Numerical Investigation On Re-oxygenation Efficiency Of Stepped Overflow Weir In Urban Stream

The concentration of dissolved oxygen in water is one of the important indexes to measure the self-purification ability of water body.The re-oxygenation process directly affects the migration,diffusion and degradation of organic pollutants in the water body,and also affects the self-purification process of the whole water body.To solve the problem of water pollution and restore the ecological and social functions of urban rivers has become the key to ensure the sustainable development of cities.Many overflow weirs have been built along urban streams in northern China to retain landscape water.In this paper,a two-dimensional numerical model of the free surface flow aeration process of stepped overflow weir is established,which is verified and analyzed by the experimental data in the literature.The aerodynamic characteristics and aeration effect are analyzed from the aspects of flow pattern,air concentration distribution,flow field and velocity field.On this basis,the hydraulic characteristics and reaeration efficiency of stepped overflow weir are studied The effect of gradient,number of steps and height of stepped weir on its reaeration efficiency under the same flow condition.The calculation results show that the flow on the the stepped overflow weir can show three flow patterns of nappe flow,transitional flow and skimming flow according to the shape of the step and the unit flow rate.For a constant discharge,changing the size of the step can realize the mutual conversion of nappe flow and skimming flow.The air concentration distribution of the stepped overflow weir presents an S-shaped curve,and the velocity satisfies the power-law velocity distribution and Prandtl-von Karman general logarithmic velocity distribution of the boundary layer,and the curve fitting method is used to determine the correlation coefficient.For a constant weir head,the air concentration is positively correlated with the normal distance from the pseudo-bottom.The air concentration near the pseudo-bottom is close to zero,which means that there is almost no air near the pseudo-bottom.The two-dimensional numerical model of the re-oxygenation process on the free surface established in this paper is feasible for the analysis of the hydraulic characteristics of the stepped overflow weir.It can obtain each hydraulic parameter effectively and supplement the data that cannot be observed in the model test.Combined with numerical simulation and experimental study,some morphological characteristics of the stepped overflow weir under different flow conditions are determined,which can make the urban stream obtain higher re-oxygenation performance and provide a basis for the optimal design of the stepped overflow weir.The results show that the re-oxygenation efficiency of a nappe flow is higher than that of a skimming flow or a transition flow.Re-oxygenation efficiency is sensitive to changes in unit discharge and reaches its peak value when the unit discharge of an inlet is about 0.06 m2/s.A comparison analysis of our present experimental simulation data shows that the slope for stepped overflow weirs for optimum re-oxygenation efficiency is 51.3°.For small flow conditions(q<0.10 m2/s),stepped overflow weirs with fewer steps and greater weir height contribute to higher re-oxygenation efficiency.For large flow conditions(q>0.10 m2/s),unit discharge,number of steps,and total weir height have little effect on the re-oxygenation efficiency of stepped overflow weirs.