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

Abstract

[1] The overall objective of this study is to identify the physical mechanisms responsible for the entrainment of an exposed particle subject to rapidly fluctuating hydrodynamic forces in the case of channel flow with a fully rough boundary. This is pursued here by examining particle dislodgment under uniform and cylinder wake-flow experiments. The critical impulse concept is investigated more rigorously by measuring directly the pressures at four points on the surface of a fixed test grain. The number of impulse events determined from these experiments increases by more than an order of magnitude, over a modest change of roughness Reynolds number. Furthermore, they are well described by a lognormal probability density function. Both results are consistent with those obtained from similar experiments via indirect (velocity-based) impulse calculations and reported in a prior contribution. This comparison supports the use of the velocity record for determining instantaneous hydrodynamic forces and impulses instead of the more difficult approach of measuring the pressure fluctuations directly. The present results demonstrate the dominant role the local, streamwise velocity component plays on particle dislodgment. This is attributed to the large impulse content and occasionally strong positive lift force associated with flow events, exhibiting pronounced positive streamwise velocity fluctuations. The majority ($70%) of these events occur in the fourth quadrant, while a significant number ($22%) appear as first-quadrant episodes. It was also determined that wake flows can increase substantially particle entrainment via enhanced lift and increased turbulence intensity.Citation: Celik, A. O., P. Diplas, and C. L. Dancey (2013), Instantaneous turbulent forces and impulse on a rough bed: Implications for initiation of bed material movement, Water Resour.

A micromechanical modelling approach for predicting particle dislodgement

“…As shown in the marked section, negative lift forces (downward) induce the extra resistance by increasing the counter moment to prevent particle motion. A similar observation was reported by Celik et al (2013). Note that the moment of lift forces is incorporated in the torque computation (equation (3)).…”

“…To determine the hydrodynamic forces applied on a sediment particle, the results from a laboratory study reported in Celik et al (2013) are employed in this paper. In their experiments, four synchronized pressure transducers are mounted on a target sphere of 12.7 mm in diameter, which is fixed within a pocket formed by three hexagonal, well-packed identical spheres ( Fig.…”

“…As a result, the critical condition of dislodgement is not a clear-cut boundary but instead a narrow band of impulse within which both movement and non-movement could occur. That leaves uncertainties in the prediction of particle migration (Celik et al, 2010(Celik et al, , 2013. A more refined and general approach, accounting for a sequence of consecutive fluid parcels, is considered here in an effort to describe a wider range of possible particle movement scenarios.…”

“…In this model, a perfect rolling mode is assumed for particle movement, and the applied forces upon the particle are computed from hydrodynamic pressure data measured by Celik et al (2013). In addition to the migration trajectory, the corresponding variation in particle resistance is shown here.…”

Taking the river inside: Fundamental advances from laboratory experiments in measuring and understanding bedload transport processes

“…Numerous studies have demonstrated that sweeps and inward interactions are correlated with the initial entrainment of sediment (Fig. 7) and/or produce larger bedload fluxes than other events (bursts, inward interactions) of the same magnitude (e.g., Nelson et al, 1995;Dwivedi et al, 2011;Wu and Shih, 2012;Celik et al, 2013). It has been hypothesized that because bursts and outward interactions are associated with positive streamwise velocity fluctuations, the instantaneous streamwise velocity rather than the vertical velocity dictates particle motion.…”