Characterization of a Bubble Curtain for PIV Measurements - Selected Topics of Computational and Experimental Fluid Mechanics - page 257

the values of the amplitude are very close and independent of the height. The same
thing happens with the velocity values.
The bubble density per unit area changes as the pressure is increased. In Fig. 7
(upper right) the average density of particles per image is shown. Data obtained for
pressures greater than 14.5 cmHg are clustered, consistent with the Fig. 7 (upper left
and bottom).
The analysis of the graphs above determined that the optimal pressure must be
greater than 14.5 cmHg regardless of height. In this area the speeds, densities and
dispersions stay in a similar range. The average displacement on the horizontal axis is
zero. Therefore, it is only relevant to note that there are periods of oscillation within
the range of 0.16-0.25 s for the bubbles. This oscillation will be taken into account
in the final analysis, and will not affect considerably the measurements.
4.2 Results for Three Bubbles
Measurements of the speeds of bubbles from three nearby holes are used to determine
whether the interactions between the bubbles affect their behavior. These measure-
ments were compared with those obtained for bubbles produced by one hole only.
TheleftsideofFig. 8 shows that the bubbles have speeds close to 25 cm/s, which
means that the increase in the number of bubbles does not induce any change in its
upward velocity. The device was maintained at pressures higher than 14.5 cmHg.
Fig. 8 Left speeds in the upward direction for three holes compared to one hole. Right upward
speeds of bubbles when waves were produced in the flume