Vehicle aerodynamics- CE00566-5Sam Lindop L018852A

12/12/2012

ContentsTypes of aerodynamic testing................................................................................................................2

Wind Tunnels.........................................................................................................................................5

Wing Test...............................................................................................................................................6

conclusion............................................................................................................................................11

References...........................................................................................................................................14

Types of aerodynamic testingAerodynamics is the way air flows around objects. It is very important to understand how the work when flowing around cars. This is because when you find ways to reduce drag you can you can improve other areas of the car such as fuel economy and speed. Modern technology has made it possible to accurately measure and test the aerodynamics of automobiles and aeroplanes. There are many ways to test and record the aerodynamics of vehicles, one of these ways is to use a wind tunnel. The earliest wind tunnels were developed in the 1930’s to test commercial air lines and then later developed to test fighter planes for World War 2. They work with the idea that a stationery object with air blown around it will react the same way a full working model would react the same way moving through stationery air. When used in motorsport they often test a section at a time for example a front wing and then they put all the parts together after they have tested all the separate parts they then bring them together. The first wind tunnel specifically built for cars was built in 1960 at MIRA. As computer power has increased they have been able to use computational fluid dynamics (CFD) to analyse design and styling before physical models have been produced. This can reduce the design period, cut down costs and give a detailed analysis on what the airflow is doing around an object in a very small time period. After you have done all the theoretical testing you can test aerodynamics on the road with flow visualisation paint and wool tuft testing. These flow with the airflow over the body of a car giving you real time visualisation of what is happening with the air flow over the car.

Wind tunnels can be classified by the speed in the test section and on the shape of them. Subsonic wind tunnels usually have a maximum speed of 135m/s and are the most cost effective type of wind tunnel due to the size and simplicity, therefore are commonly found in schools or universities, it is these types of wind tunnels that automotive companies use because they can test at a maximum of 305MPH which is more than adequate for a car companies. Transonic wind tunnels operate at about 350m/s and are most commonly used by the air craft industry because it is at these speeds that aircraft operate in. supersonic and hypersonic wind tunnels operate at very high speeds, they can reach Mach 15 and it achieves these speeds by using convergent and divergent nozzles. The power to produce such speeds is very high therefore they are not widely used. The two types of wind tunnel that can be classified on shape are open circuit and closed circuit. Open circuit wind tunnels are open at both ends and have honeycomb mesh to prevent dirt coming into the systems. Open circuit wind tunnels can be separated into two further categories, suck down tunnels and blower. Suck down tunnels have an inlet open to atmosphere and there is an axial fan or a centrifugal blower placed after the test section. These types of wind tunnel are not preferred because the entering wind enters with a significant swirl. Blower wind tunnels have a blower at the inlet of the tunnel which blows air in; they are still susceptible to swirl however much less sensitive to it. Open wind tunnels are not preferred because they have high operating costs because they are constantly accelerating the air because both of the ends are open to the atmosphere; they are also noisy when they are in operation because they are operating at high speeds for a long time. Closed circuit wind tunnels have the outlet connected to the inlet so is therefore it is the same air that is regulated. There is less chance of dirt getting in, the flow is more uniform and they are more economical than open wind tunnels because once the air is circulated they do not have to overcome any loss of flow other than that turning the corners and friction against the walls, whereas the open wind tunnels constantly need to accelerate the air. It is because of these reasons that closed wind tunnels are the first choice for larger companies.

The basic designs for the two types of wind tunnels are:

Figure 1-open wind tunnel

Figure 2- closed wind tinnel

The contraction cone is also known as the effuser.

