Hydraulic Characteristics Of Stepped Release Structure

As a kind of new structure, the stepped release structure attracts more and more attention of researchers in the fields such as hydrotechnics, landscape design of city, river sediment and fishway as well. The hydraulic characteristics are too complicated to clarify thoroughly for the time being due to its nature of two-phase flow of liquid and gas, accompanying some complicated boundary conditions. In this paper, the hydraulic characteristics of stepped release structure are researched and tested systematically and the following achievements are obtained.1. The author analyzes these key points as follows: flow the mechanism causing jump flow, the range of jump flow and how much the step height and the channel slope affect the jump flow under nappe flow and transition. Correspondently, the ways to reduce and eliminate the jump flow are also discussed, and the empirical equation of forming jump flow in the range of maximum and minimum is finally developed.2. The flow patterns of stepped release structure are categorized, and the limits of nappe flow, transition flow and skimming flow are also given for the first time in this paper in China.3. Under each flow pattern, the energy dissipation ratio at the steps of stepped release structure (the channel slope 0 = 5.7° ~ 60°) is in reverse ratio with the channel slope and the discharge, while it is in direct ratio with relative dam height. The paper offers an empirical equation in calculating the energy dissipation ratio of cross sections before and after jump. The increased impact on the bottom of stilling pool that the curving of stream line of cross section before jump causes is also considered in the equation. It demonstrates that the step number does not affect the energy dissipation ratio when the height of release structure and the channel slope keep unchanged, and the energy dissipation ratio at the jump part of stilling pool augments with the increase of channeldissipation ratio at the jump part of stilling pool increases with rising of channel slope.4. For the first time, the author analyzes the mean pressure, maximum instantaneous pressure, minimum instantaneous pressure and the strength of pulsating pressure which act on vertical plane and horizontal plane of steps of the stepped release structure (the channel slope 9 = 40o~60°), and the study shows that the maximum negative pressure occurs at the salient angle of the step vertical plane and the maximum positive pressure appears at the point where the nappe flow falls on the horizontal plane of steps; furthermore, the pulsation pressure intensity on the step horizontal plane is much larger than which on vertical plane. The test results manifest that under the self-aerated flow, the minimum negative pressure and maximum positive pressure occurring on the face of step both are within the limit of compression resistance of concrete.5. Under the condition of self-aerated, the aerated inception point moves toward the downstream along with the increase of discharge. The empirical equation for the aerated inception point is reasonable, for the test results from it approximate to the conclusion of former researchers.6. Comparing the equations of jump range of smooth channel, the author first correspondingly releases the empirical equations after testing the relations of range of critical jump respectively with the critical water depth of crest, water depth after jump and the section pressure before jump under the stepped release structure (channel slope of 40°"-60°). Meanwhile, respectively under modular flow and critical jump, the mean pressure at the bottom of stilling pool at jump part is also studied, and the changing trends of mean pressure at the jump part and within the range of downstream are further revealed.7. The author first provides the criteria of optimal flow patterns in stepped landscape design. With these criteria, the author studies the discharge suitable to stepped structure, and offers the empirical equations respectively used to estimate maximum and minimum discharge.