71th Annual Meeting of the APS Division of Fluid Dynamics (November 18, 2018 — November 20, 2018)

V0032: Nitrogen swirl: creating rotating polygons in a boiling liquid

  • Alexis Duchesne, Physics Department, Technical University of Denmark
  • Barbara Bohr, Asger Rygsgade, Copenhagen, Denmark
  • Laust Tophøj, Næstved Gymnasium, Næstved, Denmark
  • Tomas Bohr, Physics Department, Technical University of Denmark

Rotating flows are interesting and surprising, and also very relevant for us as human beings living on a rotating Earth. When we learn about rotating flows, we usually start with the most famous example: Newton’s rotating bucket. Newton attached a bucket of water to the ceiling by a rope, and twisted the rope.  As he let go the bucket started rotating and after a while the water came to rest again. This time, however, with respect to the rotating bucket, and with a water surface that was no longer flat but curved upward at the rim like a paraboloid. From this, Newton deduced that the rotating reference frame of the bucket is not an “inertial frame”, and that the simple form of the equations of motion that he had developed therefore did not suffice there. Many years later Mach and Einstein asked the question of why one could not just as well say that we were rotating with respect to the bucket, and from there came a whole new theory of gravitation!

In the video we see a very low viscosity liquid (liquid nitrogen) set into rotation in a warm pot. Since the pot is not rotating, the liquid cannot “come to rest” in any rotating frame, so contrary to Newton’s example the flow remains strongly turbulent, with a surface curved in the opposite direction. Moreover, an instability sets in which spontaneously generates large deformations of the surface, that break the axial symmetry and look like rotating polygons - despite the strong turbulence. This phenomenon illustrates the ability of rotating flows to generate order even in very turbulent states, and is therefore close to such large scale planetary phenomena as Jupiter’s red spot and Saturn’s north pole hexagon.

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