Hydraulic modelling of instabilities on half-round circular weirs at three scales

The half-round circular weir is a seminal design that has been extensively studied for relatively small discharges. Recent experimental works reported a number of instabilities, nonlinearities and hysteresis at medium to large discharges. Detailed physical investigations with a half-round circular weir were undertaken in the absence of ventilation at three geometric scales for an identical relative height and breadth. Three basic flow patterns were observed. The flow conditions for the change in flow patterns did not scale according to a Froude similitude. Cyclic flow patterns were observed for a range of constant discharges at all scales, and these nonlinearities were caused by the lack of ventilation. The quantitative measurements also indicated some hysteresis, with different data obtained whether for increasing or decreasing discharge experimental conditions, underlying the limitations of the Froude similitude.