MODELING AND ANALYSIS OF AERODYNAMIC BUS BODY USING CFD SOFTWARE

B.E. (2010) : SARDAR RAJA COLLEGE OF ENGINEERING

Guide: Mr.K.RAMSANKAR M.E.,	Head: Rameshkumar
Members: M.RAMESH KUMAR A.SELVARAJ A.SUNDARA MAHA LINGAM J.SURIYA NARAYANAN

To save the energy and to protect the Global environment, fuel consumption reduction is a primary concern of the modern bus manufacturers. Drag reduction is essential for reducing the fuel consumption. Designing a vehicle with a minimized Drag resistance provides economical and performance advantages. Decreased resistance to forward motion allows higher speeds for the same fuel consumption, or lower fuel consumption for the same speeds. The shape is an important factor for drag reduction. To design an efficient shape of the bus that will offer a low resistance to the forward motion, the most important functional requirement today is the low fuel consumption. The resistance, termed as the drag force (or the drag coefficient in non dimensional terms), is a strong function of the shape of the bus.  This suggests it is important how the fluid particles move about the bus and how fast they move along their path and the pressure created in the frontal area.            The main intention behind this project is to compute the pressure on medium mass vehicle by using CFD (Computational Fluid Dynamics) in ANSYS software.

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CHAPTER 1

1. INTRODUCTION

1.1. CONCEPT OF OUR PROJECT

Embracing the 21^st century in stride, virtually all manufacturers have adopted some form of computer aided design, Engineering, Manufacturing and Analysis. It is a common belief that they can stay ahead by continuously introducing new products that are differentiated by the latest Technology revolution, innovative designs, higher functionality and superior quality. The best technology to integrate expertise knowledge and companies unique practice is the CAD/CAM and analysis software packages; a comprehensive suite of solution to meet the present day demand.

In general term CAD/CAM means computer assistance whilst a designer converts his ideas and knowledge into a graphical model and graphical model into physical model. After that analyses the physical model to meet the Environmental conditions.

The automotive industry is a large user of commercial CFD packages. The advantage of CFD results in better designs and reduced time for the automotive manufacturers. CFD is not only used to improve the aerodynamics of vehicles, but also for the optimization of domains such as engine cooling, brake cooling, airbags, lighting and fuel system. During the development of new vehicles, understanding the flow phenomena and how aerodynamic forces are influenced by changes in body shape are very important. A large variety of complex flow properties such as three-dimensional turbulent boundary layer on the body surfaces, longitudinal vortices induced by three-dimensional separation, recirculation flows caused by separation and the ground plane boundary layer and their interaction are important to be understood. However, using CFD is a good way for designers to obtain results in a shorter time.

The job of the aerodynamics engineer is to create a body shape that maximizes the force in direction of the ground, the down force, and minimizes the force that opposes the movement, the drag force.

Drag Force analysis of bus comprises of following steps:

1) Geometric modeling of bus in 3D is created in PRO/E Wildfire 4

2) The model of bus is imported to FLOATRON CFD in ANSYS to analyze the bus to find out the pressure difference of air at Atmospheric temperature.

Calculate the Drag Co-efficient by using the values obtained from analysis

One of the objectives of this project is to conceive a body which is optimized to have good performance and also satisfies the regulations. The project is focused on the outer body shape of the bus. The CFD tools have been used to assess the quality of the design. The main purpose behind this project is to analyze the shape of bus in 2D and find out the drag co-efficient which will help to modify the bus shape or can help in design of new bus aerodynamically.

Indeed, it is worth noticing that the changes on the drag when changing the shape deal with pressure force. Moreover as only 30% of the Drag Co-efficient depends on the front of the shape, the slanted surfaces and vertical base surface of the rear end will contribute strongly to pressure drag.

A way to have a good working definition of what CFD is to break down the word. “CFD is the acronym of Computational Fluid Dynamics. Computational means having to do with mathematics, computation and Fluid Dynamics refers to the dynamics of things that flow.”

So, CFD is a computational technology that enables us to study things that flow. CFD not only predicts fluid flow behavior, but also the transfer of heat, mass, phase change, chemical reaction, mechanical movement and stress or deformation of related solid structures.

