Giovanna Darvini

Examples of subsurface solute spreading driven by inhomogeneous velocity fields

Most of solutions of flow and transport problems available in literature are obtained under assumptions of stationarity of flow field and/or ergodicity of transport, even though in real-world analyses these hypotheses cannot be always verified. For instance, into problems that are met in practical applications, the statistics of flow are often location dependent and the stationarity of flow is violated. The nonstationarity of velocity field may originate from finite domain boundaries, complex flow configurations (pumping and injecting), nonstationarity of medium properties or conditioning of the log conductivity field to measurements of head or conductivity. Moreover the lack of ergodicity related to the finite size of solute sources makes difficult transport analyses that are often carried out by rough schematizations. Some examples of solute dispersion of nonergodic passive solute plume in heterogeneous formations with nonstationary flow conditions are here considered and solved by a new approach. By this method the spatial moments of finite initial volume of solute are obtained from the statistics of velocity field evaluated by first-order perturbation expansion of steady state flow equation in Taylor series combined with a finite element discretization. The approach allows to handle nonstationarity due to several causes and is here applied to some test cases in bounded domains. A comparison of numerical results in terms of particle displacements moments with known solutions available in literature and with Monte Carlo simulations gives a measure of the effect related to the lack of plume ergodicity and of flow spatial stationarity in real-world transport analyses.

The Boundaries Influence in Trending Hydraulic Conductivity Fields

We analyze the impact of the boundary conditions on flow and transport solutions for the case of a two-dimensional heterogeneous medium with a linear trend in the mean log-conductivity. The influence of the boundaries is analyzed for different values of the domain size and aspect ratio, and different directions of the trend respect to the mean flow direction. The analysis involving a steady, spatially nonstationary velocity field, is developed by using the stochastic finite element method. Results of this study show that the impact of the trend in finite domains may disagree with that obtained in unbounded domains and it is more complex. The trend alters the result obtained for the medium with stationary conductivity, leading to a larger or smaller value of the macrodispersion coefficients, depending on the size and aspect ratio of the domain, and this effect may vary with the travel distance from the solute source. Moreover in limited domains here analyzed also for particles starting at distance of several integral scales from the boundaries and traveling in the middle of the domain, the behavior of macrodispersion coefficients is affected by the boundary conditions and the influence of the trend on longitudinal and transverse macrodispersion coefficients may vary with the imposed constraints.

Losses identification in water distribution networks through EnKF and ES

This study proposes a method for the identification of the spatial distribution of water losses in water distribution networks through the use of pressure head measurements. The proper identification of areas most prone to water losses reduces the costs associated with acoustic surveys both in terms of number of pipes to be examined and working time. To get the best estimate of the water losses spatial distribution, data assimilation techniques based on the Kalman Filter approach (Ensemble Kalman Filter and Ensemble Smoother) are coupled with the hydraulic network model (EPANET). The coupled model performances are investigated on the Anytown benchmark system with both a known and an unknown consumption pattern. A method to identify the most effective network monitoring locations is also proposed. Despite the fact that the method is tested on a single synthetic network, the results suggest that the tool is promising for water losses identification.