Motor vehicle bodywork varies widely depending on the purpose of the vehicle. Within motorsport there are conflicting needs to provide adequate safety to the driver but yet be lightweight, efficient and competitive.
To enable a competitive car to be designed vast amount of money is spent by teams for engineers to carry out aerodynamic testing including wind tunnel tests, computational tests and live track testing.
The vehicle discussed in depth within this thesis is a custom vehicle currently competing in the UK Shell Eco marathon.
The Shell Eco marathon is a competition that involves the design, build and driving of the most fuel efficient car, unlike conventional racing speed isn't a factor. The vehicle that is discussed and analyzed within this thesis is the current vehicle used by Kingdown School.
Although two very different competitions both vehicles rely upon aerodynamic factors and an emphasis is put on this when the bodywork for these vehicles is designed. This gives rise to an area of great importance in which the current bodywork for these vehicles can be examined, analyzed and improved. Reword as no longer including the Formula ford
Aims and Objectives
The project aims
To understand the design requirements for bodywork for open wheeled race cars.
To develop an understanding of aerodynamic testing and become a competent user of Computational fluid dynamic packages (CFD).
To improve the efficiency of Kingdown School's current Shell Eco Marathon car.
To have recognised any critical aerodynamic areas within the current Formula Ford bodywork.
To produce a written document that can be referred to and followed to understand and improve aerodynamic factors within open wheeled race cars.
To create a computer aided design (CAD) model of the current Formula Ford bodywork in preparation for proposals for level M.
The project objectives
To carry out research of previous work and create a report documenting relevant findings.
To create a CAD model of the current Shell Eco marathon car that can be used for aerodynamic analysis.
To carry out aerodynamic analysis of the current Shell Eco marathon car using CFD and compare results to wind tunnel testing.
To redesign the current Shell Eco marathon car and create a CAD model.
To carry out CFD testing in the aim to make the car more efficient.
Aerodynamics is the science that deals with the flow of a fluid around a body and the forces exerted on the body due to the relative motion between the fluid and the body. The total aerodynamic force can be resolved into two main components. The component that acts perpendicular to the oncoming flow is called lift force and the force component that acts parallel to the air flow is called drag. BERTIN & SMITH (1998) confirm it is conventional to resolve the forces into a component perpendicular and parallel.
HOUGHTON & CARPENTER (1993) describe drag as the component that acts in the same direction as the motion of the undisturbed stream and that it is the force that resists the motion of the body through the fluid. McCORMICK (1995) explains that there are many different forms of drag depending on the body and the type of flow that the body is immersed in.
In competitions such as the Shell Eco marathon requiring a fuel efficient vehicle the reduction of drag is vital as a reduction in drag will in turn make the car more efficient. McCORMICK (1995) highlights the importance of reducing drag explaining that a reduction of the drag force represents either a direct saving in fuel or an increase in performance. HUCHO (1983) confirms this and explains that a ten percent reduction in drag will result in around four percent reduction in fuel consumption and that no other measurement such as weight offers a comparable prospect. It is recognized that a curved shape allows air to flow smoothly around it. A flat shape fights air flow and causes more drag or resistance. To produce less resistance, the front of the object should be well rounded and the body should gradually curve back from the midsection to a tapered rear section. BERTIN & SMITH (1998) explain that a body shaped in this way is given the term streamlined.
As discussed before lift force is the force that acts on a body perpendicular to the oncoming flow. The force is generated due to air molecules having a greater velocity on the upper surface of the body than the lower. The difference in velocity creates a low pressure on the upper surface and this in turn creates an upwards force perpendicular to the air flow KATZ (1995). It is possible and favourable to create negative lift for certain bodies such as race cars.
BERTIN & SMITH (1998) discuss that the parameters that govern the magnitude of the aerodynamic forces include the following;
The vehicle size
Free stream velocity
Density of the undisturbed air
Reynolds number is the name given to a number that represents the ratio between inertial and viscous forces created in the air and is defined by the following formula:
Ï = Density of the fluid V= Velocity L=Characteristic length Î¼= Viscosity of the fluid
Knowing the magnitude of the Reynolds number can indicate the type of flow that will be prevailing.yuhil()
Types of air flow
It is important when carrying out analysis to know the type of flow that the body is operating in as factors such as lift and drag can change dramatically. A flow can either be laminar, transitional or turbulent. Laminar flow is the state of flow in which the various fluid sheets do not mix with each other the air molecules tend to flow parallel to the average velocity. Turbulent flow is the state of flow in which the various fluid sheets have the same average sheet but the molecules will momentarily move in the opposite direction. KATZ (1995) SMITH (1978) Discuss transitional flow and its effects.
