CFD Simulation And Validation Of Aerodynamic Over Ahmed Model.

CFD SIMULATION AND VALIDATION OF AERODYNAMIC OVER AHMED MODEL

ROSLINA AIDA BT. RAHIMI

This report is submitted as partial fulfillment of the requirement for the Degree of Bachelor of Mechanical Engineering (Thermal Fluid)

Faculty of Mechanical Engineering Universiti Teknikal Malaysia Melaka

CFD SIMULATION AND VALIDATION OF AERODYNAMIC OVER AHMED MODEL

ROSLINA AIDA BT. RAHIMI

I / We* admit that had read this dissertation and in my / our*opinion this dissertation is satisfactory in the aspect of scope and quality for the bestowal of

Bachelor of Mechanical Engineering(Thermal Fluid)

Signature : ………..

Name of 1st Supervisor :………...

Date :………

Signature : ………

Name of 2nd Supervisor :………....

“I hereby verify that I have read this report and I find it sufficient in term of quality and scope to be awarded with the Bachelor Degree in Mechanical Engineering “

Signature : ………

Supervisor Name : ………

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ACKNOWLEDGEMENT

All praises be to God, The Most Gracious, The Most Merciful for His Guidance and Blessing.

Then goes my deepest appreciation to my first supervisor Mr. Ahmad Kamal who have been very committed in giving me the best guidance in preparing this dissertation through out the first and second semester of my final year. Not to forget my family that gave me endlessly aspiration and strength.

Endless gratitude goes to another lecturer that directly and indirectly involved in the preparation of my dissertation, and also technicians En. Razmi,and En. Kamarudin and few others that helped much through completion of this dissertation .

Last but not least, bundle of thanks to my friend, Mohd Fadhlan that always being with me through out the laboratories sessions and others that had been supporting me directly and indirectly completing the study for this dissertation .

ABSTRACT

Nowadays, computational Fluid Dynamics (CFD) method is widely used as a tool to virtually analyze the external flow problem. However, when it come to vehicle aerodynamics , the visibility of the CFD approach as a virtual wind tunnel to substitute physical wind tunnel has not yet been recognized. Hence, this project revealed the capability of a CFD tool to predict the aerodynamic characteristic of a vehicle. In this project, a simplified vehicle model called Ahmed model was chosen. The result is validated as CD's value for CFD is 0.50 while for experiment wind tunnel is 0.54. It had proven that Spalart-Allmaras able to validate data between numerical and experimental method with only 7.4% different between both method used. Thus, the capability and consistency of the Spalart-Allmaras equation proof its potential in developing automotive industries.

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ABSTRAK

Sejak kebelakangan ini, CFD telah digunakan secara meluas sebagai alat untuk menganalisa secara maya masalah aliran udara. Walaubagaimanapun, di dalam bidang aerodinamik kenderaan, kebolehan pendekatan CFD di dalam menggantikan kajian terowong udara maya seoah-olah sama seperti kajian sebenar terowong udara masih belum lagi mendapat perhatian. Oleh itu, projek ini dibuat bagi mendedahkan kebolehan CFD untuk mendapatkan gambaran karektor aerodinamik bagi kenderaan. Sebuah model ringkas kenderaan telah dipilih iaitu model Ahmed. Berdasarkan keputusan yang diperolehi, nilai pekali geseran bagi kaedah CFD adalah 0.50 manakala nilai pekali geseran bagi kaedah eksperimen terowong udara adalah 0.54. Ini telah membuktikan kebolehan kebolehan Spalart-Allmaras dalam mengesahkan nilai data antara kedua-dua kaedah yang digunakan dengan hanya menunjukkan perbezaan peratusan sebanyak 7.4%. Dengan itu, kebolehan dan keberkesanan Spalart-Allmaras ini telah membuktikan potensinya di dalam pembangunan industri automotif.

