Asymmetric Flow in Symmetric Expansions

Asymmetric flow patterns may occur in perfectly symmetric abrupt expansions, in which the main flow deflects and attaches arbitrarily to one wall of the expansion. With the notable exception of “fluidic” devices, asymmetric flow patterns are detrimental in most civil engineering applications. The adverse effects include lengthening of the distance to full-width flow in expansions, inefficient operation of screens in screen channels, and increasing the potential for vortexing in pump intakes. No good explanation or prediction of the onset of asymmetric flow patterns is known to exist. This paper attributes the asymmetric behavior to a static instability of the system occurring under certain conditions of expansion geometry. The stability is investigated by analyzing the forces acting on the system when it undergoes a small deflection from its symmetric position. A mathematical stability criterion is developed and applied to subcritical, horizontal, rectangular expansions. The results are in excellent agreement with experimental observations (limited to small Froude Numbers). The predictive method is extended theoretically to the analysis of certain preventive and corrective measures. Means of extending the analyses to more complex two-dimensional and three-dimensional configurations are examined.