Design criteria for rotating flow cells in rectangular aquaculture tanks

Abstract

This work analyzes the simplest inlet and outlet configurations that create homogeneous rotating flow cells in rectangular aquaculture tanks, in order to combine the advantages of rectangular and circular tanks. All the configurations analyzed had a single jet discharge per rotating flow cell, with the drain placed in the center of each rotating flow cell. Length/width ratios ( L/ W) of 0.95, 1.43 and 1.91 were tested. In addition, the effect of placing oblique baffles in the walls to redirect the water currents was assessed. Experiments were conducted in a laboratory-scale tank with a Reynolds number of approximately 6000. Particle tracking velocimetry techniques were used to characterize the flow pattern in a horizontal cross-section at the midpoint of the water depth. A tank resistance coefficient ( C t) was defined in order to characterize the resistance offered by each tank configuration to the circulation of water. Results indicated that when L/ W was increased from 0.95 to 1.43, the main vortex that was formed occupied most of the rotating cell area and did not create significant dead volumes in the tank. A L/ W ratio of 1.91 dramatically reduced flow uniformity and hardly increased C t values. The presence of baffles contributed to high velocities in the area around the center drains and decreased C t values by 30–35%. Higher velocities are critical to the self-cleaning properties of the tank. The calculation of a C t value makes it easier to obtain the desired average velocities in the tank by adjusting the water exchange rate and the water jet discharge velocity.