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CONTENTS

CHAPTER TITLE PAGE

ACKNOWLEDGEMENT iii

ABSTRACT iv

ABSTRAK v

CONTENT vi

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF ABBREVIATIONS xv

CHAPTER 1 INTRODUCTION 1

1.1 Background Research 1 1.2 Problem Statement 3

1.3 Objective 3 1.4 Scopes 3 1.5 Summary 4

CHAPTER 2 LITERATURE REVIEW 6

2.1 Aerodynamics Background History 6 2.2 Fundamentals of Aerodynamics 10

2.2.1 Boundary layer 10 2.2.2 Reynolds number 11

2.2.3 Laminar and Turbulent Boundary 12 layers

2.2.4 Flow separation and reattachment 13 2.2.5 Aerodynamics Drag and Lift 14

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CHAPTER TITLE PAGE

CHAPTER 2 2.3 Background History of Ahmed 16

model 2.3.1 Ahmed model 16

2.4 Wind Tunnel 17

2.5 Computational Fluid Dynamics 18

(CFD) 2.6 Fundamentals in Computational 19

Fluid Dynamics (CFD) 2.6.1 Elements in CFD 19

2.6.2 Turbulence models 20

2.6.3 The Spalart-Allmaras Model 23

CHAPTER 3 METHODOLOGY 25

3.1 Modeling 25

3.1.1 Model design in Solidworks V5 26

3.1.1.1 Wind tunnel testing 27

3.1.1.2 CFD validation purposes 28

3.2 CFD Model Development 28

3.3 Grid Independence Test 29

3.4 Wind Tunnel Model Development 29

3.4.1 Rapid Prototyping 30

3.5 Wind Tunnel Test 31

3.5.1 Analysis of Wind Tunnel Test 32

3.6 Model Verification 33

CHAPTER 4 GRID GENERATION 34

4.1 Model Scale 34

4.2 CAD drawing 35

CHAPTER TITLE PAGE

CHAPTER 4 4.4 Splitting the Volume 36

4.5 Edge Meshing 38

4.6 Volume meshing 39

4.7 Mesh Quality Check 39

4.7.1 Mesh quality at x-direction 40

4.7.2 Mesh quality at y-direction 41

4.7.3 Mesh quality at z-direction 41

4.7.4 Mesh quality of combination of 42

x, y and z-direction 4.8 Mesh export 42

CHAPTER 5 MODEL SETUP 43

5.1 Model Setup Step 43

5.1.1 Selecting viscous model 43

5.1.2 Determine the equation into 44

Simulation 5.1.3 Creating Material 45

5.1.4 Boundary condition setup 46

5.1.5 Iteration 49

CHAPTER 6 WIND TUNNEL TESTING 50

6.1 Introduction 50

6.2 Model Fabrication 51

6.3 Preparation process of Wind Tunnel Testing 54

6.4 Calibration 55

6.5 Wind Tunnel Testing 57

CHAPTER 7 RESULT AND DISCUSSION 58

7.1 Introduction 59

7.2 Computational Fluid Dynamics (CFD) 59

Result 7.2.1 Drag Force Result 62

ix

7.3 Wind Tunnel Testing Result 64

7.3.1 Coefficient of drag calculation 65

7.4 Discussion 66

7.4.1 Comparison of Drag Force 66

7.5 Difference between result between CFD 67

and Wind Tunnel Testing CHAPTER 8 CONCLUSION 68

8.1 Conclusion 68

8.2 Recommendations 69

REFERENCES 70

BIBLIOGRAFI 72

APPENDICES 73

LIST OF TABLE

NO. TITLE PAGE

1 Model data 50

2 Spring balance reading 56

3 Technical Data for UteM Wind Tunnel 57 4 Wind Tunnel Testing Data 64

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LIST OF FIGURES

NO. TITLE PAGE

2.1 Figure of Ahmed model 5

2.2 Boundary layer velocity gradient 10

2.3 Lamina/turbulent boundary layer transition 12 (Low speed) 2.4 Lamina/turbulent boundary layer transition 13 (High speed)

