1,721,140 research outputs found
On the propagation of viscous gravity currents of non-Newtonian fluids in channels with varying cross section and inclination
No full textThis paper presents a model for the laminar propagation of gravity currents of rheologically complex fluids over natural slopes. The study is motivated by the ubiquitous occurrence of gravity currents in environmental applications that are confined by channels that widen and have reduced slopes in the flow direction; typical examples are mud and lava flows. In these applications, many fluids exhibit nonlinear relationships between shear stress and shear rate, with or without the appearance of a yield stress. We consider Ostwald-de Waele and Herschel-Bulkley fluids. A power-law equation is used to capture the variations in the channel shape and slope in the flow direction. We study the motion of constant and time-dependent volumes of these fluids on smoothly varying topographies. Approximate similarity solutions are obtained for Ostwald-de Waele fluids, while for HB fluids, we use the methods of characteristics to compute front propagation. Constant volume and constant influx tests were conducted in a channel with a widening parabolic cross-section and an inclination decreasing downstream from to . The front position was measured continuously over time, and the current thickness and the surface velocity were recorded for a subset of experiments in some cross sections. The experimental study confirms the theoretical formulation, with a better agreement for constant influx than constant volume currents
A dipole solution for power-law gravity currents in porous formations
Full text linkA theoretical and experimental analysis of non-Newtonian gravity currents in porous media with variable properties is presented. A mound of a power-law fluid of flow behaviour index is released into a semi-infinite saturated porous medium above a horizontal bed, and can drain freely out of the formation at the origin. The porous medium permeability and porosity vary along the vertical as and , respectively, being the vertical coordinate and and constant numerical coefficients. A self-similar solution describing the space-time evolution of the resulting gravity current is derived for shear-thinning fluids with n < 1, generalizing earlier results for Newtonian fluids. The solution conserves a generalized dipole moment of the mound. The spreading of the current front is proportional to . Expressions for the time evolution of outgoing flux at the origin and of the current volume are derived in closed form. The Hele-Shaw analogue is derived for flow of a power-law fluid in a porous medium with vertically variable properties. Results from laboratory experiments conducted in two Hele-Shaw cells confirm the constancy of the dipole moment and compare successfully with the theoretical formulation
Unsteady flow of shear-thinning fluids in porous media with pressure-dependent properties
Full text linkIn this paper a model for injection of a power-law shear-thinning fluid in a medium with pressure dependent properties is developed in a generalized geometry (plane, radial and spherical). Permeability and porosity are taken to be power functions of the pressure increment with respect to the ambient value. The model mimics the injection of non-Newtonian fluids in fractured systems, in which fractures are already present and are enlarged and eventually extended and opened by the fluid pressure, as typical of fracing technology. Empiric equations are combined with the fundamental mass balance equation. A reduced model is adopted, where the medium permeability resides mainly in the fractures; the fluid and porous medium compressibility coefficients are neglected with respect to the effects induced by pressure variations. At early and intermediate time, the flow interests only the fractures. In these conditions, the problem admits a self-similar solution, derived in closed form for an instantaneous injection (or drop-off) of the fluid, and obtained numerically for a generic monomial law of injection. At late times, the leak-off of the fluid toward the porous matrix is taken into account via a sink term in the mass balance equation. In this case, the original set of governing equations needs to be solved numerically; an approximate self-similar solution valid for a special combination of parameters is developed by rescaling time. An example of application in a radial geometry is provided without and with leak-off. The system behaviour is analysed considering the speed of the pressure front and the variation of the pressure within the domain over time, as influenced by the domain and fluid parameters
Asymptotic stability of bounded granular shear flow.
No full textTechnical report on research activity, University of Florence
Vorticity and intermittency within the pre-breaking region of spilling breakers
No full textThis paper presents measurements and analysis of fluid velocity within the context of spilling waves. The data have been collected using 2-D Laser Doppler Velocimetry in pre-breaking monochromatic waves generated in a wave tank. The analysis is performed using orthogonal wavelets and, in addition to the
classical criterion adopted in applying Taylor's hypothesis, a new algorithm is proposed for the eduction of eddies at different length scales. The contribution of different scale vortices is computed, and phase is resolved. Microvortices (smaller than the breaker height but larger than the dissipative vortices) and midsize vortices (with length ranging from the breaker height to the wave length) carry out most turbulence energy under wave crest. The phase average vorticity and strain rate is computed at different wave lengths, with the analysis of intermittence. The intermittency factor shows spikes in the wave crest, especially for
turbulence in small vortices
Experimental Investigation of Turbulence and Vorticity within the pre-breaking of Spilling Breakers
No full textRoll waves on a shallow layer of debris modelled as a dilatant fluid
No full textA shallow layer of dilatant fluid (tau = alpha(partial derivativeu/partial derivativey)(n) with n = 2) in laminar motion is considered to study finite amplitude roll waves down a slope, simplified by Karman's momentum integral approach. The existence of a condition of a periodic discontinuous solution is derived, neglecting the finite length of the shock, the curvature of trajectories and surface tension effects. Energy dissipation in the body of the stream and in the discontinuity are analyzed and discussed
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