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Vortex flow

The vortex flow exhibits rotational symmetry (see Fluid mechanics) with tangential components. For vortex flow in pipes, flow velocity often comprises both tangential and axial components. Particles carried in the flow thus move along helical paths, and compared with vortex-free flow, they have to negotiate a longer path along the pipe wall for a given length of piping at the same flow velocity, resulting in a higher pressure loss.

The tangential component (vu) at radius (r) in a vortex flow usually follows the law applicable to potential flow,

Drallströmung_Formel_1

and the flow is then known as a potential vortex flow. The above relationship is however not valid for very small radii, as in this case the tangential component of the vortex flow would have to increase to infinity as the radius decreases. In fact, a vortex core is present at the centre of a vortex
flow below a given limit diameter (Rankine vortex), in which the axial component is notably smaller in the area of the potential vortex, while the tangential component (vu) of flow velocity follows the law for solid body vortices:

Drallströmung_Formel_2

The static pressure in a vortex flow increases with the radius. A vortex flow is always present at the outlet of a rotating impeller, and the tangential components of this vortex flow can be partially converted into static pressure by suitable elements arranged downstream (e. g. diffuser).

In a vortex flow encompassed within solid walls in a pipe or annular chamber, the intensity of the swirl decreases slowly along its travel path (as the distance increases) due to wall friction.

Undesirable vortex flow in pipes, e.g. upstream of a measuring point or in the approach flow to a pump (see Inlet conditions) can be suppressed by fitting flow straighteners, for example of the cruciform flat plate type.

Vortex flow