GROUND VEHICLE AERODYNAMICS

M59MAE GROUND VEHICLE AERODYNAMICS

C O V E N T R Y U N I V E R S I T Y

M S C A U T O M O T I V E

E N G I N E E R I N G 2 0 1 4 - 1 5

This document contains the Wind Tunnel Test

results of the given Lynne Turner Model. A CFD

analysis using STAR CCM+ was conducted on the

model and a detail description of both the test

results has been illustrated in this report.

Imran S. Rajela – ( 5914621)


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1 M59MAE- GROUND VEHICLE AERODYNAMICS

Table of Contents 1. INTRODUCTION .................................................................................................................................................................. 6

2. LITERATURE REVIEW ...................................................................................................................................................... 7

2.1. BERNOULLI’S PRINCIPLE ....................................................................................................................................... 7

2.2. REYNOLDS NUMBER ................................................................................................................................................ 8

2.3. Mach number ............................................................................................................................................................... 8

2.4. DRAG & LIFT FORCES ............................................................................................................................................... 9

3. INFLUNCE OF AIR FLOW AROUND GROUND VEHICLE ...................................................................................... 9

3.1 INTERNAL FLOW....................................................................................................................................................... 10

3.2 EXTERNAL FLOW ...................................................................................................................................................... 10

3.3. BOUNDARY LAYER .................................................................................................................................................. 10

3.4 LAMINAR BOUNDARY LAYER FLOW ................................................................................................................ 11

3.5. TURBULENT BOUNDARY LAYER FLOW ......................................................................................................... 11

4. METHODOLGY .................................................................................................................................................................... 12

4.1. Wind Tunnel Analysis ............................................................................................................................................. 12

Advantages of a closed type wind tunnels......................................................................................................... 13

Disadvantages of a closed type wind tunnel ..................................................................................................... 13

4.3. WIND TUNNEL TEST: ............................................................................................................................................. 13

4.4. Blockage ratio calculation for wind tunnel: .................................................................................................. 14

5. CFD ANALYSIS: .................................................................................................................................................................. 15

K-EPSILON TURBULANCE MODEL: ...................................................................................................................... 15

Advantages of K- Epsilon Turbulence model: .................................................................................................. 15

Disadvantage of K-Epsilon Turbulence model: ............................................................................................... 15

Reynolds Average Navier Stoke ............................................................................................................................. 15

WALL Y+: ......................................................................................................................................................................... 16


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5.1. Simulation setups: .................................................................................................................................................... 17

5.1.1. Blockage ratio calculation for CFD wind tunnel setup: .................................................................... 17

5.1.2. Pc Performance Parameter Consideration ................................................................................................. 18

5.1.3. Mesh parameters: ................................................................................................................................................. 18

5.2. PHYSICS MODELS: ................................................................................................................................................... 20

6. WIND TUNNEL RESULTS AND DISCUSSIONS: ..................................................................................................... 20

Coefficient of Drag: ...................................................................................................................................................... 20

Coefficient of Lift: ......................................................................................................................................................... 21

Coefficient of Side Force: .......................................................................................................................................... 21

Pitching Moment Coefficient: .................................................................................................................................. 21

Rolling Moment Coefficient: .................................................................................................................................... 21

Yawing Moment Coefficient: ................................................................................................................................... 22

Calculating Reynolds Number: ............................................................................................................................... 22

7.2. Wind tunnel data plots: ......................................................................................................................................... 22

7.2.1. Force coefficients vs Yaw angle: ................................................................................................................ 24

7.2.2. Moment Coefficient vs Yaw angle: ............................................................................................................ 25

7.2.3. Force and moment coefficients vs slant angle: .................................................................................... 26

8. CFD ANALYSIS RESULTS AND DISCUSSIONS: ...................................................................................................... 27

8.1. GRID INDEPENDENCY STUDY: ........................................................................................................................... 27

8.2. CFD RESULTS VS WIND TUNNEL TEST RESULTS: ..................................................................................... 28

8.3. POST PROCESSING................................................................................................................................................... 30

Cell Relative Velocity ........................................................................................................................................ 30

Pressure Coefficient. ......................................................................................................................................... 30

Wall Y+ .................................................................................................................................................................... 31

Iso-Pressure Coefficients ................................................................................................................................ 32


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Velocity magnitude ............................................................................................................................................ 33

10. CONCLUSION .................................................................................................................................................................... 34

Bibliography ............................................................................................................................................................................ 35

11. APPENDIX ......................................................................................................................................................................... 37

List of figures Figure 1 -Air Flow Around Vehicle ............................................................................................................... 6

Figure 2- BOUNDARY LAYERS ...................................................................................................................... 11

Figure 3- WIND TUNNEL MODEL WITH OPEN SECTION AND CLOSED LOOP .............................................. 12

Figure 4 - Image of Test Piece ..................................................................................................................... 13

Figure 5-Wall Y+ (CD-ADAPCO, 2012) ......................................................................................................... 16

Figure 6Pc Performance Graph (CD-ADAPCO, 2012) .................................................................................. 18

Figure 7- CFD Mesh model .......................................................................................................................... 19

Figure 8- Mesh Properties ........................................................................................................................... 19

Figure 9- Drag force Vs Re ........................................................................................................................... 22

Figure 10-Lift force Vs Re ............................................................................................................................ 23

Figure 11-Pitching coefficient Vs Re ........................................................................................................... 23

Figure 12-force coefficient Vs Yaw Angle ................................................................................................... 24

