Importance of the diffuser?

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Post Sat Jun 06, 2009 7:04 pm

riff_raff wrote:The underbody of an F1 car is more efficient at producing downforce (ie. it generally has a more favorable L/D coefficient) than the front and rear wings. But the front and rear wing elements are still useful, in spite of their less favorable L/D characteristics, because they can be adjusted to balance the front-to-rear aero grip of the chassis, and the underwing cannot.

All aerodynamicists strive to produce the most lift (or downforce) for the least drag penalty. And since the power required to overcome drag increases exponentially with speed, that is why F1 teams spend so much money on wind tunnel work. It gives the most bang for the buck.

The reason the underbody is more efficient at producing downforce than the front or rear wing is that the basic laws of physics dictate that accelerating a large mass of air at a small rate is more efficient than accelerating a smaller mass of air at a higher rate. Thus the larger mass flow rate at a lower delta V across the under body will produce a given downforce with less drag penalty, than the smaller mass flow rate with a greater delta V across the front or rear wing airfoils.

If you look at some of the unrestricted ground effect F1 cars of the 70's, you'll note that they eliminated the front wing completely.

Image

Regards,
Terry


Yes Terry, it is more efficent for the underbody to be the primary source of force production towards better L/D, especially in the days with skids. It is my belief that had skids or "sealers" continued into the active suspension days, that we may have seen cars without wings at all and cars generating 6g's + lateral.
It was also interesting that in this era, that passing and drafting was at it's highest point vs before and after the ground effect tunnel.
There are a few things to adjusting a "non-skid" GE tunnel in regards to chassis tuning, mainly in adjusting the chassis rake, tunnel height (physically moving and reattaching), reshaping the tunnel, length, strakes, vortex generation, exterior body sculpting,entrance vortices (for sealing and stream guidance purposes), exit vortices (from the top of the exit,ie-very similar to current double diffusers) mostly in aiding and controling movement/location and size of the center of pressure.
But non-skid tunnels are not without trade offs, with the air gap to the ground. While moving the tunnel closer to the ground nets large gains for small adjustments in height, the same is true with moving away from the ground, with large losses. Change it's gap height just far enough and you can lose 80-90-100% of it's ability, with horrific results to the chassis and loss of control to the driver. Losing one tunnel's strength and increasing the other, in a roll situation (say running over a high curb at high cornering speed) is equally undesirable.
Maintaining the tunnel side gap and removing inbound side turbulance is of major interest to one tuning a GE tunnel.
All of these apply to tuning a diffuser, with rules removing most the adjustablity, shortening of the tunnel length (thereby increasing CP importance and making the diffuser very critcal and sensitive to pitch movement.

IMHO- I'm not convinced you can "stall" a F1 diffuser with the stepped undertray, as has been stated by a few, though you can very easily return a diffuser to it's truest diffuser state, thereby greatly decreasing it's low pressure area throat size and rendering the diffuser ineffective
"Driving a car as fast as possible (in a race) is all about maintaining the highest possible acceleration level in the appropriate direction." Peter Wright,Techical Director, Team Lotus
speedsense
 
Joined: 31 May 2009
Location: California, USA

Post Tue Jun 09, 2009 2:46 pm

To explain why I'm referring to Kutta conditions.

Originally I stated:

Since both are located towards the lateral extremeties of the floor, their effects on downforce production will be small due to an effective Kutta condition existing around the floor and sidepod of the car.


Now, Ciro has produced an explanation involving velocities, Kutta also applies to pressures, which is where I was driving at.



The floor is not sealed, therefore it suffers from the exact same problems as an aircraft wing, i.e. zero lift at its wingtip (or zero downforce at the side of the floor in this instance). This is due to continuous variation in pressure rather than discontinuities.

The front wing end fence, and the small wings the teams run at ground level ahead of the sidepod exist to try and reduce or eliminate this effect... similar to a winglet I suppose.

But taking that a step further - the teams would move said wing backwards into the floor itself (and narrow the sidepod locally if necessary for shadow plate reasons) if it was only sealing an upforce producing area at the leading edge.

Note also - turbulence in itself does not 'seal' the floor, it requires specific vortical formations to do that. The wheel won't - it will have an approx homogenous turbulent field - but the endplate and floor wing/sidepod leading edge may induce them.
kilcoo316
 
Joined: 9 Mar 2005
Location: Kilcoo, Ireland

Post Sun Mar 07, 2010 8:18 am

xpensive wrote:This subject became more interesting and should thus be rewarded a thread of its own, or so I believe anyway.

