For a work project I need to revive my aerodynamics knowledge again. Can somebody help me with the distinction between vorticity and viscosity.
If a flow vorticity is not equal to zero, the flow is called rotational and the flow elements have a finite angular velocity. But does this also imply the flow is viscous. And vice versa, if viscosity is taken into account, can a flow be irrotational?
Best Answer
Vorticity describes whether the flow is rotational - that is, if you follow a path along the flow and return to the same point, is the net velocity zero or finite. This is expressed with the curl ($\nabla \times$) operator. This is a property of the flow.
Viscosity is a property of the medium, not the flow. Most fluids have viscosity - that is, if there is a velocity gradient in the flow, this will set up a shear stress which tends to try to counter the gradient.
An interesting side effect of viscosity is that it allows you to set up vorticity in a liquid - when there is no viscosity, "stirring" a liquid will cause no net vorticity.