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An acoustic Doppler velocimeter (ADV) was used to measure detailed vertical profiles of mean velocity and (for the first time) turbulence characteristics at the center of the quiescent zone (QZ) of operating aquaculture raceways. Instantaneous velocities were recorded at 10 Hz and averaged over a 360-s time interval in order to derive mean velocities, root-mean-square velocities, and turbulence intensities. The type and cleanliness of the screen separating the QZ from the fish rearing zone was found to be significant. When screens are free of growth and debris, smaller screens promote more uniform mean streamwise velocity profiles, reduce the streamwise root-mean-square (rms) velocities near the floor, and greatly reduce the turbulence intensities in the lower half of the profile. These effects suggest that smaller screens may be used to enhance the setting efficiency of QZs, but this conclusion may not be valid when the screen accumulates significant amounts of debris and algal growth. This condition dramatically alters both the magnitude and the shape of the mean and turbulence profiles. The slower mean velocity produced by a dirty screen should tend to enhance settling, but the higher turbulence has the opposite effect. It is not clear which effect is dominant. The resolution of these questions will require more comprehensive measurements of velocity and solids concentration. |
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