Experimental Research On Hydraulic Characteristics When Discharging Silty Flow In Flood Tunnel With Orifices Plates

Up until now,the researches on the hydraulic characteristics of the discharge structures are mainly concentrated in the clear water.There are few studies on the turbulent mechanism when discharging hyper-concentration flow in the tunnel with orifices plate.However,the time-average and fluctuating pressure acting on the tunnel wall will be influenced to different degree when the flow carries sediment.As a result,to study the effect of hyper-concentrated flow on the hydrodynamic characteristics of the orifice plate tunnel,and the turbulent mechanism when the orifice plate tunnel discharging flow with hyper-concentration of sediment,has guiding significance for flood discharge and energy dissipation of water conservancy project.Based on the hydraulic model test,the time-average and fluctuating pressure of different flow conditions,sediment concentrations and orifice plate forms is measured and analyzed.The main results and conclusions are listed as following:(1)The time-average pressure distribution regularities along the middle line of the tunnel wall are similar on different working conditions with same orifice plate form.The time-average pressure declines sharply downstream the orifice plate,and then rises gradually.It increases with the augment of the silt content or flow intensity.The ratio of the minimum diameter of the orifice plate and the inner diameter of the tunnel,d/D,plays an important role in the time-average pressure distribution regularities.With the decrease of the ratio d/D,the time-average pressure upstream the first orifice plate increases rapidly,and which between the two orifice plates increases slowly,while that downstream the second orifice plate decreases gradually.The ratio of the distance of two orifice plates and the tunnel inner diameter,L/D,has little influence on the distribution pattern.(2)The fluctuating pressure distribution regularities along the middle line of the tunnel wall are similar on different working conditions with same orifice plate form.The fluctuating pressure reaches a peak downstream each orifice plate,and the second peak is much larger than the first one.The amplitude value of the fluctuating pressure is influenced by both the silt content and upper stream conditions.Under the same upper flow conditions,the maximum of the fluctuating pressure increases at first and then decreases with the increase of the ratio d/D.On the other hand,it decreases gradually with the increase of the ratio L/D.(3)The time-scale correlation is decreased with the sediment in water,indicating that the average scale of the large vortex is reduced.Integral time scale decreases at first and then increases near the first orifice plate,while decreases at first and then stays the same downstream the second orifice plate.It indicates that the average scale of the vortex varies complicatedly with the increase of the sediment concentration under different boundary and incoming flow conditions,which may be bigger,smaller,or unchanged.(4)The distribution regularities in the frequency domain of fluctuating pressure is similar with different sand content.The dominant frequency of the fluctuating wallpressure is very low,which indicates that it is low-frequency,large-scale,and coherent vortex that plays an important part in the fluctuating pressure.Other peaks in the NPSD(normalized power spectral density)curves of the silty flow also prove the reduction of the vortex scale.