Experimental study on radial gravity currents flowing in a vegetated channel

We present an experimental study of gravity currents in a cylindrical geometry, in the presence of vegetation. Forty tests were performed with a brine advancing in a fresh water ambient fluid, in lock release, and with a constant and time-varying flow rate. The tank is a circular sector of angle $30^circ$ with radius equal to 180 cm. Two different densities of the vegetation were simulated by vertical plastic rods with diameter $D=1.6; extrm{cm}$. We marked the height of the current as a function of radius and time and the position of the front as a function of time. The results indicate a self-similar structure, with lateral profiles that after an initial adjustment collapse to a single curve in scaled variables. The propagation of the front is well described by a power law function of time. The existence of self-similarity on an experimental basis corroborates a simple theoretical model with the following assumptions: (i) the dominant balance is between buoyancy and drag, parameterized by a power law of the current velocity $sim |u|^{lambda-1}u$; (ii) the current advances in shallow-water conditions; (iii) ambient-fluid dynamics is negligible. In order to evaluate the value of ${lambda}$ (the only tuning parameter of the theoretical model), we performed two additional series of measurements. We found that $lambda$ increased from 1 to 2 while the Reynolds number increased from 100 to approximately $6cdot10^3$, the drag coefficient and the transition from $lambda=1$ to $lambda=2$ are quantitatively affected by $D$, but the structure of the model is not

PROPAGATION OF GRAVITY CURRENTS OF NON-NEWTONIAN POWER-LAW FLUIDS IN POROUS MEDIA

A comprehensive analytical and experimental framework is presented to describe gravity-driven motions of rheologically complex fluids through porous media. These phenomena are relevant in geophysical, environmental, industrial and biological applications. The fluid is characterized by an Ostwald-DeWaele constitutive equation with behaviour index n. The flow is driven by the release of fluid at the origin of an infinite porous domain. In order to represent several possible spreading scenarios, we consider: i) different domain geometries: plane, radial, and channelized, with the channel shape parameterized by ; ii) instantaneous or continuous injection, depending on the time exponent of the volume of fluid in the current, ; iii) horizontal or inclined impermeable boundaries. Systematic heterogeneity along the streamwise and/or transverse direction is added to the conceptualization upon considering a power-law permeability variation governed by two additional parameters  and . Scalings for current length and thickness are derived in self similar form coupling the modified Darcy’s law accounting for the fluid rheology with the mass balance equation. The speed, thickness, and aspect ratio of the current are studied as a function of model parameters; several different critical values of  emerge and govern the type of dependency, as well as the tendency of the current to accelerate or decelerate and become thicker or thinner at a given point. The asymptotic validity of the solutions is limited to certain ranges of model parameters. Experimental validation is performed under constant volume, constant and variable flux regimes in tanks/channels filled with transparent glass beads of uniform or variable diameter, using shear-thinning suspensions and Newtonian mixtures. The experimental results for the length and profile of the current agree well with the self-similar solutions at intermediate and late times

Invariants of Turbulence Reynolds Stress and of Dissipation Tensors in Regular Breaking Waves

A series of measurements in a flume with a particle-tracking system in three dimensions applied to breaking waves is used to analyse the structure of turbulence with a full set of variables that usually are available only in numerical simulations. After extracting turbulence, in addition to the standard analysis aiming to quantify the fluxes, i.e., the time-average and the phase-average levels of turbulence and vorticity (details are given in two former papers), a more in-depth description of the structure of turbulence Reynolds stress tensor is given, focussing on the invariants evolution in time and in the vertical. A relation between the components of the Reynolds stress tensor and of the dissipation tensor is depicted. This relation is finalised to possible models of turbulence in breaking waves

Porous gravity currents: A survey to determine the joint influence of fluid rheology and variations of medium properties

We develop a model to grasp the combined effect of rheology and spatial stratifications on two- dimensional non-Newtonian gravity-driven flow in porous media. We consider a power-law constitutive equation for the fluid, and a monomial variation of permeability and porosity along the vertical direction (transverse to the flow) or horizontal direction (parallel to the flow). Under these assumptions, similar- ity solutions are derived in semi-analytical form for thin gravity currents injected into a two-dimensional porous medium and having constant or time-varying volume. The extent and shape of the porous domain affected by the injection is significantly influenced by the interplay of model parameters. These describe the fluid (flow behaviour index n ), the spatial heterogeneity (coefficients β, γ, δ, ω for variations of per- meability and porosity in the horizontal or vertical direction), and the type of release (volume exponent α). Theoretical results are validated against two sets of experiments with α= 1 (constant inflow) con- ducted with a stratified porous medium (simulated by superimposing layers of glass beads of different diameter) and a Hele-Shaw analogue for power-law fluid flow, respectively. In the latter case, a recently established Hele-Shaw analogy is extended to the variation of properties parallel to the flow direction. Comparison with experimental results shows that the proposed model is able to capture the propagation of the current front and the current profile

Wave-current interaction in the Porto di Lido entrance of the Venice Lagoon

Water Qualit

Buoyancy transfer in a two-layer system in transient and steady state. Experiments in a wave flume