Industrial Impact: Aerodynamic forces on low mass vehicles and trucks are now the subjects of much interest not only to bus manufacturers but also to the U.S. government since they strongly affect fuel economy and safety.

Thus, it would be plausible to apply the techniques obtained from this project to other problems such as drag reduction for trucks.

CHAPTER 2

2. LITERATURE REVIEW

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 C_d 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. 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, 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 revealed 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. G 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 hucko and H J Emmelmann [8] found that detailed shape optimization of parts such as roof radii, rain channels, headlights will result in reduction of drag force.

CHAPTER 3

3. AERODYNAMICS AND CFD

Decreasing the fuel consumption of road vehicles, due to environmental and selling arguments reasons, concerns bus manufacturers. Consequently the improvement of the aerodynamics of bus shapes, more precisely the reduction of their drag coefficient, becomes one of the main topics of the automotive research sectors. Designing a vehicle with a minimized Drag resistance provides economical and performance advantages. Decreased resistance to forward motion allows higher speeds for the same power output, or lower power output for the same speeds.

The main motivation for reducing drag resistance is:

Ø Fuel consumption reduction and

Ø Performance increasing.

3.1 REGIMES OF EXTERNAL FLOW

Consider the external flow of real fluids. The potential flow and boundary layer theory makes it possible to treat on external flow problem as consisting broadly of two distinct regimes, that immediately adjacent to the body’s surface, where viscosity is predominant and where frictional forces are generated, and that outside the boundary layer, where viscosity is neglected but velocities and pressure are affected by the physical presence of the body together with its associated boundary layer. In addition, there is the stagnation point at the front of the body and there is the flow region behind the body (known as the wake). These flow regimes are shown in Fig.1.

Fig.1: Flow Regimes around an immersed body

The wake starts from the points “S” at which the boundary layer separation occurs. Separation occurs due to adverse pressure gradient, which combined with the viscous forces on the surface produces flow reversal, thus causes the stream to detach itself from the surface. The same situation exists at the rear edge of a body as it represents a physical discontinuity of the solid surface.

The flow in the wake is thus highly turbulent and consist of large scale eddies. High rate energy dissipation takes place there, with the result that the pressure in the wake is reduced. A situation is created whereby the pressure acting on the body (stagnation pressure) is in excess of that acting on the rear of the body so that resultant force acting on the body in the direction of relative fluid motion exerts. The force acting on the body due to the pressure difference is called pressure drag.

3.2 TOTAL DRAG

Any object that moves through a fluid (water/air) can get a decrease in form drag by streamlining. Automobiles are streamlined, which translates (allows) better gas mileage; there is less drag so less fuel is required to "push" the car forward. Buses, vans, and large trucks are less streamlined, and this is the reason why they use more fuel than smaller, streamlined cars (weight is another reason).

The drag is a resistance force. This force works to slow the forward motion of an object, including planes. There are four types of drag: friction drag, form drag, induced drag and wave drag. These drag types develop around the shape of the body, the smoothness of the surfaces, and the velocity of the plane. All four sum together for the Total drag force. The total drag on the body, often called profile drag is therefore, made up of two contributors namely the pressure drag and the friction drag. The drag forces are the opposite of thrust. If the thrust force is greater than the drag force, the vehicle goes forward, but if the drag force exceeds the thrust, the vehicle will slow down and stop.

The friction drag is sometimes also called the skin friction drag. The force on the body acting in the direction of relative motion due to fluid shear stress is known as Frictional drag. Thus in external flow the immersed body is subjected to functional drag over its entire surface. Fig.2 shows the situation where air flowing along a surface will create lot of friction drag. There is a large fast-moving air next to the non-moving surface. In contrast, there will be little pressure drag because there is very little frontal area for anything to push against.

Fig.2: Friction Drag Airflow Orientation

The form drag, or pressure drag as it is sometimes called, is directly related to the shape of the body of the vehicle. Fig.3 shows the situation where air flowing along a surface will create lots of friction drag.

Fig.3: Pressure Drag Airflow orientation

3.3. MINIMIZING DRAG ON A MEDIUM MASS VEHICLE (BUS)

There are several ways to reduce the C_d of a vehicle. The vehicle’s global shape has the most direct influence in the final value of C_d . Still, improvement in details like the rearview mirrors, underbody, or the use of some particular devices may contribute to a lower final C_d.

