Study On Flows And Design Optimation Of Control Section For Water Discharge Strucures

The chute or tunnel spillway is most often used water discharge structure in hydropower projects, along which the flow is usual not aligned with the general direction of the flow in the reservoir owing to the limit of terrain. This may lead to transverse gradient over free water surface, and even strong vortex. If the action reach to the supercritical flow section downstream discharge chute, there may produce shock wave in water surface, which may directly threat the safety of discharge structure itself and even dam.Eccentric hinge and side expansion water seal for arc gate both require abrupt enlarging in gate seat and bottom, so combining gate seal and aeration devices effectively. After the flow leaving from gate seat and ejecting out from pressure tunnel, it diffuses along the horizontal and vertical directions, and forms side cavities with side wall and bottom cavities with bottom. The bottom cavities and side cavities mutually complement, turning into effective aerator which protects the rigid wall in downstream. But the water-wing generated from impaction between flow and sidewall, and the low pressure generated from impact echo may worsen the flow pattern, even lead to cavitation.In this thesis, the complex flow in control sections of both bi-tunnel spillways and abrupt enlarging and abrupt falling segment of arc gate are investigated with numerical simulation and model test. The main contents of the studies and achievements are as following:(1) The fractional volume of fluid model and the k-e turbulence model are used to study the three dimensional free water surface flows over bi-tunnel spillway. Obtained hydraulic parameters include velocities and distribution of pressure, etc. The simulation also reveals the distort form of free water surface in weir crest. In order to improving inlet flow pattern, eliminating large negative pressure in weir crest and balancing the flow rate distribution between two channels, numerical simulation and model test are combined to get prospective inlet configuration and overflow weir shape, and a new type of water guide wall is proposed.(2) The hydraulic characteristics in abrupt enlarging and abrupt falling segment of arc gate are studied by means of model tests. The experiment shows that negative pressure exists in the roof of pressure tunnel outlet and open channel segment, strong water-wing on sidewall, unstable flow in open channel. And also low cavitation number, small bottom cavity and insufficient aeration exist in original design. Aiming at these problems, two schemes for shape optimization are recommended.(3) Experimental results for the two recommended schemes include flow pattern, size of the water-wing, length of bottom cavity, size of un-airflow area, pressure distribution of sidewall and bottom, cavitation number and aeration performance, and so on. The influence of different shape factors on hydrodynamic characteristics of abrupt enlargement and abrupt fall section are analyzed, especially influences of side expansion on pressure distribution of sidewall,size of un-airflow area and flow pattern. From the view of minimum pressure due to jet impact and reflection, unfavorable side expansion width is put forward. Experimental research with partial opening shows that some specific partial opening conditions may result in the worst condition.(4) Because there exists un-airflow area on sidewall which is difficult to be aerated. Cavitation characteristics of abrupt spreading or abrupt falling segment of arc gate is studied by mean of vacuum model test. It shows that cavitation mechanism in sidewall impacting reflection area is complex.(5) Cavitation characteristics of sidewall under different water level and different similarity vacuum pressures are discussed for different roughness and smooth wall. Under specific water level or specific opening, flow cavitation performance may be the worst, and noise pressure level and relative noise energy increase quickly. When the height of convex bodies exceeds certain value, cavitation may occur under all different water levels.(6) The numerical simulation was used to investigate three dimensional flow of outlet of abrupt spreading or abrupt falling segment of arc gate. The simulation found cavity structure, water-wing of sidewall and pressure distribution. Influence of vortex in jet impact region on aeration is discussed. The result shows that the jet can form a strong vortex at impact point both on bed and side walls. On the bed, the vortex suck into air, on the side, the vortex may become a source of cavation.