exterior styling of an intercity bus for improved aerodynamics

[email protected] http://www.wix.com/arunrave/frontpage

MSc (Engg) in APD 1 FT07

Exterior Styling of an Intercity Transport Bus for

Improved Aerodynamic Performance Arun Raveendran

1, D. Rakesh

2, S.N. Sridhara

3

1- (Engg) student, 2-Senior lecturer, 3-Professor and HOD,

Automotive Engineering Centre

M.S.Ramaiah School of Advanced Studies, Bangalore

Abstract Intercity buses travel about 250 to 350 km in a stretch and usually are of sleeper coach mode. The exterior styling,

sleeper comfort and aerodynamically efficient design for reduced fuel consumption are the three essential factors for a

successful operation in the competitive world. The bus body building companies prioritizes the exterior looks of the bus and

ignore the aerodynamic aspect. Scientific design of sleepers for increased comfort of the passengers is seldom seen. The

overall aim of this project was to redesign an intercity bus with enhanced exterior styling, reduced aerodynamic drag and

increased comfort for the passengers.

Extensive product study and market study were carried out and aspirations and frustrations of commuters were

recorded. An operating intercity bus was benchmarked and analyzed for styling, aerodynamic performance and comfort.

Fluent, a commercial CFD code was used to evaluate the aerodynamic performance. Principles of product design were used

to analyse the styling and comfort. The benchmarked high floor bus was redesigned with low - floor for reduced aerodynamic

drag. The exterior was redesigned with emphasis on improvised aerodynamic performance and appealing looks. The interior

was modified to meet aspirations of the commuters.

The results of the redesigned exterior body showed a reduction of Cd from 0.53 to 0.29 and overall aerodynamic

drag reduction by 60% due to combined effect of reduced Cd and frontal area. The redesigned interior was found to be at the

satisfaction of commuters.

Key Words: Bus Aerodynamics, Drag Reduction, Low Floor Bus Design, Sleeper Coach

Nomenclature

A Frontal projected area (m2)

Cd Coefficient of drag

I Turbulent intensity (m)

k Turbulent kinetic energy (m2/s2)

ρ Density of air (kg/m3)

ε Dissipation rate (m2/s3)

µ Dynamic viscosity (Ns/m2)

Abbreviations

CFD Computational fluid dynamics

CAD Computer aided design

PDS Product design specification

QFD Quality function deployment

1. INTRODUCTION

Buses are used as means for transporting large amount

of people from one place to other. All the states governments

are having its own intercity bus fleet in India which provides

mobility for the people at a reasonable cost. Huge numbers

of private bus firms are also in operation and are efficient in

reducing the dependency on trains. Indian road conditions

are significantly improved for the past 10 years and intercity

bus travel time is reduced as they can travel with high

speeds. In order to keep a low operating cost these buses

have to deliver high efficiency at these speeds. Rising fuel

prices and stringent government regulations force the vehicle

manufactures and operators to produce and operate fuel

efficient buses. The power generated in the engine is mainly

used to overcome the rolling resistance, aerodynamic drag

and climbing resistance. Out of these three components

aerodynamic drag increases with respect to the vehicle

speed. At high speeds at about 100 Km/hr the drag force

exceeds the power spend on overcoming the rolling

resistance [1]. So reducing the aerodynamic drag is of prime

importance to achieve fuel efficiency. Vehicle aerodynamics

deals with the study of forces acting on a vehicle body when

it moves through air [2].Drag and lift are the two main

phenomena observed on the vehicle body due to the effect of

the wind. About 90% of drag is due to the pressure

difference created between the various areas of the vehicle

[2].

1.1 Styling and aerodynamics

Drag force acting on the vehicle depends on

frontal projected area and the coefficient of drag value of the

vehicle. Any reduction in these values will directly reduce

drag force experienced by the vehicle. Frontal projected area

of the intercity bus is decided by the interior packaging of

the bus. Coefficient of drag value is determined by the shape

of the vehicle. These two factors influence the exterior

styling of the vehicle. Exterior styling of the vehicle is

important due to the fact that the vehicle has to attract

customers. The vehicle should project its performance and

comfort capabilities through its exterior design. Finding

harmony with the aerodynamic requirements and customer

oriented styling will lead to a successful vehicle with low

fuel consumption.



