Examination of v -velocity fluctuations in a turbulent channel flow in the context of sediment transport

Abstract

This paper is a report on the mechanism of turbulent momentum transport normal to the wall in a turbulent wall-bounded flow. The objective of this study is to examine the ‘background’ turbulent flow field as a first step toward understanding suspended sediment transport. Specifically, the hypothesis that fine grain particles can be kept in suspension through a net upward momentum flux in a turbulent boundary layer is examined. The net momentum flux can arise if the probability density distribution of the fluctuating v–signal is positively skewed; i.e. the positive v–fluctuations are predominantly of large amplitude and short duration while the negative v-fluctuations are of small amplitude and long duration. High-resolution, two-component laser-doppler anemometer measurements of the v-velocity component in a fully developed turbulent water channel flow were examined spanning a Reynolds number range of 3000 to 23000. Averages of these signals demonstrate that, for very small particles, there is net upward momentum flux in the range y+ > 30, while there is a net downward momentum flux in the range 10 [les ] y+ [les ] 30. Preliminary results which categorize the normal velocity according to quadrants of motion are also included.