Watch any Formula 1 practice session or in particular pre-season test and you’ll likely see some framework hanging off the cars at some point.
That is an aero rake. A piece of equipment designed specifically for that car to take aerodynamic measurements at the track, positioned in areas of interest such as behind a wheel or diffuser.
These aero rakes are an array of Kiel probes. A Kiel probe is essentially a Pitot tube with a shroud around the inlet to reduce the yaw sensitivity of the measurement, which is useful since during cornering yaw is constantly changing.
This shroud essentially deflects the airflow into the axial direction when the probe is at an angle of yaw, reducing the impact of this yaw. The array of multiple probes can be used to measure the pressure field and obtain the velocity field in critical areas, for example the wheel wake.
This information can tell the trackside aerodynamicist if the new components fitted to the car are working as expected, if the real world matches the CFD or wind tunnel results or just provide extra information to the factory to improve the car.
How is velocity obtained? Well, total pressure can, with some assumptions, be thought of as the sum of static pressure (the pressure exerted by the air) and dynamic pressure (a measure of the kinetic energy of the airflow). For those who like maths here is the equation used to calculate the velocity using this method:
— Tobi Grüner 🏁 (@tgruener) November 24, 2017
The probes are connected to a transducer located elsewhere on the car via small diameter plastic tubes.
This causes a problem as the pressure measured at the probe will be different than that at the transducer, so a method called a transfer function, which uses relatively complex mathematics account for pressure losses in the tube and the time lag of the flow travelling through the tube, is used to correct the measurement.