This research is aimed to deliver an aerodynamically

improved bus design with user oriented exterior styling. The

popular Volvo 9400 bus was evaluated for its aerodynamic

performance and guidelines for better aerodynamics were

collected from literature survey. Based on these guidelines

and user study concepts were generated. The model was

analyzed using fluent and improvements in drag values were

predicted.

1.2 Literature study

Edwin J Saltzman and Robert R Meyer [3] carried out

studies on reducing the drag of trucks and buses. The final

model equipped with rounded horizontal and vertical

corners, smoothed under body and a boat tail achieved Cd

value of 0.242. Ludovico Consano and Davide Lucarelli [4]

at IVECO truck building company came up with an

aerodynamically efficient truck. They paid particular

attention on the corner surfaces of the vehicle. A higher and

smoother roof has been designed with DAM fully integrated

into the frontal bumper. Moreover the lateral lowered side

skirts have been added to mask the tanks, rear wheels and

axles. To prevent flow detachment, many rounded surfaces

have been added to the exposed surfaces, such as the roof

window, side mirrors, sun visor, etc. The test results

revealed a fuel reduction of 8%. R. Mc Callen, K. Salari, J.

et al [5] in their experiments found out removal of rear view

mirror alone will bring down the drag of the vehicle by

4.5%. Any gap in the vehicle body will result in flow

separation and flow circulation. A Gilhaus [6] investigation

reveled a reduction in drag value until the front leading edge

radii value reaches 150 mm. Further increase in the radius

did not affect the drag value of the bus. C W Carr [7]

investigated the effects of streamlining the front end of the

rectangular bodies in ground proximity. Experiments shown

a stream lined front end with low leading edge resulted in a

drag coefficient of 0.21. W H huco and H J Emmelmann [8]

found that detailed shape optimisation of parts such as roof

radii, rain channels, headlights will result in reduction of

drag force. W T mason and P S Beebe [1] carried out

experiments using horizontal and vertical splitter panels

extending from vehicle body at the rear end, vanes and non

ventilated cavities close to vehicle bodies. Splitter panels

had no affect on the drag value and the vane arrangement

increased the drag. The addition of non ventilated cavities

reduced the drag coefficient by 5%.

2. Product Survey

Under AIS-052 code of practice for bus body design

and approval, present intercity buses comes under type 3 and

4. These are designed and constructed for long distance

passenger transport, exclusively designed for comfort of

seated passengers and not intended for carrying standing

passengers. Type 4 buses are special purpose buses

exclusively sleeper coaches which are getting popularized in

these days. Intercity buses are classified according to the

occupancy level as medium capacity buses as it can carry 35

to 50 passengers. Intercity buses are again classified

according to the comfort level as non deluxe bus (NDX),

semi deluxe bus (SDX), deluxe bus (DLX) and A/C deluxe

bus (ACX). Non Deluxe Bus is designed for basic minimum

comfort level. Semi Deluxe Bus is designed for a slightly

higher comfort level and with provision for ergonomically

designed seats. Deluxe Bus is designed for a high comfort

level and individual seats and adjustable seat backs,

improved ventilation and pleasing interiors. A.C. Deluxe

Bus is Deluxe Bus which is air conditioned. The present

intercity buses operate in India mainly comes under the

deluxe and A/C deluxe class.

The main parts which defines the exterior styling of an

intercity bus are the windshield, grill, front bumper,

headlights, indicators, wipers, side windows, passenger

doors, driver door, luggage space, engine space, back

windshield, number plate, brake light, back indicator, back

bumper and radiator grill. Interior of the bus consists of

driver’s cabin and passenger compartment. The driver’s

cabin consists of seating for the driver, his assistant, dash

board and steering. The passenger compartment consists of

rows of seats or beads according to the type of bus. All the

intercity buses are high floor buses with a floor height of

1200 mm from the road. Luggage space is provided under

the floor with opening from both the side of the bus. Four

steps are provided for boarding the bus. In most of the

present bus design the passenger door is located at the front

side corner of the bus.

