effect of vortex generators on drag of a car

EFFECT OF VORTEX GENERATORS ON DRAG OF A CAR A Major Project Report submitted in partial fulfilment of the requirements for the award of the degree of Bachelor of Technology in MECHANICAL ENGINEERING by N Sumanth 1230811135 Jaydeep Lella 1230811121 Jagadish Bhyri 1230811110

DEPARTMENT OF MECHANICAL ENGINEERING GITAM INSTITUTE OF TECHNOLOGY GITAM UNIVERSITY VISAKHAPATNAM – 530045 A.P., (INDIA) April, 2015.

DECLARATION We hereby declare that the Project report entitled “EFFECT OF VORTEX GENERATORS ON DRAG OF A CAR” under the guidance of Dr.RAJESH GHOSH, Assistant

Professor. It is an original authentic work done in the

Department of Mechanical Engineering, GITAM Institute of Technology, GITAM University, Rushikonda, Visakhapatnam, submitted in partial fulfilment of the requirements for the award of Bachelor of Technology degree in Mechanical Engineering. The matter embodied in this project work has not been submitted earlier for award of any degree or diploma to the best of my knowledge.

N SUMANTH 1230811135 JAYDEEP LELLA 1230811121 B JAGADISH 1230811110

ACKNOWLEDGEMENT The satisfaction that accompanies the successful completion of any task would be incomplete without the mention of people who made it possible and whose constant guidance and encouragement crown all the effort with success. I express our deep sense of gratitude to Dr. RAJESH GHOSH, Assistant Professor, Department of Mechanical Engineering, GITAM Institute of Technology, for the esteem guidance and encouragement he provided throughout the project. I would like to thank our Professor & Head of Department, Dr. V. SRINIVAS for his encouragement, moral support, for providing all the facilities in the department and for the smooth functioning of the project. I are grateful to all the faculty Members of Department of Mechanical Engineering for their kind help during the whole development of the project. I would like to thank our friends and family members for their valuable cooperation.

ABSTRACT Aerodynamic drag is the force that opposes the direction of thrust of a car and is not a desirable force, as it eventually depreciates the mechanical efficiency of the car. One of the main causes of aerodynamic drag is boundary layer separation at the trailing end of the car. To minimize the effect drag, vortex generators are tested at the rear end of the car. Vortex generators delay local flow separation, which otherwise, can often result in increased drag, particularly pressure drag which is caused by the pressure differential between the front and rear surfaces of the object as it travels through the fluid. The model of the car is first tested in a wind tunnel, with and without the vortex generators. Subsequently, CFD simulations are used to carry out flow analysis around the car, and the post-processing tool is employed to enhance the results. Keywords: Aerodynamic drag, mechanical efficiency, boundary layer separation, pressure drag, Vortex generators

I.

II.

OVERVIEW A.

INTRODUCTION

B.

PROJECT OBJECTIVE

C.

PRE-REQUISITES

D.

PROBLEM STATEMENT

BACKGROUND A.

AERODYNAMICS

B.

DRAG

C.

D.

1.

DEFINITION

2.

TYPES OF DRAG

3.

AERODYNAMIC DRAG (IN CARS)

4.

EFFECTS OF DRAG

VORTEX GENERATORS 1.

OBJECTIVE OF USING VG

2.

MECHANISM AND WORKING

3.

DESIGN ASPECTS

COMPUTATIONAL FLUID DYNAMICS

III. METHODOLOGY A.

COURSE OF ACTION

B.

ANALYSIS USING WIND-TUNNEL

C.

COMPARISON

D.

ANALYSIS USING FLUENT SOFTWARE

IV. RESULT & DISCUSSION V.

CONCLUSIONS

I.

A.

OVERVIEW

INTRODUCTION

The performance of any automobile is mainly dependent on two factors: The ability of the engine to produce power and the aerodynamic efficiency. With the power being constant for a given automobile, the only way to improve the performance would be to enhance its aerodynamic properties. Drag, one of the properties of flow that impedes the performance from reaching the ideal state, has to be minimized so as to increase the mechanical efficiency. One of the main causes of aerodynamic drag is boundary layer separation at the trailing end of a car. When the boundary layer separates, its displacement thickness increases sharply, which modifies the outside potential flow and pressure field. In the case of airfoils, the pressure field modification results in an increase in pressure drag, and if severe enough will also result in loss of lift and stall (in case of aircrafts), all of which are undesirable. For internal flows, flow separation produces an increase in the flow losses, and stall-type phenomena such as compressor surge, both undesirable phenomena. A simple method to decrease the effects of Drag is to alter the build of the automobile body to a very streamlined shape, as in a Formula One car or a salt lake racing car. This streamlined shape would offer minimum resistance to forward thrust generated by the car, and hence would be prone to minimum drag. But, such streamlined shaped cars would be very expensive to manufacture and hence do not have any commercial application. The key to manufacturing an efficient passenger vehicle is to strike a balance between the performance aspects and the ergonomic aspects. For all practical purposes most passenger vehicles do not have ideal streamlined shapes, and these cars have coefficient of drag in the range of 0.2 to 0.5 (0.1 - least bluff bullets and 1 - most bluff solid cube). Hence, we need to find another method to reduce the effects of drag.

This is where Vortex Generators (VGs) come in. A vortex generator (VG) is an aerodynamic device, consisting of a small vane usually attached to a lifting surface (or airfoil, such as an aircraft wing or a rotor blade of a wind turbine. VGs may also be attached to some part of an aerodynamic vehicle such as an aircraft fuselage or a car. As experimental studies have shown that 40% of the drag generated is concentrated at the rear geometry, the vortex generators are tested only at the rear end (roof of the rear windshield). When a golf ball, an airplane, or any solid object moves through the air, a small layer of air called a boundary layer surrounds the object. Since the boundary layer is somewhat viscous, or sticky, and slow-moving compared to the moving golf ball or flying plane, it falls behind and separates from the object, creating a wake. This wake creates drag on the object and slows it down, resulting in shorter drives for golfers and higher energy requirements, as well as the potential for loss of lift, for airplanes. To combat this problem, the dimples on the golf ball and the vortex generators on an airplane's wings can delay the separation of the boundary layer by creating minor turbulence, which gives the boundary layer more energy and enables it to move a little faster to keep up with the object. Similar effect is expected when vortex generators are fitted onto a car.

Figure: Aerodynamic Study of a Golf Ball

B.

PROJECT OBJECTIVE

o To analyze the model sports car with and without vortex generator, using wind tunnel experiments and CFD analysis. o To reduce flow separation for normal sports car by using vortex generators. o Decrease lift at rear end and reduce overall the drag of car.

C.

PRE-REQUISITES

To successfully carry out the project, we need to understand and strengthen our foundations on some core topics. Cogent understanding of the basics of Fluid Dynamics is essential, without which the project would be futile. Concepts such as Boundary layer separation, Adverse pressure gradient, Turbulent and Laminar flow, Drag and Lift are crucial for the unhindered progress of this project.Experimental analysis using the Wind-tunnel requires us to have a theoretical understanding of the machine, before we go ahead with the analysis. Then, the operational knowledge

D.