Figure 13- Moment Coefficient Vs Yaw angle ............................................................................................. 25

Figure 14-force coefficient Vs Slant Angle .................................................................................................. 26

Figure 15-Moment Coefficient Vs Slant angle ............................................................................................ 26

Figure 16: Coefficient of drag for 19 degree ........................................................................................... 37

Figure 17: Drag Force for 19 degree ........................................................................................................ 38

Figure 18: Iso-Pressure Coefficient for 19 degree .................................................................................. 38

Figure 19: Coefficient of lift for 19 degree............................................................................................... 39

Figure 20: Lift force for 19 degree ........................................................................................................... 39

Figure 21: Mesh Scene for 19 degree ....................................................................................................... 40

Figure 22: Pressure Coefficient for 19 degree ........................................................................................ 40

Figure 23: Cell Relative velocity for 19 degree ....................................................................................... 41

Figure 24: Velocity Magnitude for 19 degree .......................................................................................... 41

Figure 25: Velocity for 19 degree ............................................................................................................. 42

Figure 26: Wall Y+ for 19 degree ............................................................................................................. 42

Figure 27: Total Pressure coefficient for 25 degree ............................................................................... 43

Figure 28: Coefficient of lift for for 25 degree ......................................................................................... 43

Figure 29: Lift force for 25 degree ........................................................................................................... 44


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Figure 31: Residuals for 25 degree .......................................................................................................... 45

Figure 32: Velocity Magnitude for 25 degree .......................................................................................... 45




Figure 36: Cell relative velocity for 25 degree ........................................................................................ 47




Figure 40: Velocity magnitude for 31 degree .......................................................................................... 49

Figure 41: Velocity for for 31 degree ....................................................................................................... 50



Figure 44: Drag force for 31 degree ......................................................................................................... 51


Figure 46: Coefficient of Lift for 31 degree ............................................................................................. 52

Figure 47: Lift Force for 31 degree .......................................................................................................... 53




Figure 51: Velocity magnitude for 37 degree .......................................................................................... 55



Figure 54: Drag Coefficient for 37 degree ............................................................................................... 58


Figure 56: pressure coefficients for 37 degree ....................................................................................... 59

Figure 57: Lift Coefficient for 37 degree .................................................................................................. 59

Figure 58: Lift Force for 37 degree .......................................................................................................... 60






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Abstract

In the present time of modern computer age, a computation fluid dynamic analysis has

surfaced as a successful substitute of actual wind tunnel test because of multiple reasons of cost,

time and resources available. The development of super computers and modern networking

architecture supporting the high tech processors, Computational Fluid Dynamics has gained a lot of

popularity.

This report focusses on the aerodynamic investigation of various forces acting on the

Lynne Turner model of the car provided in this coursework. The magnitude and impact of the

forces had been analyzed through a wind tunnel test and a simulation through the computational

fluid dynamics software called STAR CCM+.

In the following report a brief description of the wind tunnel test, Computational fluid

dynamics, types of flows and boundary layer is given along with the comparison of results of CFD

analysis and actual wind tunnel test. Remaining part of the report provides a detailed description of

the test set up, its methodology and physical conditions of the wind tunnel. The report also

provides information about the wind tunnel testing procedure and CFD analysis procedure. The

comparison of the test results has been done in the later part of report by the help of suitable graph

and excels data spreadsheet


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1. INTRODUCTION

The term aerodynamics refers to the study of flow of air around

any given body (car in this case). It is one of the most sensitive areas of the vehicular development

as it incorporates both the performance and design. Aerodynamics has been closely associated

more with performance than style. Thus it becomes very important for the designers to come up

with a design which is aerodynamically successful and easy to manufacture. Years of research and

performance valuation has shown that aerodynamically efficient vehicles provides better driving

experience, improved performance at high speeds and cornering and a better fuel economy. Hence,

on considering the aforementioned, aerodynamic performance consideration of the vehicle

becomes very important.

Through this report the objective of measurement of aerodynamic forces, the

moments & their coefficients on Lynne turner model has been accomplished and interpreted

through the CFD analysis and its comparison with the actual wind tunnel test. The benchmark study

of the influence of yaw angle and Reynolds number on the aerodynamic has also been described in

the following report

Figure 1 -Air Flow Around Vehicle


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According to (Hucho, 2013) a moving vehicle is subjected to the flow of air which can be classified

as following –

Air Flow around the vehicle

Air Flow through the body

Air flow within the machine/vehicle’s engine and transmission

The study of above given parameters is imperative while studying the aerodynamic analysis of

the vehicle. The vehicle is also subjected externally to air. The external exposure of the air has a

direct impact on the performance of the vehicle and its directional stability.

2. LITERATURE REVIEW

The fluid flow in the aerodynamic analysis study is based on certain principles which are

discussed below.

2.1. BERNOULLI’S PRINCIPLE

According to the Bernoulli’s principle for a fluid flow without any viscosity, any

increase in the speed of the fluid occurs at the same time of decrease in the fluid’s potential energy.

This principle is applicable in various fluid flow applications and is valid for incompressible flows.

Bernoulli’s principle can be also be derived from the conservation of energy theory, according to

which sum of all the energy in a fluid along a streamline is same at all points. This can also be

represented by the help of following equation (Princeton uiversity, n.d.)-

𝒑 +𝟏

𝟐𝝆𝒗𝟐 + 𝝆𝒈𝒉 = 𝒄𝒐𝒏𝒔𝒕𝒂𝒏𝒕

Where, p= pressure v= velocity g= gravitational acceleration h = Elevation.