2.5 Flow separation and reattachment 13

2.6 Project Flow Chart 24

2.7 Rounded boundaries and rear end sector of 26 Ahmed model

2.8 Front and back body of the model with a 26 rectangular cross section

2.9 One side of half model 27

3.0 The two parts of the model will be combined at 28 the dwell, denoted in dotted lines in the center

3.1 Fused Deposition Modeling machine 31 3.2 Model tested at 0o of angle of attack and 31 zero yaw (plan view)

3.3 Subsonic Wind Tunnel 32 3.4 Ahmed model in Solidworks 35

3.5 Ahmed model in GAMBIT before 36

3.6 The Ahmed model is sweep from selected 36 face to form a volume

3.7 The new volume is split rom its main 36

volume

3.8 The extra volume out of the geometry is 37

deleted

3.9 New volume created from sweep the face 37

selected by reference to vertex

4.0 Complete volume of Ahmed model CFD 38

simulation

4.1 Plotting node on the edge 38

4.2 3 edges selected before meshing the volume 38

4.3 Volume meshed (side view) 39

4.4 Volume meshed 39

4.5 Cell aspect ratio 39

4.6 Cell skewness 39

4.7 Cell growth rate 40

4.8 Alignment to the flow 40

4.9 View of mesh quality at equiangle skew 40

5.0 View of mesh quality at aspect ratio 40

5.1 View of mesh quality at equiangle skew 41

5.2 View of mesh quality at aspect ratio 41

5.3 View of mesh quality at equiangle skew 41

5.4 View of mesh quality at aspect ratio 41

5.5 View of mesh quality at equiangle skew 42

(full volume)

5.6 Figure of complete mesh model 42

xiii

5.8 Energy Equation screen 45

5.9 Materials screen 45

6.0 Boundary conditions on Ahmed model CFD geometry 46 6.1 Boundary conditions screen 47 6.2 Velocity Inlet screen 47

6.3 Ahmed model 48

6.4 Iterate screen 49

6.5 CAD model drawing 51

6.6 ABS plastic

6.7 Process of making the model in rapid prototyping machine 52 6.8 Rapid prototyping machine 53

6.9 The 1:9 scale of Ahmed model 53

7.0 Side view of Ahmed model 54

7.1 Mounting is made for center of gravity position 54

7.2 Lift Calibration 55

7.3 Drag calibration 55

7.4 Drag calibration 55

7.5 Calibration Graph for Wind Tunnel Test 56

7.6 Wind Tunnel 57

7.7 Wind Tunnel Testing 58

7.8 The converged result 60

7.9 Contours of Velocity Magnitude 61

8.0 Contours of Static Pressure on Ahmed model 62

8.1 Value of Ahmed model drag at 30m/s 63

8.3 Comparison of Drag force 66

xv

LIST OF ABBREVIATION

CHAPTER 1

INTRODUCTION

1.1 Background research

Aerodynamics is the study of a solid body moving through the atmosphere and the interaction which takes place between the body surfaces and the surrounding air with varying relative speeds and wind direction.( Heinz Heisler, 2002) Currently, study of the aerodynamics is the main concern for the vehicle manufacturers in fulfilling the customer needs. People nowadays are always searching for a vehicle that is designed with low fuel consumption over mileage used with the outstanding vehicle performance, less wind noise occurrence, good development of road holding and can constantly maintain the stability of the vehicle on the move. (Heinz Heisler, 2002)

Due to that, there are studies made specifically to focus on road vehicle aerodynamics which is called automotive aerodynamics. Automotive aerodynamics usually focus on the performance of the vehicle in reducing drag, decreasing of wind noise level and more importantly, putting off the undesired lift forces at high speeds. Studies of the automotive aerodynamics were based on two methods which are computer modeling and simulation together with wind tunnel testing.

Forrester T. Johnson *, Edward N. Tinoco, N. Jong Yu had affirmed in their journals that the tremendous cost involved in testing had caused the use of CFD and

2 wind tunnel in developing the aerodynamics design. While Gerardo Franck and Jorge D’El´ stated in their journal, that aerodynamics specialists are constantly seeking new ideas and solutions capable of providing a fast and accurate answer to the design targets which including shorten lead time, lower experimental work and lower the costs associated with vehicle design work. One way to obtain this goal is by combining numerical simulation with experimental measurements in wind tunnel tests. However, the current state of the art in the computational fluid dynamics shows that over the last years accurate results for the automotive aerodynamics expectations have appeared. One of the key points in the development of computational codes is its validation with experimental results

In order to prove the validation of results from computed solution and wind tunnel testing, various types of standards have been invented. One of them was Ahmed model which is designed as a simplified car used in automotive industry to investigate the influence of the flow structure on the drag. The Ahmed model was chosen in this project for its geometrical simplicity, and because of the body that retains the main flow features, especially the vortex wake flow where most part of the drag is concentrated and it is a good candidate to be used as a benchmark test (Franck G. and D’El´ J.)

