fan cars: what's it all about

August 2014

Josep Mª Carbonell Oyonarte

Fan cars: What’s it all about? Aerodynamic study of the implementation of a fan to a racing car

Fan cars: What’s it all about? Aerodynamic study of the implementation of a fan to a race car

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About the author

This project was performed in the CFD and

Race car aerodynamics course imparted by

Tecnica F1. The author is Josep Mª Carbonell,

aeronautical student at UPC in Terrassa

(Barcelona), and a race cars enthusiast.

That’s the reason behind this project, the

objective was to put all this knowledge into

designing a race winning car concept.

Contact Details

Mail: [email protected]

Phone number: +0034687688061

Linkedin: Josep M Carbonell Oyonarte

mailto:[email protected]


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Contents

Concept

1- Introduction ............................................................................................. 3

2- Reviewing the concept ........................................................................ 6

Preparation and results

3- Targets of the study .............................................................................. 9

4- Meshing ...................................................................................................... 9

5- Simulations and results .................................................................... 10

5.1- Downforce .................................................................................... 14

5.2- Drag ................................................................................................. 15

Resolutions

6- Conclusions ........................................................................................... 16


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1- Introduction

In the world of motorsport, there have been a lot of developments benefiting maximum speed, resistance and speed cornering. This document deals with the later. Generally speaking there are 2 main ways to increase cornering speed:

· Increasing mechanical grip through improvements of shock absorbers, springs, anti-roll bars, camber and castor angles, etc.

· Increase the downforce of the car through aerodynamic work with spoilers, wings, air flow around and under the car, etc.

Since mid-60’s, the most popular way to obtain downforce have been the implementation and fine tuning of aileron, flaps and spoilers but there have been some others paths to create downforce from the air. One of these ways is creating a low pressure area under the car, as a high pressure are always try to move towards a low pressure area the car is effectively sucked towards the ground. The basic idea is to accelerate the air under the car, the first step is adding a rear diffuser which accelerates the air exiting the rear. Also, we have to ensure that as little air as possible enters the bottom of the car through the sides and that there is the least possible air under the car. There are two ways to make that:

· Putting skirts to the floor.

· Creating an aerodynamic wall along the car.

In 1970 a new concept was born. Jim Hall and his team created a new car, the Chaparral 2J. it was a pioneer of one of the most incredible


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inventions and pieces of lateral thiniking in racing history: placing a fan at the rear of the car in order to suck the air from under the car increasing the pressure differences and effectively sucking the car to the tarmac. This development was quickly banned in the Can-Am category, due to the large performance advantage of this concept.

Eight years after, Gordon Murray and his Brabham team, run by Bernie

Ecclestone, resurrected this concept in Formula 1 with the BT46B,

famously remembered as the “Fan car”. The story of this car was the

same than Chaparral, it was banned after the first race because of the

overwhelming dominance.


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Figure 1- Chaparral 2J

Figure 2- Brabham BT46B “Fan car”

The fan of the Brabham was

connected to the crankshaft

by a small gearbox, so the

velocity of the fan could be

modified.

The two 17-inch fans

were driven by a 45 hp

two strike twin

snowmobile engine.


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2- Reviewing the concept

After so many years without any advances on this concept, a new car

has been developed. This car is different, because it’s a virtual car, the

Red Bull X1. The designer, Adrian Newey, used all the advances and

concepts possible to improve the car, even if they aren’t allowed in any

category of motorsport.

Our objective was to study a car similar to Red Bull X1, and quantify

the Fan Effect and it’s on the performance of the car.

This is the car used:

Figure 3- Car used renderized in Loews turn


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And this is the fan placed behind:

Figure 4- Fan

As we can see, the car has wheel fairings and a close cockpit, two

improvements in terms of drag reduction banned in Formula 1. Also, it

counts with ground effect, because it is a so-called “wing car”. This

design allows increasing total downforce, and combined with the fan,

the magnitude of downforce can be stunning.

We can see the low

pressure around the fan

due to the high velocity of

the suctioned air.


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The definitive improvement of the car is its skirts, closing the area for a

better suction.


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3- Targets of the study

The main targets of the study are as follows:

Minimum fan speed in order to improve downforce.