3.4 EXPRESSION OF DRAG FORCE

The Drag force value for a moving vehicle is given by the following expression:

(1)

This equation shows important point aerodynamic forces are proportional to the square of the speed. That means you quadruple the drag or lift when you double the speed. Since speed is never the item that is pretended to be decreased and the density of air is not even possible to change, the aerodynamic resistance can only be reduced or minimized by manipulating the vehicle’s design characteristics, obtaining lower C_d, or even reducing the vehicle’s frontal area.

3.5 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 will lead to a successful vehicle with low fuel consumption. 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 CFD and improvements in drag values were predicted.

3.5.1. Bus Aerodynamics

Drag force acting on the bus body is given by the equation (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.

Ø Luggage space reduction

Ø Large wheel arches reduces the number seats

Ø Divides the floor area in to two decks

Ø Less space for fuel tanks leads to low capacity

Ø Difficult to reach the engine compartment for repairs

Ø Psychologically people like t sit at high floor than the low floor area

Fig 4: Types of chassis

3.5.2. Shape of the vehicle’s body

The vehicle’s global body shape states the average aerodynamic performance, so it is the most important design feature. The main requirement is that the shape should maintain attached flow over most of the surface. This is accomplished by having a streamlined shape. The most efficient shapes are the simple ‘teardrop’ or the airfoil based one.

Fig.5: Shape of Vehicle’s Body

Unfortunately, these two body shapes are very hard to put in practice because they have very little or sometimes non acceptance by the customers. Furthermore, these shapes tend to cause obstacles to some functional aspects of the vehicles, like storing space, and dynamic stability.

3.5.3. Stream-lined body and bluff body

A bluff body is a body where the boundary layer separates permanently from the surface. This is in contrast to a streamlined body where the boundary layer either does not separate, or possibly separates temporarily, and reattaches further downstream.

A stream lined body is defined as that body whose surface coincides with the stream lines, when the body is placed in a flow. In this case the separation of flow will take place only at the trailing edge (or rearmost part of the body). Though the boundary layer will start at the leading edge, will become turbulent from laminar, yet it does not separate up to the rearmost part of the body in case of stream-lined body.

Bluff body is defined as that body whose surface does not coincide with the streamlines, when placed in a flow, then the flow is separated from the surface of the body much ahead of its trailing edge with the result of a very large wake formation zone. Then the drag due to pressure will be very large as compared to the drag due to friction on the body. Thus, the bodies of such a shape in which the pressure drag is very large as compared to friction drag are called bluff bodies.

Fig.6: Stream-lined Body and Bluff Body

When the drag is dominated by viscous drag, we say the body is streamlined, and when it is dominated by pressure drag, we say the body is bluff. Whether the flow is viscous-drag dominated or pressure-drag dominated depends entirely on the shape of the body. A streamlined body looks like a fish, or an airfoil at small angles of attack, whereas a bluff body looks like a brick, a cylinder, or airfoil at large angles of attack. For streamlined bodies, frictional drag is the dominant source of air resistance. For a bluff body, the dominant source of drag is pressure drag.

Since flow separation depends on many parameters in addition to pressure gradient, bluff body flows are often somewhat unpredictable. There can be sudden changes in flow pattern for relatively minor changes in geometry or orientation. It is also possible to get flows in which two relatively stable states exist; the flow can switch between them as a result of minor external effects, either on a random or a periodic basis. This may sometimes involve reattachment of flow. After the boundary layers have separated, they are known as free shear layers, or sometimes dividing or separating streamlines.

The region of flow outside the free shear layers is known as the free stream, while that between them is the wake. The part of the body upstream of the separation points, exposed to the free stream flow is known as the fore body. That after them, exposed to the wake, is the after body or base.

3.5.4. Drag

For bluff bodies where the drag is mainly due to direct stresses, the drag coefficient is defined in terms of a dimension normal to the flow. This is in contrast to streamlined bodies where the drag is mainly due to shear stress, where a dimension parallel to the flow is used. This is discussed further in the Appendix on drag coefficients and their definitions.

The drag is sometimes divided into fore body drag, due to the pressure distribution around the front, and base drag, due to that round the back. This is useful because, by and large what goes on in the wake does not depend to any extent on the fore body, provided separation occurs at the same place, and similarly the flow over the fore body is not influenced much by changes in the base region.

