科研之友
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摘要
An experimental study of lift force on a spherical sediment particle at different exposures was conducted in a laboratory flume. It is found that the functional form of the theoretical probability density pdf) of lift force similar to that of drag force is inadequate. An improved expression for the pdf based on the normal error law is provided which matches the measured data up to +/- 3 times the standard deviation of lift force for all exposures. The skewness of the measured lift force is found to increase with increase in exposure. In addition, the kurtosis of the measured lift force is found to be higher than that for a normal distribution for all exposures, resulting in higher probability of occurrence of extreme events (beyond +/- 3 times standard deviation). A spectral density function of lift force as a function of streamwise and stream-normal velocity is developed and validated using the measured lift force data. The spectrum predicted by the proposed model closely follows the measured spectrum for exposures 7.5 mm and 10 mm while it over predicts at frequencies below 1 Hz for exposures 0 mm and 2.5 mm because of poor coherence between lift force and streamwise velocity. The time history plots of measured lift at lower exposures showed that the high-frequency positive and negative fluctuations of lift correspond to strongly positive streamwise velocity fluctuations, a phenomenon not captured by the predictive model. Predominance of fluctuating Bernoulli's lift over lift due to stream-normal velocity fluctuations is observed at all exposures, more so in the case of lower exposures.
- 出版日期2011-5-28
