The Role of Turbulence on the Initiation of Sediment Motion

Abstract

The present study examines the role of turbulence on the

incipient motion of sediment. For this purpose,

well-controlled experiments are performed at the

laboratory in a tilting flume. In these tests glass beads of

the same size and density are used as the testing material

to isolate the role of turbulence. State of the art

equipment are used during the course of this study.

Specifically, a 3-D Laser Doppler Velocimetry system is

employed to measure the instantaneous velocity

components at different points near the vicinity of a ball

while the ball motion is monitored with a video camera.

An image analysis program is developed here to analyze

the motion of the particles within a test area. To examine

the importance of the different stress components in the

entrainment of sediment, five tests of different packing

configuration are performed. Specifically three different

roughness regimes are examined namely, the isolated, the

wake interference, and the skimming flow. The results

reveal that the instantaneous normal stress in the

streamwise direction is the most dominant component of

the instantaneous stress tensor. The backbone of this

study is the development of a methodology to link the

effects of turbulence with the commencement of

sediment motion. It is considered that the metastable

bursting cycle (i.e. sweeps, ejections, inward and

outward interactions) is responsible for the sediment

entrainment. And that the sediment entrainment, if any,

occurs within a bursting period. The main concept

behind the determination of the critical conditions is that

the probability of the entrainment of sediment (effect) is

equal to the probability of occurrence of these highly

energetic turbulent events that have magnitude greater

than the critical (cause). The probability of sediment

entrainment is computed by means of the image analysis

tool. The balance of moments is obtained here to

determine the minimum moment that is required for the

commencement of sediment motion. The balance of

moments yields the deduction of a new variable that is

used to describe the probability of occurrence of the

different turbulent events. This variable is the summation

of the instantaneous normal stresses in the streamwise

and vertical direction. It is shown here that a

two-parameter gamma density function describes quite

well the statistical behavior of this variable. The results

that are obtained from the existing model suggest that the

present methodology can adequately describe the

commencement of sediment motion. It is shown here that

the traditionally used shear stress term uw may not be

the appropriate measure for the determination of the

critical conditions.