1.2 Problem Statement

This report was made to compare and validate the credibility between numerical calculated data with actual experimental data with a known model. The model chosen to be assessed is the Ahmed model. Previous literature on Ahmed model shows the difficulty of convergence between numerical data and experimental data. The contributing factor for the validation of the numerical vs calculated data has been identified as the Drag Coefficient (CD). This in turn will effect the Reynolds number (Re) which will be used to determine the validity of the data

1.3 Objective

To appraise the capability of CFD method as a virtual wind tunnel.

1.4 Scope

i ) develop a numerical CFD model & physical mode of 1:9 scale ii) zero-yaw characteristic study

iii) carry out wind tunnel tests to validate the CFD simulation iv) Simulation at Re = 2.30 x 105

4 1.5 Summary

This research is mainly focused on study of aerodynamics on Ahmed model. According to Guilmineau E., Ahmed’s body is 1044mm long (L), 288mm high and 389mm width. The slant part is 222mm long with 30o of angle. This geometry is represented in Fig. 2.1. In this research, the Ahmed design shape will be formed in CAD software which then would be used in extension of manufacturing the model by using Rapid Prototyping machine. The Rapid Prototyping complete model will then be put in as the test specimen to be tested in the wind tunnel on several different configurations. The data gained from the testing would be used as the benchmark for the CFD validation purposes. The wind tunnel testing is important as it determines the measure of accuracy of a real test specimen in comparison to the computed modeling solution. In this case, the test model would be experimented in wind tunnel at zero yaw condition with certain velocity that can be provided by wind tunnel utilized. Application of the state-of-the-art CFD software, GAMBIT and FLUENT with application of Spalart-Allmaras viscous model would be used for modeling and analysis of aerodynamics characteristic over the Ahmed structure, similar to the one tested in the wind tunnel. Several major factors have been identified and will be researched in this study. The most significant factor is the flow region which presents the major contribution to a car’s drag is the wake flow behind the vehicle. Other than that, there would be an investigation on the location at which the flow separates which determines the size of the separation zone, and consequently the drag force. Then, a more exact simulation of the wake flow and of the separation process will be made and is essential for the accuracy of drag predictions. After the simulation is made and taking into account all the contributing parameters, certain output data will put into related equation to figure out value of coefficient of drag. The value of coefficient of drag gained from simulation will then validate with the value of CD from the wind tunnel testing result by making the comparison from the established theoretical value of CD as reference.

Figure 2.1: Figure of Ahmed model

6

CHAPTER 2

LITERATURE REVIEW

2.1 Aerodynamics Background History

The history of the word “aerodynamics” actually came from the Greek philosopher Aristotle in 350 B.C. by his observation of fluids and their effect on objects. Aristotle conceived the notion that air has weight and observed that a body moving through a fluid encounters resistance.(www.wikipedia.org)

Archimedes, another Greek philosopher, also has a place in the history of aerodynamics. A hundred years later, in 250 B.C, he presented his law of floating bodies that formed a basic principle of lighter-than-air vehicles and stated that a fluid— either in a liquid or a gaseous form—is continuous, basically restating Aristotle's theory of a hundred years earlier. He comprehended that every point on the surface of a body immersed in a fluid was subject to some force due to the fluid. He stated that, in a fluid, “each part is always pressed by the whole weight of the column perpendicularly above it.” He observed that the pressure exerted on an object immersed in a fluid is directly proportional to its depth in the fluid. A direct proportional relationship means that if one part increases, the other will increase by the same factor. In other words, the deeper the object is in the fluid, the greater the pressure .(www.wikipedia.org)

