Vibrationof gates, if it occurs, can be dangerous – seal leakage, inter-
mittent flow attachment, inadequate venting being the main causes. Some
mention of vibration and flow instabilities already has been made in previ-
ous sections.
In principle the fluctuating components of hydrodynamic forces can
be induced by different excitation mechanisms and are closely related to
the structural vibrations which they in turn may induce. The main cat-
egories of these excitation mechanisms are extraneously induced excitation
(EIE) (e.g. pressure fluctuations due to the turbulent flow downstream of
a control gate and impinging on the gate), instability induced excitation
(IIE) (e.g. flow instabilities due to vortex shedding), movement induced
excitation(MIE) (e.g. self-excitation due to the fluctuation of a gate seal)
andexcitation due to (resonating) fluid oscillator(EFO) (e.g. surging water
mass in a shaft downstream of a tunnel gate). Flow induced vibrations can
be forced by turbulence or flow structure – see EIE and IIE above; they
can also be amplified by body vibrations synchronizing with random turbu-
lence excitation or, the flow excitation is purely induced by the body vibra-
tion itself – self-excitationornegative damping –see MIE and EFO above.
In the design of gates it is desirable for their excitation frequencies to be
remote from resonance frequencies (unless there is high damping) and
negative damping (self-excitation) should be avoided.
Further treatment of the subject involving the computation of flow
induced gate vibrations including the bulk modulus of water, Young’s
modulus of the gate material, the excitation and natural frequencies as
well as all relevant hydrodynamic parameters is beyond the scope of this
text; for this and further details see Kolkman (1984), Naudascher and
Rockwell (1994), ICOLD (1996) and Lewin (2001). For a brief mention of
physical modelling of hydraulic structures vibrations see Chapter 16.
6.7 Automation, control and reliability
There are two aspects of automationin connection with dam crest gates:
gates that move automatically according to reservoir level and installations
with automatic control of gate movement. In the former category are, e.g.
drum and sector gates, some types of fabric and flap gates and crest gates
using fluid dynamics systems (Townshend, 2000) (gates with buoyancy
tanks connected by ducted radial arms to an upstream axle). Fusegates
(see Section 6.2.8) can also be classed in this category. Automatic gates are
advantageous in remote locations but are not restricted to them.
Incontrol systems electromechanical controls have largely been
replaced by electronic closed-loop systems. Programmable logic control
(PLC) is now general with duplicated controllers for the control of critical
functions. Di Stefano et al. (1976) deal comprehensively with feedback and
284 GATES AND VALVES
