Study On Hydraulic Characteristics Of The Multi-slot And Weir Combined Fishway

As a channel connecting the upstream and downstream of the river,the fishway is of great significance for restoring the ecological environment of the river.The vertical slot fishway has been widely used in practical fish passing projects,and there are many studies on its structural size,vertical slot arrangement and hydraulic characteristics.On the basis of the previous research,this paper proposes the same-side multi-slot fishway and the multi-slot and weir combined fishway.The time-average flow structure,the velocity distribution characteristics,the turbulent kinetic energy and the Reynolds shear stress within the middle pool are analyzed in detail by establishing the mathematical model and numerically simulating the flow field.The optimal layout of the fishway is recommended by studying the influence of the weir height and the slot width on the hydraulic characteristics within the pool.The main research results are as follows:(1)After comparing and analyzing the numerical results of the standard vertical slot fishway,the same-side multi-slot fishway and the multi-slot and weir combined fishway,it is found that the flow velocity value at the vertical slot can be effectively reduced after doubling the number of slots,and the combination of multi-vertical-slot and weir can further reduce the flow velocity value within the pool.(2)The multi-slot and weir combined fishways with different weir heights show similar flow regimes.As the weir height increases from 0.5m to 0.8m,the flow quantity passing the weir gradually diminishes,the mean time-averaged water depth of the whole pool gradually increases,and the mean time-averaged flow velocity of the whole pool gradually decreases.However,the maximum time-averaged flow velocity within the pool is always located in the main flow area around the weir for all the four cases.When the weir height is equal to h=0.6m,the maximum time-averaged flow velocity,the maximum time-averaged turbulent kinetic energy and the maximum time-averaged Reynolds shear stress within the middle pool are always the smallest among the four test cases.(3)Based on the optimum height of the weir,the vertical-slot width of the MSWF is further optimized.The numerical results indicate that the time-averaged flow structure has similar features for MSWF fishways with different vertical-slot widths.With the increase of the vertical-slot width from 0.55 m to 0.66 m,the mean time-averaged water depth of the whole pool gradually decreases,the mean time-averaged flow velocity of the whole pool slightly increases,and the location of the maximum time-averaged flow velocity will be shifted from the mainflow region around the weir to that around the vertical slot.Besides,the difference of the maximum time-averaged flow velocity in the mainflow region around the vertical-slot is approximately 8%,the difference of the maximum time-averaged flow velocity in the mainflow region around the weir is about 6%,the difference of the maximum time-averaged turbulent kinetic energy is equal to 20% and the difference of the maximum time-averaged Reynolds shear stress reaches up to 27%.All the three parameters,namely the maximum time-averaged flow velocity,the maximum time-averaged turbulent kinetic energy and the maximum time-averaged Reynolds shear stress,are firstly decreased and then increased,which means that the vertical-slot cannot be continuously widened.(4)For all the MSWF fishways considered in this thesis,according to the analysis of the hydrodynamic parameters in the vertical direction,it can be concluded that generally the flow velocity around the weir region increases with the decrease of the water depth(namely when approaching the water surface),but the flow velocity around the slot region only changes little along the water-depth direction(showing obvious two-dimensional characteristics).Around the weir region,with the increase of the water-depth(namely when approaching the bottom wall),the time-averaged turbulent kinetic energy firstly increases and then decreases,however,around the slot region,the level of turbulent kinetic energy becomes higher when approaching the water surface.In addition,around the weir region the time-averaged Reynolds shear stress is relatively higher near the water surface,but,on the contrary,around the slot region the time-averaged Reynolds shear stress is relatively larger near the bottom wall.(5)With the purpose of proving that,relative to the standard vertical slot fishway,the optimal scheme recommended in this thesis always has a more fish-friendly water flow under different operation conditions,the time-averaged flow field characteristics of the aforementioned two schemes under four flow rates are further analyzed and compared.The results clearly demonstrate that,for all the four flow rates considered,MSWF-6 is consistently superior to VSF in terms of the time-averaged flow velocity,the time-averaged turbulent kinetic energy level and the time-averaged Reynolds shear stress distribution.