PROBLEM STATEMENT

A simple method to decrease the effects of Drag is to alter the build of the automobile body to a very streamlined shape, as in a Formula One car or a salt lakeracing car. This streamlined shape would offer minimum resistance to forward thrust generated by the car, and hence would be prone to minimum drag.But, such streamlined shaped cars would be very expensive to manufacture and hence do not have any commercial application. The key to manufacturing an efficient passenger vehicle is to strike a balance between the performance aspects and the ergonomic aspects.

II. BACKGROUND A. AERODYNAMICS  Introduction To Aerodynamics Most of us don't think of air or wind as a wall. At low speeds and on days when it's not very windy outside, it's hard to notice the way air interacts with our vehicles. But at high speeds, and on exceptionally windy days, air resistance (the forces acted upon a moving object by the air -- also defined as drag) has a tremendous effect on the way a car accelerates, handles and achieves fuel mileage. This is where the science of aerodynamics comes into play. Aerodynamics is the study of forces and the resulting motion of objects through the air. For several decades, cars have been designed with aerodynamics in mind, and carmakers have come up with a variety of innovations that make cutting through that "wall" of air easier and less of an impact on daily driving. Essentially, having a car designed with airflow in mind means it has less difficulty accelerating

and

can

achieve

better

fuel

economy

numbers

because

the engine doesn't have to work nearly as hard to push the car through the wall of air. Some high-performance cars even have parts that move air smoothly across the underside of the car. Many also include a spoiler -- also known as a rear wing -- to keep the air from lifting the car's wheels and making it unstable at high speeds.

Figure 1: representation of aerodynamics of a car.

 Comparison with aircraft aerodynamics o Automotive aerodynamics differs from aircraft aerodynamics in several ways. o First, the characteristic shape of a road vehicle is much less streamlined compared to an aircraft. o Second, the vehicle operates very close to the ground, rather than in free air. o Third, the operating speeds are lower (and aerodynamic drag varies as the square of speed). o Fourth, a ground vehicle has fewer degrees of freedom than an aircraft, and its motion is less affected by aerodynamic forces. o Fifth, passenger and commercial ground vehicles have very specific design constraints such as their intended purpose, high safety standards (requiring, for example, more 'dead' structural space to act as crumple zones), and certain regulations.

 Methods of studying aerodynamics Automotive aerodynamics is studied using both computer modelling and wind tunnel testing. For the most accurate results from a wind tunnel test, the tunnel is sometimes equipped with a rolling road. This is a movable floor for the working section, which moves at the same speed as the air flow. This prevents a boundary layer from forming on the floor of the working section and affecting the results.

Figure 2: Air flow (blue arrows) and Drag (red arrows) representation over a sedan car.

B. DRAG  Definition Drag is the aerodynamic force that opposes an vehicle's motion through the air. Drag is generated by every part of the vehicle. Drag is a mechanical force. It is generated by the interaction and contact of a solid body with a fluid (liquid or gas). For drag to be generated, the solid body must be in contact with the fluid. If there is no fluid, there is no drag. Drag is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid. If there is no motion, there is no drag. Drag is a force and is therefore a vector quantity having both a magnitude and a direction. Drag acts in a direction that is opposite to the motion of the aircraft. Lift acts perpendicular to the motion. There are many factors that affect the magnitude of the drag. Many of the factors also affect lift but there are some factors that are unique to vehicle drag. As an object moves through the atmosphere, it displaces the air that surrounds it. The object is also subjected to gravity and drag. Drag is generated when a solid object moves through a fluid medium such as water or air. Drag increases with velocity -- the faster the object travels, the more drag it experiences.

CD, FRICTION = FD, FRICTION /0.5 ρ*V2*A CD, PRESSURE = FD, PRESSURE /0.5 ρ*V2*A When the friction and pressure drag coefficients or forces are available, the total drag coefficient or drag force can be determined by simply adding them, CD =CD, FRICTION + CD, PRESSURE

and

FD = FD, FRICTION + FD,PRESSURE

 Types of drag

Types of drag are generally divided into the following categories: o -

Parasitic drag, consisting of: Form drag: This type of drag arises because of the shape of the object. The general size and shape of the body are the most important factors in form drag; bodies with a larger presented cross-section will have a higher drag than thinner bodies; sleek ("streamlined") objects have lower form drag.

-

Skin friction: Skin friction drag arises from the friction of the fluid against the "skin" of the object that is moving through it. Skin friction arises from the interaction between the fluid and the skin of the body, and is directly related to the wetted surface, the area of the surface of the body that is in contact with the fluid.

-

Interference drag: Interference Drag is drag that is generated by the mixing of airflow streamlines between airframe components such as the wing and the fuselage, the engine pylon and the wing.

o

Induced drag: That part of the drag caused by the downflow or downwash of the airstream passing over the wing of an aircraft, equal to the lift times the tangent of the induced angle o f attack.

o

Wave drag: An additional drag caused by the air becoming sonic over any portion of the body. T wo things cause this drag: first, when the air crosses the shock wave, some energy is lost as drag because the temperature rises across theshock wave. Second, a shockinduced boundary-layer separation increases the drag.

Graph 1: Variation of types of drags with air speed.

 Equation for Drag: In fluid dynamics, the drag equation is a formula used to calculate the force of drag experienced by an object due to movement through a fully enclosing fluid.

o

is the drag force, which is by definition the force component in the direction of the flow velocity

o

is the mass density of the fluid

o

is the flow velocity relative to the object,

o

is the reference area, and

o

is the drag coefficient – a dimensionless coefficient related to the object's geometry and taking into account both skin friction and form drag. The drag equation may be derived to within a multiplicative constant by the method of dimensional analysis. If a moving fluid meets an object, it exerts a force on the object. Suppose that the variables involved – under some conditions – are the:

o

speed u,

o

fluid density ρ,

o

viscosity ν of the fluid,

o

size of the body, expressed in terms of its frontal area A, and

o

drag force FD. Using the algorithm of the Buckingham π theorem, these five variables can be reduced to two dimensionless parameters:

o

drag coefficient CD and

o

Reynolds number Re. Alternatively, the dimensionless parameters via direct manipulation of the underlying differential equations.

