Clinton L. Dancey

Impulse and particle dislodgement under turbulent flow conditions

In this study, we investigated the role of turbulence fluctuations on the entrainment of a fully exposed grain near threshold flow conditions. Experiments were carried out to measure synchronously the near bed flow velocity and the particle movement for a range of flow conditions and resulting particle entrainment frequencies. We used a simplified bed geometry consisted of spherical particles to reduce the complexities associated with the variations in the bed and flow details in an effort to identify the underlying dominant physical mechanism. An analysis was performed based on common force approximations using near bed flow velocity. Turbulence fluctuations were treated as impulses, which are products of magnitude and duration of applied force. It is demonstrated that besides the magnitude of the instantaneous forces applied on a sediment grain, their duration is important as well in determining whether a particle will be entrained by a turbulent flow event. Frequency of particle entrainment varied rema...

Nonintrusive Method for Detecting Particle Movement Characteristics near Threshold Flow Conditions

Bed-load measurements comprise an important component in the development of reliable formulas, in an effort to obtain the necessary constitutive relations between the amount of transported material and flow parameters. The uncertainty of such measurements is rather well known, being much more pronounced at lower transport rates. This uncertainty stems from the multitude of factors affecting bed-load transport and the lack of available trustworthy measuring technologies. Predictions of the limiting case of nearly zero bed-load transport, typically reported in literature as threshold of motion or critical condition, are even more challenging. The purpose of this contribution is twofold. First, to examine the sensitivity of bed-load transport measurements at conditions moderately higher than critical, to the presence of a rather unobtrusive trap, designed through several iterations. Even under relatively simple laboratory flume channel and flow conditions, it proved difficult to measure the bed-load transport rate in a completely unbiased way. Second, to develop a methodology, together with the appropriate instrumentation, for determining the condition of incipient motion. The nonintrusive approach described here proved to be reliable in detecting even the slightest movements of a particle. At the same time, it demonstrates the complexity of the problem due to the highly fluctuating nature of turbulent flow.

The Probability of Sediment Movement at the Threshold of Motion and Time Dependent Fluid Processes

The characterization and prediction of the motion of sediment near the threshold are long-standing problems in the study of sediment transport. In this paper a new criterion for the threshold condition, recently introduced by the authors, is reviewed and relevant physical mechanisms that may be necessary for the accurate prediction of sediment motion near the threshold are elaborated. The new criterion is based upon an operational definition of the probability of grain movement. The relevant physical mechanisms to be discussed relate to inherently time-dependent, transient flow effects-inertia forces and impulse (force/time history) effects.

Particle movement subject to a fluctuating fluid force

The movement of particles from an erodible surface by a flowing fluid is a fundamental physical phen…

A simple model for the average local entrainment rate

A new expression for the mean local entrainment rate in a turbulent intermittent flow is obtained. This expression is used to obtain a simple model for the entrainment rate assuming that an indicator function can be defined for the flow, that the interface defined by the indicator function is homogeneous in two directions, and that the turbulent Reynolds number is very large. A particularly simple form is obtained if the intermittency is in a scalar imbedded in a turbulence field and the correlation coefficient for the indicator function is self‐similar. The final expression compares favorably with the limited existing data and with other model expressions. Recommendations for experimental verification are presented.