Wind tunnels can vary in mass, area and cross sectional area depending on what is intended to test. For example the wind tunnel at the motor industry research association (MIRA) the wind tunnel is specifically designed to test cars. The size of the wind tunnel is 15 meters in length X 7.9 meters wide X 4.4 meters high. However they have also used the wind tunnel for other aspects other automotive sectors such as aerospace, motorsport, wind engineering and some sport sectors. Another wind tunnel designed for automotive purposes is a wind tunnel found at Tech which has the specifications of 2.23 meters high X 2.23 meters wide by 7 meters long, this differs to the wind tunnel at MIRA because this is only specifically used for automotive purposes which is why it is smaller than the wind tunnel at MIRA. Wind tunnels that are used for high speed testing such as supersonic wind tunnel testing the sizes are much smaller, for example a wind tunnel used by NASA is 2.73 meters wide X 1.36 meters high X 5.48 in length. The smaller cross sectional means that the pressure is going from a higher pressure to a very low pressure which increases the speed of the air passing through which means that it can achieve its speed of Mach 2.55. Subsonic wind tunnels can also be very large to enable large scale testing. For example the research and development centre Ames in the USA is 12 meters by 24 meters and another section is 24 meters by 36 meters. This enables them to test full scale models to how they work in as close to real life conditions as they can get without actually flying. To find out the size of the model that needs to be tested and the size of the wind tunnel companies have to consider the Reynolds. This is the ratio of the inertial forces to the viscous forces. If the Reynolds number is too low the air will not be flowing fast enough to break the boundary layer therefor the flow of the air will be turbulent. This means that your results not be valid because they will vary too much. The boundary layer is where the air has been slowing down because of the friction from the object. This loss in velocity means that the air above it is going faster which creates a pressure gradient which will encourage the air to become turbulent as it flows around the object. The boundary layer becomes a higher pressure which means that the object will gain an extra layer therefore changing the characteristics of the object.

Figure 3- Reynolds number

Computational fluid dynamics is predicting the fluid and air flows around an object using computational methods. It is used in a wide range of industries which involve automotive, motorsport, aeronautical and in many aspects of engineering. Using CFD you build a computational model that you want to test, you then apply the fluid flow physics you wish to test on the prototype and then the software outputs a prediction of the fluid dynamics.

CFD can do much more than predict the fluid flow around an object. It can predict the transfer of heat, change in mass e.g. down force, lift and perspiration. Phase change such as freezing or boiling, chemical reactions, mechanical movement such as an impeller turning and stress or deformation of an object. CFD gives you an insight into what you are testing, as it is difficult to make prototypes of some models so therefore CFD enables you to see how different mechanisms work under different air flows and different stress levels therefore gives you a better way of visualizing and improving and improving our design ideas. CFD is a tool used for predicting what will happen, therefore can answer many questions, in a sort time of testing you can predict how your model will perform therefore you can test many variations so that you can get an optimal result. The work that you do on CFD before physical building and testing means that you can design the model better and faster. CFD can improve the efficiency of the design and building stages because you don’t have to keep altering the various different parts to get the best product, the testing has already been done and you know the results therefore have to do minimal testing to confirm the results. Time and money is saved because the design time is reduced and the cost of materials is reduced. All of these mean that the product can bet to the market faster therefore start earning back the money it cost to build much faster.

Once manufactures have done all the testing from CFD and wind tunnels and have made the first prototypes they can do more road testing to validate and confirm their results. Motorsport has been at the forefront of aerodynamic testing for many years to improve their cars, and one of the ways that they test the airflow around the car is by using flow visualisation paint. Flow visualisation paint is a paraffin-based light solution that is applied to cars. This is paint that takes a while to dry; therefore they apply it before the car goes out to do a set number of laps. Where it subsequently dries shows the air flow patterns across the car, from this you can analyse where the flow goes and

what happens when you make changes. The advantages of this are this it gives you a full scale of what is actually happening. If you make changes you can see how it affects the air flow. However because it is on the car as it is going around everyone can see what the airflow is doing, for example if you have a problem everyone can see what it is. Another problem with the flow viz paint is that it makes a mess of all the equipment there you can waste a lot of

time cleaning it up when you could spend more time working on the cars improving them.

As we can see from figure 4 the McLaren team are testing the airflow over the spine of the car.

Another way to test air flow is wool tuft testing. This is where you take many pieces of wool equal in length and tape them to the area that is being tested. Then you take then and drive it at a set speed and from another car record what is happening to the pieces of wool. This gives a good visualization of what is happening to the

airflow and is very inexpensive therefore anyone can do it. A disadvantage of this is that if you do this testing at a public test day everyone can see what is happening.

Wind TunnelsThe type of aerodynamic testing I am going to analyse is wind tunnel testing. There are many different types of wind tunnels which have many different uses. Subsonic wind tunnels are what the automotive sector uses because they have the ability to get to the correct speed to test the cars. An advantage of this is that they are readily available to use because so many car manufactures have them. However they are very expensive to run because they are so large and use up a lot of energy to get the wind up to speed. This is a disadvantage to all wind tunnels because even though they get smaller in size they have to use more energy to get the wind up to speed. However a large benefit of this energy being used is that it can give you very detailed results in a quick amount of time which is another advantage of this type of testing. Subsonic wind tunnels are relatively inexpensive compared to the other types which means that some schools and universities have them installed which means that they can educate students for the future as well as testing cars for the future.

