The Ekman number (Ek) is a dimensionless quantity in fluid dynamics that characterizes the relative importance of the viscous forces to the Coriolis forces in a rotating fluid system.
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The ratio of viscous force to Coriolis force in a rotating fluid at length scale . This convention uses rotation frequency ; conventions omitting the factor two are also common. For small , bulk viscosity is weak but no-slip boundaries can form Ekman layers of thickness of order , with relatively larger boundary shear stress.