Containment has been good for some researchers. Some caught up on late readings of research articles. Others experimented at home with what they had on hand. Thus a team from the Technical University of Denmark, in Lyngby, studied the closing by sliding of the lids on rectangular cardboard game boxes to understand, as everyone could observe, why some close quickly and others slowly. Up to 50 seconds! “We also had boxes at the university. And we have supplemented this with experiments in the laboratory on better controlled devices,” says Kaare Jensen, teacher-researcher at Lyngby, specialist in fluid mechanics and transport in plants. Nevertheless, among the thirteen boxes tested, nine are board games, as they detail in Physical Review Fluids from 1is avril.
After learning to avoid the sometimes oscillating movement of the falling lid by being careful to place it above its base, the researchers identified three speed regimes. One is a descent at a constant speed, the other two are done with an increasingly slow speed but at different rhythms. In question, a small detail that is not always seen: the more or less perfect parallelism between the walls of the lid and those of the box. The lid can indeed be “flared” (the wall deviates from the box), “returning” (the opposite), or “perfect” (the parallel case).
Simple and serious
This changes everything because the evolution of the fall obviously depends on the weight of the lid, but above all on the speed at which the air escapes through a space of less than one millimeter between the two interlocking parts. The smaller this space, the less easily the fluid escapes and the slower the fall. In the “re-entrant” case, the air escapes through a space whose diameter does not change during the fall, therefore with the same flow, so that the cover falls at a constant speed.
In the “perfect” case, the air also escapes through a space whose diameter does not change. But the more the cover goes down, the more air there is “stuck” between the walls and therefore the more the descent is slowed down.
Finally, for the “flared” case, the exhaust space, large at the start, narrows and therefore the box is slowed down more and more, but according to a different law from the previous case. The researchers even calculated that this type of profile allows the fastest total descents: 15% faster than the “perfect” case, if the angle is 2 degrees.
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