Typical values for drag coefficient are:

Table 1: C_dfor various objects

PARTICULARS	VALUES OF C_d
Circular cylinder normal to stream	0.35 to 1.2
Flat plate normal to stream	2.0
Rectangular section	0.9 to 3
Sphere	0.1 to 0.4
Disk normal to stream	1.2
Cube, face on to stream	1.1
Saloon car	0.35
Articulated container truck	0.7

3.5.5. Components of drag

There are mainly four components of drag. These are:

1. Driving Force

2. Resistance Force

3. Weight

4. Lift/Reaction Force.

1. Thrust/Driving Force: - Driving Force is the Force Developed by the motion of the vehicle in forward Direction.

2. Drag/Resistance Force: - Resistance Force is the resistance developed by the motion of the air in the opposite direction of the motion of the vehicle.

3. Weight: - Weight is nothing but the weight of the vehicle itself, acting in the downward direction.

4. Lift/Reaction Force: - Lift is upward force acting on the vehicle. It acts on both the front and rear wheels of the vehicle. It is induced due to the vortices created on the back of the vehicle and it is the reason that the lift on the rear wheel is more than the lift on the front wheel.

3.6. COMPUTATIONAL FLUID DYNAMICS

3.6.1. Introduction

So, CFD is a computational technology that enables you to study things that flow. CFD not only predicts fluid flow behavior, but also the transfer of heat, mass, phase change, chemical reaction, mechanical movement and stress or deformation of related solid structures.

Computational Fluid Dynamics or simply CFD is concerned with obtaining numerical solution to fluid flow problems by using computers. The advent of high-speed and large-memory computers has enabled CFD to obtain solutions to many flow problems including those that are compressible or incompressible, laminar or turbulent, chemically reacting or non-reacting. Computational Fluid Dynamics (CFD) is the science of predicting fluid flow, heat transfer, mass transfer, chemical reactions, and related phenomena by solving the mathematical equations which govern these processes using computational methods.

CFD is the art of replacing the differential equation governing the Fluid Flow, with a set of algebraic equations (the process is called discretization), which in turn can be solved with the aid of a digital computer to get an approximate solution. The well known discretization methods used in CFD are, Finite Volume Method (FVM), Finite Element Method (FEM), and Boundary Element Method (BEM).

3.6.2. The benefits of CFD

Insight: There are many devices and systems that are very difficult to prototype. Often, CFD analysis shows parts of the system or phenomena happening within the system that would not otherwise be visible through any other means. CFD gives a means of visualizing and an enhanced understanding of the various designs.

Foresight: Because CFD is a tool for predicting what will happen under a given set of circumstances, it can answer many ‘what if?’ questions very quickly. We give it variables. It gives us outcomes. In a short time, we can predict how the design will perform, and many variations may be tested until you arrive at an optimal result. All of this is done before physical prototyping and testing. The foresight we gain from CFD helps us to design better and faster.

Efficiency: Better and faster design or analysis leads to shorter design cycles. Time and money are saved. Products get to market faster. Equipment improvements are built and installed with minimal downtime. CFD is a tool for compressing the design and development cycle.

3.6.3. Applications of CFD

CFD is interdisciplinary cutting across fields of aerospace, mechanical, civil, chemical, electrical engineering as well as physics and chemistry. CFD has been widely used in industry in the past decade. It is certainly fun for fluids enthusiasts, but where exactly can CFD be applied - the following are areas of applications of CFD to date.

Ø Automobile and Engine

Aerodynamics, Engines, Turbochargers, Intake/Exhaust Heating/Cooling

Systems, Brakes etc.

Ø Industrial Manufacturing

Aerospace, Aerodynamics. Gas Turbines, Rockets etc.

Ø Mechanical

Pumps, Compressors, Heat Exchangers, Furnaces, Nuclear Reactors etc.

Ø Chemical

Mixers (multiphase), Chemical Reactors, Separators, Boilers, Condensers etc.

Ø Environmental Engineering

Weather prediction, River and Tidal flows, Wind and Water-borne pollution,

Fire and Smoke spread, Wind loading etc.