Correlation between downforce and fan speed.

Correlation between drag and fan speed.

We are not looking for quantitative results, because the resources able

for this project don’t permit more precision on the data resulted, but

we are able to obtain a good qualitative results of the targets of this

study.

4- Meshing

We have configured a polyhedral mesh and a boundary layer around

the car to obtain more precisely the downforce and the drag induced

on the body. Before configure the boundary layer, we calculate first

layer thickness based on the air speed and the y+ desired. Also, we

added a block mesh on the entire region around the bottom of the car

and the diffuser. This block mesh has smaller mesh size than the rest of

the volume and gives us more control and precision around this area,

the most important of the car in this study.


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5- Simulations and results

The first step was to set up the basic parameters:

Air velocity: 220 Km/h

Rolling ground and wheels to make the simulation and its results

more realistic.

The next step was to identify the minimum fan speed which increases

downforce. After several simulations at different fan speeds, the

minimum speed was determined as 25000 rpm. Under this speed, the

fan decelerates air passing under the car and downforce falls.

This is the pressure distribution over the car:

Figure 5-Pression distribution over the car

These points have high

pressure because the air is

decelerated when hit the

car.


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And this is the pressure distribution under the car:

Figure 6-Pression distribution under the car

We can see the depression created by the diffuser and the fan, which

starts just at the front skirt of the car. This is the reason of the large

improvement in terms of downforce.

As we can see in this

picture, the low pressure at

the bottom of the car has

the meaning of a high speed

of air or the vacuum effect

created. In other words, we

can see how the ground

effect is working.

We can see how the front

wheels produce a large

increase in pressure and a

resulting large increase in drag.

The rear wheels produce much

less drag as they are better

protected by the car’s body.


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These are the streamlines over the car, and we can see how the air

interacts with the car:

Figure 7- Streamlines over the car


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Another interesting visualization is the velocity vectors on side plane

for a future upgrade of the aerodynamic design of the car:

Figure 8- Air speed vector side view

After this previous study, we proceeded to compare the drag and

downforce of different fan speeds and, of course, the base scenario

where there is no fan.

This picture proves turbulences

created by the fan.


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5.1- Downforce

When simulations had been made, we analyze the outcomes of

downforce of each speed, and compared them with the downforce of

the car without fan as we were looking the improvement made by

using a fan.

We can see now the effects of using a fan in terms of downforce

improvement. We can say that the magnitude of improvement is

remarkable in terms of speed through the corners, but before

definitive conclusions we must analyze what is the effect on the

maximum speed, since a large increase in drag would penalize us much

on the straights.

0

5000

10000

15000

20000

25000

30000

30000 35000 40000 45000 50000

Increased downforce (N)

Fan speed (Rpm)

Table 1- Increase of downforce related with fan speed


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5.2- Drag

Here we analyze the results of the drag in each fan speed. We did the

same as for the downforce, so we compared the drag with fan and

without it.

The study demonstrated an increase proportional to the fan speed, an

increase in downforce always causes more drag. Maybe the most

important fact is that the increase in drag is very small compared with

the gains in downforce.

80

90

100

110

120

130

140

150

160

170

30000 35000 40000 45000 50000

Increased drag (N)

Fan speed (Rpm) Table 2- Increase of drag related with fan speed


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6- Conclusions

The results show the very interesting use of a fan, it produces a large

gain in downforce but with only a small increase in drag. The use of the

fan is recommended in all circuits except very bumpy circuits where

large changes in distance of the floor to the ground could suddenly

produce an alarming loss of downforce, resulting in a potentially

uncontrollable and dangerous car. Moreover, the use of ground effect

needs stiffer suspensions as the distance of the floor to the ground has

to be the controlled as much as possible. This means a more difficult

work for the driver.

One of the possible ways to improve this concept is to design a system

which allows transmitting the best fan velocity, or the right grade of

blades, for each condition and positon of the car. The next step is to

stop the fan in the straights because its effect is detrimental for the

tires and it harms maximum speed.

As a conclusion, it’s clear that, with some investment and fine tuning of

the system, this concept can be very worthwhile.


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fan cars: what's it all about

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