Transport in Porous Media最新文献

This study examines penetrative electrothermal convection driven by uniform internal heat generation in a horizontal layer of dielectric fluid-saturated anisotropic Brinkman porous medium subjected to a uniform AC electric field. Anisotropy in permeability and thermal diffusivity is incorporated, with the vertical permeability fixed at twice the horizontal value, while the thermal anisotropy parameter is varied. The boundaries are taken to be either rigid or stress-free, with an adiabatic lower surface and an upper surface subject to a Robin thermal condition. A linear stability analysis yields a generalized eigenvalue problem, which is solved numerically using a Galerkin method. The principle of exchange of stabilities is shown to hold for rigid–rigid, rigid–free and free–free boundary combinations. Increasing the AC electric Rayleigh number and the Darcy number advances the onset of convection, whereas larger thermal anisotropy and Biot numbers delay instability. Velocity boundary conditions exert a significant quantitative influence, with rigid boundaries being the most stabilizing. As a limiting case, pure electroconvection in the absence of buoyancy is also considered. Although the qualitative trends remain similar to those in the coupled electrothermal regime, substantially higher critical AC electric Rayleigh numbers and wavenumbers are required to trigger instability, reflecting the need for electric forcing alone to overcome viscous and porous dissipation.

The linear and weakly nonlinear instability of triple-diffusive convection in a Darcy–Brinkman porous layer saturated with a Navier–Stokes–Voigt fluid is investigated analytically. The instability is driven by competing thermal and solutal buoyancy forces in the presence of viscoelastic stresses arising from a Voigt-type strain-rate regularization. Linear instability analysis reveals that the Kelvin–Voigt parameter plays a decisive role in oscillatory convection, exerting either stabilizing or destabilizing effects depending on the solute Darcy–Rayleigh number. A distinctive feature of the system is the emergence, under certain parametric conditions, of a disconnected closed oscillatory instability branch separate from the stationary one. In this regime, the neutral stability boundary is defined by three distinct thermal Darcy–Rayleigh numbers, in sharp contrast to the single threshold that characterizes the corresponding double-diffusive system. A weakly nonlinear stability analysis yields a cubic complex Landau amplitude equation governing the evolution of the disturbance amplitude, suggesting the possibility of subcritical bifurcations under certain parametric conditions. Heat and mass transport are quantified using time- and area-averaged Nusselt and Sherwood numbers, which increase with the Darcy–Prandtl number but decrease with increasing Darcy number and the effective heat capacity group, while variations in the Kelvin–Voigt parameter induce crossover behavior in the transport characteristics. These results clarify the subtle interplay between buoyancy, diffusion, and viscoelastic effects in regulating convection in triply diffusive porous media.

The inherent instability and risk of liquefaction associated with unstable glaciomarine clay are both scientifically intriguing and societally important. Stabilizing sensitive soils is increasingly necessary with the unfolding climate crisis, which leads to wilder and wetter weather. The relevant length scales extend from nanometer-sized pores to kilometer-sized geological features. Laboratory experiments are important for gaining a better understanding of these phenomena. Here, we present a study of the ultra-soft sandstone Saltwash South, which essentially consists of quartz particles glued together by clay, as a proxy for nanoscopically fine-grained sensitive soil to facilitate time-resolved imaging of the onset of activation and liquefaction. By employing in situ time-resolved combined X-ray and neutron computed tomography (CT), we visualize the structural deformation caused by the presence of water and salts. While neutron imaging was sensitive to the presence of normal and heavy water, simultaneous X-ray imaging was used to measure the porous structure, swelling, and initial liquefaction response of the consolidated rock. Uniform expansion was observed in regions exposed to water, reflecting clay swelling and disaggregation. In tightly confined samples, the swelling and disaggregation were suppressed. Finally, we discuss future perspectives of this promising approach to studying liquefaction phenomena in porous media.

