It is only through dissipation of a vortex due to viscosity that a vortex line can end in the fluid, rather than at the boundary of the fluid. The local Reynolds number is the ratio of the inertial force and viscous force in the fluid.
For an example, if a water bucket is rotated or spun constantly, it will rotate around an invisible line called the axis line. Examples for forced vortex flow is rotating a vessel containing a liquid with constant angular velocity, flow inside the centrifugal pump.
In general, vortex tubes are nested around the axis of rotation.
The free vortex flow is irrotationaland therefore, also known as the irrotational vortex. In this example the rotation of the bucket creates extra force.
Precautions: Do not run the pump at low voltage i. At the bottom centre of the graduated carriage an orifice discharge has been given to create free vortex. The water moves spirally towards the centre with stream line in motion, so that, neglecting losses caused by viscosity, the energy per unit mass remains constant.