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Perrochet, Pierre

Nom
Perrochet, Pierre
Affiliation principale
Fonction
Professeur ordinaire
Email
pierre.perrochet@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/144

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  • Publication
    Métadonnées seulement
  • Publication
    Métadonnées seulement
    Influence of capillarity on a simple harmonic oscillating water table: Sand column experiments and modeling
    (2005)
    Cartwright, Nick
    ;
    Nielsen, Peter
    ;
    Perrochet, Pierre 
    [1] Comprehensive measurements of the water table response to simple harmonic forcing at the base of a sand column are presented and discussed. In similar experiments, Nielsen and Perrochet ( 2000) observed that fluctuations in the total moisture were both damped and lagged relative to the water table fluctuations. As a result, the concept of a complex effective porosity was proposed as a convenient means to account for the damping and phase lag through its magnitude and argument, respectively. The complex effective porosity then enables simple analytical solutions for the water table ( and total moisture) dynamics including hysteresis. In this paper, these previous experiments are extended to cover a wider range of oscillation frequencies and are conducted for three well-sorted materials with median grain diameters of 0.082, 0.2, and 0.78 mm, respectively. In agreement with existing theory, the influence of the capillary fringe is shown to increase with the oscillation frequency. However, the complex effective porosity model corresponding to the classical Green and Ampt (1911) capillary tube approximations is shown to be inadequate when compared to the data. These limitations are overcome by the provision of an empirical, frequency-dependent complex effective porosity model fit to the data. Using measured moisture retention parameters, numerical simulation of the data solving a nonhysteretic van Genuchten - Richards' equation type model is unable to replicate the observations. Existing results of a hysteretic numerical model are shown to be in good agreement with the extended database.
  • Publication
    Accès libre
    Watertable dynamics under capillary fringes : experiments and modelling
    (2000-02-20)
    Nielsen, Peter
    ;
    Perrochet, Pierre 
    Watertable heights and total moisture content were measured in a sand column where the piezometric head at the base (“the driving head”) varied as a simple harmonic with periods in the range from 14.5 min to 6.5 h. The watertable height h(t) responded very closely to the driving head compared with the predictions of previous analytical and numerical models. The total moisture quantified as an equivalent, saturated height htot(t) varied very little compared with the watertable height. Neither h(t) nor htot(t) deviated significantly from simple harmonics when the driving head was simple harmonic. This indicates that non-linear effects are weak and hence that analysis based on linear solutions have fairly broad applicability. When h(t) and htot(t) are simple harmonic, the ratio nd=[dhtot/dt]/[dh/dt] is a constant in the complex formalism. Its magnitude
  • Publication
    Métadonnées seulement
  • Publication
    Métadonnées seulement
    Groundwater waves in aquifers of intermediate depths
    (1997)
    Nielsen, Peter
    ;
    Aseervatham, Raj
    ;
    Fenton, John D
    ;
    Perrochet, Pierre 
    In order to model recent observations of groundwater dynamics in beaches, a system of equations is derived for the propagation of periodic watertable waves in unconfined aquifers of intermediate depths, i.e. for finite values of the dimensionless aquifer depth n omega d/K which is assumed small under the Dupuit-Forchheimer approach that leads to the Boussinesq equation. Detailed consideration is given to equations of second- and infinite-order in this parameter. In each case, small amplitude (eta/d much less than 1) as well as finite amplitude versions are discussed. The small amplitude equations have solutions of the form eta(x, t) = eta(0)e(-kx)e(i omega t) in analogy with the linearized Boussinesq equation but the complex wave numbers k are different. These new wave numbers compare well with observations from a Hele-Shaw cell which were previously unexplained. The ''exact'' velocity potential for small amplitude watertable waves, the equivalent of Airy waves, is presented. These waves show a number of remarkable features. They become non-dispersive in the short-wave limit with a finite and quite slow decay rate affording an explanation for observed behaviour of wave-induced porewater pressure fluctuations in beaches. They also show an increasing amplitude of pressure fluctuations towards the base, in analogy with the evanescent modes of linear surface gravity waves. Copyright (C) 1996 Elsevier Science Ltd
  • Publication
    Accès libre
    Behavior of a shallow water table under periodic flow conditions
    Cartwright, Nick
    ;
    Nielsen, Peter
    ;
    Perrochet, Pierre 
    A new laboratory data set on the behavior of a shallow water table in a sand column aquifer subject to simple harmonic periodic forcing at its base is presented and discussed. The data are analyzed using the dynamic effective porosity, which is defined as the ratio of the rate of change in total moisture to the rate of change in water table elevation; thus, a reduction in this parameter means that the extent of moisture exchange has been reduced relative to a given water table fluctuation. The data show a clear decrease in the dynamic effective porosity with increasing proximity of the water table to the sand surface, which is consistent with previous research under a steadily rising or falling shallow water table. The observed reduction in moisture exchange due to shallowness of the water table has implications for periodic flow scenarios such as the propagation of water table waves in coastal and beach groundwater systems. That is, as moisture exchange is reduced, less work is being done by the flow, and thus, energy dissipation rates for shallow water tables will be reduced relative to the case of a deeper water table. At present no account of the influence of water table shallowness has been included in theories describing water table wave dispersion. The present experiments, in conjunction with the dynamic effective porosity concept, provide a framework in which this gap in knowledge can be further investigated. Additional experiments were designed such that the free surface transgressed the sand surface for part of the oscillation period to investigate the influence of meniscus formation and deformation at the sand surface on periodic flow dynamics. The observed behavior is consistent with previous observations of steady infiltration above shallow water tables, namely, a rapid drop (rise) in pore pressure with the onset of meniscus formation (deformation). A simple “wetting and drying” model is derived, accounting for the variation in effective porosity caused by the free surface transgressing the sand surface, which is shown to accurately capture the observed behavior. A finite element solution of the Richards equation in which the transient upper boundary condition is easily mimicked by means of a surface element with special storage features also shows excellent agreement with the observed data.