Presentation in a conferenc

Strong turbulence and free surface interaction in a grid-stirred tank

In this work the experimental analysis of the interaction between turbulence and free surface is detailed. The topic is interesting in many natural flows and industrial processes. Turbulence is generated by a vertically oscillating grid moving beneath the free surface, as widely reported in several experiments. Fluid velocity is measured through a hot film anemometer. This instrument is able to measure vertical velocity fluctuation to within 0.5 mm of the surface, from which profiles of r.m.s. velocity fluctuation, integral length scales and several turbulence estimators can be calculated. The free surface elevation is measured by using an ultrasonic sensor based on flight-time with a response time of 10 milliseconds and an overall error equal to 0.2 mm The present analyses follow previous analyses on different data regarding some experiment carried out in a flume, with free surface turbulence generated by a Crump weir. In the experiments in the flume a relevant evidence of free surface waves suggested a separation between potential and turbulence contributions. Free surface waves seem almost not present in the grid stirred tank and a pure turbulent flow is forecast. Aiming to detect the source effects of turbulence near the free surface, the correlation between free surface elevation and the underneath flow velocity have been studied, and the time lag between turbulence and free surface has been evaluated. As observed in the Crump weir generated turbulence, resonance is expected with turbulence excited by free surface growing at a specific frequency of the grid

An Experimental Setup to Investigate Non-Newtonian Fluid Flow in Variable Aperture Channels

Non-Newtonian fluid flow in porous and fractured media is of considerable technical and environmental interest. Here, the flow of a non-Newtonian fluid in a variable aperture fracture is studied theoretically, experimentally and numerically. We consider a shear-thinning power-law fluid with flow behavior index n. The natural logarithm of the fracture aperture is a two-dimensional, spatially homogeneous and correlated Gaussian random field. An experimental device has been conceived and realized to allow the validation of the theory, and several tests are conducted with Newtonian and shear-thinning fluids and different combinations of parameters to validate the model. For Newtonian fluids, experimental results match quite well the theoretical predictions, mostly with a slight overestimation. For non-Newtonian fluids, the discrepancy between experiments and theory is larger, with an underestimation of the experimental flow rate. We bear in mind the high shear-rates involved in the experiments, covering a large range where simple models seldom are effective in reproducing the process, and possible interferences like slip at the wall. For all test conditions, the comparison between analytical and numerical model is fairly good

Application of Modular Underground Monitoring System (MUMS) to Landslides Monitoring: Evaluation and New Insights

The paper describes the application of the Modular Underground Monitoring Sys-tem (further referred as MUMS) (Segalini et al., 2013) in two active landslides lo-cated in the Northern Italian Apennines. In particular, the aim of the paper is to demonstrate the efficiency and accuracy of the system and to examine the ad-vantages of an automated semi-continuous monitoring for the comprehension of the mechanical behavior of landslides, the definition of their triggering factors and the correct evaluation of their short term velocity. The mechanical behavior of slow moving landslides is generally evaluated on the basis of traditional surveys which are carried out at long time interval and therefore are lacking of detailed information about the links between triggering causes and mechanical effects. The main ad-vantage of an automated monitoring system resides in the observation frequency and in the simultaneous recording of several physical entities such as deformation, precipitation, pore pressure and so on. This large amount of data can be used for the numerical evaluation of the landslide behavior and for the definition of the most significant triggering cause(s). The obtained knowledge is of fundamental im-portance when there is a need of establishing an hazard threshold for the particular landslide and, when this assumption is made, the automated monitoring system can immediately become a real time control and alarm triggering device. For this pur-pose, it is important that the lifespan of the instrumentation is as long as possible, in relation with the expected displacements, in order to maintain the real time monitoring effective and economical as well as to build a reliable database for statistical analysis

Experiments on Buoyancy Transfer in a two-layer System in Transient State

We are interested in the study of mixing with salinity flux in coastal areas due to turbulence and wave action. Previous studies (Petrolo & Woods, 2019, Petrolo & Longo, 2020) had shown the efficiency of turbulence due to a stirrer in generating a salinity flow capable of mixing, in the absence of fluid inputs and subtractions. The mixing also occurs, depending on the level of turbulence, for low flows of fresh/saline water, while high flows promote a more stable stratification, with vertical flow that can generally be limited by the intensity of the sources and the intensity of the turbulence. The problem is analysed here in the case where the source of turbulence is wave action. The wave field is affected by turbulence generated at the surface, very intense in the presence of breakers, and at the bottom if the waves are in shallow water; the turbulence can spread affecting part or all of the liquid column. In a focial scheme, with fresh water from the river and salt water in the sea basin, the action of the waves on the mixing of salinity and, with different intensity on the mixing of chemicals and gas, is of great importance. The study was carried out in the laboratory in a channel with a wave generator generating regular waves in a fluid column initially stratified with salt water at the bottom and fresh water at the top. The surface dynamics are accompanied by varied interface dynamics, with slow and progressive diffusion of the salty water into the fresh water. Measurements were made photographically to identify the interface and density gradients; with a Doppler profilometer for velocity field; and with a conductivity meter to measure local salinity at a data rate of 20 Hz. The synthetic schlieren method described in Longo et al (2016) was used, with a mask of randomized array of dots mimicking the tracers in Particle Image Velocimetry (PIV) technique and positioned between the light source and the channel. A PIV software was adopted to measure the apparent displacement of the dots, which is proportional to the square of the buoyancy frequency