If you behold an air-stream separated by an object such as a wing-profile, or an F1 car for that matter:

Total pressure-pt, is static pressure-ps plus dynamic pressure-pd. Then if total pressure is held constant:
ps1 + pd1 = ps2 + pd2
The above gives a static pressure differential, ps1 - ps2 (which creates Force), equal to pd2 - pd1.

When pd is density * speed squared / 2 and Force is static pressure differential times Area, the resulting force is:
area * density * (speed2^2 - speed1^2)/2.

This force can be substantial, if you can arrange the air-speed under the car to be 90 m/s, while the speed on top is only 60 (216 km/h), downforce would be about 5400 N with a floor-area of 2 sq. meters and a density of 1.2 kg/m^3.
But if you find a way, through a novel diffuser for example, to speed up the air under the car with 10%, downforce increases to 7440 N, or with 38%.


I was going through this thread from a year ago, when it suddenly struck me that the easiest way to control down-force from under the car without limiting innovation, would be to limit floor-area, rather than the diffuser itself.
Any impressions?
"I spent most of my money on wine and women...I wasted the rest"
xpensive
 
Joined: 22 Nov 2008
Location: Somewhere in Scandinavia

Post Sun Mar 07, 2010 8:40 am

+1

I have to say that this is the obvious solution ,funny enough nobody ever really brought up the matter in this effective way .
Wish I had done that post.

reducing floor areq (as an equivalent to reducing wingarea you will inevitably cap
the maximum achievable downforce. a reduction in area should equal a reduction in DF
coming from this component .Additionally the interaction with the other DF producing components will be diminished so more losses.
marcush.
 
Joined: 9 Mar 2004

Post Sun Mar 07, 2010 10:49 am

marcush. wrote:+1

I have to say that this is the obvious solution ,funny enough nobody ever really brought up the matter in this effective way .
Wish I had done that post.

reducing floor areq (as an equivalent to reducing wingarea you will inevitably cap
the maximum achievable downforce. a reduction in area should equal a reduction in DF
coming from this component .Additionally the interaction with the other DF producing components will be diminished so more losses.


Surely the FIA have this available with the step between the skid plate and the floor, which has a regulated dimension that could be changed?
autogyro
 
Joined: 4 Oct 2009

Post Sun Mar 07, 2010 11:11 am

autogyro wrote:
marcush. wrote:+1

I have to say that this is the obvious solution ,funny enough nobody ever really brought up the matter in this effective way .
Wish I had done that post.

reducing floor areq (as an equivalent to reducing wingarea you will inevitably cap
the maximum achievable downforce. a reduction in area should equal a reduction in DF
coming from this component .Additionally the interaction with the other DF producing components will be diminished so more losses.


Surely the FIA have this available with the step between the skid plate and the floor, which has a regulated dimension that could be changed?


the step is reducing the effectiveness and this would have worked if they had not found theDD diffuser as a loophole.
Raising the step to 100mm and there is no downforce to speak off ,no matter how big or effective the difusser would be ,that was the point.
marcush.
 
Joined: 9 Mar 2004

Post Sun Mar 07, 2010 11:56 am

xpensive wrote:I was going through this thread from a year ago, when it suddenly struck me that the easiest way to control down-force from under the car without limiting innovation, would be to limit floor-area, rather than the diffuser itself.
Any impressions?


The non-dimensional quantity used to describe ground effect for racing cars is h/L where h is the ride height and L is the length of the undertray. When h/L is small, say < 0.2 you get significant gains in DF. Thus, reducing L or increasing h will both raise the value of h/L, reducing downforce. So, yeah, floor length is a good method for reducing downforce from diffusers. Obviously, the angle of the diffuser makes a difference too, with a steeper angle giving more as long as the ride height is not too low to stall it.
"Words are for meaning: when you've got the meaning, you can forget the words." - Chuang Tzu
horse
 
Joined: 23 Oct 2009
Location: Edinburgh, UK

Post Wed Aug 18, 2010 5:58 am

I was thinking more along the lines of limiting the width of the entire bodywork, which should have a proportional relation to downforce. This was actually where it all began in the 70s, by widening said bodywork to increase floor area.

Then in 1983, with the flat bottom rule, the pendelum swung back to slimmer designs for a while, like the Williams FW08C, Toleman TG183B, Ligier JS21, Tyrrell 012 and the gorgeous championship winning Brabham BT52. Until ground effects were re-discovered when the FIA failed to realize that the flat-bottom should have been xtended behind the rear-wheel centerline, it's been a mess ever since.

