| Summary: | High flow velocity causes a low pressure even negative, especially in steep
channel bed. Force caused by negative pressure will lift bed surface and side walls
of the building hydraulic structures, which can lead to flaking on the bed surface
and side walls of the building. In supercritical flow, air from the atmosphere enter
into the body even reach the bed flow. Air entrainment enter into the body and
bed flow can prevent or reduce the phenomenon of cavitation in channels bed.
This research focuses on five things, were: (1) concentration distribution
of air bubbles in the developing regions of the self air entrainment, (2) location of
the initial point of inception, (3) pressure in the bed flow, (4) configuration and
dimensions of the aerator to reduce the phenomenon of cavitation, and (5)
concentration distribution of air bubbles in the artificial air entrainment. Five
research focus were achieved using experimental methods implemented in the
Hydraulics Laboratory of the Department of Civil and Environmental Engineering
(JTSL), Faculty of Engineering Gadjah Mada University (GMU). In this study,
the steep channel having a length of 10 m, width 0.2 m, height 0.4 m with slope
varies from 15º, 20º, and 25º, which was fused with the wall of the water tank.
Discharge varied from 9.4 l/s, 11.5 l/s and 20.9 l/s. Initial location of air
entrainment during the experiment compared with the initial determination of the
location of air entrainment on calculations using the four equations, namely (1)
Wood, Ackers, and Loveless (1983), (2) Feranndo and Rico (2002), (3) Hager and
Blaser (1998), and (4) Bauer (1954). Concentration distribution of air bubbles in
condition of self air entrainment experimental results compared with the results of
calculations using the equations Chanson (1995) and Straub and Anderson (1958).
Concentration distribution of air bubbles in condition of artificial air entrainment
experimental results compared with the results of calculations using the equations
Chanson (1995).
Conclusion of this research were: (1) concentration distribution of air
bubbles on the condition of self air entrainment in slope channel bed 25º and
discharge=20.9 l/s can be predicted using the results of the calculation equation
Straub and Anderson (1958) modification, (2) concentration distribution of air
bubbles on the condition of self air entrainment in slope channel bed=25º can be
predicted using the results of the calculation equation Chanson (1995)
modification, (3) initial location of the air entrainment experimental results closest
to the initial location of the air entrainment on calculations using the method of
Hager and Blaser (1998) compared to three other methods, (4) concentration
distribution of artificial air entrainment on the slope of the channel bed=25º at a
point 7.2 m from the inlet flume and in the free surface aeration, was the Gaussian
or normal, and (5) on the slope of the channel bed=25°, at a point 7.2 m from the
inlet flume on the condition of artificial air entrainment in the down stream flow
region after the first aerator installed a new aerator is not required anymore.
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