Boundary Layers and flow separation
As fluid passes over a body the fluid particles immediately in contact with the surface are brought to a rest. Moving away from the surface of the body successive layers of the fluid are slowed down due to the shearing stress produced by the inner layers. The result of this is that there is a thin layer adjacent to the surface of slow moving fluid; this is known as the boundary layer. McCORMICK (1995) explains that near the front of the body the layer is thin and the flow is laminar. As the flow continues along the body the boundary layer under goes transition to a turbulent state this is due to factors such as surface roughness. BERTIN & SMITH (1998) agree and explain that once the flow is fully turbulent the shear forces at the surface increase resulting in a larger boundary layer. This change in boundary layer increases drag, the term given to this type of drag is surface friction drag.
As the flow continues towards the rear of the body it continues to thicken and approaching an area of static pressure that is opposing the flow. As the fluid in the boundary layer is slow moving it may not be able to overcome the adverse pressure gradient this causes the flow to begin to separate from the body surface. This results in an increase in drag that is called form drag.
HOUGHTON & CARPENTER (1993) explains that a laminar boundary layer will separate more rapidly than a turbulent layer, this causes confliction because having turbulent layers as already discussed causes an increase in friction drag. BERTIN & SMITH (1998) discuss this confliction and explain that although having a turbulent layer will increase the friction drag the reduction in form drag will dominate.
Aerodynamics in motorsport
Aerodynamics has been a concern within motorsport since the beginning of the 1900's with evidence of race cars having streamlined shapes in the aim to reduce drag. It wasn't until around the end of the 1960's that engineers began to realise the importance of reducing lift and this resulted in a sharp increase in recorded lap records at an Indianapolis track as discussed by KATZ (1995). It can be seen from comparing a modern race car with one from earlier in the 1900's that aerodynamics plays a key part when designing a modern race car. To be competitive the car has to be designed to the best possible aerodynamic efficiency. There has to be a compromise between some areas at the expense of the other as explained by WRIGHT (2000).
The primary aim of this investigation is to improve the fuel efficiency of a Shell Eco marathon vehicle therefore as explained by KATZ (1995) and SANTIN (2007) drag reduction is the primary concern. The vehicle will be subject to three main types of drag, form drag, friction drag and parasite drag.
Form drag (also known as pressure drag) is the force that is created due to the vehicle displacing the air as it moves along SMITH (1978). As seen in figure 1 as the air arrives at the leading edge of the vehicle there is a high pressure and therefore the air has a low velocity. As the flow continues over the vehicle the pressure becomes negative and the flow velocity increases. At the rear of the vehicle the flow is approaching an area of static pressure opposing the flow. As the flow is slow moving it isn't able to overcome the adverse pressure gradient this results in boundary separation. Due to the flow separation the pressure doesn't recover to the pressure at the leading edge and is therefore lower at the rear of the rear of the vehicle. This lower pressure at the rear creates a net rearward drag force. BARNARD (2001) explains that at the rear of the vehicle a wake is formed due to the flow separation, the size of the wake and in turn the amount of boundary layer pressure produced depends on where the flow separation occurs. This highlights the need in redesigning the Shell Eco marathon car to have a streamlined shape that the flow stays attached for as long as possible. SMITH (1978) confirms this and adds that with streamlined vehicles friction drag will be larger than the form drag however it is not possible to eliminate all form drag. Talk about pressure gradients favourable and unfavourable.
Figure -Distribution of measured pressure co-efficient
Friction drag is the force that is created due to the difference in the motion between the air and the surface. The surface of the vehicle will slow down the velocity of the air molecules as it passes and also the air will try to drag the surface along with the flow, this process creates viscous friction. It is important to understand friction drag because as already stated it will be the larger drag component on a streamlined vehicle and this is what is needed to create a low drag vehicle such is needed for the Shell Eco marathon competition. The magnitude of the force depends on the rate at which the layers right next to the surface are trying to slide relative to each other. It is important to understand the difference of laminar and turbulent flows at this stage as they have different effects on friction drag. BARNARD (2001) states that within laminar flows the relative speed decreases steadily through the layer, however within turbulent flows the outer edge is continually mixed in with the slower moving air so the average speed close to the surface rises rapidly with distance from the surface, thus producing a greater amount of drag for a given thickness of layer. Rephrase and add more references
Parasite drag is the drag that is produced by the pressure and friction caused by the addition of external protrusions such as mirrors and air scoops explained by SMITH (1978). The shell eco marathon design only has the addition of mirrors and this is seen as minimal. Need talking about more??
