Use of a Modified Quadrant Analysis for the Prediction of Grain Entrainment

Abstract:

The incipient motion of sediment grains is of importance to quantifying the fluvial process in natural rivers. Due to its significance, the entrainment problem has been extensively investigated and a number of models have been proposed in an effort to better determine particle threshold conditions. Among various approaches, quadrant analysis is employed to relate grain activity to turbulent flow motions; for instance, entrainment is commonly referred to the turbulence quadrant event “sweep”. Nevertheless, such quadrant description based on the instantaneous velocity cannot fully resolve the mechanism of grain movement, since the velocity fluctuates more violently than the hydrodynamic force does and thus creates the “noise quadrants” that are not as effective in producing hydrodynamic forces upon sediment particles. The focus of the present work is on the development of an improved technique capable of filtering out noise quadrants and thus making the analysis more reliable.

A synchronous velocity and pressure data set is employed to generate quadrant events and their corresponding flow forces experienced by a test grain. Then, the contribution of forces to grain movement can be determined theoretically on the basis of force balance of the grain. With so obtained mechanical representation, the effectiveness of individual quadrant events is examined on an instantaneous basis and over the duration of full particle dislodgement as well.