Hele-Shaw flow past a circle

Dye shows the streamlines in water flowing at 1 mm per second  between glass plates spaced 1 mm apart. It is a first sight paradoxical that  the best way of producing the unseparated pattern of plane potencial flow past  a bluff object, which would be spoiled by separation in a real fluid of even the slightest viscosity , is to go to the opposite extreme  of creeping flow in a narrow gap, which is dominate by viscous forces. Photograph by D. H. Peregrine.