Papers by Keyword: Porous Media

Abstract: Inlet and outlet pressure drop effects can contribute significantly to the total pressure drop in porous media if thin solid matrices are used. However, these effects are usually ignored and few are the studies that focus on this topic. This paper uses a numerical simulation approach to determine the importance of the inlet and outlet pressure drop effects on the total pressure drop in a staggered arrangement of square cylinders with equal sizes, d_c. The Navier-Stokes equations are solved at the pore level for several matrix lengths (from d_c to 34d_c) and for several Reynolds numbers based on d_c and maximum velocity of the velocity inlet profile (from 36 to 120). Accurate results of the velocity and pressure fields are obtained through the use of the immersed boundary method in combination with the finite differences method, 4^th-order compact schemes for spatial discretization and 4^th-order Runge-Kutta temporal discretization. The results presented in this paper confirm that the classical models (e.g., Hazen-Dupuit-Darcy model) are only valid when the solid matrix has a length above a certain value, called the critical length. For shorter porous media, the pressure drop does not vary linearly with the matrix length. The deviations to the model that occur at the shortest porous media are explained by the entrance and exit contributions to the total pressure drop that, in these cases, are not negligible when compared to the bulk pressure drop. For the staggered array of square cylinders and range of Reynolds numbers considered, the critical porous medium length is 16d_c. A practical outcome of the present study is the quantification of the influence of the pressure tap locations on the measurements of pressure drop in porous media. When the matrix is short when compared to the particle diameter, care must be taken with the pressure taps placement: they should be located outside the porous matrix and not too close to its inlet and outlet sections. If the matrix is thick enough when compared to the particle diameter, the taps can be placed either inside or outside the matrix. Also, if the influence of the side walls on the total pressure drop is not high (i.e., the walls are at a relative large distance when compared to the particle diameter), there is no practical need to correct the measured pressure values to account for the influence of the walls. This correction should be considered for the shortest matrices though.

Abstract: In this paper, we focus on the numerical simulation of degradation of concrete structures subject to External Sulfate Attack (ESA). A diffusion-reaction model is used to describe the diffusion of sulfates inside the material and the reaction with the reactive constituents of the cement paste (calcium aluminates). The mechanical analysis is based on a new bi-phase chemo-elastic model with chemical and mechanical damage. The results obtained with the proposed approach are compared with experimental data on a reduced scale tunnel lining structure subject to ESA.

Abstract: A set of cell-based smoothed point interpolation methods are proposed for the numerical analysis of Biot’s formulation. In the proposed methods, the problem domain is discretized using a triangular background mesh. Shape functions are constructed using either polynomial or radial point interpolation method (PIM), leading to the delta function property of shape functions and consequently, easy implementation of essential boundary conditions. The Biot’s equations are discretised in space and time. A variety of support domain selection schemes (T-schemes) are investigated. The accuracy and convergence rate of the proposed methods are examined by comparing the numerical results with the analytical solution for the benchmark problem of one dimensional consolidation.

Abstract: Partial differential equations such as models for flow in unsaturated porous media are difficult to be solved when space-time variable boundary conditions are included. A general solution to this problem is discussed in this contribution and is devised in such a way that the face with variable boundary condition can be subjected to Dirichlet, Neumann or the so-called Signorini/ambiguous boundary conditions, considering the transition from one type to another. A method based on the enrichment of finite elements that is able to accurately model seepage with these complex boundary conditions is discussed. Simulations are presented illustrating the capabilities of the new method in 2D and 3D, including cases where the free surface varies due to rain.