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2.2. REYNOLDS NUMBER

In simple terms Reynolds number is a dimensionless quantity which is used in the

determination of flow patterns at different points/stages during any flow. It can also be expressed

as the ratio of inertia forces to viscous forces (NASA, 2014). Reynolds number is very useful in the

study of fluid flow dynamics. It is sometimes used to classify the difference between laminar and

turbulent flow. Laminar flow occurs at Lower values of Reynolds Number which consists of air

molecules in smooth and continuous motion and Turbulent flow occurs at higher values of

Reynolds characterized by air molecules in random/distorted motion. (NASA, 2014)

In the aerodynamic analysis of any body, the boundary layer and its thickness are

one of the key factors which govern the drag and lift on the body. Reynolds number holds a relation

between the values of density, size and the viscosity of the flow which is given in the form of an

equation-

𝑹𝒆 =𝝆 ∗ 𝑽 ∗ 𝑳

𝝁

Where, ρ – Density of air

V – Velocity of air L – Length of vehicle 𝜇 – Dynamic viscosity of air.

2.3. Mach number

Mach number is defined as the ratio between the velocity of air and velocity of sound. The

Mach number is used to find the state of the flow whether it should be considered as compressible

flow or an incompressible flow. (Atlee M. Cunningham, 1987)

𝑴𝒂𝒄𝒉 𝒏𝒐 = 𝒖

𝑪

Where, u - Air flow velocity C – Speed of sound.


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M < 1 Subsonic

M = 1 Transonic

1< M < 3 Supersonic

M > 5 Hypersonic

Table 1- MACH NUMBER (Benson, 2014)

Calculations: Velocity of air = 45 m/s Speed of sound = 330 m/s

𝑀 = 45

330

𝑴 = 𝟎. 𝟏𝟑𝟔

(M = 0.136) < 0.2, this is subsonic. Therefore it can be consider as an incompressible flow.

2.4. DRAG & LIFT FORCES

Drag is force which is created by an object moving in the parallel direction of the

fluid flow. It is an inevitable consequence of the flow of any liquid. Drag is of two types form drag

which is dependent on the shape of the object and Induced Drag which is dependent on the viscous

friction between the object and fluid flow.

Lift is a downward acting force which generated in the perpendicular direction of

the travel of the object moving through a fluid. (Zuo, n.d.)

3. INFLUNCE OF AIR FLOW AROUND GROUND VEHICLE

The air flow on the body of a vehicle is a very important factor to be considered

while performing the aerodynamic analysis. The flow of air during the study of an aerodynamics

analysis can be of two types internal and external. The internal flow of air is the flow of air inside

the engine and transmission whereas the external flow of air is basically the flow of air outside the

body of a vehicle.


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3.1 INTERNAL FLOW

Internal flow of air is the flow inside the car’s body, around the engine and

transmission lines. According to (Hucho, 2013) the internal flow cannot be divided into viscous

boundary layer flow close to the walls of flow and far away from the walls of flow. This viscous

boundary layer flow is characterized by Reynolds Number.

3.2 EXTERNAL FLOW

The external flow is the flow of air outside the body of car. At a constant velocity of

air when no separation of the layer of air is taking place, the viscous effects of the flow is restricted

inside that layer. But at higher velocities the boundary layer gets imposed by a higher pressure

layer which gets separated from the surface of the car due to high velocities. At the point where the

flow separation takes place the boundary layer gets dispersed. The separation of boundary layer

takes place generally at the end/rear part of vehicle. (Hucho, 2013)

At some distance from the vehicle the difference between the velocity of air and the

free stream becomes zero because of which there exists no boundary layer. This point is very

important considering the fact that aerodynamic analysis of a vehicle depends completely on the

formation of a boundary layer.

3.3. BOUNDARY LAYER

Boundary layer is a layer which is formed right next to the surface of the subject body where the

effect of viscosity is imperative. The boundary stars growing from zero at the beginning/start of the

body under study. As the flow velocity increases the friction between surface and the fluid layer

increases which results in the growth of boundary layer. As the thickness of the boundary layer

increases the velocity gradient starts diminishing. The molecules of the air flow are in rapid motion

at the top end of the boundary layer as compared to the molecules near to the surface of the vehicle.

The flow outside the boundary layer constantly tries to change the shape of boundary layer because


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of high velocity gradient. (Karthik, 2011)

Figure 2- BOUNDARY LAYERS

3.4 LAMINAR BOUNDARY LAYER FLOW

In a boundary layer at the initial stages laminar flow exists in which the flow of air is

uniform. This flow is usually present in the front of the vehicle which causes the layers of air to slide

over each other. It’s because of this fact the skin friction drag is least in the front of car. (Karthik,

2011)

3.5. TURBULENT BOUNDARY LAYER FLOW

As the boundary layer grows the flow of layers of air gains velocity and instability in

the layer starts developing. This results in the transition from laminar to turbulent flow. In this

layer the flow is usually stuck to the stuck of the subject body and is in a stream lined shape. The

skin friction drag increases with the turbulent layer because of the motion of the air particles.

(Karthik, 2011)


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4. METHODOLGY

There are mainly three methods for studying the air flow around the vehicle.

Driving on-road

Wind tunnel testing

CFD Analysis.

4.1. Wind Tunnel Analysis

A wind tunnel is the testing ground of aerodynamic properties of any

subject body. With increase in the demand of highly efficient vehicles in terms of aerodynamic

performance, fuel consumption and driving experience it has become a matter of immense

importance that all sorts of real time testing possible is done on the vehicle prior to the production.

