The holiday route, an opportunity to study the resonant properties of air masses

2023-07-12 12:00:05

And off we go again for the holiday highway: first go see grandma in the Vaucluse, cousin Thierry in Saint-Etienne to continue with the wedding of the great-niece at the end of Brittany, the scenario is often written in advance… Each year, many of us make this kind of family pilgrimage, during which our carbon footprint, virtuously worked on during the year on our bike, takes its place. The growing guilt we feel will certainly lead us to use the air conditioning less, and to open the windows…

In doing so, we will not fail to rediscover this surprising and unpleasant phenomenon of infrasonic beats which can manifest itself when driving fast, with only one window open at the rear, and which gives the impression that a helicopter transporting troops wants to land on the car. With an open window, the air at the boundary between inside and outside is sheared significantly, which gives rise to vortices upstream of the window which develop going downstream. Upon encountering the downstream jamb of the window, they can go either inward or outward (to simplify), which they tend to do at a certain frequency, a phenomenon called “sheartone”.

Now it turns out that the mass of air in the car can behave collectively if it is excited at its resonant frequency (it becomes what is called a “Helmholtz resonator “). In this vibration, the air then compresses globally in the passenger compartment at low frequency (typically 10-20 beats per second). When the emission frequency of the vortices upstream of the window is not very different from this frequency, frequency locking occurs : the significant response of the passenger compartment is sufficient to in turn influence the phenomenon which gives rise to it and force the emission of the vortices to settle at its “preferred” frequency!

Equilibrium with two openings

This phenomenon occurs more by opening the rear windows (and sunroofs), as the shape of the mirrors is optimized to limit its appearance in the front. Moreover, the solution to this problem does not consist in closing the window and turning on the air conditioning, it suffices to open a window on the opposite side to stop the beatings. Indeed, the air in the passenger compartment then no longer behaves like a Helmholtz resonator, since the pressure is balanced by the two openings.

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