Manufacturing of intercity buses are carried out in two

stages. The OEM manufactures drive away chassis of the

bus which include the engine, transmission and chassis of

the vehicle. The bodies of the buses are manufactured by

body building companies. In order to regulate the design of

the bus coaches, Indian Ministry of Shipping, Road

Transport & Highways introduced the standard AIS-052

which was published in September 2001. The major rules

which are to be considered while designing an intercity bus

are listed below

Parameters Regulations

Width of the bus Shall not exceed 2.6 m

Length of the bus Maximum 12 meters for transport

vehicle with rigid frame having

two or more axles,

Gangway Minimum of 1800 mm height and

300 mm wide

Service doors Minimum 1

Width of door Minimum 650 mm

Height of service door Minimum 1650 mm

Width of windows

Minimum 550 mm (sliding type

except for ACX)

Emergency exit 2 numbers ( 1 at front half

opposite to service door next one

at rear with area not less than

4000 cm2)

Height of first step 425 mm maximum

Height of second

steps

350 mm maximum

Intrusion above seat 100 mm at height 1350 from

floor

Wheel arch intrusion 200 mm from the seat front

Table 1 bus regulations as per AIS - 052



2.1Bus aerodynamics

Drag force acting on the bus body is given by the

formula

Drag force = ½ ρv2 A Cd (1)

It is evident that the drag force acting on the vehicle

depends on the density of the air, velocity of the vehicle,

frontal projected area and the coefficient of drag value of the

vehicle. Reduction in air density or the vehicle speed is not a

viable solution for reducing the drag value of the bus.

Reducing the frontal projected area is a viable solution as it

will directly reduce the drag significantly.

It is found in the product study a huge number of low

floor buses were operating in the urban areas for transporting

people. These buses are having a low deck with a height of

350 mm or less compared to the 1200 mm of high floor

buses as shown in fig1. Low floor buses are having kneeling

mechanisms which can further reduce the overall height.

These buses are having interior height of more than 1800

over 60 % of its inner space. By incorporating the low floor

bus chassis in intercity bus design will reduce present bus

height. A low floor intercity bus design will have low

projected area which in terms results in reduction in drag

force. Interaction with the bus manufacturers revealed that

the Coach manufactures were hesitating to use a low floor

chassis due to the following reasons listed below. A low

floor bus design which overcomes the below listed

difficulties will reduce the bus height from 3400 mm to 2600

mm.

Luggage space reduction

Large wheel arches reduces the number seats

Divides the floor area in to two decks

Less space for fuel tanks leads to low capacity

Difficult to reach the engine compartment for

repairs

Psychologically people like t sit at high floor than

the low floor area

Figure 1 Types of bus chassis

3. User study (Gemba study)

User study was conducted to select the target customer

group, understand user frustrations and aspirations. A survey

was conducted among the users. Users want a comfortable

speedy commute which is reasonably priced and looks good.

It was found that intercity bus users consist of mainly

professionals working in other cities (45%) and students

(34%). Business trips contribute to 16% of the seat

occupancy. 93% people prefer to travel in night compared to

7% day travelers.72% prefer to sleep while only 26% people

like do other activities. If money is not a constrain, 82%

people like to travel in sleeper coaches and rest preferred AC

deluxe buses. Most of the people adopted bus travel over

train due to the easy availability and comfort of the bus.

Safety, speed and flexible pick up and drop points are also

influence the decision. Maximum seat occupancy was

observed on Fridays and Saturdays. Occupancy levels of

sleeper coaches and AC deluxe coaches were high compared

to deluxe and semi deluxe coaches for buses staring from

Bangalore. People prefer to occupy the front rows seats than

the rear. More than 80% of private intercity buses are

operating at night between 7 pm and morning 10 am.

It was evident from the study that the majority of

intercity bus users comes under the age group of 18 to 35

and consists of middle class professionals and students. It

was also found that people prefer sleeper coaches over semi

sleeper (reclining seat) coaches. The target customer group

was finalized as 18 to 35 and the coach arrangement was

selected as sleeper coach. The major user frustrations

gathered from the user survey are listed below

Old aged and disabled persons find it difficult to

board the bus due to high step height.

