---
title: "Vortex formation in intake"
subject: "Hydropower Engineering"
chapter: 5
key: "semester_7/hydropower_engineering:intake-vortex-formation-no-vortex"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/intake-vortex-formation-no-vortex
site: BCT Engineering
---

# Vortex formation in intake

Hydropower Engineering · Chapter 5: Headworks of Run-of- River (RoR) Plants

- Asked in: 2079 Bhadra, 2+4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/intake-vortex-formation-no-vortex
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-5

## Question

Why is a vortex formed in an intake? Discuss the hydraulic conditions for no vortex formation.

## Short answer

A vortex forms from angular momentum in approach flow combined with insufficient submergence, drawing air into the intake. Avoid it with adequate submergence S=C V sqrt D, low velocity below about 1 m/s, symmetrical smooth approach, guide walls and anti-vortex devices.

## Answer

#### 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What causes a vortex at an intake?
   A. Excess submergence
   B. Circulation in the approach flow with insufficient submergence
   C. Perfectly symmetric flow
   D. Low velocity
   Answer: B. Angular momentum plus low submergence draws a rotating core that entrains air. (Key point: Cause)

2. In S = C V sqrt(D), what is S?
   A. Submergence from water surface to the crown of the intake
   B. Intake sill height
   C. Spacing of rack bars
   D. Settling velocity
   Answer: A. C is 0.7245 for symmetric approach and 0.5434 for asymmetric. (Key point: Gordon formula)

3. Which is an effect of vortices?
   A. Higher efficiency
   B. Reduced trash
   C. Air entrainment, vibration and cavitation
   D. Lower velocity
   Answer: C. Air reduces capacity and efficiency and causes surging. (Key point: Effects)

4. Which device prevents vortices?
   A. A higher velocity
   B. Sharp edges
   C. Asymmetric approach
   D. Floating rafts, splitter walls or baffles
   Answer: D. These devices break up rotation. (Key point: Prevention)

## Related in Hydropower Engineering

- [Side intake sketch; trash rack loss](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aintake-side-sketch-trashrack-loss) — Hydropower Engineering, chapter 5

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