The Impact of Wake Turbulence on Downforce.

[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.

[miqi23]

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.

[horse]

cars at half car length behind the lead car lose around 55% of their down-force

Well, I guess this is also what makes a "tow" effective. Losing the downforce is also reducing the drag penalty, which is beneficial in a straight line.

cars at half car length behind the lead car [have] a 10% balance shift to the rear

The mechanics of this relate to the method by which downforce is generated. The rear and centre downforce generation comes from the floor and rear wing whereas the front downforce is solely the responsibility of the front wing. As the floor is less effected by the turbulent length scales within the wake of the forward car it continues to work better than the smaller aerodynamic surfaces. Thus the contribution from the diffuser will shift the balance rearward.

[miqi23]

cars at half car length behind the lead car lose around 55% of their down-force

Well, I guess this is also what makes a "tow" effective. Losing the downforce is also reducing the drag penalty, which is beneficial in a straight line.

cars at half car length behind the lead car [have] a 10% balance shift to the rear

The mechanics of this relate to the method by which downforce is generated. The rear and centre downforce generation comes from the floor and rear wing whereas the front downforce is solely the responsibility of the front wing. As the floor is less effected by the turbulent length scales within the wake of the forward car it continues to work better than the smaller aerodynamic surfaces. Thus the contribution from the diffuser will shift the balance rearward.

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.

[Tim.Wright]

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.

[andylaurence]

The cars have a couple of hundred pressure taps on the bodywork. From this, they can make a pretty good estimate of the downforce. However, two car simulations are carried out in CFD, but not as much as they used to be before the restrictions came in.

[Tim.Wright]

I have seen windtunnel models with hundereds of pressure taps but not running cars.

Wings can be relatively easily strain gauged to measure drag + downforce but the floor is practically impossible to do this on.

This is why I'm interested in where the data came from.

[livinglikethathuh]

They could be measuring suspension compression and comparing it to clean air values. Road irregularities may be averaged/filtered out of the data.

[Tim.Wright]

That's not going to give you the split between front wing, rear wing and floor.

[horse]

@Tim.Wright I have read a few academic papers that discuss the results of numerical models and some basic physical experiments for sports cars. I can look them up for you, if you like? My experience is entirely academic, however.

My belief is that where the key low pressure zones for the floor (the leading edge and the diffuser transition) is fairly common knowledge, nonetheless.

[Just_a_fan]

The downforce available to the floor is governed by the air flow at its leading edge. The front wing and associated under-nose devices affect the flow under the leading edge of the floor hence the bargeboards etc. If the front with is disrupted then it will affect the underfloor quite dramatically. This is very different to the old days where ground effect was the result if heavily sculptured undersides of sidepods.

If the front wing and under-nose are affected by disrupted air flow then the air flow to the underbody will also be disrupted. Downforce will suffer. On a straight this is beneficial hence "the tow"; in a corner it's a very big problem however.

[andylaurence]

I have seen windtunnel models with hundereds of pressure taps but not running cars.

According to Willem Toet (previous Head of Aero at Sauber), Red Bull started the trend of running the car with pressure taps during the race and it's now commonplace. I was surprised, but I have no reason to doubt him.

[horse]

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.

[trinidefender]

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.

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

That's why when they were looking to remove downforce on the cars for 2009 one of the things they did was largely reduce the size of the bargeboards.

[graham.reeds]

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