ABSTRACT
This research work focused on the use of an hybrid RANS
LES turbulence model in
simulating and checking the drag performance characteristics of an aerodynamic vehicle.
The Spalart Allmaras shear stress transport ( Scale A daptive Simulation (SAS)
Turbulence model was selected for use, and with the help of OpenFOAM Finite Volume
Computational Fluid Dynamic Solver the simulation algorithm was set and carried out in a total
time step of 4000. Convergence was achi eved under the stipulated time step based on our set
convergence criteria. The geometries were downloaded in their stereolitography (STL) format, then
SnappyHex Mesh meshing tool was use d to generate our mesh grid. The first simulation whic h took
674870sec s to run gave a coefficient of drag value of 0.522497 . Then with the help of FreeCAD
Open Source Software, the vehicle shape was ca refully altered and on running the simulation on the
new shape it produced a drag coefficient value of 0.415411 in 669116secs , which represents a
coefficient of drag reduction of about 20.49%. Thus OpenFOAM CFD Solver helped to carry out
analysis on the turbulent fl ow over a motorbike and aided in reducing the drag coefficient of the
motorbike. Further optimal shape analysis stu dies if carried out can help an aerodynamic engineer
produce more fuel economical vehicles for consumer use.
TABLE OF CONTENTS
Content
Page
Title Page
i
Certification
Page ii
Dedication
iii
Acknowledgement
iv
Abstract
v
Table of Contents
vi
Nomenclature
viii
List of Figures
ix
List of Tables
x
CHAPTER ONE:
INTRODUCTION 1
1.1 What is Aerodynamics?
2
1.2 Scope of Automobile Aerodynamics
4
1.3 External Flow Phenomena of Automobile
6
1.4 Factors contributing
to flow field around vehicles 7
1.5 Aerodynamic Drag
8
1.6 Statement of Problem
10
1.7 Aims and Objectives of Research
11
1.8 Scope of Research
11
CHAPTER TWO:
LIT ERATURE REVIEW ……………………………….. 12
2.1 Computational Fluid Dynamics (CFD) 12
2.2 CFD Governing Equations 13
2.3 Outline of Computational Fluid Dynamic Process 14
2.4 Discretization Methods 14
2.5 Meshing and Pre Processing 16
2.6 Turbulence 16
2.7 Turbulence Modeling 20
2.8 Reynolds Averaged Navier Stokes [RANS] Models ……………………………. 20
2.81 Boussinesq Assumption Models (Eddy Viscosi ty Models) …………………………. 25
2.82 Reynolds Stress Transport Models 29
2.9 Computation of Fluctuating Quantites 29
2.91 Large Eddy Simulation [LES] Models 30
2.92 Detached Eddy Simulation [DES] Models …………………………………………………………. 30
2.93 Hybrid RANS LES Models 30
2.10 Post Processing Stage 31
2.11 CFD Softwares 31
2.12 Previous Related Research Carried Out 35
2.13 Knowledge Gap ……………………………….. 35
CHAPTER THREE:
METHODOLOGY 36
3.1 OpenFOAM Finite Volume CFD Solver 36
3.2 Parrallelization and CPU Specification 36
3.3 Geometry of Shape 36
3.4 Meshing and Pre Processing 38
3.5 Computational Domain and Boundary Conditions 40
9
3.63.6 Hybrid SpallatallmarasHybrid Spallatallmaras–LES SASLES SAS–SST ModelSST Model ………………………………..……………………………….. 4141
3.73.7 Solution Algorithm SettingSolution Algorithm Setting ………………………………..……………………………….. 4141
3.73.71 The SIMPLE Algorithm1 The SIMPLE Algorithm ………………………………..……………………………….. 4242
3.73.72 Th2 The PIMPLE Algorithme PIMPLE Algorithm ………………………………..……………………………….. 4545
3.83.8 ParaView PostParaView Post–ProcessingProcessing ………………………………..……………………………….. 4545
3.9 Geometry Shape Change3.9 Geometry Shape Change ………………………………..……………………………….. 4545
CHAPTER FOUR:
CHAPTER FOUR: RESULTSRESULTS ………………………………..……………………………….. 4747
4.14.1 Results of flow characterizationResults of flow characterization ………………………………..……………………………….. 4747
4.2 Convergence4.2 Convergence TestTest ……………………………….. 48……………………………….. 48
4.34.3 Coefficient of Drag ResultsCoefficient of Drag Results ………………………………..……………………………….. 5151
CHAPTER FIVE:
CHAPTER FIVE: RESULTRESULTS ANDS AND DISCUSSIONDISCUSSION …………………………………………………………………. 5555
5.15.1 Effect of ConvergenceEffect of Convergence ………………………………..……………………………….. 5555
5.25.2 Effect of Shape Tweak on Coefficient of DragEffect of Shape Tweak on Coefficient of Drag ………………………………..……………………………….. 5656
CHAPTER SIX:
CHAPTER SIX: CONCLUSION AND RECOMMENDATIONCONCLUSION AND RECOMMENDATION …………………………….. 57…………………………….. 57
6.1 Conclusion6.1 Conclusion ………………………………..……………………………….. 5757
6.2 Recommendation6.2 Recommendation ………………………………..……………………………….. 5757
References
References ………………………………..……………………………….. 5858
APPENDICES
APPENDICES ………………………………..……………………………….. 6464
Appendix 1: Spallat
Appendix 1: Spallat Allmaras Algorithm SettingsAllmaras Algorithm Settings ………………………………..……………………………….. 6464
Appendix 2:
Appendix 2: CFDCFD ResultResultss ………………………………..……………………………….. 9696
CHAPTER ONE
1.
