Aerodynamics:
Right, this is going to become a big post over time so I will add little by little and more and more as time goes on, probably becoming more and more advanced as it comes. And I will also ad things as i learn them.
Now I’m not saying I have unsurpassed knowledge in this field, I still have a lot to learn and there is nobody on earth that fully understands aerodynamics and thermo dynamics in full. So if there are any suggestions or corrections please say or pm me as I would like to keep it all in the first post.
All drawings done on autodesk inventor, and aero in paint, as I can’t run cfd on my computer. It’ll have to do. It’s not great but I will get better at using paint, it’s a black art lol.
Ok
Section 1: Principles of drag and pressure differentiation.
These are the basic principles and the basic goals of automotive aerodynamics. The least drag and the highest pressure differentiation (in the vertical axis).
Pressure:
To get your head around what pressure actually is, it is best to think about it on an atomic level. If you think of a balloon that is blown up. What is actually stopping the material from returning to normal size? What it is is all of the molecules of air hitting against the sidewall of the balloon, the more they move the harder they will hit this wall, so the greater the pressure. Atmospheric pressure is 1bar and this is the other side of the balloon wall. If the pressure inside the balloon is greater than 1 bar it will expand. If below then it will contract. Simple balance of forces.Always remember that atmoshperic pressure is 0 on a gauge and may seem like nothing to us walking through it all day but is a
very substantial force.
If you increase the amount of air in the balloon (increase the pressure) then the molecules will hit the wall more often.
Do not associate pressure with the mass of air, in a lot of cases this is true, but not all. If you tie a balloon so the mass of the air stays the same, then heat it up it will increase in size as the pressure goes up. I won’t go on forever about pressure, as it will just turn into thermodynamics, and that will bore the world.
Pressure differentiation:
This is the relationship between pressures as I touched on above. It’s the difference between pressures or fields of pressures that act upon an object in some way. This is the main principle of the much desired property of “down” force. This is the way of designing a component (for simplicity) that can manipulate the air flowing over it in such a way so as to create an area of high pressure over it, and low pressure under it. It can often be more efficient in terms of drag to just create the high pressure on top and leave the underside neutral, or low pressure below and the above neutral. But in most cases both sides of the plane (of the component) are used.
To achieve this a simple wing element design is used. If the speed of the air increases the pressure will have to decrease if the temperature remains the same. So as the flow goes over the wing it slows down and the pressure increases. The air going under the wing has lots of room to expand so the pressure decreases. This in turn accelerates the air coming under the wing further, lowering the pressure. This part is a similar principle used at the rear section of rear diffusers. This pressure difference creates a greater force above the wing than below, so the wing will be forced downwards, and as it’s attached to the car, the car downwards.
Drag:
Drag is a positive pressure difference in the horizontal axis from front to back. It is created by an object passing through air and cutting a “hole” in the air, as it is sometimes put. As the air passes over the back of the object it fills the gap behind it. The faster the object is travelling the bigger this gap will be. This gap in the air obviously is a vacuum, as there is no air there. So this is very low pressure, sucking the car backwards and acting against the forwards motion of the car. The front of the object is also contributing to this drag by creating high pressure as the air hits into the object and has to slow down and redirect around the body of the object. This area is called the frontal area, and is a key focus for race car designers in reducing overall drag.
The faster the car travels the greater the drag becomes and the more power is needed to overcome it and keep accelerating, and then the drag will get even greater and even more power is needed, until it will reach it’s terminal velocity for the power it can deliver.
A simple diagram of an object travelling through a fluid is shown below (something like a bullet)
You can see the low pressure area behind the object and the compacted air around the front of the object.
To work out the drag efficiency of an object there is a formula used to work out what is known as the “drag coefficient”
Drag coefficient = 
f_d = The force of the drag. Or “drag force”
p = The mass density of the fluid
v = The relative speed of the object in the fluid
A = The reference area
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Section 2: Basic aerofoil profiles and design concepts
This section will discuss the various design concepts and the theories behind them.
Type 1: Asymmetrical profile
This profile uses the shape of the profile to make the air flowing under it have to travel further than the the air flowing over it. It has to travel further in the same time, so it's speed must increase. The increase in speed decreases the pressure of the air. This creates a pressure differential and downforce is produced. This design is becoming more uncommon in circuit motorsport as you can't run the profile at a very high angle of attack without the air flowing under the wing stalling. (I will cover stalling, boundary layers and flow separation at a later date). These wings are usually run at zero or low angles of attack as they are hard to keep efficient at higher angles prior to stall.
Type 2: Symmetrical profile
These are the more common profile shapes found in motorsport as they are more versatile to angle changes. These can be altered by angle to change the downforce level, where as the above profiles often have to be a different profile swapped in to change the figures. At zero angle of attack these will produce no downforce at all. The way they produce downforce is partially in the same way as above, but there is high pressure created above the wing as well as low pressure below the wing, so they are far more efficient downforce producers. Although make the sacrifice of producing more drag. The air flowing below expands into the void created behind the angled wing, lowering it's pressure, because the same volume of air is flowing into a larger area from a smaller area. The air flowing above changes direction as it hits the face of the wing and compacts, producing high pressure. Downforce is created.
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Section 3: Aerofoil and multi element wing design
In the real world of wing design most elements use a blend of the above methods. A "symmetrical" form and a "asymmetrical" curve.
The profile pictured below is probably about as aggressive you could take the wing angle of a single element wing without any flow separation.
This second picture shows the same profile at a higher angle of attack and a scenario of flow separation.
The arrows show the flow under the wing separating part way up the profile. This separates as the air de energizes when the angle becomes to great. I will go into how this is actually to do with "viscosity" at a later date. (And yes i do mean viscosity). The reason this flow separation is so detrimental to aerodynamics is because it creates a massive amount of drag and reduces the effective area of the wing. The reason this drag is created is due to the area highlighted by the green dot in the pic. Or more the lack of this important area when separation occurs. Usually at this point both high pressure and low pressure meet, equalising the pressure. When flow separation occurs the low pressure doesn't meet the high pressure so does not equalise. This creates a bigger low pressure void behind the wing than usual, so more drag is the result.
You can see this meeting point in many places as a good example. On f1 cars when they go through smoke you can see 2 vortices coming from the rear wing tips. spinning. And also on plane wing tips. These vortices are when the high pressure and low pressure meet very fast. Very similar to how tornado's are formed in nature, high and low pressures. The air spins. The same thing is happening here.
Twin element wings
A twin (or multi) element wing is the easiest way to prevent flow separation.
The main element is the same profile as above when it was stable. As you know it can't be taken any further without separation. You want more downforce from the wing. So....
Adding a second element at a much higher angle of attack is how.
To stop the flow from de energizing and separating from the wing you need to re energize it. simple.
To do this you can add the second element just above the first with the top surface of the main plane flowing underneath it. This introduces a strong high pressure stream into the de energizes low pressure and keep it attached all the way up the (now very steep) second element.
The picture will explain all.
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That now covers the fundamentals of wing design. The next section will discuss under car aerodynamics
PROJECT GALLERY
I know i haven't done the next section yet. But i thought i'd keep a little log of various things i have made for fun, or cars.
This is a hillclimb wing i created for the formula renault at college. As it's current wing doesn't produce good downforce ratings.
Aerodynamics -section 2 done
Linear Mode