A wind tunnel is usually a circular shape passage wherein a vehicle is mounted in the middle

against the flow of air. The flow of air created by the help of powerful fans and they are configured

accordingly to support the required amount of air pressure. The wind tunnels can be either closed

type or open type. The wind tunnel used in this coursework is a closed type because it offers a great

amount of energy conservation and sustenance in the wind pressure. Though one of the problem

associated with such types of wind tunnels is the high maintenance cost of the fans and cooling

utilities to cool the motors.

Figure 3- WIND TUNNEL MODEL WITH OPEN SECTION AND CLOSED LOOP


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Advantages of a closed type wind tunnels

Following are the advantages of a closed type wind tunnel (NASA, 2014)

It offers a superior flow in the test facility.

It has a low cost of operation.

The entire operation is generally quite.

Disadvantages of a closed type wind tunnel

Following are the disadvantages of a closed type of wind tunnel (NASA, 2014)

The cost of setting up of test facility is higher.

This type of design is not suitable for smoke and emission visualization.

During the course of testing the temperature tends to rise because of constant air flow.

4.3. WIND TUNNEL TEST:

The wind tunnel test section used for the testing is a closed loop open section wind

tunnel. The Lynne Turner model is fastened to the two pole balancing strut on the test table. The

force acting on the models are read by the moments acting on the balancing strut.

Figure 4 - Image of Test Piece

The wind tunnel test experiment is conducted by the following procedure;

The base line model is fasted to the balancing strut with 0 yaw angle; the model is exposed

to different velocities of 15, 25, 35 and 45m/s respectively. The corresponding forces,

moments and its respective coefficients are noted through the computerized output.


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The same procedure is followed for the same model with different yaw angles of 2, 4, 6,

8 and 10 respectively for a constant velocity of 45m/s. The corresponding computerized

outputs are noted.

Keeping the velocity at a constant 45m/s the Lynne Turner model is configured for

different slant angles as specified below.

S.NO Air velocity

(m/s)

Diffuser angle (Deg) Boat-tail angle (Deg) Slant angle (Deg)

1 45 0 0 0

2 45 0 0 19

3 45 0 0 25

4 45 0 0 31

5 45 0 0 37

Table 2-Wind tunnel experimental Input Datas

4.4. Blockage ratio calculation for wind tunnel:

The wind tunnel has some limitations when compared to a real road, its dimension

of frontal area influence the flow velocity over the body of the vehicle which is subjected to air flow.

According to Bernoulli’s equation for a constant density, the velocity of air changes with area. Thus

the recruitment of blockage correction is crucial for the accuracy of results.

Blockage ratio of the wind tunnel is a percentage value that gives out the ratio of

frontal area of car to the frontal area of wind tunnel. The blockage ratio calculation is important for

the scaling of the model, depending on the available wind tunnel test section. (CD-ADAPCO, 2012)

Blockage Ratio = 𝑭𝒓𝒐𝒏𝒕𝒂𝒍 𝒂𝒓𝒆𝒂 𝒐𝒇 𝑪𝒂𝒓

𝑨𝒓𝒆𝒂 𝒐𝒇 𝒘𝒊𝒏𝒅 𝒕𝒖𝒏𝒏𝒆𝒍 𝑶𝒖𝒕𝒍𝒆𝒕 x 100

Frontal area of Car = 0.0292m2

Area of wind tunnel outlet = 1.3m x 1m

=1.3m2

Blockage Ratio = 0.029

1.3 x 100 =2.23


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5. CFD ANALYSIS:

Since the On-road testing and Wind tunnel testing were proved to be a tedious and a

costly method of finding the air flow around the vehicle. CFD was introduced to find the air flow

around the vehicle by mathematical computational equations which were comparatively cheaper,

easier and time efficient providing comparative results.

The CFD analysis for this simulation uses the following physics parameters which

are explained as follows.

K-EPSILON TURBULANCE MODEL:

The K-EPSILON model is used to solve the air flow around the bluff bodies; since it

performs well with the external flow of air through complex bodies. The K stands for the turbulent

kinetic energy and the Epsilon stands for the Dissipation of the kinetic energy. The K-Epsilon model

uses wall functions and has good convergence rate giving this model a good advantage for external

flow problem solving. If K and Epsilon are known we can model the turbulence viscosity as the

equation given below.

𝑽𝒕 ∝ 𝝑𝒍 ∝ 𝒌𝟏

𝟐⁄ 𝒌𝟑

𝟐⁄

𝜺=

𝒌𝟐

𝜺 (Karthik, 2011)

Advantages of K- Epsilon Turbulence model:

Relatively simple implement

It leads to a stable computing which can converge result relatively easily.

It has reasonable predictions for many flows (Karthik, 2011)

Disadvantage of K-Epsilon Turbulence model:

Require wall function for implementation.

Only fully turbulent flows are validated.

Poor prediction for flow with strong separations. (Karthik, 2011)

Reynolds Average Navier Stoke

The Reynolds average Navier stokes calculates the velocity directly without the computational

requirement for series of repetitive steps during the simulation solving, thus reducing the


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magnitude of original Navier Stokes equation. The RANS model uses a time dependent velocity

fluctuation which separates the mean flow velocity. (symscape, 2006).

