ROTATING FLUID POLYGONS - TDU

Most of us know what happens, when a bucket of water is rotated around its central axis: if the rotation is even, the water will slowly come to rest in the rotating frame, and the only effect of the rotation is that the surface will be curved into the shape of a paraboloid.

Now, what happens if only the bottom rotates? Recently, a team at the Physics Department at the Technical University of Denmark performed such an experiment. A cylindrical container, where the bottom plate can turn, is filled with water up to a certain height, and a motor sets the bottom plate rotating. Right away, the situation looks like what we are used to: the surface starts to bend since the water is forced out against the sides. Surprisingly enough, however, the surface can spontaneously deform, break the axial symmetry and create shapes that are most aptly described as “polygons”. They are stationary in a rotating frame - not, however, one that rotates with the speed of the bottom, but considerably slower.

The team consisted of three bachelor-students from the University of Copenhagen - Thomas Jansson, Kåre Hartvig Jensen and Martin Haspang - a visitor from Paris - Pascal Hersen - and Tomas Bohr from the Physics Department.

Currently there is no theory for this experiment, so it is a challenge for all those who believe that Fluid Dynamics is a closed, well-understood part of physics.

[+ pictures]
viaFluid-TDU