Last time we looked at the difference between mechanical and aerodynamic grip, how aerodynamic grip is generated, and how the various appendages on the car affect aerodynamic performance.

That should give you a reasonable overview of the aero elements of the car — as we continue to expand this series we'll look at each of these elements in more detail.

The science of overtaking

Those of you who are old enough probably remember the heady days of the late 1970s-early 1980s when there was overtaking was an accepted part of the sport. Not so much these days. What has changed?

To understand what is going on think back to how downforce is created. Air flows over a cambered surface. The shape of the surface causes air to change speed, which creates a pressure difference. This pressure difference creates downforce.

What is the one thing that can go wrong with this equation? Yes, that's right — its the type of air coming towards the wing (no British Rail jokes please).

Given that the airflow around the wing is responsible for the pressure difference that creates downforce, it isn't too much of a stretch to see that if the air approaching the wing isn't stable then the pressure difference is compromised.

When the wing is working in turbulent (dirty) air, its aerodynamic efficiency is reduced. This is because the air will not be directed parallel to the wing so will have to work harder to achieve the same effect. Add in the fact that turbulent air is associated with eddies and backward flow the effects are compounded.

When an F1 car (or any car) cuts through the air the rear wing creates vortices and upwash which severely disrupts the airflow behind it. Why? The car and rear wing manipulate the air into high and low pressure zones, which have a habit of mixing after passing over the rear wing. This is an unintended consequence of making the back of the car as aerodynamically efficient as possible — saloon cars suffer a lot less from this phenomenon.

In practice what this means is that for any car within about 1 second of this dirty air will suffer from reduced downforce at the front of the car. This reduction in aerodynamic grip will also increase drag (unless you can position the car under the upwash in which case slipstreaming is possible) and make it virtually impossible to pass.

Treating a car as a system

Take a car in isolation running through clean air. To generate maximum downforce the car is sculptured so that the front of the car modifies the air to cause minimum disruption to the airflow at the back of the car.

This means that designers have to understand the implications of making aerodynamic alterations to the car. For instance, take the design of the McLaren bridge wing. Although it may have a positive downforce effect at the front of the car, if the airflow to the back of the car is disrupted then the total downforce equation may actually be worse.

How, in practice is this done?

Ostensibly there are three ways that teams drive incremental aerodynamic performance. One is on the track; two is in the wind tunnel; and three is by computer modeling.

The most important thing is that all of these sources correlate. In other words, the computer results tie up with what is observed in the wind tunnel, which tie up with track performance. If one of these is out of balance, the consequences may be disastrous.

We saw this at Renault last year where they had erroneous wind tunnel measurements. This put the whole aero programme out of sync for the rest of the year and development is so quick they are still suffering the ill effects in 2008.

Computer modeling is probably where the most gains are to be had, but it isn't easy. The equations governing air flow are called the Navier-Stokes equations. For the physicists among you these are a set of non-linear, partial differential continuity and conservation equations. This discipline is called Computational Fluid Dynamics, or CFD for short.

The equations are very hard to solve and so require very powerful computers. To give you an idea of how much progress there still is to be made in this field is will probably be 30-40 years before we'll be able to aerodynamically model a car driving around one lap of a race track!!

BMW are perhaps the team that has invested the most in computer modeling and is probably one of the reasons why they have been so aggressive with some of their aerodynamic appendages (for instance, the rather unsightly viking horns).

The typical manufacturing process is that (once the team is happy that the wind tunnel, computer and track all correlate) to test concepts with CFD, then transfer them to the wing tunnel before finally trialing them on track.

Modern aerodynamics

From the description above you'd think that turbulence is exclusively a bad thing. Not true. A skilled aerodynamicist can actually harness some aspects of turbulence to his advantage.

Consider the edge of a wing (with no end plate). We know that this creates downforce by inducing a pressure differential over the wing. But what happens at the end tip of the wing?

Beyond the tip the pressure differential still exists but there is no wing to sustain it. The physics tells us that the higher pressure air wants to move to the lower pressure zone. This creates an energetic swirl of air commonly called a vortex.

Vortices do induce more drag so from that point might seem undesirable, but if harnessed can be very important enablers of downforce.

The ugly viking horns are a case in point. These are most likely vortex generators which increase the reliability of airflow to the rear of the car and allow the rear wing to work more effectively (by sending fast, lower pressure air to the lower part of the rear wing thus allowing it to work more effectively). That's right — by creating controlled turbulence at the front of the car we can make the rear of the car more efficient.

The barge boards are also another fine example of vortex generation. The barge boards are curved shape, which induces a pressure differential (both through a Bernoulli and a Venturi effect). As the barge board tapers down we get a stream of downward flowing vortices which get swept under the car to increase the efficiency of the diffuser. Some of the jagged edges that you'll see on the Ferrari and Red Bull barge boards are specifically designed to aim the vortices more appropriately.

Vortex generation and its role in aerodynamics is a topic we'll continue to revisit as this series progresses.

What will 2009 bring

Okay, what's the deal in 2009?

This could take an entire article in itself and I have already wittered on far too long.

In order to try to boost overtaking opportunities the FIA is changing the aero and mechanical regulations quite a lot. The overall objective is to reduce the amount of downforce, increase the mechanical grip, and when the car is in rough air give it the best opportunity to grip the tarmac.

To reduce overall aero grip the FIA has outlawed various aerodynamic devices, like barge boards and winglets. These are often the main vortex generators so should reduce turbulence significantly.

In addition the rear wing will be raised and made narrower, while the diffuser will be shortened and made a little steeper. This will tidy up the airflow leaving the car and reduce the downforce generated by the diffuser. Ultimately the plan is to make it easier for a trailing car to get a slipstream effect.

The front wing will be lowered and widened and made more standard. The idea here is that while reducing downforce in general, the wider (and lower) front wing will increase efficiency in dirty air, thus making it easier to overtake.

There will also be a driver-adjustable flap on the front wing so he can steepen the angle of attack — again this will increase downforce when running in dirty air.

On the mechanical side of things the main change is the move back to slicks. Slick tyres have a greater surface area in contact with the road so engender more grip.

KERS and push to pass are also designed to increase the propensity to overtake.

It is worth saying that nothing is set in stone as yet. When the regulations are finally agreed we'll have an in depth series of articles about what we can expect to see next season.

One thing is for sure, these changes should mix the 2009 grid up nicely.