High Performance Vehicle Aerodynamics. (Copyright Torque Developments International)
Torque Developments International’s Technical Director wals us through this highly complex subject.
Part 1: The how and why?
This series will look at the importance of aerodynamics in motorsport and talk through few of the key concepts involved. The aim is to run through the main glossary of terms that you may already be familiar with and describe the meanings in some detail as we go along, with this first post we’ll look at some of the fundamentals of motorsport aerodynamics. The aerodynamic design method
CFD – this is short for computational fluid dynamics, which is the process of simulating fluid movement with computer based mathematical models. As computer power and programming skill have increased so has the accuracy of this development tool, CFD has become a real buzz word in the motorsport industry but the basic idea is just to simulate the airflow over or around a part of your car before either new parts or wind tunnel models are actually built.
- Formula Student CFD pressure plot with streamlines.jpg (59.93 KiB) Viewed 974 times
Wind Tunnel – For the testing of either scale models of devices or the testing of full size vehicles. The theory is simple enough, create a good fast clean air movement through a controlled environment so that the effect of the air acting on the test piece can be directly measured by the operators. Many designs use just moving air to perform measurements but in order to accurately measure under body aerodynamic effects the relative movement of the ground must be included and this done by way of a rolling road.
- Rolling road wind tunnel – Windshear.JPG (28.04 KiB) Viewed 979 times
Track Testing – Even though CFD is getting increasingly accurate and modern wind tunnel results are getting ever more trustworthy there is still no escaping the need for on track validation. The “real world” performance of devices can be assessed and any unintended side effects realised, most importantly new developments can hopefully be passed safe to go racing.
- Track testing ALMS Acura.jpg (82.82 KiB) Viewed 974 times
Aerodynamics in Motorsports
- Picture3 – Year VS latAcc Aero vs Mech.jpg (13.15 KiB) Viewed 979 times
As this graphic illustrates the power of aerodynamics to increase average speed and reduce lap times is absolutely massive. But as the picture below shows getting it wrong can be costly, we all know the term “sailing too close to the wind”.
- Picture2 – Mulsanne merc flip.jpg (21.76 KiB) Viewed 985 times
Due to the safety issues connected to the instability of certain types of aerodynamic effects the aero development of competition vehicles is something that motorsports governing bodies monitor extremely carefully and in many cases it is an area of development that is regulated heavily. Heavy regulation of the boundaries which may legally be explored by the engineers tends to result in this area very quickly becoming highly developed. As the winning cars successful design features are observed and subsequently copied by the other competitors they then also quickly catch up with fastest teams developments, so this potentially race result deciding aspect of car design becomes increasingly competitive and along with it more secretive.
- Picture4 – Redbull DTM car.jpg (33.04 KiB) Viewed 979 times
Part 2: The Tools of the Trade:
In this post I’m going to run through the basic details and idea’s behind some of the common aerodynamic devices found on performance focused vehicles.
The forces produced via aerodynamics ultimately come from a mix of the redirection of a significant amount of air mass, and generating differential pressures over significant area plan area’s.
Differential pressure generation becomes increasingly important when an object is operating close to the surface of the ground, and as cars do operate close to the ground I should mention Bernoulli’s principle at this point. This is a key scientific principle which is fundamental to all fluidic motion and as air is a fluid it is a principal which is crucial to aerodynamics. The most basic description of Bernoulli’s principle is to say that any change in the speed of a fluid flow must be accompanied by a corresponding change in its pressure. That is to say that if you speed a flow of air up then its pressure must drop accordingly, and it logically follows that if you slow a flow of air down then its pressure must rise accordingly.
The basis of this principle is routed in the conservation of energy, a core principle of thermodynamics making it a core principle of all modern sciences. Over 100yrs of rigorous scientific testing has proved Bernoulli’s principle correct, and after all if it were not correct a Boeing 747 would never get off the ground but as most of us will have probably witnessed at some point in our lives 747’s do regularly get all 400+Tons up off the ground and fly all over the world. So let’s state now that this principle and the relationship between air speed and pressure can be considered beyond question.