WALL Y+:

“The law of the wall explains that the average velocity of a turbulent flow at some point is

proportional to the logarithm of the distance from that point to the wall”. (CD-ADAPCO, 2014) The

Y+ stands for the wall coordinates. Y is dimensionless because of the velocity 𝑢𝑟 and kinematic

viscosity 𝜈

𝒚+ =𝒚𝒖𝝉

𝝂 , 𝒖𝝉 = √

𝝉𝝎

𝝆

𝒖+ =𝒖

𝒖𝝉 , 𝒖+ =

𝟏

𝒌𝒍𝒏𝒚+ + 𝑩. (CD-ADAPCO, 2014).

Y= the distance of the wall. u= friction velocity or shear. V= kinematic velocity. = wall shear stress. = fluid density. U+ = dimensionless velocity. U= velocity parallel to the wall. K= Von Karman’s constant (≈ .41) B= Constant (≈ 5.1)

Figure 5-Wall Y+ (CD-ADAPCO, 2012)


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5.1. Simulation setups:

For CFD analysis, the Lynne Turner model, created using CAD software is been

imported in Star CCM+. The model dimensions are been verified to that of the wind tunnel test

piece using the scale operation.

The frontal area of the Lynne turner model is calculated using the dimensions measured.

Based on the dimensions measured the frontal area of the wind tunnel is calculated using

the Blockage ratio. The blockage ratio was assumed to 5%. The Blockage ratio is considered

since the model occupies a partial space inside the tunnel thus confining the space inside

the tunnel. This in turn increases the air velocity giving very large force values.

5.1.1. Blockage ratio calculation for CFD wind tunnel setup:

The Blockage ratio of the wind tunnel was assumed to be 5%,

Blockage ratio = 𝒇𝒓𝒐𝒏𝒕𝒂𝒍 𝒂𝒓𝒆𝒂 𝒐𝒇 𝒕𝒉𝒆 𝒄𝒂𝒓

𝒇𝒓𝒐𝒏𝒕𝒂𝒍 𝒂𝒓𝒆𝒂 𝒐𝒇 𝒕𝒉𝒆 𝒘𝒊𝒏𝒅 𝒕𝒖𝒏𝒏𝒆𝒍 (CD-ADAPCO, 2012)

Frontal area of the car = 0.0292m2.

Frontal area of the wind tunnel = 0.0292

0.05

Frontal area of wind tunnel = 0.584m2

From the Frontal area of the tunnel the height and width of the tunnel is calculated for its

creation.

The length of the wind tunnel is determined by using the length of the model used; keeping

twice the length of the model in front and five times the length at the back.

The symmetry plane is created to reduce the computational time and memory.

The Volume of interest is created using the subtract option.

The VOI created is meshed using the following parameters.


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5.1.2. Pc Performance Parameter Consideration

Based on number of mesh cells pc performance evaluation is shown below.

Figure 6Pc Performance Graph (CD-ADAPCO, 2012)

5.1.3. Mesh parameters:

1. The meshes selected for the computation are Surface remesher, automatic surface repair for

surface meshing, polyhedral meshes for volumetric meshing and prism layer meshes for

refining the targeted surface.


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Figure 7- CFD Mesh model

2. The base mesh size is taken as 0.2m based on the grid independency study.

3. The surface growth rate is taken as 1.1 to produce a gradual increase in the mesh size.

4. The prism layer meshes helps in blending the walls from closer range to far fields. (CD-

ADAPCO, 2012). The number of Prism layer is given as 14 so as to increase the refinement

of the prism layer mesh cells to improve the computational results on the targeted surface

and to get a lower wall Y+.

5. The prism layer thickness is taken as 2mm and stretching of 1.3 to obtain a finer prism

layer, low Y+ value and better pressure coefficient.

6. The custom controls functions are created with two surface controls that occupy the

parental values for the model and the wind tunnel surface. Three volumetric controls are

created for getting a well refined gradually growing mesh.

7.

Figure 8- Mesh Properties


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5.2. PHYSICS MODELS:

1. Since there is no temperature variance in this simulation constant density is selected.

2. The flow is considered to be segregated flow since the flow is subsonic and incompressible.

The boundary layer is assigned to be turbulent as the velocity through the tunnel is high

since the calculated Reynolds number is Re≥ 5 × 105. (refer Apendix)

3. Since it is a volumetric analysis three dimensional gradients is selected.

4. The K-Epsilon turbulence modeled is chosen for this simulation and is explain earlier.

5. Realizable K-Epsilon turbulence model is selected for improving the boundary layer

performance and to enhance the streamline curvature.

6. WIND TUNNEL RESULTS AND DISCUSSIONS:

Below given are the values of forces and moments obtained from the wind tunnel test.

Forces Obtained: Moments Obtained:

Pitching Moment -0.97 Nm

Rolling Moment 1.1 Nm

Yawing Moment 0.084 Nm

Known data:

Air density 𝜌 = 1.225 𝑘𝑔/𝑚3

Wind tunnel speed = 45 m/s

Frontal area of the Lynne Turner model = 0.0292m2.

Length of the Lynne Turner model L = 0.525m.

