ADESS AG, LOTUS REVOLUTIONISELE MANS PROTOTYPE RACING

AUTHORS

STEPHEN FERGUSON

leads the Corporate Marketing Com-

munications team at CD-adapco

INTRODUCTION

The Lotus T128 is set to revolutionise the

Le Mans Prototype (LMP2) category of

the World Endurance Championship.

Borrowing heavily from a process per-

fected in Formula 1, the T128 will be the

first LMP2 car to be designed using an

intensive engineering simulation process.

The LMP cars are the fastest closed cars

in circuit racing. Despite being around 40

% heavier than Formula 1 cars, LMP cars

are often capable of reaching comparable

top speeds.

Motorsports and computational fluid

dynamics (CFD) are synonymous. Since

someone first had the idea of bolting a

wing onto a racing car, over 45 years ago,

much of the design effort in motor racing

has been focused on aerodynamic devel-

opment. Together with tyre and engine

performance, differences in aerodynamic

performance are often the margin

between victory and defeat. It is likely

that more computer cycles are burned in

the pursuit of motor-racing glory, than in

any other endeavour. This article tries to

find how CFD has influenced the aerody-

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T ECHNOL OGY RACING

namic design of this exciting new race car.

In 2009 the FIA recognised the impact

of CFD on the design of F1 racing cars, by

including it in the Resource Restriction

Agreement, the intention of which was to

level the playing field by preventing the

better financed teams from purchasing

huge computer resources and simply “out

simulating” their smaller competitors. The

current RRA (which is currently under

renegotiation) limits each team to 40 tera-

flops of CFD, over an eight-week period.

It is no coincidence that the richest teams

are the ones that have invested in devel-

oping the best simulation process, getting

“more simulation bang per teraflop” and

maintaining their advantage in spite of

the intended level playing field.

In March 2013, the Kodewa racing

team ran their new T128 LMP2 car at the

Twelve Hours of Sebring race in Florida.

LMP2 is a cost-capped racing category,

specifically designed to allow private rac-

ing teams compete in same races as man-

ufacturer based LMP1 cars, which have

less strict regulations, and an almost

unlimited budget. LMP2 is not for the

faint hearted, with each chassis costing a

maximum of € 355,000 and engine price

limited at € 75,000. LMP2 represents one

of the very top levels of motor sport.

Although many of the other LMP2

chassis that will contest the 2013 season

have undoubtedly been subjected to the

odd CFD simulation or two, none has

been designed using the same intensive

CFD process common in F1. That is, until

now. The T128 is the first LMP2 car to be

completely designed using an engineering

simulation driven process. It has been

developed for Lotus LMP2 Kodewa racing

team by Munich-based ADESS AG, a

design and engineering office staffed by

engineers experienced in Le Mans and F1

race car design. Stéphane Chosse, in-

charge of the design and engineering

office at Adess AG, gives us more details.

IMPORTANCE OF AERODYNAMICS

Together with tyre performance, aerody-

namic downforce is the primary determi-

nant of the stability and speed of a race

car. More downforce results in better tyre

grip, allowing faster cornering speeds and

more effective acceleration and decelera-

tion (under braking). Increased downforce

also usually comes at the cost of

increased drag, which reduces straight-

line speed and fuel efficiency, but the ben-

efit of increased cornering speed easily

makes up for deficit. The absolute magni-

tude of the downforce is most important

for relatively slow corners, which involve

smaller lateral forces and relatively little

steering. In a fast corner, the distribution

of downforce between the front and the

back wheels – the so-called “aero bal-

ance” – is a more important determinant

of the drivability of the race car. The ideal

is a perfectly aero-balanced “neutral” car

that neither understeers nor oversteers. If

there is not enough downforce acting

through the front wheels, then the car will

understeer in fast corners. Not enough

downforce to the rear wheels and the car

will oversteer.

Compared with downforce and aero-

balance, although important, the minimi-

sation drag is not usually a primary objec-

tive in racing car design. However, drag is

more of a consideration in the endurance

based LMP than in other categories. Small

reductions in drag penalty result in incre-

mental savings in fuel consumption. Inte-

grated over the whole of a 24-hour endur-

ance race, better economy means fewer

fuel stops, and more track time for the car

between them. This ultimately can be the

difference between winning and losing an

endurance race.

A particular challenge of the LMP2

category, is that the same cars need to be

competitive in both endurance races –

the 24 H eures du Mans, which require

low-drag, and much shorter sprint races,

which last just six hours and require

much higher downforce. The regulations

recognise the need to reconcile these

contradictory requirements, by allowing

the teams to utilise a single lowdrag

body kit for endurance races (even this is

cost capped at maximum price of €

10,000), the aerodynamic performance of

which also needs to be included in the

design process.

Another consideration is cooling –

both to the engine (Lotus Kodewa uses a

3,600 cc V8, but the regulations also

allow for a turbocharged 2,000 cc V6) and

to the brake ducts, both of which must be

supplied with a constant ventilation under

a variety of racing conditions to prevent

catastrophic (race-ending) overheating.

Since the LMP2 regulations dictate that

the design of the chassis is fixed for the

whole season, the engineers from ADESS

AG do not have the luxury of tweaking

the car design on a race-by-race basis.

