In order to investigate the effect of sampling frequency and time on pressure fluctuations, the three-dimensional unsteady numerical simulations were conducted in a circulating water pump. Through comparison of turbul...In order to investigate the effect of sampling frequency and time on pressure fluctuations, the three-dimensional unsteady numerical simulations were conducted in a circulating water pump. Through comparison of turbulence models with hydraulic performance experiment, SST k-co model was confirmed to study the rational determination of sampling frequency and time better. The Fast Fourier Transform （FFT） technology was then adopted to process those fluctuating pressure signals obtained. On these bases, the characteristics of pressure fluctuations acting on the tongue were discussed. It is found that aliasing errors decrease at higher sampling frequency of 17 640 Hz, but not at a lower sampling frequency of 1 764 Hz. Correspondingly, an output frequency range ten-times wider is obtained at 17 640 Hz. Compared with 8R, when the sampling time is shorter, the amplitudes may be overvalued, and the frequencies and amplitudes of low-frequency fluctuations can not be well predicted. The frequencies at the tongue are in good agreement with the values calculated by formula and the frequency compositions less than the blade passing frequency are accurately predicted.展开更多
The scenario of bypass transition is generally described theoretically as follows: low-frequency disturbances in the free-stream would generate long stream-wise streaks in the boundary layer, which later would induce ...The scenario of bypass transition is generally described theoretically as follows: low-frequency disturbances in the free-stream would generate long stream-wise streaks in the boundary layer, which later would induce secondary instability, leading to rapid increase of high-frequency disturbances, then possibly turbulent spots would emerge, and through their merging, fully developed turbulence appears. This description, however, is insufficient in the sense that it does not provide the explanation on why during the breakdown stage, a large number of waves with different frequencies and wave numbers would appear almost simultaneously, leading to a swift change of the mean flow profile. In this paper, the mechanism leading to this phenomenon is found to be the change of the stability characteristics of mean flow profile, which has a positive feedback effect on the change of the profile itself. And another interesting finding is that, during the transition, the unstable disturbance waves which appear first belong to a branch of inviscid modes, while following the change of the stability characteristics of the mean flow profile, the disturbance waves will change to another branch of inviscid modes, and the latter play the key role in bypass transition.展开更多
基金Project supported by the Priority Academic Development Program of Jiangsu Higher Education Institutions, ChinaProject(CXZZ12_0680) supported by Postgraduate Innovation Foundation of Jiangsu Province, ChinaProject(12JDG082) supported by the Advanced Talent Foundation of Jiangsu University, China
文摘In order to investigate the effect of sampling frequency and time on pressure fluctuations, the three-dimensional unsteady numerical simulations were conducted in a circulating water pump. Through comparison of turbulence models with hydraulic performance experiment, SST k-co model was confirmed to study the rational determination of sampling frequency and time better. The Fast Fourier Transform （FFT） technology was then adopted to process those fluctuating pressure signals obtained. On these bases, the characteristics of pressure fluctuations acting on the tongue were discussed. It is found that aliasing errors decrease at higher sampling frequency of 17 640 Hz, but not at a lower sampling frequency of 1 764 Hz. Correspondingly, an output frequency range ten-times wider is obtained at 17 640 Hz. Compared with 8R, when the sampling time is shorter, the amplitudes may be overvalued, and the frequencies and amplitudes of low-frequency fluctuations can not be well predicted. The frequencies at the tongue are in good agreement with the values calculated by formula and the frequency compositions less than the blade passing frequency are accurately predicted.
基金supported by the National Natural Science Foundation of China (Grant No. 11102131)the National Basic Research Program of China (Grant No. 2009CB724103)+1 种基金the National Aeronautics Basic Science Foundation of China (Grant No. 2010ZA48002)the Doctoral fund of Ministry of Education of China (Grant No. 20110032120003)
文摘The scenario of bypass transition is generally described theoretically as follows: low-frequency disturbances in the free-stream would generate long stream-wise streaks in the boundary layer, which later would induce secondary instability, leading to rapid increase of high-frequency disturbances, then possibly turbulent spots would emerge, and through their merging, fully developed turbulence appears. This description, however, is insufficient in the sense that it does not provide the explanation on why during the breakdown stage, a large number of waves with different frequencies and wave numbers would appear almost simultaneously, leading to a swift change of the mean flow profile. In this paper, the mechanism leading to this phenomenon is found to be the change of the stability characteristics of mean flow profile, which has a positive feedback effect on the change of the profile itself. And another interesting finding is that, during the transition, the unstable disturbance waves which appear first belong to a branch of inviscid modes, while following the change of the stability characteristics of the mean flow profile, the disturbance waves will change to another branch of inviscid modes, and the latter play the key role in bypass transition.