INTRODUCTION
To save energy and to protect the global environment, fuel consumption reduction is a primary
concern of automotive development. In vehicle body development, reduction of drag is essential for
improving fuel consumption and driving performance, and if an aerodynamically refined body is
also aesthetically attractive, it will contribute much to increase the vehicle’s appeal to potential
customers.
The main driver for lower ae
rodynamic drag is fuel economy (Kranson, 1985)1985). As long as s tandards
for fuel economy increase and fuel costs go up, aerodynamic drag will have to be improved .
However, as the passenger car must have enough capacity to accommodate passengers and baggage
in addition to minimum necessary space for its engine and othe r components, it is extremely
difficult to realize an aerodynamically ideal body shape. The car is therefore obliged to have a body
shape that is rather aerodynamically bluff, not an ideal streamline shape as seen on fish and birds.
Such a body shape is in evitably accompanied by f low separation at the rear end. Two elements that
have major influence on the drag coefficient of a bluff object are the roundness of its front corners
and the degree of taper at its rear end.
When
aerodynamics is considered from a fuel economy standpoint, the primary focus is the
coefficient o f drag “”. Essentially, this is how easily a vehicle moves through the air, though drag
isn’t the only factor that is considered. There’s more to aerodynamics than just drag, t here’s down
force and lift a nd there is yawing mome nt, which is basically when in a crosswind, how much the
vehicle get s steered by the wind that is pushing on it, a n d then there’s noise (Kranson, 1985)
For aerody
namic drag force a nd fuel consumption reduction, ac c ording to Richard (2001) and
Kambiz 2003)
𝑟𝑎𝑔12 𝜌 𝑆 𝑉2= (1 .1
Δ𝐹𝑢𝑒𝑙 𝐶𝑜𝑛𝑠𝑢𝑚𝑝𝑡 𝑜𝑛𝐹𝑢𝑒𝑙 𝐶𝑜𝑛𝑠𝑢𝑚𝑝𝑡 𝑜𝑛= 𝜂 ×[ Δ𝐶𝐷𝐶𝐷+ Δ𝑆𝑆+3Δ𝑉𝑉 ] 𝜂~ 0.5−0.7 1. 2)
From the above formula, to achieve low fuel consumption,
either of the following can be done to
Shaping
Devices
Flow Control
Cross Section Speed
14
the Aerodynamic Vehicle
the Aerodynamic Vehicle;;
1. TThe crosshe cross–sectionsection of Aerodynaof Aerodynamic Vehicle can be reduced.mic Vehicle can be reduced.
2. TThe speed he speed of movement can be reduced.of movement can be reduced.
3. The The Coefficient of Drag (Coefficient of Drag ( ) can be reduced by ; ) can be reduced by ;
i. Improving tImproving the shaping of thehe shaping of the Aerodynamic vehicle.Aerodynamic vehicle.
ii. Addition of Drag Reduction devices to the Aerodynamic vehicle, e.gAddition of Drag Reduction devices to the Aerodynamic vehicle, e.g, Spoilers, , Spoilers, Vortex Stake Device (VCD)Vortex Stake Device (VCD),, UnderUnder–Carriage Flow Device (UFD),Carriage Flow Device (UFD), e.t.c.e.t.c.
iii. Better contrBetter control of flow around Aerodynamic vehicle.ol of flow around Aerodynamic vehicle.
Thus, better aerodynamic s
Thus, better aerodynamic shapinghaping of vof vehiclesehicles, addition of drag r, addition of drag reduction devices oreduction devices or better flowbetter flow control can be used to lower vehiclescontrol can be used to lower vehicles ..
This research work is
This research work is primarily concerprimarily concerned with fuel consumption reduction achieved by reducing ned with fuel consumption reduction achieved by reducing the the Coefficient of DragCoefficient of Drag (( )) of Aerodynamic Vehicles through improvement of their Aerodynamic of Aerodynamic Vehicles through improvement of their Aerodynamic ShapeShape GeometryGeometry with the help with the help of Computational Fluid Dynamicsof Computational Fluid Dynamics (CFD).(CFD).
Today’s wisdom says that an en
Today’s wisdom says that an engineer cangineer can start measurstart measuring a vehicle’s aerodynamics verying a vehicle’s aerodynamics very early in the early in the initial initial design process. From the earliest conceptual stages on through the workingdesign process. From the earliest conceptual stages on through the working–prototype stage, prototype stage, automakers rely on computer software and wind tunnels to ensure vehicles automakers rely on computer software and wind tunnels to ensure vehicles meet their aemeet their aerodynamic rodynamic targets.targets. The aerodynamic improvements produced by The aerodynamic improvements produced by shape shape geometry changes can generate fuel geometry changes can generate fuel savings of assavings of as mumuch as six to sech as six to seven percent (Kirk and John, 2005ven percent (Kirk and John, 2005).).
1.1 What is Aerodynamics?What is Aerodynamics?
Aerodynamics is a
Aerodynamics is a word originaword originating from the words air and power ting from the words air and power (Kranson, 1985)(Kranson, 1985). This name has . This name has been given to a science that, being a part of mechanicsbeen given to a science that, being a part of mechanics, is , is the science of the science of the motion of bodies in the motion of bodies in general and general and studies the laws of motion of air depending on the acting forces and on their basis studies the laws of motion of air depending on the acting forces and on their basis establishes special laws of thestablishes special laws of the interaction between air and a solid body moving in it.e interaction between air and a solid body moving in it.
