The Impact of Wake Turbulence on Downforce.

[horse]

They are very important in the aerodynamics of open wheeled race cars.

Don´t get me wrong, I am not saying they are not important to the performance of the floor, I am just saying that their operation does not rely on clean air, as they are not lifting surfaces.

[Just_a_fan]

it is at the floor inlet which is driven by the barge-boards vortices and other interacting flow structures which has lost most of its energy by being in a wake of another car.

The front wing and associated under-nose devices affect the flow under the leading edge of the floor hence the bargeboards etc.

Are the bargeboards really affected by turbulent air that much? They are not really aerodynamic, hence their name. I would have thought they would help tidy up the airflow to the floor leading edge in this situation.

The bargeboards are there to condition the flow and to ensure the flow goes where it is supposed to. They are big vortex generators too. The point I was making is that they exist to help the underside of the car. I have no doubt that they work best with clean air as designed and developed in the wind tunnel. The problem with dirty air on the front wing is that it will change the air hitting the bargeboards too. It's a cascade failure, in effect.

[horse]

The problem with dirty air on the front wing is that it will change the air hitting the bargeboards too. It's a cascade failure, in effect.

The bargeboards are there to condition the flow...

You've argued against yourself a bit there, as the technical description of flow conditioning is to create laboratory conditions in a real world environment.

[miqi23]

Interesting... I assume that these numbers are from a CFD study? Otherwise I'm extremely dubious about how you calculate the drag force of the floor on a running car.

If these figures are accurate they go against the popular theory that ground effects are less susceptible to turbulence than wings.

Yes, Tim, these numbers are from a CFD study and these numbers do correlate well with tunnel and track data.

As far as ground effects is concerned, the general consensus that you mentioned in your post above is really simplistic and the truth is far from it. Ground effects, if done right, can help with the balance shift issue that F1 faces these days. The loss of down-force experienced by a ground effect floor is even when we look at the pressure distribution, which in return maintains the balance. It is definitively a good tool for helping the balance problem and if combined with the right wing package and bodywork design can result in a really useful Formula which would be miles better in promoting close racing.

it is at the floor inlet which is driven by the barge-boards vortices and other interacting flow structures which has lost most of its energy by being in a wake of another car.

The front wing and associated under-nose devices affect the flow under the leading edge of the floor hence the bargeboards etc.

Are the bargeboards really affected by turbulent air that much? They are not really aerodynamic, hence their name. I would have thought they would help tidy up the airflow to the floor leading edge in this situation.

Yes, they are affected by turbulent air and depending on the turbulence levels their performance can reduce radically even if you have high energy flow around the barge board. The increase in turbulence affects the boundary layer and cause early separation if your geometry is too aggressive (which barge boards are) and you would want an attached flow in regions of interest (which are detailed small shapes with specific features) to aid the formation of high strength vortices - a separated flow in regions of such geometry does not give you the required vortices.

Front wings are similar, too complex and you are risking losing performance - this is why simplified front wings perform better in higher turbulence environments.

[horse]

Front wings are similar, too complex and you are risking losing performance - this is why simplified front wings perform better in higher turbulence environments.

That's really interesting. So it's been a bit of a double whammy since 2009. As the cars have developed aerodynamically they are generating more turbulence and their ability to cope with turbulence has in turn decreased.

[mrluke]

Hence the difficulties faced by the two Mercedes cars when following one another

[OO7]

It all really depends on the car.

Current F1 cars lose most of their down-force at the front which results in a horrible balance shift to the rear making the car behave like a shopping trolley. The balance shift gets slightly better as you increase the distance between the lead and following car but is far from ideal. Overall down-force gets better as well as you increase the distance between the two cars. From experience, current cars at half car length behind the lead car lose around 55% of their down-force with a 10% balance shift to the rear which is massive.

Thanks miqi23, some great information there.

I must say those figures are pretty disappointing. I was thinking/hoping a 50% loss would be an absolute, rare, worst case scenario, but was expecting a figure more like 30% for total loss.

[turbof1]

It's not completely the end of the world. Having lost 50% of the downforce does not mean you are suddenly 50% slower. It means you have 50% less aerodynamic load transferred to the wheels - and in that transfer some of it goes lost as well- and ultimately to grip, which is the goal of downforce. Grip comes from the tyres as well, along other mechanical sources of load.

So while it does look like and perhaps is an astonishing loss, one should look in the grand picture of grip. Usually it translates to 1 second a lap loss on higher speed circuits, less on low speed circuits. 1s is a lot if you are driving the same car, I do want to underline that.

[OO7]

Yes, but the loss in drag is not 55% behind the lead car at half car length, it is around 30% - however, it is still good enough for slip streaming. That loss translates to an efficiency reduction of around 20% behind the lead car at half car length.

If we look at the mechanics of down-force reduction in a bit more detail, at half car length the front wing of a following car generally loses around 45% of its down-force, the rear wing around 42% and the underfloor around 48%. The balance shift to the rear is a result of not just the front wing losing its down-force but a combination of the three major down-force producing components.

