Aerodynamics

[gixxer]

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The lotus glass back looks soomth, like an Audi TT looks smooth...

But you may remember that the rounded shape of the TT caused a rear lift condition due to seperation of the air at the top of the car, leaving the air over the rear of the car turbulent.

Some of the theories of flight still apply to cars--

If the air separates (a stall) at the top of the car it will reduce lift and add wake drag. If the air stays attached all the way to the back of the car it has the potential to add the maximum amount of lift and the lowest amount of drag.

Buuuut---

Nothing is cast in concrete 'tho. Each car is different. Underbody effects count too.

To have somebody do a 2 dimensional CFD analysis and conclude that the tunnel back is preferable over the glassback, without considering that the car is a 3 dimensional form moving through the air is pretty questionable. One only wonders what the air flow field-- as it's moving down the sides and around the greenhouse is up to. To provoke the air into a vortex that will pull it down into the tunnel is a reasonable thing to do, or try, and it's reasonable that Lotus is doing that on the later cars. And it's reasonable to wonder why they ditched the glass---maybe weight and poor visibility?

The guy that did the CFD might be correct--but all the information is not in, and he shouldn't be so willing to claim that the tunnel back form is the Holy Grail of aerodynamics.

What the airflow is doing at the top as it passes over the glassback is affected by how the tailgate is rigged--how much of a gap between the body work, how much of a step exists. Some manufacturers have left a bit of an extra gap up there and initiated a vortex that helps to pull the air back down to the surface of the tailgate/hatchback. I think the early turbo cars had a bit of a bump back there at the start of the tailgate.......

So my opinion is - if you can't do a CFD analysis of the total car, better hike on over to the wind tunnel.

Good luck at that!

[wiseraven]

Gordon Bennett! Is it just me, or is Google losing it? I've just spent 2 hours searching for the guy with the long article on his wide-body V8 conversion that I mentioned. No joy. If I'd have thought it was going to be so hard to find again I'd have bookmarked it. Mea culpa. If anyone reading this knows the one I mean (it's yellow in the US somewhere), I'd appreciate a pointer.

Anyway...

I've hacked your sketch a bit to show what I was trying to explain last night about the end plates. I've also added a curve to show a suggested profile for the inside of the front splitter (i.e. the front diffuser).

One thing not discussed so far (it doesn't really affect downforce) - what are you intending to do about airflow through the bonnet. I'm guessing you'll be fitting a bigger rad, and you'll obviously be ditching the spare. The reason I ask is that you obviously want lots of airflow through the rad, but that needs to exit somewhere. Although I said it doesn't really affect downforce, get it wrong and you could upset the airflow over the car.

Harking back to the start of the thread, I contacted this guy (i.e. "someone who knows what he's talking about" and not some Walter Mitty handbag designer ) for his comments. This is what he had to say:

The Lotus Esprit is a classic sports car. Lotus was right at the forefront of aerodynamics research at the time, but they could only do wind tunnel testing. Only recently it has become possible to run CFD calculations of full car configurations, with some interesting results. First of all, the calculation of the aerodynamic drag is quite difficult and relies on the accuracy of the surface shear forces (viscosity) and the bluff body separation (unsteady). In fact, it is not uncommon to get results wrong by a factor 2. Experts in the field recognise the drawback, and calculations are usually refined by various methods, including grid refinement, unsteady runs, time averaging, etc.

I estimate a CD = 0.30 for this car, based on frontal area; in the most optimistic case it would be 0.29, in the worst case scenario it would be 0.34. Note that the use of aerodynamic surfaces such a rear spoiler tend to increase the drag.

No earth shattering insights, I think the main point is that CFD results need to be treated with caution.

He did say that he's not interested in joining the discussion. A pity, because he's worked on race aerodynamics. I've no idea how much he charges for consultancy, but I bet he's not cheap.

I mentioned the discussion on glass vs. notch to him to see if he'll bite, but I don't know if he'll feel inclined to comment.

Phil

[WayneB]

Gordon Bennett! Is it just me, or is Google losing it? I've just spent 2 hours searching for the guy with the long article on his wide-body V8 conversion that I mentioned. No joy. If I'd have thought it was going to be so hard to find again I'd have bookmarked it. Mea culpa. If anyone reading this knows the one I mean (it's yellow in the US somewhere), I'd appreciate a pointer.

Extreme Esprit?

[wiseraven]

Extreme Esprit?

Thanks Wayne, that's the guy. He must have been going too fast for google to spot him

The wide-body conversion is here

Phil

[M Blur]

Tks Phil/Wayne/Travis etc etc..

I'll send on that revised drawing to my carbon man and see what he thinks.. Mucho appreciated.

