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

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 log-normal 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 (approximate to 70%) of these events occur in the fourth quadrant, while a significant number (approximate to 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.