..and from a down-force contribution point of view the floor produces around 55% of the total down-force, the front wing around 30% and the rear wing around 25%. Losing 48% of the total 55% floor down-force is still a massive loss and it is not an even loss, it is at the floor inlet which is driven by the barge-boards vortices and other interacting flow structures which has lost most of its energy by being in a wake of another car.

This is very interesting because it shows how downforce loss (as a percentage) across the three components is similar, contrary to the general belief that the front wing suffers the most. The data available to you shows the largest deficit occurring with the floor which is surprising, but I guess the interaction between the front wing and subsequent components downstream is so intertwined these days.

[OO7]

It's not completely the end of the world. Having lost 50% of the downforce does not mean you are suddenly 50% slower. It means you have 50% less aerodynamic load transferred to the wheels - and in that transfer some of it goes lost as well- and ultimately to grip, which is the goal of downforce. Grip comes from the tyres as well, along other mechanical sources of load.

So while it does look like and perhaps is an astonishing loss, one should look in the grand picture of grip. Usually it translates to 1 second a lap loss on higher speed circuits, less on low speed circuits. 1s is a lot if you are driving the same car, I do want to underline that.

I would have expected a 50% loss in downforce to cost far more than a second a lap and even moreso when you account for the balance shift. Then again what you lose in downforce you gain in drag reduction so perhaps I'm looking at it too simplistically.

[OO7]

Has anyone seen any studies or does anyone have any information regarding how much downforce a trailing car loses in wake turbulence?
I understand the difficulties with this because the results will vary depending on:
1) The distance between the cars
2) The level of downforce being generated (wing angles)
3) The speed the cars are travelling

It would be interesting also to understand what sort of balance shift occurs if any?
We often hear drivers complain of excessive understeer in wake turbulence, however the symptoms would appear rather similar to negotiating a corner at unabated speed (while in clean air). Wake turbulence negatively impacts all aero surfaces, so I wonder if there is much of a balance shift.

Nicolas Perrinn did a series of webcasts on this subject.

Fundamentals of F1 aerodynamics
Driving in the slipstream
Overtaking maneuver

I appreciate those links Graham, I managed to watch all three seminars.

[Dynamicflow]

It's not completely the end of the world. Having lost 50% of the downforce does not mean you are suddenly 50% slower. It means you have 50% less aerodynamic load transferred to the wheels - and in that transfer some of it goes lost as well- and ultimately to grip, which is the goal of downforce. Grip comes from the tyres as well, along other mechanical sources of load.

So while it does look like and perhaps is an astonishing loss, one should look in the grand picture of grip. Usually it translates to 1 second a lap loss on higher speed circuits, less on low speed circuits. 1s is a lot if you are driving the same car, I do want to underline that.

I would have expected a 50% loss in downforce to cost far more than a second a lap and even moreso when you account for the balance shift. Then again what you lose in downforce you gain in drag reduction so perhaps I'm looking at it too simplistically.

Well, that 50% loss is at half car length behind the lead car and to be honest not many cars follow each other that closely at high speed corners, its simply not safe with current F1 cars. A more realistic condition to look at would be something at two car length where overall down-force loss is around 35% which is still a lot by current F1 standards.

[OO7]

It's not completely the end of the world. Having lost 50% of the downforce does not mean you are suddenly 50% slower. It means you have 50% less aerodynamic load transferred to the wheels - and in that transfer some of it goes lost as well- and ultimately to grip, which is the goal of downforce. Grip comes from the tyres as well, along other mechanical sources of load.

So while it does look like and perhaps is an astonishing loss, one should look in the grand picture of grip. Usually it translates to 1 second a lap loss on higher speed circuits, less on low speed circuits. 1s is a lot if you are driving the same car, I do want to underline that.

I would have expected a 50% loss in downforce to cost far more than a second a lap and even moreso when you account for the balance shift. Then again what you lose in downforce you gain in drag reduction so perhaps I'm looking at it too simplistically.

Well, that 50% loss is at half car length behind the lead car and to be honest not many cars follow each other that closely at high speed corners, its simply not safe with current F1 cars. A more realistic condition to look at would be something at two car length where overall down-force loss is around 35% which is still a lot by current F1 standards.

It the relationship between downforce in wake turbulence and distance exponential?

[dans79]

It the relationship between downforce in wake turbulence and distance exponential?

it could probably be simplified to that, but like so many other dynamic problems, it's not really something that can be written as an equation, only approximated.

[second]

One thing that gets mentioned a lot is that cars with ground effects should lose less downforce when following others. Is that true? For example if we take an extreme type of car which has no front wing, makes ~80% of it overall downforce from the ground effects and uses the rear wing to mainly to balance the car (let's just imagine the aero balance would be good for the sake of argument).

Would that kind of car actually create better racing? Would the car behind keep most of its downforce? Would the car behind get drag benefit from running in lower pressure air? How would the turbulence affect the car behind?

For the 2017 season I was hoping the regulations would move lots of the downforce to the the bottom of the car but for racing and overtaking what would the data say about that kind of aero arrangement?