Re the front end the red line indicates how we are going for a smooth frontal area with an imsa lower intake (very low) that will feed a small remote oil cooler at a 45 degree angle. The rads will be in side pods to the sides of the engine and as close as possible to the engine/pump. The front cover is already vented and lightweight so any airflow will be deflected up and over the front screen. The intake area is also cowled and deflected. Currently in the front will reside fuel cell, extonguisher (may be relocated to pax compartment, brake cylinder and oil cooler.

imho you guys haven't seen an extreme esprit yet but give me 2 months or so.. however for a road car there's some cool work going on there and does give me some ideas for phase2 for 08.

BR,

Jon

Edited February 2, 2007 by M Blur

[Vulcan Grey]

Thanks Wayne, that's the guy. He must have been going too fast for google to spot him

The wide-body conversion is here

Phil

Phil,

If you look really closely you might see a picture of me helping cut the ExtremeEsprit apart to make the widebody modification....

[M Blur]

I think the plan is to use Group4 GTA arches for 2008 if we decide to run more track...vented wider front arches would be particuarly useful..

[Paul93Lotus]

Aerodynamics are serious business. Look what can happen if you don't know what you're doing.....

Teigan's Undertray Prototype

Edited February 2, 2007 by Paul93Lotus

[M Blur]

Aerodynamics are serious business. Look what can happen if you don't know what you're doing.....

Teigan's Undertray Prototype

Not unlike the double Le Mans CLK_GTR flip for both Peter Drumbeck and Mark Webber.. along the Mulsanne.. around 98?

Thankfully MVS works a lot on radicals and other racers so know tricks like the circular holes in the diffusers, which apparently create additional effect and release any pressure build up.. (I'm sure that's a bad technical explanation for what they do) very much a work in progress - happy to over-shoot the drag initially to get a good amount of downforce and then tweak it back. He also had use of a S4 to see the overall shapes on the floorplan etc.

Hopefully we'll get more insight on the aero side re the above design and see where we can make further improvements..

Edited February 2, 2007 by M Blur

[wiseraven]

Tks Phil/Wayne/Travis etc etc..

I'll send on that revised drawing to my carbon man and see what he thinks.. Mucho appreciated.

Re the front end the red line indicates how we are going for a smooth frontal area with an imsa lower intake (very low) that will feed a small remote oil cooler at a 45 degree angle. The rads will be in side pods to the sides of the engine and as close as possible to the engine/pump. The front cover is already vented and lightweight so any airflow will be deflected up and over the front screen. The intake area is also cowled and deflected. Currently in the front will reside fuel cell, extonguisher (may be relocated to pax compartment, brake cylinder and oil cooler.

imho you guys haven't seen an extreme esprit yet but give me 2 months or so.. however for a road car there's some cool work going on there and does give me some ideas for phase2 for 08.

BR,

Jon

Now look what you've made me do - drool all over my keyboard. I'm getting the picture. Save yourself time & just list the bits you're not changing. I can't wait to see you at Donny.

All of the car flips (Merc/Porsche/BMW) were identical - a fast circuit, so relatively low drag/downforce setting, a fast uphill section with the foot to the floor and still accelerating (so the nose is already slightly high, then topping a crest so the front goes even lighter. When you're running with a flat, wide undertray and minimal ground clearance, that's all it can take to convert downforce into lift. There's a pretty good analysis in laymans terms here

@Travis - really nice work. I hadn't realised that was you.

Phil

[Vulcan Grey]

@Travis - really nice work. I hadn't realised that was you.

Phil

Johan did all the work. I just showed up at his house one day and he handed me a dremel with a cutoff wheel, and said "here, now cut that 1/4 of the car off"!

Edited February 3, 2007 by Vulcan Grey

[WayneB]

tks guys,

bear in mind I need to power bulge the oem hatch up to clear the engine, filter etc.. intention was then to taper this back to the original line to the tailgate.. we'll try and make as smooth a design as poss tho will be compromised by the sheer engine.. we'll raise the spoiler so it gets smoother air flow from above the roof panel.. how high should we go.>?? We're taking a mould from the OEM hatch but then splicing in a GTA hatch for cooling vents.

The plan for next year will be composite wings to reduce wheel arch pressure and allow wider track - I think MVS was going to start with Renault GTA wings/arches.. go for a Group4 look.

Jon, this might give you some ideasBMW M1 Group 5

[M Blur]

not far off Wayne - tks - the front wings we'll address for next year.. frotn end wil be similar but smoother with less intake bar for front angle oil cooler, brakes etc. I expect more louvres in the engine cover and a ram intake. The spoiler will be a more modern GT design. But overall it's same ball park and the sort of cars I'd like to race ultimately..