Velocity field measurement of a scroll vortex intake flow

Abstract

A scroll vortex intake is a hydraulic structure that transfers water stably from one

elevation to a lower one by generating a swirling vortex flow down a vertical drop-

shaft. Scroll vortex intakes are applied widely in water supply, stormwater drainage

and sewerage systems. For a good engineering design, a sufficiently large and stable

air core needs to be maintained within the dropshaft. Although a number of the-

oretical and experimental investigations have been conducted, the understanding

and predictions of the vortex flow is still far from complete due to a lack of de-

tailed velocity field and air core measurements. This study aims to achieve a better

understanding of the scroll vortex intake flow. The hydraulic theory of scroll vortex

intake is revisited and detailed measurements of air core and velocity field of the

vortex flow is conducted.

A 1:15 physical model of a scroll vortex intake has been designed according

to dynamic Froude similitude and constructed. Experiments have been conducted

to measure the head-discharge relation. Piezometric head and air core size are

measured at the throat of the vortex flow. Velocity fields are measured using Laser

Doppler Anemometry (LDA).

The measurements show that the vortex flow in the chamber resembles a free

vortex and the circulation is approximately equal to that at the inlet to chamber.

The chamber flow is not affected by the bottom boundary effect at bottom above

a depth of the order of the dropshaft diameter. The throat section of the vor-

tex flow is located slightly below the chamber bottom and within the bellmouth

at the entrance to dropshaft. For the vortex flow in and downstream of the bell-

mouth, the tangential velocity distribution can be described by a Rankine vortex

(combination of forced and free vortex); the transition from forced to free vortex

occurs at around the middle of the vortex flow layer. The pressure is positive for

all locations and all discharges. Due to viscous effect, the maximum circulation is

found to be lower than the inlet circulation. Consistent with the free vortex theory,

the vertical velocity in the dropshaft is approximately constant. By accounting for

the loss of circulation between chamber inlet and the dropshaft, a new 1D theory

is proposed. Unlike previous models, the new theory gives good predictions of

head-discharge relation and minimum air core size without the need of physically

unrealistic assumptions.

This study has revealed the structure of a scroll vortex intake flow for the first

time. Characteristic flow features of the scroll vortex intake have been elucidated.

The findings have helped to explain and resolve the long-standing discrepancies

between the theoretical predictions of three representative 1D hydraulic theories.

The vortex flow measurements also provide a basis for the development of a new

theory and the validation of 3D numerical models.