Difficult to sleep in the present adjustable seats as

it will not allow any body movement.

Results in body pain and neck pain after long

travel.

Outside lights often disturb the sleep.

Less leg room in semi deluxe and deluxe buses.

Lack of toilets results in journey brakes and user

discomfort.

Platforms at the bus stations are not able to reduce

the boarding height due to current door position.

Figure 2 QFD matrix



A quality function deployment matrix was prepared by

converting the user frustrations and aspirations in to

technical voices.QFD revels the areas of importance to

achieve maximum user satisfaction. Low floor, kneeling

mechanism, sleeper coach, door position and aerodynamic

shape got highest points.

A low floor bus design will eliminate the boarding

problem of aged and disabled users. It is also favourable as

the bus height can be reduced to improve the aerodynamics

of the vehicle without reducing the interior space. A low

floor sleeper coach eliminates the difficulty in sleeping due

to reclining seat, arrested body movement, body pain and

low leg room. Area below the bed arrangement inside the

coach can be easily converted as luggage space and fuel

tank. Incorporating electro sensitive side windows allows the

users to adjust the opacity of their windows and will reduce

the light disturbances from outside. A wash room with toilet

system has to be incorporated inside the bus cabin.

Positioning the door behind front wheel arches will reduce

the boarding height with respect to the bus stop platforms.

4. Bench marking and baseline simulation

The Volvo 9400 intercity bus was selected as bench

marked model. It is the latest model in the Indian market and

is very popular in the segment. Engineering parameters of

this model was kept as same for the new bus design and was

selected as the baseline for studying the aerodynamic

performances. This vehicle model was used to understand

the flow behaviour Pressure distribution, Coefficient of drag

value, Contribution of different parts, Drag force acting at

different speeds, Flow separation and pressure stagnation

areas

Figure 3 Volvo 9400 intercity bus and CAD model

4.1Geometry and mesh generation

Three dimensional model of the baseline model was

created using Alias studio tools and Catia as shown in fig 3.

Small details and gaps in the vehicle body were eliminated

as the purpose of the analysis was to understand the overall

aerodynamic performance of the basic bus shape

Fluid domain of 96m x 12.75m x 17 m was created

around the bus model which was 10 times the length, 5 times

the width and height of the vehicle. Bus model was placed

inside this domain in such a way that 1/3 length was kept in

front of the vehicle. The larger domain was kept at the rear

to capture the essential flow features. A smaller domain was

created inside this domain to generate fine mesh in and

around the bus body. Gambit pre processor was the software

tool used to generate the mesh. Outer volume was meshed

with coarse elements. 21,47,716 Unstructured tetrahedral

hybrid elements were used to mesh the entire fluid domain.

4.2Boundary conditions

Boundary conditions were applied on the meshed

model using the Gambit pre processor. The analysis was

carried out in moving road and rotating wheel condition. In

the simulation only straight wind condition was considered

at 3 different vehicle speed of 80, 100, 120 Km/hr. Constant

velocity inlet condition was applied at the inlet to replicate

the constant wind velocity conditions same as wind tunnel

tests. Zero gauge pressure was applied at the outlet with

operating pressure as atmospheric pressure. All the boundary

conditions used in the analysis are listed in table 2

Boundary Boundary condition value

inlet

Constant velocity

Turbulent intensity

Length scale

V= 22.22 m/s I = 1.97

V= 27.78 m/s I = 1.92

V= 33.33 m/s I = 1.88

Outlet

Pressure outlet

Constant pressure = 0

pa

Road

Moving wall

No slip

V= 22.22 m/s

V= 27.78 m/s

V= 33.33 m/s

Tyres

Rotating wall

No slip

Ang. V = 404.36 rpm

Ang. V = 505.55 rpm

Ang. V = 606.55 rpm

Bus body No slip – Stationery

wall

Domain

top and

side

Stationery wall

Specified shear

Shear stress = 0

Table 2 Boundary conditions

4.3 Turbulent model

The solver used for the analysis was Fluent 6.3.26 and

it uses a control-volume-method to solve the governing

equations that can be solved numerically. The solver

selected was the pressure based implicit solver. In this type

the equations of continuity and momentum are solved

sequentially. This is used for incompressible flows where the

density is constant and not related to pressure. This reduces

the computational time when compared to the other

methods. The flow is considered to be steady in nature and

thus the equations are solved using implicit iterative

methods. Reynolds number of the flow was calculated and

found out to be fully turbulent. The turbulence model

selected as k-ε model which is well known for its robustness.