WHY CFD?

Before the advent of affordable CFD, aero-

dynamic development of race cars was

principally based around the wind tunnel.

However, wind tunnel testing is both

expensive and time-consuming, and not

well suited for the type of iterative design

exploration needed to design an effective

race car on a limited budget. Although it

still plays a large role in design validation

and parametric design, most of the heavy

design work is now performed using CFD.

CFD basically acts as a filter for the

wind tunnel, allowing engineers to rap-

idly investigate multiple design con-

cepts, weeding out the worst and evolv-

ing the best.

Stéphane Chosse compares the process

to Darwinian evolution: “It’s literally sur-

Pressure contours viewed from the front of the car

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vival of the fittest. Using CFD we are able

to rapidly identify the best features of

potential designs and weed out the worst.

The best designs evolve over multiple iter-

ations, and only the very best survive to

be tested in the wind tunnel.”

In designing the T128, ADESS AG

investigated over 50 different design con-

figurations using STAR-CCM+, eventually

selecting the three of four best for further

investigation in the wind tunnel. The

team was able to set-up, compute, and

analyse a single design configuration

within a 24 hour window, with all but a

few minutes of this time taken up by the

actual number-crunching, and very little

manual interaction.

This is the heart of the automated pro-

cess that Chosse and his team have bor-

rowed from F1. Rather than manually

importing geometries, making meshes

and running simulations, everything is

controlled in batch using Java macros to

control STAR-CCM+ automatically. In

principle, all the CFD engineer needs to

do is identify a CAD model for the part of

the car to be modified, and a few hours

(and much behind the scenes number

crunching) later, a report will be automat-

ically generated containing the simulation

results, including aerodynamic coeffi-

cients, and a set of specially chosen post-

processing plots.

The net result is that engineers are able

to explore the entire design space, instead

of a few points. Better still, since the pro-

cess requires very little manual interven-

tion, engineers are free to concentrate on

analysing the simulation data, rather than

performing repetitive simulation tasks.

Each CFD simulation uses about 60

mn computational cells to represent the

air around half the vehicle (taking advan-

tage of that the flow around the car is per-

fectly symmetrical for a zero yaw case).

The mesh is locally refined to capture

areas of high flow gradient, resulting in

smaller cell sizes close to the surface of

the car, in and around the separation

regions behind it, and through the

expected locations of the trailing vortices.

Race car aerodynamics is increasingly

about vortex management, said Jan Czar-

nota, CFD engineer, ADESS AG. “CFD

simulation allows us to visualise vortex

development and predict vortex combina-

tion and bursting; all of which are critical

in being able to deliver a high-downforce

race car that is stable under a range of

racing conditions.”

The CFD simulation is also vital for

predicting the flow in parts of the design

that would not otherwise be well pre-

dicted by wind-tunnel simulation alone,

such as the mandatory fender holes,

which are prescribed by the LMP2 regula-

tions to relieve the pressure within the

wheel arch, and the prediction of engine

and brake cooling, all of which would be

difficult to perform at model scale in the

wind tunnel.

Relying on wind tunnel analysis

alone would have led to at least ten

times longer design process for this car,

been much more expensive, and still

resulted in a slower, less drivable LMP2

car, said Chosse.

INTO THE TUNNEL

The wind tunnel testing is performed

using a 50 % scale model of the car, 2.5

m long, and about 1.5 m wide. The aim of

the wind tunnel testing is two-fold – the

first objective is to offer some validation

to the CFD simulation runs, by perform-

ing the first wind tunnel runs on any

design under the same design conditions

as the CFD calculations – 250 km/h at a

single ride height.

A significant advantage of the wind

tunnel is that, once you have gone to the

expense of creating a model and hiring a

tunnel, it is relatively easy to test that

model under a variety of experimental

conditions. With that in mind, the second

objective is to perform a complete design

exploration, analysing each of the chosen

designs at a variety of speeds, as well as

performing a complete sweep of ride-

heights roll angles, yaw angles and steer-

ing conditions.

“These testing periods will be punctu-

ated by further periods of intense CFD

simulation as the team attempt to explain,

and visualise, any unexpected results, for

example an anomaly at a particular yaw

angle or speed. This process will ensure

that the final vehicle design is optimised

for the complete range of possible race

conditions,” Chosse said.

ONTO THE TRACK

The Lotus T128 hit the test track in

December, moving on to making its racing

debut at the Twelve Hours of Sebring in

Florida in March 2013. As well as racing

two T128s themselves, Kodewa will also

sell the T128 chassis to other teams com-

peting in the Le Mans Series and the

World Endurance Championships.

What next? Once they have completed

the T128, ADESS AG will turn their

attention to the design of an LMP1 car

using a similar process. “If anything, the

LMP2 project has convinced me that we

are just skimming the surface of what is

possible with an effective CFD process,”

said Chosse. “I’m looking forward to

implementing the lessons that we

learned from the T128 in the less con-

strained environment of designing a

brand new LMP1 car.”

Streamlines illustrating the flow escaping from the wheels

T ECHNOL OGY RACING

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