That this is so becomes apparent when the drag force FD is expressed as part of a function of the other variables in the problem:

This rather odd form of expression is used because it does not assume a one-to-one relationship. Here, fa is some (as-yet-unknown) function that takes five arguments. Now the right-hand side is zero in any system of units; so it should be possible to express the relationship described by fa in terms of only dimensionless groups. There are many ways of combining the five arguments of fa to form dimensionless groups, but the Buckingham π theorem states that there will be two such groups. The most appropriate are the Reynolds number, given by

and the drag coefficient, given by

Thus the function of five variables may be replaced by another function of only two variables:

where fb is some function of two arguments. The original law is then reduced to a law involving only these two numbers. Because the only unknown in the above equation is the drag force FD, it is possible to express it as

or and with

 Flow separation

When driving on a country roads, it is a common safety measure to slow down at sharp turns in order to avoid being thrown off the road. Many drivers have learned the hard way that a car will refuse to comply when forced to turn curves at excessive speeds. We can view this phenomenon as “the separation of cars” from roads. This phenomenon is also observed when fast vehicles jump off hills. At low velocities, the wheels of the vehicle of always remain in contact with the road surface. But at high velocities, the vehicle is too fast to follow the curvature of the road and takes off at the hill, losing contact with the road. A fluid acts much the same way when forced to flow over a curved surface at high velocities. A fluid climbs the uphill portion of the curved surface with no problem, but it has difficulty remaining attached to the surface on the downhill side. At sufficiently high velocities, the fluid streams detaches itself from the surface of the body. This is called flow separation. When a fluid separates from a body, it forms a separated region between the body and the fluid stream. This low-pressure region behind the body, where re-circulating and backflows occur is called the separated region. Pressure drag increases with increase in separated region. The region of flow trailing the body where the effect of the body on velocity are felt is called wake. The occurrence of separation is not limited to blunt bodies. Complete separation over the entire back surface may also occur on a streamlined body such as an airplane wing at sufficiently large angle of attack, which is the angle the incoming fluid stream makes with the chord of the wing. Flow separation on the top surface of a wing reduces lift drastically and may cause the airplane to stall. Stalling has been blamed for many airplane accidents and

loss of efficiency in turbo

machinery.An important consequence of flow separation is the formation and shedding of circulation fluid chunks, called vortices, in the wake region. the periodic

generation of these vortices shedding. This phenomenon usually occurs during normal flow over long cylinder or sphere for Re >90.

 Effects of Drag Drag has a direct effect on acceleration. The acceleration (a) of an object is its weight (W) minus drag (D) divided by its mass (m). Remember, weight is an object's mass times the force of gravity acting on it. Your weight would change on the moon because of lesser gravity, but your mass stays the same. To put it more simply: a = (W - D) / m As an object accelerates, its velocity and drag increase, eventually to the point where drag becomes equal to weight -- in which case no further acceleration can occur. Let's say our object in this equation is a car. This means that as the car travels faster and faster, more and more air pushes against it, limiting how much more it can accelerate and restricting it to a certain speed. How does all of this apply to car design? Well, it's useful for figuring out an important number -- drag coefficient. This is one of the primary factors that determine how easily an object moves through the air. The drag coefficient (Cd) is equal to the drag (D), divided by the quantity of the density (r), times half the velocity (V) squared times the area (A). To make that more readable: Cd = D / (A * .5 * r * V^2)

 Drag forces Drag, in vehicle aerodynamics, is comprised primarily of two forces. 

Frontal pressure Frontal pressure is caused by the air attempting to flow around the front of the car. As millions of air molecules approach the front grill of the car, they begin to compress, and in doing so raise the air pressure in front of the car. At the same time, the air molecules travelling along the sides of the car are at atmospheric pressure, a lower pressure compared to the molecules at the front of the car. Just like an air tank, if the valve to the lower pressure atmosphere outside the tank is opened, the air molecules will naturally flow to the lower pressure area, eventually equalizing the pressure inside and outside the tank. The same rules apply to cars. The compressed molecules of air naturally seek a way out of the high pressure zone in front of the car, and they find it around the sides, top and bottom of the car.



Rear vacuum Rear vacuum (a non-technical term, but very descriptive) is caused by the "hole" left in the air as the car passes through it. To visualize this, imagine a bus driving down a road. The blocky shape of the bus punches a big hole in the air, with the air rushing around the body, as mentioned above. At speeds above a crawl, the space directly behind the bus is "empty" or like a vacuum. This empty area is a result of the air molecules not being able to fill the hole as quickly as the bus can make it. The air molecules attempt to fill in to this area, but the bus is always one step ahead, and as a result, a continuous vacuum sucks in the opposite direction of the bus. This inability to fill the hole left by the bus is technically called Flow detachment.

Flow detachment applies only to the "rear vacuum" portion of the drag equation, and it is really about giving the air molecules time to follow the contours of a car's bodywork, and to fill the hole left by the vehicle, it's tires, it's suspension and protrusions (i.e. mirrors, roll bars). The extra bodywork allows the air molecules to

converge back into the vacuum smoothly along the body into the hole left by the car's cockpit, and front area, instead of having to suddenly fill a large empty space. The reason keeping flow attachment is so important is that the force created by the vacuum far exceeds that created by frontal pressure, and this can be attributed to the Turbulence created by the detachment. Turbulence generally affects the "rear vacuum" portion of the drag equation, but if we look at a protrusion from the race car such as a mirror, we see a compounding effect. For instance, the air flow detaches from the flat side of the mirror, which of course faces toward the back of the car. The turbulence created by this detachment can then affect the air flow to parts of the car which lie behind the mirror. Intake ducts, for instance, function best when the air entering them flows smoothly. Therefore, the entire length of the car really needs to be optimized (within reason) to provide the least amount of turbulence at high speed. 

Lift and Downforce One term very often heard in race car circles is Downforce. Downforce is the same as the lift experienced by airplane wings, only it acts to press down, instead of lifting up. Every object travelling through air creates either a lifting or downforce situation. Race cars, of course use things like inverted wings to force the car down onto the track, increasing traction. The average street car however tends to create lift. This is because the car body shape itself generates a low pressure area above itself. How does a car generate this low pressure area? According to Bernoulli, the man who defined the basic rules of fluid dynamics, for a given volume of air, the higher the speed the air molecules are travelling, the lower the pressure becomes. Likewise, for a given volume of air, the lower the speed of the air molecules, the higher the pressure becomes. This of course only applies to air in motion across a still body, or to a vehicle in motion, moving through still air.

When we discussed Frontal Pressure, above, we said that the air pressure was high as the air rammed into the front grill of the car. What is really happening is that the air slows down as it approaches the front of the car, and as a result more molecules are packed into a smaller space. Once the air Stagnates at the point in front of the car, it seeks a lower pressure area, such as the sides, top and bottom of the car. Now, as the air flows over the hood of the car, it's loses pressure, but when it reaches the windscreen, it again comes up against a barrier, and briefly reaches a higher pressure. The lower pressure area above the hood of the car creates a small lifting force that acts upon the area of the hood. The higher pressure area in front of the windscreen creates a small (or not so small) downforce. This is akin to pressing down on the windshield. Where most road cars get into trouble is the fact that there is a large surface area on top of the car's roof. As the higher pressure air in front of the wind screen travels over the windscreen, it accelerates, causing the pressure to drop. This lower pressure literally lifts on the car's roof as the air passes over it. The flow is said to detach and the resulting lower pressure creates lift that then acts upon the surface area of the trunk. This can be seen in old racing cars, where the driver would feel the car becoming "light" in the rear when travelling at high speeds. All the above mentioned forces on a car can be observed in the following figure 3:

Figure 3: Forces acting on a car

C. VORTEX GENERATORS 

Objective of using Vortex Generators

Making a correlation between drag and fuel economy: In order to decrease the aerodynamic drag on a vehicle, the sources of drag must be analyzed. As mentioned previously, aerodynamic drag is the force that opposes the direction of thrust of a car and is not a desirable force. Given a set of vehicle conditions, the drag force can be calculated. Drag is a function of the frontal area of the vehicle, the density of the air, the coefficient of drag of the vehicle, and the vehicle speed squared. The effects of drag on a vehicle become even more prominent, however, when the engine power needed to overcome drag forces is realized. The engine power needed as a function of drag depends on the frontal area of the car, the density of the air, the coefficient of drag of the vehicle, and the vehicle speed cubed. The fact that the vehicle speed has a cubic relation to the force of drag reveals that a small change in the speed of the car can require an enormous amount of engine power to overcome the forces of drag. In addition, the relation between drag and speed shows that aerodynamics of vehicles do not matter so much at lower speeds; they have a much more profound effect at highway speeds. Now that the factors involved in creating drag have been analyzed, what can be done to decrease drag? While the density of air and the vehicle speed cannot be altered by the design of the vehicle, the frontal area and coefficient of drag can. Reducing the height and width of the car can reduce the frontal area, but there is a limit to how small this area can be since people must be able to sit comfortably inside the vehicle. Therefore, the easiest method of decreasing drag is to lower the coefficient of drag of the car. The coefficient of drag of a vehicle depends predominantly on the shape . Therefore, vehicle designers change specific aspects of the shape of the body of the vehicle in order to reduce the total aerodynamic drag and thus increase fuel economy.

When the boundary layer separates, its displacement thickness increases sharply, this modifies the outside potential flow and pressure field. A simple method to decrease the effects of Drag is to alter the build of the automobile body to a very streamlined shape, as in a Formula One car or a salt lake racing car. This streamlined shape would offer minimum resistance to forward thrust generated by the car, and hence would be prone to minimum drag. For all practical purposes most passenger vehicles do not have ideal streamlined shapes, and these cars have coefficient of drag in the range of 0.2 to 0.5 (0.1 - least bluff bullets and 1 - most bluff solid cube). Hence, we need to find another method to reduce the effects of drag.

A vortex generator (VG) is an aerodynamic device, consisting of a small vane usually attached to a lifting surface (or airfoil, such as an aircraft wing or a rotor blade of a wind turbine. VGs may also be attached to some part of an aerodynamic vehicle such as an aircraft fuselage or a car.

As experimental studies have shown that 40% of the drag generated is concentrated at the rear geometry, the vortex generators are tested only at the rear end (roof of the rear windshield).

Vortex generators on an airplane's wings can delay the separation of the boundary layer by creating minor turbulence, which gives the boundary layer more energy and enables it to move a little faster to keep up with the object. Similar effect is expected when vortex generators are fitted onto a car. Following figure 4 shows the vortex generators fitted onto a car.

 Mechanism and working of Vortex Generators: -

Flow Separation Flow separation occurs down the middle of the rear window and partly onto the boot. However, the flow re-attaches itself on the boot lid. The fact that the flow does re-attach is significant because the overall size of the wake (the area of disturbed, turbulent air being dragged along behind the car) is much smaller than if the flow separated from the trailing edge of the roof.

As the height of the car at rear end reduces the flow area increases, due to which the air expands and thus its velocity drops and pressure increases. This increased downstream pressure creates the force in opposite direction generating the reverse flow at point ‘C’, acting against the air movement. There is no reversal of air flow at point ‘A’ which is upstream of point ‘C’ since momentum of the boundary layer is prevailing over the pressure gradient (dp/dx). At point ‘B’ the momentum of the boundary layer and pressure gradient balance each other. The airflow near the lower end and close to the vehicle surface, within boundary layer losses its momentum due to the viscosity.

Figure 4 shows the wind velocity profile over the roof end of a car.

Figure 4: wind velocity profile at rear end

The Vortex Generators placed just before the separation points, supply the loss in momentum by generating stream wise vortices. Thus separation point will be shifted further into the downstream and allows the expanded airflow to persist proportionally longer and hence the velocity of flow at the separation point reduces with an increase in static pressure. Figure 5 shows the flow separation delay profile when vortex Generators are placed. This static pressure reduces the control of overall pressure in the entire flow separation region. As a result of increased back pressure, the drag force is reduced. Thus shifting the separation points provides advantage in drag reduction first is to narrow the separation region in which low pressure constitutes the cause of drag; another is to raise the pressure of the flow separation region. A combination of these two effects reduces the drag acting on the vehicle. But the Vortex Generators itself produces the drag. So the total effect is calculated by subtracting the drag produced by itself from the reduction in drag caused by shifting of the separation point downstream. Larger the size of Vortex Generators larger is the effect. But the effect will be optimized for a certain size of the Vortex Generator.

Figure 5: flow profile arounf VG’s

 Design aspects of Vortex Generators: Two completely different designs of vortex generators were used:

-

Triangular Vortex Generators.

Figure 6: Triangular vortex Generators

Dimensions of Triangular VGs: Height=6mm (h) Length=12mm (2h) Spacing=15mm Number=5

Figure 7: Dimensions for Triangular VGs

-

Delta winglet pair of Vortex Generators.

Figure 8: Delta winglet pair

Dimensions of Delta winglet type: Height=6mm Base Length=12mm The height chosen should be less than the boundary layer thickness. These VGs are to be fabricated using soft plastic and arranged using a template, which is designed based on proportions. The VGs should be installed at x/L = 0.733

Figure 9: Dimensions of a Delta winglet pair VGs.

D. COMPUTATION FLUID DYNAMICS Computational Fluid Dynamics (CFD) provides a qualitative (and sometimes even quantitative) prediction of fluid flows by means of

- mathematical modeling (partial differential equations)

- numerical methods (discretization and solution techniques)

- software tools (solvers, pre and post-processing utilities) CFD enables scientists and engineers to perform ‘numerical experiments’ (i.e. computer simulations) in a ‘virtual flow laboratory. Numerical simulations of fluid flow (will) enable

- architects to design comfortable and safe living environments

- designers of vehicles to improve the aerodynamic characteristics

- surgeons to cure arterial diseases (computational hydrodynamics)

- meteorologists to forecast the weather and warn of natural disasters

- safety experts to reduce health risks from radiation and other hazards

- military organizations to develop weapons and estimate the damage

- CFD practitioners to make big bucks by selling colorful pictures

CFD analysis process:

Problem statement

information about the flow

Mathematical model

IBVP = PDE + IC + BC

Mesh generation

nodes/cells, time instants

Space discretization

coupled ODE/DAE systems

Time discretization

algebraic system Ax = b

Iterative solver

discrete function values

CFD software

implementation, debugging

Simulation run

parameters, stopping criteria

Post-processing

visualization, analysis of data

Verification

model validation / adjustment

2 . 3 . 4 . 5 . 6 . 7 . 8 . 9 . 1 0 .

 Mathematical model: 1.

Choose a suitable flow model (viewpoint) and reference frame.

2.

Identify the forces which cause and influence the fluid motion.

3.

Define the computational domain in which to solve the problem.

4.

