1,721,334 research outputs found
Trasporto di soluti reattivi in acquiferi
No full textSi presentano alcuni recenti sviluppi relativi alla modellazione di processi di trasporto di soluti reattivi in acquiferi sotterranei. Molte reazioni di interesse applicativo avvengono in presenza di sistemi geochimicamente complessi. La formulazione generale di un problema di trasporto reattivo si articola su molteplici aspetti e richiede di specificare un numero elevato di specie acquose e non acquose, nonché di identificare la distribuzione spazio-temporale dei tassi delle reazioni. La metodologia proposta da De Simoni et al. (2005, 2007) consente il calcolo diretto dei tassi di reazione associati a scenari di trasporto multi-specie mediante il disaccoppiamento del sistema di equazioni che descrive l’evoluzione dei soluti nel dominio. Uno degli obiettivi del presente lavoro consiste nel presentare l’applicazione di tale metodologia disaccoppiata per descrivere le caratteristiche salienti associate al mescolamento di acque a diversa composizione chimica in un mezzo poroso. Si discutono a tal fine i risultati della modellazione numerica di esperimenti di dissoluzione condotti in un sistema carbonatico omogeneo alla scala di laboratorio, in cui si induce il mescolamento di acqua dolce e salata. Si illustrano, quindi, possibilità e prospettive di estensione della metodologia investigata. In particolare, si presentano espressioni per la funzione di densità di probabilità e gli associati momenti (di insieme) di primo e secondo ordine dei tassi delle reazioni che hanno luogo in un acquifero stratificato ad eterogeneità aleatoria
Closure to "Hydrodynamic loading on river bridges" by Stefano Malavasi and Alberto Guadagnini
No full textEffects of evolving scales of heterogeneity on hydraulic head predictions under convergent flow conditions
No full textWe consider two-dimensional steady state flow towards a well that fully penetrates a randomly heterogeneous aquifer, with deterministically prescribed constant head boundary. Flow occurs over an infinite hierarchy of mutually uncorrelated, statistically homogeneous and isotropic random fields (modes) of natural log-transmissivity, Y, each of which is associated with a Gaussian variogram. We consider a lower and upper cut-off of the hierarchy, respectively related to the length scale of the domain and data support (sample volume). This allows directly incorporating the scale dependence of the integral scale of Y into groundwater flow (ensemble) moments and leads to a geostatistical description of the system in terms of a (stationary) Truncated Power Variogram (TPV). We then develop an analytical solution for hydraulic head mean and variance based on recursive approximations of exact nonlocal moment equations. Our solution allows to assess functional dependences of the distribution of the leading (statistical) moments of hydraulic head on parameters of the variogram associated with the hierarchy of log-transmissivity modes. The latter can be determined, for instance, along the lines of Neuman et al. (2008)
Probabilistic estimation of well catchments in heterogeneous aquifers
No full textEvaluation of solute travel times to wells is an environmental problem of high relevance. It is usually accomplished by means of models that rely on the assumption of uniform and isotropic conductivity field and general rules can be derived only for simple cases. We study the travel time of an ideal tracer to a single, steady-state pumping well, fully penetrating a confined aquifer, with uniform background flow, to analyse the influence of natural heterogeneity on the spatial location of the isochrones, that are the boundaries of time-related well capture zones. In a uniform aquifer the location of the isochrones is deterministic while heterogeneity of the hydraulic conductivity is the main source of natural uncertainty in the estimate of their spatial distribution. Local dispersion is not considered and the problem is stated in terms of dimensionless variables. The analysis is performed via a Monte Carlo procedure in conjunction with FFT-based spectral methods. The log-conductivity field is assumed to be Gaussian and stationary, with variance SIGMA2_Y and isotropic exponential correlation. Various degrees of domain heterogeneity are considered and stability and accuracy of the MC procedure is examined. The total probability that a particle starting from a point in the acquifer is pumped within a given time is identified. It is demonstrated that due to heterogeneity the location of an isochrone becomes uncertain and it is strongly influenced by SIGMA2_Y. We postulate an expression for the probabilistic spatial distribution of the isochrones which is formally similar to that derived by Bear and Jacobs (1965) for a uniform medium and introduce an additional coefficient, numerically determined, to reproduce the effect of heterogeneity. Conceptually consistent equations of practical use for engineering purposes are proposed
Recharge fronts and stagnation areas for pumping wells
No full textWe consider a single pumping well within a two-dimensional
heterogeneous aquifer, in the presence of a mean uniform background gradient. We analyse the uncertainty associated with the location of the stagnation point which is generated downstream of the well and the maximum lateral extent of the well recharge area. The study is relevant for risk assessment practice, since it allows to estimate the maximum width of the region contributing to the well and to properly locate regions of inversion of