Abstract: This paper deals with the large deformation analysis of partially saturated soils subjected to dynamic loading. The so-called ‘mixture’ theory is employed to consider the hydro-mechanical coupling involved in this kind of problem. The finite element method is used to discretise the problem domain and the generalized-α algorithm is employed to integrate the governing equations over time. Some of the most challenging aspects of dynamic analysis of partially saturated soils will be discussed. One of the key challenges is selecting a consistent constitutive model within the theory of mixtures that can incorporate the pore suction forces into the description of stress. The necessity of such incorporation has frequently been reported in experimental studies of unsaturated soils. To tackle this problem, a unique strategy for integrating the constitutive model for unsaturated soils is adopted. Moreover, an absorbing boundary condition, which prevents wave reflection from rigid boundaries, is introduced and implemented into the numerical algorithm. Finally, a solution for the problem of dynamic compaction of soil in a partially saturated condition is presented.

Abstract: Heat transfer enhancement technology has the aim to develop more efficient systems as demanded in many applications in the fields of automotive, aerospace, electronics and process industry. A possible solution to obtain efficient cooling systems is represented by the use of confined impinging jets. Moreover, the introduction of nanoparticles in the working fluids can be considered in order to improve the thermal performances of the base fluids. In this paper a numerical investigation on mixed convection in confined slot jets impinging on a porous media by considering pure water or Al₂O₃/water based nanofluids is described. A two-dimensional model is developed and different Peclet numbers and Rayleigh numbers were considered. The particle volume concentrations ranged from 0% to 4% and the particle diameter is equal to 30 nm. The target surface is heated by a constant temperature value, calculated according to the value of Rayleigh number. The distance of the target surface is five times greater than the slot jet width. A single-phase model approach has been adopted in order to describe the nanofluid behaviour while the hypothesis of non-local thermal equilibrium is considered in order to simulate the behaviour in the porous media which is featured by a porosity value of 0.87. The aim consists into study the thermal and fluid-dynamic behaviour of the system. Results show increasing values of the convective heat transfer coefficients for increasing values of Peclet number and particle concentration. This behaviour is more evident at low Peclet number values and Rayleigh number ones.

Abstract: The main objective of this paper is to give more insight on colloids deposition and re-entrainment in presence of a rough surface. Experiments on retention and release of colloids in a porous medium are first presented. The influence of physicochemical and hydrodynamic conditions is investigated. The experimental results cannot be qualitatively interpreted using the DLVO theory and knowledges at pore scale are then needed. A 3D numerical simulation approach at the pore scale is therefore proposed where the motion of colloids is solved in presence of collector surfaces bearing various kinds of asperities and by taking into account physico-chemical interactions calculated at each time step during colloid movement. It is obviously observed that both deposition and mobilization of particles are dependent on solution chemistry and hydrodynamic conditions and are significantly affected by the form and size of the local roughness of the pore surface. Therefore, depending on solution ionic strength and surface topography, colloids may be adsorbed or not and when a particle is retained an increase of flow strength is then needed to remove it and such an increase is specific to the location of occurrence of the adsorption step. In general, simulation results allow us to explain our experimental results that show that by steeply increasing the flow strength, more and more fractions of particles retained inside the porous medium are released until all particles are removed.

Abstract: Free convection on a vertical surface with Newtonian heating of the form proposed by Merkin (1994) in the fluid-filled porous medium is considered on the basis of the full equations of a viscous liquid. Using dimensional analysis a set of criteria that define the characteristics of flow and heat transfer was derived. Asymptotic analysis of the full equations allowed us to determine the region of applicability of the boundary layer approximation, which was used in the previous studies of this problem. Darcy parameter influence was studied; the composite numerical and analytical solution for stream function and temperature was derived.

Abstract: Present paper is an attempt to illustrate a specific character of gas combustion and thermo-electrical processes in small-sized power energy-supply devices with thermoelectric methods of heat conversion. The efficiency of the heat conversion into electricity by using of thermo-elements embedded in the solid walls is estimated for a system with heat regeneration by means of the heat conductivity of the solid elements.

Abstract: The 3-D OpenMP parallel version of an algorithm for modeling dynamic processes in porous media with heat-evolution sources is presented. Parallel methods and techniques which were used for significant speed up of final results obtaining are described in detail. The performance results of parallel program for specific configuration of porous object are presented.