At the beginning of its development, aerodynamics dealt with the investigation of the motion of air
At the beginning of its development, aerodynamics dealt with the investigation of the motion of air at quite low speeds because aircraft at that time had a low flight speed. It is quite natural thatat quite low speeds because aircraft at that time had a low flight speed. It is quite natural that aerodynamics was founded theoretically on hydrodynamicsaerodynamics was founded theoretically on hydrodynamics–the science dealing with the motion of a the science dealing with the motion of a dropping (incompressible) liquid. The cornerstones of this science were laid in the 18th centdropping (incompressible) liquid. The cornerstones of this science were laid in the 18th century by ury by Euler (1707Euler (1707–1783) and1783) and Bernoulli (‘1700Bernoulli (‘1700–1782)1782)..
15
The mechanic
The mechanics of a gas differs from that of a liquid when thes of a gas differs from that of a liquid when the gas has a high speed. At such gas has a high speed. At such speeds, speeds, a gas flowing over a craft experiences not only a change in its density, but also an increase in its a gas flowing over a craft experiences not only a change in its density, but also an increase in its temperature that may result in various physicochemical transformatitemperature that may result in various physicochemical transformations in it. A substantial part of ons in it. A substantial part of the kinetic energy associated with the speed of a flight is converted into heat and chemical energy. the kinetic energy associated with the speed of a flight is converted into heat and chemical energy. All these features of motion of a gas resulted in the appearance of highAll these features of motion of a gas resulted in the appearance of high–speed aerodynamics or gas speed aerodynamics or gas dynamicsdynamics–a special braa special branch of aerodynamics studying the laws of motion of air (a gas) at high nch of aerodynamics studying the laws of motion of air (a gas) at high subsonic and supersonic speeds, and also the laws of interaction between a gas and a body traveling subsonic and supersonic speeds, and also the laws of interaction between a gas and a body traveling in it at such speedsin it at such speeds. . The modern methods of studying the flow of a gas are based on aThe modern methods of studying the flow of a gas are based on a number of number of principles and hypotheses established in aerodynamics. One of them is the continuum hypothesisprinciples and hypotheses established in aerodynamics. One of them is the continuum hypothesis–the assumption of the continuity of a gas flow according to which we may disregard the the assumption of the continuity of a gas flow according to which we may disregard the intermolecular distances and molecular movements and consider tintermolecular distances and molecular movements and consider the continuous changes of the he continuous changes of the basic propertiesbasic properties of a gas in space and in timeof a gas in space and in time. .
In aerodynamic investigations, the interaction between a gas and a body moving in it is based on the
In aerodynamic investigations, the interaction between a gas and a body moving in it is based on the principle of inverted flow according to which a system consisting of a gas (aprinciple of inverted flow according to which a system consisting of a gas (air) at rest and a moving ir) at rest and a moving body is replaced with a system consisting of a moving gas and a body at restbody is replaced with a system consisting of a moving gas and a body at rest (Kranson, 1985)(Kranson, 1985). . When one system is replaced with the other, the condition must be satisfied that the freeWhen one system is replaced with the other, the condition must be satisfied that the free–stream stream speed of the gas relative to the body atspeed of the gas relative to the body at rest equals the speed of this body in the gas at rest. The rest equals the speed of this body in the gas at rest. The principle of inverted motion follows from the general principle of relativity of classical mechanics principle of inverted motion follows from the general principle of relativity of classical mechanics according to which forces do not depend on which of two interacting bodies (in our case the gasaccording to which forces do not depend on which of two interacting bodies (in our case the gas or or craft) is at rest and which is perforcraft) is at rest and which is performing uniform rectilinear motionming uniform rectilinear motion. Aerodynamics, . Aerodynamics, figurativelyfiguratively speaking, is a multispeaking, is a multi–branch science. In accordance with the needs of the rapidly developing branch science. In accordance with the needs of the rapidly developing aviation, rocket, and cosmic engineering, more or less clearly aviation, rocket, and cosmic engineering, more or less clearly expressed basic scientific trends have expressed basic scientific trends have taken shape in aerodynamics. They are associated with the aerodynamic investigations of craft as a taken shape in aerodynamics. They are associated with the aerodynamic investigations of craft as a whole and their individual structural elements, and also of the most characteristic kinds of gas flows whole and their individual structural elements, and also of the most characteristic kinds of gas flows and processes aand processes attending the flow over a body. If we have in view the range of air speeds from low ttending the flow over a body. If we have in view the range of air speeds from low subsonic to vsubsonic to very high supersonic ones, then,ery high supersonic ones, then, we can separate the following basic regions in the we can separate the following basic regions in the science of investigating flow: aerodynamics of an incompressible fluid, or flscience of investigating flow: aerodynamics of an incompressible fluid, or fluid mechanics (the uid mechanics (the Mach number of the flow is M = 0), and highMach number of the flow is M = 0), and high–speed aerodynamics. The latter, in turn, is divided speed aerodynamics. The latter, in turn, is divided into subsonic (M < 1), transonic (M ~ 1), superinto subsonic (M < 1), transonic (M ~ 1), supersonic (M> 1) and hypersonic (M >>sonic (M> 1) and hypersonic (M >> 1) 1) aerodynamics. It must be noted that each of these brancheaerodynamics. It must be noted that each of these branches studies flow processes that are s studies flow processes that are characterized by certain specific features of flows with the indicatedcharacterized by certain specific features of flows with the indicated Mach numbers.Mach numbers. Aerodynamics Aerodynamics of a boundary layer is one of the broadest and most developed sections of the science of a fluid in of a boundary layer is one of the broadest and most developed sections of the science of a fluid in motion. It studies viscomotion. It studies viscous gas flow in a boundary layer. The solution of the problem of flow in a us gas flow in a boundary layer. The solution of the problem of flow in a boundary layer makes it possible to find the distribution of the shearing stresses and, consequently, boundary layer makes it possible to find the distribution of the shearing stresses and, consequently,
16
of the resultant aerodynamic forces and moments caused by friction. It also make
of the resultant aerodynamic forces and moments caused by friction. It also makes it possible to s it possible to calculate the transfer of heat from the gas flowing over a body to a boundary. The conclusions of the calculate the transfer of heat from the gas flowing over a body to a boundary. The conclusions of the boundary layer theory can also be used for correcting the solution on inviscid flow, particularly for boundary layer theory can also be used for correcting the solution on inviscid flow, particularly for finding the correction to the pressfinding the correction to the pressure distribution due to the iure distribution due to the influence of the boundary layer.nfluence of the boundary layer.