the flow direction. The problem is approached within a numerical Monte Carlo framework and the dependence of the main statistics of the quantities of interest on the moments of the log-conductivity field is studied. An assessment of the impact of one conductivity datum at the pumping location is performed
Interactions between a rectangular cylinder and a free-surface flow
No full textThe salient features of the interaction between a free-surface flow and a cylinder of rectangular cross-section are investigated and discussed. Laboratory-scale experiments are performed in a water channel under various flow conditions and elevations of the cylinder above the channel floor. The flow field is characterized on the basis of time averaged and fluctuating local velocity measurements. Dynamic loadings on the cylinder are measured by two water-insulated dynamometers placed inside the cylinder structure. Starting from frequency and spectral analyses of the force signals, insights on the relationship between force dominant frequencies and the Strouhal number of the vortex shedding phenomenon are provided. Experimental results highlight the strong influence of the asymmetric configuration imposed by the two different boundary conditions (free surface and channel floor) on (i) the mean force coefficients and (ii) the vortex shedding frequencies. We provide an analysis of the nature of the dependence of average force coefficients on relevant dimensionless groups, i.e., the Reynolds number, normalized flow depth and cylinder submersion
Time-related capture zones for contaminants in randomly heterogeneous formations
No full textPrediction of solute travel distance based on homogeneous, and isotropic hydraulic conductivity may lead to substantial differences in terms of the actual travel distance in a heterogeneous system. The effect of randomly varying hydraulic conductivity on the spatial location of time-related well capture zones for a non-reactive tracer in a confined aquifer with uniform base flow is considered. A numerical Monte Carlo procedure is used in conjunction with Fast Fourier Transform-based spectral methods. The log hydraulic conductivity field is assumed to be Gaussian and stationary, with isotropic exponential autocovariance. Various degrees of domain heterogeneity are considered and stability and accuracy of the numerical procedure are examined. The total probability that a particle injected at a point in the aquifer is extracted by the well within a given time is identified. The concept of probabilistic isochrone, which is the boundary of a given time-related capture zone, is introduced. A simple analytical model that extends the deterministic capture-zone model for uniform media is derived, by taking into account the effect of random variations of hydraulic conductivity. Knowledge of the spatial pattern of probabilistic isochrones allows reconstructing the breakthrough curve at the well for solute injections at selected locations in the aquifer
Convergence assessment of numerical Monte Carlo simulations in groundwater hydrology
No full textNumerical Monte Carlo simulation is considered to be one of the main tools to be used in groundwater hydrology (1) to quantify the uncertainty in the flow predictions due to imperfect knowledge of aquifer architecture, hydraulic parameters, and forcing terms or (2) to assess the reliability of approximated moment-based equations for flow and/or transport. While the Monte Carlo framework is conceptually straightforward and very flexible, it is recognized as lacking well-established convergence criteria. Here we propose a methodology for convergence analysis of Monte Carlo simulations and therefore for the reliability assessment of the inferred statistical moments. The methodology, based on simple rules of statistical inference, is described with reference to a typical groundwater flow problem and can be extended to different application fields
Travel time and trajectory moments of conservative solutes in three dimensional heterogeneous porous media under mean uniform flow
No full textWe present expressions satisfied by the first statistical moments (mean and variance–covariance) of travel time and trajectory of conservative solute particles advected in a three-dimensional heterogeneous aquifer under uniform in the mean flow conditions. Closure of the model is obtained by means of a consistent second-order expansion in Sigma_Y (standard deviation of the log hydraulic conductivity) of (statistical) moments of quantities of interest. As such, the results obtained are nominally limited to mildly non-uniform fields, with Sigma_Y < 1. Resulting mean and variance of particles travel time and trajectory are functions of first and second moments and cross-moments of trajectory and velocity components. Our solution is applicable to infinite domains and is free of distributional assumptions. As an important application of the methodology we obtain closed-form expressions for the unconditional mean and variance of travel time and particle trajectory for isotropic log-conductivity domain characterized by an exponential variogram. This allows us to recover the non linear behavior of mean travel time versus distance, in agreement with numerical results published in the literature, as well as a non-linear effect in the mean trajectory. The analysis of trajectory variance allows recovering some known results regarding transverse macro-dispersion, evidencing some limitations typical of perturbation theory
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