Study Of The Mechanism Of Scour By Free Jets On Rocky River Beds

With main researched results of free impinging jets, the diffusive characteristics for an oblique-submerged turbulent impinging jet in a plunge pool and variations of main physical variables in the different jet sub-regions are analyzed in detail?and the lifting conditions for rock blocks in a scour pit are investigated preliminarily. By use of the model for transient flow, the mechanism of fluctuating pressure transmission in cracks of the bed rock and the cause of generating fluctuating uplift are investigated,and the predicting formula for the maximum fluctuating uplift is deduced. To confirm the theoretical results, the fluctuating uplift on variously dimensional rock blocks at the bottom of the scour pit are measured in the Three-Gorge spillway model.A ccording to the theory of random vibration, the process of random vibration for a rock block subjected to a fluctuating uplift is analyzed by the one-dimensional vibration equation and the frequency response term for a fluctuating uplift is introduced into the stability conditions of rock blocks or slabs. The predicting formulae for the limiting scour depth, the limiting scouring time and the development of scour depth are obtained theoretically. To verify above analytical results, the random vibration process for a rock block at the bottom of the scour pit is measured under different tail-water elevations in the Three-Gorge spillway model, and the vibrating frequency, the pulling out time, the pulling out velocity and acceleration for a rock block are obtained experimentally.Based on the dissipating mechanism in a plunge pool, diffusive characteristics of jets and the random vibration for a rock block, the predictng formulae for the limiting scour depth are presented, with the information of prototype and model for a number of hydroelectric power projects, the above formulae are confirmed and popularited. A AWith above main results, the Uniting scour depth for the Xiaowan project is predicted. Simultaneously, the scour experiment is carried out by the enlarged rock block method in the Xiaowan model, and experimental and predicted results are compared.