Vortex formation in intake

Why a vortex forms

A vortex forms when water approaches a submerged intake with angular momentum (circulation) and the submergence is too small. As the flow converges towards the opening its velocity rises and the low pressure at the centre draws the surface down, so a rotating core develops and may draw air into the intake. Typical causes:

• small submergence of the intake below the water surface (measured to its crown);
• asymmetric approach flow, or flow past piers, abutments or the bank, which creates circulation;
• high intake velocity and high Froude number;
• sharp edges, no guide walls, and sudden changes in flow direction.

Vortices cause air entrainment (reducing capacity and turbine efficiency), vibration, surging and cavitation, and they carry floating trash into the intake.

Conditions for no vortex formation

1. Adequate submergence (Gordon's formula):

$$S=C\,V\sqrt{D}$$

where $S$ is the submergence (water surface to the crown of the intake, m), $V$ the velocity in the intake (m/s), $D$ the height or diameter of the intake (m), and $C=0.5434$ for a symmetrical approach flow and $0.7245$ for an asymmetrical approach (which needs more submergence). An alternative criterion (Knauss) is $S/D\ge1+2.3\,Fr$ with $Fr=V/\sqrt{gD}$. 2. Low intake velocity (less than about 1 m/s at the rack, with a low Froude number). 3. Symmetrical, smooth approach flow, with guide walls to remove circulation; avoid sharp bends and eddies. 4. Anti-vortex devices: floating rafts or beams, vertical splitter walls or baffles above the intake, and a trash rack that breaks up the rotation. 5. A well-shaped bell-mouth entry and a sufficient height of the intake sill above the bed.