This model assumes the flow to be fully turbulent and is

based on the turbulence kinetic energy and its dissipation

rate.



Figure 4 Boundary conditions and meshed bus model

k-ε model was selected because of its ability to

converge to a moderately accurate result in comparatively

less time. Pressure velocity coupling was used to calculate

the pressure field. As the flow is considered as

incompressible there is no independent equation to calculate

pressure. Hence the pressure velocity coupling was used to

derive the pressure equations. The algorithm used was the

semi implicit method for pressure linked equations

(SIMPLE). This is based on the concept of mass flow

between the cells and the flow occurs when there is a

pressure difference. The method applied for the solution of

the momentum, kinetic energy and the dissipation rate was

the first order upwind method.

4.4 Results and discussion

Static pressure distribution plot (fig 5) of the bus body

at speed of 100 Km/hr reveals pressure concentration in

front region of the vehicle as the air flow strikes at the front

and brought momentarily to rest. Front mirrors show high

static pressure stagnation. Rear wall of the bus experiences

low pressure compared to the front due to the flow

separation and circulation. This pressure difference leads to

high pressure drag on the body.

Figure 5 Static pressure distribution in Pa

Figure 6 Flow circulation - mirrors and wheels

Figure 6 shows the path lines of flow around the bus

body. The flow gets stagnated at the frontal area and gets

accelerated at the front radius area. Flow separation was

observed behind the mirrors. Flow remains attached along

the sides and roof. Flow eventually gets separated at the rear

and forms vortices. Another major flow separation area

where vortices generation observed was at the rotating tyre

region.

Figure 7 Path lines of velocity magnitude

Drag force acting on the vehicle increases with the

speed of the vehicle. Front area of the bus experience

maximum pressure stagnation. A streamlined front end

which allows better air flow will effectively improve the

drag performance of the vehicle. Windshield angle, bonnet

shape, rear view mirror and radius at the front corners were

identified as areas to be improved in the new design for

better aerodynamics. The rear low pressure region created by

the flow separation must be brought to minimum for

improving the pressure drag acting on the body.

Incorporating optimum values obtained from the literature

review and features such as roof tapering, roof end lowering,

boat tailing, and radius improvements will reduce the drag of

bus. The main values obtained from the analysis which are

taken as reference for the new bus design are listed in table 3

Parameters 80Km/hr 100Km/hr 120Km/hr

Cd value 0.539 0.538 0.537

Projected area 9.47 m2 9.47 m2 9.47 m2

Pressure drag 1427.79 N 2230.94 N 3212.25 N

Skin friction drag 119.33 N 180.52 N 253.12 N

Total drag force 1547.13 N 2411.46 N 3465.38 N



Table 3 Analysis results

5. PRODUCT DESIGN SPECIFICATIONS (PDS)

PDS defines all features that must be incorporated in an

aerodynamically efficient, user friendly bus design. Final

product design specifications were derived from all the

studies and analysis done up to now.

