Cars’ Aerodynamics: Here’s What You Need To Know

Photo source: http://www.buildyourownracecar.com/race-car-aerodynamics-basics-and-design/

Racers particularly need to have the knowledge about car’s aerodynamics. This is because it’s one of the key elements to pull off a good drive. In this article, we will discuss about car’s aerodynamics and what basic principles you have to take note of.

Aerodynamics is the science of the way air flows inside and around objects. Overall, it could be called “Fluid Dynamics” since air is actually just an extremely thin fluid type. Beyond slow speed, the air flow inside and around a car starts to gain a noticeable impact in terms of acceleration, duel efficiency, handling, and top speed.

Thus, to create the ideal car, there’s a need to optimize how the air flows through and around the car’s body as well as its aerodynamic devices and openings.

Aerodynamic Principles

Drag

Regardless of the speed of a car, it takes an extent of energy to get the car moving through the air. This energy is needed to overcome a kind of force called Drag.

In terms of aerodynamics, Drag consists of three forces:

• Rear vacuum—also refers to the impact created by the air being incapable to fill the hole that’s left by the body of the car.
• Frontal Pressure- refers to the impact created by a car’s body pushing the air out of the way.
• Boundary layer is about the impact of friction caused by the slow movement of air at the body’s surface.

Among these 3 forces, we could get to depict most of the airflow’s interactions with the car’s body.

This energy is required in order to overcome a type of force known as Drag.

Frontal Pressure

The frontal pressure is led by the air trying to flow through the front of the car, like what’s shown here:

As thousands and even millions of air molecules draw near the car’s front, they start compressing, thus raising the air pressure on the front of the vehicle. Likewise, the air molecules that are travelling along the car’s sides are at atmospheric pressure. This means the pressure is lower than the molecules on the vehicle’s front.

Rear Vacuum

This is caused by a “hole” left in the air as the cars pass through it. In order to visualize the concept, look at this diagram:

Photo source: http://www.buildyourownracecar.com/race-car-aerodynamics-basics-and-design/

As the car drives down the road, the blocky sedan makes a hole in the air, the air then rushes through the body as depicted above.

At certain speeds beyond a crawl, the space right behind the vehicle’s trunk and rear window is basically empty or seemingly like a vacuum. These empty spaces are caused by the air molecules not being able to fill the hole as swiftly as possible. The air molecules try to fill in to this space, but the vehicle gets one step ahead each time. Thus, a continuous vacuum gets to suck in the reverse direction of the vehicle.

This failure to fill the hole left by the vehicle is called the Flow detachment.

Flow detachment only applies to the “rear vacuum” part of the drag forces. It gets a greater and greater negative effect as the car’s speed rises.

Thus, when a car achieves high speeds, it’s a must to design the car in such a way that the areas of flow detachment are limited. Preferably, let the air molecules follow the contours of a vehicle’s body work and fill the hole the car left, its suspension its ties, and its protrusions.

If you’ve seen the Le Mans race vehicles, you’ll have witnessed how the tail of each car tends to properly extend back of the rear wheels and constricts when viewed from the top or side. This bodywork enables the air molecules to join back to the vacuum seamlessly along the body and into the hole left by the vehicle’s cockpit, and front part, instead of suddenly filling a big empty space.

The force made by the rear vacuums can exceed which is caused by the frontal pressure. Thus, there’s a good reason to keep the scale of the vacuum made at the car’s rear minimal.

Turbulence is caused by the air flow’s detachment from the vehicle. The main inevitable detachment at the rear part of the car causes the so-called turbulent wake.

As the flow detaches, air flow gets very chaotic and turbulent in comparison to the seamless flow on the front part.

When we take a look at a particular protrusion from the vehicle, we can see flow turbulence and flow detachment coming into play.

The turbulence made by this detachment could affect the air flow to the sections of the vehicle. For example, intake ducts do best when the air which enters them could flow seamlessly. Wings can produce way more downforce with smoother flows over them. Thus, the whole length of the vehicle needs optimization to offer the least extent of turbulence at higher speed.

Drag Coefficient

To compare the drag created by a car versus another, the notion of Coefficient of Drag or Cd was made. Each car has a Cd that could be gauged by the use of wind tunnel data. The Cd could be maximized in drag equation to see the drag force at difference speeds.

The best Cd can be achieved if a car has these characteristics:

• Got a small grill/ nose to keep the frontal pressure minimal
• Got a minimal ground clearance below the grill to keep the air flow under a vehicle minimal.
• Got a windshield that’s steeply raked to prevent build-up at the front.
• Got a “fastback” style sloped bodywork or rear window or deck to keep the air flow attached.
• Got a converging tail in order to make the air flow stay attached.