Ø Physiological

Cardiovascular flows (Heart, major vessels), Flow in Lungs and breathing passages etc.

Ø Naval Architecture

Ship building etc.

CHAPTER 4

4.MODELING USING PRO/E WILDFIRE 4 SOFTWARE

4.1. BUS BODY PART MODEL USING PRO/E WILDFIRE 4

It scaled 3-D model of bus with its actual dimensions in table 2, by using the CAD software. We know the outer dimensions of the bus then the model is modeled. It found out the external shape of the bus. After, for the analysis the 2D format is a compatible one to view the result. After the bus model is converted in to IGES (.igs) format, this file can be used in the CFD software to analyze the external shape of the vehicle for the analysis. The specifications of aerodynamic bus is in table 3.

Table2: Bus specifications

S.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	Engine placement	Rear mounted
8	Output	213 kw(290hp)

Table 3: Aerodynamic specifications

S.NO	AERODYNAMIC SPECIFICATIONS
1	Minimum front corner radius of 150 mm
2	Minimum wind shield angle 15 degrees
3	Smooth and covered under body
4	Minimum trailing edge radius of 150mm
5	Side panel tapering
6	Rear roof tapering
7	Curved front end
8	Roof end lowering

4.2. ORDINARY BUS

Fig 7: Volvo bus model

4.3. AERODYNAMIC BUS

Fig 8: Aerodynamic bus model

CHAPTER 5

5.FLOTRAN CFD

Flotran CFD is the essential technique to solve fluid dynamics problems. The two dimensional flotran element (2D flotran 141) was selected to solve the problem of our content.

5.1. BOUNDARY CONDITIONS

Boundary conditions were applied on the meshed model using the flotran CFD preprocessor. 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 60, 80, 100 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 4

Table 4: boundary conditions

BOUNDARY	BOUNDARY CONDITIONS	VALUES
Inlet	Constant velocity Turbulent intensity Length scale	V=16.66 m/s V=22.22m/s V=27.77m/s
Outlet	Pressure outlet	Constant pressure= 0 N/m²
Road	Moving wall No slip	V=16.66 m/s V=22.22m/s V=27.77m/s
Bus body	No slip – stationary wall	-
Domain top and side	Stationary wall Specified shear	Shear stress = 0

5.2 TURBULENT MODEL

The solver used for the analysis was flotran cfd 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. This model assumes the flow to be fully turbulent and is based on the turbulence kinetic energy and its dissipation rate.

CHAPTER 6

6. CFD RESULTS AND DISCUSSION

Fig 9(a) : Pressure distribution of volvo at 60 Kmph

Fig 9(b) : Pressure distribution of Aero at 60 Kmph

Fig 9(c) : Graph for Pressure distribution of volvo at 60 Kmph

Fig 9(d) : Graph for Pressure distribution of Aero at 60 Kmph

Fig 10(a) : velocity distribution of volvo at 60 Kmph

Fig 10(b) : Velocity distribution of Aero at 60 Kmph

Fig 10(c) : Graph for velocity distribution of volvo at 60 Kmph

Fig 10(d) : Graph for Velocity distribution of Aero at 60 Kmph

Fig 11(a) : Pressure distribution of volvo at 80 Kmph

Fig 11(b) : Pressure distribution of Aero at 80 Kmph

Fig 11(c) : Graph for Pressure distribution of volvo at 80 Kmph

Fig 11(d) : Graph for Pressure distribution of Aero at 80 Kmph

Fig 12(a) : velocity distribution of volvo at 80 Kmph

Fig 12(b) : Velocity distribution of Aero at 80 Kmph

Fig 12(c) : Graph for velocity distribution of volvo at 80 Kmph

Fig 12(d) : Graph for Velocity distribution of Aero at 80 Kmph

Fig 13(a) : Pressure distribution of volvo at 100 Kmph

Fig 13(b) : Pressure distribution of Aero at 100 Kmph

Fig 13(c) : Graph for Pressure distribution of volvo at 100 Kmph

Fig 13(d) : Graph for Pressure distribution of Aero at 100 Kmph

Fig 14(a) : velocity distribution of volvo at 100 Kmph

Fig 14(b) : Velocity distribution of Aero at 100 Kmph

Fig 14(c) : Graph for velocity distribution of volvo at 100 Kmph

Fig 14(c) : Graph for Velocity distribution of Aero at 100 Kmph

6.1.CFD RESULT DISCUSSION

VOLVO BUS

The figures of 9(a),11(a),13(a) are show the pressure distribution of the volvo bus in various speeds. It shows the pressure act on the body is high at maximum frontal area.

