Yarko Niño

Experiments on particle-turbulence interactions in the near-wall region of an open channel flow: Implications for sediment transport

A high-speed video system was used to study the interaction between sediment particles and turbulence in the wall region of an open channel flow with both smooth and transitionally rough beds. In smooth flows, particles immersed within the viscous sublayer were seen to accumulate along low-speed wall streaks; apparently due to the presence of quasi-streamwise vortices in the wall region. Larger particles did not tend to group along streaks, however their velocity was observed to respond to the streaky structure of the flow velocity in the wall region. In transitionally rough flows particle sorting was not observed. Coherent flow structures in the form of shear layers typically observed in the near-wall region interacted with sediment particles lying on the channel bottom, resulting in the particles being entrained into suspension. Although there has been some speculation that this process would not be effective in entraining particles totally immersed in the viscous sublayer, the results obtained demonstrate the opposite. The entrainment mechanism appears to be the same independent of the roughness condition of the bottom wall, smooth or transitionally rough. In the latter case, however, hiding effects tend to preclude the entrainment of particles with sizes finer than that of the roughness elements. The analysis of particle velocity during entrainment shows that the streamwise component tends to be much smaller than the local mean flow velocity, while the vertical component tends to be much larger than the local standard deviation of the vertical flow velocity fluctuations, which would indicate that such particles are responding to rather extreme flow ejection events.

Gravel saltation: 1. Experiments

Laboratory observations of the saltation of natural gravel particles in a steep, movable-bed channel are reported. Standard video-imaging techniques were used to measure and analyze particle motion. The saltation of gravel particles is described in terms of statistical properties of particle trajectories, such as mean values and standard deviations of saltation length, height, and streamwise particle velocity. The results obtained are compared with available empirical data, and a general good agreement is obtained. Particle collision with the bed is also analyzed, and friction and restitution coefficients are estimated from the experimental observations. Nonvanishing values of the restitution coefficient and values of the friction coefficient lower than unity are obtained, which contradicts previous discussions on the subject. The dynamic friction coefficient associated with particle motion is also estimated from the experimental data, and a mean value of 0.3 is obtained, which is about half of that proposed by Bagnold (1973) but similar to those found in previous experiments.

Gravel Saltation: 2. Modeling

A Lagrangian model for the motion of a sediment particle in a water flow is proposed. The particle motion equation is averaged over flow turbulence, specialized to the case of coarse sediment particles saltating in water, and coupled with a stochastic model for particle collision with the bed. Model predictions of statistics of gravel saltation generally agree well with experimental observations. As an application of the saltation model, bed load transport rates are estimated from modeled mean saltation streamwise velocity and a dynamic friction coefficient. The results obtained compare favorably with typical bed load equations, but they overestimate experimental measurements of gravel transport, which would suggest that a Bagnoldean formulation for bed load transport of coarse material by saltating particles may not be valid.

Using Lagrangian particle saltation observations for bedload sediment transport modelling

A Lagrangian model for the saltation of sand in water is proposed. Simulated saltation trajectories neglecting particle rotation and turbulence effects compare fairly well with experimental observations. The model for particle motion is coupled with a stochastic model for particle collision with the bed, such that a number of realizations of the saltation process can be simulated numerically. Model predictions of mean values and standard deviations of saltation height, length and streamwise particle velocity agree fairly well with experimental observations. Model predictions of the dynamic friction coefficient are also in good agreement with experimental observations, but they underestimate the value of 0·63 proposed by Bagnold for this coefficient. The saltation model is applied to the estimation of bedload transport rates of sand using a Bagnoldean formulation. Modelled values of the bedload transport rates overestimate those predicted by commonly used bedload formulae, which appears to be a consequence of problems in the definition of the dynamic friction coefficient. These results seem to indicate a few problems with the Bagnoldean formulation, particularly regarding the continuum assumption for the bedload layer, which would be valid only for very high particle concentrations and small particle diameters, and also regarding the evaluation of the shear stress exerted on the bed by the saltating particles.

Experimental observations of water‐like behavior of initially fluidized, dam break granular flows and their relevance for the propagation of ash‐rich pyroclastic flows

The physics of ash-rich pyroclastic flows were investigated through laboratory dam break experiments using both granular material and water. Flows of glass beads of 60–90 μm in diameter generated from the release of initially fluidized, slightly expanded (2.5–4.5%) columns behave as their inertial water counterparts for most of their emplacement. For a range of initial column height to length ratios of 0.5–3, both types of flows propagate in three stages, controlled by the time scale of column free fall ∼(h0/g)1/2, where h0 denotes column height and g denotes gravitational acceleration. Flows first accelerate as the column collapses. Transition to a second, constant velocity phase occurs at a time t/(h0/g)1/2 ∼ 1.5. The flow velocity is then U ∼ equation image(gh0)1/2, larger than that for dry (initially nonfluidized) granular flows. Transition to a last, third phase occurs at t/(h0/g)1/2 ∼ 4. Granular flow behavior then departs from that of water flows as the former steadily decelerates and the front position varies as t1/3, as in dry flows. Motion ceases at t/(h0/g)1/2 ∼ 6.5 with normalized runout x/h0 ∼ 5.5–6. The equivalent behavior of water and highly concentrated granular flows up to the end of the second phase indicates a similar overall bulk resistance, although mechanisms of energy dissipation in both cases would be different. Interstitial air-particle viscous interactions can be dominant and generate pore fluid pressure sufficient to confer a fluid-inertial behavior to the dense granular flows before they enter a granular-frictional regime at late stages. Efficient gas-particle interactions in dense, ash-rich pyroclastic flows may promote a water-like behavior during most of their propagation.

