Close to 50% of the world's oil and gas reserves are held in naturally fractured carbonate reservoirs. In addition to the presence of fractures, vugs at different scales and distributions are scattered throughout the porous matrix. These cavities cause fluid flow characteristics to significantly differ from those of conventional homogeneous pore structure reservoirs and bring the need to evaluate equivalent petrophysical properties of the heterogeneous medium. In this study, a microfluidic approach is used to determine water and oil relative permeability curves and phase distribution in two-dimensional micromodels of vugular porous media. Steady-state water–oil injection experiments were performed at different fractional flows, while monitoring the dynamics of the pressure drop and visualizing the fluid displacement at the pore scale. Live-image acquisition through fluorescence microscopy made it possible to examine the evolution of the saturation of water and oil phases. A direct comparison between the relative permeability curves of well-characterized vugular porous media and their porous matrix showed that the incorporation of vugs leads to (i) higher equivalent absolute permeability, especially with longer cavities and higher vug density, (ii) increased oil occupancy in the porous matrix, due to less efficient water invasion into the porous matrix, and (iii) higher relative permeability to water, which flows preferentially through the vugular space. These results are consistent with the oil-wet nature of micromodels, since the vugs offer less capillary resistance to the flow of the non-wetting phase.
Thin liquid films are ubiquitous in nature and have many practical applications. From biological films to the curtain coating process, thin films are present in both large and small scales. Despite their importance, understanding the stability of these films remains a significant challenge due to the fluid–fluid interface that is free to deform, affected by interfacial tension and complex rheological behavior. Instabilities in thin films are often caused by van der Waals attractions, which can lead to the rupture of the layer. To investigate the rupture dynamics, numerical methods are commonly used, such as asymptotic derivations of the lubrication theory or interface tracking methods. In this paper, we present a computational study of the breakup dynamics of a stationary thin liquid sheet bounded by a passive gas with a viscous interface, using the arbitrary Lagrangian–Eulerian method and the Boussinesq–Scriven constitutive law to model the rheological behavior. Our results demonstrate that the stability of thin liquid films is influenced by both surface rheology and disjoining effects and that the viscous character of the interface can delay sheet breakup, leading to more stable films.
For investigating the nonlinear acoustic behavior of circular orifices under high amplitude sound excitation, an approach based on the three-dimensional time-domain computational fluid dynamics simulation is proposed to extract the nonlinear acoustic impedance of circular orifices. By solving laminar flow equations, the propagation of the acoustic signal in the vicinity of orifice is simulated, and the effect of the interaction among adjacent orifices is considered by using the lateral periodic boundary condition. The effects of diameter, thickness, and porosity on acoustic impedance under different amplitude sound excitations are studied. By using nonlinear regression analysis of the dimensionless parameters composed of particle velocity amplitude, frequency, and geometric parameters, fitting formulas of nonlinear acoustic impedance of circular orifices are presented. As an application of engineering computation, transmission loss of perforated silencers under low and high amplitude sound excitations is predicted by using the present acoustic impedance fitting formulas and via the finite element computation. By comparing the predicted and measured results, the accuracy and practicability of fitting formulas of nonlinear acoustic impedance of circular orifices are validated.
Numerical investigation of the coupled Kelvin-Helmholtz Rayleigh-Taylor instability (KHRTI) is presented here by solving the compressible Navier-Stokes equations for two air streams differentially heated in two halves of a three-dimensional (3D) box. Here, we explore the role of a stabilizing and destabilizing thermal gradient and that of reversing the direction of the air streams considered for Atwood numbers of {plus minus}0.1567 and dimensionless tangential shear of ΔU = 0.68 and 4.1. The onset of the KHRTI and development of the turbulent mixing layer are explored via time-resolved and instantaneous distributions of temperature and vorticity. Early stages of the KHRTI with reversed air streams follows a Kelvin-Helmholtz (KH) mechanism, with Rayleigh-Taylor (RT) dynamics becoming important at later times. This leads to an earlier development of the turbulent mixing layer. The KHRTI with stabilizing or destabilizing thermal gradients shows a dominance of the buoyancy-driven mechanism, right from the onset. The transition from laminar to turbulent mixing layer involves the creation of coherent structures of spikes, bubbles and KH whirls for the destabilizing, stabilizing thermal gradient and reversed shear cases, respectively. The spectra of the turbulent signals reveal a -5/3 scaling when the shear-driven mechanism is prevalent in the flow and -11/5 scaling when the buoyancy-driven effects become prominent. The compressible enstrophy budget of the KHRTI shows that the onset process is dominated by vortex stretching or compressibility effects, followed by a sharp rise in baroclinic torque contribution once the buoyancy effects become relevant.
We use large-eddy simulation (LES) and theoretical analyses to study the turbulent flow over fast opposing water waves. A novel nonlinear viscous model for the airflow perturbations induced by Stokes waves is developed, which can account for the wave-perturbation viscous stress and the nonlinear forcing by the multimode interactions of wave-correlated quantities in the wind field. Overall, the nonlinear viscous model can describe the wave-coherent airflow in the LES results for high-order Stokes waves, which demonstrates the negligible effects of wave-perturbation turbulent stress in the wind. According to the model, the dominant components of the fundamental mode of the airflow perturbation primarily result from the linear response of the wind to the wave, and, thus, are not substantially affected by the nonlinear forcing. However, the weak components of the fundamental mode, which produce the form drag on the wave, are created by the combined effects of the nonlinear forcing and the wave-perturbation viscous stress. We found that the main mechanism for generating the nonlinear forcing is the interaction between the second harmonic and the fundamental mode of the wave-correlated quantities in the air. In this mechanism, wave nonlinearity exerts its effects mainly through the second harmonic of the wave surface, instead of the second harmonic of the wave kinematics. Therefore, it is further demonstrated that a second-order Stokes wave is sufficient to capture the wave nonlinearity effects on the form drag.
