Numerical Study of Flow Characteristics on Bottom Axis-driven Overflow Weirs

Plain river network areas in China, such as the Yangtze River Delta or the Pearl River Delta, are prosperous in economy dense in population and closely distributed with water network. However, due to its small riverbed slope and river reflux, the bad hydrodynamic condition of urban river endangers the water renew of the river network and the adjoining structures. Therefore, improving its hydrodynamic conditions is of high significance in the plain river network areas.

The bottom axis-driven overflow weir (BAOW) is currently widely used in plain river network areas of China, because of its simple structure, convenient construction, economy and applicability. The flood discharge and navigation will not be affected when it completely lies down (opening degree is 0°), and a small water fall landscape will be formed when opening degree is 90°. The BAOW can get different weir head and flow discharge by adjusting opening degree from 0^o to 90^o which is considered superior fit and high integration with the urban water environment.

Although numerical studies on BAOW were proposed for its discharge capacities, geometry and downstream scour, several problems have so far not been systematically addressed. Standard information such as when it uprights completely (opening degree is 90^o) as a sharp-cresed weir and the series rules when it adjusts from 0^o to 90^o are not available.

In present study, κ-ε turbulence model has been tested for the flow characteristics at different opening degree (i.e., 90^o, 75°, 60°, 45°, 30°, 15°, 0^o). The prediction models are validated by using past experimental data and theoretical expressions of opening degree are 0 °and 90°. The flow properties including flow patterns, velocity distributions, streamline, and water surface profiles of BAOW at different opening (i.e., 75°, 60°, 45°, 30°, 15°) are discussed in this paper. Detailed relationships between water discharge with weir head and opening degree were performed and its overflow capacity comparative performance were also examined.

The main conclusions indicated that: (1) The flow discharge coefficient increases at first but then a decrease in it with increasing opening degree, and the discharge coefficient at 0° is less than that at 90°. (2) The relationship between opening degree and flow coefficient becomes more unstable as well as smaller water head. (3) Both the weir head and the opening degree are supposed to certain influence on the overflow capability of the BAOW. And the influence of the weir head is more complicated.