Time to scroll back to that period perhaps?
"I spent most of my money on wine and women...I wasted the rest"
xpensive
 
Joined: 22 Nov 2008
Location: Somewhere in Scandinavia

Post Wed Aug 18, 2010 8:08 am

xpensive; You're right, but only if you want ground effect, openly that is, instead of this farce we have now. There are those that would see the diffuser and all ground effect gone.
"To race is to live. But those who
died while racing knew, perhaps,
how to live more than all others."
~ Juan Fangio
strad
 
Joined: 2 Jan 2010

Post Wed Aug 18, 2010 8:57 am

That would of course be the simpliest rule of them all, strad, flat bottom as long as there is car to measure.
"I spent most of my money on wine and women...I wasted the rest"
xpensive
 
Joined: 22 Nov 2008
Location: Somewhere in Scandinavia

Post Wed Aug 18, 2010 3:22 pm

I always thought that F1 cars look uninspiring from below with these huge floors they add to the real shape of the car.
Formula One's fundamental ethos is about success coming to those with the most ingenious engineering and best .............................. organization, not to those with the biggest budget. (Dave Richards)
WhiteBlue
 
Joined: 14 Apr 2008
Location: WhiteBlue Country

Post Wed Aug 18, 2010 4:09 pm

xpensive wrote:That would of course be the simpliest rule of them all, strad, flat bottom as long as there is car to measure.


This would cause in much weirder and more complex floors, teams will run more rake etc. to just increase its efficiency. In a flat floor itself there is still an ground effect, due to the pressure increase and the sudden increase when exiting the floor, an simple gurney could increase such floors downforce with alot.

IMO the diffuser is something that cars should have as it is the cleanest way to get the downforce and still maintain an good enough wake to slipstream, thus my idea was always to just create underbody tunnels(one on each side) with an centre 20cm wich has to be flat. from what I have read was that certain tunnels are much more efficient and have cleaner flow then normal diffusers. If you have these tunnels starting around the cars centreline and end them in front of the rear wheels then you simply have an great balanced car. Then the front and rear wing come much more to tweaking the car's aerobalance, wich also increases its overal drivability as there wouldnt be any problem anymore like schumacher is having, you could simply balance it out much more easily. also such an underbody is just out of sight of the rear wing, thus reducing the Rear wings effect on the diffuser.
wesley123
 
Joined: 23 Feb 2008

Post Wed Aug 18, 2010 4:37 pm

xpensive wrote:I was thinking more along the lines of limiting the width of the entire bodywork, which should have a proportional relation to downforce. This was actually where it all began in the 70s, by widening said bodywork to increase floor area.

Then in 1983, with the flat bottom rule, the pendelum swung back to slimmer designs for a while, like the Williams FW08C, Toleman TG183B, Ligier JS21, Tyrrell 012 and the gorgeous championship winning Brabham BT52. Until ground effects were re-discovered when the FIA failed to realize that the flat-bottom should have been xtended behind the rear-wheel centerline, it's been a mess ever since.

Time to scroll back to that period perhaps?


you are correct Xpensive.
F=p*A
SO if you can reduce A you will reduce F as well, for a given pressure difference.
Now, that would/could mean reducing the width/length of the car´s, with some implications of side impact protection etc.

There is not that much "free/just floor" area.
You could get rid of the splitter and the "free floor" in front of the rear wheels, but not much more IMO.

The FIA went the other way trying to reduce p with their limeted diffusor size and step in the floor (reducing the sealing effect - making it harder to generate p)
There (FIA) biggest "mistake" was to allow DDD at the beginning of 2009, this nullified their intention to limit downforce.
So if they get a handle on diffusor size, they will limit p and therefore F (amount of downforce).
You could demand some holes in the floor, that would reduce delta p as well, significant. ( unless you use them to feed your DD :) )

I´m not sure if making the cars smaller in size is what we want to see (could help in overtaking).
With the more compact engine/drive trains fom 2013 onwards it´s maybe worth a consideration.
"Make the suspension adjustable and they will adjust it wrong ......
look what they can do to a carburetor in just a few moments of stupidity with a screwdriver."
- Colin Chapman

“Simplicity is the ultimate sophistication.” - Leonardo da Vinci
747heavy
 
Joined: 6 Jul 2010

Post Wed Aug 18, 2010 6:28 pm

IMO the diffuser is something that cars should have as it is the cleanest way to get the downforce and still maintain an good enough wake to slipstream

But wesley, The diffuser is what causes much of the disruptive wake. I have told you guys..They design the diffuser to do it's job sure...but with as much turbulence as they can create for the following car. It's not bad enough on it's own,,,they work to make it worse. That turbulence these days can be seen for a hundred yards behing the car. Clean that up and you'll clean up the racing..that and the friggin tires.
"To race is to live. But those who
died while racing knew, perhaps,
how to live more than all others."
~ Juan Fangio
strad
 
Joined: 2 Jan 2010

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