Breaking the basics – Aerodynamics

Breaking the basics – Aerodynamics

Enthusiast or not, the world is filled with people who love automobiles that are quick and fast (Quick referring to acceleration and Fast meaning having high top-speeds). No matter how impractical, uneconomical or otherwise pinned as “just making no sense”, they are the most desirable ones. With increase in speed comes one substantial subject, namely, Aerodynamics.

Aerodynamics plays an immensely major role when dealing with high-speeds or motion in general. It is basically a study of the effect of any given fluid (liquid or gaseous) on a body moving at considerable speeds. Let us get some basics straight then, shall we?

Aerodynamic forces interact with a vehicle in motion causing drag, lift, lateral forces, and moments in roll, pitch & yaw.

We all have experienced the feeling of getting pushed backwards when the automobile accelerates, pushed forward under braking, pushed left when it is turning right or the other way round. Why does this happen? When a vehicle accelerates, the weight of the vehicle shifts to the rear and so does ours (due to inertia). Opposite, when braking; the weight shifts towards the front here. Both these forward and backward shifts in weight make up the term “pitching of a vehicle”. The side-ward shift in weight during turning or cornering results in “rolling of a vehicle”.  While, yawing is simply the rotating of a vehicle about an imaginary axis that’s passing vertically through its center.

1

Aerodynamic Lift

2

Down-force

Lift and down-force are two sides of a coin. In very simple words, down-force is the force exerted by air on any surface pushing it in the downward direction. Lift is generated by air exerting force on any surface from the bottom (in the upward direction). The wing shown in both diagrams could be a car’s spoiler or the wing of an aircraft. Let us understand how air causes lift. When an airplane gathers speed on the runway, air passes from both above and below its wings. The air passing above the wing has high velocity & low pressure and the air passing from the bottom has low velocity & high pressure. Because of the high pressure created below the wing, more air molecules gather and push it upwards. Since the pressure on the top of the wing is comparatively lesser, it isn’t enough to counter the lift that is being generated. Down-force is exactly the opposite of this. By controlling lift & down-force using electronic flaps on different wings of the aircraft, it is able to make turns(rolling moment). It should be noted here that a wing is never literally straight, its shape is always smooth and flowing for lift or down-force efficiency.

What is air resistance in general? Take a square piece of plywood, (say 1 sq.ft) and place it vertically facing a table fan. The force in the direction of the wood surface pushing it away from the fan is the Air Resistance here.

A very common misconception that most of us have today is this; mistaking aerodynamic drag for air resistance. Let’s say that a vehicle is travelling at 100 km/h in a straight path. Air will flow from the front of the vehicle to its back through the top, bottom and the sides. This force of air falling on the surface of the body & resisting the vehicle’s motion is air resistance. Hence, faster the vehicle travels, more is the air resistance.

Aerodynamic drag on the other hand, comes into play at the back of the automobile. Let’s see how. When air flows from the front of the vehicle to its back, there is a point where all the air meets again (the air from the top, bottom and sides of the vehicle). This point is called the point of separation. The area between this point and the back-end of the car does not contain air; making it vacuum. This vacuum area is called the wake area. Because of this vacuum in the wake area, a suction effect gets created at the back end of the vehicle. And it is this suction effect that pulls the vehicle backwards, hence creating aerodynamic drag.

72321main_tennisball090500

Streamlining of a tennis ball (air passing from left to right)

To understand drag better, let’s look at the streamlining of a tennis ball. Air is passing from the left to the right. As the tennis ball cuts through air, some air passes from over the top, some from the bottom and some falls right on the center of the ball. The ball is hence facing air resistance on its left side. The cloud of smoke that we see on the right side of the ball is the wake area. Since the wake area is vacuum, it attracts air from the top and bottom causing turbulence which in turn causes a suction effect that resists the ball’s motion towards the left.

A tremendous amount of money is being spent in the research and development of aerodynamics. “Fast” and “Aerodynamic” mean the same thing today. It is not feasible to make new models and do real world tests every single time to see how aerodynamically efficient they are. So if an automobile can’t be moved through air every time, why not move air past the surface of it? Hence manufacturers make several scale design models and put them in wind tunnels to check how good the design will perform aerodynamically. This way they save the trouble of making a full-sized model every time. A wind tunnel generally is a closed setup containing a massive fan that can blow or suck air at insane speeds (depending on its type). The test model is placed in the center of the setup. And streamlines of smoke are released with air to see how the air passes over the body of the car. The smoke helps make the motion of air visible over the surface of the body. Accordingly, either different parts are added/removed from the vehicle or the design is changed.

maxresdefault

Streamline passing over a test model in a wind tunnel

Parts like fenders, bumpers, wheel arches, under-body design, fore-body design, spoilers, air dams, diffusers, winglets, fairings, etcetera are played with to attain the desirable performance. Even the layout of the engine or every other component that is placed in an automobile, is placed in such a way that it causes the lowest possible amount of air resistance.  Even parts as small as side-mirrors are designed to be aerodynamically sound. An engineer always tries for a performance vehicle to cut air as much as possible. Bumpers and front diffusers are designed such that maximum air passes over or through the front of the vehicle instead of colliding with it. Air dams are given at various places in a car to direct air flow to places where air pressure needs changing i.e. from the bumper to the sides through the wheel arches or from the rear fenders to the back and so forth. Rear diffusers are meant to direct air towards the wake area and reduce drag.

Spoilers are a very common term. But why are spoilers called spoilers? A spoiler is a wing-like structure installed at an angle, generally added to the rear of a vehicle, to ‘spoil’ the air flow passing over the surface of the vehicle. Spoilers are installed to create ‘forced air resistance’ or ‘down-force’. When a vehicle is travelling at very high-speeds, the front end (bonnet) receives immense amounts of air resistance causing strong down-force. In some cases, the down-force on the front end could make it a pivot point for the rear end to completely lift off ground and topple the vehicle body. Spoilers resist air at the back of the vehicle creating enough down-force to counter this lift off. Plus, an appropriate amount of down-force on the front and rear of the vehicle increases the amount of traction and ultimately optimizes high-speed performance. For example, the spoiler in a McLaren P1 rises till speeds of 250 km/h to increase down-force. But past that speed it has to lower down a bit. Why? Because the amount of down-force generated at that speed is capable of breaking the rear suspension of the P1. This gives us an idea of how important aerodynamic forces are at high-speeds.

Every sector in the automobile industry is working towards aerodynamic perfection. The world has long before understood that a box-like machine loaded with a tonne of horsepower isn’t going to give an optimum output. Instead, the world has become a sensible place where ‘real performance’ is being tried to achieve. Aerodynamics plays a majorly crucial role in fuel economy, handling and NVH. Most of all, ‘High-Speed’. ‘Record-Breaking High-Speed’.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s