Ultrasonic techniques have been used to directly and non-intrusively measure the circulation of free surface vortices. All experiments were performed in a vertical cylindrical tank with a central drain and a tangential inlet The circulation was measured on a closed triangular path by measuring the difference in upstream and downstream transit-times. Circulation was measured as a function of the Reynolds and Froude numbers and was found to increase as the Reynolds and Froude numbers increased. The circulation was also found to be proportional to the square of the ratio of the drain diameter to cylinder diameter while the ratio of fluid depth to cylinder diameter was held constant. Minimum surface elevations were measured at various conditions and attempts were made to correlate them with measured circulation.
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