When we split the two types of wind tunnel we can compare them both. The advantages of an open wind tunnel is that are that there are less construction costs because there is less tunnelling to build and another advantage is that when you use visualisation aids such as smoke, it is more visible to see because the air is not being recirculated. Advantages to closed wind tunnels are a better flow of air because flow vanes in the corners and straighteners before the test area mean the flow is more uniform than open ended wind tunnels. These types of wind tunnels have lower operating costs because once the air is flowing at the optimum speed you don’t have to keep accelerating it, only maintain it because of the loss from the friction against the wall. This reduction in energy needed also means that the operation noise of closed wind tunnels is a lot less than that of open wind tunnels.

The disadvantages of open ended wind tunnels are that they use a lot of energy because they are not recycling air therefore they have to be constantly accelerating air. Another disadvantage is that because the air is not coming from a closed are the wind can be turbulent. The disadvantages of closed wind tunnels are that there are higher construction costs because there is more tunnelling to build to circulate the air, and there are also vanes and ducts to produce to help the air flow around corners and to create a more uniform flow of air. There are also hotter running conditions because there is nowhere for the air to lose heat, this could mean that you have to spend more money on cooling systems. The air in closed tunnel systems is also

Figure 4

not as clean as open wind tunnels because it picks up dust and fur from clothes and there is no way in which it can be filtered out. Another disadvantage to wind tunnel testing is that if you are testing a model you need to spend time making the model and calculating the Reynolds number which can be time consuming if you get it wrong.

Wing TestThe CFD system that we used was CHAM Phoenix VR. With this programme we can set the domain size to what is needed and also set it only test in 2D rather than 3D and this programme also enables us to change the velocity if needed and the angle in which the wing is at. The speed of which that I did my tests is 35.76m/s (80MPH).the reason why I did it at this speed is because it is at this speed when we can start to see visible changes in down force and drag. I did four tests starting with an angle of attacks of 0˚ going up in increments of 5˚ to a maximum of 15˚. I did this because it gives an even spread of results across the range and it doesn’t overcomplicate the results, if I did any less I wouldn’t have seen enough variation, and anymore there would be no significant difference until later on in the test. Interations are the number of calculations the system does in each section of the grid across the labyrinth. I had 50 boxes X 50 boxes making 2500 boxes total and I had the number of interations at 700.

What we can see in figure 5 and figure 6 is at the front of the wing there is an increase in pressure where the wind is hitting the front of the wing. On top of the wing there is more pressure a less velocity as the wind passes over, this means that the air is pushing down on the wing creating downforce. The downforce produced by this wing angle is Fz = -4.902815E+02 N. this is a negative number because the force works in lift, and negative lift is downforce. This wing angle also has the least amount of drag, 2.315529E+02N which is the lowest value from all the tests.

Figure 5- wing angle 0, pressure

What we can see in figures 7 and 8 is that as the wing angle increases the velocity below the wing increases and the pressure below the wing increase. The reason why the air is flowing faster underneath the wind is because the lower pressure creates a large pressure gradient so the air will be flowing faster to get out of the high pressure and into the low pressure. The downforce produced for this wing angle is Fz = -3.851457E+02 N. this is less than when the angle was at 0because the wing angle has increased therefore should increase downforce, this has not occurred because the angle means that the air is passing over and under the wing at more of an equal speed therefore reducing the pressure gradient is reduced therefore reducing down force.

Figure 7- wing angle 5 degrees, pressure

Figure 8- wing angle 5 degrees, velocity

Figure 9- wing angle 10 degrees, pressure

From figure 6 we can see how the pressure changes across the labyrinth as the wing angle is increased. Because the wing is at more of an angle, it is creating more of a blockage therefore the air gets pushed back on its self-creating high pressure in the top left corner. This high pressure means that the air below the wing is flowing even faster which is creating lower pressures and therefore increasing the overall downforce on the wing. The downforce produced at this angle is Fz = -1.650145E+03N. This is a high amount of downforce and it is what we would expect at the angle of attack. We can also start to see that there is a noticeable amount of blue at the tip of the wing in figure 10 which indicates there increasing amounts of drag that is starting to become more visible therefore the effectiveness of the wing is decreasing.