Formulate conservation laws for the mass, momentum, and energy.

5.

Simplify the governing equations to reduce the computational effort:

-

use available information about the prevailing flow regime

-

check for symmetries and predominant flow directions (1D/2D)

-

neglect the terms which have little or no influence on the results

-

model the effect of small-scale fluctuations that cannot be capture

-

incorporate a priori knowledge (measurement data, CFD results)

Add corresponding relations and specify initial/boundary conditions.

 Discretization process: The PDE system is transformed into a set of algebraic equations

1.

Mesh generation (decomposition into cells/elements)

-

structured or unstructured, triangular or quadrilateral?

-

CAD tools + grid generators (Delaunay, advancing front)

-

mesh size, adaptive refinement in ‘interesting’ flow regions

2.

Space discretization (approximation of spatial derivatives)

-

finite differences/volumes/elements

-

high- vs. low-order approximations

3. -

Time discretization (approximation of temporal derivatives) explicit vs. implicit schemes, stability constraints

Local time-stepping, adaptive time step control Iterative solution strategy: The coupled nonlinear algebraic equations must be solved iteratively

Outer iterations: the coefficients of the discrete problem are updated using the solution values from the previous iteration so as to o get rid of the nonlinearities by a Newton-like method o solve the governing equations in a segregated fashion

-

Inner iterations: the resulting sequence of linear sub-problems is typically solved by an iterative method (conjugate gradients, multi-grid) because direct solvers (Gaussian elimination) are prohibitively expensive

-

Convergence criteria: it is necessary to check the residuals, relative solution changes and other indicators to make sure that the iterations converge.

As a rule, the algebraic systems to be solved are very large (millions of unknowns) but sparse, i.e., most of the matrix coefficients are equal to zero.

 CFD simulations: The computing times for a flow simulation depend on

-

the choice of numerical algorithms and data structures

-

linear algebra tools, stopping criteria for iterative solvers

-

discretization parameters (mesh quality, mesh size, time step)

-

cost per time step and convergence rates for outer iterations

-

programming language (most CFD codes are written in Fortran)

-

Many other things (hardware, vectorization, parallelization etc.)

The quality of simulation results depends on •

the mathematical model and underlying assumptions

•

approximation type, stability of the numerical scheme mesh, time step, error indicators, stopping criteria .

III. METHODOLOGY A. COURSE OF ACTION (FABRICATION OF VORTEX

GENERATORS AND SCALE MODEL CAR) 

Fabrication of car model

Vehicle aerodynamics began with selecting scale model w h i c h should obey both geometric similarity and kinematic similarity. In the present study, the chosen model is streamlined sports car, FERRARI ENZO.

IMAGE 1: Scale model of Ferrari enzo

The actual dimension of the car is:

Length of the car

=

520 mm

Width of the car

=

225 mm

Height of the car

=

127 mm

Based on the consideration of blockage ratio in wind tunnel test section area, the scale model is as 1:10 which is exactly matched with test conditions of wind tunnel. The model is initially smoothened, to reduce induced frictional drag using PVC tape, araldite and sand paper as shown in image 2. The model is made hollow below it, in order to accommodate pressure sensors and also to use short pressure tubes. A base plate is suitably fixed at the bottom to facilitate measurement of only the external surface pressure for the car model.



Preparation of specimen model:

-

Several air taps are drilled on the surface of the car’s body, and 4x2 mm nylon pipes are attached to these taps.

-

All the pipes are catalogued to prevent any confusion in the future.

-

All the parts are reassembled and the gaps are sealed using m-seal.

-

The final model is smoothened using sandpaper to eliminate any uneven surfaces.

IMAGE 2: Air taps to be drilled are marked

IMAGE 3: Pressure pipes fitted on bonnet of model

IMAGE 4: Pressure pipes fitted on rear windshield

IMAGE 6: Specimen model with pressure pipes(side view)

IMAGE 7: Specimen with fully fitted pressure pipes

IMAGE 8: Finished model of specimen with pressure pipes



Fabrication of Vortex Generators As per the design aspects we have learned earlier this fabrication of VGs have been done. Triangle shaped VGs and Delta winglet pair VGs are designed and fabricated for the present study. Typical view of triangular vortex generator fixed in rear side of a truck is also shown by an image. These VGs are fabricated using soft plastic and arranged using a template, which is designed based on proportions

IMAGE 9: Triangular shaped vortex generators

IMAGE 10: Delta shaped vortex generators

IMAGE 11: VGs fixed onto specimen model

B. ANALYSIS USING WIND TUNNEL The aerodynamic study of the vortex generator in car model of scale ratio 1:10 was conducted using the wind tunnel facility available at GIT, Mechanical Department. The wind tunnel is of open-circuit and blower type, with the maximum wind speed of about 30 m/s (108 km/hr) attainable at the test section. The wind tunnel with long test section is preferred to achieve natural development of equilibrium boundary layer. Considering the scale ratio of 1:10, it is decided to conduct the experiment with low level of turbulence intensity. Pitot static tube is used for measuring the wind velocity and tube manometers are used to simultaneously display up to 16 measured pressures in millimetres water gauge, figure 7. The manometer utilises the principle of communicating tubes. The zero point is adjusted using a vertically sliding compensation chamber as a function of the measuring task. The resolution can (with a simultaneous reduction of the measuring range) be increased by tilting the unit. To ensure that the working environment is optimised, the height of the manometer can also be adjusted. Model is and it is fixed at zero degree i.e., the length of the car being parallel to the wind flow. Following test cases have been carried out : 1.

Experiment on model without Vortex Generators.

2.

Model with delta shaped Vortex Generators.

3.

Model with right angled Vortex Generators.

Figure 10: Wind Tunnel

Procedure of experimentation in wind tunnel:

o Total pressure taps are connected to t u b e s a t t h e i r r e s p e c t i v e h o l e s . o Initial velocit y of wind is found using the pitot tube. o R e q u i r e d p r e s s u r e t a p s a r e c o n n e c t e d t o 1 6 channel capacity pressure scanners of manometer tubes, d e p e nding on critical regions at required points where the pressure distribution should be known for the analysis. o

All the pressure tap locations are divided into 5 regions, namely,



Bonnet,



Front Wind shield,



Top,



Rear wind shield, and



Back.

o Pressures of the corresponding 5 regions are respectively averaged, after evaluating mean pressure coefficients. The pressures are normally expressed in terms of a non-dimensional pressure coefficient. o Experiment has been conducted for different velocity readings. o Velocity of the wind has been verified along with digital electronic anemometer, at different velocities.

IMAGE 12: Specimen placed in wind tunnel for experimentation IMAGE 13: Specimen under analysis for wind speed

C. COMPARISON Comparison of experiment and simulation Experiments

Simulations

Quantitative

Quantitative prediction of

description of flow

flow

phenomena using

phenomena using CFD

measurements

software

• for one quantity at a time

• for all desired quantities

• at a limited number of points

• with high resolution in

and time instants

space and time

• for a laboratoryscale model

• for the actual flow domain

• for a limited range of problems and

• for virtually any problem

operating condition

and realistic operating conditions

Error sources:

Error sources: modeling,

measurement errors,

discretization,

flow disturbances by the probes

iteration, implementation

As a rule, CFD does not replace the measurements completely but the amount of experimentation and the overall cost can be significantly reduced.