The hydrodynamic performance of porous structures is paramount for the efficiency of compact heat exchangers. This study investigates the fluid transport characteristics of three types of triply periodic minimal surface (TPMS) structures (Diamond, Primitive, and IWP) fabricated from CuCrZr alloy via selective laser melting (SLM). Combining computational fluid dynamics (CFD) simulations with experimental validation, we systematically elucidate the influence of structure and unit cell density on key performance metrics, including pressure drop, friction factor, and permeability. Results reveal that the Diamond structure, with the lowest inlet porosity (23.07%), exhibits the most complex flow patterns—characterized by significant flow separation, enhanced vorticity, and superior flow uniformity—leading to the highest pressure drop and friction factor, but the lowest permeability. In contrast, the primitive and IWP structures, with higher inlet porosities (46.34% and 50.07%, respectively), offer lower flow resistance. Furthermore, increasing the unit cell count for a fixed overall porosity amplifies these effects, resulting in a higher pressure drop per unit length and reduced permeability due to more frequent flow disturbances. The experimental measurements of pressure drop align well with numerical predictions, with discrepancies attributed to a reduction in effective functional porosity caused by adherent unmelted particles during the SLM process. This work provides fundamental insights into the structure-transport relationships in TPMS architectures and offers practical guidelines for designing high-performance heat exchangers for applications under extreme conditions, such as deep-sea thermal management.

Thermal gradients in many natural environments, such as undersea hot springs and hydrothermal vents, are often accompanied by variations in the concentration of chemical compounds carried into the surrounding seawater. This interaction gives rise to a phenomenon known as thermosolutal mixed convection. To investigate the underlying physical mechanisms in such systems, a vertical pipe filled with saline water through a porous medium is considered. The non-Darcy Brinkman–Forchheimer extended model is employed, assuming that the saturated porous medium is in a thermal equilibrium state. The instability boundary curve reveals three distinct regimes: (i) the Rayleigh–Taylor (R–T) phenomenon, (ii) a nonlinear variation of mixed convection on a log–log scale and (iii) a linear variation on a log–log scale in the pipe. These regimes occur at specific Reynolds numbers. The instability mechanism of the base flow is examined using the spectral collocation technique. The present study focuses on the instability of a dense porous medium, where the Darcy–Forchheimer drag is incorporated in the momentum equation. A linear stability analysis is performed for a wide range of Darcy numbers ((10^{-8}) to (10^{-5})). Similar to cross-diffusive free convection in a vertical slot, a relationship is observed between the solutal Rayleigh number ((Ra_S)) and the thermal Rayleigh number ((Ra_T)). A hyperbolic relationship between the threshold values of (Ra_S) and Da is found at (Re = 1000) for the upper limit of (Ra_S) in the first regime, expressed as (Ra_S) Da =4.9 x (10^{-4}). Simulations of secondary flow profiles are also presented at the critical state for all three regimes.

Soil carbon flux is the main source of atmospheric CO₂. An accurate understanding of the soil CO₂ transport process is of substantial significance for the study of global carbon flux and the prediction of future climate change. In this study, data on CO₂ concentrations over time in five media under the influence of surface winds and pore air pressure fluctuations were analysed using (1) an analytical solution of the dispersive transport model and (2) a numerical solution of the advection transport model to accurately delineate the transport mechanisms that control soil gas transport. For loam and sandy soils, gas transport is mainly dominated by diffusion, and in highly permeable sand, gas transport changes from diffusion–dispersion transport to advection transport dominated by wind speed and pore air pressure fluctuations. The effects of surface wind and pore pressure fluctuations on advection velocity differed with depth. The pore pressure fluctuations had a stronger penetrating power. The ratio of the enhanced diffusion coefficient (D_e) to advection velocity(v) was effective in determining the transport mechanism driving gas transport within the medium, which is conducive to accurately modelling the release of CO₂ from soils to improve the understanding and assessment of the global carbon cycle.