Aerodynamics is therefore that branch of fluid dynamics concerned with studying the motion of air,
Aerodynamics is therefore that branch of fluid dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is often used synonymously particularly when it interacts with a moving object. Aerodynamics is often used synonymously with with gas dynamics, with the difference being that gas dynamics applies to all gases. Understanding the gas dynamics, with the difference being that gas dynamics applies to all gases. Understanding the motion of air (often called a flow field) around an object enables the calculation of forces and motion of air (often called a flow field) around an object enables the calculation of forces and moments acting on the object. Typical properties calculamoments acting on the object. Typical properties calculated for a flow field include velocity, ted for a flow field include velocity, pressure, density and temperature as a function of position and time. By defining a control volume pressure, density and temperature as a function of position and time. By defining a control volume around the flow field, equations for the conservation of mass, momentum, and energy can be around the flow field, equations for the conservation of mass, momentum, and energy can be defined and used to solve fdefined and used to solve for the properties. The use of aerodynamics through mathematical or the properties. The use of aerodynamics through mathematical analysis, empirical approximation and wind tunnel experimentation form the scientific basis. analysis, empirical approximation and wind tunnel experimentation form the scientific basis. Aerodynamics and its analysis are basically divided into two major subAerodynamics and its analysis are basically divided into two major sub–categories, namely the categories, namely the exterexternal and internal aerodynamics. External aerodynamics is the study of flow around solid objects nal and internal aerodynamics. External aerodynamics is the study of flow around solid objects of various shapes. Evof various shapes. Evaluating the lift and drag on aaluating the lift and drag on a car or airplane,car or airplane, the shock waves that form in the shock waves that form in front of the nose of a rocket, or the flow of air over a wind front of the nose of a rocket, or the flow of air over a wind turbine blade are examples of external turbine blade are examples of external aerodynamics. On the other hand, internal aerodynamics is the study of flow through passages in aerodynamics. On the other hand, internal aerodynamics is the study of flow through passages in solid objects. For instance, internal aerodynamics encompasses the study of the airflow throughsolid objects. For instance, internal aerodynamics encompasses the study of the airflow through pipes and ducts,pipes and ducts, a jet ena jet engine or through an air conditioning pipe.gine or through an air conditioning pipe.
Apparently, this
Apparently, this research workresearch work concentrateconcentratedd more on the external category of the aerodynamics more on the external category of the aerodynamics related to vehiclesrelated to vehicles..
1.2
1.2 Scope oScope of Automobile Aerodynamics f Automobile Aerodynamics
The rapidly increasing fuel prices and the regulation of gr
The rapidly increasing fuel prices and the regulation of green house gasses to control global een house gasses to control global warming have warming have given tremendous pressure ongiven tremendous pressure on design engineers to endesign engineers to enhance the current designs ofhance the current designs of automobileautomobiles using minimal changes in theirs using minimal changes in their shapes. To shapes. To ffulfulfillill the above requirements, design the above requirements, design engineers have been using the conceengineers have been using the concepts of aerodynamics to enhancepts of aerodynamics to enhance tthe efficiency of automobiles he efficiency of automobiles (Wolf(Wolf–Heinrich, 1998)Heinrich, 1998)
The
The FigFigure 1.1ure 1.1 shows the spectrum of task for vehicle aerodynamics. The shows the spectrum of task for vehicle aerodynamics. The FigFigure illustrates the ure illustrates the various problems which can be solved using the aerodynamics of the vehicle. Aerodynamics is used various problems which can be solved using the aerodynamics of the vehicle. Aerodynamics is used by design engineers for cooling the engines, improving the performance of the vehicle, enhancing by design engineers for cooling the engines, improving the performance of the vehicle, enhancing the comfort of the rider, stabilizing the the comfort of the rider, stabilizing the car in external wind conditions and also increasing the car in external wind conditions and also increasing the
17
visibility of the rider.
Figure 1.1 Spectrum of Task for vehicle Aerodynamics (Wolf-Heinrich, 1998)
Figure 1.2(a) Typical Fuel Energy usage at Urban driving (Wolf-Heinrich, 1998)
Standby
17%
Accessories
2%
Engine Losses
62%
Engine Drive train
Drivetrain Loss
6%
Aerodynamic
3%
Rolling
4%
Braking
6%
Fuel
100%
19% 13%
18
Fig
Figure 1.2(bure 1.2(b) ) TypicalTypical Fuel Energy usage Fuel Energy usage at Highway driving at Highway driving (Wolf(Wolf–Heinrich, 1998)Heinrich, 1998)
The
The FigFigure 1.2ure 1.2(a) and 1.2(b)(a) and 1.2(b) shows the description of the fuel energy used in a modern vehicle at shows the description of the fuel energy used in a modern vehicle at urban driving and highway driving. urban driving and highway driving. From the description above, we observe that the From the description above, we observe that the shape of the shape of the vehicle uses about 3 % of fuel to overcome the resistance in urban driving, while it takes vehicle uses about 3 % of fuel to overcome the resistance in urban driving, while it takes 11% of 11% of fuel for the highway driving. This considerable high value of fuel usage in highway driving attracts fuel for the highway driving. This considerable high value of fuel usage in highway driving attracts several design engineers to enhance the aerodynamics of the vehicle using minimal design changes. several design engineers to enhance the aerodynamics of the vehicle using minimal design changes.
