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F1 S-duct

The F1 S-duct is a clever aerodynamic trick that has been on Formula 1 cars since 2012 when Sauber introduced it. However, the details of how it works can be easily misinterpreted so we spoke to F1 aerodynamicists to find out the real reason behind the S-duct.

‘Dirty’ air

Whenever air flows over a surface, it loses energy, which causes the flow to slow down and become turbulent. Therefore, once the flow has travelled over the elements in the front wing, it becomes ‘dirty’. In particular the gap between the underside of the nose, the upper surface of the front wing and the inner faces of the front wing pillars can cause an expanding tube of turbulent air. Add to this the fact that the air hitting the top corners of the nose can also accelerate round and roll underneath, and the airflow under the nose can become extremely turbulent.

This flow not only feeds the main turning vanes but also the leading edge of the underfloor, so the cleaner the teams can get this airflow, the more performance they can extract from the other aero devices rearwards of the nose such as the turning vanes, bargeboards, underfloor and the diffuser.

The area highlighted in blue, can cause an expanding tube of turbulent air underneath the nose

‘The airflow under the nose is ‘dirty’ which means it is a slower speed flow that has been worked by the presence of the nose and the front wing,’ explains Arron Melvin, Principal Aerodynamicist at Haas F1 Team. ‘To be legal, it is necessary to have certain nose volumes and inevitably there is a boundary layer growth due to the front wing and nose expansions and you can also get acceleration around the shoulder of the nose which leads to a high curvature flow.’

Boundary Layer

When air flows over an object, the molecules closest to the surface slow down, which then causes the molecules just above them to slow down also. As you move away from the surface, the molecules gradually increase in speed up to the speed of the main flow. This thin layer of fluid where the velocity increases from zero at the surface to the free stream velocity is called the boundary layer and its thickness depends on the viscosity of the fluid and the characteristics of the surface it is travelling over.

S-duct

‘We have introduced an S-duct for the first time this year and essentially we ingest this dirty flow from under the nose through two pairs of NACA ducts and then release this flow on top of the chassis, rather than letting it travel underneath the car,’ highlights Melvin. ‘If we let it go underneath the car, the lower speed flow would arrive at the main turning vane, whereas now it goes through the inlets, into the cockpit and over the sidepod and does less harm. It is very much about where to place loss.’

The S-duct ingests ‘dirty’ airflow from under the nose via small NACA inlets (blue arrows) and distributes this flow into the cockpit via an outlet at the bridge of the nose (green arrows) where it has less negative effect on the overall aerodynamics

S-duct inlet – NACA ducts

The geometry of the NACA duct is essential in achieving the necessary vortices which help to ingest airflow efficiently. CREDIT: http://bit.ly/2Td3C6r

A NACA duct is a type of inlet which allows the air to be drawn in with high efficiency and minimal drag. To achieve this, NACA ducts are usually placed parallel to the local airflow and in locations where the boundary layer is relatively thin. The theory is that the shape of these ducts encourage vortices to form, reducing static pressure and enhancing the efficiency of the flow through the inlet. As air flows towards the narrow end of the duct, it flows down the gentle slope and into the inlet. But the air that approaches from outside the inlet has to flow over the edges which causes a vortex. This results in the formation of two counter-rotating longitudinal vortices which then induce more air to flow down the duct.

An area of low static pressure can be seen at the edges of the duct (blue) as the rolling vortex is formed. CREDIT: Simon McBeath & ANSYS

The two longitudinal vortices along the edges of the NACA duct induce more air into the inlet, as demonstrated by the yellow sections of the streamlines as they decrease in velocity. CREDIT: Simon McBeath & ANSYS

S-duct outlet

Quite often the S-duct outlet at the bridge of the nose can be misinterpreted as a device to help avoid flow separation due to the angle change from the steep nose, to the flatter monocoque. However, this outlet is simply about extracting the dirty airflow in a place where it will do the least damage to the overall car’s aerodynamic performance.

There are a variety of designs for the S-duct outlet as shown here on the Mercedes W10 (left) and the Haas VF-19

‘It’s a very clear but ultimately relatively subtle technology and for a small team such as Haas, we had to be sure of the benefit to justify the additional costs,’ says Melvin. ‘The nose is a lot more complicated to design and slightly heavier – it is an aerodynamic vs structural trade-off.’

This more complex design relates to the fact that the air has to be channelled through the nose, up to the outlet. But these channels have to be incorporated into the nose in such a way that it retains its structural requirements to pass the FIA crash safety tests.

This is the most likely reason behind why not all teams have adopted this technology. As mentioned above, this is the first year that Haas are running with an S-duct, and from pre-season testing, McLaren, Sauber and Racing Point have opted to run without it. However, this may change when the cars line up on the grid in Melbourne.

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