Coefficient of Drag:

𝑫𝒓𝒂𝒈 𝑭𝒐𝒓𝒄𝒆 =𝟏

𝟐𝑪𝑫 𝝆 𝑨 𝑽𝟐 (K. V. S. PAVAN, 2012)

𝐶𝐷 =Drag Force

12 𝜌 𝐴 𝑉2

𝐶𝐷 =13.96

0.5 𝑥 1.225 𝑥 0.029 𝑥 452 CD = 0.385

Drag Force 13.96 N

Lift Force -1.609 N

Side Force 0.855 N


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Coefficient of Lift:

𝑳𝒊𝒇𝒕 𝑭𝒐𝒓𝒄𝒆 =𝟏

𝟐 𝑪𝑳 𝝆 𝑨 𝑽𝟐 (K.V.S.PAVAN, 2012)

𝐶𝐷 =Lift Force

12

𝜌 𝐴 𝑉2

𝐶𝐿 =−1.609

0.5 𝑥 1.225 𝑥 0.029 𝑥 452 CL = -0.44

Coefficient of Side Force:

𝑺𝒊𝒅𝒆 𝑭𝒐𝒓𝒄𝒆 =𝟏

𝟐 𝑪𝑺 𝝆 𝑨 (K.V.S.PAVAN, 2012)

𝐶𝑆 =Side Force

12

𝜌 𝐴 𝑉2

𝐶𝑆 =0.855

0.5 𝑥 1.225 𝑥 0.029 𝑥 452 CS = 0.023

Pitching Moment Coefficient:

𝑷𝒊𝒕𝒄𝒉𝒊𝒏𝒈 𝑴𝒐𝒎𝒆𝒏𝒕 =𝟏

𝟐 𝑪𝑴 𝝆 𝑨 𝑳 𝑽𝟐 (K.V.S.PAVAN, 2012)

𝐶𝑀 =Pitching Moment

12 𝜌 𝐴 𝐿 𝑉2

𝐶𝑀 =−0.97

0.5 𝑥 1.225 𝑥 0.0292 𝑥 0.52 𝑥 452 CM = -0.0051

Rolling Moment Coefficient:

𝑹𝒐𝒍𝒍𝒊𝒏𝒈 𝑴𝒐𝒎𝒆𝒏𝒕 =𝟏

𝟐 𝑪𝑹 𝝆 𝑨 𝑳 𝑽𝟐 (K.V.S.PAVAN, 2012)

𝐶𝑅 =Pitching Moment

12 𝜌 𝐴 𝐿 𝑉2


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𝐶𝑅 =1.1

0.5 𝑥 1.225 𝑥 0.0292 𝑥 0.52 𝑥 452 CM = 0.058

Yawing Moment Coefficient:

𝒀𝒂𝒘𝒊𝒏𝒈 𝑴𝒐𝒎𝒆𝒏𝒕 =𝟏

𝟐 𝑪𝒀 𝝆 𝑨 𝑳 𝑽𝟐 (K. V. S. PAVAN, 2012)

𝐶𝑌 =Yawing Moment

12

𝜌 𝐴 𝐿 𝑉2

𝐶𝑅 =0.084

0.5 𝑥 1.225 𝑥 0.0292 𝑥 0.52 𝑥 452 CM = 0.00446

Calculating Reynolds Number:

𝑹𝒆 = 𝝆 𝒙 𝑽 𝒙 𝑳

𝝁 (K.V.S.PAVAN, 2012)

𝑅𝑒 = 1.225 𝑥 45 𝑥 0.52

1.983 𝑥 10−5 Re = 1.44x10-6

7.2. Wind tunnel data plots:

From the output of wind tunnel results the following graphs are plotted.

Figure 9- Drag force Vs Re

5.31E+05, 0.342

8.85E+05, 0.335

1.24E+06, 0.342

1.59E+06, 0.3270.326

0.328

0.33

0.332

0.334

0.336

0.338

0.34

0.342

0.344

5.00E+05 1.00E+06 1.50E+06

Dra

g Fo

rce

Co

eff

icie

nt

Reynolds number

Drag force VS Reynolds number

Drag force VS Reynoldsnumber


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Figure 10-Lift force Vs Re

Figure 11-Pitching coefficient Vs Re

5.31E+05, 0.1

8.85E+05, 0.101

1.24E+06, 0.098

1.59E+06, 0.098

0.0975

0.098

0.0985

0.099

0.0995

0.1

0.1005

0.101

0.1015

5.00E+05 1.00E+06 1.50E+06

Lift

Fo

rce

Co

eff

icie

nt

Reynolds number

Lift force VS Reynolds number

Lift force VS Reynolds number

5.31E+05, 0.053

8.85E+05, 0.054

1.24E+06, 0.056

1.59E+06, 0.054

0.0525

0.053

0.0535

0.054

0.0545

0.055

0.0555

0.056

0.0565

5.00E+05 1.00E+06 1.50E+06

Lift

Fo

rce

Co

eff

icie

nt

Reynolds number

Pitching coefficient VS Reynolds number

Pitchingcoefficient VS…


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7.2.1. Force coefficients vs Yaw angle:

Figure 12-force coefficient Vs Yaw Angle

When the yaw angle is increased from 0 to 10 the drag force coefficient has very

negligible variance. Whereas the side force coefficient has a linear depletion on the negative axis

corresponding to the change in yaw angles. From the graph it is clear that the lift force coefficient

has a minimal variance due to the induced lift caused by the side force.

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 2 4 6 8 10 12

Fro

ce c

oe

ffic

ien

t

Yaw angle

Froce coefficient Vs Yaw Angle

Drag Force Coefficient

Side Force Coefficient

Lift Force Coefficient


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7.2.2. Moment Coefficient vs Yaw angle:

Figure 13- Moment Coefficient Vs Yaw angle

As mentioned above in case of moment coefficient the pitching moment stays stable

with a very slight variance. Whereas the rolling moment increases linearly with the change in

angles; thus it can be deduced that the side force is responsible for rolling. Also a slight change in

yawing moment is observed due to the change in yaw angles. This change is due to the induced lift

caused by the rolling moment.

