride height is equal to or smaller than boudary layer thickness, downforce will decrease and the floor+diffuser system would be more likely to stall.

A commonly adopted solution to decrease boudary layer thickness, energizing the airflow (which gives you better chances to exploit Venturi effect) is generating vortices at the leading edge of the floor. Those are created thanks to bargeboards or similar devices (one of the few good things about that McLaren concept is that it shows such vortex generating devices under the leading edge of the floor).

[godlameroso]

This is true, and unfortunately the rear tire is there to destroy any and all vortex that's trying to seal the floor.

[bhall II]

Yes, I considered that ride height sensitivity could play a part, but noticed that no pre-1993 IMSA GTPs (prior to the mandated flat floor section under the driver) were designed with raised tunnel throats. There is a video interview with the designer of the Porsche 956, where he stated that unlike single seaters, it was necessary for the throat of the tunnel in this specification of car, to be fed from airflow along the sides of the car. That simply feeding from air entering under the nose had pretty much no impact. Perhaps this may be one of the reasons that a lower throat was necessary?

It's difficult to speak to a designer's intentions with regard to underbody flow in the early '90s, because the subject wasn't particularly well understood at the time - and it remains the least understood piece of the puzzle today.

My best guess is that the IMSA guys were perhaps exploring force enhancement from edge vortices to varying degrees - overview below - while the F1 guys were busy with active suspensions. But, that's just a guess.


From Ground Effect Aerodynamics of Race Cars



Incidentally, that highlights the misnomer of exhaust-blown diffusers, especially designs from 2010 to 2013. The concept was never intended to "seal" the floor; rather, exhaust gasses were used to energize edge vortices, which allowed for more aggressive diffusers and rake angles.

I think the "specialized press" was way off on that one.





As for McLaren's design study, it just looks like a funky, highly stylized double-diffuser to me. Air flow would be accelerated by the floor's curvature and turning vanes around the leading edge before being extracted by the secondary diffuser at the rear of the car. If effective - big if - it would just move the floor's aerodynamic center to an area that's closer to the middle of the car.

[Just_a_fan]

This is an interesting piece:
https://www.linkedin.com/pulse/how-do-m ... plash=true

[wesley123]

I think a video would explain it in the best way;

[riff_raff]

The IMSA GTP regs in the late 80's were wide open. They allowed GTP cars to compete with Group C cars. And GTP cars could run at various combinations of engine type and weight. Some of the undertrays used on early GTP cars had massive tunnels. I believe the IMSA GTP regs in the late 80s/early 90s limited the tunnel height to 18 inches. The group C cars later on were required to run much smaller diffusers.

[Just_a_fan]

Many of those huge diffuser cars suffered from huge drag too. Look at the rear of that car - there is little attempt at keeping the airflow happy once it's gone through the initial part of the diffuser.

A good book on these cars is Ian Bamsey's
The Anatomy & Development of the Sports Prototype Racing Car. A bit old now but covers the development up to the early 90s.

[OO7]

I've never heard of a fundamental principal that would favor channeling airflow directly at the contact patch of the front tires.

Looks to me like someone said "Hey new guy, come up with something cool and futuristic looking to keep the promotion and marketing guys happy."

I must admit I have no idea what is going on around the front end and was really focusing on the tunnel. Regarding channelling airflow directly at the front wheels, I may have this wrong but I vaguely remember reading somewhere that vortices aimed at the front wheels were designed to reduce drag. Perhaps that is part of the McLaren concept?