1.3 External Flow
1.3 External Flow Phenomena oPhenomena of Automobile f Automobile
The
The FigFigure 1.3ure 1.3 shows the streamline of an external flow around a stationary vehicle.shows the streamline of an external flow around a stationary vehicle. When the When the vehicle is moving at an undistributed velocity, the viscous effects in the fluidvehicle is moving at an undistributed velocity, the viscous effects in the fluid are restricted to a thin are restricted to a thin layer called boundary layer. Outside the boundary layer is tlayer called boundary layer. Outside the boundary layer is thehe inviscid flow. This fluid flow inviscid flow. This fluid flow imposes pressure force on the boundary layer. When theimposes pressure force on the boundary layer. When the air reaches the rear part of the vehicle, the air reaches the rear part of the vehicle, the fluid gets detached. Within the boundary layer,fluid gets detached. Within the boundary layer, the movement of the fluid is totally governed by the the movement of the fluid is totally governed by the viscous effects of viscous effects of the fluid.the fluid.
Standby
4%
Accessories
2%
Engine Losses
69%
Engine
Drive train
Drivetrain Loss
5%
Aerodynamic
11%
Rolling
7%
Braking
2%
Fuel
Fuel 100%100%
25
25%%
20
20%%
19
FigFigure 1.3ure 1.3 Streamline of external flows around a stationary vehicleStreamline of external flows around a stationary vehicle (Pramod, 2006)(Pramod, 2006)
The boundary does not exist for the Reynolds Number which is lower than
The boundary does not exist for the Reynolds Number which is lower than 104. The. The Reynolds Reynolds number is dependent on the characteristic length of the number is dependent on the characteristic length of the vehicle, the kinematicvehicle, the kinematic viscosity and the viscosity and the speed of the vehicle. Apparently, the fluid moving around the vehicle isspeed of the vehicle. Apparently, the fluid moving around the vehicle is dependent on the shape of dependent on the shape of the vehicle and the Reynolds number. There is anotherthe vehicle and the Reynolds number. There is another important phenomenon which affects the important phenomenon which affects the flow of the car and theflow of the car and the performance of theperformance of the vehicle. This phenomenon is commonly known as vehicle. This phenomenon is commonly known as ‘Wake’ of the vehicle. When the air„Wake‟ of the vehicle. When the air moving over the vehicle is separated at the rear end, it leaves a moving over the vehicle is separated at the rear end, it leaves a large low pressurelarge low pressure turbulent region behind the vehicle known as the wake. This wake contturbulent region behind the vehicle known as the wake. This wake contributes to ributes to thethe formation of pressure drag, which eventually reduces the vehicle performance.formation of pressure drag, which eventually reduces the vehicle performance.
1.4
1.4 Factors Contributing To Flow Field Factors Contributing To Flow Field aroundaround Vehicle Vehicle
The major factors which affect the flow field around the vehicle are the boundary layers, separation
The major factors which affect the flow field around the vehicle are the boundary layers, separation of flof flow field, friction drag and lastly the pressure drag. ow field, friction drag and lastly the pressure drag.
BOUNDARY LAYER
BOUNDARY LAYER:: The Aerodynamics boundary layer was first defined by the Aerodynamic The Aerodynamics boundary layer was first defined by the Aerodynamic engineer ‘Ludwig Prandengineer „Ludwig Prandtl’tl‟. This allows aerodynamicists to simplify the equations of fluid flow by . This allows aerodynamicists to simplify the equations of fluid flow by dividing the flow dividing the flow field into two areas: one inside the boundary layer and the one outside the field into two areas: one inside the boundary layer and the one outside the boundary layer. In this boundary layer around the vehicle, the viscosity is dominant and it plays a boundary layer. In this boundary layer around the vehicle, the viscosity is dominant and it plays a major role in drag of the vehicle. The viscosity is neglected in the fluid regiomajor role in drag of the vehicle. The viscosity is neglected in the fluid regions outside this ns outside this boundary layer since it does not have significant effect on the solution. In the design of the body boundary layer since it does not have significant effect on the solution. In the design of the body shape, the boundary layer is given high attention to reduce drag. There are two reasons why shape, the boundary layer is given high attention to reduce drag. There are two reasons why
20
designers consider the boundary layer as a major
designers consider the boundary layer as a major factor in aerodynamic drag. The first is that the factor in aerodynamic drag. The first is that the boundary layer adds to the effective thickness of the body, through the displacement thickness, boundary layer adds to the effective thickness of the body, through the displacement thickness, hence increasing the pressure drag. The second reason is that the shear forces at the surface of the hence increasing the pressure drag. The second reason is that the shear forces at the surface of the vehicle vehicle causes skin friction drag.causes skin friction drag.
SEPARATION
SEPARATION:: During the flow over the surface of the vehicle, there is a point when the change in During the flow over the surface of the vehicle, there is a point when the change in velocity comes to stall and the fluid starts flowing in reverse direction. This phenomenon is called velocity comes to stall and the fluid starts flowing in reverse direction. This phenomenon is called ‘Separa„Separation’ of the fluid flow.tion‟ of the fluid flow. This usually occursThis usually occurs at the rear part of the vehicle. This separation is at the rear part of the vehicle. This separation is highly dependent on the pressure distribution which is imposed by the outer layer of the flow. The highly dependent on the pressure distribution which is imposed by the outer layer of the flow. The turbulent boundary layer canturbulent boundary layer can withstand much higher pressure without separating as cowithstand much higher pressure without separating as compared to mpared to laminar flow. This separation causes the flow to change its behavior behind the vehicle and thereby laminar flow. This separation causes the flow to change its behavior behind the vehicle and thereby affect the flow field around the vehicle. This phenomenon is the major factor to be considered while affect the flow field around the vehicle. This phenomenon is the major factor to be considered while studying the wake of the vehicle.studying the wake of the vehicle.
