Wavelet analysis of dynamic response of floc in turbulent shear flows: - settling velocity of floc in turbulent flows (II) -

摘要

Numerical simulations were performed for settling velocities of floc in the turbulent shear flow generated by the Monte-Carlo simulation, using probabilistic and statistical knowledge of turbulent structure in open channel flows. Firstly, the dynamic response of floc was examined utilizing wavelet analysis. As a result, it was found that floc would easily follow the low-frequency components 6.25 and 12.5Hz in fluctuations of vertical flow velocities, and its motion would be affected by the high-frequency components in which the product of frequency and diameter of floc was larger than 2.0cm/s. Furthermore, we appraised the retardation of the mean settling velocity for shear flow. Consequently, it was found that the retardation rates of the mean settling velocities could be expressed by dimensionless parameters of the particle Reynolds number, the difference in the specific gravity between floc and water, the Strouhal number using a reciprocal of an integral time scale as the frequency and the ratio of micro-scale to the diameter of floc. In summary, the retardation of the mean settling velocity in shear flow would arise from the negative result of following, which would be attributed to an increase in the high-frequency components in fluctuations of vertical flow velocities.