Kinematics of Particles at Entrainment and Disentrainment

We address the issue of characterizing experimentally entrainment and disentrainment of sediment particles of cohesionless granular beds in turbulent open channel flows. Employing Particle Image Velocimetry, we identify episodes of entrainment and of disentrainment of bed particles by analysing the raw PIV images. We define a reference velocity for entrainment or disentrainment by space-averaging the flow field in the vicinity of the (entrained or disentrainned) particle and by time-averaging that space-average over a short duration encompassing the observed episode. All observations and measurements took place under generalized movement conditions and in non-controlled geometrical set-ups, resulting in unique databases of conditionally sampled turbulent flow kinematics associated with episodes of particle entrainment and of particle disentrainment. Exploring this database, the objective of this paper is to prove further insights on the dynamics of fluid-particle and particle-particle interactions at entrainment and disentrainment and to polemicize the use of a reference velocity to serve as a proxy for hydrodynamics actions responsible for entrainment or disentrainment. In particular, we quantify the reference velocity associated with entrainment and disentrainment episodes and discuss its potential to describe the observed motion vis-a-vis local bed micro-topography and the type of entrainment or disentrainment event. Entrainment may occur at a wide range of reference velocities, including smaller than mean (double-averaged) velocities. Anecdotal evidence was collected for some typologies of entrainment: (i) momentum transfer from flow to a single particle, (ii) momentum transfer from a perturbed local flow to a single particle, (iii) collective entrainment associated to momentum transfer between a moving and a resting particle and (iv) collective entrainment considered to be a dislodgment of several particles involving momentum transfer from other particles. In some of these cases, e.g., (ii) and (iii), the use of a reference velocity seems inadequate to characterize the entrainment episode. A word of caution about the use of entrainment models based on reference velocities is henceforth issued and contextualized. In the case of disentrainment, a reference velocity seems to constitute a better descriptor of the observed behaviour. The scatter in the observed values seems to express the contribution of bed micro-topography. All particles were found to experience frictional contacts with the resting bed surface particles, but some particles were stopped more abruptly due to the presence of an obstacle along their path. Most disentrainment of particles took place when the near-bed flow was featuring ejection events

LES modelling of a flow within an infinite array of randomly placed cylinders: Anisotropy characterization

The LES approach is employed to model flows within random arrays of emergent cylinders. The model is validated against laboratory data acquired with a 2D-2C Particle Image Velocimetry system. The main goals are: i) discussion of the effect of the numerical domain size and the grid resolution on the predicted flow variables; and ii) spatial characterization of the flow anisotropy. Three domains of different sizes (16 to 36 cylinders) and four grid resolutions were independently tested. A 2D methodology was proposed to characterize the flow anisotropy on the horizontal plane. The results show that the first and second order moments were not significantly affected by the size of the tested numerical domains or by the grid resolution. The comparison with laboratory data showed a fair agreement confirming that the numerical model was able to adequately reproduce all the components of the Reynolds stress tensor. The results show that turbulence is of axisymmetric expansion nature in this type of flow. Relatively to the degree of anisotropy, the highest values were found close to the cylinder, decreasing gradually downstream towards the isotropy state. However, a truly isotropic turbulence state is not reached

Depth-averaged flow in presence of submerged cylindrical elements

The hydraulic resistance of vegetation can play a major role in the hydrodynamics of rivers with extensive natural floodplains. Vegetation penetrates the flow field and thereby causes drag that, in addition to the flow resistance at the bed level, causes energy losses and slows down the flow. Here, these flow processes are studied in an idealized form by treating vegetation as cylindrical roughness elements with homogeneous geometrical dimensions. Based on scaling considerations of the forces involved, depth-averaged flow velocities within the resistance layer and in the free flowing layer above the roughness elements are estimated. This yields a new description of the overall average flow field, which is entirely determined by measurable geometrical boundaries. We tested the new relation against laboratory flume experiments and found very good agreement (R2 =0.98). The new description even showed realistic results when the depth of flow is of similar size as the height of the roughness elements themselves. This result demonstrates its superiority over commonly used wall-roughness methods