FRICTION
FRICTION DRAGDRAG: Each wall surface or material has a distinct friction which resists the flow of : Each wall surface or material has a distinct friction which resists the flow of fluids. Due to molecular friction, a stress acts on every surface of the vehicle. The integration of the fluids. Due to molecular friction, a stress acts on every surface of the vehicle. The integration of the corresponding force component in the free stream direction leads corresponding force component in the free stream direction leads to a friction drag. If the separation to a friction drag. If the separation does not occur, then friction drag is one of the main reasons to cause overall drag. does not occur, then friction drag is one of the main reasons to cause overall drag.
PRESSURE DRAG
PRESSURE DRAG:: The blunt bodies like large size vehicle show different drag characteristics. The blunt bodies like large size vehicle show different drag characteristics. On the rear part of such vehicles, therOn the rear part of such vehicles, there is an extremely steep pressure gradient which leads to the e is an extremely steep pressure gradient which leads to the separation of the flow separation in viscous flow. The front part of the flow field shows high separation of the flow separation in viscous flow. The front part of the flow field shows high pressure value, whereas on the rear part flow separates leading to a high suction in the area. As wepressure value, whereas on the rear part flow separates leading to a high suction in the area. As we integrate the force component created by such high change in pressure, the resultant is called as integrate the force component created by such high change in pressure, the resultant is called as ‘Pressure Drag’. This factor is affected by the height of the vehicle as well as the separation of the „Pressure Drag‟. This factor is affected by the height of the vehicle as well as the separation of the flow fieldflow field..
1.5
1.5 Aerodynamic DragAerodynamic Drag
Aerodynamic drag is
Aerodynamic drag is the fluid drag force that acts on any moving solid body in the direction of the the fluid drag force that acts on any moving solid body in the direction of the fluid freefluid free stream flow. From the body’s perspective (nearstream flow. From the body’s perspective (near–field approach), the drag comes from field approach), the drag comes from forces due to pressure distrforces due to pressure distribibutions over the body surface (Kranson, 1985)utions over the body surface (Kranson, 1985). . AA bodybody movingmoving throughthrough aa fluidfluid experiencesexperiences aa dragdrag force,force, whichwhich isis usuallyusually divideddivided intointo twotwo components:components: FFictionalictional DDragrag,, andand PPressureressure DDragrag.. FrictionalFrictional dragdrag comescomes fromfrom frictionfriction betweenbetween thethe fluidfluid andand thethe surfacessurfaces overover whichwhich itit isis flowing.flowing. ThisThis frictionfriction isis associatedassociated withwith thethe developmentdevelopment ofof boundaryboundary layers,layers, andand itit scalesscales withwith ReynoldsReynolds number.number. PressurePressure dragdrag comescomes fromfrom thethe eddyingeddying motionsmotions thatthat areare setset upup inin thethe fluidfluid byby thethe passagepassage ofof thethe body.body. ThisThis dragdrag isis associatedassociated withwith thethe formationformation ofof aa wake,wake, whichwhich cancan bebe readilyreadily seenseen behindbehind aa passingpassing boat,boat, andand itit isis usuallyusually lessless sensitivesensitive toto ReynoldsReynolds numbernumber thanthan thethe frictionalfrictional drag.drag.
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Formally,
Formally, bothboth typestypes ofof dragdrag areare duedue toto viscosityviscosity (if(if thethe bodybody waswas movingmoving throughthrough anan inviscidinviscid fluidfluid therethere wouldwould bebe nono dragdrag atat all),all), butbut thethe distinctiondistinction isis usefuluseful becausebecause thethe twotwo typestypes ofof dragdrag areare duedue toto differentdifferent flowflow phenomena.phenomena. FrictionalFrictional dragdrag isis importantimportant forfor attachedattached flowsflows (that(that is,is, therethere isis nono separation),separation), andand itit isis relatedrelated toto thethe surfacesurface areaarea exposedexposed toto thethe flow.flow. PressurePressure dragdrag isis importantimportant forfor separatedseparated flows,flows, andand itit isis relatedrelated toto thethe crosscross–sectionalsectional areaarea ofof thethe body.body.
