Flow Control Techniques – from Aerospace to Motorsport

Boundary layer flow control in the aeronautical world mainly serves for delaying transition and controlling separation. In the motorsport world dealing with major bluff bodies, transition and separation happens all the time, so that the purpose of flow control varies. However, many techniques manipulating fluid flow have shared the same essence. This post will introduce some basic boundary layer control techniques used in the aeronautical world and how these ideas have migrated and developed to serve different goals in motorsport.

For people new to boundary layer, have a quick look here. Basically boundary layer separates when flow gets ‘tired’ so that by finding a way to re-energise the boundary layer, flow can possibly stay attached.

Vortex Generators

VGs are known as the first fix for flow separation in aerospace applications. The idea is to redistribute momentum by bringing high momentum flow outside the boundary layer to the inside. For this purpose, VGs are typically of the height of the local boundary layer and placed about 20 times local boundary layer height ahead of the separation point. The boundary layer scale VGs are used in the automotive applications as well to prevent flow separation.

VGs on car roof to reduce separation
VGs on car roof to reduce separation

However other than the traditional boundary layer VG, large scale VGs can often be seen on racing cars. These VGs interact with the mean flow, instead of the viscous boundary layer. They can be used to generate vortex suction (add downforce) and help turning/guiding the flow. Various devices, such as barge boards, turning vanes, front wing strake, etc., can be possibly generalised into large VG category (writer’s point of view).

Delicate front wing feature - VG ahead of downstream strakes
Delicate front wing feature – VG ahead of downstream strakes

Blowing/Suction

Blowing aims to inject high energy air into the boundary layer while suction aims to remove the tired boundary layer. Blowing and suction are not widely used because of the power requirement and weight penalties. Suction can also suffer from the dirt in the air blocking the porous areas. However, both ideas can work efficiently with careful design of the blowing/suction slot.

Mechanism of blowing
Mechanism of blowing
Mechanism of suction
Mechanism of suction

Blowing and suction on race cars can be a different story. Blowing was known as a way to deliberately stall the wing in order to reduce drag on straights. Instead of blowing the air tangentially, air is blown out in different directions to the surface in order to upset the main flow. Famous examples include the McLaren F-duct and Lotus drag reduction device.

The Red Bull S duct is an interesting demonstration of blowing and suction, where flow is scooped under the nose and channeled to the top of the chassis. This would probably help remove some thick boundary layer  under the nose and aid flow attachment on the chassis.

Red Bull S Duct
Red Bull S Duct

Moving Surface

Moving surface looks like one of the most extreme ways to manage boundary layer flow. It basically eliminates or alleviates the non-slip condition by reducing the relative motion between the surface and the flow. It could have practical use on large square shaped vehicles to prevent separation, as shown in the figure below. The moving ground wind tunnel that teams use to test their cars is a good example of using the moving surface to eliminate boundary layer. However its use on race cars may be a bit unpractical due to the high rotational speed required for the surface.

Rotating surfaces at the corners can help reduce separation
Rotating surfaces at the corners can help reduce separation
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Formula 1 Car Vortices

The damp weather in US Grand Prix gave us a great chance to see some ‘aero porn’. Overnight everyone’s talking about the Y250 vortex as something magical that again Red Bull managed better than others. However Y250 vortex is not a new concept – check out Scarbs post back in 2011 and SomersF1 update after Austin. Here I’m going to introduce some basic principles on vortices and explain why Y250 vortex is important. This would contain some text book material for engineering students but hopefully it will be helpful for people with general interest in aerodynamics.

Wing Tip Vortices

Let’s start with a classic textbook picture of wing tip vortices…

trailing-vortex
Schematic description of wing tip vortices

Wing tip vortices are formed because of the pressure difference between the  top and bottom of the wing. For a wing generating lift, there’s high pressure at the bottom, low pressure at the top. So at the wing tip, the high pressure air below the wing tends to roll up to the low pressure region above the wing, forming two tip vortices running off trailing edge. This would subsequently cause a downwash in the middle of the span, which bends the air down as it comes off the wing. Recall that AoA is the angle between flow path and chord line, as the downwash bends the flow, effective AoA is reduced so that lift is reduced. It would also introduce a drag term called vortex drag, which is proportional to the lift and inversely proportional to aspect ratio.

On a formula 1 car, wing tip vortices can be often seen at the rear wing. However, the race car world is somehow an inversion of aeronautical world, whereby downforce and ‘upwash’ is generated.

Vortices coming off Ferrari rear wing
Vortices coming off Ferrari rear wing (Source: Suttton Image)

Vortex Lift

While wing tip vortices are associated with loss of lift, vortices can be used to generate lift as well. A typical case here is slender delta wings, where lift is generated from two strong leading edge vortices forming along the sharp edges. The high speed vortices reduce pressure above the wing based on Bernoulli equation so that more lift is generated from a suction effect. The vortex lift idea can also be applied to race cars on some plate components to add downforce.