Groundwater flow in an unconfined aquifer resting on a horizontal impermeable layer with a boundary condition of a rapid increase in the source water level is considered in this work. The newly introduced condition, referred to as the backward power law head condition, represents a situation where the water level in the adjacent water body increases more rapidly than do conventional problems, which can only represent a situation akin to a traveling wave under rising water level conditions, given its consideration of infinite time. This problem admits the similarity transformation which allows the nonlinear partial differential equation to be converted into a nonlinear ordinary differential equation via the Boltzmann transformation. The reduced boundary value problem is interpreted as the initial value problem for a system of ordinary differential equations, which can be numerically solved via Shampine’s method. The numerical solutions are in good agreement with Kalashinikov’s special solution, which is also introduced into the Boussinesq equation. We find that the solution is consistent with the limit of the forward power law head condition. The new approximate analytical solution and the associated wetting front position are derived by assuming that the solution has the form of quadratic polynomials, which enables the description of the time progression of a real front position. The obtained approximation is compared to Shampine’s solution to check the accuracy. Furthermore, the finite element method is applied to the original partial differential equation, which validates Shampine’s solution for use as a benchmark.

With the aim of remediating soils eroded by suffusion, three different techniques were explored and compared at the laboratory scale in this study to effectively infiltrate fine grains into a porous granular filter. In the first technique, which serves as a reference owing to its simple implementation in the field conditions, a pure layer of fine grains is deposited on the upstream side of the filter. However, this technique is found inefficient due to a collective effects between the fine grains which induces formation of surface cake, preventing thus their penetration into the filter. A second technique is therefore proposed in which the fine grains are mixed homogeneously with coarse particles and deposited on the upstream of the filter. Besides, a third technique was designed, where the fine grains are put in suspension in the water seeping through the filter in order to limit their collective clogging. The influence of size ratio and fine grains shape for both the mixture and suspension techniques, and fine grains shape in the case of suspension technique were also explored. For the latter, the cumulative efficiency was further investigated through multiple rounds of infiltration. The results showed that, for a given size ratio between the coarse and fine grains, there is a significant increase in the amount of infiltrated fine grains for the mixture and suspension techniques, that is respectively twice and eight times larger than the value obtained with the reference one, respectively. In the mixture technique, fine grains infiltration is found to increase with the size ratio up to a limiting value whereas, in the suspension method, it decreases as the size ratio increases. Moreover, rounded fine grains are found to infiltrate more easily than angular ones. This work should contribute to improve engineering protocols for achieving deep infiltration through the selection of an appropriate fine grains infiltration technique, with the objective of developing an effective remediation method.

The study investigates the linear instability of multi-diffusive convection in a horizontal Darcy–Brinkman porous layer saturated with a zero-order Kelvin–Voigt fluid (Navier–Stokes–Voigt fluid), heated and salted from below, under the framework of the local thermal non-equilibrium (LTNE) model. The momentum transfer is modelled by adopting a modified Darcy–Brinkman model, thus including the viscoelastic and viscous diffusion contributions. The stress-free boundaries are assumed to be perfect conductors of heat and solute concentrations. The stability eigenvalue problem is solved using normal-mode analysis procedure, and the onset conditions for both stationary and oscillatory convection are determined in a closed form. It is found that, in some cases, three values of the thermal Darcy–Rayleigh number are required to specify the linear instability criteria owing to the existence of disconnected closed convex oscillatory neutral curves from the stationary one. The sensitivity of the Kelvin–Voigt viscoelastic parameter on the topology of the neutral curves is examined, while its influence on the critical parameters exhibits a mixed trend—either stabilizing or destabilizing depending on the relative strength of the solutal buoyancy forces. Increased interphase heat exchange delays oscillatory convection, with both the critical thermal Darcy–Rayleigh number and the oscillation frequency approaching different limits at its asymptotic extremes. The convection cell size remains invariant at the asymptotic limits of the interphase heat transfer coefficient for all viscoelastic and Darcy numbers.