T
Thehe rolerole playedplayed byby frictionfriction dragdrag (sometimes(sometimes calledcalled viscousviscous drag)drag) andand pressurepressure dragdrag (sometimes(sometimes calledcalled formform dragdrag oror profileprofile drag)drag) is observed clearlyis observed clearly byby consideringconsidering anan airfoil airfoil atat differentdifferent anglesangles ofof attackattack (Williamson, 2005)(Williamson, 2005).. AtAt smallsmall anglesangles ofof attackattack,, thethe boundaryboundary layerslayers onon thethe toptop andand bottombottom surfacesurface experienceexperience onlyonly mildmild prpressure gessure gradients,radients, andand theythey remainremain attachedattached alongalong almostalmost thethe entireentire chordchord length.length. TheThe wakewake isis veryvery small,small, andand thethe dragdrag isis dominateddominated byby thethe viscousviscous frictionfriction insideinside thethe boundaryboundary layers.layers. HoweverHowever,, asas thethe angleangle ofof attackattack increasesincreases,, thethe pressurepressure gradientsgradients onon thethe airfoilairfoil increaseincrease inin magnitude.magnitude. InIn particular,particular, thethe adverseadverse pressurepressure gradientgradient onon thethe toptop rearrear portionportion ofof thethe airfoil airfoil maymay becomebecome sufficientlysufficiently strongstrong toto produceproduce aa separatedseparated flow.flow. ThisThis separationseparation willwill increaseincrease thethe sizesize ofof thethe wake,wake, andand thethe pressurepressure losseslosses inin thethe wakewake duedue toto eddyeddy formation.formation. ThereforeTherefore thethe pressurepressure dragdrag increases.increases. AtAt aa higherhigher angleangle ofof attack,attack, aa largelarge fractionfraction ofof thethe flowflow overover thethe toptop surfacesurface ofof thethe airfoilairfoil maymay bebe separated,separated, andand thethe airfoilairfoil isis saidsaid toto bebe stalled.stalled. AtAt thisthis stage,stage, thethe pressurepressure dragdrag isis muchmuch greatergreater thanthan thethe viscousviscous drag.drag. WhenWhen thethe dragdrag isis dominateddominated byby viscousviscous drag,drag, wewe saysay thethe bodybody isis streamlinedstreamlined,, andand whenwhen itit isis dominateddominated byby pressurepressure drag,drag, wewe saysay thethe bodybody isis bluffbluff.. WhetherWhether thethe flowflow isis viscousviscous–dragdrag dominateddominated oror pressurepressure–dragdrag dominateddominated dependsdepends entirelyentirely onon thethe shapeshape ofof thethe body.body. AA streamlinedstreamlined bodybody lookslooks likelike aa fish,fish, oror anan airfoilairfoil atat smallsmall anglesangles ofof attack,attack, whereaswhereas aa bluffbluff bodybody lookslooks likelike aa brick,brick, aa cylinder,cylinder, oror airfoilairfoil atat largelarge anglesangles ofof attack.attack. ForFor streamlinedstreamlined bodiesbodies as shown in as shown in FigureFigure 1.41.4,, frictionalfrictional dragdrag isis thethe dominantdominant sourcesource ofof airair resistance, whileresistance, while fforor bluffbluff bodies as shown bodies as shown in in FigureFigure 1.51.5,, thethe dominantdominant sourcesource ofof dragdrag isis pressurepressure drag.drag. ForFor aa givengiven frontalfrontal arearea a andand velocity,velocity, aa streamlinedstreamlined bodybody willwill alwaysalways havehave aa lowerlower resistanceresistance thanthan aa bluffbluff bodybody..
Here
Here FlowFlow followsfollows contourscontours ofof thethe BodyBody
Figure
Figure 1.41.4:: StreamlinedStreamlined BodyBody (Pramod, 2006)(Pramod, 2006)
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Here
Here FlowFlow SeparatesSeparates
Figure
Figure 1.51.5: Bluff: Bluff BodyBody (Pramod, 2006)(Pramod, 2006)
1.6 Statement of
1.6 Statement of Problem:Problem:
In
In thethe historyhistory ofof aerodynamicaerodynamic researchresearch aroundaround bluffbluff bodybody Vehicles,Vehicles, itit hashas alwaysalways beenbeen observedobserved thatthat theirtheir shapesshapes withwith squaresquare basebase havehave servedserved asas anan obstacleobstacle toto improvingimproving vehicle vehicle fuelfuel economy.economy. WhenWhen thethe airair passespasses overover thethe vehiclevehicle surface,surface, itit makesmakes thethe airair overover thethe surfacesurface changechange itsits behavior,behavior, resultingresulting inin aa lowlow pressurepressure regionregion andand aa highhigh pressurepressure region.region. ThisThis pressurepressure differencedifference alongalong withwith vortexvortex sheddingshedding causescauses dragdrag, , therebythereby increasingincreasing fuelfuel consumptionconsumption (Williamson, 2008(Williamson, 2008).). ResearchersResearchers aroundaround thethe worldworld havehave triedtried toto reducereduce thethe dragdrag ofof vehicles, i.e,vehicles, i.e, sedan cars, sedan cars, trailerstrailers,, trucktruckss, , motorbikes, etc,motorbikes, etc, either by optimizing its aerodynamic shape or either by optimizing its aerodynamic shape or byby usingusing externalexternal devicesdevices likelike spoilers,spoilers, vortexvortex strakestrake devicedevice (VCD)(VCD) andand underunder–carriagecarriage flowflow devicedevice (UFD).(UFD). OneOne ofof thethe mainmain causescauses ofof aerodynamicaerodynamic dragdrag forfor sedansedan vehiclesvehicles and motorbikesand motorbikes isis thethe separationseparation ofof flowflow nearnear thethe vehiclevehicle’‟ss rearrear end.end. ToTo delaydelay flowflow separation,separation, bumpbump–shapedshaped vortexvortex generatorsgenerators areare oftenoften testedtested forfor applicationapplication toto thethe roofroof endend ofof aa sedan.sedan. CommonlyCommonly usedused onon aircraftaircraft toto preventprevent flowflow separation,separation, vortexvortex generatorsgenerators themselvesthemselves createcreate drag,drag, butbut theythey alsoalso reducereduce dragdrag byby preventing fpreventing flowlow separationseparation atat downstreamdownstream (Jeff (Jeff and Adrian, 2004and Adrian, 2004).).
The
The ShapeShape ofof aa vehiclevehicle (four legged or two legged)(four legged or two legged) playsplays anan importantimportant rolerole inin itsits dragdrag reduction.reduction. LowLow dragdrag isis achievedachieved byby aa shapeshape whichwhich avoidsavoids suddensudden changeschanges inin thethe crosscross sectionalsectional areaarea andand hashas aa degreedegree ofof tapering ttapering towardsowards thethe basebase ofof thethe vehicle.vehicle. InIn practicalpractical designdesign environment,environment, dragdrag reductionreduction comescomes fromfrom attentionattention toto shape design shape design detaildetailss (Jeff and Adrian, 2004(Jeff and Adrian, 2004).).
