Dry planetary surfaces, sea floors and riverscapes are excellent examples of constantly altering surface appearance as a result of sand and gravel sediments being transported by turbulent flows of air or liquid. Sediment transport in air, called saltation, involves energetic particles moving in large ballistic trajectories that visibly entrain particles resting at the bed surface upon impact onto the sediment bed. In contrast, in sediment transport in liquids, called bedload, entrainment of bed surface particles is thought to be a sole consequence of the action of the turbulent flow as bedload involves rolling, sliding, and hopping of comparably very slow particles in the vicinity of the sediment bed. However, recent studies have questioned the legitimacy of this generally accepted hypothesis by pointing out the potential role that the particle inertia plays in sustaining bedload transport.
Recently, Professor Thomas Pähtz from the Institute of Port, Coastal and Offshore Engineering, Zhejiang University, Hangzhou, China in collaboration with Professor Orencio Durán (currently Research Assistant Professor at Texas A&M University) determined the role of impact entrainment of bed surface particles relative to their entrainment by the action of the mean turbulent flow using direct sediment transport simulations in a Newtonian fluid with a numerical model that belongs to a novel generation of sophisticated grain-scale models of sediment transport. They also hoped to clarify on the possible links between impact entrainment and average transport characteristics. Their work is now published in the research journal, Physical Review Fluids.
Briefly, Pähtz and Durán initiated their empirical procedure by visually inspecting particle impacts onto the bed surface occurring in their simulations of steady bedload and saltation transport. They then quantitatively analyzed their simulation data of the average particle velocity gradient within the sediment bed, which serves as a proxy to determine the relative role of impact entrainment. Eventually, they investigated possible links between impact entrainment and average transport characteristics.
From the numerical simulations, the authors of this paper observed that continuous sediment transport driven by nearly any kind of Newtonian fluid (except viscous liquids) was fully sustained through entrainment by particle-bed impacts. This finding challenges the paradigm that sediment transport driven by water or heavy air, like on Venus and Titan, is sustained through entrainment by the mean turbulent flow. However, Pähtz and Durán noted that this finding does not imply that entrainment due to turbulence does not take place at all because the turbulent flow undergoes large fluctuations around its mean that can result in sediment entrainment due to occasional spikes of the fluid forces acting on bed surface particles. Entrainment by such turbulent spikes is, in fact, prevalent in natural rivers because bedload transport along river beds is usually very weak and significantly below the continuous transport threshold.
One of the reasons why the role of impact entrainment for sustaining bedload transport has been underestimated in the past is the experimental observation that the average velocity of transported particles increases with the flow velocity. In fact, previous theoretical studies argued that predominant impact entrainment necessarily results in a constant average particle velocity. In their study, Pähtz and Durán showed that these theoretical deductions were flawed because they did not consider the effects of a fully mobile bed surface in bedload transport, which allows the average particle velocity to increase with the flow velocity despite predominating impact entrainment.
Thomas Pähtz and Orencio Durán. Fluid forces or impacts: What governs the entrainment of soil particles in sediment transport mediated by a Newtonian fluid? Physical Review Fluids 2, 074303 (2017)Go To Physical Review Fluids