Front Air Dam – The front air dam is a piece of front bodywork which runs close to the road and by doing so limits the amount of air that can pass beneath the vehicle. In operation at high speeds this contributes to the higher pressures in front of the car whilst causing lower pressure underneath the car body.
- Picture5 – Nascar front air dam in action.jpg (39.45 KiB) Viewed 1815 times
Front Splitter – A basic front splitter protrudes forward from the bottom of the front air dam capturing some of the high pressure created by the air dam causing down-force on the splitter. More advanced versions can become just the leading edge of an entire front-end aerodynamic solution.
- Picture6 – Aston DBRS9 front splitter.jpg (26.43 KiB) Viewed 1815 times
Side Skirts – These are pieces of body work similar to the front air dam in that they run close to the road, these essentially guard the precious and fragile low pressure area underneath the car by preventing any relatively high pressure air from the sides of the vehicle mixing with it and raising the under body pressure.
- Picture7 – GT3RSR side skirts.jpg (34.08 KiB) Viewed 1815 times
Canards – From the French word for duck, these aerodynamic devices can generate a minor amount of high drag downforce simply by redirecting air flow upwards but this is not their main function. The main function of these devices is as “vortex generators” this is to say that they create high energy high speed spirals of airflow downwind of the device. In the specific case of canards the aim is to generate a high energy vortex which runs the full length of the car in order to help aid the side skirts in their mission, this is by adding a further barrier between the higher pressure air flowing above the vehicle and our precious and fragile low pressure area beneath the car body.
- Picture8 – Nissan R34 GTR front Canards.jpg (24.77 KiB) Viewed 1815 times
- Picture9 – Nissan R34 GTR front Canards illustrating side flow vortex.jpg (25.08 KiB) Viewed 1815 times
Ground Effect – Experienced motorsport fans will be familiar with the term “ground effect” from when it became a massive buzz word and area of contention in formula one during the late 70’s and early 1980’s. A development frenzy started by the engineering team supporting the motorsport visionary Colin Chapman found something very interesting during some routine wind tunnel testing. This major breakthrough resulted in the cars becoming able to generate previously unthinkable of levels G-force through corners by virtue of literally tons of highly efficient downforce coming from the cars cleverly shaped under floors. Unfortunately it would soon transpire that this downforce produced via the vehicle floor working in ground effect was in practice highly sensitive to key variables such as the amount of ground separation (or ride height), and also very sensitive to pitch and roll in the cars body. When you combine all of these inherent issues with the rushed implementation for competitive reasons, the lack of real understanding of the effect in three dimensions and the raw fact that most race circuits at this time were not all that flat or all that smooth we have the makings of a major problem. The problem came as unpredictability, and having unpredictability plaguing such a major component of the cars limit grip handling all too often proved to have fatal consequences, after a series of horrific high speed crashes the FIA brought down the axe on the time of free rein aero development in formula one and so began a regime of heavy regulation of race car aerodynamics which today effects all FIA sanctioned motorsports.
- Picture11 – Graph showing effect of underfloor and diffuser vs ride-height and length.jpg (28.13 KiB) Viewed 1815 times
In spite of heavy technical regulation the underneath of a race car remains the most drag efficient area for the production of downforce. So race car development engineers are forced in order to be competitive to walk a fine line at all times trying to maximise the amount of ground effect downforce they can get whilst not putting their car in breach of the technical regulations. A really good example of motorsport engineers walking this tight rope is the case of the double diffuser development which dominated the 2009 formula one season when some engineering teams spotted an area of under floor development not actually constrained by the prevailing technical rules (Brawn, Williams, Toyota) and other rather embarrassed engineering teams were very publicly caught napping (Ferrari, Redbull, McLaren).
- Picture12 – Typical pressure diagram of an LMP car.jpg (17.38 KiB) Viewed 1815 times
Rear Diffuser – You might have noticed that in both of the above graphics a rear diffuser is implied, this is because the rear diffuser is a crucial part of the whole under floor system, it’s not useful to think of it as a separate aero dynamic device as is the common perception. The rear diffuser is crucial because it performs multiple functions, its main job is to expand the fast but thin under body flow into the cars wake, as it does this it slows it down and as the Bernoulli principle tells us that this will increases the air’s pressure somewhat. Increasing the pressure of the air coming out of the diffuser is a good thing as it helps fill the wake of low pressure that the moving car is constantly leaving behind it, this “pressure drag” often represents a significant portion of the overall aerodynamic drag so it’s very much a case of every little helps.