D. ANALYSIS USING FLUENT SOFTWARE

The Analysis is carried out at various speeds such as 20m/s, 30m/s, 40m/s and 60m/s in ANSYS-FLUENT and the results such as contours, vector plots, and turbulent kinetic energy and streamlines plots are plotted. The surface pressure contour is also observed in the analysis. The analysis includes process of five steps such as 

Geometry



Mesh



Setup



Solution



Results.

- Geometry: The modelling of the car was completed in Rhinoceros-3D and the .stp file format was then imported to Design Modeller in ANSYS.

- Mesh: The imported file geometry is imported into the Meshing module of Ansys. Here, the default mesh given by Ansys is modified such that the Orthogonal quality of the modified mesh is closer to unity. Ansys Meshing allows the user to control every element of the mesh, but it also has a predetermined settings template for ready-to-use purposes. The following image will you give you an idea of the settings we used for meshing our specimen car.

Project First Saved

Friday, April 10, 2015

Last Saved

Friday, April 10, 2015

Product Version

15.0 Release

Save Project Before Solution

No

Save Project After Solution

No

Units TABLE 1 Metric (mm, kg, N, s, mV, mA) Degrees rad/s

Unit System

Celsius

Angle

Degrees

Rotational

rad/s

Velocity Temperature

Celsius

Model (A3) Geometry TABLE 2 Model (A3) > Geometry Geometry

Object Name State

Fully Defined Definition

Source

D:\Proj\main\1_files\dp0\FFF\DM\FFF.agdb

Type

DesignModeler

Length Unit

Meters Bounding Box

Length X

5073. mm

Length Y

885.27 mm

Length Z

1362. mm Properties

Volume

6.1114e+009 mm³

Scale Factor Value

1. Statistics

Bodies

1

Active Bodies

1

Nodes

291735

Elements

989750

Mesh Metric

None Basic Geometry Options

Parameters

Yes

Parameter Key

DS

Attributes

No

Named Selections

No

Material Properties

No Advanced Geometry Options

Use Associativity

Yes

Coordinate Systems

No

Reader Mode Saves

No

Updated File Use Instances

Yes

Smart CAD Update

No

Compare Parts On

No

Update Attach File Via Temp File

Yes

Temporary Directory

C:\Users\User\AppData\Local\Temp

Analysis Type

3-D

Decompose Disjoint

Yes

Geometry Enclosure and Symmetry

No

Processing TABLE 3 Model (A3) > Geometry > Parts Object Name

air

State

Meshed Graphics Properties

Visible

Yes

Transparency

0.1 Definition

Suppressed

No

Coordinate System

Default Coordinate System

Reference Frame

Lagrangian Material

Fluid/Solid

Defined By Geometry (Fluid) Bounding Box

Length X

5073. mm

Length Y

885.27 mm

Length Z

1362. mm Properties

Volume

6.1114e+009 mm³

Centroid X

1237.3 mm

Centroid Y

442.97 mm

Centroid Z

681.55 mm Statistics

Nodes

291735

Elements

989750

Mesh Metric

None

Coordinate Systems TABLE 4 Model (A3) > Coordinate Systems > Coordinate System SOI Object Name Global Coordinate System XYPlane ZXPlane State Fully Defined Definition Type Cartesian Coordinate System ID 0. Coordinate System Program Controlled Suppressed No Origin Origin X 0. mm 493.54 mm Origin Y 0. mm 96.265 mm Origin Z 0. mm 112. mm Define By Global Coordinates Geometry Selection Location Defined Geometry Defined Directional Vectors X Axis Data [ 1. 0. 0. ] [ 0. 0. 1. ] [ 1. 0. 0. ]

Y Axis Data Z Axis Data Axis Define By

[ 0. 1. 0. ] [ 0. 0. 1. ] Principal Axis

[ 1. 0. 0. ] [ 0. 1. 0. ]

[ 0. 1. 0. ] [ 0. 0. 1. ] X

Fixed Vector Orientation About Principal Axis

Axis Define By

Global X Axis Y

Fixed Vector

Default

Transformations Base Configuration Transformed Configuration

[ 0. 0. 0. ]

Absolute [ 493.54 96.265 112. ]

Connections TABLE 5 Model (A3) > Connections Object Name

Connections

State

Fully Defined

Auto Detection Generate Automatic Connection On Refresh

Yes

Transparency Enabled

Yes

Mesh TABLE 6 Model (A3) > Mesh Object Name State

Mesh Solved

Defaults Physics Preference Solver Preference Relevance

CFD Fluent 0

Sizing Use Advanced Size Function Relevance Center Initial Size Seed

On: Proximity and Curvature Fine Active Assembly

Smoothing

High

Transition

Slow

Span Angle Center

Fine

Curvature Normal Angle Num Cells Across Gap

Default (18.0 °) Default (3)

Min Size

Default (0.777630 mm)

Proximity Min Size

Default (0.777630 mm)

Max Face Size

Default (77.7630 mm)

Max Size

Default (155.530 mm)

Growth Rate Minimum Edge Length

Default (1.20 ) 8.95670 mm

Inflation Use Automatic Inflation

Program Controlled

Inflation Option

Smooth Transition

Transition Ratio

0.272

Maximum Layers

5

Growth Rate

1.2

Inflation Algorithm

Pre

View Advanced Options

Yes

Collision Avoidance

Layer Compression

Fix First Layer

No

Gap Factor

0.5

Maximum Height over Base Growth Rate Type Maximum Angle Fillet Ratio

1 Geometric 140.0 ° 1

Use Post Smoothing

Yes

Smoothing Iterations

10

Assembly Meshing Method

None

Patch Conforming Options Triangle Surface Mesher

Program Controlled

Patch Independent Options Topology Checking

Yes

Advanced Number of CPUs for Parallel Part Meshing Shape Checking Element Midside Nodes

Program Controlled CFD Dropped

Straight Sided Elements Number of Retries

0

Extra Retries For Assembly

Yes

Rigid Body Behavior

Dimensionally Reduced

Mesh Morphing

Disabled

Defeaturing Pinch Tolerance

Default (0.699860 mm)

Generate Pinch on Refresh

No

Automatic Mesh Based Defeaturing

Off

Statistics Nodes

291735

Elements

989750

Mesh Metric

None

TABLE 7 Model (A3) > Mesh > Mesh Controls Object Name

Edge Sizing

State

Body Sizing Fully Defined

Scope Scoping Method Geometry

Geometry Selection 2 Edges

1 Body

Definition Suppressed Type Element Size Behavior Curvature Normal Angle

No Element Size Default

Bias Type

No Bias

Named Selections

5. mm

Default Default

Sphere Center

Influence

Soft

Growth Rate

Local Min Size

Sphere of

Default (0. mm)