Coefficient of drag
BARNARD (2001) explains to compare the aerodynamic drag produced by a vehicle independent of the vehicle's a factor called the coefficient of drag is used (abbreviated to CD). The CD of a vehicle is independent of the vehicle's speed and is only related to the shape. The CD of a vehicle is given by the following equation;
Where; D= Drag force, Ï= Density of the fluid, V=Velocity of vehicle relative to the air, A= The reference area
KATZ (1995) explains that the reference area used is usually the frontal area of the vehicle; this is the area that will be used when calculating the CD of the Shell Eco marathon vehicle. The frontal area is measured as shown in Figure 2
Open wheel race cars
In this investigation it is important to note that the current design used by Kingdown School has an open front wheel layout. Exposed wheels produce a significant amount of drag due to the frontal area of each wheel and the fact that the flow separates behind each wheel. BARNARD (2001) explains that flow around the wheel separates early as shown in Figure 3 resulting in a large wake and that the flow around the side of the wheels produces strong vortices both creating drag. DOMINY (1992) underlines the importance of investigating the open wheel layout currently used stating that for an open wheel layout the wheel drag as a percentage of the overall vehicle drag can be between 35 and 50 percent.
With the knowledge gained its imperative to understand how to reduce the drag types that have been discussed. BARNARD (2001) explains in order to minimise form drag it is essential to keep the flow attached as far along the vehicle as possible. KATZ (1995) agrees and adds that to keep the flow attached the body should have areas of favourable pressure gradients
To understand the effects of aerodynamics testing needs to be conducted. BARNARD (1996) states that 'road vehicle aerodynamics does not lend itself to traditional methods of mathematical analysis".
The two most used testing methods are a wind tunnel or carrying out computations. Due to the complexity of road testing cars, trying to get readable data is hard to achieve. Also, before the car has been manufactured there is no car to be road tested KATZ (1995).
Wind tunnels allow testing in controlled conditions so that aerodynamicists can create conditions such as will be experienced. To get an accurate reading a full scale wind tunnel is desirable due to the effects of walls creating larger lift and drag values. However the cost of set up and running increases as a compromise of a smaller model and wind tunnel is used KATZ (1995).
CFD is widely used within motorsport, it is however time consuming and hard to obtain a detailed simulation of the partially separated flow field over a race car. Instead it is used to study localized areas such as a wing shape. The advantages of using CFD are that computations can be carried out without the need for any manufacture. KATZ(1995)
CFD is normally used alongside wind tunnels or as an initial testing procedure of designs as discussed by Waldemar Klem, Toyota F1:"We need to have at least one wind tunnel. No one could really decide whether CFD results have any correlation to the real world. So to gain this correlation means you need to get nearly the same results by a wind tunnel test."MEHTA (2006)
Analytical plan of current design
In order to carry out testing of the current design used by Kingdown School it was necessary to create a plan to follow;
Understand and learn the function of CFD software with no previous knowledge
Complete tutorials and carry out basic analysis of a blunt body to develop understanding
Complete a CAD model of the current chassis and bodywork
Carry out CFD analysis of the current design at various speed conditions experienced during the Shell Eco marathon competition
Carry out full analysis of the findings ensuring validity of results
Complete wind tunnel testing of the current vehicle
A complete comparison and discussion of CFD and wind tunnel testing.
Having no previous experience of using CFD software it is imperative to start at a basic level of understanding and progress. It is recommended to begin by completing a tutorial within the Ansys software, "The flow around a blunt body".
To begin the learning process the following is examined;
Domain- The domain is the area that is to be investigated. The domain size is dependent on the size of the body and the distance from the body that the user wishes to analyze.
Model meshing- The number of nodes used within the mesh model dictates the accuracy of the solution. A greater level of accuracy results in a longer time and more computer power required to compute the solution. The accuracy of the mesh is dependent on the complexity of the design therefore a more dense mesh is used in complex areas and areas of great interest.
Boundary conditions- The domain and body need to be divided into subparts called boundaries. At these points boundary settings are applied, assumptions such as symmetry can be applied to minimize computing time. It is important that the settings accurately match those the vehicle experiences to enable the collection of accurate results.
Flow model- There is various flow models that can be used that are based on different assumptions. The assumptions make the models more accurate in different flows.
Analysis- The analysis is a numerical method that approximates a solution by completing a user defined number of iterations.
Evaluation of results- The coefficients of drag calculated should closely match to those expected and those of other low drag vehicles.
Analytical findings of the current design
Show findings in tables, graphs etc and talk about the findings
Comparison of CFD findings, wind tunnel (compare streamlines) and analytical findings
Comparison of drag at different speeds- acceleration run, average speed, max speed and min speed.
Comparisons of different meshes if possible produce graphs.
Process of designing new bodywork
Talk about the process that will be used to design the new body work.
Idealised design of bodywork
Realistic design of bodywork taking into account constraints
Analytical plan for new design
Analytical findings of new bodywork
CFD and analytical findings
Reduction/ elimination of critical areas found in current design.