Aerodynamics: Frontal drag

[SuperArt]

Recently I've become very interested in the study of automotive aerodynamics.

After watching a video on a young chap and his self-built VX220 with home-cook aero creating real downforce while lapping the 'ring without a wing got me thinking about applications for the MR2.

A flat floor is not too difficult, but how does one manage airflow in the front?
Once air enters the bumper where does it go?
Where is it optimal to vent this air pressure?
If directing and venting air up through bonnet mods, how does one measure this effect on downforce/drag and how does one go about balancing the front to the back.

The most obvious answer to me is to spend time in a wind tunnel. Though surely it's possible to do something meaningful at home without one.

[Carolyn]

Ideally one would vent out of the side of the car, rather than the bonnet, as extra air over a curved top generates lift.  On a racecar we use gurney flaps at the rear of curved roof sections to break up the flow and minimise lift. They don't have to be big an inch high, in the right spot, does the trick.  Not easy on a convertible, and would you want to ruin a hardtop gluing flaps to it?

Lotus went for bonnet vents on the Elise, on the theory that out is better than in!

[Petrus]

Have you gone through Belle´s thread?

Summed up simply:
Stock all air through the radiator leaves únder the car creating lift.
The rubber chin lip ever so slightly reduces the air flow under the front bumper skirt, thus reducing lift.
A vent in the hood will redirect some of the flow from under the car to over the car = double effect on reducing lift: Even only 5% redirect = 10% less lift.

At the rear there are several options too.
Balancing front to rear is trial and error* with the caveat that it will shift with speed and hood up down, hard top makes a mess of it   
I´d say reduce sóme lift at the front, reduce sóme drag at the rear, add sóme negative lift there and enjoy!

* been at it for some four years and ok with the result now.

[Petrus]

Ideally one would vent out of the side of the car, rather than the bonnet, as extra air over a curved top generates lift. 

I drive just about exclusively top, even windows down, thwarting any air foil effect of the car shape.

[Ardent]

@SuperArt
I certainly don't know the answer,  but have similar curiosity myself. Though more rear than front.
To the point as someone that has a dislike to nappies. I ended up buying one.

We all have our own ideas on what functions the nappy provides. But having one off the car and being able to study it, I am leaning more to it, providing an aero function.
Comes back to your opening post. Flat flooring.
When you look at the moulding, (vanes) it clearly shows evidence of guiding the air in a soft diffuser like manner.
Guiding the flow of air out behind the bumper rather than the bumper acting like huge air scoop.
As well as providing some rigidity to the lower bumper by linking to the sub frame.

I would like that function, but in a more attractive package.

[Petrus]

Guiding the flow of air out behind the bumper rather than the bumper acting like huge air scoop.
As well as providing some rigidity to the lower bumper by linking to the sub frame.

For said reason I cut the rear skirt higher. No more air brake ánd free flow through the boiler room.  Last thing was fitting the Swiss crash bar, allowing for even more flow than when you saw it.

Imo one either has the nappy or goes the whole hog.

Your TTE has an xtra thingie; you have the air to air cooler in the front nappy. Adding some more exit to the rear will increase flow through the cooler. In your case with the sports cat I´d fit the nappy and open the rear. P.e. dremel the inserts and cut matching holes in the crash bar.

[SuperArt]

Ideally one would vent out of the side of the car, rather than the bonnet, as extra air over a curved top generates lift.  On a racecar we use gurney flaps at the rear of curved roof sections to break up the flow and minimise lift. They don't have to be big an inch high, in the right spot, does the trick.  Not easy on a convertible, and would you want to ruin a hardtop gluing flaps to it?

Lotus went for bonnet vents on the Elise, on the theory that out is better than in!

The lotus vent is the first image called to mind when I read your opening sentence.
'Surely lotus wouldn't have designed a bonnet that would generate lift!' I thought to myself.
Googling the terms 'WTCC cars' and 'SuperGT cars' to take a look at examples show no rhyme nor reason whether engine location determines the presence of bonnet vents. Some have them, some not and not all for the same drive layouts either! The mystery deepens, perhaps old hat to some but for me a new baffling world.
Side venting sounds like an idea, would that not then create turbulence down the side of the car and thus drag?

Have you gone through Belle´s thread?

Summed up simply:
Stock all air through the radiator leaves únder the car creating lift.
The rubber chin lip ever so slightly reduces the air flow under the front bumper skirt, thus reducing lift.
A vent in the hood will redirect some of the flow from under the car to over the car = double effect on reducing lift: Even only 5% redirect = 10% less lift.

At the rear there are several options too.
Balancing front to rear is trial and error* with the caveat that it will shift with speed and hood up down, hard top makes a mess of it   
I´d say reduce sóme lift at the front, reduce sóme drag at the rear, add sóme negative lift there and enjoy!

* been at it for some four years and ok with the result now.

With regret, I have not had the luxury of pouring over many of the interesting reader's rides thread save for peeking at some latest comments. So while I have seen some of your pages I've not read the proverbial book cover to cover.
In stock form, I think the frunk bin must play a part in directing air down no? Its absence I presume would just play havoc on the airflow but it's also not ideal to have air pushed beneath the car like that.
If a stock car was fitted with a full flat floor, where then would the air go (other than wherever it can)?

I guess I'm not too concerned about the rear as flaps/spoilers/wings are known solutions. The front however is a black box for me.

