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.

Laboratory observations regarding the limit conditions for particle entrainment into suspension are presented. A high‐speed video system was used to investigate conditions for the entrainment of sediment particles and glass beads lying over a smooth boundary as well as over a rough bed. The results extend experimental conditions of previous studies towards finer particle sizes. A criterion for the limit of entrainment into suspension is proposed which is a function of the ratio between the flow shear velocity and particle settling velocity. Observations indicate that particles totally immersed within the viscous sublayer can be entrained into suspension by the flow, which contradicts the conclusions of previous researchers. A theoretical analysis of the entrainment process within the viscous sublayer, based on force–balance considerations, is used to show that this phenomenon is related to turbulent flow events of high instantaneous values of the Reynolds stress, in agreement with previous observations. In the case of experiments with a rough bed, a hiding effect was observed, which tends to preclude the entrainment of particles finer than the roughness elements. This implies that, as the ratio between particle and roughness element sizes becomes smaller, progressively higher bed shear stresses are required to entrain particles into suspension. On the other hand, an overexposure effect was also observed, which indicates that a particle moving on a smooth bed is more prone to be entrained than the same particle moving on a bed formed by identical particles.

[1] 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 mm 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 $(h 0 /g) 1/2 , where h 0 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/(h 0 /g) 1/2 $ 1.5. The flow velocity is then U $ ffiffi ffi 2 p, larger than that for dry (initially nonfluidized) granular flows. Transition to a last, third phase occurs at t/(h 0 /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 t 1/3 , as in dry flows. Motion ceases at t/(h 0 /g) 1/2 $ 6.5 with normalized runout x/h 0 $ 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.

[1] The emplacement dynamics of pyroclastic flows were investigated through noninvasive measurements of the pore fluid pressure in laboratory air-particle flows generated from the release of fluidized and nonfluidized granular columns. Analyses of high-speed videos allowed for correlation of the pressure signal with the flow structure. The flows consisted of a sliding head that caused underpressure relative to the ambient, followed by a body that generated overpressure and at the base of which a deposit aggraded. For initially fluidized flows, overpressure in the body derived from advection of the pore pressure generated in the initial column and decreased by diffusion during propagation. Relatively slow diffusion caused the pore pressure in the thinner flow to be larger than lithostatic at early stages. Furthermore, partial auto-fluidization, revealed in initially nonfluidized flows, also occurred and contributed to maintain high pore pressure, whereas dilation or contraction of the air-particle mixture with associated drag and/or pore volume variation transiently led the pressure to decrease or increase, respectively. The combination of all these processes resulted in long-lived high pore fluid pressure in the body of the flows during most of their emplacement. In the case of the initially fluidized and slightly expanded (∼3-4%) flows, (at least) ∼70%-100% of the weight of the particles was supported by pore pressure, which is consistent with their inertial fluid-like behavior. Dense pyroclastic flows on subhorizontal slopes are expected to propagate as inertial fluidized gas-particle mixtures consisting of a sliding head, possibly entraining basement-derived clasts, and of a gradually depositing body.Citation: Roche, O., S. Montserrat, Y. Niño, and A. Tamburrino (2010), Pore fluid pressure and internal kinematics of gravitational laboratory air-particle flows: Insights into the emplacement dynamics of pyroclastic flows,

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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.

An integrated cross-correlation/relaxation algorithm for particle tracking velocimetry is presented. The aim of this integration is to provide a flexible methodology able to analyze images with different seeding and flow conditions. The method is based on the improvement of the individual performance of both matching methods by combining their characteristics in a two-stage process. Analogous to the hybrid particle image velocimetry method, the combined algorithm starts with a solution obtained by the cross-correlation algorithm, which is further refined by the application of the relaxation algorithm in the zones where the cross-correlation method shows low reliability. The performance of the three algorithms, crosscorrelation, relaxation method and the integrated crosscorrelation/ relaxation algorithm, is compared and analyzed using synthetic and large-scale experimental images. The results show that in case of high velocity gradients and heterogeneous seeding, the integrated algorithm improves the overall performance of the individual algorithms on which it is based, in terms of number of valid recovered vectors, with a lower sensitivity to the individual control parameters.The University of Chile and the Karlsruhe Institute of Technology supported this work. The authors gratefully acknowledge the support provided by the German Science Foundation (DFG Grant JI 18/18-1), the scholarship program of the National (Chilean) Commission of Science and Technology research, CONICYT, and the support from Fondecyt Project 1080617

The capability of acoustic Doppler velocimeters to resolve flow turbulence is analyzed. Acoustic Doppler velocimeter performance curves ͑APCs͒ are introduced to define optimal flow and sampling conditions for measuring turbulence. To generate the APCs, a conceptual model is developed which simulates different flow conditions as well as the instrument operation. Different scenarios are simulated using the conceptual model to generate synthetic time series of water velocity and the corresponding sampled signals. Main turbulence statistics of the synthetically generated, sampled, and nonsampled time series are plotted in dimensionless form ͑APCs͒. The relative importance of the Doppler noise on the total measured energy is also evaluated for different noise energy levels and flow conditions. The proposed methodology can be used for the design of experimental measurements, as well as for the interpretation of both field and laboratory observations using acoustic Doppler velocimeters.