Recirculating flow in oscillatory U-tubes

Wastewater treatment facilities often experience undesirable oscillations in water surface elevations due to the potential of unsteady outlet flow. Self-induced oscillations, particularly those generated by the geometry and flow conditions of treatment facilities with a U-tube component, are of particular interest. This study focuses on the U-tube component of an existing wastewater treatment facility where oscillations of constant amplitude and period were observed shortly after the facility was operational. It is assumed that an oscillating U-tube manometer represents this problem, and a numerical model was developed to predict the amplitude and period of the oscillations using Newton's second law of motion and momentum analysis. A 1-to-4.88-scale physical model of the existing facility was constructed and tested. Numerical model results show good agreement with measured oscillatory characteristics of the existing facility and the physical model.