Research On The Length Of A Small Flow Free Overfall And Energy Dissipation

The geometric construction of free overfall is simple, but the flow is very complex. It flows with the complicated free surface. This flow problem with free surface has always been a hot research. This paper introduced the structure of the free overfall. And on the basis of understanding the research background, we reviewed the research status of Formula,experimental investigation and numerical simulation in the domestic and overseas. Using acrylic to make different size of sink, and this sinks made up the model. At the time of model experiment, under the different height of drop, carried out more groups of experiment including different flow to ensure the comprehensiveness and rationality of the experimental investigation. Now the hydraulics formulas mostly were empirical formula. In this paper, on the basis of summarizing predecessors’ research results, put forward a fast and convenient formula to calculate the length of free overfall and validated it. Using software FLUENT to set up three-dimensional model in this paper, and a series of studies were carried out.⑴ According to the projectile motion, we can deduce a formula to calculate the length of free overfall. The formula is, A is experience factor, the range is1.5~1.6;K is air resistance coefficient, K(28)1; is the kinetic energy correction coefficient,a(28)1; q is unit discharge;H is the height of drop.⑵ The VOF method is used to deal with the interface between gas and water. And it is combined with k(25)e turbulence model to simulate free overfall can get a very good effect.It fit well with the actual situation.⑶ By analyzing the flow pattern, pressure distribution, velocity distribution, streamline distribution of water in the channel, we can judge that the numerical simulation method was reliable.⑷ The comparison results of formula, model experiment and numerical simulation showed that the result of formula was very close to model experiment, the errors were 17.6%、22.4% and 14.6% between numerical simulation and model experiment. According to the previous experience and actual test conditions, this error was within an acceptable range. So the formula can be applied to practical engineering.⑸ In order to reduce the destruction in the channel bottom slabs and banks, we can set stilling wall in the downstream to improve the rate of energy dissipation. In this paper,different height of stilling wall was set up in the proper position. Combined velocity with dynamic pressure to choose the most optimal height of stilling wall. As for the model of this paper, choosing 3cm high of drop was the optimal conditions. This result provided some design basis for practical engineering application in the future.