Delta Wing Vortices
Delta wing vortices
delta wing cp
Delta wing pressure distribution (suction effect at the tip)

Vortices on F1 Cars

Here is a screenshot from one of the Sauber tech videos – Tech Bites: CFD – side wind, spinning car – Sauber F1 Team

Sauber CFD modelling of vorticity on F1 car
Sauber CFD modelling of vorticity on F1 car

The picture here shows vorticity in the air as the car heads towards wind coming from left. Highlighted by red regions, there is high vorticity flow coming off front wing, mirror, airbox, exhaust and rear wing/diffuser. There is highly turbulent, not necessarily vortex flow coming off front and rear wheels. The famous Y250 vortex sits in the middle 250mm from centreline, governing flow towards rear of the car. There are also vortices from front wing cascades/ auxiliary wings hanging from the endplates, which manages flow above the wheel. The endplates, correspondingly, guide the airflow outbound around the wheel. With regulation change in 2014, front wing will be 75mm narrower on each sides, which would make it tricker to direct the flow around. However there’s no change related to the Y250 vortex so we can still see them next year.

Y250 Vortex

The Y250 vortex, similarly to wing tip vortices is generated because of a pressure difference, in this case between the neutral middle section (250mm from centreline) and the rest of the wing. It is very important for controlling flow approaching leading edge of the floor. Here in analogy with the delta wing vortex lift, the Y250 vortex can potentially extract air at the edge of the floor, therefore producing a suction effect that improves aerodynamics efficiency in this area. On the other hand, the Y250 vortex may also have a benefit on managing front wheel wake by pushing it away from the car.

Comparison of Y250 vortices coming of Red Bull and Ferrari
Comparison of Y250 vortex coming of Red Bull and Ferrari

Skysports has edited a good video comparing Y250 coming of RBR and Ferrari and talking about how well-controlled the RBR vortices is. However ‘well-controlled’ is never a good word for engineers to use. To estimate vortices, we need to use some parameters like vortex core position, vortex strength and cleanness. These parameters are what the design of complex flaps/cascades is based on. I can’t agree with the Skysports’ explanation of vortices travelling down over sidepots as the vortex strength cannot be strong enough to affect area so far downstream. Nevertheless the Y250 vortex do interact with different parts of the car, which requires careful consideration in designing of front wing, floor and turning vanes, etc.

Aerodynamic Components on F1 Car

A F1 car is made of thousands of components and nearly every part need to take aerodynamics into consideration. However, there’re some major aerodynamics components that make huge difference to car aerodynamic performance.

Example of F1 Car Major Components – Exploded View of a BMW

These aero components get mentioned again and again in various F1 technical analysis assuming that people know what they are, but the fact is most people have no idea on those terms! I’m going through some major ones starting from the front of the car to the rear.

Front Wing

The first part we see on the front is definitely the front wing. Being the first means that it’s the first part on the car that interacts with the air, therefore having an important job to determine the under stream flow through the rest of car. The front wing generates 25% to 40% total downforce. Major design modification lies on the endplates and flaps of the wing, aiming to reduce tip vortex and wake of front wheel, which is one of the biggest drag components. In addition, ducts and slots are becoming popular in recent years, as can be seen in Mercedes W duct in 2011 and DDRS in 2012.

Sophisticated Front Wing Flaps and Endplate of MP4-27

Barge Board

This are vertical panels located between the front wheels and sidepods. It deals with the dirty air produced by the front wheels, guiding and smoothing air flow into the sidepod. In recent years’ designs, it may also have the function of feeding more air into the diffuser.

ferrari f2012 side view

Ferrari F2012 Side-view: Barge Board in White

Sidepod

Sidepod is the part alongside the cockpit that accommodates the radiator and engine exhaust. Main Function of Sidepod is to 1) cool down the engine and gearbox; 2) control underbody flow to generate desired downforce. The profile of sidepods are varied significantly on different cars based on different aerodynamics configuration. A memorable design is McLaren L-shaped sidepod on MP4-26 in 2011.

MP4-26 L-shaped Sidepod

Airbox

The opening channel above drivers head that guides fresh and cold air to the cylinder for cooling purpose. Nevertheless, besides the conventional aim of cooling, the air flow through airbox can be utilised to generate more downforce/reduce drag by guiding it later to the desired parts on the rear wing assembly. F duct is a good example making advantage of this air flow. It’s also suspected that the Lotus E20 DDRS/ Super DRS has a tricky design of ‘ear’ inside the airbox.

‘Ear’ Inside Lotus E20 Airbox to Help Guiding the Air

Rear Wing

With the use of F duct and DRS, rear wing is always under spotlight in recent seasons. We’re talking about rear wing assembly here which normally consists of two sets of airfoil. The upper set is the main downforce generator including DRS, while the lower set is known as the beam wing. The whole rear wing sets generate to 30% to 40% total downforce.

Adjustable Rear Flap (DRS)

Flap on the rear wing whose angle of attack can be adjusted by the driver in order to reduce drag. Check out more about DRS here.

F duct

A driver controlled drag reduction system, in which a slot gap is opened on the rear wing flap. This air flow through the gap is able to stall the wing, therefore reducing drag.