If it seems like we’re describing a sports car here, you are right. For a car to be ideal, the body must be shaped like a tear drop since even the most expensive sports cars could encounter flow detachment. Nevertheless, tear drop shapes aren’t conducive to the part where a vehicle runs, and that’s close to the ground. Planes do not have this restriction. Thus, teardrop shapes work for them.

The most idea road cars now can manage a Cd of around 0.28. F1 vehicles, with their open wheels and wings, can manage at least 0.75.

If we suppose that a flat plate got a Cd of around 1.0, a Formula One vehicle can hardly be efficient. However, while a Formula 1 car doesn’t have the absolute aerodynamic drag efficiency, it can cover it up with its horsepower and downforce.

Frontal Area

Drag coefficient is itself useful in identifying how slippery a car is. To grasp the whole thing about the aerodynamic impact of a body shape of the car, we have to consider the car’s frontal area. It defines the actual size of the hole a car makes in the air when it passes through.

In the image below, you can see that the sedan creates a smaller hole into the air as compared to the truck.

Photo source: http://www.buildyourownracecar.com/race-car-aerodynamics-basics-and-design/

Thus, it’s by emerging the Cd w/ the frontal area wherein we reach the final amount of drag made by a car.

Downforce/ Lift

Downforce refers to the exact same force airplane wings experience as they lift. Each object that travels by air makes either a downforce or a lifting scene. Most of the race cars or road cars utilize aerodynamic devices like the inverted wings to force cars down to the road, facilitating the traction. The normal street vehicle has the tendency to experience lift or downforce. It is due to the car’s body shape which produces a low pressure area on top of itself.

As stated in Bernoulli’s principle, for a certain air volume, the higher the travelling air molecules’ velocity, the lower the pressure gets. Similarly, for a certain air volume, the lower the air molecules’ velocity, the higher the pressure gets. This principle can be applied to air in motion across a motionless object or to a car in motion, moving through a stagnant air.

In the discussion of frontal pressure above, we have mentioned that as the air rammed to the car’s front grill, the air pressure was high. The actual situation is that the air tends to slow down as it draws near the car’s front; thus, more molecules get constricted into a more limited area. Once the air becomes still, it will seek a lower pressure area like the top, bottom, or sides of the car.

You can see a demonstration of this effect in the diagram below:

Photo source: http://www.buildyourownracecar.com/race-car-aerodynamics-basics-and-design/

The body shape is the one that causes the downforce and lift from the air flow. The drag becomes cumulative as the air starts to flow to the rear from the front of the car.

When the air starts to flow over the car’s hood, it loses pressure. However, when the air goes through the windscreen, it tends to come up against the barrier again and momentarily achieves a high pressure. The low pressure area above the car’s hood makes a small lifting force which acts upon the hood area. The high pressure area of the windscreen’s front part then paves the way for a downforce. It’s the same thing with pressing down on your windshield.

The thing that causes road cars the most trouble is that there’s a huge surface area on top of the roof of the vehicle. As the high pressure air of the wind screen’s front travels through the windscreen, it then accelerates,  eventually causing the pressure to significantly drop. This low pressure tends to lift on the vehicle’s roof as the air flows over it.

The worst case—once the air goes through the rear windows, the notch made by the window going down to your trunk leads to the creation of a vacuum (low pressure space) which the air can’t properly fill. The flow then detaches and the low pressure causes a lift that acts upon the trunk’s surface area. Before the use of the aerodynamic devices to minimize this sort of effect, race car drivers can sense the car being light in the rear when at high speeds.

Aerodynamic Devices

Aerodynamic devices offered a way of making use of airflow through a car. Certain devices boost the efficiency of air flows in the car’s body. They help cars achieve their maximum potential as they hit the road. Other aerodynamic devices can also boost the traction.

Scoops/Positive pressure intakes

Scoops refer to positive pressure intakes that are beneficial in offering a subtle “ram air” or “supercharging” effect to a combustion engine. This works on the notion that the air flow constructs inside the “air box” if subjected to a steady and oncoming air flow. This air box got an opening which permits a sufficient air volume to get inside. The expanding air box then slows down the air flow to boost the pressure in the box. The faster the car runs, the faster the pressure boosts and air volume in the box.

NACA Ducts

NACA Ducts are beneficial when the air has to be drawn to a space that is not exposed to the directly approaching air flow that the scoop has access to. Usually, NACA ducts are made use along the car’s sides. The NACA duct makes use of the Boundary layer. This is a layer of slow moving air which “clings” to the car’ bodywork, particularly where the body work tends to flatten or doesn’t decelerate or accelerate the air flow. Areas, such as the side body panels and the roof can be cited as examples. The longer body panels or the roof, the thicker this layer gets.

The NACA duct rummages this slower moving space through an exclusively shaped intake. This intake shape drops toward the inner part of the bodywok. This draws the slow moving air into the opening towards the end of the NACA duct. The vortices are made by the walls of the duct shape as well, facilitating in the scavenging. The depth and the shape of the duct are crucial for correct operations.

