Aldo Tamburrino

Large flow structures in a turbulent open channel flow

The outer region flow structure in an open channel flow is studied by means of flow visualization and velocity measurements. The structure can be thought of as formed by large streamwise vortices scaling with the flow depth, creating upwelling and downwelling motions at the free surface. Associated with the downwelling motion induced by these vortices are regions with higher longitudinal velocity on the free surface. Similarly, “boils” and eddies with a vertical axis are detected in the zones corresponding to the upwelling. Three temporal components are used to analyze velocity measurements: a temporal mean value, a component associated with slow fluctuations, and a third component associated with fast fluctuations. It is shown that the contribution of fast fluctuations to the turbulent intensities is important near the wall, at z+ < 1000. Slow fluctuations are the primary contributors in the remainder of the boundary layers outer region. It is proposed that the large streamwise vortices, a manifestation...

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.

On the role of the ambient fluid on gravitational granular flow dynamics

The effects of the ambient fluid on granular flow dynamics are poorly understood and commonly ignored in analyses. In this article, we characterize and quantify these effects by combining theoretical and experimental analyses. Starting with the mixture theory, we derive a set of two-phase continuum equations for studying a compressible granular flow composed of homogenous solid particles and a Newtonian ambient fluid. The role of the ambient fluid is then investigated by studying the collapse and spreading of two-dimensional granular columns in air or water, for different solid particle sizes and column aspect (height to length) ratios, in which the front speed is used to describe the flow. The combined analysis of experimental measurements and numerical solutions shows that the dynamics of the solid phase cannot be explained if the hydrodynamic fluid pressure and the drag interactions are not included in the analysis. For instance, hydrodynamic fluid pressure can hold the reduced weight of the solids, thus inducing a transition from dense-compacted to dense-suspended granular flows, whereas drag forces counteract the solids movement, especially within the near-wall viscous layer. We conclude that in order to obtain a realistic representation of gravitational granular flow dynamics, the ambient fluid cannot be neglected.

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

Comparative flow characteristics of a moving-bed flume

1. Pore pressure diffusion in our mixtures was modeled by a simple diffusion equation, taking into account a variable diffusion coefficient. When mixtures were expanded (bo > 0.8–0.9), continuous consolidation introduced nonlinearities in the diffusion coefficients, which retarded the decay of pore pressure. In contrast, for non-expanded mixtures, the diffusion coefficient remained constant (linear diffusion). Our results highlight that mixture compressibility can effectively reduce the pressure diffusion coefficient in initially expanded granular mixtures, thus increasing the duration of pressure diffusion. In our experiments, as well as for most self-consolidating natural granular mixtures, changes in permeability due to mixture consolidation appear to be negligible for the defluidizing process, as they are counteracted by changes in porosity and because the fluid behaves as incompressible, even when the fluid is air.

Roll wave appearance in bentonite suspensions flowing down inclined planes

Moving-bed flumes (flumes that have a moving bottom, usually a belt) have been used as an alternative to standard water flumes in some laboratories. Although a moving-bed flume is a more elaborate facility than a traditional flume, it presents several advantages that make it a valuable experimental tool. In this paper, the flow characteristics in one moving-bed flume (velocity distribution, turbulence intensities, outer layer flow structure) are presented and compared with those corresponding to traditional laboratory flumes.

Water Technology in Ancient Mesopotamia

As in Newtonian fluids, unconfined yield stress fluid slender flows are prone to become unstable and host travelling waves. We propose a non-local dimensionless number ψ, obtained from a physical order-of-magnitude analysis, to predict, for the first time, the transition that marks the onset of roll waves in unconfined discharges of bentonite, whose rheology was assumed as a Bingham plastic. Involved variables include the discharge volume flow, Bingham viscosity, mixture density, gravity acceleration, plane inclination and yield stress. Experimental verification considering various discharge flow-plane inclinations and concentrations of the mixture indicate that for values of ψ exceeding the unity, single or multiple roll waves appear.

Particle organization after viscous sedimentation in tilted containers

Mesopotamia is in the east side of the region named “fertile crescent”, were agriculture flourished and the earliest civilizations were born more than eight thousand years ago. In the alluvial plain of Lower Mesopotamia agriculture based on irrigation developed, in contrast to the Upper Mesopotamia, where dry-farming was possible. A complex system of canals and waterworks developed, with the dual function to ensure irrigation and to be used as waterways. Control of water was decisive as a way to guarantee economic prosperity, but also was a source of inter-state conflicts and a political tool. Water technology was not limited to irrigation, Mesopotamians also pioneered in sanitary engineering, with many cities presenting networks of wastewater and stormwater drainage systems. Overexploitation of land and water resources for agriculture affected the environment, resulting in silting and soil salinisation, matter that has been recorded since the earliest cuneiform writings.

Digital image correlation applied to the calculation of the out-of-plane deformation induced by the formation of roll waves in a non-Newtonian fluid

A series of sedimentation experiments and numerical simulations have been conducted to understand the factors that control the final angle of a static sediment layer formed by quasi-monodisperse particles settling in an inclined container. The set of experiments includes several combinations of fluid viscosity, container angle, and solids concentration. A comparison between the experiments and a set of two-dimensional numerical simulations shows that the physical mechanism responsible for the energy dissipation in the system is the collision between the particles. The results provide new insights into the mechanism that sets the morphology of the sediment layer formed by the settling of quasi-monodisperse particles onto the bottom of an inclined container. Tracking the interface between the suspension solids and the clear fluid zone reveals that the final angle adopted by the sediment layer shows strong dependencies on the initial particle concentration and the container inclination, but not the fluid viscosity. It is concluded that (1) the hindrance function plays an important role on the sediment bed angle, (2) the relation between the friction effect and the slope may be explained as a quasi-linear function of the projected velocity along the container bottom, and (3) prior to the end of settling there is a significant interparticle interaction through the fluid affecting to the final bed organization. We can express the sediment bed slope as a function of two dimensionless numbers, a version of the inertial number and the particle concentration. The present experiments confirm some previous results on the role of the interstitial fluid on low Stokes number flows of particulate matter.

Visualization of 2-D Divergence on the Free Surface and its Relation to Gas Transfer

Abstract. A method based on digital image correlation (DIC) is implemented for measuring the height of the roll waves developed in a non-Newtonian fluid flowing on an inclined channel. A projector and a high-resolution digital camera, placed vertically above the fluid surface, are used to project and record a random speckle pattern located on the free liquid surface, where the pattern is deformed due to the developed roll waves. According to the experimental geometry, the height of the roll waves associated to the out-of-plane deformation of the dots is obtained through a quantitative relationship between the experimental parameters and the in-plane displacement field in the flow direction. In terms of this, the out-of-plane deformation is found using a DIC criterion based on the speckle comparison between a reference image without the deformed pattern and an image with a deformed pattern. The maximum height of the roll waves computed with this technique is compared with the height measured using a lateral camera, with both results differing by <10% over the set of experimental instances.