In 1490, the Italian painter, sculptor, and thinker Leonardo da Vinci began documenting his aerodynamics theories and ideas for flying machines in personal

notebooks. He first believed that birds fly by flapping their wings, and thought that this motion would have to occur for manmade aircraft to rise. He later correctly concluded that the flapping of the wings created forward motion which allowed air to pass across the bird's wings to create lift. It was the movement of the wing relative to the air and the resulting reaction that produced the lift necessary to fly. He then noticed that water in a river moved faster at a greater velocity where the river narrowed. In numerical terms, the area of a cross-section of a river multiplied by the velocity of the water flowing through that section equals the same number at any point in the river. This is known as the law of continuity (Area x Velocity = constant or AV = constant). The law of continuity demonstrates the conservation of mass, which is a fundamental principal in modern aerodynamics. He also observed the different ways in which a fluid flowed around an object—called a flow field. (www.wikipedia.org)

Leonardo also stated that the aerodynamics results are the same if an object moves through the fluid at a given velocity or if the fluid flows past the object at rest at the same velocity. This became known as the “wind tunnel principal.” For example, the results are the same aerodynamically whether a runner moves at 10 miles per hour in calm air and if the wind is blowing at 10 miles per hour past a stationary person. He also determined that drag on an object is directly proportional to the area of the object. The greater the area of an object, the greater the drag it will produce. Further, Leonardo pointed out the benefits of streamlining as a way to reduce an object's drag. (www.wikipedia.org)

Scientists working in the 17th century contributed several theories relating to drag. The Italian mathematician and inventor Galileo Galilei built on Archimedes' work and discovered that the drag exerted on a body from a moving fluid is directly proportional to the density of the fluid. Density describes the mass of an object per unit volume. A very dense fluid produces more drag on objects passing through it than a less dense fluid. The density of air (a fluid) changes with its distance from the Earth's surface, becoming less dense the farther it is above the Earth's surface and, as such, exerting less pressure. .(www.wikipedia.org)

wind tunnel in developing the aerodynamics design. While Gerardo Franck and Jorge D’El´ stated in their journal, that aerodynamics specialists are constantly seeking new ideas and solutions capable of providing a fast and accurate answer to the design targets which including shorten lead time, lower experimental work and lower the costs associated with vehicle design work. One way to obtain this goal is by combining numerical simulation with experimental measurements in wind tunnel tests. However, the current state of the art in the computational fluid dynamics shows that over the last years accurate results for the automotive aerodynamics expectations have appeared. One of the key points in the development of computational codes is its validation with experimental results

In order to prove the validation of results from computed solution and wind tunnel testing, various types of standards have been invented. One of them was Ahmed model which is designed as a simplified car used in automotive industry to investigate the influence of the flow structure on the drag. The Ahmed model was chosen in this project for its geometrical simplicity, and because of the body that retains the main flow features, especially the vortex wake flow where most part of the drag is concentrated and it is a good candidate to be used as a benchmark test (Franck G. and D’El´ J.)

1.2Problem Statement

This report was made to compare and validate the credibility between numerical calculated data with actual experimental data with a known model. The model chosen to be assessed is the Ahmed model. Previous literature on Ahmed model shows the difficulty of convergence between numerical data and experimental data. The contributing factor for the validation of the numerical vs calculated data has been identified as the Drag Coefficient (CD). This in turn will effect the Reynolds number (Re) which will be used to determine the validity of the data

1.3Objective

To appraise the capability of CFD method as a virtual wind tunnel.

1.4Scope

i ) develop a numerical CFD model & physical mode of 1:9 scale ii) zero-yaw characteristic study

iii) carry out wind tunnel tests to validate the CFD simulation iv) Simulation at Re = 2.30 x 105