Characterization of near-bed coherent structures in turbulent open channel flow using synchronized high-speed video and hot-film measurements

High-speed video recordings (500 Hz) of flow visualizations in the near wall region of a turbulent open channel flow were synchronized with hot-film measurements of flow velocity and bed shear stress. Analysis of the video images provided information about the main characteristics of coherent flow structures associated with the occurrence of low-speed streak ejections near the bed. These structures consisted mainly of oscillating shear layers that were converted in the downstream direction and lifted away from the bed. A visual detection criterion was developed to obtain ensemble averaged profiles of the velocity and shear stress data during ejection events, allowing for the characterization of the associated flow field during the occurrence of coherent structures. Conditional averaging suggests that the occurrence of such coherent patterns affects mainly the turbulence structure in the wall region, and that the observed events reveal a plausible mechanism by which energy is extracted from the mean flow by large scale turbulent fluctuations, and then further transferred towards smaller eddies, while the structures lose their coherence. The intermittent nature of production and dissipation of turbulent energy becomes noticeable, taking place about 21% of the time. The results obtained also provide evidence that seems to link the structures responsible for the turbulent vertical transport of momentum, and for the maintenance of the turbulent state, with the mechanism that triggers the entrainment of sediment into suspension. Comparison of present results with other experiments conducted in different types of flows strongly confirms a universal structure of coherent events in wall bounded flows.

Dynamics of sediment bars in straight and meandering channels: experiments on the resonance phenomenon

The results of an experimental study on the formation and development of sediment bars in straight and meandering channels, are reported. The laboratory observations are used to analyze recently developed linear and nonlinear theoretical models for the formation, geometrical properties, and migration characteristics of alternate bars. The theoretical predictions are found to be in good agreement with the observations of height, wavelength, and celerity of alternate bars. The theoretical conditions for which the suppression of migrating bars in meandering channels takes place, seem to agree qualitatively well with the experimental observations made in the laboratory.

Dynamic pore-pressure variations induce substrate erosion by pyroclastic flows

Field evidence shows that pyroclastic flows can entrain blocks from underlying substrates formed by earlier geological events, yet, counterintuitively, they are less likely to erode unconsolidated layers of fine particles. Here we report laboratory experiments that reproduce these seemingly contradictory observations and also offer a means to infer pyroclastic flow velocity. Experiments demonstrate that the sliding head of a granular flow generates a dynamic upward pore-pressure gradient at the flow-substrate interface. Associated upward air flux is enough to fluidize a substrate of fines, so that particles are not entrained individually but the substrate instead is subject to small shear instabilities. In contrast, coarse particles forming a non-fluidized substrate are lifted at a critical upward force due to the pore-pressure gradient, according to their individual masses, which provides a basis for a model to calculate the flow velocity. Application to the 18 May 1980 pyroclastic flow deposits at Mount St. Helens (Washington State, USA) gives velocities of ∼9-13 m s-1 at ∼6-7 km from the vent on gentle slopes (<4°-6°), in agreement with field observations at this volcano and at others.

Experimental and large eddy simulation study of the flow developed by a sequence of lateral obstacles

In this paper we provide a description of the three-dimensional flow induced by a sequence of lateral obstacles in a straight shallow open-channel flow with flat bathymetry. The obstacles are modelled as rectangular blocks and are located at one channel wall, perpendicular to the main stream direction. Two aspect ratios of the resulting dead zones are analysed. The flow structure is experimentally characterised by particle image velocimetry measurements in a laboratory flume and simulated using three-dimensional Large Eddy Simulations. Good agreement between experimental measurements and numerical results is obtained. The results show that the effect of the obstacles in the main channel is observed up to one obstacle length in the spanwise direction. The spacing between obstacles does not seem to have a large influence in the outer flow. The mean flow within the dead zone is characterised by a large recirculation region and several additional vortex systems. They are discussed in the paper, as well as the mean and root-mean-square wall shear-stresses.

Pore fluid pressure diffusion in defluidizing granular columns

Pore fluid pressure variations play an important role in the motion of natural granular flows like debris and pyroclastic flows. Pore pressure in a defluidizing air-particle bed was investigated by means of experiments and numerical modeling. Experiments consisted of recording the defluidization process, measured as the decay of the basal pore fluid pressure in initially aerated granular mixtures. Mixtures were aerated to different degrees of fluidization by introducing a vertical air flux at the base of a granular column. The degree of fluidization was characterized by the parameter bo (pore fluid pressure/lithostatic pressure). Bed expansion occurred for bo > 0.8–0.9, with maximum expansions near 8% at bo