I'd like a flat floor, but it seems more complicated than just bolting a few sheets of ally to the chassis.

@SuperArt
I certainly don't know the answer,  but have similar curiosity myself. Though more rear than front.
To the point as someone that has a dislike to nappies. I ended up buying one.

We all have our own ideas on what functions the nappy provides. But having one off the car and being able to study it, I am leaning more to it, providing an aero function.
Comes back to your opening post. Flat flooring.
When you look at the moulding, (vanes) it clearly shows evidence of guiding the air in a soft diffuser like manner.
Guiding the flow of air out behind the bumper rather than the bumper acting like huge air scoop.
As well as providing some rigidity to the lower bumper by linking to the sub frame.

I would like that function, but in a more attractive package.

While I like to think the nappy serves some performance function I would guess the design is more for economy.
In an ideal world I'd buy an off-the-shelf flat floor for our cars and get it installed today. I like the idea of protecting my intercooler a little (I occasionally scrape), minimise the amount of road debris shot blasting the undercarriage and MAYBE benefit a little from 0.1 MPG more economy.

But no such off-the-shelf solution exists so time to hit the books and learn how to DIY. I'd like not to alter the body of the car, but as I'm learning from @Carolyn and @Petrus the air needs to go somewhere, if not up, then sides or below; and if I'm blocking off the below then there needs to be holes installed in the sides or bonnet!

[jvanzyl]

@lgriffiths is the one you want to speak to since it's his job for F1 and all...

[Petrus]

While I like to think the nappy serves some performance function I would guess the design is more for economy.

Toyota just about sealed the boiler room to meet noise regulations. The underside is particularly important because the sound bounces off the hard road surface.

As to the front you can see in photos with the bucket removed that Toyota left a gap between the front lower plastic bottom panel and the rocker panel for the airflow to go underneath the car.
If you look at airflow like a thin liquid you will see that the presence of the bucket makes very little difference; it is about pressure differences.
Any opening in the bonnet (and the vanity panel out) will see air flowing out the top and thus less air flow under the car.

I had a neat indirect proof of the pudding: With the bonnet not tight on the rubbers, it lifted when I did not have the vents in. It did not with the vents.

A bit of rake will do wonders for less lift too btw. See F1 aero set up too   

All in all I found the little cabrio véry responsive to aero changes. The reasons are that it is relatively light so forces will have relatively strong influance and because of the shape the standard aero is not very good.
Toyota themselves added the chin lip and the little air dams in front of the rear wheels p.e. Two minor things that they still thought worthwhile.
The effect of the flip up ´airdam´ behind the seats and that of ´mongos´ are perfect illustrations too of how effective small aero thingamies are.
My car being ligher makes it even more fun 


 

[AdamR28]

Some ideas in here (long thread): https://www.mr2roc.org/index.php?topic=69843.0

The rough data I have reckons the car makes 150-200kg downforce.

But yep. Flat floor is easy. Block up as much of the radiator opening as you can, and make the air exit up and through the bonnet rather than stalling into the frunk.

[Alex Knight]

I've often wondered if you could evacuate the radiator spent air through a duct in the flat floor, rather than venting the bonnet?

[AdamR28]

Don't see why you couldn't! Dont know enough about aero to speculate what that might do to handling balance, though.

[Nvy]

I've often wondered if you could evacuate the radiator spent air through a duct in the flat floor, rather than venting the bonnet?

Its what v mount setup does but it could be from the sides. Haven't read enough on the topic.

[Petrus]

I've often wondered if you could evacuate the radiator spent air through a duct in the flat floor, rather than venting the bonnet?

I have been venting the bonnets since the eighties. First tried the traditional scoop to cool a turbo duct. Was before intercoolers became common. It redúced air flow through the radiators. Took me a while to simply put the same thing on the same hole the other way round. Did the trick.
The only a bit tricky thing is the slightly higher pressure area in front of the windshield but the ´flatter´ the windshield the less pressure and the closer to it. It is no issue on the ZZW30.
Although they extract better, I do not like the looks of the reverse scoops on the MR2 bonnet so went for sunken vents.

As to the flat floor I simply do with a bit (about 15 mm) more rake.

As Adam´s calculations show, the effects on less lift/more downforce are staggering. The reason being the large surface areas of the under and upper sides of the car: The slightest difference in pressure(s) are multiplied (force per cm²).


I lóve aero. For mé it was an ´aha-erlebnis´ when the penny dropped about air behaving very much like water. TRhge same thing helped with ´getting´ the flow through the engine. Especially n.a.

[Petrus]

Ideally one would vent out of the side of the car, rather than the bonnet, as extra air over a curved top generates lift. 

Well, yes, the shape or a car means a longer travel top side than under making it like an air foil.
More air on top rather than under... not necessarily increasing air foil effect.
More air under will raise pressure because the space is restricted, not like above.
Top side it does not necessarily increase speed because it will partially upset any laminar flow. It is more likely to increase pressure.
Also because of the air foil effect pressure on top is indeed lower than under, meaning that a vent will see air flow directed directed to up, probably increase flow through the rads, making a reduction of opening as suggested by Adam an option without reducing cooling.
 

Imo the equasion is very simple: any air above instead of under is ditto difference in pressure and times the surfaces involved is the lift reduced.