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0 2 4 6 8 10 12

Mo

me

nt

coe

ffic

ien

t

Yaw angle

Moment coefficient Vs Yaw Angle

Rolling Moment Coefficient

Pitching Moment Coefficient

Yawing Moment Coefficient


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7.2.3. Force and moment coefficients vs slant angle:

Figure 14-force coefficient Vs Slant Angle

Figure 15-Moment Coefficient Vs Slant angle

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 10 20 30 40 50

Fro

ce c

oe

ffic

ien

t

Slant angle

Froce coefficient Vs Slant Angle

Drag Force Coefficient

Side Force Coefficient

Lift Force Coefficient

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 10 20 30 40 50

Mo

me

nt

coe

ffic

ien

t

Slant angle

Moment coefficient Vs Slant Angle

Rolling Moment Coefficient

Pitching Moment Coefficient

Yawing Moment Coefficient


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The graph trend shows that side force and drag force coefficients are maintained

constant. Whereas the lift force coefficient increases linearly up to 19 degrees from the baseline

model. After which the lift force coefficient decreases slightly and remains constant. From here the

baseline model has lower lift force whereas the model with 19 deg slant angle has the highest lift

force this may be due to the flow separation.

The graph trend lines shows that the yawing and the rolling moment coefficients are

stable with the change in slant angles. Whereas the pitching moment is high on 19 deg slant angle

because the air velocity on the surface will be higher till the beginning of slant angle once it passes

the slant angle the flow separates because of this a low pressure zone is formed at the rear which is

the reason for pitching moment variance. (dileep.P menon, 2014)

8. CFD ANALYSIS RESULTS AND DISCUSSIONS:

8.1. GRID INDEPENDENCY STUDY:

The grid independency study is performed before simulating the model to find an

appropriate mesh cell size that can reduce the computational effort and time. The grid

independency study is carried out by simulating a single model with different base mesh size.

While comparing the Cd values of all the simulation at particular mesh size the Cd value remains

constant, this shows the appropriate mesh size for the model to be simulated. (CD-ADAPCO, 2014)

Sr. No. BASE SIZE COEFFICIENT

OF DRAG

NUMBER OF

CELLS

1 0.30 0.45 415617

2 0.35 0.43 549376

3 0.20 0.425 1123077

4 0.18 0.423 1494707

Table 3- Grid Independency


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From the mesh independence study it has been concluded that a base size of 0.20 could be an ideal

base size. The aforementioned is based on the observation of the values of coefficient of drag. From

the above table it can be seen that the value of coefficient of drag starts repeating at a base size of

0.2, thus the corresponding values of number cells obtained direct us towards the selection of

coarser mesh for reducing time and memory.

8.2. CFD RESULTS VS WIND TUNNEL TEST RESULTS:

ANGLES 0 19 25 31 37

COFFIECIENT OF DRAG 0.327 0.292 0.288 0.312 0.319 WIND

TUNNEL

0.4036 0.3821 0.4084 0.3745 0.4105 CFD

COFFIECIENT OF LIFT 0.098 0.405 0.354 0.34 0.327 WIND

TUNNEL

0.00363 0.3692 0.3002 0.2445 0.2847 CFD

DRAG FORCE (In

Newton)

13.961 12.5 12.352 13.438 13.789 WIND

TUNNEL

14.64 13.86 14.8 13.58 14.88 CFD

LIFT FORCE (In Newton) -1.606 12.295 10.025 9.364 9.386 WIND

TUNNEL

0.135 13.39 10.88 8.86 10.32 CFD


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The change in values when compared with wind tunnel and CFD analysis is mainly because of the

following mentioned point –

1. Type of test set up: Since the wind tunnel was an open section so the air velocity wasn’t

constant throughout but was decreasing by a very small magnitude. In CFD analysis the

wind was a completely closed set up wherein the natural factors of diminishing air velocity

doesn’t affects the values.

2. The actual blockage ratio was calculated to be 2.2% for the wind tunnel set up whereas in

case of CFD analysis the blockage ratio was assumed to be 5% due to the CFD processing set

up which affects the air velocity directly thus giving higher values.

3. According to (dileep.P menon, 2014)40% of the drag is happening on the rear end of the

vehicle. While investigating the given model with slant angles as 0,19,25,31 and 47 degrees,

it can be concluded that the coefficient of lift is lower for the vehicle which comes under the

category of SUV which have a slant angle of 0 degree. Whereas the hatch back is considered

to have the worst value of coefficient of lift with the angle of 19 degree and without a boot

the flow around the vehicle crates a sudden change in flow separation which creates a

negative force behind the vehicle which leads to the generation of increased lift force of the

vehicle.

4. On investigating the remaining slant angles though there was a flow separation due to the

presence of boot section a separation bubble is formed between the slant angle and the boot

section which gives a reaction force against the lift. Hence it provides a better lift to drag

ratio and gives a good stability


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8.3. POST PROCESSING

In this section of the report the post processing of the base line model has been discussed.

Cell Relative Velocity

From the below given image it can interpreted that the relative velocity around the vehicle body

is smooth and has a constant distribution which ranges from 0 to 45.

Pressure Coefficient.


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From the above given image it can interpreted that the pressure coefficient around the body of

the given model varies which ranges between -3 to 1. It can be seen from the above figure that the

pressure value sis maximum at the intersecting point of bonnet and wind screen.

Wall Y+

As the model has enhanced wall treatment and a very refined prism layers formed around the

body thus it has obtained a low Y+ value. It should be technically in the region of 0 to 5.