Beam Wing

A single element wing at the lower part of the rear wing that helps regulate the air below the upper rear wing sets and improves diffuser performance. As F duct mounted on the upper flap is banned, there is now more aerodynamics consideration taken into the beam wing design. E.g. Lotus DDRS system which utilise beam wing to further reduce drag.

Source: Sutton Image

Lotus E20 Rear Wing Assembly

Gurney Flap

An L-shaped strip along the trailing edge of the wing, commonly on rear wings. With the use of gurney flap, flow separation can be delayed at high angle of attack so that more downforce can be generated. Gurney are used more in wet weather where more grip (downforce) is needed. FIA regulates that Gurneys on the rear wing should not exceed 20mm.

Source: ScarbsF1

Gurney flap along the edge of rear wing

Diffuser

The rear element at the underbody of F1 car close to the floor, from which air exits the car. This is the last components where air interacts with the car. The speed of air flow can significantly influence downforce, whereby the faster the flow exits, the more downforce is generated. Most famous designs in recent years are Brawn GP double diffuser in 2009 and exhaust blown diffuser which many teams used in 2011.

Brawn GP double diffuser

Source: ScarbsF1

Exhaust Blown Diffuser

General Talk: Aerodynamics and F1

There is no need to explain how important aerodynamics is to F1 cars. This is the word that we hear everyday but may not have a clear clue of what it is. As there is quite limited space for the development on engine, tyre and other mechanical components, aerodynamics is the single most important performance factor for F1 cars nowadays.

Aerodynamic Impact on F1 Cars

We may take it for granted, but think of those simple airplane wings that hold hundreds of people in the sky… isn’t aerodynamic force astonishing?

The basic idea of aerodynamics in F1 is to find the best compromise between higher downforce and lower drag. As a surprise to most people, passenger car that we use everyday produces lift force rather than downforce due to the shape of it. However, the bodyweight itself generates enough force to keep it on the ground. Nevertheless, additional downforce is essential for F1 cars as the speed of it requires huge amount of grip to enhance its stability, especially at corners, to allow high cornering speed.

WIth higher speed than common cars, most sports cars need rear spoilers to counteract the lift force generated by car body. Shown here – Bugatti Veyron Super Sport 2011
Huge grip needed for F1 cars to maintain high speed at corners

On the other hand, minimizing drag is the priority to production cars considering its benefit for fuel economy. It is also preferable for F1 cars although not of same stress. The trick here is to find the best downforce and drag combination.

Creating Downforce

While increasing weight is clearly not a clever idea to add up downforce, force exerted by the air becomes the major downforce source. Aerodynamic downforce can be either generated by streamlining the whole car or adding on extra aerodynamic features (wings, spoilers, etc.).

The most famous case for streamlining the car is known as ground effect, which suggests that as the car body approaches the ground, higher downforce is generated. In modern IndyCar race, underbody tunnels are carefully allowed to suck it down to the ground. However, the application of ground effect is almost banned in F1 since early 1980s as the car was capable of reaching dangerously high speed at corners. At present, diffuser design is permitted (although with strict regulations) and crucial for F1 engineers to do take some advantage from the ground.

Lotus79 – Old day F1 champion, making full use of ground effect

Compared to streamlining effects, aerodynamics features are more noticeable on a F1 cars. Aerodynamics consideration must be taken into the design of front wing, nose cone, rear wing, etc. These parts are adjusted in each race to match with the circuit condition. Basically, on circuits with long straights (e.g. Monza, Italy) , lower downforce is needed, minimizing drag becomes more important, whereas on circuits with substantial corners (e.g. Monte Carlo, Monaco), downforce is definitely the priority.

Monte Carlo definitely needs more downforce!

 

Measuring Downforce

Due to the complexity of F1 car shape, it’s almost impossible to calculate downforce generated on each parts directly from fluid mechanics formulas. Computational approach, CFD and experimental approach, wind tunnel testing, are both critical method to measure aerodynamic force.

CFD is the abbreviation of Computational Fluid Dynamics. It is based on numerical method and algorithm while utilising computer to manipulate calculation and simulation. It’s integrated with CAD (Computer Aided Design) so that engineers can test their virtual 3D models in simulated air flow. 

CFD demonstrating pressure and air flow direction through the car body

A wind tunnel is basically a tunnel big enough to hold the testing car model, with a powerful axial fan to produce desired type of air flow. It can nearly simulate all kinds of real circuit conditions by adjusting temperature of air, flow direction and speed, ground inclination, etc. Aerodynamic force is measured by sensitive beam balance attached to the test model, while pressure distribution is obtained by pressure taps mounted along different positions of car body. In addition, flow motion can be observed by injecting smoke into the air. There are various scales of wind tunnels. Although full scale wind tunnel may produce the most accurate measurements, considering the huge cost of it, most teams are using 60% or 70% scale models.

A nice video here of Lotus wind tunnel testing:

 

So these are some general ideas of F1 aerodynamics. Each aspect of it can be dug much deeper in detail, which is what I’m trying to do in the following posts. Enjoy Aero 🙂