The figures of 10(a),12(a),14(a) are show the velocity distribution of the volvo bus in various speeds. It shows the velocity distribution over the body.

The graphs of 9(c),11(c),13(c) are show the pressure with respect to distance. It shows the pressure values at the frontal area nodes.

The graphs of 10(c),12(c),14(c) are show the velocity with respect to distance. It shows the velocity values at the frontal area nodes.

AERODYNAMIC BUS

The figures of 9(b),11(b),13(b) are show the pressure distribution of the volvo bus in various speeds. . It shows the pressure act on the body is high at maximum frontal area.

The figures of 10(a),12(a),14(a) are show the velocity distribution of the volvo bus in various speeds. It shows the velocity distribution over the body

The graphs of 9(d),11(d),13(d) are show the pressure with respect to distance. It shows the pressure values at the frontal area nodes.

The graphs of 10(d),12(d),14(d) are show the velocity with respect to distance. It shows the velocity values at the frontal area nodes.

CHAPTER 7

7. CALCULATION

Table 5: Pressure values from CFD results

	Pmin			Pmax
MODEL	60kmph	80kmph	100kmph	60kmph	80kmph	100kmph
VOLVO	-234.49	-421.689	-541.172	96.195	172.105	269.172
AERO	-32.145	-57.66	-102.76	36.5	65.665	102.917

Pressure values are in N/mm²

7.1. FORMULAS

Drag Force is given by the equation (1)

Coefficient of drag,
(2)

Pressure difference is given by

(3)

7.2. VOLVO BUS

At 60 kmph

Pmin = -234.49N/m² ; Pmax = 96.195N/m²

Substitute the values in the equation (3) to find out the pressure difference,

= 96.195-(-234.49)

∆P=330.685 N/m²

To find out the value of Coefficient of drag , Substitute apply the values in equation (2),

                                   C_d= 2.047

To find out the drag force , Substitute the values in equation (1)

.
Fd₌2529.6N

Similarly,

C_d= 2.006 and Fd₌4541.53 Nat 80kmph

C_d= 1.806 and Fd₌6200.88 N at 100kmph

Average drag coefficient and drag force is 1.953 and 4424 N respectively.

7.3. AERODYNAMIC BUS

At 60 kmph

Pmin = -32.145 N/m² ; Pmax = 36.5N/m²

Substitute the values in the equation (3) to find out the pressure difference,

= 36.5-(-32.145)

∆P=68.9145 N/mm²

To find out the value of Coefficient of drag , Substitute apply the values in equation (2),

C_d= 0.426

To find out the drag force , Substitute the values in equation (1)

=
350.95 N

Similarly,

C_d= 0.4289 and Fd₌630.81 N at 80kmph

C_d= 0.458 and Fd₌1048.35 N at 100kmph

Average drag coefficient and drag force is 0.437 and 676.77 N respectively

7.4. RESULT COMPARISION

Table 5: Result comparison of values of coefficient of drag and drag force

	60 kmph		80 kmph		100 kmph
MODEL	C_d		C_d		C_d
VOLVO	2.047	2529.6	2.006	4541.53	1.806	6200.88
AERO	0.426	350.95	0.4289	630.81	0.458	1048.35

From the above results the percentage of drag force reduction from Volvo 9400 to the aerodynamic bus body is approximately 65 %

CHAPTER 8

8. CONCLUSION

A detailed investigation of the Volvo 9400 bus body and aerodynamic bus body 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 bus body design.

The following conclusions are drawn from this study.

Ø Bus body was modified to a low floor version with substantially reduced aerodynamic drag.

Ø The drag coefficient 1.953 of Volvo bus body was found to be reduced as 0.437 in the modified bus body design.

Ø The exterior body was redesigned giving emphasis to both aerodynamics and aesthetics.

1. The fuel consumption is a reduced one in the aerodynamic bus than the Volvo 9400 bus for the same speed.

9. 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.

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