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Iso-Pressure Coefficients

From the above image it can be seen that the Iso-pressure coefficients acts uniformly

throughout the body, except from the spaces between the bonnet and wind shield. The

pressure coefficient has extended in the rear end of the model due to the vacuum created as

well as near the wheels.

Velocity

From the above given image it can be seen that the major portion of air stagnation occurs at the

front bumper and gradually deflects to the upper surface and the lower body. There are certain

separation bubbles formed near the intersection of bonnet and wind screen and at the rear end

because of the vacuum space created, thus allowing the circulation of air to cause the vortex

formation (in blue color)


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Velocity magnitude

From the above image it can be seen that the air flows around the body; in the regions of

separation recirculation air bubbles or vortices are created due to the presence of vacuum. The side

s of the model clearly shows that the velocity is higher because of the stagnation on the frontal area.


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10. CONCLUSION

From the CFD analysis and wind tunnel tests and evaluation it has been concluded that the

aerodynamic forces act majorly on the rear end. On comparing the results of both the tests the

magnitude in the values of results obtained are mainly because of the set up conditions and the

physical environmental set up.

On comparing the values of different models the baseline model has found to be having the

lowest lift and high drag force whereas the 19 degree slant angled hatch back model has the highest

lift force because of the inappropriate pressure gradient found near the rear end. The models of the

remaining slant angles have average lift, rolling and pitching moments. The entire coursework

exercise provided the group with a better understanding of the air flow on the vehicle body and the

impacts of modern day wind tunnel & CFD based tests facilities on the vehicle performance.


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Bibliography

Anon., 1993. turbulence modelling for CFD. D.C: Wilcox.

Atlee M. Cunningham, G. S. S., 1987. A Study of the effect of Reyonlds Number and Mach Number on

Constant Pressure Coefficient Jump for Shock-incluced Triangle- edge Seperation. NASA Contractor

Report, p. 79.

Benson, T., 2014. NASA. [Online]

Available at: http://www.grc.nasa.gov/WWW/k-12/airplane/mach.html

[Accessed 2015 4 20].

CD-ADAPCO, 2012. star south east asian confrence. [Online]

Available at: http://www.cd-

adapco.com/sites/default/files/Presentation/SEA%20Conference%202012_CDadapco_VolumeMes

hing_Used_KM.pdf

[Accessed 22 4 2015].

CD-ADAPCO, 2014. youtube. [Online]

Available at: https://www.youtube.com/watch?v=f1UL4o9rdJs

[Accessed 2015 4 22].

Clancy, L. I., 1975. Aerodynamics. Newyork: Wiley.

dileep.P menon, s. k. G. S. K., 2014. to improve the aerodynamic performance of hatchback car with

the addition of a rear roof spoiler, BANGALORE: CFD SYMPO.

Hucho, W. H., 2013. Aerodynamics of road vehicles. s.l.:s.n.

K.V.S.PAVAN, 2012. CFD MODELING OF FLOW AROUND AHMED BODY, HYDERABAD: CD -ADAPCO

BANGLOORE.

Karthik, T., 2011. turbulence models and there applications, Chennai: Dpt of mechanical engineering

IIT Madras.

NASA, 2014. Reynolds Number. [Online]

Available at: http://www.grc.nasa.gov/WWW/BGH/reynolds.html


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Princeton uiversity, n.d. Bernoulli's Equation. [Online]

Available at: http://www.princeton.edu/~asmits/Bicycle_web/Bernoulli.html

symscape, 2006. computational fluid dynamics software for all. [Online]

Available at: www.symscape.com/reynolds-averaged-navier-stokes-eqautions

[Accessed 22 4 2015].

V.Easwaran, G. a., 2002. turbulent flows- fundamentals, experiments and modelling.. s.l.:Narosa

Publishing house..

Zuo, W., n.d. Introduction of computational fluid dynamics. Jass 05, St. petersburg, p. 8.


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11. APPENDIX

Supporting Images and Graphs for The Lynne Turner model with a slant angle of 19 degree.

Figure 16: Coefficient of drag for 19 degree


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Figure 17: Drag Force for 19 degree

Figure 18: Iso-Pressure Coefficient for 19 degree


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Figure 19: Coefficient of lift for 19 degree

Figure 20: Lift force for 19 degree


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Figure 21: Mesh Scene for 19 degree

Figure 22: Pressure Coefficient for 19 degree


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Figure 23: Cell Relative velocity for 19 degree

Figure 24: Velocity Magnitude for 19 degree


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Figure 25: Velocity for 19 degree

Figure 26: Wall Y+ for 19 degree



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Figure 27: Total Pressure coefficient for 25 degree

Figure 28: Coefficient of lift for for 25 degree


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Figure 29: Lift force for 25 degree



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Figure 31: Residuals for 25 degree

Figure 32: Velocity Magnitude for 25 degree


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Figure 36: Cell relative velocity for 25 degree


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Figure 40: Velocity magnitude for 31 degree


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Figure 41: Velocity for for 31 degree



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Figure 44: Drag force for 31 degree


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Figure 46: Coefficient of Lift for 31 degree


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Figure 47: Lift Force for 31 degree



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Figure 51: Velocity magnitude for 37 degree



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Figure 54: Drag Coefficient for 37 degree



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Figure 56: pressure coefficients for 37 degree

Figure 57: Lift Coefficient for 37 degree


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Figure 58: Lift Force for 37 degree



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GROUND VEHICLE AERODYNAMICS

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