Fluctuating pressure characteristics of stilling basin with drop sill and sudden expansion
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Abstract:
Traditional underflow energy dissipators were mostly used in small and medium-sized projects,and they were rarely used in high dam discharge energy dissipation.In recent years,as people′s requirements for the ecological environment have increased,environmentally-friendly stilling basins with drop sill and sudden expansion had been well used in many projects in high-head and large-unit-wide flow projects.The fluctuating water pressure on the bottom of the stilling pool is extremely destructive.Many of the bottoms of the stilling pool had been damaged during operation in the domestic and foreign-built projects.There were many results of previous studies on the fluctuating pressure characteristics of the stilling pool,but there are few studies on the fluctuating pressure characteristics of the stilling pool with drop sill and sudden expansion,especially the related characteristics and spectral characteristics are not involved. Based on the results of physical model tests,the fluctuating pressure characteristics of the bottom plate of the stilling pool were studied with drop sill and sudden expansion and the stilling pool were analyzed with drop sill and sudden expansion (β=1.3,1.6) and the non-sudden expansion stilling pool(β=1.0).The research results of the amplitude characteristics,correlation characteristics,and frequency characteristics of the fluctuating pressure of the bottom plate of the stilling pool can provide a reference and guidance for engineering design. It was found that under different hydraulic conditions,the fluctuating pressure coefficient of the midline of the bottom of the suddenly and non-suddenly expanded stilling pools showed a trend of attenuation along the way,the peak appeared at the head of the stilling pool,and the fluctuating pressure coefficient increased with the increase of the flow energy ratio.Compared with the non-sudden expansion stilling pool,the stilling pool with drop sill and sudden expansion can significantly reduce the fluctuating pressure level of the bottom plate.The probability density of fluctuating pressure in the impingement zone and the wall jet zone conforms to the normal distribution.Under the conditions of different flow energy ratios,the spatial integration scale distribution law of the fluctuating pressure of the bottom of the sudden expansion stilling pool was the same,and all increased along the flow direction,and the spatial integration scale and the fluctuating pressure energy of the sudden expansion stilling pool were significantly lower than that of the non-sudden expansion stilling pool. Under the same hydraulic conditions,the central line fluctuating pressure coefficient of the stilling pool with drop sill and sudden expansion was the same as the non-sudden expansion-type distribution law,and the maximum value of the fluctuating pressure coefficient appeared at the head of the pool.It decayed gradually along the way,and the stilling pool with drop sill and sudden expansion decayed more rapidly.Compared with the non-sudden expansion stilling pool,the sudden expansion stilling pool can significantly reduce the fluctuating pressure level of the bottom plate.The maximum value of the sudden expansion ratio of 1.3 is reduced by 63%,and the maximum value of the sudden expansion ratio of 1.6 is reduced by 42%.The probability density of the fluctuating pressure of the sudden expansion stilling pool conforms to the normal distribution.In the impact area,the spatial integral scale of the sudden expansion type stilling pool was significantly lower than that of the non-sudden expansion type.Its vortex retention was low,accompanied by the continuous generation and fragmentation of the vortex.The predominant frequency of the fluctuating pressure of the sudden expansion stilling pool shifted to high frequency,from 4 Hz to 8 Hz,the possibility of pressure pulsation causing the bottom plate to resonate was very small.However,there was the possibility of resonance in the drop and sudden expansion parts,which should be paid attention.In the engineering design,the relationship between the strength of the fluctuating pressure and the dominant frequency should be comprehensively considered.Under the premise of no resonance,it was beneficial to the project to appropriately reduce the fluctuating pressure.
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