A numerical study of pyroclastic flow dynamics: A shallow-water model for gravity currents with wide ranges of density differences
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Gravity current
Dynamics
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Wind Stress
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A three-dimensional model is developed for the prediction of steady-state circulation induced by the discharges, wind stresses, and horizontal temperature gradients in the far field regions of shallow lakes. For the case of lakes having rectangular geometries and uniform depth explicit analytical solutions are presented for the velocity distribution. The results of the analysis show that a negative temperature gradient along the lake in a given direction gives rise to a flow in the same direction at the upper layer of the lake and to a reverse flow at the lower layer. The circulations induced by horizontal temperature gradients may be as important as those generated by wind stresses.
Circulation (fluid dynamics)
Temperature Gradient
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AbstractFlow velocity and sedimentation patterns were investigated experimentally and numerically in shallow rectangular reservoirs with different asymmetric locations of the inlet and outlet channels. Velocity fields were measured in the entire reservoir, both for clear water flow and with suspended sediments. Thickness of sediment deposits was mapped in the whole reservoir by means of a laser light method. In one of the studied geometric configurations, injection of suspended sediments led to a complete change in the observed flow field. Experimental results were compared with numerical simulations performed with the depth-averaged flow model WOLF 2D, using a depth-averaged k–ϵ turbulence model. The simulations lead to accurate predictions of the velocity profiles and the change in flow pattern as a result of sediment deposits was successfully reproduced.Keywords: Flow stabilitymorphodynamic evolutionnumerical modellingreservoir sedimentationshallow reservoir
Sedimentation
Deposition
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Tidal current
Numerical models
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We present theoretical and experimental analyses of the critical condition where the inertial–buoyancy or viscous–buoyancy regime is preserved in a uniform-density gravity current (which propagates over a horizontal plane) of time-variable volume ${\mathcal{V}}=qt^{\unicode[STIX]{x1D6FF}}$ in a power-law cross-section (with width described by $f(z)=bz^{\unicode[STIX]{x1D6FC}}$ , where $z$ is the vertical coordinate, $b$ and $q$ are positive real numbers, and $\unicode[STIX]{x1D6FC}$ and $\unicode[STIX]{x1D6FF}$ are non-negative real numbers) occupied by homogeneous or linearly stratified ambient fluid. The magnitude of the ambient stratification is represented by the parameter $S$ , with $S=0$ and $S=1$ describing the homogeneous and maximum stratification cases respectively. Earlier theoretical and experimental results valid for a rectangular cross-section ( $\unicode[STIX]{x1D6FC}=0$ ) and uniform ambient fluid are generalized here to a power-law cross-section and stratified ambient. Novel time scalings, obtained for inertial and viscous regimes, allow a derivation of the critical flow parameter $\unicode[STIX]{x1D6FF}_{c}$ and the corresponding propagation rate as $Kt^{\unicode[STIX]{x1D6FD}_{c}}$ as a function of the problem parameters. Estimates of the transition length between the inertial and viscous regimes are also derived. A series of experiments conducted in a semicircular cross-section ( $\unicode[STIX]{x1D6FC}=1/2$ ) validate the critical values $\unicode[STIX]{x1D6FF}_{c}=2$ and $\unicode[STIX]{x1D6FF}_{c}=9/4$ for the two cases $S=0$ and $1$ . The ratio between the inertial and viscous forces is determined by an effective Reynolds number proportional to $q$ at some power. The threshold value of this number, which enables a determination of the regime of the current (inertial–buoyancy or viscous–buoyancy) in critical conditions, is determined experimentally for both $S=0$ and $S=1$ . We conclude that a very significant generalization of the insights and results from two-dimensional (rectangular cross-section channel) gravity currents to power-law cross-sections is available.
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The anatomy and propagation dynamics of non-Newtonian fluid mud gravity currents through emergent aquatic vegetation were investigated experimentally. The motivation of this study was related to the pipeline disposal of the dredged fluid mud into vegetated wetlands and near-shore areas, during which bottom gravity currents form. Our experimental observations showed that the presence of vegetation affects the propagation dynamics, hence the anatomy, of the gravity currents significantly. Vegetation-induced drag force dominated the resisting forces acting on the gravity current, forcing the current to transition into a drag-dominated propagation phase. During this transition, the gravity current profile evolved into a well-defined triangular/wedge shape. The onset of the fully established drag-dominated propagation phase was marked by the establishment of an equilibrium slope angle for the upper interface of the current with the ambient fluid. This equilibrium/terminal slope angle value remained constant throughout the rest of the drag-dominated propagation phase. Parameterizations for the required propagation distance for the onset of the fully established drag-dominated propagation phase, the array-averaged drag coefficient at the onset of this propagation phase, and the value of the terminal slope angle were proposed. Our experimental observations on the anatomy of gravity currents during the drag-dominated propagation phase were discussed in detail. This study documented significant effects of the vegetation in the propagation dynamics and anatomy of gravity currents, which warrants future detailed studies.
Gravity current
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We present a numerical model of the dynamics of Lake Donuzlav, which enables one to perform simultaneous numerical analyses of the currents, sea level, waves, and sediment transport. The model is based on the hydrodynamic block and the spectral wave model. For typical storm situations, we study the specific features of the integral circulation of waters and the three-dimensional structure of currents, investigate the wind-induced wave fields, and evaluate the flows of sediments and deformations of the bottom. The presence of intense eddy structures is revealed in the field of currents caused by the bottom topography. A significant intensification of waves in the south part of the lake is established in the case of penetration of storm waves through the strait. It is shown that the account of waves leads to qualitative changes in the structure of circulation in the lake and to the formation of well-pronounced areas of wave-induced elevations and lowerings of the sea level.
Kondratiev wave
Wave model
Circulation (fluid dynamics)
Numerical models
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Gravity currents are flows driven by buoyancy differences between two contacting fluids caused by differences in temperature, salinity, or by the presence of suspended particles. Such flows can reach high velocities near the bed, especially on the area behind the front of the current. As a result, rapid morphological changes may take place in river and estuarine beds due to the passage of these flows. Essential to determine the erosion induced by the current, are the spatial and temporal distributions of the bed shear stress. However, these are troublesome to measure in laboratory or in the field. To bridge this difficulty, the eddy-solving numerical simulations may be used. This study presents here the three-dimensional numerical simulations of lock-exchange salinity currents flowing over a mobile bed. It is aimed at the characterization of the sediment entrainment capacity of the current. The large eddy simulation technique is employed for analyzing the evolution and the structure of the current. For the sediment simulation, an Euler-Euler methodology based on a single phase approach is used. The main features of the current are compared with experimental data obtained in the laboratory. Velocity fields and bed shear stress distributions for different initial current densities are analyzed and linked to entrainment scenarios. The influence of small variations in particle size of the mobile bed is also discussed.
Gravity current
Entrainment (biomusicology)
Large-Eddy Simulation
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