1.5Summary

This research is mainly focused on study of aerodynamics on Ahmed model. According to Guilmineau E., Ahmed’s body is 1044mm long (L), 288mm high and 389mm width. The slant part is 222mm long with 30o of angle. This geometry is represented in Fig. 2.1. In this research, the Ahmed design shape will be formed in CAD software which then would be used in extension of manufacturing the model by using Rapid Prototyping machine. The Rapid Prototyping complete model will then be put in as the test specimen to be tested in the wind tunnel on several different configurations. The data gained from the testing would be used as the benchmark for the CFD validation purposes. The wind tunnel testing is important as it determines the measure of accuracy of a real test specimen in comparison to the computed modeling solution. In this case, the test model would be experimented in wind tunnel at zero yaw condition with certain velocity that can be provided by wind tunnel utilized. Application of the state-of-the-art CFD software, GAMBIT and FLUENT with application of Spalart-Allmaras viscous model would be used for modeling and analysis of aerodynamics characteristic over the Ahmed structure, similar to the one tested in the wind tunnel. Several major factors have been identified and will be researched in this study. The most significant factor is the flow region which presents the major contribution to a car’s drag is the wake flow behind the vehicle. Other than that, there would be an investigation on the location at which the flow separates which determines the size of the separation zone, and consequently the drag force. Then, a more exact simulation of the wake flow and of the separation process will be made and is essential for the accuracy of drag predictions. After the simulation is made and taking into account all the contributing parameters, certain output data will put into related equation to figure out value of coefficient of drag. The value of coefficient of drag gained from simulation will then validate with the value of CD from the wind tunnel testing result by making the comparison from the established theoretical value of CD as reference.

Figure 2.1: Figure of Ahmed model

CHAPTER 2

LITERATURE REVIEW

2.1 Aerodynamics Background History

The history of the word “aerodynamics” actually came from the Greek

philosopher Aristotle in 350 B.C. by his observation of fluids and their effect on objects. Aristotle conceived the notion that air has weight and observed that a body moving through a fluid encounters resistance.(www.wikipedia.org)

Archimedes, another Greek philosopher, also has a place in the history of aerodynamics. A hundred years later, in 250 B.C, he presented his law of floating bodies that formed a basic principle of lighter-than-air vehicles and stated that a fluid— either in a liquid or a gaseous form—is continuous, basically restating Aristotle's theory of a hundred years earlier. He comprehended that every point on the surface of a body immersed in a fluid was subject to some force due to the fluid. He stated that, in a fluid, “each part is always pressed by the whole weight of the column perpendicularly above it.” He observed that the pressure exerted on an object immersed in a fluid is directly proportional to its depth in the fluid. A direct proportional relationship means that if one part increases, the other will increase by the same factor. In other words, the deeper the object is in the fluid, the greater the pressure .(www.wikipedia.org)

In 1490, the Italian painter, sculptor, and thinker Leonardo da Vinci began documenting his aerodynamics theories and ideas for flying machines in personal

notebooks. He first believed that birds fly by flapping their wings, and thought that this motion would have to occur for manmade aircraft to rise. He later correctly concluded that the flapping of the wings created forward motion which allowed air to pass across the bird's wings to create lift. It was the movement of the wing relative to the air and the resulting reaction that produced the lift necessary to fly. He then noticed that water in a river moved faster at a greater velocity where the river narrowed. In numerical terms, the area of a cross-section of a river multiplied by the velocity of the water flowing through that section equals the same number at any point in the river. This is known as the law of continuity (Area x Velocity = constant or AV = constant). The law of continuity demonstrates the conservation of mass, which is a fundamental principal in modern aerodynamics. He also observed the different ways in which a fluid flowed around an object—called a flow field. (www.wikipedia.org)

Leonardo also stated that the aerodynamics results are the same if an object moves through the fluid at a given velocity or if the fluid flows past the object at rest at the same velocity. This became known as the “wind tunnel principal.” For example, the results are the same aerodynamically whether a runner moves at 10 miles per hour in calm air and if the wind is blowing at 10 miles per hour past a stationary person. He also determined that drag on an object is directly proportional to the area of the object. The greater the area of an object, the greater the drag it will produce. Further, Leonardo pointed out the benefits of streamlining as a way to reduce an object's drag. (www.wikipedia.org)

Scientists working in the 17th century contributed several theories relating to drag. The Italian mathematician and inventor Galileo Galilei built on Archimedes' work and discovered that the drag exerted on a body from a moving fluid is directly proportional to the density of the fluid. Density describes the mass of an object per unit volume. A very dense fluid produces more drag on objects passing through it than a less dense fluid. The density of air (a fluid) changes with its distance from the Earth's surface, becoming less dense the farther it is above the Earth's surface and, as such, exerting less pressure. .(www.wikipedia.org)