5. mm SOI

TABLE 8 Model (A3) > Named Selections > Named Selections carvelocity- symmetry- symmetrysymmetry Object Name body inlet top side State Fully Defined Scope Scoping Method Geometry Selection 11 Geometry 1 Face Faces Definition Send to Solver Yes Visible Yes Program Controlled Include Exclude Inflation Statistics Type Manual 11 Total Selection 1 Face Faces Suppressed 0 Used by Mesh No Worksheet

pressureoutlet

road

Include

Another way of checking the mesh quality is by plotting the Skewness in the statistics table. Skewness Skewness Skewness is defined as center of

where e is inner face center, e’ is the connect

and A is area of face e. Theinner faces may be triangles or

quadrangles. P and E are centers of cells adjacent to face e. Cells may be tetra, pyramid, prism or hexa solid elements. The quality of skewness indicates the distance between the connect center and face center. And its value is normalized by the square root of inner face area. If these two centers, e and e’, are coincident, the skewness is equal to 1. The skewness is influenced by the area of inner face. The smaller skewness implies a bigger distance between two centers. By definition, the skewness may be negative. As shown in Fig 6108, the skewness is good for the left face, and is bad for the right one.

Fig. 4: The definition of skewness

Fig. 5: Good skewness V.S. poor skewness

After using different Sizing techniques and Inflation parameters, we succeeded in achieving a Skewness value of 0.8754. The Skewness value should be less than 0.95, otherwise the meshing is not accurate and the solution will not converge.

Setup: After the meshing, the Fluent module is used to set the testing environment parameters such as Boundary conditions, Solution Methods, Solution initialization, Monitors, Calculation activities, etc.. We used the following settings for our simulation:

Fluent Version: 3d, pbns, rke (3d, pressure-based, realizable k-epsilon) Release: 15.0.0 Title:

Models ------

Model

Settings

-----------------------------------------------------------------Space

3D

Time

Steady

Viscous

Realizable k-epsilon turbulence model

Wall Treatment

Non-Equilibrium Wall Functions

Heat Transfer

Disabled

Solidification and Melting

Disabled

Species

Disabled

Coupled Dispersed Phase

Disabled

NOx Pollutants

Disabled

SOx Pollutants

Disabled

Soot

Disabled

Mercury Pollutants

Disabled

Material Properties -------------------

Material: air (fluid)

Property

Units

Method

Value(s)

---------------------------------------------------------------Density

kg/m3

constant 1.225

Cp (Specific Heat)

j/kg-k

Thermal Conductivity Viscosity

constant

w/m-k kg/m-s

Molecular Weight

constant 0.0242

constant 1.7894e-05

kg/kgmol constant 28.966

Thermal Expansion Coefficient 1/k Speed of Sound

1006.43

m/s

constant 0

none

#f

Setup Conditions

air

Condition

Value

--------------------------------------------------------------Material Name

air

Specify source terms?

no

Source Terms

()

Specify fixed values?

no

Local Coordinate System for Fixed Velocities

no

Fixed Values

()

Frame Motion?

no

Relative To Cell Zone

-1

Reference Frame Rotation Speed (rad/s)

0

Reference Frame X-Velocity Of Zone (m/s)

0

Reference Frame Y-Velocity Of Zone (m/s)

0

Reference Frame Z-Velocity Of Zone (m/s)

0

Reference Frame X-Origin of Rotation-Axis (m)

0

Reference Frame Y-Origin of Rotation-Axis (m)

0

Reference Frame Z-Origin of Rotation-Axis (m)

0

Reference Frame X-Component of Rotation-Axis

0

Reference Frame Y-Component of Rotation-Axis

0

Reference Frame Z-Component of Rotation-Axis

1

Reference Frame User Defined Zone Motion Function

none

Mesh Motion?

no

Relative To Cell Zone

-1

Moving Mesh Rotation Speed (rad/s)

0

Moving Mesh X-Velocity Of Zone (m/s)

0

Moving Mesh Y-Velocity Of Zone (m/s)

0

Moving Mesh Z-Velocity Of Zone (m/s)

0

Moving Mesh X-Origin of Rotation-Axis (m)

0

Moving Mesh Y-Origin of Rotation-Axis (m)

0

Moving Mesh Z-Origin of Rotation-Axis (m)

0

Moving Mesh X-Component of Rotation-Axis

0

Moving Mesh Y-Component of Rotation-Axis

0

Moving Mesh Z-Component of Rotation-Axis

1

Moving Mesh User Defined Zone Motion Function

none

Deactivated Thread

no

Laminar zone?

no

Set Turbulent Viscosity to zero within laminar zone? Boundary Conditions

Zones

name

id type

-------------------------------------symmetry-side

5

symmetry

pressure-outlet

6

pressure-outlet

symmtery-top

7

symmetry

velocity-inlet

8

velocity-inlet

raod

9

wall

symmetry

10 symmetry

car

11 wall

Setup Conditions

Symmetry-side

Condition Value -----------------

pressure-outlet

Condition

Value

----------------------------------------------------------Gauge Pressure (Pascal)

0

Backflow Direction Specification Method

1

Coordinate System

0

X-Component of Flow Direction

1

Y-Component of Flow Direction

0

yes

Z-Component of Flow Direction

0

X-Component of Axis Direction

1

Y-Component of Axis Direction

0

Z-Component of Axis Direction

0

X-Coordinate of Axis Origin (m)

0

Y-Coordinate of Axis Origin (m)

0

Z-Coordinate of Axis Origin (m)

0

Turbulent Specification Method

2

Backflow Turbulent Kinetic Energy (m2/s2)

1

Backflow Turbulent Dissipation Rate (m2/s3)

1

Backflow Turbulent Intensity (%)

9.9999998

Backflow Turbulent Length Scale (m)

1

Backflow Hydraulic Diameter (m)

1

Backflow Turbulent Viscosity Ratio

10

is zone used in mixing-plane model?

no

Radial Equilibrium Pressure Distribution

no

Average Pressure Specification?

no

Specify targeted mass flow rate

no

Targeted mass flow (kg/s)

1

Upper Limit of Absolute Pressure Value (pascal)

5000000

Lower Limit of Absolute Pressure Value (pascal)

1

symmtery-top

Condition Value -----------------

velocity-inlet

Condition

Value

---------------------------------------------------------Velocity Specification Method

0

Reference Frame

0

Velocity Magnitude (m/s)

40

Supersonic/Initial Gauge Pressure (pascal)

0

Coordinate System

0

X-Velocity (m/s)

0

Y-Velocity (m/s)

0

Z-Velocity (m/s)

0

X-Component of Flow Direction

1

Y-Component of Flow Direction

0

Z-Component of Flow Direction

0

X-Component of Axis Direction

1

Y-Component of Axis Direction

0

Z-Component of Axis Direction

0

X-Coordinate of Axis Origin (m)

0

Y-Coordinate of Axis Origin (m)

0

Z-Coordinate of Axis Origin (m)

0

Angular velocity (rad/s)

0

Turbulent Specification Method

2

Turbulent Kinetic Energy (m2/s2)

1

Turbulent Dissipation Rate (m2/s3)

