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Image by H. Ait Abderrahmane and A. Kasimov/KAUST
The photograph shows the flow structure of a spinning pentagonal hydraulic jump and logarithmic spiral waves of fluid elevation downstream of the jump which are produced when a jet of water-glycerol mixture hits a horizontal glass plate and starts spreading radially out. The picture is taken from below the plate. The spiral waves in this experiment bear striking resemblance to the spiral density waves in galaxies. We found that the structure of the flow in the hydraulic jump exhibits counterparts of such additional salient morphological features of galactic flows as the outflow from the center, the jets, circum-nuclear rings, gas inflows toward the galactic center, and vortices. The hydrodynamic instability that is responsible for the formation of these features in the hydraulic jump may have a counterpart that plays an important role in the formation and persistence of the galactic spiral arms.