- Picture10 – Diffuser tunnels.jpg (28.44 KiB) Viewed 1815 times
The diffuser simply doesn’t have enough under body airflow at its disposal to fill the cars entire wake with high pressure air, because of this when in motion the diffuser behalves a bit like a pump, that is it’s essentially pulling air from the under body to try to fill the wake all the time increasing the underfloor air flow speeds and thus lowering pressures in underfloor area which translates as an increase in “negative lift coefficient” or as we commonly term it, downforce.
- Picture13 – Aston DBRS9 rear diffuser.jpg (25.82 KiB) Viewed 1815 times
The last important thing to just briefly mention about the rear diffuser is its relationship with the rear wing. The air rotation generated by the rear wing can help drive extra airflow through the rear diffuser owing to the close proximity of the ring wing’s low pressure surface to the exit of the diffuser, now this effect does vary greatly from one car design to another but this effect can amplify the diffusers effectiveness generating even more under body flow, realising these gains however does become rather dependant on the real time effectiveness of the rear wing (angle of attack, dirty air, etc).
P art 3: The Devil in the Details:
When most people think of aerodynamics they first tend think of smooth airflows and their focus tends to fall on the aspect of drag reduction, their minds eye might conjure up images of sleekly designed cars from the 1930’s or perhaps tear dropped shaped bicycle racing helmets. This isn’t wrong as drag reduction is a highly valid application for aerodynamics but by no means is it the whole story, especially when you’re looking to generate lift, and by that I mean either positive lift on the body of an aircraft or negative lift on the body of our performance car.
In our field we will we often find ourselves tuning the airflow around a rigid and pre-defined shape and it can be beneficial to deliberately induce turbulence in specific area’s to shape the way in which air flows around our car by forcing it to move in ways that would not come about naturally. One of the most useful types of turbulence available in the race car engineer’s aerodynamic tool box is a cyclonic turbulence pattern which spirals out in a cone shape downwind, the engineering term given to this cyclonic type of turbulence is a Vortex.
Diffuser Vortices– So carrying on from where we left off at the end of the last post we’ll look at how vortices can be used to make a rear diffusers work better. As we mentioned before, the rear diffuser performs multiple roles in that it helps fill the cars low pressure wake and in doing so drags more airflow under the car body, the angle of the rear diffuser effects how it interacts with the cars low pressure wake. In most cases a higher exit for the rear diffuser increases aero performance but only as long as air flow actually follows the path defined by the upper portions of the diffuser, motorsport engineer speak for this would be “keeping the air attached to the diffuser”. This is where vortices can help us out because by spinning the air in a cyclone shape we actually speed portions of the air up and this as Bernoulli’s principle tells us reduces its pressure and produces suction loadings along the trailing vortex, the interaction between our smooth airflow and our generated vortex makes our vortex stick to the edges of the diffuser and ultimately keeps airflow attached.
- Picture14 – Diffuser or Venturi tunnel vortices.jpg (14.5 KiB) Viewed 1172 times
Our Vortex is actually generated at the leading edge of the diffuser but then re-attached to the diffuser by the naturally occurring side vortices.
- Picture15 – Diffuser or Venturi tunnel vortices CFD plot.jpg (14.55 KiB) Viewed 1172 times
Leaving enough air gap underneath the diffuser edges to enable these vortices to form is an important consideration when designing and tuning the vehicles suspension as the loss of these vortices will result in an extremely rapid reduction of downforce from the underfloor. No problem if it was hardly working in the first place but if it was working well then you can be damn sure you’re drivers going to miss it and they’ll absolutely hate that unpredictable loss of grip.
Here is a picture nicely illustrating the use of a strong stable vortex across the top wing surface of an F16 fighter to help keep airflow attached to the wings surfaces and in doing so create more lift.