1

Turbulent Intensity (%)

5

Turbulent Length Scale (m)

1

Hydraulic Diameter (m)

1

Turbulent Viscosity Ratio

10

is zone used in mixing-plane model?

no

Solver Settings --------------Equations Equation

Solved

------------------Flow

yes

Turbulence yes Numerics Numeric

Enabled

--------------------------------------Absolute Velocity Formulation yes Relaxation Variable

Relaxation Factor

---------------------------------------------Density

1

Body Forces

1

Turbulent Kinetic Energy

0.8

Turbulent Dissipation Rate

0.8

Turbulent Viscosity

0.94999999

Linear Solver

Solver Variable

Termination Residual Reduction

Type

Criterion

Tolerance

-----------------------------------------------------------------------Flow

F-Cycle

Turbulent Kinetic Energy

0.1

Flexible 0.1

0.7

Turbulent Dissipation Rate Flexible 0.1

0.7

Pressure-Velocity Coupling

Parameter

Value

-------------------------------------------Type

Coupled

Pseudo Transient

no

Flow Courant Number

50

Explicit momentum under-relaxation 0.25 Explicit pressure under-relaxation

0.25

Discretization Scheme

Variable

Scheme

----------------------------------------------Pressure

Standard

Momentum

First Order Upwind

Turbulent Kinetic Energy

First Order Upwind

Turbulent Dissipation Rate First Order Upwind

Solution Limits

Quantity

Limit

----------------------------------------Minimum Absolute Pressure

1

Maximum Absolute Pressure

5e+10

Minimum Temperature

1

Maximum Temperature

5000

Minimum Turb. Kinetic Energy

1e-14

Minimum Turb. Dissipation Rate 1e-20 Maximum Turb. Viscosity Ratio

10000000

Solution: We initially used First order Upwind method for doing 100 iterations. This is recommended as this method is better for approximation. Then, we shifted to Second order upwind and used these settings to do 500 iterations. As you can infer from the below image, our solution converged after 88 iterations (without Vortex generators) and 182 iterations (with vortex generators).

IV. RESULTS AND CONCLUSION

RESULTS: Without Vortex Generators: Contours around the car at 20m/s :

a) We can clearly observe that the flow over the car at this speed is laminar. (Velocity Magnitude)

b) There is no adverse pressure gradient behind the car and as a result reverse flow is not generated.

c) Dynamic Pressure contour

From the above contour maps it is clear that simulations to show the effect of Vortex generators cannot be done at this velocity.

Contours around the car at 30m/s:

a) The velocity magnitude contour below, shows us there is turbulent flow generated at this speed. The flow separation point is indicated.

b) The vortexes which are formed behind the car, because of adverse pressure gradient, is clearly visible in this picture.

c) Velocity magnitude contour: Low pressure

Contours around the car at 40m/s:

a) Low pressure area behind the car. (The main reason for pressure drag)

b) The stagnation point at which the velocity is zero, is indicated.

c) The reverse flow because of vortex formation continues to persist.

Drag force vs Velocity

Cd vs velocity

We decided to continue our simulations with vortex generators, at 40m/s. This is because at this speed the turbulent flow is fully developed, and the effects of any accessory can be clearly visualized.

With Vortex Generators: Contours at 40m/s with Vortex Generators:

a) The main function of Vortex generators is to generate vortices which reattach the flow. The vortices are distinctly visible in this image.

b) Separation point without vortex generators

c) Separation point with vortex generators

With the use of VGs, it is distinctly visible that the separation point has been shifted downstream and hence the separation has been delayed. Because the separation point is shifted further into the downstream and allows the expanded airflow to persist proportionally longer, the velocity of flow at the separation point reduces with an increase in static pressure and a decrease in drag.

Static Pressure Plots:

0.38m

a) Static pressure without VG

0.38m

b) Static Pressure with VG

The red line in the images indicates the immediate region after the VGs (which is at 0.37m from the leading edge). As we can clearly observe, there is a spike in the static pressure in the wake region of the VGs. This is the desired result.

Dynamic Pressure Contours:

a) Dynamic Pressure without VGs Low Pressure region in the wake of the roof results in drag.

b) Dynamic Pressure with VGs After the installation of VGs, the pressure in the wake region of the roof is increased. This increase in pressure results in the reduction of drag.

Drag Force Table:

Drag Force

Without

With Vortex

Vortex

Generators

generators Pressure

4.0609307

4.089835

Viscous

0.70691288

0.53595095

Total

4.7678436

4.62578595 values are in Newtons

The above table clearly illustrates the Drag force values before and after the installation of Vortex generators. With VGs the Total Drag Force has been reduced by 2.9%

V.CONCLUSION

Drag loading on automobiles due to wind loading is extremely important for achieving fuel efficiency. Local vortex generators have been used in aircraft wings to reduce the drag loading. In a similar concept, it is considered feasible that the drag loading on car can also be reduced by provision of vortex generating devices. A 1:10 scale model of a car was fabricated in plastic and instrumented with pressure taps over the entire top region of the car. Area averaged pressures were measured in regions where the sensitivity of pressure loading is insignificant towards the drag on the car. At other locations, individual pressure taps were provided to measure the pressures. Measurements were carried out at relatively low turbulence in the on-coming wind. Two types of vortex generators were investigated. The locations of the vortex generators were also varied. One of the configurations of the vortex generators, when located near end of the roof of the car, led to a reduction in drag force, for the case of wind acting parallel to the length of the car.

REFERENCES [1] Masaru Koike, Tsunehisa Nagayoshi, Naoki Hamamoto, “Research on Aerodynamic Drag Reduction by Vortex Generators”, Mitsubishi Motors Technical Review, pp. 11-16, No.16, 2004. [2] K. Sai Sujith, G.Ravindra Reddy, “CFD analysis of sedan car with vortex generators”, International Journal of Mechanical Engineering applications Research – IJMEAR, pp. 179-184, Vol 03, Issue 03, July 2012. [3] Darko Damjanovi�, et al., “Car design as a new conceptual solution and CFD analysis in purpose of improving aerodynamics”, Josip Juraj Strossmayer University of Osijek, Croatia. [4] Manan Desai, S. A. Channiwala, H.J. Nagarsheth, “Experimental and Computational Aerodynamic Investigations of a Car”, WSEAS Transactions on Fluid Mechanics, pp. 359- 368, Issue 4, Volume 3, October 2008. [5] Gu Zheng-qi, et al., “Numerical Simulation of Airflow Around the Car Body”, 2001-01-3086, SAE Technical paper series, International Body Engineering Conference and Exhibition Detroit, Michigan, October 1618, 2001. [6] John C. Lin, “Review of research on low-profile vortex generators to control boundary- layer separation”, Progress in Aerospace Sciences, pp. 389-420, No. 38, 2002. [7] Chainani. A, Perera. N, “CFD Investigation of Airflow on a Model Radio Control Race Car”, Proceedings of the World Congress on Engineering, Vol 2, July 2-4 2008. [8] Suhas V. Patankar, “Numerical Heat Transfer and Fluid Flow”, Taylor and Francis.