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Tow and wake turbulence

Aerodynamics: Lesson 8

Chasing a car: wake turbulence.

A moving vehicle interacts with the air it encounters, and moves it. A turbulence and a low-pressure zone are created behind the vehicle in which the air re-occupies the region of space from which the car has just left. This occurs, for example, due to the different pressure between the low surface and the upper surface of the wing which takes the air to compensate the areas of depression passing through the wingtips. Another way to create strong vortices is due to wheels rotation. Due to the motion of the vehicle, the vortices generated by the vehicle go towards the back.

This turbulence zone, which consists of a series of swirling motions of the fluid that loses its laminar flow, is in continuous movement and evolution and follows the car that generates it: it is called aerodynamic slipstream or tow.

Wake turbulence

Image produced by NASA able to illustrate the phenomenon of wake turbulence.

The Tow: a drag benefit and a downforce handicap .

The aerodynamic slipstream of a car significantly influences the aerodynamic force produced by the car that follows it. The effect of the tow is stronger as the cars get closer, while it weakens with the passage of time and with the increasing of space.

When one car approaches another, the aerodynamic surfaces of the “tracker” work in the tow generated by the preceding car, and therefore encounter turbulent air in movement and with less pressure than that in which they work in ideal conditions.

Aerodynamic slipstream

Aerodynamic slipstream effects on nearby cars

The speed of race cars is higher, with the same engine power engaged, when they run in pairs one behind the other at a very short distance, compared to when they run separate, because the drag opposed by the air decreases. Depending on the speed, when the two cars run a few lengths apart, both are affected by the aerodynamic advantages of the other car, especially the one that follows, which manages to run at the same speed as the first car by engaging a lower engine power, because behind the first car an aerodynamic slipstream of depression is formed which “sucks” the second car. This “reserve” of engine power allows the second car to overtake the first.

The tow, in other words, inhibits the functionality of ailerons and flaps that no longer generate the same downforce by not being hit by the energized undisturbed flow.

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