No Parameters Specifications

1 Length 12000 mm

2 Width 2550 mm

3 Height 2600 mm

4 Wheel base 6200 mm

5 Front over hang 2590 mm

6 Rear over hang 3210 mm

7 Chassis B7R LE ( Low entry)

8 Capacity 30 + 2

9 Interior Fully Sleeper coach

10 Interior seating Foldable seats

11 Coach type Air conditioned

12 Entry height 340 mm

13 Interior height 1800 mm

14 Door position Behind front wheel

15 Door width 650 mm

16 Door height 1800 mm

17 Luggage space Middle of bus , min 5000 L

18 Side windows Electro sensitive glasses

19 Toilet system Behind drivers cabin

20

Engine

Rear-mounted, 6-cylinder, 7-litre

diesel

21 Output 213 kW (290 hp) @ 2100 rpm

22 Power Torque 1200 nm @ 1050 - 1650 rpm

23 Emissions class Euro 3

24 Gearbox 6-speed manual

24 Suspension Full Air

25 Tyres Tubeless (295/80 R 22.5")

Table 4 product design specifications

No Aerodynamic Specifications

1 Minimum front corner radius of 150 mm

2 Minimum windshield angle 15 degrees

3 Smooth and covered under body

4 Minimum trailing edge radius of 150 mm

5 Side panel tapering

6 Rear roof tapering

7 Diffuser

8 Rear view mirror elimination

9 Curved front end

10 Boat tailing

11 Roof end lowering

Table 5 Specifications for aerodynamics improvements

6. CONCEPT GENERATION

Using the data collected from the user study the target

customer selected was students and working professionals at

the age of 18 to 35.To understand the different activities in

their day to day life a lifestyle board was prepared. It was

observed that they lead an energetic, fast and fun loving life

in this age period. They never want to waste any time in

their life and were constantly on the move between college,

office, friends and family. Most of them were obsessed with

travelling, adventure trips, fast bike and cars. Most of them

travel in the intercity bus to their vacation destination or to a

busy business meeting. After the completing the journey in

the bus most of them go to office or to their planned work

just after a quick fresh up. So they don’t want to waste the

rest of the day in their busy life. This age group people have

lots of energy in them and are at the peaks of their life.

From the life style board a mood board was prepared

which shows the different emotions and situation this age

group goes through in their life. From the mood board the

most suitable theme for the bus design was found to be

adventure, fast and speed. This theme was also selected

because it directly related with the aerodynamics. A theme

board was prepared which was related to the adventure

theme. The theme board includes objects and things directly

related to speed and adventure. Nature is composed of

animals which are very fast and have good aerodynamic

shape. Even though Killer whales are fearsome animals they

are always the main attractions in big aquariums on the

vacation trips. They can move through water very easily and

are having huge body which is similar to the requirements of

an intercity bus.

The shape of the bus is derived from the killer whale

shape. Constrains listed in the PDS and aerodynamic

specifications were incorporated on the concept. Maroon

colour was selected for the exterior as it truly expresses the

adventure theme. The windshield is curved and angled. Front

area is curved to improve the flow in that area which is

derived from killer whale face. Front corner radii were kept

more than 150 mm, roof tapering, roof end lowering, side

panel tapering and boat tailing also incorporated.

Figure 8 New bus concept design



Figure 9 Comparison of new interior with the present design

Interior of the bus was designed for transporting 30

passengers, one driver and his assistant. Coats for sleeping

were arranged on each side of the gangway of width 600

mm. Each side was having two rows of beds stacked on

another. A comfortable gap of 600 mm was kept between

coats. On the left side of the gangway two beds were

arranged and right side single bed was arranged. Beads were

made foldable so that it will converts as temporary seats if

the passenger wants to sit. Interiors were made up of wood

finish materials to induce the feeling of being at home bed.

Each cabin was equipped with individual bead lamps and

fall protection guide rails. A minimum of 1800mm roof

height was maintained throughout the gangway. The luggage

space was incorporated below the coats in the middle of the

bus with a capacity of 4914 L.

A survey was conducted among the users of the

intercity bus to understand the acceptance level of the new

design. Majority of the users were found to be happy with

the new design as the problems listed by them are solved in

the new design. People were found to be excited with the

low floor bus design and the toilet system inside which was

not present in the existing intercity bus. These added features

found to attract older people as they found it difficult to

board the existing bus. Incorporation of the toilet system was

appreciated by the users as many of them ignored the bus

travel due to lack of sanitation in the present design.

7. Simulation

The new bus design was analysed for its aerodynamic

efficiency. Model was meshed using the same procedure

explained in section 3. Fluid domain of 96m x 12.75m x

13m was created around the vehicle. Area around the bus

model was meshed with fine elements and coarse elements

were used at the outer areas. 24,36,871 Unstructured

tetrahedral hybrid elements were used to mesh the fluid

domain fig 9. The only boundary condition which differs

from the baseline analysis was the length scale value which

is 0.91.Grid independent study was carried out on the modal.

It was found that a difference in 8,68,073 elements produced

a change in Cd value of 0.023 which was well within the

acceptable limits.

