Hydraulic Structures: Fourth Edition

plicated with the shape and size of the passage between the two gates
playing an important part.

6.6 Cavitation, aeration, vibration of gates

Cavitation, air demand and vibration of gates are closely connected and
influence each other.
For a general discussion of cavitationsee Section 4.6; some instances
of cavitation at gates and valves also have been mentioned in previous sec-
tions. In principle cavitation danger exists in any situation where there is
flow separation without sufficient air supply, e.g. in case of flow under a
gate in a high-head conduit or downstream of gate slots (see Section 6.2.2).
The determination of the air demandand the associated design of air
ventsis important both for crest gates with an overfalling nappe and for
high-head gates and their downstream conduits.
For crest gates Novak and Cˇábelka (1981) quote for the air demand
the equation proposed by Hickox, giving an air vent diameter D:

DC

H

p

1.8

0

2

.8

b

2

0.5

 (6.6)

where for a flow over the gate only C0.004 for all parameters in metres (pis
the negative head underneath the overfall jet), or the procedure of Wisner and
Mitric, which takes into account the length of contact of the overfall jet with air
and which, for p/H0.1, gives about the same results as equation (6.6). The air
demand both for flow in a conduit with a hydraulic jump and for flow under
pressure downstream of the jump has been discussed in Section 4.8.
For air vents of conduit gates, Smith (1978) quotes the equation of
Campbell and Guyton for the ratio of air and water discharge :

0.04(Fr 1)0.85 (6.7)

whereFrrefers to the flow just downstream of the gate. In using equation
(6.7) it is assumed that the maximum air demand occurs with the conduit
flowing half full. When dimensioning the air vent the air velocity should not
exceed 45 m s^1 and the pressure drop should be limited to 2 m (of water).
Using the coefficient 0.015 in equation (4.71) gives identical results to
equation (6.7) for Froude numbers about 6.
Aydin (2002) after investigating on a model the air demand required
to keep the pressure drop downstream of a high-head gate during emer-
gency closure to relatively insignificant values concludes that a ratio of the
effective air shaft length (i.e. accounting also for head losses) to the
conduit diameter should be less than 40.

CAVITATION, AERATION, VIBRATION OF GATES 283