For a
For a fullfull–size truck, a reductionsize truck, a reduction in drag coefficient of 0.01 is approximately equalin drag coefficient of 0.01 is approximately equalss to an to an improvement in fuel economy of improvement in fuel economy of 0.0425 kilometer per liter (0.0425 kilometer per liter (0.1 m0.1 miles iles pper er ggallonallon)) on the combion the combined ned city/highway driving cycle. city/highway driving cycle. The same drag coefficient reduction can imprThe same drag coefficient reduction can improve a car’s fuel eove a car’s fuel econoconomy my
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by approximately
by approximately 0.0850Km/l (0.0850Km/l (0.2 mpg0.2 mpg)) ((edmundsedmunds, 2013), 2013).. Comparing drag Comparing drag coefficient coefficient numbers fornumbers for VolVolvo sedans,vo sedans, during Volvo’s boxyduring Volvo’s boxy–butbut–safe era, a safe era, a of 0.36 for the 960 model was typical for a of 0.36 for the 960 model was typical for a sedan. Today’s Volvos have come a long way, with the much sleesedan. Today’s Volvos have come a long way, with the much sleeker ker Volvo Volvo S80 coming in at just S80 coming in at just 0.28. 0.28. Not too long ago, anything below 0.3 was considereNot too long ago, anything below 0.3 was considered a sports car like vehicle. d a sports car like vehicle. Now, Toyota Now, Toyota has several vehicleshas several vehicles in the subin the sub–0.3 range, including the Avalon and Camry at 0.28 and the Solara at 0.3 range, including the Avalon and Camry at 0.28 and the Solara at 0.29.0.29. Vehicles for which fuel economy is a primary goal receive even more focus on aerodynamics. Vehicles for which fuel economy is a primary goal receive even more focus on aerodynamics. For instance, Toyota’s Prius is rated atFor instance, Toyota’s Prius is rated at 23.382Km/l23.382Km/l ((55 mpg55 mpg)) (combined), and it(combined), and it has an outstanding has an outstanding drag coefficient of just 0.26.drag coefficient of just 0.26. CarCars typically have a much lower drag coefficients typically have a much lower drag coefficient than pickups and than pickups and SUVs, which sit higher, are bigger and haveSUVs, which sit higher, are bigger and have greater cgreater cooling needs. For trucks,ooling needs. For trucks, the range of 0.40 to the range of 0.40 to 0.43, 0.440.43, 0.44 is availableis available.. Cars are in tCars are in the order of 0.30 he order of 0.30 — 0.34, a0.34, and SUVs and SUVs are somewhere in between re somewhere in between 0.360.36–0.41.”0.41.” Motorbikes have a drag Motorbikes have a drag coefficient in the range of 0.35coefficient in the range of 0.35–0.67 0.67 (Bahram, 2005)(Bahram, 2005)..
Thus,
Thus, in in usingusing CFDCFD toto simulatesimulate andand determinedetermine thethe amountamount andand naturenature ofof dragdrag developeddeveloped aroundaround aa vehiclevehicle or motorbikeor motorbike,, wewe cancan optimizeoptimize aa shapeshape thatthat cancan givegive thethe leasleast t possiblepossible drag,drag, withoutwithout actuallyactually buildingbuilding thesethese vehiclesvehicles.. Therefore,Therefore, CFDCFD providesprovides researchersresearchers andand vehicle vehicle manufacturersmanufacturers withwith aa viableviable tooltool toto produceproduce shapesshapes ofof vehicles vehicles thatthat wouldwould generategenerate minimalminimal dragdrag duringduring operation,operation, andand hencehence economizeeconomize fuelfuel usageusage (Bahram, (Bahram, Zhang, Koromilas and Bernal, 2001)Zhang, Koromilas and Bernal, 2001)..
1.7 Aims and
1.7 Aims and ObjectivesObjectives of Researchof Research::
The main aim of this research work is to check the extent to which shape change of an aerodynamic
The main aim of this research work is to check the extent to which shape change of an aerodynamic vehicle influvehicle influences the coefficient of drag values of the vehicle.ences the coefficient of drag values of the vehicle.
To achieve this aim, the
To achieve this aim, the ffollowingollowing Objectives Objectives will be carried outwill be carried out;;
(1) ToTo SimulateSimulate thethe AerodynamicAerodynamic flowflow aroundaround aa Aerodynamic Aerodynamic BodyBody (Car and Motorbike), (Car and Motorbike), usingusing SpalartSpalart–Allmaras (SA)Allmaras (SA)–SSTSST–Scale Adaptive Simulation (SAS) Hybrid Scale Adaptive Simulation (SAS) Hybrid RANSRANS–LES LES TurbulenceTurbulence model, with the help of OpenFOAM CFD Solver.model, with the help of OpenFOAM CFD Solver.
(2) To obtain To obtain shape change shape change drag parametersdrag parameters of each aerodynamic objectof each aerodynamic object..
1.8 Scope of Research
1.8 Scope of Research
This research is limited to CFD Analysis of Aerodynamic
This research is limited to CFD Analysis of Aerodynamic objects, without wind tunnel experimentobjects, without wind tunnel experimentss, , and will be done using Spalartand will be done using Spalart–Allmaras (SA)Allmaras (SA)– Shear Stress Transport (SST), Scale Adaptive Shear Stress Transport (SST), Scale Adaptive Simulation (SAS) Hybid Turbulence Model, with the help of OpenFOAM Computational Fluid Simulation (SAS) Hybid Turbulence Model, with the help of OpenFOAM Computational Fluid Dynamic (CFD) Dynamic (CFD) Finite Volume Finite Volume SSolver.olver.
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