- Picture17 – F16 fighter jet vortex generators.jpg (16.84 KiB) Viewed 1172 times
Vortex Generator Strips – This device trips up laminar airflow which in this case is moving just above the boundary layer along the surface of our vehicles roof, the local air pressures in the air flow directly around the strakes becomes imbalanced causing the flow from there on to become turbulent.
- Picture19 – EVO style roof mounted vortex generators.jpg (15.38 KiB) Viewed 1172 times
- Picture20 – Evo style roof mounted vortex gen CFD in operation, showing effects.jpg (21.82 KiB) Viewed 1172 times
Above you can see two CFD plots comparing air flow patterns with and without a vortex generator, both plots are simulating flow from the roof of a Mitsubishi Evo down on to the rear screen. By tripping the boundary layer air and spinning it in a cyclonic fashion the pressure of some portions of the vortex is reduced and this helps the laminar flows change direction downwards at the end of the roof and so stay attached to the rear screen for longer. But why is that important?
- Picture21 – Evo style roof mounted vortex gen CFD in operation, showing effects from side.jpg (12.14 KiB) Viewed 1172 times
On the left is a CFD plot simulating the air pressure pattern around the rear of the Mitsubishi Evo with a vortex generator fitted and on right the same plot without a vortex generator fitted. The smaller area of blue at the tail indicates a reduction in the size of the cars low pressure wake, so as a direct result the car fitted with the vortex generator will suffer less power loss through pressure drag, this has exactly the same effect as finding extra power in the engine. In this instance the reduction in pressure drag will more than out weigh the small amount of extra drag caused directly by the vortex generator so the net effect in this instance is an increase in aerodynamic efficiency. As a secondary effect of keeping the airflow better attached to the cars profile the airflow on to and around the rear wing improves, which translates into potentially better negative lift performance and efficiency .
Part 4: The Learning Curve:
The Gurney Flap – Sharp roof mounted teeth aren’t the only example of real world aero dynamics responding positively to sharp structures, chord section wings can be made to work at greater angles of attack by adding a small flap to the upper rear edge of the wing, a concept invented by legendary racing driver and motorsport engineer Dan Gurney.
- Picture23 – Chord wing with Gurney.jpg (29.67 KiB) Viewed 1031 times
The idea of an aerodynamic wing is straight forward enough, the aim is to create an imbalance of pressure from one side of the wing to the other and in doing so make the air flow apply a force to the wing in the direction we require. Wings achieve this pressure imbalance by creating a rotation in the air flow as it passes by the wing making air flow around one side of the wing travel further and faster than on the other side.
In order to create higher levels of aero force the wing must impart larger amounts of rotation to the airflow, this can be achieved by operating the wing at greater angles of attack or building cambered shapes into the design, eventually the limiting factor is being able to keep air attached to the low pressure surface of the wing. It is easier to keep air attached to a longer section of wing as the air is asked to change direction less aggressively, and it is in this crucial way that the Gurney flap helps us generate more downforce.
- Picture24 – Two diagram of gurney flap effect.jpg (17.93 KiB) Viewed 1031 times
As you can see the Gurney flap enables us to use angles of attack that would otherwise be counter productive, or in other words it enables a shorter wing section to perform as though it were longer. In the chart below you can see how a sample chord section responds to the application of a Gurney flap in terms of it’s lift coefficient (downforce vs chord area) for a given angle of attack, the % number is the size of the Gurney flap as a percentage of the wings chord length.
- Picture26 – chart of gurney flap effect.jpg (16.24 KiB) Viewed 1031 times
In wings operating at the point of stalling rotation the addition of a Gurney flap generates vortices which can re-attach airflow to the wing restoring rotation. Also due to the same vortices the trailing airflow from the low pressure side of the wing is redirected upward slightly imparting even greater rotation, using a Gurney flap it’s sometimes possible to generate equal levels of downforce with up to 3deg’s less wing angle of attack.
- Picture25 – Two diagram of gurney flap effect more detail.jpg (14.59 KiB) Viewed 1031 times
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