The usual uses of NACA ducts include engine cooling and air intakes.

Spoilers

Spoilers are mainly used on sedan road or race cars to offer downforce. They’re also used in counteracting the tendency of this car-type to become “light” in the rear because of the lift produced by the rear body shape.

Spoilers serve as the barriers to the air flow to build up higher air pressure of the spoiler’s front. The high pressure area then acts upon the trunk/ deck’s space to give way to a downforce—as shown below:

Photo source: http://www.buildyourownracecar.com/race-car-aerodynamics-basics-and-design/

Front Air Dam

A Front air dam is usually used to avoid air from flowing beneath a car. This is done by make a “dam” or wall across the car’s front which extends close down to the road and typically along the sides to a certain extent. This paves way to an area of low pressure or vacuum beneath the car. This vacuum area, coupled with the high pressures above the vehicle and top of the front, produces a downforce at the car’s front.

The front air dam gets to block the air from going beneath the car. This leads to a low pressure space right behind the air dam. This then leads to downforce. However, with no side skirts, the air shortly enters from the car sides in order to equalize the pressure beneath the vehicle that reduces the downforce back.

If we extend the air dam along the car’s sides to turn into “skirts”, we could extend the low pressure or vacuum area produced beneath the car by the air dam too.

Wings

This is perhaps the most famous form of aerodynamic device. Wings do well by producing a great deal of downforce for a little penalty in drag. Spoilers might not be as efficient at all. However, because of their practical value, they’re used widely on sedans wherein wings can be somewhat less effective.

Wings produce downforce by a pressure difference between the bottom and top surfaces. This air pressure difference is caused by how the air flows around the wing shape.

As stated by Bernoulli’s principle, for a certain air volume, the higher the travelling air molecules’ velocity, the lower the pressure gets. Thus, to lower the air pressure, the air flow has to be speeded up.

A wing could do this by compelling the air molecules to travel various distant locations to the trailing edge from the leading edge. The wing’s long underside needs the air flowing on that side to move at a much higher speed in order to join up with air flowing at a lower speed.

The low pressure area beneath the wing enables the high pressure area above it to push down on the wings. The wing angle or angle of attack could be boosted to cause even bigger pressure differences. Eventually, the wing stalls and loses downforce. Drag also gets to increase with high angled attacks.

The downforce could be further increased without having to stall the wing through the use of multi-element wings which position one or more small wings at the rear of a larger wing.

Take note that the wing in the diagram below is displayed upside down in comparison to how it’s usually mounted on a race car. This diagram is intended to plot the negative pressure coefficients from the wing’s front to its rear.

Photo source: http://www.buildyourownracecar.com/race-car-aerodynamics-basics-and-design/

Venturi Tunnels

It somewhat is similar to the venture tube that’s usually observed in a laboratory. Venturi tunnels use the compression of a flow to produce high speed, low air pressure beneath the vehicle.

On race cars, the venture is formed by enabling the car’s undertray to shape like an inverted wing. The distance from the undertray to the road gives way to a compression and then expands to activate the low pressure made possible by the compression to act upon the car’s middle and rear. Venturi tunnels are really effective devices. However, they can also be prone to changes in car ride height.

Diffusers

Diffusers make downforce at the vehicle’s rear. Just like a venture tunnel, it shapes a curvature akin to the underside of the wing right before the low pressure surface at the car’s back, leading to a downforce. Just like the venture tunnels, diffusers tend to leverage the low pressure area at the back of the car and can even leverage high speed exhaust gases to the diffusers to make even lower air pressure.

Diffusers use the underside of a vehicle’s body to imitate the wing’s underside. The diffuser’s expanding opening paves the way for a low pressure area beneath the car’s rear that produces downforce.

 

Overall, the essence of aerodynamics can be mainly due to the downforce it creates. Many racers seek greater downforce for a better drive. However, the downforce isn’t everything. The final recipe of success is to figure out the best solution from the best possible downforce to the lowest air resistance possible, there is actually no best set-up to fit each racetrack. Thus, the fight is all about getting as close to the ideal one as possible. This is never an easy task as there could be over 20 possible settings for a rear wing and around 100 probably settings for the front wing.

In Formula 1 cars, the aerodynamics serves as the most essential factor in terms of design. The air duct panel from the front wheel to the side panel, for example, adds better speed than 2 or even 3 additional horsepower. Only those teams that have their own wind tunnel can manage to keep up with the quick advancements of this field. Many engineers dedicate over 15000 hours each year at the wind tunnel, and every complex could cost around 45 million euros.

In spite of all the changes to the regulations, aerodynamics still remains the dominant factor in designing F1 vehicles. Its developers are seemingly far from perfecting everything on this field. However, developments are going really fast. So, in no time, we can surely feel the biggest difference already. The best thing to do right now is simply to study the details to maximize its boons.