Figure 10 Meshed bus model

Static pressure contours of the bus model at vehicle

speed of 100 km/hr are shown in fig 11. The plot shows

considerable reduction in pressure stagnation area in the

front of the vehicle. The static pressure value also reduced

compared to the baseline model. Static pressure plot at the

rear end of the vehicle reveals an increase in pressure. The

pressure difference between the front and the rear area was

reduced which reduces the pressure drag acting on the body

Figure 11 static pressure contours in Pa

Large numbers of vortices were generated and

flow separation was observed at the rear of the baseline

model fig 12. In the new design these were brought to a

minimum value. It is evident from the fig 13 that the low

velocity area behind the new bus design is considerably

reduced due to the effect of roof tapering, roof lowering boat

tailing and diffuser at the rear. Flow is directed at the rear to

minimise the low pressure area behind the vehicle. No flow

separation is occurring at the front of the vehicle as due to

the improvement in the front corner radii and elimination of

the rear view mirror



Figure 12 Comparison of vortices at the rear

Figure 13 Velocity contours comparison

608947

13571547

2411

3465

0

500

1000

1500

2000

2500

3000

3500

4000

80 Km/hr 100 Km/hr 120 Km/hr

Vehicle speed

Dra

g f

orc

e in

N

New design

Volvo 9400

Figure 14 Drag force comparison

The coefficient of drag value of the new design is found

to be 0.296 which 44 % improvement compared to the

baseline model. Use of low floor bus chassis led to the

height reduction of bus from 3.4 m to 2.6 m which reduced

the projected area to 6.75 m2. Clear decrease in the drag

force is visible in the analysis and the total drag force is

reduced from 2411 N to 955N.This is an improvement of

60.39 %.figure 14 shows the comparison of drag force acting

on the base line model and the new design at different

vehicle speeds.

8. Conclusion

A detailed investigation of the present bus in the

field of styling and aerodynamics was carried out. The

results of these investigations were used to come up with an

aerodynamically efficient user friendly intercity bus design.

The flowing conclusions are drawn from the studies.

The present high floor sleeper coach was modified to a

low floor version with substantially reduced

aerodynamic drag.

The drag coefficient of 0.53 of present bus was found to

be reduced to 0.29 in the modified design.

The exterior was redesigned giving emphasis to both

aerodynamics and aesthetics.

The interior was also modified to improve the comfort of

the commuters.

The proposed concept was well received by the

commuters.

7. References

[1] Wolf Heinrich Hucho., (2001), “Aerodynamics of

road vehicles”, 4 th edition, SAE International,

vol.1, pp. 11-88.

[2] Fred Browand., (2005), “Reducing the drag and

fuel consumption”, Advanced transportation

workshop. October, 10 -11.

[3] Edwin. J. Asltzman and Robert. R. Meyer., (1999),

“A reassessment of heavy duty truck

aerodynamic design features and priorities”,

NASA/tp-1999-206574

[4] Mr. Ludovico Consano and Davide Lucarelli.,

(2007), “Fuel Reduction on a Tractor-Trailer Truck

at IVECO IVECO S.p.A”, 3 rd European automotive

CFD conference, EACC 2007

[5] R.. Mc. Callen, K. Salari, J. Ortega, F. Browand, M.

Hammache, T. Hsu., (2004), “Effort to Reduce Truck

Aerodynamic Drag – Joint Experiments and

Computations Lead to Smart Design”, AIAA Fluid

Dynamics Conference, June 28 – July 1,

[6] Gilhaus A., “Main parameters determining the

aerodynamic drag of buses, colloque construire avec

le vent, vol 2,

[7] Carr.G.W., (1982), “The aerodynamics of basic

shapes of road vehicles, part 1, Simple rectangular

bodies”, MIRA report No.1982/2

[8] Hucko,W.H Emmelmann. H.J (1977)

“Aerodynamiche Formoptimierung,ein weg zur

steigerung der wirtschsftlichkeit von

nutzfahrzeugen,” Series.12, NO.31 1977.

Base line – Mid plane

New design – Mid plane

exterior styling of an intercity bus for improved aerodynamics

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