Role of instantaneous force magnitude and duration on particle entrainment

Abstract

[1] A new criterion for the onset of entrainment of coarse sediment grains is presented here. It is hypothesized that not only the magnitude, but also the duration of energetic near bed turbulent events is relevant in predicting grain removal from the bed surface. It is therefore proposed that the product of force and its duration, or impulse, is a more appropriate and universal criterion for identifying conditions for particle dislodgement. This conjecture is investigated utilizing two theoretical models, representative of two modes of entrainment: saltation and rolling. In these models, instantaneous, highly fluctuating turbulent forces are simulated as short-lived pulses of characteristic magnitude and duration, which transfer adequate fluid momentum to the particle, to trigger its entrainment. The analytical solution of the respective equations of motion is employed in deriving representations of threshold conditions in terms of the impulse characteristics. It is shown that hydrodynamic forces of sufficiently high magnitude are capable of entraining a particle only when they last long enough so that their impulse exceeds a critical value. To illustrate further the validity of the critical impulse concept, as well as extend and generalize its application to different entrainment levels of an individual grain, a novel experimental setup is utilized. This setup facilitates observations of angular displacement of a steel mobile particle in air due to electromagnetic pulses of different magnitude and duration. The experimentally obtained conditions for partial or complete entrainment support the concept of a critical impulse.

Instantaneous turbulent forces and impulse on a rough bed: Implications for initiation of bed material movement

“…We note that during entrainment the unbalanced force acting on the grain may change with time (due to changing resistance and applied hydrodynamic force) ; however, the entrainment of a grain is still well captured by a critical value of the impulse, which accounts for the accumulated time integral of the varying unbalanced force. The existence of such a critical impulse as well as the wide range of combinations of force magnitude and duration that can cause entrainment have been demonstrated in Diplas et al [2008], Valyrakis et al [2010], and Celik [2011]. This assertion was independently validated later by the work of Dwivedi et al [2011a, Figure 19].…”

“…[18] The emphasis of this investigation is on flow impulse as a mechanism for initiating grain entrainment (as detailed in Diplas et al [2008]; Valyrakis et al [2010], and…”

Flow Hydrodynamics across Open Channel Flows with Riparian Zones: Implications for Riverbank Stability

“…This makes it of even greater utility and urgency to identify relevant criteria, which can be used and associated to riverbank stability. Further analysis using recently suggested event-based criteria for sediment transport, which appropriately accounts for the scales of coherent turbulent flow structures [53,54] and relevant modelling tools [24,25,55], would be also relevant to pursue in future studies. Such efforts should focus on investigating the potential of such criteria to better characterize the erosive capacity of flow turbulence on riverbanks, and find practical applications on vegetation flow hydrodynamics, engineering ecohydraulics and fluvial geomorphology.…”

Gravel threshold of motion: a state function of sediment transport disequilibrium?

“…Flow characteristics influencing τ * c include particle Reynolds number, flow depth relative to grain size, the intensity of turbulence, the history of prior flow both above and below transport thresholds, and the partitioning of stress into form drag and skin friction (e.g., Shvidchenko and Pender, 2000;Ockelford and Haynes, 2013;Schneider et al, 2015;Valyrakis et al, 2010;Celik et al, 2010). Most flowdependent controls are not independent of the bed surface controls.…”