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.

2013 Monza Aero Analysis

As the fastest race of the year, Monza is always very interesting to watch technically as teams bring their completely new setups to meet the low downforce/ drag requirement for this circuit. Massive difference in front wing and rear wing design can be clearly seen, that mainly involves usage of smaller shallower wing flaps, removal of cascades, endplates slots, etc.

Front Wing

Front wings appeared in Monza all look much simpler than they normally are in other races, with the removal of cascades and use of fewer elements. The usage of cascades in most races aims to better manage tyre wake by lifting the air up around the tyre. However they can also introduce some drag together with downforce, that are necessarily needed in Monza. Many teams have used a no cascade front wing here, while some others trim off their cascades to reduce drag.

Ferrari’s front wing looks like the most interesting one, with removal of cascades, they fit on two vertical vortex generators instead aligned outwards on the wing. This would help flow over/around the tyres (what cascades are used for) by 1) better directing the air outwards from the wheel; 2) generating two vortices behind it that re-energise the airflow for better management of tyre wake. They have also simplified the upper flap to make it into one piece instead of two in other races.

Ferrari Front Wing Monza 2013
Ferrari Front Wing Monza 2013

Red Bull has trimmed all their cascades to shorter span therefore reducing drag.

Red Bull Front Wing Monza 2013
Red Bull Front Wing Monza 2013

McLaren used a classic three tier front wing without any cascades. They’ve been using a quite simple classic design in recent few races already. Reportedly they’ve not managed to find a way to get  aero right to meet up with the new pull-rod suspension used this year, which is the main reason for their performance drop. In this case, a classic design is a reasonably good solution, while they can shift more effort to next year’s design.

McLaren Front Wing Monza 2013
McLaren Front Wing Monza 2013

Rear Wing

Rear wing design has been made quite complicated in recent years with the deployment of endplates slats and slots (louvres). Basically pressure difference over two sides of the wing surface (how downforce is generated) would cause a spiralling air over the wing tip, that is called wing tip vortices. This also happens to the endplates as pressure outside the endplates would be lower than pressure in between. To decrease the induced drag caused by wing tip vortices, slots and slats are used to even out the pressure difference on two sides.  However in Monza, with small shallow rear wings, pressure difference is not as significant as that in other races. Slats/slots are therefore removed since reduced pressure difference also reduces downforce. In addition, there isn’t much space on the endplates to add in louvres above a small main plane. Another notable influence is that DRS is not very powerful on this circuit with already small and flat rear wings.

Red Bull has removed all the slots on their endplates:

Red Bull Rear Wing Monza 2013
Red Bull Rear Wing Monza 2013

Mercedes Monza rear wing: only two slots above. Leading edge slots used to direct turbulent air coming off the rear tyre inside the endplates for pressure balance. Also worth noting that their beam wing is running at lower AoA to shed off downforce/drag.

Mercedes Rear Wing Monza 2013
Mercedes Rear Wing Monza 2013

Lotus Long Wheel Base

Lotus has tested their long wheel base car in practice. They basically reduced the degree at which the front wishbone is angled towards the mainbody, therefore moving front tyre forwards about 10cm (Sketch from @TechF1LES). This would change the front-rear balance/ weight distribution of the car. It should also give an aerodynamic benefit by allowing more space for front tyre wake to settle before it reaches the sidepod. Lotus will continue their development on LWB as not much advantage was noted in Monza practice.

Lotus Long Wheel Base (@TechF1LES)
Lotus Long Wheel Base (Source: @TechF1LES)

*Images from AMuS unless otherwise stated

Mercedes DRD tested in Nurburgring

Following Lotus’ use of DRD in British Grand Prix, Mercedes tested their updated DRD system in Germany GP practice. The basic idea of Mercedes DRD is similar to that of Lotus, although the mechanism for turning on/off the air switch might be different.

Mercedes DRD Inlet
Mercedes DRD Inlet (Source: @tgruener)

Instead of having two ears besides the airbox as Lotus did, Mercedes has two additional inlets at the back of airbox. This is easily fitted as Mercedes has integrated some removable design to their bodywork around roll hoop this season. The combination of a big and a small inlet may act as the control system for the air switch.

Mercedes DRD outlet
Mercedes DRD Outlet (Source: F1 technical)

In terms of outlet, there are two outlets with the upper one below the main plain of the rear wing and lower one below the monkey seat/ beam wing. Above a certain high speed, such as long straights and high speed corners where less downforce is needed, the upper outlet switches on. The air blowing out under the rear wing disturbs the flow and therefore stall the wing for less downforce and drag. This would give an extra boost on the speed of the car.

Another noticeable detail is that both Lotus and Mercedes now have an extra small outlet below the rear wing. This is used to let out leaked air under the rear wing when the upper outlet is not supposed to be switched on.

Lotus DRD Outlet
Lotus DRD Outlet Detail (Source: Sutton Image)