Figure 10- wing angle 10 degrees, velocity

At 15˚ the results are as similar as 10˚ however there is more uneven flow as the air goes over the tip of the wing; this creates a vortex effect which explains the circling higher amounts of pressure at this point. This pressure is pushing up on the rear of the wing which causes a drop in downforce. This explains why the downforce is Fz = -1.518089E+03N, this is 132.056N less than when the angle was 10˚. At this angle we can also see how the velocity is affected at the tip of the wing as a result of the vortex. There is much less velocity at this point than there has been throughout the testing. This angle of attack has the largest drag effect therefore has the most resistance.

Figure 11- wing angle 15 degrees, pressure

Figure

The downforce that is produced can be placed on a graph so that we can compare each result, the graph looks like this:

From the graph we can see that the angle where the most downforce is produced at 10˚. This is not what we would expect to see because the more angle you put on a wing, the more downforce it should produce. However the 15˚ could be where the wing starts to stall. Wing stalling is where the air can no longer flow around the object and therefore breaks away from it. This drastically reduces downforce, and is said to happen around the 15˚ mark, therefore what we are seeing the beginnings of the wing stalling.

Drag is a by-product of lift and related to the angle of attack, and is where the air is trying to move from low pressure to a high pressure, which is against the pressure gradient. We would expect there to be more drag with a higher angle of attack. The results for drag are shown below:

From the graph we can see that the drag is doing exactly what we would expect. This means that the more angle to wing has the less efficient it is, and the more power will be needed to overcome the effects of drag.

My tests can be improved by increasing the amount of wing angle tests I did, there is a gap between 5˚ and 10˚ where I would like to see what the trend for downforces is because the reading could be an anomaly which would explain why its downforce was higher than that of 15˚. Next time I would also add streamlines so show how the air flows over the wing, this would give me more insight as to what the air flow if doing after the wing.

conclusionTo conclude there are many types of aerodynamic testing all of them are very affective in research for the best aerodynamic package to what is needed. Aerodynamic testing can be used in all automotive aspects, for example motorsport teams can used to gain an advantage in cornering grip or straight line speed, and production cars can use it to find the set up with less drag to increase fuel economy.

The future of aerodynamic testing is getting more and more advanced. Currently the most advanced wind tunnel in the world is in Germany, owned by Mercedes-Benz. It can simulate all climate types, from -40˚C to 60˚C and achieve wind speeds of 164mph. this means that the engineers can optimise new vehicles or components for specific climatic conditions, or to ensure that they can work in all climates. The largest wind tunnel in the world in in Ames in the USA and enables NASA to test full scale models of their space shuttles so that they can get real life data which can save them time and increase accuracy because they don’t have to use as many calculations to get the results.

It is important in all aspects of the automotive industry to have a good aerodynamic strategy. A starting point would be to create the model using CAD software, for example Creo Parametric. Then once a full scale CAD model has been used, CFD software should be used to calculate how much downforce and drag is produced, then take it back to CAD to see if any improvements can be made, for example wing adjustments or body shape. Once you have optimised the model to the specifications that are needed then wind tunnel testing is the next step. First you would test a model half the size to cut down on production costs as this is only a test On this test air flow visualisation would also help so I would put wool tufts on the body and wings to see where the air is flowing and also to see if It can be improved. If the numbers do not match that of the CFD files or the air flow is different to that predicted then go back to the CAD model to see what can be improved and restart the process. Once the final ideas and additions have been made to the half scale model, then a full scale model should be made for aerodynamic testing, this should just be there to confirm the results that have already been made. After the model testing, prototypes should be made to test the wings and the body in real life conditions, and to aid the visualisation of the airflow flow viz paint should be used to see how the air is flowing over the car and then you can see if an changes can be made. The following flow chart shows the processes that should be taken:

1.

Start CAD design

CFD Testing

Are These The Results that you want?

No

Yes

Half Scale Wind Tunnel Testing

Are These The Results that you want?

Full scale wind tunnel test

Are These The Results that you want?

Track testing

Are These The Results that you want?

Yes

Yes

Yes

No

No

No

FinishYes

2. This strategy will work because all types of testing are used in the making of the final product. Each type of test can also test every part that needs to therefore no part is over looked and if there are mistakes it enables you to go back to the CAD stage to change what needs to be changed then you can go to the CFD stage to test the updates and change the angles if needed.

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